US20070041845A1

US20070041845A1 - Motor-driven metering pump

Info

Publication number: US20070041845A1
Application number: US11/506,355
Authority: US
Inventors: Thomas Freudenberger
Original assignee: Prominent Dosiertechnik GmbH
Current assignee: Prominent Dosiertechnik GmbH
Priority date: 2005-08-19
Filing date: 2006-08-18
Publication date: 2007-02-22
Also published as: ES2581979T3; EP1754891B1; EP1754891A2; DE102005039237A1; EP1754891A3; JP2007051645A

Abstract

A metering pump with a rotary drive motor and an oscillating piston, wherein the rotary motion of a drive motor is transformed into an oscillatory motion of a connecting rod by means of a gear arrangement, so that a displacement means activated thereby executes an oscillating linear motion on continuous rotation of the drive motor, that results in transfer of a medium to be metered in a metering head (12) arranged in the longitudinal axis of the connecting rod (19) cooperating alternately with an outlet and inlet valve to produce a pump stroke (pressure stroke) and a priming stroke. A reference element (35) is associated with the connecting rod, the position of which is detected by a positional sensor, wherein the positional sensor provides an actual signal (x_I) that is in a fixed relationship to the position of the reference element and thus to that of the displacement means and that provides information regarding the motion executed by the displacement means, so that the electronic control system of the metering pump can react to the conditions of the metering circuit and pump.

Description

BACKGROUND OF THE INVENTION

The invention relates to a motor-driven metering pump with a rotary drive motor and an oscillating piston. Rotary motion of a drive motor is transformed into an oscillatory motion of a connecting rod by means of a gear arrangement, so that a displacement means, e.g. a piston or diaphragm, actuated thereby executes an oscillating linear motion on continuous rotation of the drive motor that results in transfer of a medium to be metered in a metering head arranged in the longitudinal axis of the connecting rod cooperating alternately with an outlet and an inlet valve to produce a pump stroke (pressure stroke) and a priming stroke.
Motor-driven metering pumps of that type are generally known and are matched to requirements by add-ons. They operate volumetrically, wherein metering is carried out by transporting a closed volume through a displacement means. The metering volume per stroke thus corresponds to the difference in volume on movement of the displacement means.
In general, such motor-driven metering pumps convert the continuous rotary motion of a drive motor via a conversion means, e.g. a gear unit, (all such conversion means being referred to herein as a gear unit for convenience) into an oscillating linear motion of the displacement means. The rotary speed and torque of the motor may be reduced in the conversion unit and matched to speed and power requirements of the displacement means. A take-off shaft of the gear may drive a deflection device for converting the rotary motion into a lateral deflection, i.e. at right angles to the rotary axis, such as a spring/cog or cam drive. The lateral deflection actuates a connecting rod that may be slidably guided in bearings in the direction of the deflection. This transfers the motion and power to the displacement means that, in a metering head arranged in the longitudinal axis of the connecting rod, cooperating alternately with an outlet and inlet valve, produces a pump stroke (pressure stroke) and priming stroke and thus results in movement of the medium to be metered.
The various embodiments may differ firstly in the type of motor. Such motors may usually be asynchronous motors, synchronous motors and step motors, that are mounted outside or inside the actual pump housing. The individual metering pump types also differ in the type of gear unit, e.g. a worm gear, a spur gear, a belt drive or a chain drive. The drive for the connecting rod via the deflection device can be force-guided or with positive locking only on forward motion of the deflection device. The connecting rod is usually driven in the pressure stroke by the deflection device, but in the latter case on priming, it may be driven by a recuperating spring, that places it close to the reversing of the deflection device. The recuperating spring is compressed on the pressure stroke and is dimensioned to provide the required force required upon priming. The various types of pump may also differ in form of the type of power coupling from the connecting rod to a diaphragm or piston as the displacement means either via a rigid linkage or a hydraulic intermediate circuit. Since hydraulic fluid, usually oil, is not compressible, a hydraulic coupling operates like a rigid coupling. In addition to the system described here with a metering head, pump constructions are also known that operate with two or more metering heads driven by a common drive. In one example, two opposing connecting rods may be arranged on either side of a cam in a common longitudinal axis that are driven in opposite directions and each has a metering head with its own displacement means. In another example, it is also known to operate with multiple metering heads with an extended cam shaft that carries several jointly driven cams each of which drive a unit formed by a connecting rod arranged across the cam axis and a metering head with a displacement means lying in the direction of the connecting rod axis.
In the simplest case, all moving parts are mounted in ball bearings or friction bearings in a common pump housing; in other cases, the individual functional groups are collected in further housings or mountings some of which may be filled with oil and are mounted as modules. An example of this case would be a unit mounted outside the pump housing formed by the motor and reduction gear with a mounting flange and a ready mounted take-off shaft.
In the simplest case, the drive motor is on continuously for continuous metering or for a particular period for individual metering strokes. Other types control the drive motor via a frequency converter in accordance with a predetermined time profile, whereby the motor rotary speed and thus the metering power is more reproducible and independent of electrical parameters such as the frequency or actual level of the mains.
The motor rotary speed is predetermined by the electrical frequency of the motor drive and, together with the gear reduction and the gear characteristics that is sinusoidal with a cam gear, determines the period of each stroke. When continuously driven, the period per stroke is given by the effective motor rotary speed under load and the gear reduction. When in on/off operation, where single strokes or sets of strokes are carried out, between which the motor itself is stopped, for example at the priming dead centre, is stopped, startup and slowdown times must be considered and the period per stroke is correspondingly extended. In continuous operation, the stroke frequency is provided by the period per stroke and in on/off operation it is determined by the repetition rate for motor switching, that naturally cannot be faster than the time necessary for carrying out a stroke.
The stroke length can be adjusted by limiting the lateral deflection. This may be carried out by adjusting an eccentric, for example using wobble cylinders, that operate on the basis of two inclined planes that are rotatable in opposite directions. A further possibility is an adjustable buffer that can be used with unforced deflection systems. This buffer, in the form of a mechanically adjustable spindle, when adjusted limits the reverse motion of the connecting rod on priming to an adjustable position before reaching the rear dead centre of the deflection device. The buffer provides the start point for the stroke motion; the end position is the completed deflection movement. In one possible embodiment, a stroke adjustment pin is screwed into a thread in the pump housing and has a calibrated knob accessible from the outside, that constitutes the buffer for the connecting rod on priming. With hydraulic systems, stroke adjustment is, for example, carried out by means of a slidable sleeve the position of which can be adjusted by means of a calibrated knob accessible by the operator, that is screwed into a thread in the pump housing. The sleeve covers a by pass bore in the connecting rod that, after moving a certain distance, opens a shunt in the oil circuit and increases the power coupling from the connecting rod to the diaphragm.
The motion of the displacement means occurs by a combination of the gear and other mechanical components. During the forward motion, the drive operates against the recuperating spring force operating on the connecting rod via the displacement means. During the reverse motion, with forced displacement systems, the connecting rod is drawn back by the drive, and with one sided actuation, the recuperating spring pushes the connecting rod back and thus produces the power for priming the medium for metering. The motion of the connecting rod thus follows the characteristic of the deflection device; with a cam, for example, this is sinusoidal, lying between the two dead centres of the cam stroke for a full stroke length. In operation with a reduced stroke length, the motion on adjusting a cam is still purely sinusoidal with a reduced amplitude, with rigidly coupled systems with an adjustable buffer or hydraulic systems with a bypass bore, the original motion and the amplitude of the deflection device are maintained, but no longer carried out completely; moreover, the connecting rod motion is intersecting, depending on the adjusted stroke length and the coupling system, at the start or end regions (phase cut-off). The forward motion to carry out the pressure stroke is carried out during a time period which is rather less than one second (for example about 200 ms) depending on the drive of the motor. The priming stroke is carried out as set by the deflection device over a period that is similar to that for the pressure stroke. This results in relatively high instantaneous speeds for the metering medium in both stroke phases; for an eccentric drive, the maximum is about half way through the motion.
With embodiments constituted by several units consisting of connecting rod and metering head driven by a common cam shaft operating with several cams, then those cams may be arranged on the shaft in a phase displaced manner in order to distribute the peak power requirement for the individual metering heads around a full rotation of the cam shaft and thus to optimize the use of the available power of the motor.
Particular embodiments, so-called diaphragm metering pumps, use a partially flexible diaphragm as the displacement means. This is not rigid, but deforms elastically by a particular amount in the flexing region when the pressure of the metering medium operates thereon. The amount of deformation, incurred in a first part of the stroke motion that is not used for metering, is lost to the effective stroke motion and the result is that with increasing operational pressure, the metered amount reduces. This drop-off characteristic is much more prominent in normal use than allowed by the metering accuracy. Thus, motor-driven metering pumps normally cannot be adjusted over a wide range of operating pressures with the desired accuracy; moreover, the errors that arise by a calibration are exacerbated as they are included in further calculations. However, the calibration measurement must be carried out in use under actual operating conditions and particularly when using aggressive chemicals, is a step that is extremely difficult.
Current motor-driven metering pumps in general use are efficient and for many processes have good metering properties, but suffer from disadvantages as regards the hydraulic properties of the metering process compared with the ideal position. Examples that may be mentioned are the relatively strong dependency of the metered amounts on the operating pressure of the metering circuit and disadvantages such as noisy flow or pressure drops due to the high instantaneous flow speeds of the metering medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings show:
FIG. 1 shows a cross section through a motor-driven metering pump with positional sensor;
FIG. 2 shows an exploded view of the positional sensor (enlargement of section X in FIG. 1);
FIG. 3 shows components of positional control circuit;
FIG. 4 shows components of speed control circuit;
FIG. 5 shows a top view of positional sensor in axial direction;
FIG. 6 shows a side view of positional sensor at right angles to axis;
FIG. 7 shows an illustration of shadow region of positional sensor;
FIG. 8 shows brightness values for pixels in a real shadow;
FIG. 9 shows an illustration of positional sensor measurements on the basis of geometrical arrangement;
FIG. 10 shows interpolation of positional resolution;
FIG. 11 shows an illustration of calculation basis for interpolation of positional resolution;
FIG. 12 shows an illustration of metering performance as a function of mechanical stroke length and operating pressure.

BRIEF DESCRIPTION OF THE INVENTION

The invention is a metering pump with a rotary drive motor and an oscillating piston, wherein the rotary motion of a drive motor is transformed into an oscillatory motion of a connecting rod by means of a gear arrangement, so that a displacement means actuated thereby executes an oscillating linear motion on continuous rotation of the drive motor, that results in transfer of a medium to be metered in a metering head arranged in the longitudinal axis of the connecting rod cooperating alternately with an outlet and an inlet valve to produce a pump stroke (pressure stroke) and a priming stroke.
Importantly, a reference element is associated with the connecting rod, the position of which is detected by a positional sensor, wherein the positional sensor provides an actual signal (x_I) that is in a fixed relationship to the position of the reference element and thus to that of the displacement means and that provides information regarding the motion executed by the displacement means, so that an electronic control system of the metering pump can react to the operating conditions of the metering circuit and pump.
The particular aim of the invention is to overcome the known disadvantages as regards hydraulic properties of the metering process and to provide a variable, larger operational range for the motor-driven metering pump without negatively affecting manufacturing costs. Further, the motion of the connecting rod and the connected displacement means should be matched to the reference details so that the metering process itself is adjustable, and any defects caused by manufacture or disadvantageous properties of modules (for example the elastic diaphragm, if present) can be taken into account by the electronic control system. These measures should ensure accurate metering of a predetermined volume of a metering medium in a metering process by preventing or detecting defective operating conditions and compensating for manufacturing and/or in-service deficiencies using the on-board electronics.
The problem is solved by dint of a reference element linked with the connecting rod, the position of which is detected by a positional sensor, wherein the positional sensor provides an actual signal (x_I) that is in a fixed relationship to the position of the reference element and thus to that of the displacement means and that provides information regarding the motion of the displacement means, so that the electronic control system of the metering pump can react to the operating conditions of the metering circuit and pump.
The positional sensor captures the motion of the connecting rod and the electronic control system evaluates it. In this regard, starting from the limiting conditions, the control system examines the motion by comparing characteristic features and reacts thereto by influencing the motor drive so that metering is carried out in the best possible manner and inaccuracies that arise, for example due to the properties of the diaphragm, are eliminated.
The positional sensor may detect the position of the reference element in accordance with a touch free principle. If the positional sensor operates in accordance with a touch-free principle, then wear-free operation of the sensor is guaranteed, which because of the large number of strokes during the service life of a metering pump is advantageous and in fact necessary.
The reference element associated with the connecting rod and the positional sensor may be outside the metering head. If the positional element associated with the connecting rod is located outside the metering head, then flexibility as regards space for the precursor is increased.
If the reference element influences the light path of a light source and if the positional sensor cooperating therewith, that is fixed in the pump housing or on another stationary part, operates as a light-sensitive receiver, then wear free operation is ensured, which is vital because of the large number of strokes during the service life of a metering pump, and the moving parts are swept over without touching them. A further advantage of such an arrangement is that such a structure for a positional sensor is in principle insensitive to stray magnetic fields.
If the reference element is a shadow-producing body or a shadow-providing edge and the cooperating positional sensor that is fixed in the pump housing or to another stationary part is constituted by a series of light-sensitive charge coupled devices (CCD), such an arrangement has important optical properties that must be satisfied by the positional sensor. Firstly, the arrangement operates on a wear free optical functional principle and is insensitive to stray magnetic fields, and secondly, such a sensor has practically no linearity defects.
If the positional sensor is arranged on its own sensor carrier, that is fixed to the pump housing or another stationary part, such an arrangement can be pre-assembled as a module and tested, thus facilitating assembly. If the sensor carrier is formed as a non insulating plastic part, then in addition, electrical insulation of the sensor elements from the metallic parts of the housing or gears is simplified.
If the light source, the shadow-producing body or the shadow providing contour and the receiver constitute a lightbox-like arrangement and if the measurements are fed continuously or supplied stepwise to the electronic control system, such an arrangement provides the electronic control system with positional data at a suitable rate.
If the optical receiver of the positional sensor consists of a plurality of linearly arranged receivers (pixels), preferably 128 pixels, such an arrangement can readily determine the position by determining the edge of the shadow between illuminated and non-illuminated cells and thus clearly has a resolution equal to the separation of the cells of the receiver module.
If the light source is a light emitting diode (LED), that is arranged opposite the optical receiver of the positional sensor, so that its light beam directed at the receiver is not perturbed by the connecting rod, then this has the advantage that the cheap LED has a near point light source which is vital to high optical resolution, and has an almost limitless service life. Arranging it opposite the positional sensor beyond the connecting rod produces a large distance between the light source and the receiver, which makes the projection angle of the relevant light beams relatively independent of the mounting position of the elements.
If the starting value for the positional sensor is produced by interpolating the brightness values for several pixels lying in the shadow transitional region, then the resolution for the starting signal for the positional sensor is finer than when it is determined by the mechanical pitch (spacing) of the cells of the CCD receiver.
If filtering means are employed when processing the signals from the positional sensor, the resistance to interference of the positional sensor is improved.
The sensitivity of the positional sensor as regards variations in assembly and mechanical displacements during operation, for example during heating up or on wear of the bearings, is reduced if zero position errors of the positional sensor are eliminated by means of a reference memory or scaling errors of the positional sensor are eliminated by including one or more reference positions.
If variations in illumination of the positional sensors are evened out by controlling or regulating the light source using the brightness values obtained for the pixels, the sensitivity of the positional sensor to variations in module parameters is reduced.
If brightness variations between individual pixels of the optical receiver are compensated for by incorporating a reference memory for the sensitivity of each pixel, the effects of dirt on the optical receiver are reduced.
If the value to which the stroke adjustment pin is set is determined by measurement during metering directly via the positional sensor, an additional sensor for mechanically positioning the accompanying on-board elements can be dispensed with.
If the electronic control system detects a blockage in the displacement means or an incomplete stroke by evaluating the positional sensor signal, then the reliability of metering is increased. With prior art metering pumps without a positional sensor, sensors are often added that monitor the metering motion, for example by passing a reference mark per stroke and sending a check signal to the electronic control system, whereby the stroke period is measured and allows a deduction to be made that the correct metering process has been executed. In contrast to such sensors, the described use of a positional sensor has the advantage that the desired information is available at any point in the metering stroke and not simply when passing the reference mark, so that such additional sensors can readily be dispensed with.
If the drive motor operates with slippage in which, for example, an asynchronous motor is employed, and if the electronic control system determines a nominal stroke frequency or a nominal stroke period for the displacement means from the nominal rotary speed of the drive motor and the known gear characteristics and also determines the actual stroke frequency or the actual stroke period of the displacement means by evaluating the positional sensor signal, wherein the slippage in the drive motor is determined by comparing the actual stroke frequency with the nominal stroke frequency or the actual stroke period with the nominal stroke period of the displacement means, and further, if the nominal rotary speed of the drive motor is changed so that finally, the displacement means moves at the desired stroke frequency, this improves the accuracy of metering by eliminating errors in the stroke frequency that would be caused by slippage in the drive motor. Prior art metering pumps without positional sensors often employ sensors that monitor the metering motion, for example by passing a reference mark per stroke and sending a check impulse to the electronic control system, whereby the stroke period can be measured and corrected; such additional sensors can be dispensed with when using a positional sensor.
If the drive motor operates with slippage in which, for example, an asynchronous motor is employed, and if the electronic control system determines a nominal stroke frequency or a nominal stroke period for the displacement means from the nominal rotary speed of the drive motor and the known gear characteristics for the piston and also determines the actual stroke frequency or the actual stroke period of the displacement means by evaluating the positional sensor signal, wherein slippage in the drive motor is determined by comparing the actual stroke frequency with the nominal stroke frequency or the actual stroke period with the nominal stroke period of the displacement means and further, if the electronic control system determines the force on the displacement means from the observed slippage in the drive motor and the known gear characteristic and reduces the operating pressure of the metering medium, this information means that monitoring functions and compensatory functions can be executed that improve reliability and accuracy of metering. If the electronic control system determines a nominal speed for the displacement means from the nominal rotary speed of the drive motor and the known gear characteristic for every instant of the metering process and in addition, determines it by evaluating from the positional sensor signal the actual speed of the displacement means, wherein it calculates the instantaneous slippage in the drive motor by comparing the actual instantaneous speed with the nominal speed of the displacement means and thereby, again in connection with the known gear characteristic, associates it with the instantaneous energy profile in the displacement means, then the desired information regarding the energy at any point in the metering process is available and the desired monitoring and compensation functions can be executed at different times, improving reliability and accuracy of the metering.
If the electronic control system reduces the operating pressure of the metering medium from the observed power profile in the displacement means, this compensates for the damaging effects of the operating pressure on the metering process.
If the electronic control system detects operation outside the specified pressure range from the observed operating pressure of the metering medium and if it adjusts metering on exceeding a predetermined maximum allowable pressure specified for the metering pump or by the operator, or on going below a predetermined minimum pressure, then problematic operating conditions such as excess pressure situations or pressure loss due to defective pipework are detected and safety measures can be taken, such as adjusting the metering, that improves the reliability of metering. The otherwise necessary additional operational means such as an excess pressure limiter can thus be dispensed with, as long as the metering pump is the only pressure increasing system in the process. The possibility of controlling the operating pressure to values within the specified pressure range of the metering pump broadens the possibilities for pressure monitoring to situations where the monitoring system of prior art metering pumps, that only come into play when the metering pump becomes blocked, cannot be used.
If the displacement means is a partially elastic diaphragm and informs the electronic control system of a possible metering error that is caused by the elastic deformation of the diaphragm and determined from the measured operating pressure of the metering medium and the known dependency of the metering efficacy on the operating pressure, and if it influences the rotary speed of the drive motor and thus the stroke frequency so that the expected metering error is compensated for, this improves the accuracy of metering.
If the rotary speed of the drive motor is influenced by the signal (x_I) read by the positional sensor for the position of the connecting rod via a control circuit in the context of its accuracy of control and as a result influences the linear motion of the connecting rod and thus of the displacement means so that it follows a given nominal profile, this intended influence on the motion of the displacement means can be exploited to achieve or improve advantageous hydraulic properties of metering, for example on slow metering, and/or the metering accuracy for a partial stroke.
Advantageously, in addition to the positional sensor, the metering pump has a control device and it alternately influences the position (hereinafter denoted x_I), speed (hereinafter denoted v_I) or the acceleration of the displacement means via a control device by changing the rotary speed of the drive motor. Controlling the speed allows direct control of the actual flow rate of the metering medium that, for example, is necessary to avoid cavitations when priming slowly. Controlling the position, on the other hand, allows situations near to standstill to be controlled, wherein the information regarding speed, that is produced by differences in the displacement signal, become very small and cannot be processed effectively by the control device. Controlling the position gets around this problem and is advantageously employed, for example, with electronic stroke limiting or slow metering. Controlling the acceleration is advantageous for easy control of the regulation, as the acceleration of the masses to be moved constitutes a direct picture of the motor power for fast processes.
If in addition to the positional sensor the metering pump has a control device and if it reduces the v_Iof displacement means in the priming phase and/or in the pressure phase, pressure losses caused by resistance to flow or the creation of cavitations are counteracted. When metering highly viscous media, for example lecithin, large pressure drops occur in narrow places, such as in the valves, when the flow rate is too high. These pressure drops must be overcome in the form of additional power from the drive and, by employing the control of the v_Iof the displacement means, can be kept low. In addition, flow noises on reduced flow speeds can be effectively reduced. When metering media that readily evolve gas, such as chlorine bleach, cavitations frequently occurs particularly during priming, when too high a flow speed is used, by dropping below the vapour pressure of the metering medium, resulting in increased mechanical wear. Controlling the v_Iin the displacement means in the priming phase and/or in the pressure phase advantageously avoids this phenomenon.
If in addition to the positional sensor the metering pump has a control device and if the desired stroke length is communicated to the control device and the motion of the displacement means is electronically limited by the control device to the stroke length to be executed, wherein the control device stops the drive motor after executing the desired stroke length, switches it into reverse and then executes a priming stroke and then stops the motor or executes the next pressure stroke, then the mechanical adjustment elements can largely be dispensed with.
If in addition to the positional sensor the metering pump has a control device and if the control device distributes the forward motion of the displacement means during the pressure phase by driving the motor for the period given by the repetition rate of the metering stroke, so that the metering medium is dispensed in the smoothest manner possible, even with very slow metering strokes of several minutes duration, for example, then concentration variations in the metering medium can be substantially avoided.
The metering accuracy is improved if the displacement means is a partially elastic diaphragm and the electronic control system detects opening of the outlet valve from the instantaneous power profile and thus from this observation measures the dead region that is caused by the elastic deformation of the diaphragm, and then influences the actual stroke path by intentionally stopping the stroke motion as a function of the diaphragm deformation so that the dependency of the metered amount on back pressure is substantially reduced. This improvement is achieved by eliminating errors that are caused by elastic deformation of the diaphragm due to the operating pressure so that said deformation does not contribute to the metering. By dint of the reduced dependency of the metered quantity on the operating pressure, subsequent calibrations that otherwise are required when operational parameters such as operating pressure are significantly changed, are dispensed with. Compensating for the diaphragm deformation by observing the power profile is particularly advantageous when evaluating motor slippage as this is a good reflection of the actual power requirement and thus no additional measurements are required.
The metering accuracy is improved if the metering pump, in addition to the positional sensor, has a control device, the displacement means is a partially elastic diaphragm and the actual stroke path is influenced by the diaphragm deformation, wherein the control device stops the drive motor after executing the desired stroke length after opening the outlet valve, switches it into reverse and then executes a priming stroke, stops the motor or carries out the next pressure stroke so that the error contributed by the diaphragm deformation (with respect to the stroke path or the metered volume) is eliminated and the amount of this deformation does not contribute to metering. The reduced dependency of the metered quantity on the operating pressure means that subsequent calibrations that are otherwise necessary on changing the operating parameters such as the operating pressure can be dispensed with, and the linearity of the relationship between the set stroke length and the actual metered amount of the metering medium are improved. Determining the deflection of the diaphragm deformation from an observation of the power profile is particularly advantageous when evaluating the motor slippage as this is a good picture of the actual power requirement and thus does not necessitate additional measurements.
If the displacement means is a partially elastic diaphragm and the metering pump has a control device in addition to the positional sensor, and the actual stroke frequency is influenced by the determined diaphragm deformation, wherein the control device determines a correction for the error caused by the diaphragm deformation (with respect to the stroke path or the metered volume) and the nominal rotary speed of the drive motor is so changed by said correction that errors caused by the diaphragm deformation are eliminated, the dependency of the metered quantity on the operating pressure is reduced.
We shall now describe in more detail an embodiment of the invention of a motor-driven diaphragm metering pump with a cam drive and its various uses in conjunction with the drawings.
FIG. 1 shows the structure of a motor-driven metering pump (partially cut away). As is generally known, the motor-driven metering pump consists essentially of three groups of components, namely the drive motor 2 with a gear unit, the cam drive in the cam housing 1 and the electronics housing 28 with the electronic control system contained therein and the on-board electronic modules and groups installed therein. The underside of the electronics housing 28 has a foot plate 4 with fixing holes; the cam housing 1, that is set on the electronics housing and fixed thereto, carries the drive motor 2 with the gear unit, that is fixed to the cam housing using screws, for example.
The housing, formed from the cam housing 1 and the electronics housing 28, has in its upper part, namely the cam housing 1, the components of the cam drive. The components of the cam drive are housed in a cam carrier 22 which ensures that the individual components are positioned correctly and is fixed in the cam housing 1. A three phase asynchronous motor 2 is flanged from outside as a module on the cam housing 1 together with a reduction gear 11 that is formed as an angular gear and fixed with screws. The take-off shaft of the motor forms a right angle to the shaft axis of the motor and either directly forms the drive shaft for the cam drive or, as in the present embodiment, is fixed coaxially thereto via a coupling. The drive shaft of the cam drive, the cam shaft 17, is rotatably mounted in the cam carrier 22 and carries a cam fixed thereto. The cam shaft along with the cam pass through a suitably cut out push arm 20. The cam shaft 17 is rotated by the motor/gear unit via the shaft coupling for a driven motor 2 and drives the push arm 20 onto an inner surface of its cutout, namely the stop surface, with the outer surface of the cam. The push arm 20 drives a connecting rod 19, that in this example is injected, that is fixed thereto. The unit formed by the push arm 20 and the connecting rod 19 is longitudinally displaceable in two guide bushes. The axis of the cam shaft 17 and the longitudinal axis 18 of the push arm 20 and the connecting rod 19 are in the horizontal plane and form a right angle to each other. One of the two guide bushes 26 for the connecting rod 19 sits in a bearing plate 24 that is fixed to the cam carrier 22 on the pressure head side; a further guide bush 27, that takes up the spigot of the push arm 20 turned towards the metering head side, sits in the stroke adjustment pin 8. Coaxially with the longitudinal axis 18 of the connecting rod 19 is a manually actuatable adjustment means 7 to adjust the stroke adjustment pin 8 screwed into a thread of the cam carrier 22, so that the axial motion of the push arm 20 is thus limited on priming and thus the stroke of the metering pump is limited.
In its lower part in a sealed chamber, the housing also contains the electronics housing 28, i.e. the electronic control system. The housing is sealed against spray and protects the cam drive as well as the electronic control system from moisture or corrosion, as metering pumps are often used in connection with chemically aggressive media. The electronic control system consists of a horizontal control electronic element 34 with the control for the motor control 29, that is formed as an integrated frequency converter, and electronics 6 arranged in a housing cover 5 that carries the input and display elements required to operate the metering pump. The control elements are protected by a cover 9. Under the cover 9 are connections for the control wires 10 or for the power supply.
On the side to the control wires 10 or the power supply connections, coaxially with the longitudinal axis 18 of the connecting rod is a metering head 12 in which a diaphragm produced, for example, from plastic acts as the displacement means, that is fixed at its circumference. The metering head 12 also carries an inlet valve 14 and an outlet valve 15 to impel the metering medium primed into the metering chamber 16 through the inlet valve 14 between the diaphragm and metering head 12 through the outlet valve 15 into the metering tube. The motor-driven metering pump operates volumetrically, i.e. a predetermined volume is primed in and then forced out through the outlet valve 15 on each stroke. The diaphragm 13 is moved in an oscillating motion by means of the cam drive that moves the connecting rod 19 to and from in the longitudinal axis. On the stroke adjustment pin 8 side, the unit formed by push arm 20 and connecting rod 19 operates with the adjustment means 7 as a manually adjustable stroke adjustment device. At the opposite end, the part of the connecting rod 19 facing the metering head 12 is fixed to the core 30 of the diaphragm 13 and moves it in an oscillatory motion.
Between the push arm 20 and a collar 25 of the bearing sleeve 24 is a compression spring 23, for example a spiral spring, that constantly causes the push arm 20 to bear against the cam. In the forward phase of the cam motion, i.e. the movement of the connecting rod towards the metering head, the push arm is moved with the connecting rod towards the compression spring, and simultaneously the diaphragm 13 is impelled into the metering chamber 16, which means that an excess pressure occurs in the metering chamber, the outlet valve 15, for example a spring loaded ball valve, opens and the metering medium is forced into the metering tube. In the reverse phase of the cam motion, i.e. movement of the connecting rod away from the metering head, the push arm 20 is moved in the opposite direction towards the stroke adjustment pin 8 by the compressed spring 23 that may, for example, be formed as a spiral spring, following the cam motion, which means that the connecting rod 19 moves the diaphragm 13, and an under pressure occurs in the metering chamber 16, that opens the inlet valve 14 so that a further batch of metering medium can be primed into the metering chamber. The alternating oscillating motion of the diaphragm 13 by means of the cam drive drives the metering medium into the metering tube. The cam drive produces a sinusoidal motion of the unit formed form the push arm 20, connecting rod 19 and diaphragm 13 during a metering stroke. If a reduced stroke length is set using the stroke adjustment pin 8, the motion in the priming phase prematurely brakes before the dead centre is reached by means of the adjustable buffer of the stroke adjustment pin 8, whereby the sinusoidal path of the motion is intersecting and the phase of the stroke motion is altered.
The position of the unit formed by the push arm 20, connecting rod 19 and diaphragm 13 is detected by the positional sensor 36, the signal from which is in a predetermined relationship to this position; this relationship may, for example, be a strictly proportional relationship. The signal of the positional sensor 36 thus constantly relates to the position of the part of the movable unit where it is employed. This fixing point is formed by the reference element, which is abstract in this case. Depending on the requirements of the positional sensor, it may be formed as a real additional element to be built in, but it may solely consist of a characteristic shape, for example an edge or face on one of the required components, for example on the push arm 20.
In this embodiment, the cam carrier 22 has a sensor carrier 31 (see also the illustration in FIG. 6) fixed thereto, that on one side carries longitudinally orientated light-sensitive CCD cells 32 (charged coupled device) and on the other side carries a light source 33, for example a light emitting diode.
The sensor carrier 31 fixed to the cam carrier and the components fixed thereto form a lightbox, the beam from which is partially interrupted by the push arm. The reference element is formed by a shadow-providing edge 35 of the push arm 20 in the region of the lightbox arrangement. When the connecting rod 19 oscillates, the shadow-providing edge 35 passes over the light-sensitive cells 32 without touching them. As can in particular be seen in FIG. 5, which Figure shows a top view in the axial direction, the light source 33 must be arranged so that on its way to the light-sensitive cells 32, the light beam is not interrupted by the connecting rod 19; this means, for example, that the light source 33 and the lines of light-sensitive CCD cells 32 are arranged over or under the connecting rod 19. As can in particular be seen in FIG. 7, a shadow is cast by the shadow-providing edge of the light source 33 onto the light-sensitive cells 32, that divides the cells into illuminated (h) and non illuminated (d) cells. Since the row of light-sensitive cells orientated parallel to the long axis 18, for example 128 pixels, which covers a distance of about 8 mm, is only partially illuminated or in shadow in the transitional region, the transitional situation SV shown in FIG. 8 occurs. The height of the right angled surfaces shown in FIG. 8 represents the brightness of the pixels. A special process, that will be described below with reference to FIG. 10, exploits this transitional situation in order to accurately determine the position of the shadow-providing edge and thus the position of the connecting rod or the diaphragm. This measuring device, consisting of the shadow-providing edge on the push arm side and the light-sensitive CCD cells on the sensor carrier side with the opposite light source, serves to measure the actual position or speed of the oscillating connecting rod and to exploit this information to carry out the functions described.
The connecting rod that sets the diaphragm moving in an oscillatory movement, covers a distance on each stroke that corresponds to the mechanical stroke length. In order to allow for assembly variations, the longitudinal extent of the light-sensitive CCD cells must be somewhat greater. This is principally the case with all other positional sensors that may be envisaged.
When the diaphragm or, more generally, the displacement motion is to be controlled using the positional sensor signal, as explained in FIG. 3 and FIG. 4 in particular, the following mechanical and electronic components are required. The abbreviations used in the two Figures mean the following:
x_s: nominal value for position of displacement means;
x_I: actual value for position of displacement means;
x_SI: deviation for position of displacement means;
vs: nominal value for speed of displacement means;
v_I: actual value for speed of displacement means;
v_SI: deviation for speed of displacement means;
SG: controller output;
KSG: corrected controller output;
MA(U, f) motor-driven drive (voltage or frequency).
The moving parts of the drive, the motion of which is to be controlled, consists of the push arm 20 with the connecting rod 19, to which the diaphragm core 30 is fixed. The recuperating spring 23 returns the push arm after a working stroke and thus operates priming. The outer ring of the diaphragm 13 is fixedly mounted in the metering head 12, which metallic diaphragm core 30 injected into the diaphragm moves the central surface of the diaphragm in the metering head as the displacement means. The inlet valve 14 closes on the priming side, the outlet valve 15 on the pressure side of the metering head and thus offers a connection possibility for the external pipework. A reference element is, for example, connected at the end facing the metering head with the connecting rod 19 or with a module connected thereto, in this case the push arm 20, for example, the position of which in the present case is detected by a positional sensor 36 that operates without touching. In the embodiment shown, the reference element is a shadow-providing edge 35 of the push arm 20 and the positional sensor is a lightbox-like arrangement consisting of the light source 33 described above cooperating with a series of light-sensitive cells 32, that determine the position of the shadow-providing edge 35 optically, and thus without touching by its shadow formation. Since the connecting rod 19 is the actual connection and power coupling to the diaphragm 13 and push arm and connecting rod are fixed together in the present example, the description below constantly refers to the motion of the connecting rod 19, although it is really that of the shadow-providing edge 35 of the push arm 20 that is measured.
The positional sensor 36 produces an actual signal x_Ithat is proportional to the position of the reference element 35. In the case of a speed controller, in this embodiment it is fed through a time differentiator 37 (dx_I/dt) and thus additionally produces an actual signal v_Ithat is proportional to the speed. Other methods clearly would be suitable for the control step, that could produce a signal proportional to the diaphragm speed. Depending on the type of control and the metering requirements, a time dependent profile for the nominal value 38 of position x_Sor the speed v_Sis produced. A variance comparison 39 determines the variation as a positional variation x_SI=(x_S−x_I) or a speed variation v_SI=(v_S−v_I) and the result is given on a PID control (proportional, integral and differential control). The output, the controller output SG, corresponds to the value for the drive. To improve the stability of the controller, the controller output SG is processed further using a positional correction 41. The positional correction takes into account the fact that the rotary speed of the motor depends on the rotary angle of the cam (deflection from the connecting rod position) according to the sinusoidal characteristic of the cam drive is transformed into the speed at the connecting rod. The positional correction 41 then transforms the start signal for the PID controller 40 over the reverse characteristic of the cam drive into a corrected controller output, KSG, that represents the required motor drive with respect to the input to the reduction gear 11, that is required in order to obtain a motion of the connecting rod 19 at the output from the cam drive that corresponds to the desired controller output SG. An amplifier 42, that is formed as a frequency converter, holds the power levels and drives the motor at the required rotary speed with the accompanying voltage and frequency. The degree of position dependent correction, the transformation of the corrected controller output KSG into a real rotary speed for the frequency converter and if necessary the deflection constant for the formation of the whole speed signal v_Iare set by the three proportionality factors k1, k2, k3. The factor for the position-dependent correction, k1, is selected according to the characteristic of the cam drive; the two factors k2 for the amplifier or k3 for the speed signal deviation, can be selected from practical considerations, such as operation with the best available ranges for the dimensions used.
FIG. 3 shows the control circuit for a positional regulator, and FIG. 4 shows the control circuit when using a speed controller. The control circuit described transfers the predetermined time dependent profile for the nominal value for the position x_sor the speed vs, clearly in the context of its possible control regulation.
Establishing the real profile for the position, speed or acceleration and switching between these operational modes occurs as described below, for example, taking into account the functional limitations of the controller such as control speed, achievable accuracy, etc.
With such a control, a motor-driven metering pump can be used to predetermine the desired speed of the diaphragm 13, in general the displacement means, and thus to control the effective flow speed of the metering medium.
The diaphragm position can thus be directly controlled. This function allows the positions to be obtained in selected phases of the metering process and if necessary also when stationary.
By controlling the motion by means of a positional indicator, in contrast to uncontrolled operation, changes in the operational parameters can be responded to, which changes crop up over time or occur because of environmental considerations or variations, i.e. statistical deviations in the production series, and can minimize their damaging influence. Examples that can be cited are the diaphragm rigidity or the viscosity of the metering medium. Both require drive force that must be added to the force for creating the operational pressure on the diaphragm surface. By determining their effect and subsequently regulating the motor drive, these damaging influences can be compensated for. An unregulated metering pump with a preset motor speed, even if this is controlled to keep it stable, still suffers from such influences. Moreover, because of the sinusoidal characteristic of the cam drive, it is not possible to accurately predict the instantaneous speed of the displacement means without knowing the connecting rod position, i.e. the angle of the cam.
Further, controlling the movement by means of a positional indicator makes it possible, in contrast to the spontaneous metering process on unregulated operation, to react to internal and external influences that will be described below, and to establish operational conditions that can exploit or avoid particular hydraulic conditions on metering. An example is the function of the cavitations protection on priming that is described below.
Individual implementations for a motor-driven metering pump of the type described above will now be described, which pump has a positional sensor and, by evaluating the positional signal, reduces the operational conditions of the metering circuit or influences the motion of the diaphragm by controlling and adjusting the motor drive. Detection of position of adjustment controller for stroke length
Prior art metering pumps are often operated so that the metering stroke which is executed is converted directly from the dispensed volume of the piston chamber (stroke length) into a metered total volume and this is shown as the volume flow rate in the 1/h unit, for example. For such functions, knowledge of the stroke set by the operator is necessary, as the volume metered per stroke is dependent thereon. The position of the stroke adjustment device in prior art metering pumps must for this reason be transformed by a separate sensor into an electrical signal and be read into the control system. An example of a practical embodiment is a linear potentiometer on the stroke adjustment means 7, that detects the adjustment via a needle.
A metering pump that can detect the actual diaphragm path using the integrated positional sensor 36 during the stroke does not need an additional sensor. The difference between the two positional values in the end positions, that can be measured after reaching the mechanical buffer as soon as the motion stops, can be used to calculate the stroke length directly and is available for further processing.
Detection of a Blockage or Incomplete Stroke
Prior art metering pumps without a positional sensor often employ sensors that provide an impulse to the electronic control system to monitor the metering motion per stroke. A known implementation is, for example, a small permanent magnet, that is fixed to the output shaft of the gear and thus on the cam shat 17 outwardly of the axis and turns with it, in connection with a stationary Hall sensor, that produces a signal on passing the magnet at a particular angle of the cam shaft. The electronic control system measures the stroke period from this signal, that is identical to the cam shaft speed, and deduces therefrom that the metering process is being executed correctly. When a blockage occurs in the execution of the metering stroke by an excess pressure situation, for example an unintentionally closed blocking means in the metering tube, the Hall sensor signal is off and after a monitoring period elapses, an alarm is given and further actions, for example stopping the metering pump, are implemented. With such a prior system, the desired information is available only after the monitoring period has elapsed.
Using a positional sensor 36 means that the speed of the connecting rod 19 in relation to the drive of the motor 2 can be set at any time during the metering stroke and a blockage can essentially be determined at any time.
Slippage Compensation
If the drive motor 2 is a synchronous motor, for example, the effective mechanical rotary speed at the motor take-off shaft under load is always slightly smaller than given by the frequency of the electronic control system. The difference in the two rotary speeds, the so-called slippage, depends on the parameters of the motor and within a reasonable load range is almost proportional to the torque under load. The slippage can be measured using various methods that are described below. It can be used to calculate a correction that can be incorporated into said motor rotary speed in the form of an increase in the frequency using a frequency converter and thus can be compensated for.
Slippage can, for example, be determined by comparing the measured stroke period with that given by the electronic control system. This method is also applied to metering pumps of the prior art by measuring the time difference between two Hall sensor impulses. In the case of a metering pump with a positional sensor, a characteristic point in the stroke path, for example the half way point, is defined to measure the period and for consecutive metering processes the time at which this point is passed is recorded; the difference between two such times is the desired period.
In motor-driven metering pumps with positional sensor 36, a direct method can be used to determine the slippage by observing the instantaneous speed of the connecting rod 19. From the motor rotary speed obtained from the electronic control system, an ideal connecting rod speed can be calculated using the known gear and cam characteristics. Comparing the ideal with the measured speed produces the slippage at any time during the cam cycle and it can be corrected by readjusting the frequency of the motor drive.
Determination of Pressure
If an asynchronous motor is used, the slippage measured by means of one of the methods described above can be used to determine the force on the displacement means and thus the operating pressure of the metering medium can be reduced. However, it should be noted that the cam transfers the force operating on the connecting rod 19 in a sinusoidal manner that depends on the instantaneous angle via the gear 11 to the motor 2. At the two dead centres, i.e. the turning points for the stroke motion, the motor is decoupled from the connecting rod force, i.e. has no load, and at the two points exactly therebetween the cam transfers the maximum load moment to the motor. Thus for an assumed constant connecting rod force, the torque on the motor take-off shaft has the same pattern and thus the slippage also varies almost sinusoidally. The variation is thus a reflection of the connecting rod force.
If as described above the deviation in the stroke period from the ideal value is determined, this represents the slippage transferred via the sinusoidal pattern of the cam, that is also a measure of the mean stroke force, i.e. the operating pressure. If the slippage is continuously determined by comparison of the motor rotary speed given by the electronic control system with the connecting rod speed, then using the known cam characteristic and a knowledge of the instantaneous angle of the cam which follows from the connecting rod position, the force profile at the connecting rod 19 can be calculated. The force profile on the connecting rod can also be used to deduce the operating pressure.
If the deflection mechanism is provided by something other than a cam, then this characteristic can be appropriately applied thereto.
Pressure Limitation, Recognition of Pressure Drop
If the operating pressure is determined according to the methods discussed above, it can be monitored so that it is within specific limits, and inquiries of the monitoring system can generate alarms and other actions such as stopping the metering pump. Monitoring the overstepping of limits can protect the pump or other components; in certain cases, an operational limit of 130% of the maximum pressure of the metering pump can be set and monitored, but the monitored limit can also be within the specified operational limits of the metering pump if, for example, sensitive components have to be protected, and in this case can be set by the operator. It is also possible to monitor compliance with preset operational conditions; in this case, an alarm can be given if, for example, an operating pressure set as a reference rises or falls by a percentage point. If the operating pressure is monitored for compliance with a minimum pressure of 1 bar, for example, it is possible to detect a leak caused by damage to the pipework.
Pressure Compensation
The exact dosage in motor-driven metering pumps is influenced by the operating pressure in different ways depending on the embodiment employed. On the one hand, the drive motor 2, when it is an asynchronous motor, suffers increasing slippage on increasing the operating pressure, which has the effect of a drop in rotary speed and an accompanying reduced stroke frequency. On the other hand, a diaphragm used as the displacement means 13 undergoes elastic deformation under the influence of the operating pressure. At the start of the metering stroke, the interior pressure rises continuously in the metering chamber 16 when the outlet valve 15 is closed, wherein the diaphragm core 30 is moved by the connecting rod 19 into the metering chamber with an increase in pressure and the elastic deformation region of the diaphragm 13 gives under the pressure against the motion of the diaphragm core 30. The diaphragm 13 deforms into itself and overall, practically no volume change occurs, which is because the metering medium is practically incompressible and at this point both valves are closed. At the end of this deformation phase, the chamber pressure corresponds to the external operating pressure. The executed path of the connecting rod 19 corresponds to the diaphragm deformation, and so to the dead region at the start of metering, and substantially does not contribute to metering. The deformation or dead region is typically in the range from about 0.1 to 0.5 mm depending on the size of the diaphragm, operating pressure etc. At the pressure equilibrium point, the operating side outlet valve 15 opens. The pressure on the diaphragm 13 is now practically identical to the outer operating pressure and, like the diaphragm deformation, remains almost constant for the remainder of the metering stroke. The pressure equalization point at which the operating side outlet valve opens marks the actual start of metering, so the diaphragm deformation is lost to the metering stroke, i.e. the effective stroke length is determined from the mechanical length less the diaphragm deformation. Since the diaphragm deformation is itself substantially proportional to the operating pressure, the metering curve typically falls off with increasing operating pressure. The negative deviation is thus more noticeable for shorter stroke lengths.
For a prior art motor-driven metering pump metering is not only dependent on pressure but also under partial stroke conditions is not strictly proportional to the mechanical stroke length. Further, effective metering begins at the stroke only after the initial dead region from the point at which diaphragm deformation is maximal with the opening of the outlet valve 15. If a steady state characteristic is made which shows the metering profile as a function of the mechanical stroke length, a linear rising curve is produced which only shows a real dose after a minimum stroke length corresponding to the dead region of X_T1, X_T2, X_T3, . . . X_Tn(see FIG. 12). Since this minimum stroke length corresponds to the diaphragm deformation, it is thus dependent on the operating pressure p₁, p₂, p₃, . . . p_n.
This shift X_T1, X_T2, X_T3, . . . X_Tnin the steady state characteristic in the prior art means re-calibration under real working conditions, insofar as the current stroke length is substantially changed, as the new metering performance cannot be determined with sufficient accuracy by a proportional calculation from the current and new stroke lengths.
If the operating pressure is determined using one of the methods described above, it is possible, using the described dependencies that can be quantitatively determined in advance for a type of apparatus, to determine in advance and compensate for the error-producing influence of the operating pressure on the metering performance. To this end, the determined operating pressure and the employed stroke length that, as described above, can also be measured using the positional sensor, is used to calculate a correctional value calculated from the known error dependency that is added to the stroke frequency employed. Care should be taken that for practical and economic reasons, only the systematic part of the influence can be eliminated. The pressure dependent metering performance error is mainly determined by the material properties and measurements made on the components employed, that can be changed to a certain extent by alteration or caused by production variations. These variations are not taken into account by the methods described here, which correct errors caused by diaphragm deformation using predefined module parameters or series of measurements; further, the actual relationships must be defined afresh at regular intervals or at every stroke in the present example.
If the error-producing influence of the diaphragm deformation as described above is compensated for, wherein the operating pressure is determined using one of the methods described above and the stroke frequency used is altered by a correctional value, the proportional errors in partial stroke operational mode are also eliminated, and so the metering pump can be operated over practically the entire useful range of stroke lengths from 20% to 100%, for example, without having to carry out the re-calibrations necessary until now in a prior art metering pump that require an adjustment of the stroke length by more than 10%, for example, in order to ensure the specified metering accuracy.
Avoidance of Loss of Flow in Highly Viscous Media
Regulating the speed of the displacement means, in this instance diaphragm 13, in particular with highly viscous media (for example lecithin), can limit flow losses in the valves and other tight spots. High flow speeds in such media have a negative influence on the metering accuracy through additional pressure drops as a result of flow resistance. In addition, it is advantageous here if the valves have more time to open and close because of the limited speed. Both effects improve the metering accuracy in highly viscous media. To achieve this, during the entire metering process, the diaphragm speed is limited to a selectable maximum value. This maximum speed depends inter alia on the viscosity of the actual medium to be metered and is, for example, in the form of several predetermined values that depend on the application selected by the operator or are provided directly. The positional sensor and the regulation of the speed of the displacement means described above can be used to ensure the desired limitation of the diaphragm speed.
Cavitation Protection
With media that readily evolve gas (for example chlorine bleach), particularly on priming, but also in the metering stroke, too high a flow speed can cause cavitations at restricted spots by a local drop in the vapour pressure that is dependent, inter alia, on the chemical composition of the metering medium and on its temperature, resulting in increased wear. Cavitations can be avoided both in the pressure stroke and during priming, i.e. return of the diaphragm 13, by limiting the speed by regulation or simply by setting the rotary speed to a value that is substantially below a critical flow speed. The speed requirement for the control circuit or in the simplest case the motor rotary speed is thus set to limit the corresponding diaphragm speed to 1 mm/50 ms, for example.
Particularly for the priming procedure, cavitations is likely as here the static pressure is particularly low and thus the safety margin for dropping below the vapour pressure is very small. To refine the method, then, it is sensible to limit the diaphragm speed on priming to values smaller than those used in the pressure stroke. Examples of reasonable values are 1 mm/50 ms in the pressure stroke or 1 mm/100 ms during priming, however other values are clearly possible. Essential to an individual treatment of the metering phases is the fact that the positional sensor can determine the exact position of the diaphragm at any time, and so the start of the (particularly critical) priming phase can be determined reliably.
Electronic Stroke Length Adjustment
The invention allows the mechanical device for regulating the stroke length (adjustment means 7 and stroke adjustment pin 8) to be dispensed with. To this end, the control device is told the desired stroke length electronically, for example input by an operator. If the desired stroke length is executed, the position reached by the diaphragm 13 is stopped by braking the motor 2 and then the motor is reversed for the priming phase. The next stroke can occur by turning the motor through the priming dead centre in the opposite direction (pressure phase in reverse, priming in normal operation) or in the same sequence as the previous stroke; in the first case, the motor brake and start procedures between the strokes save time and energy. Care should be taken that the constant changes in direction mean that a fan fixed to the motor shaft can no longer function sufficiently, so in this case the use of an externally driven fan is vital for the motor in case cooling is necessary. Slow metering to avoid variations in concentration
For applications in which good mixing with the process medium stream is required, dispensing of the metering medium into the process must be as even as possible. Particular applications also require the possibility of metering very small amounts over very long periods as evenly as possible, to produce an almost continuous metering. For these cases, in the prior art motor-driven metering pumps are employed that, for example, have a step motor and a self locking gear. A complete stroke is carried out in such metering pumps with a reduced rotary speed or distributed over several steps with intermediate rest periods, and at the end of the complete stroke a complete (fast) priming phase is carried out, and thereafter the metering process continues in the manner described.
With a motion regulated motor metering pump, the time available, that is given by the repetition frequency of the metering stroke, can distributed so that the remaining portion following priming can be exploited to the maximum for the forward motion even up to a short rest phase. The speed to be regulated is thus calculated from the path covered (set stroke length) and the available time. In contrast to prior art motor metering pumps, using a positional sensor 36 and a regulation device means that the position of the connecting rod 19 which is known at all times can give the instantaneous angle of the cam drive that can be incorporated into the motor rotary speed so that the characteristic of the deflection device, that is sinusoidal when using a cam, is compensated for and the metering stroke is a precisely linear movement with correspondingly constant dispensing of the metering medium. The speed can be in a very wide range, from 1 mm/min to 1 mm/s and beyond, for example.
The applications described above for the positional provider together in part with control means show that using a positional sensor on the connecting rod, for example, during the entire stroke and priming procedure reveals the exact position of the displacement means and means that it can be monitored. Positioning and monitoring mean that control parameters that are dependent on the situation that lead to the described advantages can be complied with exactly by measuring the actual values.
Positional Sensor
As already discussed, the reference element for the positional indicator in the embodiment described is the shadow-providing edge 35 on the push arm 20 to detect the position, the shadow of which is cast onto the line of CCD cells 32 (charge coupled devices). The active sensor elements described in more detail in the example, that detect the position, are on the side of the push arm 20 directed towards the metering head. The light source 33 is an LED, the optical receiver is an electronic module with a CCD cell 32, which in this case is mounted on an intermediate part, namely the sensor carrier 31. Mounting the positional sensor 36 on the sensor carrier 31 enables it to be treated in the production process as a stand alone module and it can, for example, be separately pre-assembled and tested away from final construction location. Furthermore, the lightbox-like arrangement described constitutes a touch-free and thus wear-free sensor.
For the basic function, locating the sensor in the area of the moving unit formed by push arm 20 and connecting rod 19 is not significant; the location can be determined by structural considerations such as space, order of assembly etc. Further, the parts described here as being fixedly mounted (light source 33, receiver 32) and those that move with the connecting rod (shadow-providing edge 35) can exchange functions.
In this example, the CCD module 32 is controlled by an evaluation unit that contains a micro processor and produces the required control signals. Instead of a microprocessor, the evaluation unit can also be produced from a DSP (digital signal processor) or discrete technology.
Any element can be used for the light source 33 as long as it produces a sufficiently narrow light spot. Together with the geometry shown in FIG. 7, this width determines the shadow region SV (see also FIG. 8).
The light source 33 can also be constituted by several elements or a line source and the shadow SV can thus be produced to satisfy particular requirements. An example is the production of high brightness without influencing sharpness in the direction of motion.
The CCD line 32 is a linear arrangement of M optical receivers (hereinafter denoted pixels) that are arranged in a regular array with a pitch R of several μm. As an example, there are 128 pixels 64 μm apart over a total length of about 8 mm, i.e. M=128 and R=64 μm.
The control signals that are produced by the evaluation unit sets the illumination time during which the individual pixels of the CCD line 32 integrate the incident light in an amplifier in the CCD module and stores it for later processing. This integration occurs not only over the illumination period, but also over the light-sensitive surface of each pixel. After illumination, the brightness values for the pixels are successively read by further control signals as analogue values from the CCD module and captured by the evaluation unit.
Illumination and reading of the brightness values occur alternately in the simplest case. Some commercial CCD line constructions also have the possibility of simultaneously carrying out both procedures, wherein they store the integrated illumination measurements and free the integrator immediately for the next measurement. Simultaneous outputting of the results of a measurement during the illumination phase for the subsequent procedure can increase the measurement speed.
The diagram of FIG. 8 shows the integrated brightness values H of the actual shadow in the region of the affected pixels in the concrete example. The shadow region SV extends in this example from pixel # 60 to #63.
As a simple evaluation procedure, a decision threshold H_v(shown in FIG. 8 as a dashed line) is set at half the maximum brightness, for example, and the pixel is sought for which the brightness value H in the shadow transition area is the first to dip below the threshold H_v; in the example, this would be pixel # 62.
In other embodiments, the brightness can be in the opposite direction, with an increasing pixel number from the non illuminated to the illuminated CCD cells; this is dependent on the arrangement of the light source 33, CCD module 32 and shadow-producing body 35 elements and also on the internal organization of the CCD module 32 employed. In this case, the pixel with a brightness that is the fist to exceed the threshold is the one that is sought out.
After the three phases of illumination, reading and processing, a positional value is produced. The total time for the three phases determines the frequency with which positional values are obtained. The measurement resolution is the pixel pitch R of the CCD cells corrected by the geometrical relationship A that is given by the mounting distance between the individual components.
For the ratio A (see FIG. 9):
A=s′/s=x ₃ /x ₂
where:
s=actual motion of shadow-providing edge;
s′=projected motion of shadow-providing edge in plane of CCDs;
x₂=distance between optical shadow-providing edge and light source;
x₃=distance between CCD plane and light source.
This procedure determines the position by counting pixels, and is thus a digital procedure. Deviations and shifts in linear parameters such as module sensitivities have practically no effect on the result compared with analogue procedures. If the ratio A is determined for practical values, then assembly variations also only have a small influence. In a practical embodiment in which x₃=21 mm and x₂=20 mm, a nominal value for the ratio A of 1.05 is obtained; i.e. a movement of the shadow-providing edge 35 by a particular distance produces a 1.05 times higher shift in the shadow region SV in the plane of the CCD cells 32. Assuming an assembly variation factor for x₃, i.e. a possible variation in the distance of the CCD cells 32 from the light source 33, of ±0.3 mm, and assembly on the upper end of the tolerance range with x₃=21.3 mm and x₂=20 mm, then in this case the ratio A is 1.065. The ratio in this example changes by 1.065/1.05=1.014, or by +1.4%. This deviation can readily be eliminated by a single calibration, for example on production. The linearity is almost exclusively determined by the accuracy of the pixel pitch in the chip geometry and deviations are thus vanishingly small.
Although the methods described above for determining the position of the shadow-providing edge 35 and thus the position of the diaphragm 13 already gives very exact and linear positional values, interpolation can produce an even more precise positional resolution. In this broadened implementation, evaluation of the pixel brightness H produces a positional resolution, for example between pixels 61 and 62 (see FIG. 10), that is finer than the pixel pitch R, in which the brightness values of the pixels is interpolated in the region of the decision threshold. The aim is to determine the location at which the brightness profile intersects with the decision threshold H_vand to give this intersection a value on a virtual positional scale the x values of which correspond in the middle of the pixels to exactly the pixel number.
To this end, the two pixels to the left and right of the decision threshold H_vare sought and the distance ΔH of the brightness value from this threshold is determined. As shown in FIG. 10 or in FIG. 11:
ΔH ₁ =H ₁ −H _v
ΔH _r =H _r −H _v
The distances Δx, calculated from the central axis of each of the two neighbouring pixels, in this example pixels # 61 and #62, in multiples of the pixel width to the intersection point, form the following relationship with the brightness distances ΔH with respect to pixel # 61 to the left of the intersecting point (left neighbouring pixel):
Δx ₁/(Δx ₁ +Δx _r)=ΔH ₁/(ΔH ₁ +ΔH _r)
when (Δx ₁ +Δx _r)=1(1 pixel width), then:
Δx ₁ =ΔH ₁/(ΔH ₁ +ΔH _r)
With respect to pixel # 62 to the right of the sought intersecting point (right neighbouring pixel), the following relationship holds:
Δx _r/(Δx ₁ +ΔX _r)=ΔH _r/(ΔH₁ +ΔH _r)
when (Δx ₁ +Δx _r)=1(1 pixel width), then:
Δx _r =ΔH _r/(ΔH ₁ +ΔH _r)
In this example, the intersecting point is at a value of 61.7. If the brightness in the interpolation region follows an ideal straight line, both calculations produce the same result, and so in principle, one of the two calculations can be carried out. However, carrying out both calculations and averaging the results can minimize errors arising through a not exactly straight brightness profile in the transitional region under consideration or through inaccuracies in measurements, that have to be expected.
In other embodiments, the conditions either side of the intersecting point as regards non illuminated and illuminated CCD cells can be exchanged; in this case, the left and right indicators exchange their function as appropriate and the interpolation equations must be altered concomitantly.
Furthermore, other embodiments are possible, wherein brightness values from more than two pixels are used. The position can then be determined by redundant multiple calculations and averaging over several results. In another possible implementation, a linear interpolation other than that discussed here or an interpolation with data from other than directly neighbouring pixels can be used.
Deviations and shifts in linear parameters such as module sensitivities only have an effect on the result within the interpolation region. The slope of the brightness profile in the shadow transitional region resulting from the sharpness of the cast of the shadow-providing edge on the CCD plane is of minor significance as the interpolation is broadly unaffected by it; only the linearity of the brightness profile is important for the accuracy of the interpolation.
Independently of the interpolation method described above, further procedures for improving sensor properties can be used, building on the basic principle described. These procedures are described below:

- improved resistance to interference by filtering

The resistance to interference of the sensor can be improved by filter. Filtering can be applied both to the brightness values for the pixels and to the result of the positional determination itself. In the first case, the procedure operates with brightness values that are averaged over several pixels or several passes, and in the second case several initially determined positional results are collected together into a deduced positional value that is then used for further processing.

- Compensation for assembly variations

In a defined phase, for example the rest phase before the actual metering stroke, the positional value for this phase can be determined and stored in a reference memory. During the active motion phase, the positional values relative to the previously determined reference value are processed. The procedure allows assembly variations in the rest position arising during production and deviations during operation, for example heat expansion, to be automatically compensated for, thus improving accuracy.

- Compensation for scaling errors

In a further alternative, two or more known positions termed reference positions can be used to scale the positional sensor. This can occur once during the production or test procedure or repeatedly in operation.
In the first case, the reference position is provided by external apparatus, for example pitch positions or external measurement apparatus. From the positional values measured in these reference positions together with knowledge of the actual position of the reference positions, a corrected value for scaling of the positional sensor can be determined and stored for further processing.
In the second case of repeated scaling determination, known positions, for example mechanical buffers or reference signals from further available apparatus are necessary to determine the position. If the diaphragm is at such a known position during operation, the positional value measured from this location can produce a correctional value for the scaling of the positional sensor and it can be stored for further processing.

- Compensation for optical sensitivity parameters

In a further embodiment, the brightness values of the fully illuminated pixels are used to provide a representative value for the illumination strength. To this end, for example, a suitable group of pixels can be used to provide an average brightness. The illumination strength can be used to control the illumination so that the available range is optimally exploited; as an example, the brightness or on-time of the light source can be controlled so that the illumination strength of the fully illuminated pixels lies slightly below the burn-out limit for the CCD module. For each measurement, the illumination strength is corrected using the ratio obtained previously so that any variations in the illumination parameters are smoothed out, for example on ageing.

- Compensation for dirt and pixel deviations

In a further embodiment, the mechanical construction of the sensor can be structured so that in a defined phase, for example in the rest phase before executing the actual metering stroke, the complete operative pixel range or a large part thereof can be illuminated. A possible embodiment is, for example, to use a shadow-providing edge 35 facing the diaphragm for the evaluation, whereby the shadow-providing edge during the stroke motion sweeps over the sensor and darkens a region of the CCD cells that were illuminated in the previous rest position. In this phase, the brightness of all relevant pixels can be determined and stored individually in a reference memory. Deviations from the measured values for individual pixels from the ideal value can, for example, be compensated for in the form of corrections. During the active motion phase, the brightness of each pixel is first corrected and only then processed further using the reference values previously obtained for each measurement. By dint of this procedure, it is possible to compensate for deviations in the sensitivity of individual pixels brought about by the manufacturing process and also to a certain extent for dirt, and thus to improve accuracy or operational reliability.
Naturally, the CCD receiver cells may also be arranged in two or more rows to produce increased safety by redundancy against dropouts, for example because of soiling, or to increase the accuracy of measurements by averaging. For particularly large stroke lengths, two or more CCD lines can be combined in order to broaden the measurement region of an individual line beyond the functional limits of a single line.
The motor metering pump described in detail in the example may vary in the details and arrangements of components such as the motor, gear, cam drive and other constructional details. However, it is essential that the oscillating motion that is produced by the drive can be detected by a positional sensor wherein the positional sensor provides an actual signal that is in a fixed relationship to the position of the reference element and thus also to the displacement means, so that using this value, information regarding the motion of the displacement means can be obtained.

LIST OF REFERENCE NUMERALS

1 cam housing
2 motor (asynchronous motor)
3 housing ribs
4 floor plate
5 housing cover
6 electronics in housing cover
7 adjustment means
8 stroke adjustment pin
9 cover
10 control wires
11 gear (reduction gear)
12 metering head
13 diaphragm
14 inlet valve
15 outlet valve
16 metering chamber
17 cam shaft
18 longitudinal axis
19 connecting rod
20 push arm
21 attack surface for cam
22 cam carrier
23 compression spring (recuperating spring)
24 bearing plate
25 collar for compression spring
26 bush in bearing plate
27 bush in stroke adjustment pin
28 electronics housing
29 power settings for motor drive
30 diaphragm core
31 sensor carrier
32 receiver, CCD module
33 light source
34 drive electronics
35 shadow-providing edge as reference element
36 positional sensor
37 differentiator
38 nominal value setting
39 nominal-actual comparison
40 PID regulation
41 positional correction
42 amplifier
SV Shadow Profile
h lit region
d dark region
#58 . . . #65 cells (pixels) of CCD
H brightness of pixels
H_vbrightness of comparison threshold (VS)
H₁brightness of pixels to the left of intersecting point with VS (left hand side neighbouring pixel)
ΔH₁brightness difference between left hand side neighbouring pixel and brightness value of comparison threshold
H_rbrightness of pixels to the right of intersecting point with VS (right hand side neighbouring pixel)
ΔH_rbrightness difference between right hand side neighbouring pixel and brightness value of comparison threshold
Δx₁positional separation of middle line of left hand side neighbouring pixel to intersecting point with VS
Δx_rpositional separation of middle line of right hand side neighbouring pixel to intersecting point with VS
x₁distance between shadow-providing edge and CCD plane
x₂distance between shadow-providing edge and light source
x₃distance between CCD plane and light source
p₁operating pressure p₁
p₂operating pressure p₂
p₃operating pressure p₃
p₄operating pressure p₄
x_T1dead region for operating pressure p₁
x_T2dead region for operating pressure p₂
x_T3dead region for operating pressure p₃
x_T4dead region for operating pressure p₄
s actual motion of shadow-providing edge
s′ projected motion of shadow-providing edge
D metering performance
HL mechanical stroke length
SG controller output
KSG corrected controller output
MA (U, f) motor drive (voltage, frequency)
k1 factor for positional dependent positional correction
k2 factor for performance amplifier
k3 factor for deviation of speed signal
x_snominal value for position of displacement means
x actual value for position of displacement means
x_SIcontrolled deviation for position of displacement means
v_snominal value for speed of displacement means
v_Iactual value for speed of displacement means
V_SIcontrolled deviation for speed of displacement means

Claims

1. A metering pump with a rotary drive motor and an oscillating piston, wherein the rotary motion of a drive motor (2) is transformed into an oscillatory motion of a connecting rod (19) by means of a gear arrangement, so that a displacement means actuated thereby executes an oscillating linear motion on continuous rotation of the drive motor (2), that results in transfer of a medium to be metered in a metering head (12) arranged in the longitudinal axis of the connecting rod (19) cooperating alternately with an outlet and an inlet valve to produce a pump stroke (pressure stroke) and a priming stroke, wherein a reference element (35) is associated with the connecting rod (19), the position of which is detected by a positional sensor (36), wherein a positional sensor provides an actual signal (x_I) that is in a fixed relationship to the position of the reference element and thus to that of the displacement means and that provides information regarding the motion executed by the displacement means, so that the electronic control system of the metering pump can react to the operating conditions of the metering circuit and pump.

2. A metering pump according to claim 1 wherein the positional sensor (36) detects the position of the reference element (35) in accordance with a touch free principle.

3. A metering pump according to claim 1 wherein the reference element (35) associated with the connecting rod (19) and the positional sensor (36) are outside the metering head.

4. A metering pump according to claim 1 wherein the reference element (35) influences the path of a light source (33) and the positional sensor cooperating therewith (36), that is fixed in the pump housing or another stationary part, operates in a light-sensitive manner.

5. A metering pump according to claim 1 wherein the reference element (3) is a shadow-producing body or a shadow producing contour and the positional sensor (36) cooperating therewith that is fixed in the pump housing or another stationary part, consists of an optical receiver (32) in the form of a series of light sensitive charged coupled devices (CCD).

6. A metering pump according to claim 1 wherein the positional sensor (36) is arranged on its own sensor carrier (31) that is fixed to the pump housing or another stationary part.

7. A metering pump according to claim 4 wherein the light source (33), the shadow-producing body or shadow-producing contour (35) and the receiver (32) constitute a light box-like arrangement and the measured values are continuously or intermittently fed to the electronic control system.

8. A metering pump according to claim 5 wherein the optical receiver (32) of the positional sensor (36) consists of a number of linearly arranged receivers (pixels), preferably 128 pixels.

9. A metering pump according to claim 5 wherein the light source (33) is a light emitting diode (LED) that is arranged with respect to the optical receiver (32) of the positional sensor (36) so that its light beam directed at the receiver is not interrupted by the connecting rod (19).

10. A metering pump according to claim 1 wherein a start value for the positional sensor (36) is produced by interpolating the brightness of a plurality of pixels in a shadow transition region.

11. A metering pump according to claim 1 wherein when processing signals for the positional sensor (36), filtering is employed.

12. A metering pump according to claim 1 wherein zero position errors of the positional sensor (36) are eliminated by means of a reference memory.

13. A metering pump according to claim 1 wherein scaling errors of the positional sensor (36) are eliminated by using one or more reference positions.

14. A metering pump according to claim 5 wherein variations in illumination of the positional sensor (36) are compensated for by controlling or regulating the light source (33) using pixel brightness values obtained.

15. A metering pump according to claim 14 wherein variations in brightness between individual pixels of the optical receiver (32) are compensated for by using a reference memory for the sensitivity of each pixel.

16. A metering pump according to claim 1 wherein determination of a value to which a stroke adjustment apparatus (7) for adjusting stroke, should be adjusted is obtained directly by means of the positional sensor (36) by measurement during metering.

17. A metering pump according to claim 1 wherein an electronic control system recognizes a blockage in the displacement means or an incompletely executed stroke by evaluating a signal from the positional sensor (36).

18. A metering pump according to claim 1 wherein the drive motor (2) operates with slippage and an electronic control system defines a nominal stroke frequency or a nominal stroke period for the displacement means from the nominal rotary speed of the drive motor and the known gear characteristics and additionally determines the actual stroke frequency or the actual stroke period of the displacement means by evaluating the positional sensor signal (36), wherein by comparing the actual stroke frequency with the nominal stroke frequency or the actual stroke period with the nominal stroke period of the displacement means, the slippage of the drive motor can be calculated and the nominal rotary speed can be changed so that the displacement means moves at the desired stroke frequency.

19. A metering pump according to claim 1 wherein the drive motor (2) operates with slippage and an electronic control system provides a nominal stroke frequency or a nominal stroke period for the displacement means from the nominal rotary speed of the drive motor and the known gear characteristic and additionally defines the actual stroke frequency or the actual stroke period of the displacement means by evaluating the positional sensor signal (36), wherein by comparing the actual stroke frequency with the nominal stroke frequency or the actual stroke period with the nominal stroke period of the displacement means, the slippage of the drive motor can be calculated and further, the electronic control system can determine the force on the displacement means from the determined slippage of the drive motor and the known gear characteristic and thus reduce the operating pressure of the metering medium.

20. A metering pump according to claim 1 wherein the drive motor (2) operates with slippage and an electronic control system determines a nominal speed for the displacement means from a nominal rotary speed of the drive motor and a known gear characteristic for every instant of the metering procedure and additionally, determines the actual speed of the displacement means by evaluating the positional sensor signal (36), wherein by comparing the actual instantaneous speed with the nominal speed of the displacement means, the instantaneous slippage of the drive motor can be calculated and, again in connection with the known gear characteristics, the instantaneous force in the displacement means can be deduced therefrom.

21. A metering pump according to claim 20 wherein the electronic control system reduces the operating pressure of the metering medium using the observed force profile on the displacement means.

22. A metering pump according to claim 19 wherein the electronic control system recognizes operation beyond the specified pressure range from the determined metering medium operating pressure and adjusts metering when a maximum permissible pressure as dictated by the specification for the metering pump or as provided by the operator is exceeded, or when a predetermined minimum pressure is not reached.

23. A metering pump according to claim 21 wherein the electronic control system recognizes operation beyond the specified pressure range from the determined metering medium operating pressure and adjusts metering when a maximum permissible pressure as dictated by the specification for the metering pump or as provided by the operator is exceeded, or when a predetermined minimum pressure is not reached.

24. A metering pump according to claim 19 wherein the displacement means is a partially elastic diaphragm (13), wherein the electronic control system determines an expected metering error from the determined operating pressure of the metering medium and the known dependency of the metering performance on the operating pressure and influences the rotary speed of the drive motor (2) and thus the stroke frequency to work against the expected metering error.

25. A metering pump according to claim 21 wherein the displacement means is a partially elastic diaphragm (13), wherein the electronic control system determines an expected metering error from the determined operating pressure of the metering medium and the known dependency of the metering performance on the operating pressure and influences the rotary speed of the drive motor (2) and thus the stroke frequency to work against the expected metering error.

26. A metering pump according to claim 1 wherein the signal (x_I) for the position of the connecting rod (19) read from the positional sensor (36) is fed to a control circuit device in the context of its control accuracy and influences rotary speed of the drive motor (2) and thus linear motion of the connecting rod and thus of the displacement means so that it follows a predetermined nominal profile (38).

27. A metering pump according to claim 26 wherein the control device alternately influences the position (x_I), speed (v_I) or the acceleration of the displacement means by changing the rotary speed of the drive motor (2).

28. A metering pump according to claim 27 wherein the control device can deliberately decrease v_Ifor the displacement means during a priming phase and/or in a pressure phase, in order to counteract pressure drops that are caused by resistance to flow, for example the onset of cavitations.

29. A metering pump according to claim 27 wherein the desired stroke length is communicated to the control device by an operator and the control device is used to electronically limit motion of the displacement means to the stroke length to be executed, wherein the control device stops the drive motor (2) after executing the desired stroke length, switches it into reverse and then carries out a priming stroke and the motor then stops or carries out the next pressure stroke.

30. A metering pump according to claim 27 wherein the control device distributes forward motion of the displacement means during the pressure phase by driving the drive motor (2) for the time determined by the repetition rate of the metering stroke, so that the metering medium is dispensed as evenly as possible, even in the case of metering strokes that are very slow, for example of several minutes.

31. A metering pump according to claim 20 wherein the displacement means is partially in the form of an elastic diaphragm (13) and the electronic control system detects the opening of the outlet valve (15) from the instantaneous force on the diaphragm (13) and with the aid of said detection measures a dead region that arises because of elastic deformation of the diaphragm (13).

32. A metering pump according to claim 31 wherein the actual stroke is influenced independently of deduced diaphragm deformation, wherein the control device stops the drive motor (2) after reaching the desired stroke length from opening of the outlet valve (15), switches it into reverse and then executes the priming stroke and then stops the motor or executes the subsequent pressure stroke, so that error caused by the diaphragm deformation, with respect to the stroke or the metered volume, is eliminated and the dependency of metered quantity on back pressure is substantially reduced.

33. A metering pump according to claim 32 wherein actual hub frequency is influenced independently of the deduced diaphragm deformation, wherein the control device determines a correction for the error caused by the diaphragm deformation, with respect to the stroke or the metered volume, and changes nominal rotary speed of the drive motor (2) with the aid of said correction so that the error caused by the diaphragm deformation is eliminated.