CA1197550A - Linear motors and control circuitry therefor - Google Patents
Linear motors and control circuitry thereforInfo
- Publication number
- CA1197550A CA1197550A CA000410872A CA410872A CA1197550A CA 1197550 A CA1197550 A CA 1197550A CA 000410872 A CA000410872 A CA 000410872A CA 410872 A CA410872 A CA 410872A CA 1197550 A CA1197550 A CA 1197550A
- Authority
- CA
- Canada
- Prior art keywords
- motor
- transducer
- circuitry
- velocity
- output
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- G—PHYSICS
- G05—CONTROLLING; REGULATING
- G05B—CONTROL OR REGULATING SYSTEMS IN GENERAL; FUNCTIONAL ELEMENTS OF SUCH SYSTEMS; MONITORING OR TESTING ARRANGEMENTS FOR SUCH SYSTEMS OR ELEMENTS
- G05B19/00—Programme-control systems
- G05B19/02—Programme-control systems electric
- G05B19/18—Numerical control [NC], i.e. automatically operating machines, in particular machine tools, e.g. in a manufacturing environment, so as to execute positioning, movement or co-ordinated operations by means of programme data in numerical form
- G05B19/19—Numerical control [NC], i.e. automatically operating machines, in particular machine tools, e.g. in a manufacturing environment, so as to execute positioning, movement or co-ordinated operations by means of programme data in numerical form characterised by positioning or contouring control systems, e.g. to control position from one programmed point to another or to control movement along a programmed continuous path
- G05B19/39—Numerical control [NC], i.e. automatically operating machines, in particular machine tools, e.g. in a manufacturing environment, so as to execute positioning, movement or co-ordinated operations by means of programme data in numerical form characterised by positioning or contouring control systems, e.g. to control position from one programmed point to another or to control movement along a programmed continuous path using a combination of the means covered by at least two of the preceding sub-groups G05B19/21, G05B19/27, and G05B19/33
Abstract
ABSTRACT OF THE DISCLOSURE
IMPROVEMENTS IN OR RELATING TO LINEAR MOTORS AND CONTROL
CIRCUITRY THEREFOR
The relative position of the armature and stator of a linear motor is controlled using the outputs of two optical transducers 14 and 15 in phase quadrature from an optical graticule disposed along the length of the stator. An analogue, velocity-indicating signal is derived from the output of the photoelectric devices 14 and 15 which are also digitised and used to cycle an up/down counter 317 which produces as an output a ten-bit digital word representing the armature position. The outputs of this counter 317 are used as the ten higher order bits of the twelve-bit digital word representing the armature position which is compared by means of a comparator 316 with a demanded armature position. The two lower order bits of the actual-position word are derived directly from the digitised outputs of the devices 14 and 15 so as to eliminate the effect of variation in the exact position at which the counter 317 is reset to zero by a position-detecting switch 340. While the positional error is large, the motor is servoed in accordance with a predetermined velocity profile and, when the higher order bits of the position error word are zero, the circuitry switches over to a position-servo mode using as the actual position signal, the analogue output of one of the two photoelectric devices 14 and 15. The commutation of the motor coils 415 and 416 is controlled in accordance with the seventh and eighth bits of the output of counter 317, the graticule spacing being chosen so that it equals the commutation spacing divided by 2 raised to an integral power.
The polarities of the drive signals to the motor coils 415 and 416 is additionally controlled in the position servo-mode in accordance with the slope of the analogue output of the selected photoelectric device 14, 15 so as to achieve the correct, negative feedback action.
IMPROVEMENTS IN OR RELATING TO LINEAR MOTORS AND CONTROL
CIRCUITRY THEREFOR
The relative position of the armature and stator of a linear motor is controlled using the outputs of two optical transducers 14 and 15 in phase quadrature from an optical graticule disposed along the length of the stator. An analogue, velocity-indicating signal is derived from the output of the photoelectric devices 14 and 15 which are also digitised and used to cycle an up/down counter 317 which produces as an output a ten-bit digital word representing the armature position. The outputs of this counter 317 are used as the ten higher order bits of the twelve-bit digital word representing the armature position which is compared by means of a comparator 316 with a demanded armature position. The two lower order bits of the actual-position word are derived directly from the digitised outputs of the devices 14 and 15 so as to eliminate the effect of variation in the exact position at which the counter 317 is reset to zero by a position-detecting switch 340. While the positional error is large, the motor is servoed in accordance with a predetermined velocity profile and, when the higher order bits of the position error word are zero, the circuitry switches over to a position-servo mode using as the actual position signal, the analogue output of one of the two photoelectric devices 14 and 15. The commutation of the motor coils 415 and 416 is controlled in accordance with the seventh and eighth bits of the output of counter 317, the graticule spacing being chosen so that it equals the commutation spacing divided by 2 raised to an integral power.
The polarities of the drive signals to the motor coils 415 and 416 is additionally controlled in the position servo-mode in accordance with the slope of the analogue output of the selected photoelectric device 14, 15 so as to achieve the correct, negative feedback action.
Description
--L--IMPROVEMENTS IN OR RELATING TO LINEAR MOTORS AND CONTROL
CIRCVITRY THEREFOR
The present invention relates to linear ~otors and control circuitry therefor.
A form of linear motor suitable for being driven by the control circuitry of this invention is described in my copending Canadian application No: 377,774, and comprises a stator and an armature movable lengthwise relative to one another, the stator and armature each comprising a respective set of magnets disposed in a predetermined sequence in the direction of relative movement, the magnets of one set being arranged in a stack lengthwise of the direction of relative movement and the poles of the other set of magnets being annular and disposed around and slidable relative to the stack, one set of magnets being permanent magnets and the other set being provided by coil windings, the arrangement being such that by appropriate energisation of the coil windings a net thrust to cause relative movement of the armature and stator in a desired direction can be produced at any position in a range of relative armature and stator positions.
There are many forms of circuitry for controlling both rotary and linear motors. The general object of such circuitry is to enable the velocity and positional .` ~
movements of the ~otors to be controlled accurately according to externally applied parameters. Linear motors however, are not widely favoured in many applications, e.g~ hi~h speed character printers, on account of the difficulty of achieving rapid and accurate translational and positional control.
According to the present invention there is provided a servo device for controlling the operation of a motor comprising a motor position detector and circuitry for controlling the energisation and commutation of coils of the motor, the position detector comprising a position transducer having two parts movable relative to one another, and circuitry for providing a digital word representing the relative position of the two transducer parts, the digital word comprising higher order bits and at least one lower order bit, wherein the higher order bits of the digital word are derived from the outputs of a digital counter cycled by the output of the transducer and used by the commutating circuitry for commutating the motor coils and the said at least one lower order bit is/are derived directly from the output of the position transducer and used, in combiantion with higher order bits from the digital counter, by the coil energisation circuitry for controlling the energisation of the coils to enable precise control of the position of the motor.
Also accordiny to the invention there is provided in combination: a linear motor compxising a stator and an armature movable lengthwise relative to one another, the stator and armatrure each comprising a respective set of magnets disposed in a predetermined sequence in the direction of relative
CIRCVITRY THEREFOR
The present invention relates to linear ~otors and control circuitry therefor.
A form of linear motor suitable for being driven by the control circuitry of this invention is described in my copending Canadian application No: 377,774, and comprises a stator and an armature movable lengthwise relative to one another, the stator and armature each comprising a respective set of magnets disposed in a predetermined sequence in the direction of relative movement, the magnets of one set being arranged in a stack lengthwise of the direction of relative movement and the poles of the other set of magnets being annular and disposed around and slidable relative to the stack, one set of magnets being permanent magnets and the other set being provided by coil windings, the arrangement being such that by appropriate energisation of the coil windings a net thrust to cause relative movement of the armature and stator in a desired direction can be produced at any position in a range of relative armature and stator positions.
There are many forms of circuitry for controlling both rotary and linear motors. The general object of such circuitry is to enable the velocity and positional .` ~
movements of the ~otors to be controlled accurately according to externally applied parameters. Linear motors however, are not widely favoured in many applications, e.g~ hi~h speed character printers, on account of the difficulty of achieving rapid and accurate translational and positional control.
According to the present invention there is provided a servo device for controlling the operation of a motor comprising a motor position detector and circuitry for controlling the energisation and commutation of coils of the motor, the position detector comprising a position transducer having two parts movable relative to one another, and circuitry for providing a digital word representing the relative position of the two transducer parts, the digital word comprising higher order bits and at least one lower order bit, wherein the higher order bits of the digital word are derived from the outputs of a digital counter cycled by the output of the transducer and used by the commutating circuitry for commutating the motor coils and the said at least one lower order bit is/are derived directly from the output of the position transducer and used, in combiantion with higher order bits from the digital counter, by the coil energisation circuitry for controlling the energisation of the coils to enable precise control of the position of the motor.
Also accordiny to the invention there is provided in combination: a linear motor compxising a stator and an armature movable lengthwise relative to one another, the stator and armatrure each comprising a respective set of magnets disposed in a predetermined sequence in the direction of relative
- 2~
movements, the magnets of one set being arranged in a stack lengthwise of the direction of relative movement and the poles of the other set of magnets being annular and disposed around and slidable relative to the stac~, one set of magnets being permanent magnets and the other set being provided by coil windings, the arrangement being such that by appropriate energisation of the coil windings a net thrust to cause relative movement of the armature and stator in a desired direction can be produced at any position in a range of relative armature and stator positions; and a servo device for controlling the operation of the motor and comprising a motor position detector and circuitry for controlling the energisation and commutation of coils of the motor3 the position detector comprising a position transducer having two parts movable relative to one another, and circuitry for providing a digital word representing the relative position of the two trnsducer parts, the digitaL
word comprising higher order bits and at least one lower order bit, wherein higher order bits of the digital word are derived from the outputs of a digital counter cycled by the output of the transducer and used by the commutating circuitry for commutating the motor coils and the said at least one lower order bit is/are derived directly from the output of the position transducer and used, in combination with higher order bits from the digital counter, by the coil energisation circuitry for controlling the energisation of the coils to enable precise control of the position of the motor.
5uitably, closed loop control circuitry is provided for use with a linear motor especially but not exclusively of the form claimed in the aorementioned copending patent application No. 377,774, the control circuitry comprising, transducer means for deteeting the relative and absolute positions of the armature of the motor with respect to its stator and for providing a digital word or representing the said positionsl higher order bits o the digital word being stored within a primary counter and lower order bits being derived directly from the transducer means, commutation information for commutating the motor in such manner as to provide a desired thrust profile, an2 logic soft, firm and/or hardware to co-ordinate the positional information with the commuta~ion information such that in combination, the velocity and position of the motor may be controlled to a precise degree by velocity and positional information applied to the said control circuitry. The transducer means for detecting the position of the armature of the motor with respect to its stator may comprise, two relatively movable parts, one fixed relative to the armature and the other fixed relative to the stator, one of the transducer parts being a graticule having a multiplicity of cycles of a detectable marking along its length and the other of the parts comprising two sensors for sensing the markings of the graticule, the sensors being arranged so that~ as they move relative to the graticule, they produce outputs which are phase shifted relative to one another. The outputs are converted to pulses.
The present invention has a number of features, which may be used singly or in any desired combination with one another. These features are as follows.
According to a first feature of the invention~
the output pulses provided by the transducer sensors are used to clock the aforesaid primary counter for providing the higher order bits of the digital word representing the position of the armature while the lower order bits are derived directly by the digiti~ed information provided by the sensors.
In a conventional arrangement for example, all bits of the position-indicating digital word are derived from the position counter. This counter must be reset once the linear motor is switched on and to do this the armature is usually automatically moved to a known position relative to the stator to operate a reset switch which resets the counter~ However, the exact position at which the reset switch is operated may not be entirely consistent, at least the lower order bit or bits of the output of the position counter is/are sometimes suspect. -This in turn, as will hereinafter be explained in more detail, may prevent the motor from being detented at any desired position. Thus utilisation of this invention provides that in place of the lowest or lower order bits of the position counter, one or more bits obtained from the sensors are synthesised by gating circuitry which bypasses the position counter, to enable the motor to be arrested in any desired Idetent' positionO
In a second feature of the invention, digital processing circuitry is provided for providing a digital output corresponding to the difference between the detected positions and a desired position de~ined by a multiple-bit digital word and for energising the armature towards its desired position relative to the stator, commutation inEormation for the motor being provided directly by selected bits of the primary counter which in turn control drive circuitry connected to the motor's coils, further circuitry being provided to modulate the energisation of the motor coils in a continuously variable or quasi-continuously variable fashion depending upon a positional or velocity error. The circuitry may be replaced by software or firmware controlled by suitable processing po~er.
Preferably, in well known manner, the arrangement is such that while the positional error is greater than a pre-determined value, the control circuitry works in a velocity-servo fashion tending to cause the motor armature to follow a pre-determined velocity profile, and, when the positional error falls into the predetermined range, the circuit switches over to a position-servo mode in which the desired position is reached. The velocity profile is preferably defined in terms of the positional error, and for minimum transist time, is preferably a square root function of the position error.
In a third feature of the invention, the spacing of the graticule markings relative to the spacing of the commutation points of the motor is 50 chosen that the ratio of the number of graticule marking spacings within a given commutation spacing is a convenient binary multiplicand. ~y this means, the comm~tation information provided by the primary counter may, for example, be obtained simply from one or more bits of the higher order bits registered thereby. The motor may be so commutated that a desired thrust~position profile may be obtained.
By this means, an externally generated velocity profile may be applied, being matched to the expected performance from the motor at any given position, to effect precise velocity control thereof.
There now follows by way OL example a description of the invention with reference to the accompanying drawings in which:-Figure 1 is a schematic perspective of a linearmOtGr with optical transducer controlled by circuitry according to the present invention;
Figure 2a is a schematic plan view showing to an enlarged scale the optical position transducer system of $igure 1;
Figure 2b shows wave-forms produced by the transducer arrangement;
Figure 3a is a circuit diagram ~howing in block form the principal components of the circuitry used to control the linear motor;
Figure 3b shows wave-forms arising in operation of the circuitry of figure 3a;
Figure 3c shows in more detail part of the apparatus of figure 3a;
Figure 3d shows the outputs of the position sensors being used to make the final positional adjustment;
Figures 4(i) and 4 (ii) illustrate the thrust produced by the linear motor.
Figure 1 ~hows ~chematically one form of linear motor connected to control circuitry embodying ~he pres~nt inventionO The linear motor, is of the form described more fully in my copending Canadian application No: 377,774 and comprises tubular stator 10 housing permanent magnets 11 spaced equidistantly along the motor by spacing blocks not shown o~ the same length as the magnets. The stator is fixed to a suitable chassis. Free to move along the length o the motor i5 an armature 12. This armature comprises essentially two coils, shown here as coils 1 and 2. Energisation of the coils in a predetermined sequence causes movement of the armature along the statvr,. A posi'cion sensing transducer means 13 is affixed to the armature 12. This comprises two photo-electric device~ 14 and 15 mounted within a block 16.
Running along the length of the motor is an opti~al graticule 17. A light emitting source not shown causes light to pass through the graticule on to the cells 14 and 15. The graticule and transducer means provide information to control circuitry as to the position of the armature relative to ~he sta~or as well a~ the absolute po~ition of the armature along the motor.
The graticule and transducer means will now be described in more detail with reference to figure 2a.
The position transdueer, generally de~ignated 13 comprises the graticule 17 which is made up of a transparent material and has ruled on its ~rface 17a a series of regularly spaced parallel dark markings 17b;
these markings 17b are each of equal width and are spaced apart by distances equal to their widths.
D;sposed to one side of the graticule 17a are the two photo electric devices 14 and 15 preferably photo-transistors, moun ed within the block 16t see figure 17 which is fixed to the motor's armature 12. Disposed between them and the graticule 17 i5 a plate 18 of transparent material which is also fixed to the block 160 Th;s plate 18 com~rises two sections 19 and 20 which also have dark markings 21 ruled on them, corresponding to the markings 17b. The markings on the two sections 19 and 20 have a phase shift between them, lengthwise of the graticule 17, of 90,dé~rees. The photo-transistors 14 and 15 look through the respective sections 19 and 20 and the graticule 17 to light sources 22 and 23 which are disposed to the opposite side of the graticule 17 and are fixed ~o ~he ~ounting block 16 by an arm not shownO
As will be seen from figure ~b, (i) and ~ii), the output~ ~rom photo-transistors 14 and 15 vary nearly sinusoidally with distance, the two ~ignals being in phase qua~rature relative to one another.
As will become apparent from the following description, the use of the two photo transistors 14 and 15, to produce phase quadrature signals allows the resolution of the position detector 13 to be one qualter of the wavelength of the liqht and dark stripe~ on the graticule 17~ In one practical embodiment, the wavelength of these stripes was 1/80 of an inch and the resolution of the position detector a~ a whole~ 1/320 of an inch. This, in combination with control circuitry to be explained later, makes possible fine increments in the stopping position of the motor's armature, which can be accurately and repeatably determined~ This particularly suit~ the motor for such applications as an actuator for computer disc memory systems, and a~ a form of motive power for high precision printers.
The circuitry used to control the linear motor will now be explained with reference to figure 3a, which shows the principal elements in block form.
The outputs from the photo-transducers 14 and 15 are applied to respective Schmidt trigger circuits 310a and 310b to produce waveforms shown in figure 2b at iii and iv. The outputs of t}lese two Schmid~ triggers are applied to a gating circuit 311 which derives from the squared wa~e-forms, ~wo ou~put ~i~nals, one of which is delivered at output 311a and which is a direction indicating signal, i.e. whether the armature 12 i~
moving to the left or to the right and the other of which is delivered at ouput 311b being a clock signal which changes state every quarter ~ycle of the yraticule marking, i~e. once every l/320th of an inch.
Broadly, the major components of the remainder of the circuitry 303 are:
(a) A digital position error circuit 312 which produces a twelve-bit output word indicatiny the magnitude of the position error, i.e~ the distance between the actual and demanded positions and also a direction indicating signal indicating whether the actual position is to the left or to the right of the demanded position.
i5~
~b~ An analogue velocity profile circuit 313 which produces as an output a velocity profile which the linear motor is required to ~ollow over the major part of its travel to the demanded position, thi~ velocity profile being defined in terms of the positional error.
~c) A servo-mode determining circuit 314, which until the linear motvr closely approaches the demanded position, aets as a velocity servo and as the position error decreases, switches over to a position servo action.
(d) Motor coil energization circuitry 315 which control~ the magnitude and polarity of the voltages applied to the linear motor coils 415 and 416 (coil~l and ~oil 23.
The operation of the circuits 312, 313, 314 and 315 will now be described in somewhat more detail.
The digital position error circuit ~12 comprises a twelve-bit up/down counter 317 which is clocked by the circuit 311 so that it changes state up or down, the direction of the change being determined by the state of the output 311a once for every quarter wavelength o the movement relative to the graticul~ 170 The ten most significant bits of the output of counter 317 and also two lower significan~e bits generated in a manner de~cribed below are applied to one input of a binary subtractor 318~ and the other input being from the address latch 301. The twelve--bi~ output of the subtractor 318 is delivered both to a gate circuit 316 which produces a high ou~put when the eleven most signiFicant bits are zero, and ~he sign of the subtxahend i5, by convention, positive, indicating that the home position has been reached. The output is also delivered to a digital to analogue converter 319 which produces an output voltage proportional to the magnitude of the diference word.
The output of the digital to analogue convertor 319 is connected to an analogue circuit 320 which produces an output voltage which is proportional to the square root of the input voltage. This is to define a square root velocity profile in terms of the detected positional error. This square root law gives an optimum velocity profile. An analogue switch 322 selects the velocity profile signal either direct from the circuit 320 or from an invertor which serves to adjust the polarity of the velocity profi~e signal according to the direction of the position error, this being controlled by a sign output of the subtractor 318 which indicates this direction.
Within the circuitry 114 an analogue circuit 323 monitors the analogue outputs of the photo-transistors 14 and 15 to produce a voltaye proportional to velocity.
This is subtracted from the d~emanded velocity from the velocity profile circuit 313 by means of a summing ~mplifier 324 to produce a voltage representing the velocity error.
A three way electronic switch 325 is used to determine whether the servo operate~ in he velocity or po~ition-servo mode, being controlled by a circuit 326 which responds to the state of the home signal and the least significant bits of the demanded addressl When out of the nhome~ position, the switch 325 selects the output of the summing ampliier 324 so th2t the circuitry operates in a velocity servo-mode~ When, on the other hand, the home ~ignal is active, and accurate positioning of the motor is required in a ~detentW mode, the circuit 326 changes the switch 325 over to monitor the analogue output of one or other of th* photo-transistors 14 and 15. Why and how this is achieved i5 explained below. Briefly, however, the sircuit 326 is arranged so that the switch 325 monitors the selected one of the photo-transistor o~tputs on the part of its wave-form which has its next ~ero crossover, as the detent position. Thus with the switch 325 connected to one or other of the photo-transistor~ 14 and lj an _ 16 -analogue position servo-mode is achieved~ The outputs of the photo-transistors 14 and lS can be applied to the switch 325 via the circuitry 327 which modifies the position signals to yive the ~desired second order response in the position-servo mode to prevent undesirable oscill~tions.
Within the circuitry 315, the motor coils 415 and 416 have associated with them respective drive amplifiers 328a and 328b which amplify the signal from the switch 325 via respective electronic switches 329a and 329b. These are each two pole switches and connect direct with the switch 325 or via an invertor 330a and 330b. The state of these two switches is determined by commutating circuitry 331 which responds in both the velocity servo and positional servo modes, in this example, to the seventh and eighth bits from the position counter 317 so that the polarity o the motor coil voltages is appropriately adjusted to maintain the required direction of the thrust for movement of the armature 12. This is explained in further detail below.
For this purpose i~ is convenient to arrange ~he stator pole pitch of the linear motor to correspond with an integral power of two time one quarter of the graticule wavelength. It is then a simple matter to commutate the coil voltages directly using bits seven and eight of the position counter 317.
~'7~
- 17 ~
It will be appreciated that at any given detent position, the selected slope of one of the analogue outputs of the photo transistors 14 and 15 may be positive or negative depending on the demanded address.
For example, a binary address having least significant bits 00 and 01 might select positive going portions of the outputs whereas a binary address of 10 or 11 would select negative going portions, lsee figure 3d~
In the po~ition servo mode therefore, as well as responding to bits seven and eight of the position counter, the circuitry 331 also responds to the second least significant bit of the a~solute address so as to inver the polarity of the coil currents in order to correct the sign of the gradient of the selected photo-transistor output which would otherwise cau~e the motor to move out of its desired detent position.
Before the start of printing, after power-up of the printer~ the position counter is re-set. One of the coil~ of the motor 415 or 416 is then energised by æeparate circuitry. After a predetermined delay, the other coil is then energised al~o. Or the reverse of this sequence ~lay take place~ This arrangement ensures that the armature will move to an unambiguous stable position relative to the stator corresponding to a commutation position~ 9ecause the position counter is reset, bit~ 7 and 8 both have zero values, i.e. the conditions of bits 7 and 8 are in synchronism with the desired commutation position achieved by the reset routine. A velocity reference signal Vf i5 produced to cause movement of the motor to the left. At the same time, the postion counter is allowed to cycle, 80 keeping bits 7 and 8 in synch:ronism with the movement of the motor, which they then control. As a result the motor continues its motion to the left at a steady speed, until an optical or mechanical reset switch 340 is activated, which resets the actual-position counter 317 again to zeroS However, the exact position at which the limit switch 340 is operated may vary slightly from operation to operation.
The reason for this is as follows. Typically limit switches may be in the form of a paddle mounted on the armature interupting a fixed optical beam sensing device. The exact point at which the sen~ing device registers the limit of movement varies slightly according to external factors such as ambient light.
Therefore the limi~ po~ltion switch may reset the position counter with an accuracy considerably less than the resolution obtainable from the graticule and photo-transistors. For example, assuming in this case 1/80"
variance the reset might occur at a physical position a~
which the digitized output of the two photo-transistors could be at any value between 00 and 11 ~g~
-- 19 ~
depending on the ambient condi~ions. Therefore, the least significant bi~s available from the position counter cannot be depended upon to be in synchronism with the digita1ized outputs from the two photo-transistors. It fo110ws from this, by way of example, that they cannot therefore be uti1ised for such ~
function as mentioned ear1ier to correct the sign of the gradient o~ the ana10gue OUtpllts when in ~he ~home~
position according to the demanded detent address~ For reliable operation~ the digit:ised ou~puts themselves must be utilised for this type of function, and it is for this reason that the least significant bits of the digita1 word representing th~ position of the armature are derived direct1y from the position transducer.
To effect a so1ution, substitute bits derived direct1y from the position transducer are used instead as the lower order bits of the position word; the number of substituted bits should be great enough so that the corresponding distance a10ng the 1ength of the motor is greater than the positional uncertainty associated with the reset switch~ In the i11ustrated embodimentr two substitute bits ~re therefore used instead, P~ ~0) and P' (1). These are produced as shown in figure 3co The output of the Schmidt trigger 31oa associ~ted with the photo-transistor 14 is used as P' ~1) and P' (0) is produced hy combining the outputs of trigger~ ~lla and b in an exclusive-or fashion as shown in figure 3~0 The up/down and clo~k signal generating circuit 311 uses 1ip flops and an arrangement of gates to monitor the state of the outputs of photo-transistors 14 and 15 -~ ~o -at each transition of each one of them and to compare the states after a transition with those before to determine the direction of the movement of the armature and also to generate the clock pulses. A clock pulse to the position counter 317 is only produced if ~he states of P' (0~ and P' (1) were both low pr.ior to the transition and high after it or vice versa.
Figure 3d shows the intended function of the switch control circuit 32~6 when it is operating in the position servo mode and selecting between one of the wave-forms A
and B from the two photo-transistoræO
Suppose the demanded address is such that when moving to the right~ the home signal goes active at the B wave-form crossover Hlo Suppose further that for convenience the wave-form is then selected by the circuit 3~6. The linear motor will then servo to the next A crossover, i~e. H2 which i9 one position off~et from Hl. It will be appreciated that the home position spans two transitions' worth, from Hl to H3 in order to allow the motor to settle at the centre point of H2 o the A waveform. lf the home signal goes activ~ at the A
rossover position ~2, the B waveform i~ then ~elected by circuit 326 and in a similar fashion the motor will then servo to the next ~ crossover~ i~eO H30 Similarly the same actions will occur for home position H0 and H3~
~n important consideration in any elosed loop sy~tem controlling a motor is that the thrust available from the motor should be predictable for efficient ~ervo-action. With the exception of alternatin~ current type linear motors, conventional stepping motors produce non-uniform thrust/displacement: profiles. The form of motor described in my copendina application noO 377,77 however is capable of producing nearly linear thrust, when con~rolled by the circuitry described herein. This i5 explained with reference to figure 4 showing in ~olid outline thrust versus displacement profiles at ~1) and ~2) whi~h would be produced were the coils of a two coil version of the motor to be permanently energised. T~e dotted outlines ~how half phase reversal of the thrust from each coil. On account of the fact that the thrust profiles are effectively in phase quadrature9 it can be seen that by switching from one ~hrust proile to the other and effecting the half phase reversal, a nearly uniform thrus~ profile is obtained, see waveform ~3~.
In practice the variance experienced in tbis simple system is less than 18%o This commutation, as has already been explained, i~
effected by use of bits P7 and P8 which provide the nec~ssary logic combina~ion. Combinations of other bits and joint energisation of both coils can provide an almost straight line thrust profile.
- 2~. -In the event of the motor being detented at an addrescoinciding with a commutation bit change a further advantage of using bits direct from the counter arises in that the commutation is automatically synchronised with the zero crossover point of one of the photo transistor outputs, i.e~ a~ an H position as ~hown in figure 3do Thus undesirable 1back emf' ~pikes are avoided, discontinui~y in thrust displacement relation is a~oided and detenting occurs ~n the same manner at any position in between commutation points.
Thus the foregoing description in total disclose~
means for providing precise po~itional and velocity control of a linear motor making such a motor ~uitable for a range of applications. It will be appreciated that numerous variations are possible for example the use o~ microprocessor soft- and firmware to ~arry out the functions shown in figure 3a.
movements, the magnets of one set being arranged in a stack lengthwise of the direction of relative movement and the poles of the other set of magnets being annular and disposed around and slidable relative to the stac~, one set of magnets being permanent magnets and the other set being provided by coil windings, the arrangement being such that by appropriate energisation of the coil windings a net thrust to cause relative movement of the armature and stator in a desired direction can be produced at any position in a range of relative armature and stator positions; and a servo device for controlling the operation of the motor and comprising a motor position detector and circuitry for controlling the energisation and commutation of coils of the motor3 the position detector comprising a position transducer having two parts movable relative to one another, and circuitry for providing a digital word representing the relative position of the two trnsducer parts, the digitaL
word comprising higher order bits and at least one lower order bit, wherein higher order bits of the digital word are derived from the outputs of a digital counter cycled by the output of the transducer and used by the commutating circuitry for commutating the motor coils and the said at least one lower order bit is/are derived directly from the output of the position transducer and used, in combination with higher order bits from the digital counter, by the coil energisation circuitry for controlling the energisation of the coils to enable precise control of the position of the motor.
5uitably, closed loop control circuitry is provided for use with a linear motor especially but not exclusively of the form claimed in the aorementioned copending patent application No. 377,774, the control circuitry comprising, transducer means for deteeting the relative and absolute positions of the armature of the motor with respect to its stator and for providing a digital word or representing the said positionsl higher order bits o the digital word being stored within a primary counter and lower order bits being derived directly from the transducer means, commutation information for commutating the motor in such manner as to provide a desired thrust profile, an2 logic soft, firm and/or hardware to co-ordinate the positional information with the commuta~ion information such that in combination, the velocity and position of the motor may be controlled to a precise degree by velocity and positional information applied to the said control circuitry. The transducer means for detecting the position of the armature of the motor with respect to its stator may comprise, two relatively movable parts, one fixed relative to the armature and the other fixed relative to the stator, one of the transducer parts being a graticule having a multiplicity of cycles of a detectable marking along its length and the other of the parts comprising two sensors for sensing the markings of the graticule, the sensors being arranged so that~ as they move relative to the graticule, they produce outputs which are phase shifted relative to one another. The outputs are converted to pulses.
The present invention has a number of features, which may be used singly or in any desired combination with one another. These features are as follows.
According to a first feature of the invention~
the output pulses provided by the transducer sensors are used to clock the aforesaid primary counter for providing the higher order bits of the digital word representing the position of the armature while the lower order bits are derived directly by the digiti~ed information provided by the sensors.
In a conventional arrangement for example, all bits of the position-indicating digital word are derived from the position counter. This counter must be reset once the linear motor is switched on and to do this the armature is usually automatically moved to a known position relative to the stator to operate a reset switch which resets the counter~ However, the exact position at which the reset switch is operated may not be entirely consistent, at least the lower order bit or bits of the output of the position counter is/are sometimes suspect. -This in turn, as will hereinafter be explained in more detail, may prevent the motor from being detented at any desired position. Thus utilisation of this invention provides that in place of the lowest or lower order bits of the position counter, one or more bits obtained from the sensors are synthesised by gating circuitry which bypasses the position counter, to enable the motor to be arrested in any desired Idetent' positionO
In a second feature of the invention, digital processing circuitry is provided for providing a digital output corresponding to the difference between the detected positions and a desired position de~ined by a multiple-bit digital word and for energising the armature towards its desired position relative to the stator, commutation inEormation for the motor being provided directly by selected bits of the primary counter which in turn control drive circuitry connected to the motor's coils, further circuitry being provided to modulate the energisation of the motor coils in a continuously variable or quasi-continuously variable fashion depending upon a positional or velocity error. The circuitry may be replaced by software or firmware controlled by suitable processing po~er.
Preferably, in well known manner, the arrangement is such that while the positional error is greater than a pre-determined value, the control circuitry works in a velocity-servo fashion tending to cause the motor armature to follow a pre-determined velocity profile, and, when the positional error falls into the predetermined range, the circuit switches over to a position-servo mode in which the desired position is reached. The velocity profile is preferably defined in terms of the positional error, and for minimum transist time, is preferably a square root function of the position error.
In a third feature of the invention, the spacing of the graticule markings relative to the spacing of the commutation points of the motor is 50 chosen that the ratio of the number of graticule marking spacings within a given commutation spacing is a convenient binary multiplicand. ~y this means, the comm~tation information provided by the primary counter may, for example, be obtained simply from one or more bits of the higher order bits registered thereby. The motor may be so commutated that a desired thrust~position profile may be obtained.
By this means, an externally generated velocity profile may be applied, being matched to the expected performance from the motor at any given position, to effect precise velocity control thereof.
There now follows by way OL example a description of the invention with reference to the accompanying drawings in which:-Figure 1 is a schematic perspective of a linearmOtGr with optical transducer controlled by circuitry according to the present invention;
Figure 2a is a schematic plan view showing to an enlarged scale the optical position transducer system of $igure 1;
Figure 2b shows wave-forms produced by the transducer arrangement;
Figure 3a is a circuit diagram ~howing in block form the principal components of the circuitry used to control the linear motor;
Figure 3b shows wave-forms arising in operation of the circuitry of figure 3a;
Figure 3c shows in more detail part of the apparatus of figure 3a;
Figure 3d shows the outputs of the position sensors being used to make the final positional adjustment;
Figures 4(i) and 4 (ii) illustrate the thrust produced by the linear motor.
Figure 1 ~hows ~chematically one form of linear motor connected to control circuitry embodying ~he pres~nt inventionO The linear motor, is of the form described more fully in my copending Canadian application No: 377,774 and comprises tubular stator 10 housing permanent magnets 11 spaced equidistantly along the motor by spacing blocks not shown o~ the same length as the magnets. The stator is fixed to a suitable chassis. Free to move along the length o the motor i5 an armature 12. This armature comprises essentially two coils, shown here as coils 1 and 2. Energisation of the coils in a predetermined sequence causes movement of the armature along the statvr,. A posi'cion sensing transducer means 13 is affixed to the armature 12. This comprises two photo-electric device~ 14 and 15 mounted within a block 16.
Running along the length of the motor is an opti~al graticule 17. A light emitting source not shown causes light to pass through the graticule on to the cells 14 and 15. The graticule and transducer means provide information to control circuitry as to the position of the armature relative to ~he sta~or as well a~ the absolute po~ition of the armature along the motor.
The graticule and transducer means will now be described in more detail with reference to figure 2a.
The position transdueer, generally de~ignated 13 comprises the graticule 17 which is made up of a transparent material and has ruled on its ~rface 17a a series of regularly spaced parallel dark markings 17b;
these markings 17b are each of equal width and are spaced apart by distances equal to their widths.
D;sposed to one side of the graticule 17a are the two photo electric devices 14 and 15 preferably photo-transistors, moun ed within the block 16t see figure 17 which is fixed to the motor's armature 12. Disposed between them and the graticule 17 i5 a plate 18 of transparent material which is also fixed to the block 160 Th;s plate 18 com~rises two sections 19 and 20 which also have dark markings 21 ruled on them, corresponding to the markings 17b. The markings on the two sections 19 and 20 have a phase shift between them, lengthwise of the graticule 17, of 90,dé~rees. The photo-transistors 14 and 15 look through the respective sections 19 and 20 and the graticule 17 to light sources 22 and 23 which are disposed to the opposite side of the graticule 17 and are fixed ~o ~he ~ounting block 16 by an arm not shownO
As will be seen from figure ~b, (i) and ~ii), the output~ ~rom photo-transistors 14 and 15 vary nearly sinusoidally with distance, the two ~ignals being in phase qua~rature relative to one another.
As will become apparent from the following description, the use of the two photo transistors 14 and 15, to produce phase quadrature signals allows the resolution of the position detector 13 to be one qualter of the wavelength of the liqht and dark stripe~ on the graticule 17~ In one practical embodiment, the wavelength of these stripes was 1/80 of an inch and the resolution of the position detector a~ a whole~ 1/320 of an inch. This, in combination with control circuitry to be explained later, makes possible fine increments in the stopping position of the motor's armature, which can be accurately and repeatably determined~ This particularly suit~ the motor for such applications as an actuator for computer disc memory systems, and a~ a form of motive power for high precision printers.
The circuitry used to control the linear motor will now be explained with reference to figure 3a, which shows the principal elements in block form.
The outputs from the photo-transducers 14 and 15 are applied to respective Schmidt trigger circuits 310a and 310b to produce waveforms shown in figure 2b at iii and iv. The outputs of t}lese two Schmid~ triggers are applied to a gating circuit 311 which derives from the squared wa~e-forms, ~wo ou~put ~i~nals, one of which is delivered at output 311a and which is a direction indicating signal, i.e. whether the armature 12 i~
moving to the left or to the right and the other of which is delivered at ouput 311b being a clock signal which changes state every quarter ~ycle of the yraticule marking, i~e. once every l/320th of an inch.
Broadly, the major components of the remainder of the circuitry 303 are:
(a) A digital position error circuit 312 which produces a twelve-bit output word indicatiny the magnitude of the position error, i.e~ the distance between the actual and demanded positions and also a direction indicating signal indicating whether the actual position is to the left or to the right of the demanded position.
i5~
~b~ An analogue velocity profile circuit 313 which produces as an output a velocity profile which the linear motor is required to ~ollow over the major part of its travel to the demanded position, thi~ velocity profile being defined in terms of the positional error.
~c) A servo-mode determining circuit 314, which until the linear motvr closely approaches the demanded position, aets as a velocity servo and as the position error decreases, switches over to a position servo action.
(d) Motor coil energization circuitry 315 which control~ the magnitude and polarity of the voltages applied to the linear motor coils 415 and 416 (coil~l and ~oil 23.
The operation of the circuits 312, 313, 314 and 315 will now be described in somewhat more detail.
The digital position error circuit ~12 comprises a twelve-bit up/down counter 317 which is clocked by the circuit 311 so that it changes state up or down, the direction of the change being determined by the state of the output 311a once for every quarter wavelength o the movement relative to the graticul~ 170 The ten most significant bits of the output of counter 317 and also two lower significan~e bits generated in a manner de~cribed below are applied to one input of a binary subtractor 318~ and the other input being from the address latch 301. The twelve--bi~ output of the subtractor 318 is delivered both to a gate circuit 316 which produces a high ou~put when the eleven most signiFicant bits are zero, and ~he sign of the subtxahend i5, by convention, positive, indicating that the home position has been reached. The output is also delivered to a digital to analogue converter 319 which produces an output voltage proportional to the magnitude of the diference word.
The output of the digital to analogue convertor 319 is connected to an analogue circuit 320 which produces an output voltage which is proportional to the square root of the input voltage. This is to define a square root velocity profile in terms of the detected positional error. This square root law gives an optimum velocity profile. An analogue switch 322 selects the velocity profile signal either direct from the circuit 320 or from an invertor which serves to adjust the polarity of the velocity profi~e signal according to the direction of the position error, this being controlled by a sign output of the subtractor 318 which indicates this direction.
Within the circuitry 114 an analogue circuit 323 monitors the analogue outputs of the photo-transistors 14 and 15 to produce a voltaye proportional to velocity.
This is subtracted from the d~emanded velocity from the velocity profile circuit 313 by means of a summing ~mplifier 324 to produce a voltage representing the velocity error.
A three way electronic switch 325 is used to determine whether the servo operate~ in he velocity or po~ition-servo mode, being controlled by a circuit 326 which responds to the state of the home signal and the least significant bits of the demanded addressl When out of the nhome~ position, the switch 325 selects the output of the summing ampliier 324 so th2t the circuitry operates in a velocity servo-mode~ When, on the other hand, the home ~ignal is active, and accurate positioning of the motor is required in a ~detentW mode, the circuit 326 changes the switch 325 over to monitor the analogue output of one or other of th* photo-transistors 14 and 15. Why and how this is achieved i5 explained below. Briefly, however, the sircuit 326 is arranged so that the switch 325 monitors the selected one of the photo-transistor o~tputs on the part of its wave-form which has its next ~ero crossover, as the detent position. Thus with the switch 325 connected to one or other of the photo-transistor~ 14 and lj an _ 16 -analogue position servo-mode is achieved~ The outputs of the photo-transistors 14 and lS can be applied to the switch 325 via the circuitry 327 which modifies the position signals to yive the ~desired second order response in the position-servo mode to prevent undesirable oscill~tions.
Within the circuitry 315, the motor coils 415 and 416 have associated with them respective drive amplifiers 328a and 328b which amplify the signal from the switch 325 via respective electronic switches 329a and 329b. These are each two pole switches and connect direct with the switch 325 or via an invertor 330a and 330b. The state of these two switches is determined by commutating circuitry 331 which responds in both the velocity servo and positional servo modes, in this example, to the seventh and eighth bits from the position counter 317 so that the polarity o the motor coil voltages is appropriately adjusted to maintain the required direction of the thrust for movement of the armature 12. This is explained in further detail below.
For this purpose i~ is convenient to arrange ~he stator pole pitch of the linear motor to correspond with an integral power of two time one quarter of the graticule wavelength. It is then a simple matter to commutate the coil voltages directly using bits seven and eight of the position counter 317.
~'7~
- 17 ~
It will be appreciated that at any given detent position, the selected slope of one of the analogue outputs of the photo transistors 14 and 15 may be positive or negative depending on the demanded address.
For example, a binary address having least significant bits 00 and 01 might select positive going portions of the outputs whereas a binary address of 10 or 11 would select negative going portions, lsee figure 3d~
In the po~ition servo mode therefore, as well as responding to bits seven and eight of the position counter, the circuitry 331 also responds to the second least significant bit of the a~solute address so as to inver the polarity of the coil currents in order to correct the sign of the gradient of the selected photo-transistor output which would otherwise cau~e the motor to move out of its desired detent position.
Before the start of printing, after power-up of the printer~ the position counter is re-set. One of the coil~ of the motor 415 or 416 is then energised by æeparate circuitry. After a predetermined delay, the other coil is then energised al~o. Or the reverse of this sequence ~lay take place~ This arrangement ensures that the armature will move to an unambiguous stable position relative to the stator corresponding to a commutation position~ 9ecause the position counter is reset, bit~ 7 and 8 both have zero values, i.e. the conditions of bits 7 and 8 are in synchronism with the desired commutation position achieved by the reset routine. A velocity reference signal Vf i5 produced to cause movement of the motor to the left. At the same time, the postion counter is allowed to cycle, 80 keeping bits 7 and 8 in synch:ronism with the movement of the motor, which they then control. As a result the motor continues its motion to the left at a steady speed, until an optical or mechanical reset switch 340 is activated, which resets the actual-position counter 317 again to zeroS However, the exact position at which the limit switch 340 is operated may vary slightly from operation to operation.
The reason for this is as follows. Typically limit switches may be in the form of a paddle mounted on the armature interupting a fixed optical beam sensing device. The exact point at which the sen~ing device registers the limit of movement varies slightly according to external factors such as ambient light.
Therefore the limi~ po~ltion switch may reset the position counter with an accuracy considerably less than the resolution obtainable from the graticule and photo-transistors. For example, assuming in this case 1/80"
variance the reset might occur at a physical position a~
which the digitized output of the two photo-transistors could be at any value between 00 and 11 ~g~
-- 19 ~
depending on the ambient condi~ions. Therefore, the least significant bi~s available from the position counter cannot be depended upon to be in synchronism with the digita1ized outputs from the two photo-transistors. It fo110ws from this, by way of example, that they cannot therefore be uti1ised for such ~
function as mentioned ear1ier to correct the sign of the gradient o~ the ana10gue OUtpllts when in ~he ~home~
position according to the demanded detent address~ For reliable operation~ the digit:ised ou~puts themselves must be utilised for this type of function, and it is for this reason that the least significant bits of the digita1 word representing th~ position of the armature are derived direct1y from the position transducer.
To effect a so1ution, substitute bits derived direct1y from the position transducer are used instead as the lower order bits of the position word; the number of substituted bits should be great enough so that the corresponding distance a10ng the 1ength of the motor is greater than the positional uncertainty associated with the reset switch~ In the i11ustrated embodimentr two substitute bits ~re therefore used instead, P~ ~0) and P' (1). These are produced as shown in figure 3co The output of the Schmidt trigger 31oa associ~ted with the photo-transistor 14 is used as P' ~1) and P' (0) is produced hy combining the outputs of trigger~ ~lla and b in an exclusive-or fashion as shown in figure 3~0 The up/down and clo~k signal generating circuit 311 uses 1ip flops and an arrangement of gates to monitor the state of the outputs of photo-transistors 14 and 15 -~ ~o -at each transition of each one of them and to compare the states after a transition with those before to determine the direction of the movement of the armature and also to generate the clock pulses. A clock pulse to the position counter 317 is only produced if ~he states of P' (0~ and P' (1) were both low pr.ior to the transition and high after it or vice versa.
Figure 3d shows the intended function of the switch control circuit 32~6 when it is operating in the position servo mode and selecting between one of the wave-forms A
and B from the two photo-transistoræO
Suppose the demanded address is such that when moving to the right~ the home signal goes active at the B wave-form crossover Hlo Suppose further that for convenience the wave-form is then selected by the circuit 3~6. The linear motor will then servo to the next A crossover, i~e. H2 which i9 one position off~et from Hl. It will be appreciated that the home position spans two transitions' worth, from Hl to H3 in order to allow the motor to settle at the centre point of H2 o the A waveform. lf the home signal goes activ~ at the A
rossover position ~2, the B waveform i~ then ~elected by circuit 326 and in a similar fashion the motor will then servo to the next ~ crossover~ i~eO H30 Similarly the same actions will occur for home position H0 and H3~
~n important consideration in any elosed loop sy~tem controlling a motor is that the thrust available from the motor should be predictable for efficient ~ervo-action. With the exception of alternatin~ current type linear motors, conventional stepping motors produce non-uniform thrust/displacement: profiles. The form of motor described in my copendina application noO 377,77 however is capable of producing nearly linear thrust, when con~rolled by the circuitry described herein. This i5 explained with reference to figure 4 showing in ~olid outline thrust versus displacement profiles at ~1) and ~2) whi~h would be produced were the coils of a two coil version of the motor to be permanently energised. T~e dotted outlines ~how half phase reversal of the thrust from each coil. On account of the fact that the thrust profiles are effectively in phase quadrature9 it can be seen that by switching from one ~hrust proile to the other and effecting the half phase reversal, a nearly uniform thrus~ profile is obtained, see waveform ~3~.
In practice the variance experienced in tbis simple system is less than 18%o This commutation, as has already been explained, i~
effected by use of bits P7 and P8 which provide the nec~ssary logic combina~ion. Combinations of other bits and joint energisation of both coils can provide an almost straight line thrust profile.
- 2~. -In the event of the motor being detented at an addrescoinciding with a commutation bit change a further advantage of using bits direct from the counter arises in that the commutation is automatically synchronised with the zero crossover point of one of the photo transistor outputs, i.e~ a~ an H position as ~hown in figure 3do Thus undesirable 1back emf' ~pikes are avoided, discontinui~y in thrust displacement relation is a~oided and detenting occurs ~n the same manner at any position in between commutation points.
Thus the foregoing description in total disclose~
means for providing precise po~itional and velocity control of a linear motor making such a motor ~uitable for a range of applications. It will be appreciated that numerous variations are possible for example the use o~ microprocessor soft- and firmware to ~arry out the functions shown in figure 3a.
Claims (21)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A servo device for controlling the operation of a motor comprising a motor position detector and circuitry for controlling the energisation and commutation of coils of the motor, the position detector comprising a position transducer having two parts movable relative to one another, and circuitry for providing a digital word representing the relative position of the two transducer parts, the digital word comprising higher order bits and at least one lower order bit, wherein the higher order bits of the digital word are derived from the outputs of a digital counter cycled by the output of the transducer and used by the commutating circuitry for commutating the motor coils and the said at least one lower order bit is/are derived directly from the output of the position transducer and used, in combiantion with higher order bits from the digital counter, by the coil energisation circuitry for controlling the energisation of the coils to enable precise control of the position of the motor.
2. A device according to claim 1 wherein the transducer has two digital outputs with a phase shift between them and there are two such lower order bits, one of the digital outputs of the transducer being used as the lowest order bit and further comprising an exclusive-OR circuit, the other lower order bit being derived by the exclusive-OR circuit arranged to combine the two digital outputs of the transducer.
3. A device according to claim 1 and including means for comparing the digital word with a further digital word representing a demanded relative position and for producing a position error signal to control the operation of a motor.
4. A device according to claim 3 wherein the error signal is analogue.
5. A device according to claim 3 wherein the circuitry is operative to derive a velocity signal from an output of the transducer, to derive a velocity error signal representing the error between the actual velocity and a demanded velocity as determined by a predetermined velocity/position profile and to substitute the velocity error signal for the position error signal as the drive signal while the position error is greater than a predetermined value.
6. A device according to any one of claims 3 to 5 and including means for producing a "home" signal when higher order bits of the digital word are the same as the corresponding bits of the further digital word, the circuitry being adapted so that when the "home" signal is active, an analogue signal from an output of the position transducer is used as the position error signal for servo operation.
7. In combination: a linear motor comprising a stator and an armature movable lengthwise relative to one another, the stator and armatrure each comprising a respective set of magnets disposed in a predetermined sequence in the direction of relative movements, the magnets of one set being arranged in a stack lengthwise of the direction of relative movement and the poles of the other set of magnets being annular and disposed around and slidable relative to the stack, one set of magnets being permanent magnets and the other set being provided by coil windings, the arrangement being such that by appropriate energisation of the coil windings a net thrust to cause relative movement of the armature and stator in a desired direction can be produced at any position in a range of relative armature and stator positions; and a servo device for controlling the operation of the motor and comprising a motor position detector and circuitry for controlling the energisation and commutation of coils of the motor, the position detector comprising a position transducer having two parts movable relative to one another, and circuitry for providing a digital word representing the relative position of the two trnsducer parts, the digital word comprising higher order bits and at least one lower order bit, wherein higher order bits of the digital word are derived from the outputs of a digital counter cycled by the output of the transducer and used by the commutating circuitry for commutating the motor coils and the said at least one lower order bit is/are derived directly from the output of the position transducer and used, in combination with higher order bits from the digital counter, by the coil energisation circuitry for controlling the energisation of the coils to enable precise control of the position of the motor.
8. A combination according to claim 7 wherein the transducer has two digital outputs with a phase shift between them and there are two such lower order bits, one of the digital outputs of the transducer being used as the lowest order bit and further comprising an exclusive-OR circuit, the other lower order bit being derived by the exclusive-OR circuit arranged to combine the two digital outputs of the transducer in exclusive-or fashion.
9. A combination according to claim 7 and including means for comparing the digital word with a further digital word representing a demanded relative position and for producing a position error signal to control the operation of a motor.
10. A combination according to claim 9 wherein the position error signal is analogue.
11. A combination according to claim 9 or 10 wherein the circuitry is operative to derive a velocity signal from an output of the transducer, to derive a velocity error signal representing the error between the actual velocity and a demanded velocity as determined by a predetermined velocity/position profile and to substitute the velocity error signal for the position error signal as the drive signal while the position error is greater than a predetermined value.
12. A combination according to claim 9 and including means for producing a "home" signal when higher order bits of the digital word are the same as the corresponding bits of the further digital word, the circuitry being adapted so that when the "home" signal is active, an analogue signal from an output of the position transducer is used as the position error signal for servo operation.
13. A combination according to claim 7 wherein some of the higher order bits of the digital word are used to control commutation of the motor coils.
14. A combination according to claim 13 wherein commutation is effected by controlling the polarity of the drive signals applied to the coils.
15. A combination according to claims 12 and 14 wherein when using the position error signal as the motor coil drive signals, its polarity is also controlled in accordance with the slope of the output of the transducer to ensure negative feedback operation.
16. A combination according to claim 7 wherein one of the parts of the transducer has a multiplicity of cycles of a detectable marking lengthwise of the direction of relative movement of the armature and stator, the motor coils having a commutation spacing lengthwise of the motor which equals the repeat length of the detectable marking times 2 raised to the power "n", n being a positive integer or 0.
17. A combination according to claim 7 wherein the control circuitry is adapted to modulate the motor thrust by adjusting the commutation of the motor
18. A combination according to claim 17 wherein the modulation is effective to linearise the thrust/position profile of the motor.
19. A combination according to claim 17 wherein the modulation is used to achieve a desired velocity profile.
20. A device according to claim 1, wherein the motor is a kind such that reversal of polarity of the coil currents is required during motor movement in one direction and the commutation circuitry is arrangd to ensure, by processing of the lower order bit or bits, that the correct polarity of coil energisation is maintained irrespective of variations in the present position of the motor.
21 A combination according to claim 7, wherein the motor is a kind such that reversal of polarity of the coil currents is required during motor movement in one direction and the commutation circuitry is arranged to ensure, by processing of the lower order bit or bits, that the correct polarity of coil energisation is maintained irrespective of variations in the present position of the motor.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB81.27003 | 1981-09-07 | ||
| GB8127003 | 1981-09-07 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1197550A true CA1197550A (en) | 1985-12-03 |
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ID=10524348
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000410872A Expired CA1197550A (en) | 1981-09-07 | 1982-09-07 | Linear motors and control circuitry therefor |
Country Status (7)
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| US (1) | US4590411A (en) |
| EP (1) | EP0074767B1 (en) |
| JP (1) | JPS5875488A (en) |
| AU (1) | AU567862B2 (en) |
| CA (1) | CA1197550A (en) |
| DE (1) | DE3278050D1 (en) |
| GB (1) | GB2106644B (en) |
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| ATE18967T1 (en) * | 1980-05-19 | 1986-04-15 | Kelly H P G | LINEAR MOTOR. |
-
1982
- 1982-09-03 GB GB08225115A patent/GB2106644B/en not_active Expired
- 1982-09-03 DE DE8282304639T patent/DE3278050D1/en not_active Expired
- 1982-09-03 EP EP82304639A patent/EP0074767B1/en not_active Expired
- 1982-09-06 AU AU88052/82A patent/AU567862B2/en not_active Ceased
- 1982-09-07 US US06/415,383 patent/US4590411A/en not_active Expired - Lifetime
- 1982-09-07 JP JP57154734A patent/JPS5875488A/en active Granted
- 1982-09-07 CA CA000410872A patent/CA1197550A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| AU8805282A (en) | 1983-03-17 |
| GB2106644B (en) | 1985-11-13 |
| GB2106644A (en) | 1983-04-13 |
| EP0074767A3 (en) | 1983-04-20 |
| JPS5875488A (en) | 1983-05-07 |
| US4590411A (en) | 1986-05-20 |
| JPH0340592B2 (en) | 1991-06-19 |
| AU567862B2 (en) | 1987-12-10 |
| EP0074767B1 (en) | 1988-01-27 |
| EP0074767A2 (en) | 1983-03-23 |
| DE3278050D1 (en) | 1988-03-03 |
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