High resolution optical hybrid digital-analog position encoder

Abstract: An optical encoder for sensing angular position includes a rotatable optical medium having a major surface and an axis of rotation perpendicular to the surface, with a plurality of uniformly sized angular sectors defined on the surface. An annular digital track is defined on the surface concentric with the axis, a plurality of digital bits being formed on the surface in sequence along the digital track within each sector. Each such bit is optically readable as a binary 1 or a binary 0, with each plurality of bits defining a binary word. A first series of optically detectable analog marks are evenly spaced around a first analog track on the surface. The binary word in a sector is optically read to identify the angular position of that sector and indicate the approximate angular position of the medium, while the first series of analog marks can be optically detected to refine the approximate angular position indication. A source of light illuminates the surface along a fixed radius from the axis, while a digital track detector senses the illumination reflected from the digital bits and a first analog track detector senses the illumination reflected from the analog marks. Signal processing and decoding means convert the output of the digital track detector into a binary word and interpolate the precise angular position of the medium from the output of the first analog track detector. An input digital track optical fiber is connected between a digital track optical coupler and proximate the surface of the medium, while a first input analog track optical fiber is connected between a first analog track optical coupler and proximate the surface. Output digital track and first analog track optical fibers are connected between the couplers and the track detectors for conveying reflected light from the tracks to the detectors.

Description: BACKGROUND OF THE INVENTION This invention is concerned with optical encoders which are used to measure accurately the angular displacement of a shaft or rotating member. More particularly the invention relates to a technique for optically ascertaining the angular position of a rotating member using a combination of digital and analog sensing. The need for precision measurement in a variety of devices demands an inexpensive, high resolution method for ascertaining angular position. Stepper motors, for example, as used in such diverse applications as gun turrets, paper making plants, machine tools, direct drive motors, and robotics applications, require a high precision indication of angular position. Such angular displacement information may be utilized in subsequent process steps or as a feedback indicator of system error and misalignment. Prior art absolute encoders employ a disk having a series of abbreviated lines at various radii coupled with a multiple channel reader to directly read the position of the encoder in a binary format. While such a system is simple and inexpensive, it does not provide the accuracy required by modern high precision instrumentation. Other prior art encoders use laser technology, such as the laser incremental encoder model X-1, manufactured by Canon. A complex series of prisms in this device is used to produce 225,000 sinusoidal waves per revolution. The 225,000 waves are each further subdivided into 80 pulses per wave to yield 18 million pulses per revolution. The principle employed in the laser rotary encoder is that of a grating disk having a series of radially oriented bars about its outer periphery. The radially oriented bars are very small and must be positioned on the disk with great precision. A grating disk having the required grating bar tolerances for the laser rotary encoder is difficult and expensive to fabricate. Furthermore, the grating disk is required to be used in conjunction with a phase plate having similarly strict manufacturing requirements. EP-A-0 172 323 teaches a set of reference marks Rn, which are assigned a code mark Ci in binary number format, the code mark Ci uniquely identifying the reference mark Ri. The intervals a, b and r between the code mark components and the reference mark components, which are established for each copy of the measuring device when that unit is fabricated, must be known to properly read the reference mark and code mark data. These particular intervals are determined by counting the marks of graduation T and comparing to the stored interval raster in the evaluation unit W. DE-A-4 025 055 teaches a rotary position selecting device which uses a memory disk similar to compact disk technology. 22 concentric tracks on the disk contain digitally coded data, which is read in parallel to evaluate the rotary position of the disk. The cost of such a high precision rotary encoder is consequently quite prohibitive. Furthermore, because of the electronic components in this apparatus, there are limitations on the operating environment in which such an encoder may be employed. The operating temperature of the encoder must remain within a temperature range of approximately 10° C to 40° C. Otherwise, not only may inaccuracies be introduced into the measurements, but the encoder may be damaged or destroyed. Consequently there is a need in some low and high temperature position measurement applications to employ a device which does not require any electronic components in the position encoder head. What is needed is an angular displacement rotary encoder having a precision and resolution which exceeds that available for encoders in its class. The cost of such an encoder should be considerably less than that associated with a complex laser rotary encoder, yet the resolution should be much greater than that obtainable with a standard direct digital read encoder. SUMMARY OF THE INVENTION By adapting compact optical disk technology to the measurement of rotary displacement, this invention provides a new technique for achieving high precision measurement of angular position with a significant reduction in the manufacturing cost of the measurement apparatus. An optical encoder for sensing angular position includes a rotatable optical medium having a major surface and an axis of rotation perpendicular to the surface, with a plurality of uniformly sized angular sectors defined on the surface. An annular digital track is defined on the surface concentric with the axis, a plurality of digital bits being formed on the surface in sequence along the digital track within each sector. Each such bit is optically readable as a binary 1 or a binary 0, with each plurality of bits defining a binary word. A first analog track is defined on the surface concentric with the axis, a first series of optically detectable analog marks being formed on the surface and evenly spaced around the first analog track. The binary word in a sector can be optically read to identify the angular position of that sector and thereby indicate the approximate angular position of the medium, while the first series of analog marks can be optically detected to refine the approximate angular position indication and thereby indicate a more precise angular position for the medium. The annular digital track may include a binary start symbol, recorded on the digital track within each sector and having a common value in all sectors to indicate the start of a sector, and a binary sector number recorded on the digital track within each sector and having a unique value identifying the sector in which it is recorded. The optical encoder may further include a second analog track defined on the surface concentric with the axis, with a second series of optically detectable analog marks formed on the surface and evenly spaced around the second analog track, the number of marks in the second series being equal to the number of marks in the first series. In addition, each mark within the second series is displaced in a circumferential direction with respect to a corresponding mark in the first series by an angular amount equal to one eighth the spacing of the marks in the first series. The second series of marks can be optically detected to, in conjunction with the first series of marks, refine the approximate angular position indication. The annular digital track may further include a binary start symbol, recorded on the digital track within each sector and having a common value in all sectors to indicate the start of a sector, and a base 3 sector number recorded on the digital track within each sector and having a unique value identifying the sector in which it is recorded, each digit of the sector number being encoded by means of its radial alignment with a mark in the first analog track or a mark in the second analog track. The optical encoder may additionally include third and fourth analog tracks defined on the surface concentric with the axis, with a third series of optically detectable analog marks displaced with respect to the first series by one fourth the spacing of the marks and a fourth series of marks displaced by three eighths the spacing of the marks. The third and fourth series of marks can be optically read to, in conjunction with the first and second series of marks, further refine the approximate angular position indication. In this embodiment, the annular digital track may further include a binary start symbol, recorded on the digital track within each sector and having a common value in all sectors to indicate the start of a sector, and a base 5 sector number recorded on the digital track within each sector and having a unique value identifying the sector in which it is recorded, each digit of the sector number being encoded by means of its radial alignment with a mark in the first analog track, a mark in the second analog track, a mark in the third analog track, or a mark in the fourth analog track. The optical encoder may be further equipped with a source of light for illuminating the surface along a radius from the axis, the illuminated radius being fixed with respect to rotation of the medium. A digital track detector senses the portion of the illumination reflected from the bits within the digital track, while a first analog track detector senses the portion of the illumination reflected from the marks within the first analog track. Signal processing and decoding means are provided for converting the output of the digital track detector into a binary word and for interpolating the precise angular position of the medium from the output of the first analog track detector. The optical encoder may include as well an opaque mask positioned between the light source and the optical medium and proximate to the surface of the medium, with a digital track opening in the mask for permitting illumination of a precise area of the digital track and a first analog track opening in the mask for permitting illumination of a precise area of the first analog track. A non-rotating read head may slidably engage the optical medium. A digital track optical collimator is affixed to the read head for conveying light from the light source to the digital track and conveying light reflected from the digital track to the digital track detector. A digital track optical coupler is connected between the light source, the digital track optical collimator, and the digital track detector. A digital track optical fiber couples the digital track optical coupler to the digital track optical collimator. A first analog track optical collimator is affixed to the read head for conveying light from the light source to the first analog track and conveying light reflected from the first analog track to the first analog track detector. A first analog track optical coupler is connected between the light source, the first analog track optical collimator, and the first analog track detector. A first analog track optical fiber couples the first analog track optical coupler to the first analog track optical collimator. DESCRIPTION OF THE DRAWINGS Figure 1 is a plan view of an optical encoder disk constructed according to the present invention; Figure 2 is an expanded view of a portion of the encoder disk of Figure 1 illustrating the format of the digital and analog tracks on the surface of the disk. Figure 3 is a diagram schematically illustrating the relative positions of the four analog tracks in the five track embodiment of the optical encoder disk shown in Figure 1. Figure 4 schematically depicts the start symbol and encoded number for the first 20 and last 20 sectors of an exemplary base 5 encoding scheme. Figure 5 illustrates in cross sectional side view an encoder assembly housing the encoder disk shown in Figures 1 - 2. Figure 6 is a schematic diagram of the encoder system of the present invention embodying the encoder assembly of Figure 5. DESCRIPTION OF THE INVENTION This invention provides a new technique for precision sensing of angular position using a hybrid analog/digital optical encoder, with the precision depending upon the number of tracks used. In the particular embodiments discussed below, for example, angular displacement can be resolved to within 1/1,000; 1/10,000; 1/40,000; or 1/160,000 of a single rotation. Additional improvements in resolutions will be possible as advances are made in photolithography, disk fabrication, and shorter wavelength lasers. The inventive solution employs a new approach to optical rotary position encoding which takes advantage of the existing body of technology developed for the manufacture, recording, and reading of compact disks. Compact disk technology utilizes interferometric reflection. Data is stored in a compact disk by embossing, which forms a step or a pit in a plastic layer on the disk. The step or pit is formed to a depth equal to one fourth the wavelength of the light which is to be used in reading the disk. During reading of the data, a laser beam illuminates an area that includes both the step or pit and an additional area around it equal to that of the step or pit. Light which is reflected from the pit will be 180 degrees out of phase with respect to light reflected from the surrounding area. The interference between these reflections causes the net optical power reflected from that area of the disk to be near zero. In contrast, the absence of a pit in a region being viewed by the detector will produce a significant non-zero optical signal, since all of the light reflected will be in phase. Information can thus be stored in binary format on such a disk by designating either the presence or the absence of a pit as a binary "1", while the other state is designated as a binary "0". An encoder disk for use in the optical encoder of the present invention could be fabricated from glass, as is conventionally done, with the encoding accomplished by an overlayer of plastic in which the coding pits are embossed from a master die. The plastic would then be covered by a high reflectance metal layer, typically aluminum or gold. A circular disk, utilizing state of the art compact disk construction and having a radius of approximately three centimeters, can be employed satisfactorily as a position encoder to give the necessary resolution. One embodiment of such a rotatable disk, referred to hereinafter as an encoder disk, is shown in plan view in Figure 1 as disk 102, which is provided with an annular digital track 104 at a radius r and concentric with the disk's axis of rotation 106. A number of uniformly sized angular sectors, such as the sector area outlined by the lines 108, are designated for the surface of the disk 102. The disk might, for example, be divided into 1,000 such angular sectors. The choice of 1,000 sectors is based upon the limitation imposed by the radius of the encoder disk, coupled with the physical dimensions of the pits used in compact disk technology. Larger encoder disks could be divided into more sectors, while smaller disks would require fewer divisions. The disk also includes annular first, second, third, and fourth analog tracks 110, 112, 114, and 116 defined on the surface concentric with the axis 106. Figure 2 is an expanded view of Figure 1 illustrating a portion of one angular sector. Within the digital track 104, a series of digital bits are formed on the surface of the disk in sequence along the track within each sector. Each such bit is optically readable as a binary 1 or a binary 0, and each series of the bits within a sector defines a binary word. In the sector shown, for example, are ten digital bits 120-138. Each such bit also corresponds to a step or pit as referred to above, wherein "dark" bits, such as the bits 120, 124, 130, 132, 134, and 138, correspond to locations where pits produce out of phase light which interferes with light reflected from the surrounding area, producing a minimum of reflected optical energy. "Bright" bits, such as the bits 122, 126, 128, and 136, correspond to locations where there is no pit, so that a maximum amount of optical energy is reflected from these areas. As stated above, the bit "sense" could equally well be reversed, with reflected optical energy indicative of either a logical "one" bit or a logical "zero" bit. In Figure 2, note that there are ten bits within the digital word contained in the sector. The first two bits 120 and 122 are utilized for a start symbol, leaving eight bits with which position information may be encoded. Depending upon the size of the encoder disk 102 and other factors, a typical computation for the sector size is as follows. r d = [r * 360 * (π/180)] / 1000 where r d is the length of the sector and d is the angle subtended, i.e., 1/1000 of 2π. Given a radius of three centimeters, for example, which was chosen to achieve an angular resolution of greater than one in 100,000, the sector would be 188 microns long. A digital string of ten bits, as shown below, can provide an absolute address for the sector to a resolution of 1 part in 1,000. These ten bits can fit comfortably in the center of the angular sector, leaving blank space on either side of the digital code segment for word isolation. In addition to the digital track 104, the first analog track 110 and the second analog track 112 are depicted in Figure 2. The first track contains a first series of optically detectable marks, such as the mark 142, which are evenly spaced around the track. In the embodiment discussed above, wherein an encoder disk with a three centimeter radius is utilized, there are 10,000 such marks 142 positioned around the disk in the first analog track. In the preferred embodiment these marks have a center-to-center distance of about 18.8 microns and a width of about 4.0 microns, dimensions which are easily within the range of current compact disk optical lithography technology. The digital track 104 and the first analog track 110 provide incremental position information relative to the last sector word that was read, in addition to indicating the direction of angular displacement. Absolute position is therefore provided by first identifying the digital word in the last digital code segment read, which indicates the position of the corresponding angular sector. By counting the analog marks in the track 110, the incremental distance relative to the start of the sector can be determined to deduce the absolute position of the encoder. If, for example, ten analog marks were used in each sector, this combination of a digital track and a single analog track would yield an accuracy of one part in 10,000. As mentioned above, four analog tracks 110-116 are provided in the preferred embodiment. The second analog track 112, which is also shown in the expanded view of Figure 2, includes a second series of marks, such as the mark 144, which are evenly spaced around the track. The number of marks in the second series is equal to that in the first series. Moreover, the marks in the second series are displaced in a circumferential direction with respect to corresponding marks in the first series by an angular amount equal to one eighth the spacing of the marks in the first series. Similarly, the third and fourth analog tracks 114 and 116 contain a third and a fourth series of marks, each of which also includes the same number of marks as in the first series. The marks in the third series, however, are displaced in a circumferential direction with respect to a corresponding mark in the first series by an angular amount equal to one fourth the spacing of the marks in the first series, while the marks in the fourth series are displaced in a circumferential direction with respect to a corresponding mark in the first series by an angular amount equal to three eighths the spacing of the marks in the first series. Figure 3 is a diagram schematically illustrating the relative positions of the marks in the four analog tracks 110, 112, 114, and 116 in the five track embodiment of the optical encoder disk shown in Figure 1, with marks 142, 144, 143, and 145 depicted for tracks 110, 112, 114, and 116, respectively. One particular embodiment of the first digital track involves an implementation using a base 5 numbering system to indicate the absolute position and the direction of rotation of the disk. The start symbol and encoded number for the first 20 and last 20 sectors of an exemplary base 5 encoding scheme are depicted schematically in Figure 4. The technique for accomplishing this requires two or more tracks of data, i.e., one digital track and one or more analog tracks, with N tracks implying the use of a base N number system. The absolute position data must be readable for either direction of rotation and must include a unique "start symbol" to unambiguously determine the starting point of the digital information recorded on the digital track in each sector. When a total of five tracks are employed, 1,000 unique numbers (positions) are encoded in a like number of sectors on the disk. The unique numbers are ultimately translated to base 2, where ten bits are required to distinguish among the 1,000 numbers. A "start symbol" is required at the start of each number to unambiguously determine its position. The start symbol must be unique when read forward or backward, and must be placed every 1/1000 of a revolution in the exemplary embodiment. Use of the CD technology imposes certain constraints. Because a quarter-wave pit is used to reflect half the spot size back out of phase, resulting in an optical null, all marks must have the same width, although the spacing between the marks may vary. Furthermore, because of size constraints limiting the number of marks which can be placed within each sector in the digital track, a strictly binary encoding scheme may not be possible. Consequently, in order to pack the data more efficiently and simplify the decoding electronics, a base 5 number system is used. The symbol '0' is represented by a missing mark symbol '1' by a mark which is aligned with the marks in the first analog track (shown in Figure 4 as a cross-hatched rectangle in the leftmost position within a digit column), '2' by a mark which is aligned with the marks in the second analog track (in Figure 4, a rectangle shifted one position to the right within a digit column with respect to the position of a '1' symbol), '3' by a mark which is aligned with the marks in the third analog track (in Figure 4, a rectangle shifted two positions to the right within a digit column with respect to the position of a '1' symbol), and '4' by a mark which is aligned with the marks in the fourth analog track (in Figure 4, a rectangle shifted three position to the right within a digit column with respect to the position of a '1' symbol). The sequence '11' (in Figure 4, represented in each sector row by the left two cross-hatched rectangular marks) is designated as the start symbol. Seven of the remaining base 5 marks in each sector are then used to encode 1,000 unique numbers. The CD technology, however, imposes an additional constraint, which permits no two marks to be closer than the minimum spacing. This prevents, for example, a '5' being followed by a '1' (there is some flexibility in this limitation, depending on the length chosen for each mark and the number of tracks employed, since the space between marks increases as tracks are placed radially further from the center of the encoder). Furthermore, the start symbol '11' may never occur in any number, to avoid confusion with the start symbol. Assuming a conservative constraint that none of the symbols in the set (1,2,3,4) can be followed by a smaller digit except 0, there are 1,954 numbers remaining that can be represented in seven digits base 5. In this implementation, as will readily be understood by those skilled in the art, a set of adders and an EPROM can be utilized to convert the base 5 number to a binary (base 2) number. A set of numbers for a particular embodiment of this coding scheme, as illustrated by the coding for the first 20 and last 20 sectors in Figure 3, can be readily generated by one skilled in the art using the requirements and limitations discussed above. When the information available from the second analog track is combined with that from the digital track and the first analog track, the achievable resolution in the exemplary embodiment is doubled to one part in 20,000. Moreover, since the two series of marks 142 and 144 in the two analog tracks are 90° out of phase, quadrature interpolation can be utilized to further increase the resolution to one part in 40,000. Such quadrature interpolation requires only four additional bits of accuracy and is therefore easily realizable using an ordinary microprocessor or logic circuit. With the addition of the third and fourth analog tracks 114 and 116, a mark is provided for each 45° of phase change in the analog series. This is equivalent (in an embodiment having ten marks per analog track per sector) to providing forty analog marks per angular sector. The raw resolution with the additional information provided by the employment of the four analog tracks 110-116 is increased to one part in 40,000. Again, digital interpolation of the raw information can be accomplished to yield a factor of four multiplication, resulting in an accuracy in the illustrative embodiment of one part in 160,000. Each of the tracks, namely the digital track 104 and the four analog tracks 110-116, requires a channel for optical addressing. The optical channels associated with the four analog tracks in essence require only four additional bits for the final accuracy figure. Encoding is therefore accomplished with one track, the digital track 104, for an absolute digital resolution of 1,000 parts per revolution, while one, two, or four analog tracks provide incremental position information relative to the last sector position and four bit digital interpolation of the analog tracks provides the final resolving power. Within the industrial environment in which encoders frequently must operate, there are several practical problems which must be addressed. Typically, the rotating member on which the encoder disk is mounted will not consistently rotate within a single plane. Such deviations from in-plane rotation are known as "wobble", which interferes with the ability to read the encoder disk consistently and accurately. Wobble may develop for a variety of reasons, such as temperature changes in the machine driving the encoder disk or degradation of the attachment between the encoder disk and the device whose angular position is to be measured. Furthermore, when elevated temperatures are present in the machinery to be measured, the heat can degrade the electronics associated with commonly available encoders. Heat damage is especially likely when a metal rotating shaft is utilized to transmit angular displacement to the encoder device, since the metal shaft forms an efficient conductor of heat. In accord with the device and method of the present invention, these limitations arising from wobble and heat sensitivity can be ameliorated or eliminated from the encoding function. Figure 5 is a cross sectional view of an encoder assembly using the encoder disk of Figures 1 and 2. Here, first and second annular low friction coatings 146 and 148 have been applied to the surface of the encoder disk 102. A housing 150 encases the encoder and provides other structure necessary to allow angular displacement to be accurately determined despite wobble and despite a thermally harsh operating environment. The housing 150 contains an aperture at one end for rotatably admitting an encoder shaft 152 into the interior. The encoder shaft is attached to the encoder disk 102 so that both the encoder shaft and the encoder disk rotate together. Opposing the encoder disk 102 is a read head disk 154. The read head disk 154 carries a pair of skids 156 and 158 which are preferably made of a fluorocarbon material or coated with a fluorocarbon coating such as Teflon®. The skid 156 evenly opposes the low friction coating 146, while the skid 158 evenly opposes the low friction coating 148. The read head disk 154 is secured by a shaft 160. A cover 162 is rigidly attached to the housing 150 and is removable from the housing to provide service and maintenance access thereto. The inside of the cover 162 is partially packed with a filler material 164 which serves to provide additional support for the shaft 160. The shaft 160 is rigidly connected to the cover 162 and the read head disk 154 to prevent rotation of the read head disk. A compression spring 166 surrounds the shaft 160 between the filler material 164 and the read head disk 154 to urge the read head disk toward the encoder disk 102. This biasing of the read head disk brings the skids 156 and 158 into direct sliding contact with the low friction coatings 146 and 148 on the encoder disk. Alternatively, the read head could be mounted on the back side of the encoder disk to simplify the design. In addition, the shape of the marks in the digital and analog tracks can be chosen to minimize the sensitivity of the design to in-plane wobble of the encoder disk. A series of first collimating transfer optics 168, 170, 172, 174, and 176 are located in the read head disk 154. These elements are an array of optical fibers with appropriate focusing optics or alternatively are a series of gradient index (GRIN) lens rods. Each one of the first collimating transfer optics is positioned over a single track of the encoder disk 102. In the cover 162 and opposing the first collimating transfer optics is a set of second collimating transfer optics 178, 180, 182, 184, and 186 mounted within the filler material 164. Each element in the second collimating transfer optics is aligned with a corresponding element in the series of first collimating transfer optics. A series of five optical fibers 188, 190, 192, 194, and 196 extend from a distant point with respect to the rotary encoder assembly to connect with corresponding elements in the second series of collimating transfer optics. In this manner, a single optic channel or connection is made between each fiber in the set of optical fibers 188-196 and thereby to each of the five tracks of the encoder disk 102, namely the digital track 104 and the four analog tracks 110, 112, 114, and 116. It may be desirable to provide an opaque mask 234 positioned close to the surface of the encoder disk. This mask contains openings which permit illumination of a precise area of the digital track and the analog tracks in order to ensure a more effective interferometric effect in the reading of dark bits. Note the combination of clearances between the read head disk 154 and the filler material 164. The mechanical construction of the rotary encoder assembly 150 enables the read head disk to follow the encoder disk in the event that the encoder disk develops wobble. The limited support which the read head disk receives near its concentric center allows any such wobble forces to be effectively transmitted into a following motion. As will be appreciated by those skilled in the art, if the depth of field of the optical elements 178-186 is sufficiently long, the read head disk 154 and its associated collimating optics 168-176 will not be required. Moreover, if the optical fibers 188-196 can be held in close proximity to the encoder disk 102, no focussing optical elements will be needed. In the latter implementation, however, precision alignment is required. This alignment may be provided, for example, by etched grooves in a silicon or other substrate, which can be accomplished by chemical or mechanical ruling or machining. Now referring to Figure 6, a schematic diagram of the hybrid digital analog system of the present invention is shown incorporating the rotary encoder assembly of Figure 5. The set of optical fibers 188-196 shown in Figure 5 extends from the rotary encoder assembly 150 to a series of fiber optic couplers 198, 200, 202, 204, and 206. These fiber optic couplers are typically three port devices which allow light to be introduced into a first port for transmission out of a second port. Light returning to the second port is redirected to a third port. In this manner, a laser or other light source can be introduced at the first port to be routed through the second port to a remote location. Light returning to the second port is directed to the third port rather than back to the source, to better enable the returning light to be measured and detected without interference from the source. Thus the first ports of the fiber optic couplers are connected to a light source in the form of a laser 208. A fanning or dispersion coupler 210 is utilized to couple light from the laser into each of the first ports of the fiber optic couplers 198-206. In this manner, a single laser is configured to supply light in the same manner as would five separate lasers. Those skilled in the art will appreciate that an optical isolator may be necessary to prevent feedback to the laser 208 and the resulting instability in the signal level. A light emitting diode or other incoherent light source would not require an isolator. The third ports of the fiber optic couplers are each individually connected to a series of photosensitive detectors 212, 214, 216, 218, and 220 by means of a series of optical fibers 222, 224, 226, 228, and 230. The detectors convert the optical signals from the second ports of the fiber optic couplers into electrical signals. A signal processing and decoding device 232 is electrically connected to the detectors 212-220. The signal processing and decoding device is typically a computer, a microprocessor, or digital logic which is programmed to read, interpret, and output the digital signals it has received from the detectors. The second ports of the fiber optic couplers 198-206 are connected to the optical fibers 188-196 both to transmit light to, and receive digital signals from, the rotary encoder assembly 150. The optical coupling could also be achieved with an alternative arrangement which allows the three port fiber optic couplers to be eliminated and delivers the laser light to the encoder assembly through separate illumination fibers. The light reflected from the disk is collection in this scheme by a set of collection fibers which are directly connected to the detectors for each track. Depending on the application and the desired length of the fibers, this approach may provide a lower cost option by eliminating the fiber optic couplers. Referring to Figures 4 and 5, the operation of the high resolution optical hybrid digital-analog position encoder of the present invention is as follows. Light from the laser 208 is directed into the fiber optic couplers 198-206. This light propagates from the second ports of the optic couplers along the optical fibers 188-196 and into the second collimating transfer optics 178-186, where the cross sectional area in which the light is propagating is somewhat expanded. This expansion of the cross sectional area facilitates the transmission and reduces the losses of the light as it propagates between the first collimating transfer optics 168-176 and the second collimating transfer optics 178-186. An expanded cross sectional area of light in this gap serves to mitigate the loss of light through any misalignment between the first and second collimating transfer optics due to displacement of the read head disk. The optical fibers 188-196 should preferably be single mode fiber to preserve the interferometric effect caused by reflection of light from the pits and the surrounding surface. Multimode fiber will scramble and degrade the interference signal over the length of the fiber. The light propagates across the gap and into the first collimating transfer optics 168-176, from which it is positioned to focus upon the particular track on the encoder disk with which it is associated. Consequently, the first collimating transfer optics contain focussing optics on the side facing the encoder disk to efficiently direct light to the encoder disk and collect the reflected light. The second collimating transfer optics are designed to match the numerical aperture of the set of optical fibers 188-196. The first collimating transfer optics 168-176 act both to focus the downward traveling input beams on the encoder disk and to capture the beams reflecting from the surface of the encoder disk. The focal spot size on the encoder disk must be chosen for tolerance to fluctuations in the position of the encoder disk with respect to the read head disk and lateral tolerance due to circular shifts resulting from wobble. The focussing function could also be accomplished by miniature binary optic lenses, by "lensing" the ends of rods, or by using gradient index (GRIN) lenses of the correct pitch to focus light on the encoder disk and collect the reflected signals. The latter approach is more forgiving of alignment tolerances for collecting the light reflected from the encoder disk and provides for communication of an expanded collimated beam between the read head disk and any such GRIN lenses in the first collimating transfer optics. During angular displacement detection, the encoder shaft 152 turns along with the machinery to which it is connected, also rotating the encoder disk 102. Reflections from the encoder disk occur in the manner as described above for the dark bits, bright bits, and analog marks on the encoder disk. Since the read head disk 154 is slidably engaged with the encoder disk, any wobbling of the encoder disk will cause congruent wobbling of the read head disk. Even if congruent wobbling occurs, the close contact of the read head disk against the encoder disk ensures uniformity of the gap between the encoder disk and the first collimating transfer optics 168-176. The uniformity of this gap ensures that the first collimating transfer optics will retain its focus upon each respective track of the encoder disk. Light reflected from each of the tracks on the surface of the encoder disk is transmitted into the associated element of the first collimating transfer optics and thence to the second collimating transfer optics 178-186. Light from the second collimating transfer optics is impedance matched into the optical fibers 188-196. Again, the information from each track is kept segregated from the information in the other tracks, as each track of information is guided into an associated single optical fiber. Light propagated through the optical fibers is routed from the second to the third ports of the couplers 198-206 and into the detectors 212-220, where the optical signals are converted to digital electrical signals. The digital electrical signals are then directed into the signal processing and decoding device 232 for further treatment. The signal processing and decoding device is not required to be an unusually high speed device. If, for example, a maximum rotation speed of 2,500 revolutions per minute, or 42 revolutions per second, is assumed, the maximum bit sampling rate needed to read the analog tracks 110-116 at the lowest Nyquist rate would be: 2 * 42 rev / sec * 10 bits / sector * 1000 sectors = 840 kbits / sec This data rate represents a very conservative clock rate for commercially available computing systems. Packing the bits of the digital track 104 into a portion of the digital code segment, rather than evenly distributing them throughout each angular sector, to spatially isolate words would slightly increase the required read rate to the order of about one MHz. It is to be understood that the individual optical fibers 188-196 and individual fiber optic couplers 198-206 can be replaced by a single fiber used in conjunction with a series of wavelength division multiplex couplers. The use of wavelength division multiplex couplers would require that each track of the encoder disk operate on a separate wavelength. This would complicate the design somewhat, but would facilitate the use of a single fiber in place of multiple optical fibers. The preferred embodiments of this invention have been illustrated and described above. Modifications and additional embodiments, however, will undoubtedly be apparent to those skilled in the art. Changes in the materials, orientation, method of manufacture, and other characteristics and design parameters, for example, are contemplated to be within the scope of the claimed invention. Furthermore, equivalent elements may be substituted for those illustrated and described herein, parts or connections might be reversed or otherwise interchanged, and certain features of the invention may be utilized independently of other features. The digital track encoding technique, for example, can also be applied where different numbers of analog tracks are employed. Of special interest are the cases of two tracks, where the method is slightly modified, and three tracks, where conventional quadrature data can be concurrently implemented. There are cost savings to be had by minimizing the number of tracks, while performance is improved by increasing the number of tracks. In the three track implementation, the two inner tracks are conventional, with a standard 90 degree quadrature. The outer track in such an embodiment can use base 3 numbering, implemented identically to that described in the five track case but with a packing density higher than that which could be achieved with a binary or grey-code scheme. The use of two tracks represents substantial cost reduction. In this case, the encoding scheme reduces to base 2 and there is no start symbol. In this implementation, however, no quadrature is available to determine direction. A slight modification of this scheme can provide adequate performance. The start symbol can be encoded as a track in phase with the incremental track. All other bits are then encoded at a 90 degree phase shift with respect to the incremental track. This design will allow the direction in which the encoder disk is rotating to be determined on any digit but the start symbol, while only two bits are lost in the data track. Consequently, the exemplary embodiments should be considered illustrative, rather than inclusive, while the appended claims are more indicative of the full scope of the invention.

Claims: An optical encoder for sensing an angular position comprising: a rotatable optical medium (102) having a major surface and an axis of rotation (106) perpendicular to the surface, a plurality of uniformly sized angular sectors (108) being defined on the surface; an annular digital track (104) defined on the surface concentric with the axis; a plurality of digital bits (120) formed on the surface in sequence along the digital track within each sector, each such bit being optically readable as a binary 1 or a binary 0 and each such plurality of bits defining a binary word, including a start symbol to indicate a starting point for the binary word; N annular analog tracks (110) defined on the surface concentric with the axis, where N is an integer equal to or greater than 2; and N series of optically detectable analog marks (142) formed on the surface, the marks in each series being evenly spaced around the corresponding analog track, the number of marks in each series being equal and being displaced in a circumferential direction with respect to corresponding marks in a neighbouring series by an angular amount equal to a fraction of the spacing of the marks in said neighbouring series; whereby each of the digits of each binary word is aligned with the marks in one of the analog tracks to indicate the value represented by the digits, and the binary word in a sector can be optically read to identify the angular position of that sector, and thereby indicate the approximate angular position of the medium, and whereby the N series of analog marks can be optically detected to refine the approximate angular position indication and thereby indicate a more precise angular position of the medium. A method for sensing angular position, comprising the steps of: providing a rotatable optical medium (102) having a major surface and an axis of rotation (106) perpendicular to the surface; defining a plurality of uniformly sized angular sectors (108) on the surface; defining an annular digital track (104) defined on the surface concentric with the axis; forming a plurality of digital bits (120) on the surface in sequence along the digital track within each sector, each such bit being optically readable as a binary 1 or a binary 0, each such plurality of bits defining a binary word, including a start symbol to indicate a starting point for the binary word; defining N annular analog tracks (110) on the surface concentric with the axis, where N is an integer equal to or greater than 2; forming N series of optically detectable analog marks (142) on the surface, the marks in each series being evenly spaced around the corresponding analog track, the number of marks in each series being equal and being displaced in a circumferential direction with respect to corresponding marks in a neighbouring series by an angular amount equal to a fraction of the spacing of the marks in said neighbouring series; optically reading the binary word in a sector to identify the angular position of that sector, and thereby indicate the approximate angular position of the medium, where each of the digits of each binary word is aligned with the marks in one of the analog tracks to indicate the value represented by the digits; and optically detecting the N series of analog marks to refine the approximate angular position indication and thereby indicate a more precise angular position of the medium.

Inventors: BECCUE STEVE, KHOSHNEVISAN MOHSEN, KOLLOFF BRUCE G, RODE JONATHAN P, SCHOENWALD JEFFREY S, SMITH RAYMOND J D, BECCUE, STEVE, KHOSHNEVISAN, MOHSEN, KOLLOFF, BRUCE G., RODE, JONATHAN P., SCHOENWALD, JEFFREY S., SMITH, RAYMOND J.D.
Applicants: ROCKWELL INTERNATIONAL CORP, ROCKWELL INTERNATIONAL CORPORATION
Application year: 1993
Main classification: G01D5
IPC codes: G01D5, G11B19