WO1989000672A1

WO1989000672A1 - Process for detecting a nearly pinpoint, essentially force-free contact of small area between a probe and a solid object, and contact detector

Info

Publication number: WO1989000672A1
Application number: PCT/DE1988/000388
Authority: WO
Inventors: Dirk-Michael Rupp; Jürgen KISING
Original assignee: Krautkrämer Gmbh & Co.
Priority date: 1987-07-20
Filing date: 1988-06-28
Publication date: 1989-01-26
Also published as: DE3723933A1

Abstract

In a process for detecting a virtually pinpoint, essentially force-free contact of small area between a probe (28) and a solid object (70), a rod-shaped resonator (22) having a probe (28) at its free end is excited at its natural oscillation frequency in its longitudinal direction by an electrical frequency generator (30). The resonator (22) is connected to a receiver (32) which monitors the amplitude of these oscillations in order to detect changes in amplitude. A contact signal is emitted if the amplitude drops by a predetermined value, for example 1 decibel, when the probe (28) approaches the object (70).

Description

Description: Method for detecting a small-area, almost point-like and largely force-free contact between a sun and a solid object, as well as a touch detector

The invention relates to a method for detecting a small-area gene, almost punctiform and largely force-free contact between a probe and a solid object, and an apparatus for performing this method (touch detector).

In a large number of measuring methods, the detection of a possible gentle contact between a probe and an object is desirable. The hardness test and the length measurement with a mechanical measuring rod may be mentioned as an example.

In the Vickers hardness test, the surface of the permanent pyramid impression is determined for a given test force. The surface can be calculated using the depth of the impression if the length difference between the almost force-free contact and the penetration depth is known after the test force has been removed. A distinction must therefore be made between the purely elastic part and the permanent part of the deformation under test force. This distinction is also advantageous for other hardness test methods.

When measuring length, in particular an automatically performed length measurement, a defined contact force of the mechanical length measuring rod is a prerequisite for a precise measurement. With different system forces, different elastic and permanent deformations can occur at individual measuring points, as a result of which the measurement result is falsified.

A small-area, almost punctiform contact is understood as a touch, in which, for. B. the diagonal of a Vickers pyramid is in the range of one micrometer and below. These are contact surfaces that are mostly not detectable in a light microscope, at most - if at all - in a scanning electron microscope. Subs A largely force-free contact becomes contact forces in the area

-4 (milliNewtons) and below, e.g. 10 N, understood.

In the hardness tester known from DE-OS 3424514, the difference between the depth of indentation compared to the (undisturbed) surface of the object is not determined directly by contact of the probe with the surface of the object. Rather, in addition to the probe, at least two mounting feet are provided, against which the probe can be moved transversely to the surface. The starting point for the measurement of the penetration depth is the state of the probe, in which the probe tip lies on a straight line passing through the contact points of these feet. However, the measurement of the (undisturbed) surface carried out in this way is only accurate if the surface to be measured runs in a straight line between the two contact points. All deviations from a straight line connection between the two contact points lead to errors in the measurement of the impression depth. In addition, the measure of the pressing force with which the feet are placed on the surface of the object is not constant, so that further measurement errors result from the fact that the feet are more or less pressed into the surface to be measured. This can be counteracted by using relatively large feet, for example a probe foot tube, but this defines a zero line or zero plane, which generally does not have the actual point of impact (first contact) on the surface with rough surfaces to a moving probe coincides with the surface.

On the basis of the methods of the type mentioned at the outset and the devices operating according to these methods, the object of the invention is to specify a method or a device with which the detection of the first contact of a probe brought up to the surface of a solid object of the probe is possible. In other words, as small a surface area as possible and with as little contact force as possible, successful contact contact of the probe with the surface to be measured should be detected.

This object is achieved procedurally in that the probe, which is preferably designed as a diamond tip, has a free end region rod-shaped resonator is arranged, which is excited by an electrical frequency generator connected to natural vibrations in its longitudinal direction and is connected to a receiver that monitors the amplitude of these vibrations for changes in amplitude and a touch signal appears when the amplitude approaches the probe to the Object stood for a predetermined value, for example a value of one deciBel drops.

According to the invention, the contact of the probe itself with the surface to be measured is detected and displayed directly, which with an indirect measurement, as is known for example from DE-OS 34 24 514, which errors are avoided and do not occur. At the same time, small surface areas, for example flanks of gear wheels, in which the areas are too small for the attachment of two attachment feet, can also be measured. In connection with a hardness test, for example in the low-load range or for micro hardness testing, the method according to the invention enables the initial contact and thus the level of the (undisturbed) surface to be determined with the probe, which is also used later for the hardness test ( for example Vickers diamond). With length measurements, there is the possibility of obtaining reproducible system forces and in this way measuring the smallest length differences, for example steps. The method can also be used to precisely scan and reproduce a profile of a surface. The method according to the invention is particularly suitable for automatic detection, for example an automatic hardness test or an automatic measurement of an object. An arbitrarily shaped surface, for example a workpiece, can thus be scanned point by point, but it is also possible with the method according to the invention to move the probe in the surface itself, that is to say to keep it constantly in contact with the surface, with the contact force is kept constant in that the

Vibration amplitude of the rod-shaped resonator is kept within a predetermined range.

In terms of the device, the object is achieved by a touch detector which has a housing in which a rod-shaped resonator is mounted so that it can vibrate is. At its free end area it has a probe which is designed separately or is an integral part of the resonator rod. The resonator rod is connected to a frequency generator, by means of which it is excited to produce longitudinal natural vibrations. It is also connected to a receiver which has a circuit for detecting amplitude changes or for displaying the amplitude of the rod's natural vibrations.

Such rod-shaped resonators are known in principle from US Pat. No. 3,153,388. in the case of the resonators described there, however, the frequency shift of the oscillation frequency of the rod is determined when the rod tip comes into contact with a solid object. When the probe of the rod is elastically coupled to a mass, the resonance frequency of the rod-shaped resonator is shifted to higher values, since an additional restoring force acts on the oscillating system. The magnitude of this restoring force and thus the frequency shift mentioned depend on the contact area between the probe and the surface to be measured and its elastic behavior. A change in amplitude or measurement of the amplitude is not evident from this previously known method. The previously known method is suitable for being antibibinated with the method according to the invention. When measuring both an amplitude change and thus a frequency shift, a decidedly universal hardness measurement, in particular a Vickers hardness measurement using the depth measurement method (HVT), is possible. In addition, the experience gained with the known rod-shaped resonators can largely be transferred to the method according to the invention or to the touch detector. In this respect, the disclosure content of the US patent mentioned and also the disclosure content of a further development described in DE-OS 37 20 625 is included in the disclosure content of the present application.

In a further development of the touch detector, it is proposed to manufacture the rod-shaped resonator from a piezoelectric material and to attach electrodes which are connected to the frequency generator and to the receiver. Alternatively, the rod-shaped resonator can also be made of a magnetostrictive material, the excitation then taking place via coils, which in turn are connected to the frequency generator or receiver. Such training enables inexpensive resonator rods, the rod itself can form the probe, so that a separate probe is not necessary.

In another embodiment, the resonator can also be made of metal, it is then connected to ultrasonic transducers, which in turn are connected to the frequency generator or to the receiver. For construction, reference is made to the already mentioned laid-open specification 37 20 62, in which it is also stated that a preferably slim resonator has larger frequency changes when in contact with an object than thick resonators (seen in relation to the length). Experiments have shown that this relationship also applies to the change in amplitude. Slim resonators show larger changes in amplitude when in contact with an object than thicker resonators. On the basis of this fact, an amplitude detection and a frequency measurement can be combined favorably on the same resonator.

The circuit for detecting an amplitude change or for displaying the amplitudes of the rod natural vibrations in the receiver is in itself arbitrary and can be implemented according to the prior art. The amplitude can be measured digitally or analogously and, if necessary, averaging over a few or more amplitudes can be carried out. Instead of an immediate amplitude measurement, it is also possible to record the area enclosed by the signal with the zero line (via an integration). A point-by-point scanning of the elongation is also conceivable. Bridge circuits, discriminators, comparators and the like are suitable for detecting an amplitude change.

For a length measurement, it is advantageous if either the probe itself or, preferably, the rod-shaped resonator in the region of a vibration node is connected to a length meter, for example a micrometer screw, the mirror of an interferometer, an inductive displacement sensor, a line grid or the like. In particular, the arrangement of the length meter in the region of an oscillation node of the resonator is advantageous because the dimensions of the touch detector in the region of the probe remain extremely small and the smallest areas can still be measured. The amplitude of the longitudinal vibrations of the resonator are in the nm range, so that they would practically not influence the length measurement anyway. By coupling the length meter in a vibration node, however, the advantage is achieved that there is no interference with the vibration behavior of the resonator. The resonator must anyway be mechanically clamped in a partial area of its total length s, preferably at s / 4. This clamping can also be used to connect the length measuring device.

Further advantages and features of the invention emerge from the remaining subclaims and the following description of non-restrictive exemplary embodiments of the invention, which are explained in more detail with reference to the drawing. In this show:

1 is a schematic diagram of the touch detector according to the invention, the housing is indicated by the dashed square, the touch detector includes a separate display device which is connected to the housing via a connecting line,

2 shows a longitudinal section through a touch detector, which is also designed as a low-load hardness tester,

3 shows a side view of a rod-shaped resonator which is connected to a length measuring device, which is designed here as a micrometer watch for better illustration, and

Fig. 4 is a schematic representation in side view of a device for measuring a surface (a relief).

The touch detector according to FIG. 1 has a housing 20 indicated by dashed lines, which according to FIG. 2 is designed as a hand-held device with an essentially stylus shape. In this housing 20, a slim, rod-shaped resonator 22 is mounted so that it can vibrate. It is a total of around sixty millimeters long and consists of two cylindrical parts that are connected to one another by a truncated cone. In an upper, approximately thirty millimeter long section, it has a diameter of three millimeters, as a result of which there is just a sufficiently large area for attaching ultrasonic transducers 24, 26 to the metallic rod. In the lower area it has a diameter of two millimeters, this area is about twenty millimeters long. Because of this slim design of the resonator 22 is for the Measurement process takes up little space and only requires limited access. E Inaccessible parts, for example flanks of gear wheels, inner walls of pipes, blind bores and the like can also be measured.

A probe 28 is fastened to the resonator 22 at its free end region, which is located outside the housing 20 in accordance with FIG. 2, and is generally designed as a diamond tip, but in a simplified embodiment can also be a metal tip or a tip connected in one piece to the resonator. For the formation of the rod-shaped resonator 22, its ultrasonic transducers 24, 26 and the probe 28, reference is made to the literature on the UCI (Ultrasonic Contact I pedance) method.

The ultrasonic transducers 24, which are arranged opposite one another at the upper end of the resonator 22, are connected to a frequency generator 30, which is referred to in FIG. 1 as a transmitter. The two other ultrasonic transducers 26 arranged approximately in the longitudinal center of the resonator 22 are connected to a receiver 32. The metallic resonator 22 and the transmitter 30 and receiver 32 are connected to ground. The resonator 22 has an overall length of s and is clamped at s / 4 (arrow 34 in FIG. 1) and is connected to the housing 20 at this point. Because of the clamping described, only the first harmonic of a natural vibration can develop in the longitudinal direction, which is excited by the transmitter 30. The generated vibration is interrogated via the receiver. In the exemplary embodiment shown, the receiver 32 has a circuit (not shown here) for detecting the vibration amplitude or for determining a change in the vibration amplitude. In the simplest case, an oscillograph is connected to the receiver output, with which the oscillation amplitude can be monitored. In an alternative which is suitable for automatic measurement, the receiver 32 has a memory in which the amplitude of the last oscillation (or an average of the amplitudes from a number of past oscillations) is stored in each case. The current value of the amplitude is compared with this stored value, and averaging can also take place here. If the compared signals deviate from one another by a predetermined threshold value, for example a dB, the receiver 32 emits a touch signal to a control circuit 36 located in the housing 20 is separate from housing 20 and is connected to it via a line 40, is further processed.

The methods known for this purpose can be used to detect the amplitude of the vibrations detected by receiver 32, for example a peak value detector can be provided, but it is also possible to integrate the surface of the signal curve enclosed with the zero line. Individual parameters of the rod-shaped resonator 22 can be stored in the control circuit 36 in a preferably non-volatile spoke. In this way, the values characteristic of a special resonator 22 are available within the housing 20 designed as a hand-held device, so that the basic device 38 can be connected to different hand-held devices. The parameters of the rod-shaped resonator 22 also include the parameters of the ultrasonic transducers 24, 26 connected to it, the individual design of which has an influence on the vibration behavior of the resonator 22 itself. The specific conditions of the holder in the area of the clamping 34 are also taken into account.

FIG. 2 shows a sectional view of a handheld device for micro hard testing under load. Using such a combined device, the advantage of a known UCI measurement by detecting the frequency shift and monitoring the amplitude according to the invention is to be shown:

The resonator 22 protrudes with its lower end, which is connected to the probe 28, approximately ten percent of its total length s freely from the lower end of a housing 20 formed as a tubular body. The resonator 22 is surrounded within this tubular body by a guide tube 42. It forms in the lower region a plurality of threads running transversely to its axial direction, i the fixing screws 44 are screwed in, which fix the resonator 22 at the position 34. At an appropriate distance from the other end region of the resonator 22, an O-ring 46 is arranged between the guide tube 42 and the resonator 2. On both sides of it there are two ultrasonic transducers 24, 26, which are inserted into corresponding recesses in the metallic rod and are permanently connected to it.

The guide tube 42 is surrounded by two cylindrical-shaped slide bearings 48 handles, one of which is located in the lower end region and the other approximately i in the middle of the tubular body. A compression coil spring 50 is arranged between the threaded connection piece for the fixing screws 44 and the lower ring surface of the upper slide bearing 48, which presses the guide tube 42 and dami 22 against the lower end of the handle-like housing 2. There is a step inside the housing 20 which forms a stop.

During a measurement, the housing is clamped in a lowerable stand (not shown) and motorized against a surface to be measured. The movement relative to this surface is recorded by means of a length measuring device (not shown). If the probe 28 comes into contact with the surface of the object during the advance, a drop in amplitude is registered in the receiver 32. The display of the length meter reached at this time is recorded and saved. If the housing 20 is pressed increasingly against the surface in continuation of the movement, the resonator 2 and thus its guide tube 42 spring inward against the action of the spring 50.

The upper end region of the guide tube 42 is closed, it is assigned a switch 52, the switching part of which is actuated by a certain relative movement between the guide tube 42 and the housing 20. This relative position is set so that it has the desired test pressure. De switch 52 is connected to electronics 54, which is located in the upper inner space of the housing 20 and which includes the receiver 32, the transmitter 30 and the control circuit 36. It is in turn connected to a connector 56, which is arranged in the upper end region of the housing 20.

If the predetermined test force is reached, the frequency shift of the resonator frequency is determined and from this the size of the contact area between the probe 28 and the impression made by it on the surface of the object to be measured is determined. The advance of the housing 20 against the surface is not continued in order not to exceed the test force. The frequency shift is measured in a very short period of time, for example in twenty milliseconds. For this purpose, reference is made to the disclosure content of the aforementioned DE-OS 37 20 625. Un it- After the test force has been reached, the motor feed of the housing 20 is reversed, ie the housing 20 is again moved away from the surface. Attention is once again paid to a change in amplitude: if the probe 28 loses contact with the surface of the object to be measured, the amplitude increases again. The display of the length meter available at this point in time is recorded and compared with that which was ascertained during the first contact. The difference between the two displays is the depth of the permanent impression that the probe 28 has created in the surface.

The principle of length measurement is to be illustrated below with reference to FIG. 3: FIG. 3 shows a resonator 22 as used in the device according to FIG. 2. It has at its lower, free end an axial blind bore 58 into which the probe 28, which is not shown in FIG. 3, can be inserted and exchangeably inserted. In contrast to the illustration according to FIG. 2, the resonator 22 according to FIG. 3 has a collar 60 in its clamping region, which in the exemplary embodiment shown has the same diameter as the upper, cylindrical end region of the resonator 22 (three millimeters). The resonator 22 has an oscillation node in the region of the collar 60 used for clamping. A measuring plunger 62 of a micrometer watch 64 bears against the collar 60. It is attached in such a way that the relative movements of the resonator 22 with respect to a reference, for example with respect to a carriage of a tripod guide, can be detected.

In the case of a pure length measurement, for example, as is shown in more detail in FIG. 4, the resonator 22 is connected to a liftable and lowerable slide (arrow 67) of a stand 68, the micrometer clock 64 itself is connected to the carriage 76 of the stand 68, so that the displacement of the slide 66 in the direction of the arrow 67 can be detected and measured.

In the illustration according to FIG. 4, an object 70, whose surface facing upwards is to be measured, is clamped onto a plate 72. Above the object 70 there is a tripod 68, which has two lateral supports and a guide rail 74 held by them, on which a carriage 76 is slidably guided in the direction of the double arrow 78. The carriage 66, which can be moved in the sense of the double arrow 67, is arranged on it at right angles to the guide rail 74. With the aid of the device according to FIG. 4, the course of the surface of the object 70 can be detected either by point-by-point scanning or by continuous scanning. In the case of point-by-point scanning, the housing 20, which is clamped to the slide 66, is lifted from the surface of the object 70, a certain amount in the sense of the double arrow 78 (for example one millimeter) is pushed, then the slide 66 is lowered again and the distance is determined until the probe 28 of the housing 20 is again in contact with the surface of the object 70. This is followed step by step.

During the continuous scanning, the motors of the carriage 76 on the one hand and the carriage 66 on the other hand are controlled such that the probe 28 always remains in contact with the surface of the object 70. This control takes place by monitoring the oscillation amplitude of the resonator 22 received by the receiver 32. The basic device 38 is connected on the output side to the drive motors (line 80). The drive motors are operated in such a way that the oscillation amplitude received by the receiver 32 remains within a predetermined bandwidth.

Claims

Designation: Method for detecting a small-area, almost punctiform and largely force-free contact between a probe and a solid object, as well as touch detector claims

1. A method for detecting a small-area, point-like and largely force-free contact between a probe (28) and a solid object (70), characterized in that the probe (28) preferably formed as a diamond tip at the free end region of a rod-shaped resonator (22nd ) is arranged, which is excited by an electrical frequency generator (30) connected to natural vibrations in its longitudinal direction and is connected to a receiver (32), which monitors the amplitudes of these vibrations for changes in amplitude and a touch signal appears when the amplitude at The probe (28) approaches the object (70) by a predetermined value, for example a decibel.

2. The method according to claim 1, characterized in that changes in the frequency of the vibrations are simultaneously detected at the receptions.

3. The method according to claim 1, characterized in that the Ausmessun a surface of the object (70), the probe (28) relative to the surface is motorized and the motors used for this are controlled by the amplitude signal detected by the receiver so that the amplitude of the detected Vibration remains at a level corresponding to its level after an initial contact.

4. Apparatus for carrying out the method = according to claim 1 or 2, characterized by a housing (20) in which the rod-shaped resonato (22) is mounted such that it can vibrate, by means of a probe (28) on the free end region of the resonator (22) one on the rod-shaped resonato (22) connected and stimulating it to longitudinal natural vibrations F frequency generator (30) and a receiver (32) connected to the resonator (22), which has a circuit for detecting amplitude changes or for displaying the amplitudes of the rod natural vibrations

5. The device according to claim 4, characterized in that the resonate

(22) has a length s and is connected at s / 4 to the housing (20) (at 3).

6. Apparatus according to claim 4 or 5, characterized in that the resonator (22) is made of a piezoelectric material and has electrodes which are connected to the frequency generator (30) and to the receiver (32), or that it consists of a magnetostrictive material is made and connected to the frequency generator (30) or d receiver (32) via coils.

7. Apparatus according to claim 4 or 5, characterized in that d resonator (22) made of metal and connected to ultrasonic transducers (2 26), which in turn are connected to the frequency generator (30) or to the receiver (32).

8. Device according to one of claims 4 to 7, characterized in that the probe (28) is integrally or detachably connected to the resonator (22).

9. Device according to one of claims 4 to 8, characterized in that the receiver (32) has an amplitude discriminator.

10. The device according to one of claims 4 to 9, characterized in that the resonator (22) in the region of a vibration node with a length meter, for example micrometer clock (64), a mirror, an interferometer, an inductive displacement sensor or the like, is connected.

11. The device according to one of claims 4 to 10, characterized in that the resonator (22) in the housing in the longitudinally movable gel device and is supported relative to the housing by a spring (50), so that a frequency meter or a timer for the period of the oscillation frequency of the resonator rod is provided in the receiver (32).