ITBO980218A1 - CONTACT DETECTION PROBE. - Google Patents

Description

Description of the industrial invention entitled:

"Contact detection probe"

Description text

The present invention relates to a contact detection probe comprising a casing, which defines a longitudinal geometric axis and an internal support surface, a device partially housed in the casing and movable with respect to it with a support element having a portion suitable for cooperating, in a rest condition of the crew, with said support surface, and an arm connected to the support element and carrying a feeler able to touch a piece to be checked, and a detection device able to provide signals indicative of the position of the assembly with respect to the casing, comprising at least one emitting device and at least one receiving device connected to said casing.

Probes with contact detection, or "touch trigger", with mobile devices carrying feelers, are used in coordinate measuring machines and in machine tools, in particular machining centers and lathes, to carry out checks on machined or to be machined pieces. on tools, on machine tables, etc. The checks may concern, for example, the position or the geometric dimensions of the pieces. In each of these probes, the contact between the probe and, for example, a mechanical piece is signaled by suitable devices which detect certain movements of the movable unit with respect to a casing, and control the reading of transducers associated with the slides of the machine, which provide measurement values with respect to a reference position or origin.

Some of these probes make use of optical switches to signal contact between the probe and the mechanical piece to be checked.

The British patent application GB-A-2238616 shows various embodiments of touch trigger probes which comprise a movable member, with respect to a substantially longitudinal casing, to which an arm carrying a feeler is fixed and which rests, in rest condition, on surfaces of that envelope. One of the embodiments shown comprises an optical switch with a light source located in a cavity of the movable member. A translucent plate on which a grid is printed is connected to the movable member and covers the cavity, allowing partial passage of light. A receiver is arranged on a fixed part, on the opposite side of the translucent plate with respect to the light source, and is always hit by the light passing through the plate. When the arm and, consequently, the movable member are deflected with respect to the rest condition following the contact of the feeler with a piece, the characteristics of the light passing through the plate are altered by the changed arrangement of the grid printed on it and, through the receiver, such alterations are detected and contact is signaled.

In this embodiment, any deformation of mechanical parts (in particular of the grid) due to temperature variations can cause similar alterations in the characteristics of the light and cause inaccurate or false signals that contact has occurred. To overcome this problem, or to make the mechanical effects of thermal drifts negligible, it is necessary that the signaling of the contact occurs only when the detected light alterations reach relatively high amounts, and this involves a reduction in the sensitivity of the instrument.

The Soviet inventor certificate SU-A-1516786 shows a "touch trigger" probe comprising an optical system for detecting movements of a crew comprising a movable arm and a plurality of optical guides arranged in radial planes with respect to the longitudinal axis of the probe. optical guide connects a light emitter to an optical receiver and is divided into two parts, separated by a portion of the mobile crew. When, following movements of the crew, the portion that separates the two parts moves, the light can pass from one to the other of the two parts making the optical connection between emitter and receiver.

The structure of the probe, in particular the detection system with the optical guides that must be arranged with considerable precision in the generally restricted space inside the enclosure, requires complex and delicate operations for assembly and setup, and for avoid that the optical guides interfere with the correct mechanical functioning of the probe itself.

The object of the present invention is to obtain a probe which is simple to manufacture, reliable, with a considerable degree of accuracy and repeatability and which provides a signal substantially immune to the effects of thermal drift.

A contact detection probe according to claim 1 achieves this and other purposes. The invention will now be described in detail with reference to the attached drawings, given purely by way of non-limiting example, in which:

Figure 1 is a longitudinal section of a contact detection probe according to the invention;

Figure 2 is a cross section of the probe of Figure 1 along the line 11-II of Figure 1; Figure 3 is a block diagram of a first type of device for feeding, amplifying and processing the probe of Figure 1;

Figure 4 is a block diagram of a second type of device for feeding, amplifying and processing the probe of Figure 1;

Figure 5 is a longitudinal section of the probe of Figure 1 in a different operating position with respect to Figure 1;

Figure 6 is a graphical representation of some signals relating to the power supply device of Figure 3;

Figure 7 is a graphical representation of some signals relating to the power supply device of Figure 4;

Figure 8 is a longitudinal section 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 section of a probe according to a further embodiment of the invention; And

Figure 11 is a schematic representation of the optoelectronic measurement system of the probe of Figure 10.

The probe schematically illustrated in Figures 1, 2 and 5 comprises support and protection means with a casing 1, which has a substantially cylindrical shape and defines a longitudinal geometric axis, comprising a lower base, which defines an internal support surface 16 which is substantially flat and annular, and an upper base. A mobile unit 5 is partially housed in the casing 1 and comprises a support element 7 which defines a substantially cylindrical symmetrical annular portion or edge 8, an arm 11 connected to the support element 7 and partially protruding from the casing 1 through a hole in the lower base, and a feeler 13 fixed to a free end of the arm 11.

A thrust device comprises a compression spring 17 disposed between surfaces of the upper base of the casing 1 and of the support element 7 to push the annular edge 8 and the support surface 16 into mutual contact when the member 5 is in rest condition, In the absence of contact between the feeler 13 and a piece to be checked 15. In this condition, a symmetrical cavity 14 is defined between the element 7 and the casing 1.

Centering and anti-rotation devices (of a known type and not shown in the figures) are provided, for example between the mobile unit 5 and the casing 1, to prevent mutual movements of transverse translation and rotation around the longitudinal axis.

A detection device, for example of the optoelectronic type, comprises emitting devices and receiving devices. In particular, two light emitting diodes 21, 23, or "LED", are fixed to the lower base of the casing 1 and arranged in diametrically opposite positions, substantially in correspondence with the plane defined by the annular surface 16. Two receiving photodiodes 25, 27 they are also fixed to the lower base of the casing 1, substantially on the same plane as the LEDs 21, 23 and facing them. The mutual arrangement of the elements of each of the LED / photodiode pairs 21/25 and 23/27 is such that the light beam emitted by the LED it reaches the relative photodiode along a predetermined trajectory which consists of a section transversal to said longitudinal axis of the casing 1.

The two LEDs 21, 23 and the two photodiodes 25, 27 are electrically connected to a power supply and processing unit 31, through cables schematized in Figure 1 with the reference number 30.

A sealing gasket 37 has its ends fixed respectively to the lower base of the casing 1 and to the arm 11 and, in addition to protecting the internal equipment of the probe from foreign bodies, prevents the passage of light inside the probe.

Figures 3 and 4 illustrate, by means of block diagrams, two different embodiments of some elements of the power supply and processing unit 31, in cases where the two LEDs 21 and 23, connected in series, are powered by direct current and high current pulses respectively.

In the first case, illustrated in Figure 3, the two LEDs 21, 23 are powered by a direct current generator G. Amplification and comparison means comprise an amplifier A which amplifies the signal V, coming from the photodiodes 25, 27, connected in parallel, and a comparator C which compares the signal VA in output from amplifier A with a suitably predetermined threshold value VTl, 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, 23 are powered by a generator G 'which supplies them with current pulses. Means of amplification and comparison include an amplifier A 'which amplifies the signal Vr coming from the photodiodes 25, 27 and a comparator C, coupled by means of a filter F to the amplifier A', which compares the signal VA. outgoing from this with a suitably predetermined threshold VT. The Vc. at the output of the comparator C it is further processed by a timer T.

The timer T outputs a signal V „. at a low logic level, if there is a low logic level signal at its input. The V ”signal. it passes to a high logic level, and remains so for a predetermined period of time of minimum duration τγ every time the input signal to the timer T passes from a low logic level to a high logic level. When a new pulse arrives at the input of timer T and the output signal V „'is high, timer T clears the count, ie the output signal V0. it remains at a high level for at least a period τΤ starting from that moment.

The operation of the probe is as follows.

In the absence of contact between the feeler 13 and the piece 15 to be measured, the assembly 5 is in a rest condition in which the edge 8 and the support surface 16 are in annular mutual contact (Figure 1) under the action of the spring 17 In this condition, the light beams emitted by the LEDs 21, 23 do not reach the photodiodes 25, 27 since opposite portions of the edge 8 lie along the relative trajectories.

In the event that the LEDs 21, 23 are powered in direct current (Figure 3), in the rest condition at the input of the amplifier A there is a signal V, of null value (Figure 6). At the input of the comparator C there is a VA signal of null value or - as will be better described below - in any case lower than the VT threshold and, consequently, the signal V0 output from the comparator C remains at a low logic level.

During a relative movement between the probe and the piece 15 to be checked along the transverse direction X, following the contact at an instant t, between the feeler 13 and a surface of the piece 15, the arm 11 and the entire mobile device 5 incline with respect to the casing 1 (figure 5) and the edge 8 partially rises from the annular surface 16 allowing the passage of the light beam emitted by one of the LEDs, in the example of figure 5 the LED 23. The light beam strikes the relative photodiode (27) , which generates a signal V, which is amplified by amplifier A (VA) and compared, in the comparator C, with the threshold VT (figure 6). Following the exceeding of the value VT, in an instant t, + T, the output signal V "goes to a high level, thus signaling the contact made between the feeler 13 and the piece 15.

In the case of longitudinal movement between the probe and the piece 15, and contact between the feeler 13 and a transversal surface of the piece 15 along the Z direction, a lifting of the arm 11 and of the movable element 5 occurs which causes a detachment of the edge 8 from the annular surface 16. In this case the light beam of at least one of the LEDs 21, 23 reaches the relative photodiode 25, 27, and the processing of the signal V, provided by at least one of the latter, takes place as previously described.

This solution has the advantage of being simple and immune from any inductive or capacitive couplings between the power supply current of LEO 21, 23 and photodiodes 25, 27 or amplifier A.

Furthermore, possible undesired variations of the signal VA in conditions of rest of the crew 5, due to the effects of temperature variations, are limited to the so-called thermal drifts of the amplifier itself, of a generally small entity. In fact, the probe according to the present invention is insensitive to the so-called mechanical effects of thermal drifts which, by causing limited deformations in the mechanical parts of the measuring devices, alter, in known probes, the signal to be amplified, introducing consequent variations of a non-negligible entity in the signal. amplified. In the case of the probe according to the present invention, in rest conditions the signal to be amplified V has a value of zero and, in normal operating conditions, it remains so even after limited deformations of the mechanical parts involved caused by temperature variations.

In other words, indicating with K the gain of amplifier A, the signal VA subject to thermal drifts can in general be expressed as

where K, and D represent the effects of the thermal drift of amplifier A.

Since, as previously mentioned, in rest conditions V remains stably equal to zero, even after limited deformations of the mechanical parts, the signal VA can vary with respect to zero of the reduced direct component D.

To avoid false signals, in rest conditions, of the contact between feeler 13 and piece 15, or to ensure that, in such conditions, the value of the output signal V0 remains at a low logic level, it is therefore sufficient to establish a VT threshold value that takes into account only the possible limited variability of the VA signal attributable to the D component. The possibility of establishing a particularly low VT threshold value guarantees a particularly high sensitivity of the probe. If the LEDs 21, 23 are powered with pulsed current (Figure 4), in the absence of contact between the feeler 13 and the piece to be checked 15 and therefore in the absence of signals coming from the photodiodes 25, 27, the effects of any drifts in direct current of amplifier A 'are eliminated by the high pass filter F present between amplifier A' and comparator C '. Following the contact of the feeler 13 with the piece 15 at an instant tt 'along the X direction, and the consequent passage of the light beam from the LED 23 to the photodiode 27, as previously described (Figure 5), the signal Vr produced by this photodiode 23 (figure 7), suitably amplified by A '(VA.), is sent, after having eliminated the continuous component with the filter F (VF), to the comparator C'

When the peak value of the latter signal VF exceeds the threshold VT, the logic signal V? at the output of the comparator C 'it assumes a high or low value with a predetermined frequency equal to that of the current flowing through the LEDs 21, 23. If the period ττ defined by the timer T is slightly greater than the period of the supply current of the LEDs 21, 23, at the output of the timer T there is a signal V0. which, when the arm 11 is deflected, remains stably at the high logic level and which passes to the low logic level when the arm 11 returns to the rest position, with a delay which is at most equal to the period TT of the timer T.

The definition of the VT threshold value is carried out with similar criteria to those seen previously for the VT threshold which contribute to the high sensitivity of the probe.

The probe illustrated in Figure 8 is in many respects the same as that described with reference to Figures 1 and 2 and for this reason the parts in common are identified with the same reference numbers.

The fundamental difference consists in the presence of two windows 39, 41 present in the support element 7 near the edge 8 and substantially in correspondence with the LED-photodiode pairs 21, 25 and 23, 27. In the windows 39, 41 there is a material that attenuates the intensity of the light transmitted by the LEDs 21, 23 to the respective photodiodes 25, 27. This optical attenuator can, for example, be obtained in a known way by vacuum depositing a thin layer of special metal alloys on a substrate of heat treated glass.

Figure 9 shows, by means of a block diagram, the processing means of a processing unit 31 "relating to the second embodiment of the invention, in the case in which the LEDs 21, 23 are powered by a continuous supply. In particular, an amplifier A "receives the signals coming from the photodiodes 25, 27 and is connected to comparison means which comprise two comparators C1 and C2 and logic OR processing means which perform the logical sum of the output signals from the latter.

Since, in the rest condition of the mobile unit 5, the light transmitted by the LEDs 21, 23 reaches the photodiodes 25, 27, albeit with attenuated intensity, through the windows 39, 41, at the input of the amplifier A "- and consequently at its output - there is a low value signal but not zero. This signal is sent to the inverting input of the comparator C2 which compares it with a threshold V "and to the non-inverting input of the comparator C1 which compares it with a threshold VT1 + VT2.

The values of the two thresholds VT1 and VT2 are chosen in such a way that if the arm 11 is not deflected, ie when only the light passing through the windows 39 and 41 reaches the photodiodes 25 and 27, the output signal from the amplifier A " results to be higher than the VT2 threshold, and lower than the VT1 + VT2 threshold and therefore the outputs of both comparators are low and the output V0. of the OR unit is low. If, on the other hand, the arm is deflected, the output signal from the amplifier A "has a value higher than both that of the threshold VT2 and that of the threshold VT1 + V · ,, so that the output of the comparator C2 is at a low logic level, while the output of the comparator C1 is at a high logic level, therefore the the V0- output of the OR unit is at a high logic level.

In the event of failure of the detection device comprising the LEDs 21, 23 and the photodiodes 25, 27, or if an anomaly causes the signal transmitted by the photodiodes 25, 27 to the amplifier A "to have a substantially null value, the output of such amplifier A "is lower than both thresholds (VT2 and VT1 + νπ). In this case, while the output of comparator C1 remains at a low logic level, the output of comparator C2, and consequently the output VO- of The OR unit is at a high logic level In practice, a deflection of the assembly 5 is signaled in the absence of contact between the feeler 13 and the piece 15, from which the fault status of the probe is clear.

The probe illustrated in Figure 10 differs from that described with reference to Figures 1 and 2 substantially in the arrangement and some components of the optoelectronic measurement system. Both the emitters 21, 23 and the receivers 25, 27 are fixed to the upper base of the Enclosure 1. The emitters 21 and 23 are arranged in diametrically opposite positions of the upper base of the Enclosure 1 and are oriented so as to emit the light beam along a first longitudinal trajectory section directed towards the lower base of the casing 1. The receivers 25, 27 are placed side by side with the emitters 21 and 23, arranged at a certain distance from them along the same diametrical direction, and are turned towards the lower base of the casing 1 to receive the light beam along a second longitudinal trajectory section. Four reflecting elements, comprising for example mirrors 42, 43, 44, 45, are connected to the lower base of the casing 1, substantially in correspondence with the positions previously occupied by the emitters 21, 23 and by the receivers 25, 27 in the embodiment illustrated with reference to Figure 1, and are inclined, with respect to the main transverse plane of the lower base of the casing 1, by an angle of 45 degrees. The mirror 42 is oriented in such a way as to receive the light beam emitted by the emitter 21 along the first longitudinal trajectory portion, and to reflect it at least partially along a transverse trajectory portion towards the mirror 43. The latter is in turn oriented in such a way as to at least partially reflect the light beam coming along said transverse section and direct it along the second longitudinal section of the trajectory towards the receiver 25. The light beam emitted by the emitter 23 follows a predetermined trajectory which is entirely similar, which includes, in addition to longitudinal sections between the emitter 23 and the mirror 44 and between the mirror 45 and the receiver 27, a direct transverse section between the mirrors 44 and 45. The support element 7 has two longitudinal holes 46 and 47 to allow the passage of the light partially reflected by the mirrors 43 and 45 towards the receivers 25, 27 (second longitudinal sections of the respective trajectories) . The operation is completely similar to that of the probe illustrated with reference to Figures 1 and 2. In fact, even in this case, in the rest condition of the crew 5 portions of the edge 8 lie along the trajectories of the light beams, in particular in correspondence of the relative transversal portions, and following the lifting of the edge 8 from the support surface 16, the light beams can reach the relative photodiodes. The processing of the signals supplied by the photodiodes 25, 27 takes place in the manner described above.

The probe of figure 10 can be modified to include windows with optical attenuator, as described with reference to figure 8.

Although the embodiments illustrated in the drawings and described above comprise elements (the casing 1 and the movable assembly 5) which define flat annular surfaces, in the shape of a circular crown, (the internal support surface 16 and the annular edge 8 respectively) pushed to mutual contact, the invention can also be realized with the use of elements defining concave / convex surfaces with rotation symmetry pushed to mutual contact, when the probe is not subjected to forces, along a contact circumference. For example, the invention can be applied in a probe with a constraint system that includes an element with a toroidal surface, or in the shape of a spherical zone and an element with an internal surface in the shape of a truncated cone, provided that the head is made in such a way that the displacement of the feeler causes in any case, as occurs in the embodiments illustrated and described above, a separation between the two surfaces such as to allow the light beam of at least one of the LEDs 21, 23 to reach the relative photodiode 25, 27 .

The probes described and illustrated comprise two emitter / receiver pairs arranged on the same diametrical plane of Figures 1, 5, 8, 10, and allow to detect movements of the feeler 13 on this plane, in particular movements caused by contacts between the feeler 13 and workpiece 15 along the X and / or Z axes. The number and arrangement of these emitter / receiver pairs can however vary, as only one light beam is sufficient to detect contacts in one direction only (for example X), and for example being provided for detecting displacements along any direction of space, three emitter / receiver pairs and relative light beams along three distinct trajectories lying on diametrical planes angularly arranged at 120 ° from each other.

Other variations are possible with respect to what has been described and illustrated. For example, with respect to the configuration of Figures 1, 8 and 10, the position of one or both of the LEDs and the related photodiodes can be reversed. The presence of a single emitter placed inside the cavity 14 and able to diffuse the light in all directions, and of several photodiodes outside this cavity, for example fixed to the casing 1, can also be provided for detecting the light emitted by this emitter when the arm 11 is deflected and the edge 8 separates at least partially from the surface 16.

It is possible to use emitters and detectors of different types from those described, in particular the photodiodes can be replaced with CCD devices (Charge Coupled Devices) or phototransistors. It is also possible to use non-optical systems, such as ultrasound or microwave systems, instead of the LED-photodiode couplings.

The power supply and processing units 31 and 31 "illustrated in an extremely schematic way in the figures, can be housed in the casing 1, for example in suitable seats, not visible in the figures, made in the upper base of the casing 1.

The reflecting elements 42, 43, 44, 45, described with reference to figures 10 and 11, can be made integrally in the envelope 1, for example, obtaining the necessary number of suitably inclined surfaces at the lower base of the envelope 1 and lapping them to make them reflective.

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

The annular edge 8 can, for example, be replaced by a certain number of support elements separated from each other, with the LED / photodiode pairs (or the pairs of inclined mirrors) arranged on opposite sides of these elements. 'support.

Claims (20)

Claims 1. A contact sensing probe comprising • a casing (1), which defines a longitudinal geometric axis and an internal support surface (16), . a member (5) partially housed in the casing (1) and movable with respect thereto A support element (7) having a portion (8) adapted to cooperate, in a rest condition of the crew (5), with said support surface (16), and • an arm (11) connected to the support element (7) and bearing a feeler (13) designed to touch a piece (15) to be checked, and . a detection device (21, 23, 25, 27) adapted to provide signals indicative of the position of the crew (5) with respect to the casing (1), comprising at least one emitter device (21, 23) and at least one receiver device ( 25, 27) connected to said casing (1), characterized in that said emitter device (21, 23) is also connected to said casing (1) and is able to generate a light beam directed, along a predetermined trajectory, towards the receiver device (25,27), said portion (8 ) of the support element (7) being arranged, in the rest condition of the crew (5), along said trajectory, to prevent the light beam from reaching the receiver device (25,27). 2. Probe according to claim 1, wherein said trajectory comprises a transverse portion, said portion (8) of the support element (7) being arranged, in the rest condition of the assembly (5), along said transverse portion. 3. Probe according to claim 2, wherein said emitter device (21, 23) and said receiver device (25, 27) are fixed to the housing (1) at the support surface (16) on opposite sides with respect to said portion (8) of the support element (7). 4. Probe according to claim 3, wherein the rest element (7) and the casing (1) define a cavity (14), said emitter device (21, 23) and said receiver device (25, 27) being respectively housed inside and outside said cavity (14), or vice versa. 5. Probe according to claim 2, wherein the detection device comprises at least a first inclined reflecting element (42, 44), adapted to reflect the light beam generated by the emitting device (21,23) and to generate said transverse section, and at least a second inclined reflecting element (43, 45), said first (42,44) and second (43,45) reflecting element being integral with the casing (1) and arranged in correspondence with the support surface (16) on opposite sides with respect to to said portion (8) of the support element (7). 6. Probe according to claim 5, wherein the envelope (1) comprises reflecting inclined internal surfaces adapted to define said first (42,44) and second (43,45) reflecting element. 7. Probe according to claim 5 or claim 6, wherein said emitter device (21,23), first reflective element (42,44), second reflective element (43,45) and receiver device (25,27) are arranged in such a way that said trajectory comprises a first longitudinal section, between the emitter device (21,23) and the first reflecting element (42,44), the transverse section, 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. Probe according to claim 7, wherein said rest element (7) comprises at least one hole (46, 47) arranged along one of said first and second longitudinal stretches of the trajectory. 9. Probe according to claim 2, wherein said portion (8) of the support element (7) has at least one window (39, 41), substantially in proximity to said transverse portion of the trajectory, optical attenuation means being housed in said at least one window (39, 41) and being able to attenuate the light beam through the window (39, 41). 10. Probe according to claim 9, wherein said attenuation means comprises a glass and a thin layer of metal alloy deposited on the glass. 11. Probe according to one of claims 2 to 10, wherein said portion of the element of support (7) is a substantially annular edge (8). Probe according to one of claims 2 to 11, comprising a power supply unit and processing (31, 31 ") connected to said detection device and comprising a generator of current (G; G ') connected to the emitter device (21, 23) and amplification means (A; A'; A ") and comparison (C; C; C1, C2) connected to the receiver device (25, 27) and capable of providing a output (V0, V0 ·, V0.) indicative of the transmission of the light beam from said emitting device (21, 23) to said receiver device (25, 27). 13. Probe according to claim 12, wherein said generator is a current generator continues (G). 14. Probe according to claim 12, wherein said generator is a current generator ad pulses (G ')> the comparison means (C) being adapted to supply a predetermined signal frequency (Vc.), the power supply and processing unit (31) further comprising a timer; (T) connected to the comparison means (C) and adapted to receive said predetermined signal frequency (Vc) and to supply said output signal (VQ.). 15. Probe according to claim 14, wherein said timer (T) defines a time of minimum duration (ττ) of the output signal (V0.), the minimum duration time (xT) being greater than period of said signal of predetermined frequency (VC). 16. Probe according to claim 12 as dependent on claim 9, wherein said means of comparison include . first (C1) and second (C2) comparators connected to the amplification means (A ") and able to define first ones (VT1) and second (νπ) threshold values, e . logic processing means (OR) connected to the first and second comparators and adapted to supply said output signal (V0 "), the output signal (V0 ") in the rest condition of the crew (5) being indicative of the transmission of the light beam through said at least one window (39, 41). 17. Probe according to one of the preceding claims, wherein said detection device is of the optoelectronic type, and said at least one emitting device (21, 23) and at least one receiving device (25, 27) comprise, respectively, a light emitting diode (21,23) and a receiver photodiode (25,27). 18. Probe according to claim 17 wherein said detection device comprises two emitting devices (21, 23) and two receiving devices (25, 27). 19. Probe according to claim 17, wherein said detection device (21, 23, 25, 27) comprises more than two emitting devices (21, 23) and more than two receiving devices (25, 27). Probe according to one of claims 17 to 19, wherein said receiver photodiode (25, 27) is a diode of the CCD (Charge Coupled Device) type or a phototransistor.