Reflective shaft angle encoder

Abstract: An optical shaft angle encoder has a code wheel (18) with alternating reflective (32) and non-reflective (31) areas in a circumferential path on one face of the wheel. A light emitting diode (29) is spaced apart from the reflective areas (32) on the wheel. Four photodetectors (w,x,y,z) are also spaced apart from the same face of the wheel so that, as the wheel (18) rotates, the photodetectors (w,x,y,z) are alternately illuminated or not illuminated by light from the light emitting diode (29) that is reflected from the reflective areas (32). Preferably the light emitting diode (29) is relatively nearer the shaft axis than the photodetectors (28), and is in a radial direction from the shaft axis which bisects the array of photodetectors.

Description: Field of the Invention This invention relates to an optical shaft angle encoder for producing electrical signals indicative of the angular position or motion of a shaft. In particular, it employs a reflective encoder wheel so that all measurements may be made on the same face of the wheel, thereby reducing difficulty in making electrical connections. A light emitter and photodetectors are arranged on one side of the wheel. Background of the Invention Incremental optical motion encoders are used for resolving the position or measuring rotation of a shaft. Exemplary shaft angle encoders are known from U.S. Patents Nos. 4,451,731 and 4,691,101, for example. Such encoders include a light source for emitting a light beam, an encoder wheel for modulating the light beam in response to shaft rotation, and a detector assembly for receiving the modulated light and producing electrical signals indicating the amount of light received by the detectors. As the light is modulated in response to shaft rotation, each electrical signal from the detector assembly produces a wave form. The position of the shaft determines the position of each signal on its particular wave form, that is, the phase of each signal. Thus, the electrical signals from the detectors can be used to indicate shaft rotation. Two or more properly out-of-phase signals, from separate detectors, can be used to indicate both direction and magnitude of rotation. Typical shaft angle encoders have an opaque wheel with a plurality of transparent windows through which illumination passes. In the simplest embodiment, a light source is provided adjacent to one face of the wheel. A photodetector is placed on the opposite side of the wheel so that as the wheel rotates, light from the emitter is alternately passed through a window to the photodetector or occulted by an opaque area of the wheel. There may be a number of drawbacks to such an arrangement, particularly when relatively low-cost encoders are desired. Much of this can stem from having the light emitter on one side of the wheel and the light detectors on the opposite side of the wheel. This means that there are two substrates to which electrical connections must be made. Having two substrates complicates the positioning and alignment of components to obtain a desired precision. Each of these adds significant cost to the encoder. Further, to obtain precision and high contrast, it is commonly necessary to employ collimating optics for the light source. This adds an element of complexity and difficulty in assembly alignment and also enlarges the size of the encoder. In the modern world, it is almost universal that small size is desirable. It is, therefore, desirable to produce a shaft angle encoder where the light emitters and photodetectors can be mounted on a single substrate. DE-A-3 727 188 describes an optical type displacement detecting device as defined in the preamble of the independent claims. Patent Abstracts of Japan, Vol. 12, No. 361 (P-763), 28.09.88 & JP-A-63 111 418 (Sony) describes a circular encoder plate arranged to reflect a plurality of beams emitted by a laser light source to a plurality of photodetectors. A detection signal is calculated on the basis of the detection signals produced by the photodetectors. FR-A-2 602 381 describes an incremental disc encoder which is provided with transparent regions, the arrangement being such that light emitted along a path adjacent the encoder plate is reflected at a back wall and is subsequently incident on the rear face of the encoder plate. Light passing through the transparent regions of the encoder plate is detected by a photodetector. FR-A-2 619 660 is a published Certificate of Addition application relating to the above French patent application, in which the transparent regions of the encoder plate have been replaced with reflecting zones. In accordance with the present invention there is provided a shaft angle encoder as defined in claims 1, 2, 3, 5, 10 and 13. According to a preferred embodiment, there is provided a shaft angle encoder having a rotatable code wheel with a plurality of alternating reflective and non-reflective areas extending in a ciroumferential path around the wheel. Means, such as a light emitting diode, are provided for illuminating the reflective areas. A plurality of photodetectors are arrayed on the same side of the code wheel as the light emitting diode, for receiving light which is directly reflected from the reflective areas on the code wheel. This produces a plurality of electrical signals indicative of code wheel position. Brief Description of the Drawings These and other features and advantages of the present invention will be appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: FIG. 1 is a longitudinal cross-section of a shaft angle encoder constructed according to principles of this invention: FIG. 2 is a fragmentary view of one face of an exemplary code wheel; FIG. 3 is a face view of a substrate having a light emitter and photodetectors: FIG. 4 is a fragmentary, enlarged semi-schematic cross-section including a fragment of the code wheel of the encoder; FIG. 5 is a schematic illustration of illumination of photodetectors by light from such a light emitter; and FIG. 6 is a block diagram of an encoder, with illumination indicated schematically. Detailed Description An exemplary shaft angle encoder constructed according to principles of this invention is built in an opaque square housing 10 through one side of which a shaft 11 extends. The shaft enters the housing through a threaded portion 12 which permits mounting the encoder on a panel or the like. The other end 13 of the shaft fits in a bearing socket in a back plate 14 which closes the housing. The shaft may be rotated by any of a broad variety of devices, or may be rotated manually. A round hub 16 is mounted on a portion of the shaft having splines 17 which assure rotation of the hub with the shaft. In FIG. 1 of the drawings, the portion of the hub to the left of the centerline is illustrated in elevation, and to the right of the centerline, the hub is illustrated in cross-section. A code wheel 18 is held tightly against the hub, and the hub is held on the shaft by a conventional spring retention clip 19 having conical leaves 21 which frictionally engage the shaft. It should be recognized that the code wheel 18 is very thin (e.g. 50 micrometers) and is drawn somewhat exaggerated in thickness solely for purposes of illustration. To give an idea of scale, the code wheel in an exemplary embodiment has a diameter of a little over 13 millimeters. On the opposite side of the hub from the code wheel, there is a generally circular detent spring 22 bearing against the inside of the housing, for biasing a detent ball 23 toward the back of the hub. A circumferentially extending band of detent holes 24 is formed in the back of the wheel for engaging the ball and providing a tangible indication of wheel rotation. The detent ball also serves to lightly restrain the hub in any of a plurality of positions corresponding to the detent holes. Such an arrangement is appropriate where the shaft may be manually adjusted. Such detents may be absent in other embodiments. A printed circuit board 26 is mounted on the back plate and extends outside the housing. A conventional connector on the printed circuit board has a plurality of pins 27 for making electrical connections to components mounted on the board. Four such pins are sufficient when some data processing is performed, within the housing. An integrated circuit and photodetector substrate 28 and a light emitting diode (LED) 29 are mounted on the printed circuit board in a location near the perimeter of the code wheel. The LED and photodetector substrate are located 45° around the shaft axis from a flat side of the square housing to provide additional space on the printed circuit board for these components. In this embodiment, the code wheel has a highly reflective face opposite the emitter and photodetector substrate. By reflective, it is meant that the face is smooth and flat to provide specular reflection of the light, rather than diffuse reflection. A circumferentially extending band of essentially trapezoidal windows 31 are etched through the code wheel. The reflective space 32 between adjacent windows has essentially the same width as the windows. The band of windows overlies a conical groove 33 formed in the front face of the hub. Thus, the portion of the hub behind the band of windows through the code wheel has a surface that is not parallel to the reflective surface of the code wheel. Thus, any light passing through such a window will be reflected, if at all, in a direction different from light reflected specularly from the face of the code wheel. For this reason, the windows are regarded as non-reflective areas between reflective areas 32 between the windows. Instead of reflecting the light in a different direction from the specular reflection from the code wheel, the non-reflective areas of the code wheel may be diffuse reflectors or absorbers. For example, absorbing areas might be formed with a one-quarter wave length interference layer for absorption of monochromatic light. Alternatively, the hub and code wheel could be made integral, such as by injection molding plastic. This type of code wheel has recessed areas angled to reflect light in directions different from reflection from the flat face of the code wheel. The recesses define areas that, for purposes of this invention, are non-reflective. With such an arrangement, the entire face of the code wheel/hub may be metallized for good specular reflection from the reflective areas, while still retaining non-reflective areas interspersed between the reflective areas. Also, the reflective areas may be curved instead of flat for concentrating light from the emitter onto a smaller photodetector area. Such curved surfaces may also be specular reflectors when smooth. Thus, for purposes of this specification, a reflective area is regarded as an area that specularly reflects light from the emitter to the photodetectors and a non-reflective area is regarded as an area that does not so reflect, regardless of whether the light incident on the "non-reflective" area from the emitter is absorbed or reflected elsewhere. The LED and photodetector substrate are mounted on a gold grounding pad 35 on the printed circuit board. The LED 29 is positioned at a relatively smaller radial distance from the axis of the shaft than the integrated circuit chip 28 on which a plurality of photodetector areas w, x, y, z are formed. The LED is a conventional LED which emits essentially monochromatic light from a line 37 which extends in a radial direction from the shaft axis. As illustrated in FIG. 3, the line 37 has some width as it must necessarily have in any practical embodiment. In effect, the line is a point source of light elongated in a radial direction. In an exemplary embodiment, the light emitting line of the LED is about 64 micrometers wide and 320 micrometers long. The integrated circuit chip includes four elongated photodiodes w,x,y,z which are arrayed in radial directions from the axis of the shaft. In between each of the photodiode areas, and outboard of the outermost photodiodes x,z, there are dummy doped areas 38 shorted to a conventional isolation layer or barrier for minimizing leakage current to and between the photodiodes. The distance between adjacent photodiodes is the same as the width of the emitting line 37 of the LED. This helps compensate for the "non-sharp" edge of the illuminated area and minimizes dark current from the photodiodes, thereby enhancing contrast. The integrated circuit chip 28 also includes conventional signal processing circuitry 39 (indicated only schematically in FIG 6) including two comparators or operational amplifiers 41. A load resister 42 for the LED is mounted on the printed circuit board adjacent to the substrate. Other conventional individual circuit elements, wire bond pads, and electrical interconnections are omitted for clarity. Signals from the four photodiodes are processed in substantially the same manner described in U.S. Patent No. 4,691,101 by Leonard. The four signals from the photodiodes are compared by way of the operational amplifiers 41 to produce two channels of output logic by which the shaft position or motion can be determined. In the illustrated embodiment, there are four photodetector areas suitable for a code wheel having 32 reflective areas in the circumferential path around the wheel. In other embodiments with larger numbers of reflective areas, it may be desirable to have more than one cycle of four photodiodes. For example, in an embodiment having 64 reflective areas on the code wheel, it is desirable to employ eight photodetector areas connected in quadrature as described in the Leonard patent. Further, with a code wheel having 120 reflective areas, it is desirable to employ three cycles of four photodetector areas or a total of twelve. The light emitting line 37 of the LED lies in a radial direction bisecting the array of photodiodes w,x,y,z. The circumferential path of the alternating reflective and non-reflective areas on the code wheel lies at radial distance from the shaft axis mid-way between the LED and photodetectors. Thus, light emitted from the LED striking a reflective area on the code wheel is reflected for illuminating a photodetector. The photodetectors are preferably further from the axis of the shaft than the LED so that the area available for forming the photodetectors is larger. The general pattern of such illumination is illustrated schematically by dashed lines in FIGS. 4 to 6 wherein it is assumed that illumination arises from a point source (a line source is, in effect, a row of point sources). FIG. 4 illustrates the illumination in a radial plane, while FIG. 5 illustrates illumination in a circumferential direction. One thing that should be apparent from the drawings is that the cycle of four photodetector areas has approximately twice the width of each cycle of one reflective area and one non-reflective area on the code wheel. In an encoder employing collimated light and a measurement technique as described in the Leonard patent, the width of one photodetector cycle (four photodetectors) is the same as one cycle on the code wheel. In the reflective encoder provided herein, the light diverges from the light source toward the reflective areas. It is reflected at the angle of incidence and travels substantially the same distance back to the photodetectors. Thus, it continues to diverge, and the area illuminated (indicated by reference numeral 36 in FIG. 5) is twice as wide as the reflective area. Thus, for encoding in the same manner as in the Leonard patent, the width of the full cycle of four detectors should be twice as wide as the cycle on the code wheel. Likewise, each individual photodetector area is the same width as either a window or the reflective area between windows. This relation does not precise for several reasons. Although the LEDs and photodetectors in the illustrated embodiment are approximately in a plane parallel to the plane of the code wheel, they are not quite in the same plane. The divergence of the light is therefore slightly less from the reflector to the photodetectors than from the LED to the reflective area. Parallax also spreads the illuminated area on a plane parallel to illuminated area as the angle of reflection becomes increasingly different from 90°. The width of the emitting line on the LED is finite. Physical limitations on the spacing of semiconductor areas on the integrated circuit chip also influence the width of photodetectors actually used. These discrepancies are minor, however, and the width relation is nearly as stated. It will also be apparent that the photodetectors are alternately illuminated and not illuminated as the band of reflective areas rotates. Thus, the signals by the photodiodes are essentially the same as, and can be processed in the same manner as, the signals transmitted through windows in the code wheel of the Leonard patent. The photodetector areas x,y closest to and on opposite sides of the radial direction bisecting the array of photodetectors may be made slightly smaller than the outboard photodetector areas w,z. The length of the optical path between the LED and the outboard photodetector areas is slightly longer than the path to the inboard photodetectors. The light intensity at the outboard photodiodes is therefore slightly less. The photodetector area is, therefore, increased so that approximately the same total flux illuminates each of the photodetector areas. This refinement is of less significance when two or more cycles of photodetectors are used, since the flux from inboard and outboard photodetectors in the channel are integrated. Although but one embodiment of optical shaft angle encoder has been described and illustrated herein, many modifications and variations will be apparent to one skilled in the art. Thus, for example, in this embodiment, there is a single band of alternating reflective and non-reflective areas extending circumferentially on the code wheel. This is sufficient where the information desired is the magnitude of rotation of the shaft, rather than absolute shaft position. If the position is significant, index marks may also be provided on the code wheel for determining position in a conventional manner. A distinct advantage of the arrangement described and illustrated is the absence of need for any lensing associated with the light emitting device, light being emitted more or less isotropically. However, it will be recognized that light intensity may be enhanced by a concentrating lens at the LED that minimizes the amount of light illuminating areas that cannot reflect toward the photodetectors. The reflective and intervening non-reflective areas on the code wheel are illustrated as extending radially. For wheels with a small number of cycles around the circumference, it may be desirable to employ spirally extending reflective areas and non-reflective areas for greater sensitivity.

Claims: A shaft angle encoder comprising: a rotatable code wheel (18) having a plurality of reflective areas (32) arranged in a path extending circumferentially around the wheel (18); means (29) for emitting a light beam toward the circumferential path for illuminating the reflective areas (32); and at least one photodetector (w,x,y,z) arranged to be illuminated by light from said light emitting means (29) after reflection from each of a plurality of the reflective areas (32) and for producing, in response thereto, a plurality of electrical signals indicative of the position of the code wheel; wherein said light emitting means (29) is arranged so that the light beam has an elongate cross-section which extends in a direction transverse to said circumferential path; characterised in that said code wheel (18) comprises a circumferentially extending band of windows (31), said plurality of reflective areas (32) being defined by the spaces (32) between adjacent pairs of said windows (31), said windows (31) overlying a surface (33) which is not parallel to the reflective surface of the code wheel (18). A shaft angle encoder comprising: a rotatable code wheel (18) having a plurality of reflective areas (32) arranged in a path extending circumferentially around the wheel (18); means (29) for emitting a light beam toward the circumferential path for illuminating the reflective areas (32); and a plurality of photodetectors (w,x,y,z) arranged to be illuminated by light from said light emitting means (29) after reflection from each of a plurality of the reflective areas (32) and for producing, in response thereto, a plurality of electrical signals indicative of the position of the code wheel; wherein said light emitting means (29) is arranged so that the light beam is in the form of a line which extends in a direction transverse to said circumferential path; characterised in that the distance between adjacent ones of said plurality of photodetectors (w,x,y,z) is the same as the width of the emitting line of the light emitting means (29). A shaft angle encoder comprising: a rotatable code wheel (18) having a plurality of reflective areas (32) arranged in a path extending circumferentially around the wheel (18); means (29) for emitting a light beam toward the circumferential path for illuminating the reflective areas (32); and a sequence of four photodetectors (w,x,y,z) arranged to be illuminated by light from said light emitting means (29) after reflection from each of a plurality of the reflective areas (32) and for producing, in response thereto, a plurality of electrical signals indicative of the position of the code wheel; wherein said light emitting means (29) is arranged so that the light beam has an elongate cross-section which extends in a direction transverse to said circumferential path; characterised in that the areas of the inner two photodetectors (x,y) of said sequence of four photodetectors (w,x,y,z) are smaller than the areas of the two outer photodetectors (w,z) of said sequence. A shaft angle encoder according to any preceding claim, wherein said light emitting means (29) is elongated in a radial direction and each of said at least one photodetector (w,x,y,z) has a respective elongate detection area extending in a radial direction. A shaft angle encoder comprising: a rotatable code wheel (18) having a plurality of reflective areas (32) arranged in a path extending circumferentially around the wheel (18); means (29) for emitting a light beam toward the circumferential path for illuminating the reflective areas (32), said light emitting means (29) being elongated in a radial direction; and at least one photodetector (w,x,y,z) having an elongate detection area and being arranged to be illuminated by light from said light emitting means (29) after reflection from each of a plurality of the reflective areas (32) and for producing, in response thereto, a plurality of electrical signals indicative of the position of the code wheel; wherein said light emitting means (29) is arranged so that the light beam has an elongate cross-section which extends in a direction transverse to said circumferential path; characterised in that: the elongate detection area of said at least one photodetector (w,x,y,z) extends in a radial direction; and the width of said at least one photodetector (w,x,y,z) is the same as the width of each of said plurality of reflective areas (32) measured along the circumferential path. A shaft angle encoder according to any preceding claim, wherein the circumferential path is on one face of the code wheel (18). A shaft angle encoder according to claim 6, further comprising a plurality of detents on the opposite face of the code wheel (18) from the reflective areas (32) for tangibly indicating the wheel position. A shaft angle encoder according as claimed in any preceding claim, wherein said code wheel (18) comprises a plurality of non-reflective (31) areas extending in said circumferential path and alternating with said reflective areas (32). A shaft angle encoder according to claim 8, when dependent on claim 6, wherein said non-reflective areas (31) comprise windows in the code wheel (18) and a non-reflective region (33) beyond the opposite face of the code wheel (18). A shaft angle encoder comprising: a rotatable code wheel (18) having a plurality of reflective areas (32) and a plurality of non-reflective areas (31) arranged alternately in a path extending circumferentially around the wheel (18) on one face thereof; means (29) for emitting a light beam toward the circumferential path for illuminating the reflective areas (32); and at least one photodetector (w,x,y,z) arranged to be illuminated by light from said light emitting means (29) after reflection from each of a plurality of the reflective areas (32) and for producing, in response thereto, a plurality of electrical signals indicative of the position of the code wheel; wherein said light emitting means (29) is arranged so that the light beam has an elongate cross-section which extends in a direction transverse to said circumferential path; characterised in that said non-reflective areas (31) comprise windows in the code wheel (18) and a non-reflective region (33) beyond the opposite face of the code wheel (18). A shaft angle encoder as claimed in any preceding claim, wherein said at least one photodetector comprises a plurality of photodetectors (w,x,y,z) and said encoder further comprises comparator means (39,41) for comparing signals from the photodetectors and producing signals indicative of code wheel rotation. A shaft angle encoder according to claim 11, when dependent on claim 8 or claim 10, wherein the width of each cycle of photodetectors (w,x,y,z) is twice the width of each cycle of reflective (32) and non-reflective (31) areas. A shaft angle encoder comprising: a rotatable code wheel (18) having a plurality of reflective areas (32) and a plurality of non-reflective areas (31) arranged alternately in a path extending circumferentially around the wheel (18); means (29) for emitting a light beam toward the circumferential path for illuminating the reflective areas (32); and a plurality of photodetectors (w,x,y,z) arranged to be illuminated by light from said light emitting means (29) after reflection from each of a plurality of the reflective areas (32) and for producing, in response thereto, a plurality of electrical signals indicative of the position of the code wheel; and comparator means (39,41) for comparing signals from the photodetectors (w,x,y,z) and producing signals indicative of code wheel rotation; wherein said light emitting means (29) is arranged so that the light beam has an elongate cross-section which extends in a direction transverse to said circumferential path; characterised in that the width of each cycle of photodetectors (w,x,y,z) is twice the width of each cycle of reflective (32) and non-reflective (31) areas. A shaft angle encoder according to any one of claims 11 to 13, wherein the light emitting means (29) is disposed in a radial direction bisecting said plurality of photodetectors (w,x,y,z). A shaft angle encoder according to any one of claims 11 to 14, wherein said plurality of photodetectors comprises at least four photodetectors (w,x,y,z) and wherein said comparator means (39,41) is arranged to compare signals from the four photodetectors (w,x,y,z) to produce two output channels (A,B) of information indicative of shaft position. A shaft angle encoder as claimed in any preceding claim, further comprising: an opaque housing (10); a rotatable shaft (11) mounted in the housing (10); a hub (16) rotatable with the shaft (11) and including a plurality of detent holes on a back face; a detent ball (23) and means for biasing (22) the detent ball (23) toward engagement with the detent holes; said rotatable code wheel (18) being arranged on a front face of said hub (16). A shaft angle encoder according to any preceding claim, wherein said at least one photodetector (w,x,y,z) and said light emitting means (29) are arranged in a plane parallel to the plane of the code wheel (18). A shaft angle encoder according to any preceding claim, wherein the light emitting means (29) is at a first radial distance from the axis of the code wheel (18) and said at least one photodetector (w,x,y,z) is at a second, different, radial distance from the axis of the code wheel (18). A shaft angle encoder according to claim 18, wherein said first radial distance is less than said second radial distance.

Inventors: LOEWEN VICTOR D, LOEWEN, VICTOR D.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1990
Main classification: H03M1
IPC codes: H03M1, G01D5