Feeler Pin Using an Optical Contact Sensor

The invention relates to a feeler pin having an optical contact sensor.

It is known, particularly for 3D measuring machines, to use a contact detecting apparatus or feelers having a feeler pin, which upon contact with a test object, experiences a deflection from which a signal is derived, in order to detect even the most infinitesimal deflections.

In this regard, it is known from German Patent No. 2,937,431, U.S. Pat. No. 4,453,082 and U.S. Pat. No. 4,574,199 that an optical system which has a light source, a mechanically influenced optical element and a light detector can detect a contact. This light source generates a stable luminous flux, which is deflected or blanked out by the optical element, so that the position and/or intensity of the light bundle arriving at the light detector thereafter are influenced.

The U.S. publications teach the use of photodiode arrays as light detectors, whereby a quantitative measurement of the magnitude of the deflection of the contact element, "measuring probe head" is also rendered possible. It is also known from U.S. Pat. No. 4,453,082 that a semiconductor laser diode can be used as a light source.

In the arrangement according to German Patent No. 2,937,431, a mirror is set oscillating by abutting the contact element on the test object. The result of this is that a stationary contact or a deformation of the feeler pin are not detected.

The measurement of changes in position with a laser diode feedback interferometer is known from U.S. Pat. No. 4,655,597 and T. Yoshino et al., Appl. Optics 26 (1987), pages 892-897. In this process, the light emitted by a laser diode is redirected by a mirror onto the laser diode. The mirror therefore acts as a third cavity facet of a combined resonator, which additionally also contains the two end mirrors of the laser diode. A change in the position of the third cavity facet with respect to the laser diode effects a change in the quality of this combined resonator. That is to say, the light emission of the laser diode is influenced by the position of the third resonator. The luminous flux of the laser diode is detected by a photodiode. A control circuit regulates the electrical operating current of the laser diode to stabilize or stably modulate the light emission. Consequently, a relative measurement of position over several .mu.m can be done with a linear signal in an experimental setup such as that shown in T. Yoshino et al. Neither of the publications describes the effects of tilting the mirror.

It is also known from R. 0. Miles et al., J. Lightwave Technology, Vol. 1, No. 1, March 1983, pages 81-93, how to use an optical sensor with a laser diode and an external near-field cavity in order to detect a deflection. In that case, the spacing of the external reflector from the resonator of the laser diode is small by comparison to the length of the resonator, so that the acceleration sensor detects even small angular tiltings of the external reflector.

It is the object of the present invention to equip a feeler pin with an optical contact sensor which enables particularly high feeling sensitivity even in the case of stationary feeling.

By comparison with the direct optical feeling of an object, an advantage arises in that the optical system is protected against environmental influences, and the optical characteristics of the object play no role. A laser feedback interferometer is an integral component of the feeler.

The feeler pin comprises a contact element for contacting an external object and a laser feedback interferometer, whose external reflector is coupled to the contact element for sensing changes in resonator quality of the laser feedback interferometer, the resonator quality corresponding to changes in position of the contact element. The feeler pin further comprises a unit for measuring radiation flux of a laser in the laser feedback interferometer, the radiation flux being dependent upon the resonator quality. The feeler pin may also comprise a laser in the laser feedback interferometer, which is a laser diode with resistance measuring element for measuring internal resistance of the laser diode, this internal resistance corresponding to the resonator quality. The feeler pin may also comprises a laser in the laser feedback interferometer, which operates in a region of threshold between fluorescence and lasing.

It has been established that the spacing of the external reflector from the laser diode can be large, whereby the feeler is simple to construct. The principle is suitable for both switching and measuring feelers.

Objects and advantages of the invention will become apparent from the following detailed description and the drawings.

FIG. 1 shows a feeler pin according to the invention for 3D measuring machines.

FIG. 2 shows an example in which the feeler pin length and the resonator length are independent of one another.

FIG. 3 shows an example which is particularly suitable for exchangeable feeler pins,

FIG. 4 shows an example in which the abrupt change from lasing to fluorescence operation is used in order to form a trigger.

FIG. 5 shows an example with a lens in the beam path, and with the arrangement of the cavity facet on the base of the feeler pin.

FIG. 6 shows an example with an optical fiber in the beam path.

FIG. 7 shows a feeler pin in a modular system having plug-and-socket connections and branchings for optical fibers.

FIG. 1 shows, firstly, a feeler pin 1 used as a switching feeler pin with 3D measuring machines It has a contact element 2--typically a polished sapphire ball--a beam 3 and a base 4.

In order to limit the forces between ball 2 and a test object 5, the base 4 is mounted in a tilting bearing 6, and is fixed there in the rest position by retaining springs 7.

According to the invention, a laser 8 is arranged in the base 4, the beam 3 is embodied as a hollow tube, and the region of the ball 2 lying therein is used as mirror 2a.

The first and second end mirrors 8a, 8b of the laser 8, and the mirror 2a as the third cavity facet form a coupled laser resonator. The laser power is measured by a monitor photodiode 9, which is arranged behind the outer laser mirror 8b.

If the relative position of mirror 2a and laser diode 8 is altered by sliding or tilting, then the feedback, and equally the quality, of the coupled laser resonator is varied. This can be detected by the photodiode 9 as a change in radiation flux. A laser diode 8 is preferably used as laser 8. Suitable laser diodes 8 and monitor photodiodes 9 are commercially available as a complete unit, e.g. Hitachi HL 7806, Sharp LT 030 MD/MF. However, the electrical resistance of the laser diode 8 also changes with the resonator quality and hence that resistance can also be evaluated. It is then possible to omit the photodiode 9. An electronic circuit 80 having terminals 81 serves for supplying power and for evaluation.

It is also possible to modulate the wavelength of the laser diode 8 by modulating the current and/or temperature thereof, for the purpose of measuring the resonator quality.

Since only a small proportion of the luminous flux from the laser diode 8 needs to be reflected by the mirror 2a in order to achieve the above-mentioned effect, the polished sapphire ball, which the contact element 2, can also be used without further processing as the mirror 2a.

The tube used as beam 3 can be very thin and light, since it envelops only the narrow laser beam 10, and is preferably not rigid, in order to be able to affect a detectable change in position of the mirror 2a given the smallest force between contact element 2 and object 5. For this purpose, the tube is advantageous for it to have a bending point 3a near the base 4.

The interior of the tube can be provided with a particular atmosphere, including vacuum, in order to exclude the effects of dust, moisture, and fluctuations in pressure and density on the laser resonator.

FIG. 2 shows an arrangement in which the length of the beam 3 and of the feeler pin 1, and the length of the laser resonator are decoupled. The laser diode 8 is fastened to a rigid carrier 11 on the base 4, and projects into the tube up to near the mirror 2a. In the case of a small resonator length, this results in large specific deflections of the mirror 2a relative to the laser diode 8. Since the internal resistance is measured in the example, it is possible to dispense with a photodiode 9 (FIG. 1).

The example of FIG. 3 shows an embodiment which is particularly suitable for exchangeable feeler pins. Laser 8 and monitor photodiode 9 are connected with a mount 12 to the tilting bearing 6 near the base 4, and arranged in an extension of the hollow beam 3 of the feeler pin 1. The feeler pin 1 thus contains no active components, and is therefore particularly cost effective to produce as an exchangeable part. In addition, with this arrangement it is possible to detect maladjustments of the basic setting of the feeler pin in the tilting bearing 6.

Furthermore, it is shown that the contact element 2 can also be provided with a specially embodied mirror 2b, which is optimized for the optical system. For example, it is possible to provide a plane mirror 2b having a special coating. The adjustment of laser diode 8 and plane mirror 2b is simplified thereby, but the cost of producing the mirror 2b is increased.

FIG. 4 shows an arrangement of the laser diode 8 in the base 4 of the feeler 1, of the plane mirror 2b on the contact element 2, which is represented here as a cylinder/cone combination. FIG. 4 also shows monitor photodiodes 13a and 13b alongside the laser diode 8, which detect light that has taken the path from the laser diode 8 via the mirror 2b to the photodiodes 13a, 13b.

The optical construction thus initially looks like a conventional reflex light barrier. If the photodiodes 13a, 13b are part of an annular array, the tilting axis between mirror 2b and laser diode 8 can be deduced from the position of the reflex on the photodiode array in accordance with U.S. Pat. No. 4,574,199.

According to the present invention, the arrangement of the laser diode 8 with its end mirrors 8a and 8b, and of the mirror 2b is now embodied as coupled laser resonator, having three mirrors. For this purpose, the resonator quality and the operating conditions of the laser diode 8, such as current and temperature, are chosen in the basic setting such that precisely the threshold for the laser light emission is exceeded.

The laser beam 10 is narrowly bundled, so that virtually no light can fall upon the photodiodes 13a and 13b.

If the mirror 2b is tilted about a specific angle, the quality of the resonator is diminished, so, that the laser diode 8 passes over into fluorescent operation, and fluorescent light 14 of relatively high divergence is now emitted.

It follows that a considerable light intensity is incident upon the photodiode 13a which is determined by the tilting direction of the mirror 2b. Thus, the arrangement acts as a trigger.

As long as the threshold for laser light emission is not undershot owing to changes in position of the mirror 2b, this arrangement can operate like that according to FIG. 1, and the change in resonator quality can be detected by the electrical exciting current of the laser diode 8, or by an additional monitor photodiode. Because of the divergent light beam path to the photodiodes 13a, 13b, the beam 3 must be wider than the beam 3 in the embodiments of FIG. 1 to FIG. 3.

As a modification of FIG. 3, FIG. 5 shows that the third cavity facet 15 can also be separated from the contact element 2. For this purpose, it is necessary to modify the known, stiff feeler pin designs according to the invention. In this regard, the cavity facet 15 is attached to the base 4 of the feeler pin 1. In addition, the use of a lens 16 in the beam path of the laser beam 10 is shown, so that the divergence of the light emitted by the laser diode 8 can be reduced, and the feedback improved. It is especially advantageous in this arrangement to use lens elements 16 having refractive index gradients.

In FIG. 6 an optical fiber 17 is inserted in the beam path between the laser diode 8 and the third cavity facet 2a on the contact element 2.

If the optical fiber is rigidly embodied in a free-standing fashion in the region of the beam 3, then the advantages of the arrangement according to FIG. 2 are achieved but the active components of laser diode 8 and, possibly, the photodiode 9 can be withdrawn onto the tilting bearing 6, as in the case of FIG. 3. Given the choice of a suitable optical fiber 17, e.g. a single mode fiber, the laser diode 8 can be withdrawn as far as into the control cabinet of the complete 3D measuring machine, to which the feeler pin 3 is attached. Electromagnetic disturbances can be suppressed in this way.

Also represented in FIG. 6 is the arrangement of a lens element 18 between the optical fiber 17 and the third cavity facet 2a. It is thus possible, as represented, for the emergent light to be focused onto the center of the ball of the contact element 2, wherein the ball forms the cavity facet 2a, so that the light impinges vertically onto the mirror surface.

FIG. 6 also shows that the optical fiber 17 can be provided with a connector 19 and socket 20 of commercial design. The entire feeler pin can then easily be exchanged.

The optical fiber 17 can also fill the entire length between the laser 8 and the third cavity facet 2a, 2b, that is to say, take the place of the cavity in the beam 3 of FIG. 1, and thus reduce light losses. It is then possible to do without lens element 16, 18, particularly if graded-index optical fibers are used as optical fibers 17. If the feeler element 2 comes into contact with an object 5, the optical fiber 17 is then bent or compressed. In the case of embodiments of the optical fiber 17 which respond to this with changes in optical characteristics, e.g. of the damping, or changes in the polarization characteristics, these change intensifying the change in quality of the resonator during feeling.

In order to render different feeling geometries possible, feeler pins 1 are often embodied in a modular system with a base 4, various beam elements 3b, contact elements 2 and branched and angled pieces. The individual elements are then combined as appropriate for a specific measuring task.

This can also be realized in the case of a feeler pin 1 according to the invention, as the example of FIG. 7 shows.

In this figure, the base 4 and beam element 3b are provided with optical fibers 17 and interconnection devices, embodied as optical fiber connector 19 and optical fiber socket 20. Contact elements 2 with the third cavity facet 2a, 2b can be fastened directly to beam elements 3b, or likewise be provided with connectors 19. Together with branching elements 21 for optical fibers 17, which are simultaneously constructed as mechanical branching elements for the beam elements 3b, it is then possible for all required feeler pin configurations to be realized.

The represented feeler pins according to the present invention are only examples. In particular, features can be combined with one another from different figures.

The electronic circuits 80 and terminals 81 (FIG. 1) required to operate a feeler according to the present invention are not described in more detail, since they are, e.g., known from the cited prior art, and can easily be adapted to the requirements in a 3D measuring machine.

However, the application of the feeler pin according to the present invention is not limited to 3D measuring machines, rather it can be used quite generally as a sensitive tactile sensor.