WO1999051939A1

WO1999051939A1 - Contact detecting probe with light barrier

Info

Publication number: WO1999051939A1
Application number: PCT/EP1999/002253
Authority: WO
Inventors: Carlo Carli
Original assignee: Marposs Societa' Per Azioni
Priority date: 1998-04-06
Filing date: 1999-03-29
Publication date: 1999-10-14
Also published as: ITBO980218A0; ITBO980218A1; IT1299954B1

Abstract

A contact detecting probe including a casing (1) and a movable arm-set (5), housed in the casing, with a support element (7), an arm (11) and a feeler (13), coupled to the arm and adapted for contacting a mechanical piece (15) to be checked. At least a light emitting diode (21) is fixed to the casing and directs a light beam, along a predetermined path, toward a corresponding photodiode (25) also fixed to the casing. The support element, axially biased by a spring (17), includes an annular rim (8) that, when the probe is in rest condition, contacts an internal surface (16) of the casing and interrupts the path of the light beam. The occurring of contact between the feeler and the mechanical piece is detected by the photodiode that, in consequence of the at least partial detaching of the annular rim and the internal surface of the casing, receives the light emitted by the associated emitter diode.

Description

DESCRIPTION

CONTACT DETECΗNG PROBE WITH LIGHT BARRIER

Technical Field

The present invention relates to a contact detecting probe, including a casing, that defines a longitudinal geometric axis and an internal support surface, an arm-set partially housed in and movable with respect to the casing with a support element having a portion for cooperating, when the arm-set is in an inoperative condition, with said support surface, and an arm coupled to the support element and carrying a feeler for contacting a piece to be checked, and a detecting device for providing signals depending on the position of the arm-set with respect to the casing, including at least an emitter device and at least a receiver device coupled with said casing.

Background Art

Contact detecting, or "touch trigger", probes with movable arm-sets that carry feelers are used in co-ordinate measuring machines and in machine tools, particularly machining centers and turning machines, for checking machined or being machined pieces, tools, machine tables, etc. The checkings can regard, for example, the position or the geometric dimensions of pieces. In each of these probes, contact between feeler and, for example, a mechanical piece is monitored by suitable devices that detect specific displacements of the movable arm-set with respect to a casing, and control the reading of transducers associated with the machine slides, that provide measurement values with respect to a reference position or origin. Some of these probes utilize optical switches for monitoring that contact between probe and mechanical piece to be checked has occurred.

British patent application GB-A-2238616 discloses various embodiments of touch trigger probes that include an element, movable with respect to a substantially longitudinal casing, to which there is coupled an arm carrying a feeler that rests, in rest conditions, on surfaces of said casing. One of the illustrated embodiments includes an optical switch with a source of light located in a recess of the movable element. A translucid plate on which there is stamped a grid, is coupled with the movable element and covers the recess, thus enabling the partial passage of light. A receiver is located on a stationary part, at the opposite side of the translucid plate with respect to the light source, and is always irradiated by the light beam that crosses the plate. When the arm and, consequently, the movable element are deflected with respect to the inoperative condition further to contact occurring between the feeler and a piece, the characteristics of the light that crosses the plate are altered by the change in the arrangement of the grid stamped on it and these changes are detected by the receiver and the occurring of contact is monitored.

In this embodiment, possible deformations of mechanical parts (in particular of the grid) due to temperature variations can cause corresponding changes in the characteristics of the light and inaccurate or false indications of the occurring of contact. In order to overcome this problem, in other terms for minimizing the mechanical effects of the thermal drifts, it is necessary that the monitoring of the occurring of contact takes place only when the detected changes of light reach relatively high values, and this implies a reduction in the device sensitivity. Inventor's certificate SU-A-1516786 discloses a "touch trigger" probe including an optical system for detecting displacements of an arm-set including a movable arm and a plurality of optical guides located on planes radially arranged with respect to the longitudinal axis of the probe. Each optical guide connects a light emitter with an optical receiver and is divided into two parts, that are separated by a portion of the movable arm-set. When the portion separating the two parts displaces, subsequently to displacements of the arm-set, light can cross from one to the other of the two parts, hence, achieve the optical connection between emitter and receiver.

The probe structure, more specifically the detecting system with the optical guides that have to be arranged with great accuracy within the generally limited space at the interior of the casing, foresees complex and delicate operations for the assembly and the set up, and for preventing the optical guides from interfering with the correct mechanical operation of the probe.

Disclosure of Invention

Object of the present invention is to provide a probe, achieved by simple construction, reliable, with a remarkable degree of accuracy and repeatability and that provides a signal that is substantially unaffected by the effects of thermal drifts.

This and other objects are provided by a contact detecting probe according to claim 1.

Brief Description of the Drawings

The invention is now described in more detail with reference to the enclosed sheets of drawings, given by way of non limiting example, wherein: figure 1 is a longitudinal cross-sectional view of a contact detecting probe according to the invention; figure 2 is a cross-sectional view of the probe shown in figure 1 viewed along line II-II in figure 1; figure 3 is a block diagram of a first type of power supply, amplification and processing device of the probe of figure 1; figure 4 is a block diagram of a second type of power supply, amplification and processing device of the probe of figure 1; figure 5 is a longitudinal cross-sectional view of the probe of figure 1 shown in a different working position with respect to that of figure 1;

Figure 6 is a graph that shows some signals relating to the power supply device of figure 3;

Figure 7 is a graph that shows some signals that relate to the power supply device of figure 4;

Figure 8 is a longitudinal cross-sectional view of a probe according to a different embodiment of the invention;

Figure 9 is a block diagram of a part of the amplification and processing device of the probe of figure 5;

Figure 10 is a longitudinal cross-sectional view of a probe according to a further embodiment of the invention; and

Figure 11 is a schematic view of the optoelectronic measuring system of the probe of figure 10.

Best Mode for Carrying Out the Invention

The probe schematically illustrated in figures 1, 2 and 5 includes support and protection means with a casing 1, that has a substantially cylindrical shape and defines a longitudinal geometric axis, with a lower base, that defines an internal support surface 16 that is substantially plane and annular, and an upper base. A movable arm-set 5 is partially housed within casing 1 and comprises a support element 7 that defines a portion or annular rim 8 with substantially cylindrical symmetry, an - 5 -

arm 11 coupled with support element 7 and partially protruding through a hole in the lower base of casing 1, and a feeler 13 coupled to a free end of arm 11. A biasing device comprises a compression spring 17 arranged between surfaces of the upper base of casing 1 and support element 7, for urging annular rim 8 and support surface 16 into mutual contact when arm-set 5 is in rest condition, i.e., in the absence of contact between feeler 13 and a piece to be checked 15. In this condition, a symmetric recess 14 is defined between element 7 and casing 1.

Centering and antirotation devices (of a known type and not shown in the figures) are foreseen, for example between movable arm-set 5 and casing 1, for preventing mutual transversal translation and rotation displacements about the longitudinal axis.

A detecting device, for example of opto-electronic type, comprises emitter devices and receiver devices. In particular, two light emitting diodes, or "LED", 21 and 23 are fixed to the lower base of casing 1 and arranged in diametrically opposite positions, substantially in correspondence of the plane defined by annular surface 16. Two receiving photodiodes 25 and 27 are also fixed to the lower base of casing 1, substantially on the same plane as that of LEDs 21 and 23 and facing them. The mutual arrangement of the elements of each of the pairs LED/photodiodes 21/25 and 23/27 is such that the light beam emitted by the LED is directed toward the corresponding protodiode along a determined path that consists of a section transversal with respect to the longitudinal axis of casing 1.

The two LEDs 21 and 23 and the two photodiodes 25 and 27 are electrically connected to a power supply and processing unit 31, by means of cables that are schematically shown in figure 1 and identified by reference number 30. A sealing gasket 37 has its ends fixed to the lower base of casing 1 and arm 11, respectively, and - besides protecting - 6 -

the internal arm-set of the probe from foreign matter - prevents the passage of light into the probe. Figures 3 and 4 show, in the form of block diagrams, two different embodiments of some elements of the power supply and processing unit 31, when the two LEDs 21 and 23, connected in series, are fed by direct current and pulse current, respectively.

In the first case, illustrated in figure 3, the two LEDs 21 and 23 are fed by a direct current generator G. Amplification and comparison means include an amplifier A that amplifies signal V_τ sent by photodiodes 25 and 27, connected in parallel, and a comparator C that compares signal V_A output by amplifier A with a suitably predetermined threshold level V_τ, and provides a logic signal V₀ indicative of the result of this comparison.

In the second case, shown in figure 4, the two LEDs 21 and 23 are fed by a generator G' that provides them with current pulses. Amplification and comparison means include an amplifier A' that amplifies the signal V_τ, sent by photodiodes 25 and 27 and a comparator C , coupled by means of a filter F with amplifier A' , that compares signal V_A, output by the latter with a suitably predetermined threshold level V_τ. Signal V_c, output by comparator C is furtherly processed by a timer T. Timer T outputs a signal V₀, at low logic level, if at its input there is a low logic level signal. Signal V₀, changes to high logic level and remains so for a prefixed time period of a minimum duration τ_τ each time that the signal input at timer T changes from low logic level to high logic level. When a new pulse reaches the input of timer T and the output signal V₀, is at high level, timer T zero sets the counting, i.e., the output signal V₀, remains at high level for at least a period τ_τ starting from that moment. The probe operates in the following way. In the absence of contact between feeler 13 and piece 15 to be measured, arm-set 5 is in a rest condition, in which annular rim 8 and support surface 16 are into mutual - 7 -

annular contact (figure 1) due to the bias provided by spring 17. In this condition, the light beams emitted by LEDs 21 and 23 do not reach photodiodes 25 and 27, as opposite portions of rim 8 lie in their associated paths. When LEDs 21 and 23 are fed by direct current (figure 3) , in a rest condition there is a signal V_j of null value (figure 6) at the input of amplifier A. At the input of comparator C there is a signal V_A of null value or - as hereinafter more detailedly described - in any case lower than the threshold level V_τ and, consequently, signal V₀ output by comparator C is kept at low logic level. In the course of a mutual displacement between probe and piece 15 to be checked in the transversal direction X, subsequently to contact occurring between feeler 13 and a surface of piece 15 at a time instant t_{ι r} arm 11 and the whole movable arm-set 5 tilt with respect to casing 1

(figure 5) and rim 8 partially raises from the annular surface 16 thus enabling the passage of the light beam emitted by one of the LED, 23, according to the embodiment of figure 5. The light beam reaches its associated photodiode (27) , that generates a signal Vj amplified by amplifier A (V_A) and compared, in comparator C, with threshold V_τ (figure 6) . Further to the exceeding of value V_τ, at a time instant t^τ, the output signal V₀ reaches a high level thus monitoring that contact between feeler 13 and piece 15 has occurred.

In the case of longitudinal displacement between probe and piece 15, and contact between feeler 13 and a transversal surface of piece 15 in the direction Z, arm 11 and movable arm-set 5 raise thereby causing rim 8 to detach from annular surface 16. In this case the light beam of at least one of LEDs 21 and 23 reaches its associated photodiode 25 and 27 and the processing of signal V_τ provided by at least one of the latter occurs in the formerly described manner. This solution offers the advantage of being simple and unaffected by possible inductive or capacitive couplings between the power supply current of LEDs 21 and 23 and photodiodes 25 and 27 or amplifier A.

Furthermore, possible undesired variations of signal V_A when arm-set 5 is in rest conditions, due to the effects of temperature variations, are limited to the so-called thermal drifts of the amplifier, that generally are of a limited entity. In fact, the probe according to the present invention is not sensitive to the so-called mechanical effects of thermal drifts that, by causing limited deformations of the mechanical parts of the measuring devices, alter, in the known probes, the signal to be amplified, by introducing subsequent variations of a substantial entity in the amplified signal. In the case of the probe according to the invention, in rest conditions the signal to be amplified V-. has zero value and, in normal working conditions, it remains so even further to limited deformations - caused by temperature variations - that occur in the involved mechanical parts. In other words, by indicating K as the amplifier A gain, signal V_A subject to thermal drifts can in general be expressed as

V_A = (K+K^V_j+D where K_x and D represent the effects of the thermal drifts of amplifier A. As already hereinbefore stated, because in rest conditions V_j remains, even subsequently to limited deformations of the mechanical parts, permanently equal to zero, signal V_A can vary with respect to zero of the reduced direct current component D. In order to avoid false monitorings of the occurring of contact between feeler 13 and piece 15 in rest conditions, in other terms in order to guarantee that, in these conditions, the value of the output signal V₀ is held at low logic level, it is therefore sufficient to predetermine a threshold level V_τ that keeps into account just the possible limited variability of the signal V_A attributable - 9 -

to component D. The possibility of setting a particuarly low threshold level V_τ guarantees a particularly high probe sensitivity.

When LEDs 21 and 23 are fed by a pulse current (figure 4), when no contact occurs between feeler 13 and the piece to be checked 15 and therefore when there are no signals arriving from photodiodes 25 and 27, the effects of possible direct current drifts in amplifier A' are eliminated by the high pass filter F located between amplifier A' and comparator C . Further to contact occurring between feeler 13 and piece 15 at a time instant t_x ' in direction X, and upon the subsequent passage of the light beam from LED 23 to photodiode 27, as formerly described (figure 5), signal V-. provided by said photodiode 23 (figure 7), suitably amplified by A' (V_A,), is sent to comparator C after filter F (V_F) has eliminated the direct current component .

When the peak value of the latter signal V_F exceeds threshold V_τ, , logic signal V_c, output by comparator C reaches a high or low value with a pre-set frequency equal to that of the current across LEDs 21 and 23. If the period τ_τ defined by timer T is slightly longer than the period of the power supply current of LEDs 21 and 23, at the output of timer T there is a signal V₀, that, when arm 11 is deflected, permanently remains at high logic level and changes to low logic level when arm 11 returns to the rest condition, with a delay that at the most is equal to period τ_τ of timer T. The definition of the threshold level V_τ, is made by following criteria like those previously described in connection with threshold V_τ that contribute to the high sensitivity of the probe.

The probe illustrated in figure 8 is under many aspects alike the one described with reference to figures 1 and 2 and for this reason the common parts are identified with identical reference numbers. - 10 -

The fundamental difference consists in the presence of two windows 39 and 41 in the support element 7 near rim 8 and substantially at the pairs LED-photodiodes 21, 25 and 23, 27. In windows 39 and 41 there is a material that attenuates the intensity of the light transmitted by LEDs 21 and 23 to their associated photodiodes 25 and 27. This optical attenuator can, for example, be achieved in a known way by a vacuum deposition of a thin layer of special metal alloys on a lower-layer of thermically treated glass. In figure 9 a block diagram shows the processing means of a processing unit 31' ' that relates to the second embodiment of the invention, when LEDs 21 and 23 are fed by direct current. More particularly, an amplifier A'' receives the signals from photodiodes 25 and 27 and is connected with comparison means that include two comparators Cl and C2 and logic processing means OR that carry out the logic sum of the signals output by the latter.

As the light transmitted by LEDs 21 and 23 - when movable arm-set 5 is in rest condition - reaches photodiodes 25 and 27, even if with attenuated intensity, through windows 39 and 41, at the input of amplifier A'' - and consequently as its output - there is a low logic value signal, but not a null signal. This signal is sent to the inverting input of comparator C2 for its comparison with a threshold V_τ2 and at the non inverting input of comparator Cl for its comparison with a threshold V_τl+ V_τ2.

The levels of the two thresholds V_τl and V_τ2 are chosen in such a way so that if arm 11 is not deflected, in other terms when just the light that crosses windows 39 and 41 reaches photodiodes 25 and 27, the signal output by amplifier A' ' exceeds threshold V_τ2 and does not exceed threshold V_τl+ V_τ2 and hence the outputs of both comparators are low and output V₀. , of unit OR is low. If, on the contrary, the arm is deflected, the signal output by amplifier A' ' has a value that exceeds that of both threshold V_τ2 and threshold V_τl+ V_τ2 so the output of comparator C2 is at low logic level, while the output of - 11 -

comparator Cl is at high logic level, thus output V₀,, of unit OR is at high logic level .

In the event there occurs a failure of the detecting device including LEDs 21 and 23 and photodiodes 25 and 27, in other terms in the event an anomalous condition causes the signal transmitted by photodiodes 25 and 27 to amplifier A'' to have a substantially null value, the output of said amplifier A'' is lower than both thresholds (V_τ2 and V_τl+ V_τ2) . In this case, while the output of comparator Cl is at low logic level, the output of comparator C2 , and consequently output V₀,, of unit OR, is at high logic level. In practice, there is monitored a deflection of arm-set 5 in the absence of contact between feeler 13 and piece 15, hence clearly proving that there is a probe failure condition.

The probe illustrated in figure 10 differs from that described with reference to figures 1 and 2 substantially insofar as the arrangement and some components of the optoelectronic measuring system are concerned. Both emitters 21 and 23 and receivers 25 and 27 are coupled to the upper base of casing 1. Emitters 21 and 23 are arranged at diametrically opposite positions of the upper base of casing 1 and oriented so as to emit the light beam in a first longitudinal path portion directed towards the lower base of casing 1. Receivers 25 and 27 are placed beside emitters 21 and 23, at a certain distance from them along the same diametral direction, and face the lower base of casing 1 for receiving the light beam along a second longitudinal path portion. Four reflecting elements, including for example mirrors 42, 43, 44 and 45, are coupled to the lower base of casing 1, substantially at the positions previously occupied by emitters 21, 23 and receivers 25, 27 in the embodiment shown in figure 1, and are inclined, relative to the main transversal plane of the lower base of casing 1, according to a 45 degree angle. Mirror 42 is oriented so as to receive the light beam emitted by emitter 21 along the first longitudinal path - 12 -

portion and reflect it, at least partially, along a transversal path portion towards mirror 43. The latter is in turn oriented so as to reflect, at least partially, the light beam arriving from said transversal portion and direct it along the second longitudinal path portion towards receiver 25. The light beam emitted by emitter 23 follows a similar preset path, that includes, besides the longitudinal portions between emitter 23 and mirror 44 and between mirror 45 and receiver 27 a direct transversal portion between mirrors 44 and 45. The support element 7 has two longitudinal holes 46 and 47 for enabling the passage of light partially reflected by mirrors 43 and 45 towards receivers 25 and 27 (second longitudinal portions of the associated paths) . The operation is alike that of the probe shown in figures 1 and 2. In fact, in this case too, when arm-set 5 is in rest condition, portions of rim 8 lie in the paths of the light beams, in particular at their associated transversal sections, and further to the raising of rim 8 from support surface 16, the light beams can reach the associated photodiodes. The processing of the signals produced by photodiodes 25 and 27 occurs in the previously described manner.

The probe of figure 10 can be modified in order to include windows with an optical attenuator, according to the description provided with reference to figure 8.

Although the embodiments illustrated in the drawings and formerly described include elements (casing 1 and movable arm-set 5) that define annular plane surfaces, in the shape of a circular crown, (the internal surface of support 16 and annular rim 8, respectively) urged into mutual contact, the invention can be applied even by utilizing elements that define concave/convex surfaces with rotational symmetry urged into mutual contact, when the feeler is not subject to forces, along a contact circumference. For example, the invention can apply to a probe with a constraining system that includes an element with a toroidal surface, or a surface in the shape of a spherical - 13 -

zone and an element with an internal surface with a truncated cone shape, provided that the head be made in such a way so that the displacement of the feeler causes in any case, as in the formerly illustrated and described embodiments, a detaching of the two surfaces that is sufficient for enabling the light beam of at least one of LEDs 21 and 23 to reach the associated photodiode 25 and 27. The herein illustrated and described probes include two emitter/receiver pairs arranged in the same diametral plane of figures 1, 5, 8, 10, and enable to detect displacements of feeler 13 in said plane, more particularly displacements caused by contact between feeler 13 and piece 15 in the X and/or Z axes. However, the number and the arrangement of said emitter/receiver pairs can vary, as just a light beam is necessary for detecting the contacts in just one direction (for example +X) , and as there are foreseen, for example, for detecting displacements in any direction of space, three emitter/receiver pairs and associated light beams in three distinct paths lying in diametral planes angularly spaced apart at 120° from one another. Other variants can be foreseen with respect to what has been herein described and illustrated. For example, with respect to the embodiment of figures 1, 8 and 10, the position of one or both the LEDs and their associated photodiodes can be inverted. There can also be foreseen a single emitter placed at the interior of recess 14 and adapted for diffusing light in all directions, and a plurality of photodiodes at the exterior of said recess, for example coupled to casing 1, for detecting the light emitted by said emitter when arm 11 is deflected and rim 8 detaches at least partially from surface 16.

It is possible to utilize other types of emitters and receivers with respect to those described, more particularly the photodiodes can be replaced with CCD

(Charge Coupled Devices) or phototransistors . It is also possible to utilize non-optical systems, for example of - 14 -

ultrasound or microwave type, instead of the LED-photodiode couplings .

The power supply and processing units 31 and 31' ' , herein schematically illustrated in the figures, can be housed in casing 1, for example in suitable seats, not shown in the figures, achieved in the upper base of casing 1. The reflecting elements 42, 43, 44 and 45, described with reference to figures 10 and 11, can be achieved in an integral way in casing 1, for example, by obtaining the necessary number of suitably inclined surfaces at the lower base of casing 1 and lapping them so they become reflecting surfaces .

Many aspects of the mechanical structure of the probe schematically illustrated in the figures can be modified without departing from the scope of the invention.

Annular rim 8 can be replaced, for example, with a specific number of supporting elements separated from one another, with the LED/photodiode pairs (or the inclined mirror pairs) arranged at opposite sides of said support elements.

Claims

- 15 -CLAIMS

1. A contact detecting probe including

ΓÇó a casing (1) , that defines a longitudinal geometric axis and an internal support surface (16) ,

ΓÇó an arm-set (5) partially housed in and movable with respect to the casing (1) with

ΓÇó a support element (7) having a portion (8) for cooperating, when the arm-set (5) is in an inoperative condition, with said support surface

(16) , and

ΓÇó an arm (11) coupled to the support element (7) and carrying a feeler (13) for contacting a piece (15) to be checked, and ΓÇó a detecting device (21,23,25,27) for providing signals depending on the position of the arm-set (5) with respect to the casing (1) , including at least an emitter device (21,23) and at least a receiver device (25,27) coupled with said casing (1), characterized in that said emitter device (21,23) is also coupled with said casing (1) for generating a light beam directed, in a pre-set path, towards the receiver device (25,27), said portion (8) of the support element (7) being arranged, in the rest condition of the arm-set (5) , along said path, for preventing the light beam from reaching the receiver device (25,27).

2. The probe according to claim 1, wherein said path includes a transversal section, said portion (8) of the support element (7) being arranged, when the arm-set (5) is in rest condition, in said transversal section.

3. The probe according to claim 2 , wherein said emitter device (21,23) and said receiver device (25,27) are fixed to the casing (1) in correspondence of the support surface (16) at opposite sides with respect to said portion (8) of the support element (7) . - 16 -

4. The probe according to claim 3 , wherein the support element (7) and the casing (1) define a recess (14) , said emitter device (21,23) and said receiver device (25,27) being housed at the interior and at the exterior of said recess (14), respectively, or vice versa.

5. The probe according to claim 2, wherein the detecting device includes at least a first inclined, reflecting element (42,44) , for reflecting the light beam generated by the emitter device (21,23) and generating said transversal section, and at least a second inclined, reflecting element (43,45), said first (42,44) and second (43,45) reflecting elements being integral with the casing (1) and arranged in correspondence of said support surface (16) at opposite sides with respect to said portion (8) of the support element (7) .

6. The probe according to claim 5, wherein the casing (1) includes internal, inclined, reflecting surfaces for defining said first (42,44) and second (43,45) reflecting element .

7. The probe according to claim 5 or claim 6, wherein said emitter device (21,23), the first reflecting element

(42,44), the second reflecting element (43,45) and the receiver device (25,27) are arranged in such a way so that said path includes a first longitudinal section, between the emitter device (21,23) and the first reflecting element (42,44), the transversal portion, between the first (42,44) and the second reflecting element (43,45), and a second longitudinal section, between the second reflecting element

(43,45) and the receiver device (25,27).

8. The probe according to claim 7, wherein said support element (7) includes at least a hole (46,47) arranged in - 17 -

one of said first and second longitudinal section of the path.

9. The probe according to claim 2, wherein said portion (8) of the support element (7) has at least a window (39,41), substantially near said transversal section of the path, optical attenuation means being housed in said at least one window (39,41) and being adapted for attenuating the light beam through the window (39,41) .

10 The probe according to claim 9, wherein said attenuation means include a glass and a thin layer of metal alloy deposited on the glass.

11. The probe according to one of claims from 2 to 10, wherein said portion of the support element (7) is a substantially annular rim (8) .

12. The probe according to one of claims from 2 to 11, including a power supply and processing unit (31,31'') connected with said detecting device and including a current generator (G;G') connected with the emitter device (21,23) and amplification means (A;A' ;A' ' ) and comparison means (C;C';C1,C2) connected with the receiver device (25,27) for providing an output signal (V₀, V₀,, V₀,,) indicative of the transmission of the light beam from said emitter device (21,23) to said receiver device (25,27).

13. The probe according to claim 12, wherein said generator is a direct current generator (G) .

14. The probe according to claim 12, wherein said generator is a pulse current generator (G' ) , the comparison means (C ' ) providing a predetermined frequency signal (V_c,), the power supply and processing unit (31) furthermore including a timer (T) connected with the comparision means (C ) for receiving said predetermined frequency signal (V_c,) and providing said output signal (V₀.) .

15. The probe according to claim 14, wherein said timer (T) defines a time of minimum duration (╧ä_╧ä) of the output signal (V₀,), the minimum time duration (x_╧ä) being longer than the period of said predetermined frequency signal (V_c.) .

16. The probe according to claim 12 as dependent on claim 9, wherein said comparison means include

ΓÇó first (Cl) and second (C2) comparators connected with the amplification means (A'') for defining first (v_╧äl) and second (V_╧ä2) threshold levels, and ΓÇó logic processing means (OR) connected with the first and the second comparators for providing said output signal (V₀,,) , the output signal (V₀,,) being indicative, when the arm-set (5) is in the rest condition, of the transmission of the light beam through said at least one window (39,41) .

17. The probe according to one of the preceding claims, wherein said detecting device is of the opto-electronic type, and said at least one emitter device (21,23) and at least one receiver device (25,27) include a light emitting diode (21,23) and a receiver photodiode (25,27), respectively.

18. The probe according to claim 17, wherein said detecting device includes two emitter devices (21,23) and two receiver devices (25,27).

19. The probe according to claim 17, wherein said detecting device (21,23,25,27) includes more than two emitter devices (21,23) and more than two receiver devices (25,27) . - 19 -

20. The probe according to one of claims from 17 to 19, wherein said receiver photodiode (25,27) is a CCD (Charge Coupled Device) diode or a phototransistor.