FR2966239A1 - Angular position sensor for detecting angular position of rotating element, has LED isolated and placed at distance from photoreceptor so that photoreceptor receives beam of non-deviated light - Google Patents

Abstract

The sensor has a rotationally movable disk (6) centered on a rotational shaft that is driven by a moving element to measure an angular position at a given instant. The disk includes two concentric circular tracks, where each track is provided with opaque and transparent zones. A photoreceptor (4) is arranged such that light (F) emitted by an LED (3) is traverse to the disk before being received by the photoreceptor. The LED is isolated and placed at distance (C) from the photoreceptor so that the photoreceptor receives a beam of non-deviated light.

Description

ANGULAR POSITION SENSOR WITH HIGH RESOLUTION AND LOW DIMENSIONS

The invention relates to an angular position sensor with high resolution and low overall dimensions. An angular position sensor is a sensor that makes it possible to know the angular position of a member that can be rotated or translated. Such sensors are also called optical encoders. This expression will be used preferentially. Subsequently, a rotary type optical encoder is described, it being understood that the invention also relates to a rectilinear type optical encoder. An optical encoder comprises a disk integral with a rotating drive shaft. This tree is itself integral with an element whose angular position one wishes to know at a given moment. The disc is provided with a succession of opaque areas and transparent areas. A light source, stationary with respect to the disc, emits light which passes through the transparent areas of the rotating disc and illuminates photoreceptors, also immobile with respect to the disc. The signal received by the photoreceptors, after treatment, is representative of the angular position of the disk, and therefore of the angular position of the element at a given instant. The optical encoder also includes a reticle, said scanning reticle, placed before the light beam is received by the photoreceptors. This reticle forms a means of "calibrating" the light beam in the manner of a diaphragm, without deflecting the light beam. There are two types of optical encoders: incremental encoders and absolute encoders. Incremental encoders are equipped with a disc having two types of tracks. A type of track, called inner, provided with a single transparent zone and adapted to deliver a reference signal at each turn. One type of track, called outer track, is concentric with the inner track and has N 'angular portions alternately formed of opaque and transparent areas. The non-zero integer numeral value N 'represents the resolution, also referred to as the number of points or bits of the encoder disk. An incremental encoder thus provides a measurement of the displacement, linear or angular, of an element, for example of a tool of a machine tool. This type of encoder is simple to make and use. Nevertheless, it must be reset in case of disc rotation stop and is susceptible to noise.

Absolute encoders give the angular position of an element, either on a rotation turn of the latter, or on several rotational revolutions depending on whether it is an absolute single-turn or multi-turn encoder. An absolute encoder continuously provides a code representative of the actual position of the moving element. Absolute encoders are particularly used in the aerospace, oil, medical and military space fields because they are insensitive to power cuts and pests. Like incremental encoders, they include a fixed light source and photoreceptors. The light emitted by the source passing through a movable disc rotating and secured to the moving element. Absolute encoders comprise a disk provided with at least two, advantageously several, concentric circular tracks. The central track has a single transparent area extending over a half-circumference. It has been referred to as MSB (Most Significant Bit). It makes it possible to determine in which half-turn of the disk the measurement is made. The next tracks, going outward from the disc, comprise two then four, eight, sixteen transparent areas, and so on to the outer track. As you move away from the central track, the tracks determine which quarter, eighth, and sixteenth of a turn you are in, and thus to the outer track or LSB (Least Significant Bit). The non-zero integer value N is the number of tracks, so the absolute encoder disk has a precision or resolution of 2 "dots per revolution or bit. The arrangement of the opaque and transparent areas of the different tracks is carried out, in a manner known per se, according to a coding, either binary or Gray.

It turns out that, in many technical fields, it is advantageous to use optical encoders with a small footprint, for example with a diameter of less than 20 mm, for a thickness of less than 10 mm, while preserving a high resolution, advantageously greater than 16 bits. Advantageously, such an encoder is also adapted to withstand harsh environmental conditions, especially at temperatures between -55 ° C. and + 125 ° C. A set of light-emitting diodes or LEDs is generally used as a light source. To obtain a sufficiently powerful and focused light beam, so that the coding of the various tracks of the disc is read easily, it is necessary to straighten the light beam before it passes through the disc. For this, there is disposed between the light source and the disk an optical correction device, such as a ball-type or half-ball lens, forming a rectifying means of the light beam. This optical lens makes it possible to align the optical axes of the photoreceptors with that of the light source. On the other hand, the presence of an optical lens induces an oversize and an overweight of the encoder. FR-A-2 779 816 discloses an optical encoder in which the light source and the photoreceptors are located on the same side of the disk, in a plane generally parallel to the main plane of the disk. Mirrors, inclined relative to the main plane of the disc, are arranged near the source and the disc. They make it possible to direct, parallel to the main plane of the disc, the beam emitted by the source before returning it perpendicularly towards the photoreceptors. Thus, for the same length of the optical path traveled by the light beam, the total thickness of this encoder is less than the thickness of an encoder equipped with a lens. This makes it possible to reduce the size of the encoder while using an optical correction device for the light beam. Such an encoder is nevertheless of a relatively complex construction, the mirrors to be positioned precisely. It is these drawbacks that the invention intends to remedy more particularly by proposing an angular position sensor of high resolution in view of its bulk.

For this purpose, the subject of the invention is an angular position sensor of a moving element comprising at least one light source, a mobile disk in rotation centered on a rotation shaft integral with a member that is also mobile, at least in rotation, and whose angular position is to be known, the disk comprising at least two concentric circular tracks each provided with alternating opaque and transparent zones, at least one photoreceptor arranged so that the light emitted by the light source passes through the disc before to be received by at least one photoreceptor, characterized in that the light source is of point type and in that it is placed at a distance from the photoreceptor adapted for the photoreceptor to receive a non-deflected light beam.

The use of a point source of light and the absence of an optical correction device, making it possible to straighten the light beam during its path between the source and the photoreceptors, makes it possible to obtain an optical encoder with a small overall size. while preserving a high resolution. According to advantageous but not compulsory aspects of the invention, the angular position sensor may incorporate one or more of the following characteristics: the point light source is electrically powered by pulses. - The light source emits a divergent light beam. - The light beam has an emission angle greater than +/- 15 °. - The light source is formed by a light emitting diode or LED. - The distance, or optical path, between the light source and the photoreceptor is less than 10 mm. - The optical path is close to 3 mm. The sensor comprises a plurality of photoreceptors formed by photodiodes. The light-emitting diode is adapted to illuminate at least four, advantageously eight, photoreceptors simultaneously. - The sensor includes several point light sources. The invention will be better understood and other advantages thereof will appear more clearly on reading the following description of an embodiment of an angular position sensor according to the invention, given solely for example and made with reference to the accompanying drawings in which: - Figure 1 is a general perspective view of an angular position sensor according to the invention - Figure 2 is a section, at the same scale, the sensor of Figure 1 according to the plane II, some components are not illustrated for ease of reading, - Figure 3 is a sectional view, on a larger scale, of detail III of Figure 2, the path of a light beam in the sensor and the supply of the light source being schematically illustrated and - Figure 4 is a simplified representation, in perspective, of the main components of the sensor, the path of the light beam being shown. The angular position sensor 1, or optical encoder, shown in Figure 1 is made of a rigid material adapted to isolate, electromagnetically, the internal components of the encoder. Advantageously, this encoder is made of polymer or metal. The encoder 1 comprises a main body 2, hollow, globally cylindrical with a circular base. This hollow body 2 has, for example, a diameter D of between 20 mm and 60 mm for a height H advantageously less than 10 mm. The body 2 of the encoder 1 forms a receiving housing for several constituent elements of the encoder 1. In particular, it houses a light source 3, at least one photoreceptor 4, a scanning reticle 5 and a coding disc 6. rotatably mounted in the body 2 of the encoder 1. The disk 6 is centered, permanently or releasably, on a rotation shaft 7. This shaft 7 extends outside the main body 2. The shaft 7 is driven in rotation by a moving element, not shown, which is to measure the angular position at a given instant. This rotational drive of the shaft 7 is either direct in the case of a single-turn encoder 1 or is carried out by means of return devices in the case of a multi-turn encoder, that is to say ie, an encoder capable of indicating the number of rotations on 360 °, or turns, made by the moving element. In the example, members 70 for guiding rotation of the shaft 7 are shown in Figure 2. The body 2 is adapted to receive other elements, known per se and not illustrated to facilitate the reading of the various figures. These include power supply means, signal processing and sensor control. Figures 2 and 3 show, in a simplified manner, the elements 3 to 7 as arranged in the body 2. In particular, the light source 3 is fixed permanently or removably on a wall 20 of the body 2. This wall 20 corresponds to that traversed by the shaft 7.

Looking at FIG. 2, the scanning reticle 5, the disc 6 and the photoreceptors 4 are placed, in this order, above the source 3. The reticle 5 is attached to the lateral walls 21, 22 of the body 2. The photoreceptors 4 are fixed, removably or permanently, on a support 40, close to the wall 23 opposite the wall 20.

Thus, the source 3, the reticle 5 and the photoreceptors 4 are integral with the body 2 and they are immobile with respect to the shaft 7 and the disc 6. The disc 6 is, for more legibility, represented in one piece with the shaft 7. Alternatively, it can be secured to the latter removably. FIGS. 2 to 4 illustrate, schematically, a light source 3 and a group of photoreceptors 4, in this case eight, regularly distributed in two rows, a single row being visible in FIGS. 2 and 3. The light source 3 is of the type punctual. This term denotes a source of light radiation whose dimensions are small enough, relative to the distance separating the source of a receiver, so that these dimensions can be neglected in the various calculations. In other words, with this type of source, one can assimilate at a point the origin of the light emitted in the form of a light beam. This point source 3 is a source of monochrome light of a given wavelength. This given wavelength is chosen, either in the visible range, or in the near-infrared or near-ultraviolet range. In the example, the source 3 is advantageously a point source emitting in the near infrared. The source 3 is, preferably, formed of a light emitting diode (LED) emitting in the near infrared.

The LED 3 is fixed, permanently or removably, on the wall 20 so that the optical axis A of the source 3 is parallel to the axis of rotation R of the disk 6. The use of an LED 3 as point source makes it possible to emit a diverging beam of light F. In this case, the light cone has an emission angle α greater than +/- 15 °. C is the distance between the point source 3 and the photoreceptors 4.

This distance C corresponds to the optical path traveled by the light. The presence, on the one hand, of a divergent light beam F coming from a point source 3 and, on the other hand, of a short optical path C, implies that it is not necessary to straighten the beam F emitted light. In the case in point, this distance C is less than 10 mm and advantageously close to 3 mm. As illustrated in FIGS. 3 and 4, the distance C is adapted so that the photoreceptors 4 are all illuminated simultaneously and with sufficient power. The LED 3 is chosen to emit a light beam F of a luminous intensity sufficient to read, optimally, the coding of the disc 7 by exciting the photoreceptors 4, without it being necessary to straighten or focus the light to the light. using an optical correction means such as a lens. For this purpose, an LED 3 powered electrically by a pulse circuit 8 is used. The use of a pulsed power supply, as shown diagrammatically in FIG. 3, makes it possible to increase the effective brightness, that is to say say the intensity of the light beam F emitted by the LED 3, when the latter is activated to make a measurement, while significantly reducing the power consumption of the encoder 1. Such a type of power supply makes it possible to obtain a high peak current with a medium current of relatively low intensity. By way of example, for an average intensity of 3 mA, such a power supply, with pulses 80 lasting 1 ps every 18 ps, makes it possible to obtain a peak current of 50 mA. In other words, the power gain due to the peak current of the electric pulse compensates for the original weakness of the light emitted by an unreplaced point source. Indeed, in the absence of a pulsed power supply, it is not possible to obtain, with a single LED, a light beam of sufficient power to be effectively detected by the photoreceptors 4.

Above the source 3 and under the disc 6, looking at FIGS. 2 to 4, a reticle 5 is arranged. This term "reticle" denotes a flat, fixed piece provided with slots 50. In another embodiment, not shown, the reticle 5 is located above the disc 6 and below the photoreceptors 4, looking at FIGS. 4. In all cases, the reticle 5 must be placed in the path of the light beam F before it excites the photoreceptors 4. The purpose of the scanning reticle 5 is to avoid the presence of stray light arriving on the photoreceptors 4. photoreceptors 4 and to "calibrate" the light beam F, without deflecting or straightening the latter. In other words, the reticle acts as a diaphragm. The reticle 5 is arranged parallel to the main plane of the disk 6. During the rotation of the disk 6, according to the arrow T in FIG. 4, only the tracks facing the slots 50 of the reticle 5 are illuminated by the point source 3. The slots 50 are, in the example, rectangular. They have dimensions and a spacing between two adjacent slots 50 so as not to deflect the light beam F and ensure a passage of light, simultaneous and of similar intensity, through all the slots 50, so that each photoreceptor 4 receives the same light signal. In this case, the reticle 5 is illustrated with four aligned slots, visible in Figures 2 and 3. Another row of four slots 50, parallel to that shown in Figures 2 and 3, is visible in Figure 4. Above of disk 6, looking at FIGS. 2 to 4, is at least one photoreceptor 4. In this case, it is at least one group of eight photoreceptors 4, in this case identical, arranged in two parallel rows of four photoreceptors 4. These are advantageously formed by photodiodes 41 to 48.

The photodiodes 41 to 48 have a short reaction time, particularly suitable for use as a photoreceptor receiving light emitted from a pulsed light source 3. Nevertheless, it is possible to use other types of photoreceptors, for example, a combination of photodiodes and phototransistors. Subsequently, the invention is described with reference to photodiodes. The photodiodes 41 to 48 make it possible to detect an optical signal and to transcribe it into an analog signal which is transmitted to processing and calculation modules, also inserted into the encoder 1 and not shown. The photodiodes 41 to 48 are arranged so as to each receive a light signal having passed through the disc 6 and a given slot 50, located opposite the photodiode concerned.

Given the pulsed supply of the LED 3 and the optical path C, the LED 3 effectively illuminates the eight photodiodes 41 to 48 distributed in two rows. A row of four photodiodes 41 to 44 or 45 to 48 is adapted to receive the light signal of the LED 3 transmitted by four concentric tracks. For ease of reading, the coded tracks are illustrated by lines arranged in chevrons in FIG. 4. The point source 3 optimally illuminates four tracks 60 and at least four, advantageously eight photodiodes 41 to 48. In embodiments, this number is different, provided that the power and the pulse power of the point source 3 are adapted. An arrangement in two parallel rows of slots 50 and photodiodes 41 to 48 allows each track to be read twice, shifted, by two independent photodiodes. The photodiodes 41 and 45 "read" one after the other a track, that is to say receive a light signal transiting a track. The photodiodes 42 and 46, 43 and 47, 44 and 48 read, respectively, three other tracks. A double reading for each track makes it possible, in a manner known per se, to remove the ambiguities, in order to synchronize the reading of the tracks and to code, in binary code, the received signal. As illustrated in FIG. 4, the light beam F emitted by the LED 3 is split into as many beams FO to F3 as photodiodes 41 to 48 as it passes through the slots 50. For greater clarity, only two beams FO and F1 are represented at the output of the slots 50. These independent beams F0, F1 interfere respectively with the beams F2, F3 at the end of their passages through the disc 6, the latter rotating along the arrow T. Each beam F2, F3 corresponds to a coded light signal. The beams F2, F3 excite, in the example, the photodiodes 41, 45. Thus, for a track, two identical light signals, shifted, representative of the same code. The set of signals corresponding to the different tracks thus provides, after processing, an angular position of the element at a given instant. During a measurement cycle, the source 3 illuminates the disk 6 and the photodiodes 4 capture the light signal transmitted by the disk 6. The received signal is processed by a processing and calculation module, not shown. A short reaction time photodiodes allows the duration of excitation of the photodiode, so the duration of a light pulse is less than 1 ps. The power supply of the pulse circuit is wired, the energy source being external to the encoder 1. In a variant, the encoder is autonomous and comprises a power supply source, for example a battery or a rechargeable battery. Advantageously, the sensor 1 is mounted in a watertight and / or airtight manner.

Alternatively, it is possible to have several point light sources. In this case, their arrangements, as well as the reticle and the photoreceptors are adapted so that the light beams received by the photoreceptors remain separated, undivided and independent. In order to verify the correct operation of such an encoder, in particular in order to ensure that the photoreceptors 4 operate, it is advantageous, during signal processing, to apply to the signals received a treatment capable of detecting the malfunctioning of one or more photoreceptors. For this, it is possible to create, in shifted, a virtual encoder and, by comparison of the latter with the actual encoder, to deduce the state of the encoder.

Claims (10)

CLAIMS1.- An angular position sensor (1) of a moving element comprising at least one light source (3), a disk (6) movable in rotation centered on a rotation shaft (7) integral with an element also mobile, at least in rotation, and whose angular position is desired, the disk (6) comprising at least two concentric circular tracks each provided with alternating opaque and transparent zones, at least one photoreceptor (4) arranged so that the light (F, F0, F1, F2, F3) emitted by the light source (3) passes through the disk (6) before being received by at least one photoreceptor (4), characterized in that the light source (3) is of point type and in that it is placed at a distance (C) from the photoreceptor (4) adapted for the photoreceptor (4) to receive a non-deflected light beam (F, F0, F1, F2, F3). 2. A sensor according to claim 1, characterized in that the light source (3) point is electrically powered by pulses (80). 3. - Sensor according to claim 1, characterized in that the light source (3) emits a divergent light beam (F). 4. - Sensor according to claim 3, characterized in that the light beam (F) has an emission angle (a) greater than +/- 15 °. 5. Sensor according to one of the preceding claims, characterized in that the light source is formed by a light emitting diode or LED (3). 6. A sensor according to claim 1, characterized in that the distance, or optical path (C), between the light source (3) and the photoreceptor (4) is less than 10 mm. 7. A sensor according to claim 6, characterized in that the optical path (C) is close to 3 mm. 8.- sensor according to one of the preceding claims, characterized in that it comprises a plurality of photoreceptors (4) formed by photodiodes (41 to 48). 9. A sensor according to claim 8 characterized in that the light emitting diode (3) is adapted to illuminate at least four, advantageously eight, photoreceptors (41 to 48) simultaneously. 10. - Sensor according to one of the preceding claims, characterized in that it comprises several point light sources.