WO1995005707A1

WO1995005707A1 - Absolute encoder using multiphase analog signals

Info

Publication number: WO1995005707A1
Application number: PCT/US1993/007586
Authority: WO
Inventors: Carl P. Spaulding
Original assignee: Trj & Company
Priority date: 1991-06-06
Filing date: 1993-08-13
Publication date: 1995-02-23

Abstract

An absolute decoder comprises a measuring scale (1) with at least one track and a plurality of analog sensors (47, 48) for such track. The output of each sensor is modulated by the track to generate a plurality of cyclic non-sinusoidal multiphase analog signals from the sensors indicative of the relative position of the sensors and the measuring scale. The multiphase signals are converted to digital form (50, 51) and conditionally added or subtracted (58) to obtain an output that increases linearly in proportion to the position of the sensors relative to the measuring scale.

Description

ABSOLUTE ENCODER USING MULTIPHASE ANALOG SIGNALS

Field of the Invention

This invention relates in general to absolute encoders, and more particularly to an absolute encoder having multiphase analog outputs used in combination with digital circuits that linearize and correct the multiphase analog outputs. Background of the Invention

Absolute encoders are knowr. for providing an output indication of the position of a sensing head relative to a measuring scale. For sensing rotary displacement, the scale is in the form of a disc with one or more tracks with one or more sensors per track. For sensing linear displacement, the scale is an elongated member containing one or more linearly arranged parallel tracks with one or more sensors per track. The tracks are often formed of optically responsive segments, which segments are light transmissive or light reflective, but the tracks can alternatively be of other forms such as magnetic, capacitive or inductive. Higher resolution is achieved by increasing the number of tracks.

Summary of the Invention

The present invention employs two or more sensors per track. The output of each sensor is modulated by the track to generate a multiphase analog signal from each track. An analog to digital converter (ADC) converts each analog signal to a digital value. A novel method is employed to combine the multiphase cyclic digital values into a single linear output per track. Other novel methods are employed to correct, (1) zero drift in the analog signals, (2) changes in amplitude (gain) of the analog signals, (3) deviations in the linear values from a perfect straight line. These correction methods are optional and can be employed in any combination. The invention applies to one, two or more tracks per measuring scale and the invention includes novel methods of combining the signals from each track to obtain a single output that increases linearly from zero to a maximum over the full span of the measuring scale. When measuring rotary displacement the methods disclosed may be used to combine the outputs of geared scales.

The methods used to linearize and correct the digital values are illustrated and described in terms of hard wired circuits. It is clear, however, that the methods described may be executed conveniently using any general purpose microcontroller or ngital signal processor integrated circuit.

Some features of the invention A. Clipped multiphase analog signals with 1 cycle per revolution.

1. Use periphery of disc to get 1 cycle per revolution.

2. Correcting for zero drift. 3. Correcting for changes in amplitude.

4. Converting multiphase signals to single phase linear signals.

5. An example of a two phase output that is practical approximation of the requirements of this invention.

6. Two examples of three phase signals that meet the requirements of this invention. B. Correcting for non-linearity

1. Repeatability 2. Interpolation

3. Simplified correction me ods

C. An encoder that combines 1 cycle/rev. and N cycles/rev. to increase the resolution of the encoder N times. 1. Use the periphery of the disc to get 1 cycle/rev. and a circular row of N slots to get

N cycles/rev.

2. For example, combine A32, B32, Al, and Bl to obtain linear output that repeats once per revolution with 32 times the resolution.

D. An encoder that combines N cycles/rev. and N-l cycles/rev. to increase the resolution of the encoder N times.

1. Use a circular row of N slots and a second circular row of N-l (or N-+- 1) slots to get the absolute value of N cycles/rev.

2. For example, combine A50, B50, A49 and B49 to obtain a linear output that repeats once per revolution with 50 times the resolution of the 50 slot track.

LIST OF FIGURES

I. Disc and slits to generate a 2 phase analog signal with one cycle per revolution. (Model 10) 2A, B, C. Mechanical design of d e Model 10

3. Graph of the 2 phase analog signals from the Model 10. 4. Graph of the absolute value of the slope (DY/DX) of the 2 phase analog signals.

5. Comparison of the actual to the ideal graph of the slope.

6. Graph of the sum of the slopes of FIG. 4 and of d e error resulting from the non-ideal output. 7A. Block diagram of a method to combine the digitized values of the 2 phase signals of FIG. 3 to obtain a single phase linear output. 7B. FIG. 7A plus a method to correct the amplitude of the analog signals.

7C. FIG. 7B plus a method to correct for zero drift in the analog signals. 7D. An alternate method to correct the amplitude of the analog signals using an analog multiplier.

8. Disc and slits to generate a 3 phase analog signal with one cycle per revolution.

9. Graph of the 3 phase analog signals obtained from the disc and slits shown in FIG. 8. 10. One phase of the signal from FIG. 9 and the absolute value of the slope (DY/DX) the signal.

I I . Graph of the absolute value of the slopes of the 3 phase signals from FIG. 9.

12. Block diagram of a method to correct the amplitude of a 3 phase signal and to combine die signals to obtain a single phase linear output.

13. Graph of one phase of an alternate 3 phase analog signal and a graph of the absolute value of the slope of the alternate signal.

14. Graph of one cycle of an alternate 3 phase signal.

15. Graph of the absolute value of the slopes of the 3 phase signal from FIG. 14. 16A. A method of correcting the linear output using interpolation.

16B. A simplified method of correcting the linear output. 16C. Another simplified method of correcting the linear output. 17. The disc and slits of FIG. 1 with the addition of a circular row of slots and the associated slits.

18. Block diagram of a method to combine the digitized values of the analog signals obtained from the disc of FIG. 17 to obtain a single phase output with increased resolution.

19. A disc with 49 slots in the outer row and 50 slots in die inner row with the slits required to obtain a 2 phase output from each row of slots.

20. Block diagram of a method to combine die digitized values of the analog signals obtained from the disc of FIG. 19 to obtain a single phase output.

A. ENCODERS USING CLIPPED MULTIPHASE ANALOG SIGNALS WITH 1 CYCLE PER REVOLUTION

1. AN ENCODER THAT USES THE PERIPHERY OF A DISC TO GET A TWO PHASE ANALOG SIGNAL WITH 1 CYCLE PER REVOLUTION.

FIG. 1 shows a measuring scale designed to measure rotary position over a range of one revolution. The measuring scale is an opaque disc 1 with a single track formed by e periphery of the disc. There is a center hole in the disc which is used to mount the disc on a rotating shaft. All angles and radii described in diis example are measured from the center of this hole. This example employs two optical sensors that generate a two phase analog signal. The two source-sensor pairs are spaced 90 degrees apart at the periphery of the disc. The optical padi between the source and die sensor is limited by a pair of identical slit plates, one on each side of the disc, diat confine the light path to area of slits 46 and 47.

The disc 1 has a maximum radius diat is constant over the angle from 345° to 0° to 15°. The radius is a minimum over the angle from 165° to 180° to 195°. From 15° to 165° the periphery of the disc forms a spiral in which the radius (R) is reduced a constant rate with respect to the angle (0). In this example DR/DO is .00012 inch per degree. This has the result that the maximum radius is 0.18 inches greater than the minimum radius. The periphery of the disc between 195 degrees and

345 degrees is also a spiral with the radius increasing at the rate of .00012 inches per degree.

In FIG. 1, the disc is shown in the angular position where slit 48 is totally exposed and slit 47 is one-half eclipsed by die disc. I have selected the output of slit 48 to be phase A and d e output from slit 47 to be phase B. I have arbitrarily selected diis position as the zero reference position of the disc. I have also arbitrarily selected counter-clockwise rotation of die disc in d is view to produce an increasing linear output from the signal e -nditioning circuits. In other words, the signal conditioning circuits will be designed to producr ^* output that is zero and increasing when the output from phase A is a maximum and when the outp_^_ from phase B is half-way between maximum and minimum and is decreasing. The maximum radius of slits 47 and 48 is equal to die minimum radius of die disc. The maximum radius of slits 47 and 48 is equal to the maximum radius of the disc. The sides of die disc are parallel to a line through the center of the disc and die center of the slit. The area of the slit must be less than the active area of the light source and light sensor. The width is usually equal to or less than the difference between d e maximum and minimum radius of the slit. In diis example, die widdi is 0.18 inches and is equal to die difference between die maximum and minimum radius of the slit. To summarize, FIG. 1 shows a rotary optical encoder that produces two phase analog output with one cycle corresponding to one revolution of the disc.

FIGs. 2A, 2B, and 2C show a mechanical assembly of the disc 1, the two slit plates 31 and 33, the light emitting diodes 24 and 27 (the sources), and the phototransistors 36 and 43 (the sensors). The center hole of the disc locates the disc on shaft 26. The disc is fixed in position by a cylindrical keeper 22 that is cemented both to the shaft and to die disc. The shaft rotates in bearings which in this example are simply holes drilled in the source board 29 and die sensor board 35. The axial location of the shaft is determined by tiirust bearings 25 and 42 which are cemented to die shaft. Holes in the split plates 31 and 33 locate on pins 23 and 28 which in turn are a press fit in holes drilled in die source and sensor boards. The axial location of the slit plates is determined by spacers

30, 32, 34, 44, 45 and 46.

The sources are located by holes drilled in die source board and are soldered to etched foil conductors which in turn are soldered to wires 37 and 41 which supply electrical current to the sources. In this example, the sources are connected in series by a feed-thru connection 49 and die same current flows through both sources. The sensors are located by holes drilled in die sensor board and are soldered to etched foil conductors which in turn are soldered to wires 38, 39 and 40. Wire 40 is connected to a source of positive voltage, typically 5 to 25 volts. Wires 38 and 40 carry the output current from sensors 36 and 43. The ouφut analog current is proportional to the amount of light arriving at the sensors. The geometric relationship of the sources, the sensors, the slit plates, and die disc shown in

FIGS. 2 A, 2B and 2C has die result that illumination of die disc falls to zero at the edges of die shadow of die slits and rises to a maximum at die center of the illuminated area. As a consequence the change in the amount of light arriving at the sensor (DY) for a change in the illuminated area of the slit is small near the edge of die slit and is a maximum near the center of the slit. In paragraph 5, 1 discuss how this effect is used to improve the linearity of the encoder output.

2. COMPENSATING FOR ZERO DRIFT

To correct zero drift in die analog signals, I first convert the analog signals to digital values and store the minimum digital value of each analog signal. I have designed the measuring scale so that each of die two phase signals remain at the minimum value over 30 degrees of d e input. This permits me to store in fixed computer memory, a range of digital values of phase B diat identifies when phase A is at a minimum. In diis example, if phase B is between 37.5% and 62.5% of full range, then phase A is a minimum or a maximum. If phase A is greater than one-half its range, then it is stored as a maximum and if it is less than one-half its range, then it is stored as a minimum. Similarly, phase A is between 37.5% and 62.5% of full range when phase B is a minimum or a maximum. To allow for variations that will occur during the manufacturing process the range is reduced, for example to 40% to 60%.

To correct for zero drift, I subtract the stored minimum value from all subsequent digital values of the related analog signal.

3. COMPENSATING FOR CHANGES IN AMPLITUDE To correct for changes in the amplitude of die analog signals, I use the same procedure to store the maximum digital value of each analog rgnal. For each analog input, the stored maximum value is used to compute a gain factor. The numerical value of the gain factor is the desired maximum value divided by the difference of die stored maximum value and die stored minimum value.

The corrected digital output of each phase is called die normalized value. The normalized value of any phase is equal to die gain factor of that phase multiplied by die difference of current value of d e output of the analog to digital converter and die stored minimum value of diat phase.

4. CONVERTING MULTIPHASE SIGNALS TO SINGLE PHASE LINEAR SIGNALS

I have also designed die measuring scale to shape the multiphase signals so mat diey can be combined to form a single phase signal whose output increases linearly in proportion to the mechanical motion of the measuring scale. A characteristic of a linear output is that the ratio of die incremental output DY to the incremental input DX is a constant. I have discovered diat I can simplify the computation of the single phase linear output if die sum of absolute values of the ratio DY/DX for each phase of the multiphase signals is a constant.

The method used to combine die normalized multiphase signals to obtain a linear single phase output also requires that at the minimum and maximum values of die multiphase signals die ratio

DY/DX must be zero over a definite range of values of the input X. As described in the previous paragraphs, this feature is also required for the operation of the circuits used to correct for zero drift and to correct for changes in signal amplitude.

It is possible to devise many different multiphase analog signals that meet d ese two requirements.

I. that the value DY/DX be zero for at least 1/25 of a cycle at the maximum and minimum values of Y for each of the multiphase signals, and

II. that the absolute values of DY/DX for each phase when added togedier will equal a constant

5. ANEXAMPLEOFTWOPHASEENCODEROUTPUT

FIGURE 3 is an example of a set of two phase signals that meet requirement I and closely approximate requirement II. The slope DY/DX is zero over 30 degrees at bodi d e minimum and maximum values of d e signals (requirement I). The absolute value of die slopes DY/DX of phase A and phase B is shown in FIGURE 4. The shape of these curves reflects the non-uniform illumination of the disc discussed in paragraph 1. The sum of these slopes is not exactly constant, but I will show first that the error is small and secondly I will show a mediod to correct the result and eliminate the error. FIGURE 5 shows both the ideal values and the practical values of DY/DX for phase A. The curved line shows die slope obtained from die optical encoders described in paragraph 1. The ideal slope is constant at diat maximum value of die slope over 30 degrees of the input as shown by the second curve in Figure 5. I will demonstrate by diis example that small deviations from the ideal waveshape product acceptable errors in the linearized output. This disclosure will also describe methods to remove diese and od er errors from the final output.

The upper curve in FIGURE 6 shows the sum of d e slopes from Figure 4. Requirement II states that diis sum should be a constant. In this example, the sum varies from a constant by about 10%. The lower curve shows die resultant error in the linearized output obtained from the signals shown in Figure 3. The error is about + or - 1 part per 1000 (0.1 %) of the maximum linear output. This error is small compared to other errors in a typical encoder and is acceptable. This error and otiier encoder errors are repeatable and can be corrected by d e metiiods described in d is disclosure. FIGURE 7A is a block diagram of a method of combining die normalized values of a two phase encoder to obtain an output KLIN whose amplitude is proportional to the mechanical displacement of die input X. FIGURE 7B is an expansion of Figure 7A with the addition of d e blocks required to correct the amplitude of the analog signals. FIGURE 7C is an expansion of Figure 7B with die addition of die blocks required to correct for zero drift in the analog signals. FIGURE 7D is a block diagram of an alternate mediod to correct the amplitude of d e analog signal. The metiiod shown in

4D uses the analog to digital converter as an analog multiplier as a substitute for the digital multiplier shown in Figure 11B. In some applications, die mediod shown in Figure 7D may increase the speed and/or reduce d e cost of die signal processing circuits.

6. TWO EXAMPLES OF THREE PHASE SIGNALS THAT MEET THESE REQUIREMENTS

FIGURE 8 shows a disc similar to Figure 1 modified to produce a diree phase output. The disc has a maximum radius that is constant over die angle from 330 degrees to 0 degrees to 30 degrees. The radius is a minimum over the angle from 150 degrees to 180 degrees to 210 degrees. From 30 degrees to 150 degrees die periphery of the disc forms a spiral in which the radius (R) is reduced a constant rate with respect to the angle (O). In this example DR/DO is .00015 inch per degree. This has the result that the maximum radius is .018 inches greater than die minimum radius. The periphery of the disc between 210 degrees and 330 degrees is also a spiral with the radius increasing at the rate of .00015 inches per degree.

In Figure 8, the disc is shown in die angular position where slit 98 is totally exposed. Slits 97 and 99 are one-fourth eclipsed by die disc. Because of d e non-uniform illumination discussed in paragraph 1, the normalized output at diis position is approximately 7/8 of die maximum. I have selected the output of slit 98 to be phase A, the output from slit 97 to be phase B, and the output from slit 99 to be phase C. I have arbitrarily selected diis position as die zero reference position of die disc. I have also arbitrarily selected counter-clockwise rotation of die disc in diis view to produce an increasing linear output from the signal conditioning circuits. In other words, d e signal conditioning circuits are designed to produce an output that is zero and increasing when the output from phase A is a maximum and when die normalized output from phase B is about 7/8 of die maximum and is decreasing. At zero, die normalized output from phase C is equal to phase B and is increasing. The minimum radius of slits 97, 98 and 99 is equal to die minimum radius of die disc. The maximum radius of slits 97, 98, and 99 is equal to die maximum radius of d e disc. The sides of the disc are parallel to a line through the center of the disc and die center of the slit. The area of the slit must be less than the active area of the light source and light sensor. The width is usually equal to or less dian die difference between the maximum and minimum radius of die slit. In diis example, die widdi is 0.012 inches and is less dian die difference between the maximum and minimum radius of the slit.

To summarize, Figure 8 shows a rotary optical encoder disc and slit layout diat produces a diree phase analog output with one cycle corresponding to one revolution of the disc. One example of a suitable diree phase signal is shown in FIGURE 9. In this example, full scale input (X) is 360 degrees. The maximum value of die normalized output (Y) is 10,000 and die minimum value is zero.

At the maximum and minimum values, die output is constant and DY/DX is zero over a range of 60 degrees of die input.

FIGURE 10 shows one phase of the three phase signal together with the absolute value of die slope. The encoder is designed so diat die signal Y has a slope DY/DX diat rises linearly from zero to a maximum slope that occurs when die output Y is one-half die maximum. The slope the decreases linearly so that die slope again res "hes zero when the output Y reaches the maximum. An encoder similar to diat shown in Figures 2A, 2B and 2C but widi diree source sensor pairs and widi a disc as shown in Figure 8 produces an output diat closely approximates the shape shown in Figure 10. It is possible to produce similar outputs using magnetic, capacitive, or inductive transducers. FIGURE 11 shows the absolute values of DY/DX for each of die diree phase signals shown in Figure 9. From inspection of Figure 11 it can be seen diat for any value of the input X, die sum of die diree values of DY/DX is a constant value which in this example is 10,000.

FIGURE 12 is a block diagram of die mediod used to combine die signals from a three phase encoder first to correct for changes in signal amplitude and second to obtain a linear single phase output whose amplitude is proportional to the mechanical displacement of die input.

The values A', B' and C are digital values corresponding to the analog inputs A¹, B¹ and CA The values Ap, Bp, and Cp are the maximum values of A', B' and C\ The values Am, Bm, and Cm are the desired maximum values of die normalized outputs A, B and C. The values Am, Bm, and Cm, are stored in computer memory. The normalized value A is obtained by multiplying A' by die ratio Am/Ap. B and C are computed in a similar fashion. The peak value Ap is obtained by sorting the value of A' when A is greater dian Am/2 and when B and C are nearly equal (i.e., when die absolute values of die difference B-C is less dian k, where k is a number stored in computer memory. In this example, k might be chosen as 1000) Bp and Cp are stored in a similar fashion.

The linear output, KLIN, is obtained by adding or subtracting die normalized values A, B and C using die rules listed in Figure 12.

The analog signals shown in Figures 9, 10 and 11 are obtained when die encoder is designed to produce a signal such diat DY/DX has a triangular wave shape as shown in Figure 11. I have chosen this example because the optical encoders described in diis disclosure produce an output diat closely approximates diis example. Many odier waveshapes are suitable, it is only necessary (as stated above) diat die absolute values of DY/DX summed over all phases closely approximate a constant value. The closer the approximation, the more accurate the encoder.

To illustrate that odier signal wave shapes produce similar results, I have shown in Figures 13, 14 and 15 the signals from a 3 phase encoder in which DY/DX has the value 1-Cosine 3X over a 120 degree sector and has a value zero of the adjacent 60 degree sector. FIGURE 14, shows three such signals spaced at 120 degree intervals to form a three phase output. A three phase signal can also be formed using 60 degree intervals, this is equivalent to inverting phase A, i.e., A inverted = Am - A.

FIGURE 13 shows the relationship of the output Y to the slope DY/DX for phase A of figure 14. For purposes of illustration, DY/DX is 10(l-Cosine 3X). FIGURE 15 shows the absolute value of DY/DX for all diree phases. It can be seen from inspection, diat in diis example, die sum of diese three signals is 20 for any value of X.

In summary, Figures 9, 10 and 11 show a set of diree phase encoder signals that can be closely approximated by die optical encoder designs described in diis disclosure. Figures 13, 14 and 15 show an alternate set of three phase encoder signals that may be more easily created using magnetic, capacitive or inductive transducers. Beyond diese two examples, other waveshapes may be chosen to fit other requirements.

Either set of diree phase signals can be normalized and linearized using die mediod shown in Figure 12.

B. CORRECTING FOR NON LINEARITY 1. REPEATABILITY

Figure 6 shows one source of error in the encoder output KLIN. Similar errors result from mechanical imperfections in the parts or the assembly of the encoder.

Typically these errors are independent of changes in the operating environment and are constant over the life of die encoder. For this reason, it is practical, as a part of the manufacturing process, to store calibration data for the encoder. If the encoder includes a microprocessor, the calibration data are loaded directly to non-volatile memory. If not, the calibration data are shipped separately, for example, in a floppy disc to be loaded by die encoder user into his signal processing equipment.

2. INTERPOLATION

FIGURE 16A is a block diagram of a mediod to correct the output KLIN. This method uses interpolation to reduce die amount of calibration data diat must be stored in die computer memory.

The method separates KLIN into the most significant digits (KLA) and die least significant digits (KLB). For example, if KLIN ranges from 0000 to 9999, KLA has 100 possible values from 0000 to 9900. KLB also has 100 possible values from 00 to 99. The correction table stores a correction term (OFn) for each value of KLAn. To speed die computation, a second lookup table may be used to store the value DDOF = OFn+ 1-OFn. Alternately, DDOF may be computed each time from die contents of the OFn lookup table.

The corrected output (COR) is computed as follows:

COR = KLN - (OF + (KLB * DDOF))

3. SIMPLIFIED CORRECTION METHODS FIGURE 16B omits the second lookup table, the multiplier and the adder. This method is used when die encoder errors are small and DDOF is not greater than plus or minus one.

FIGURE 16C shows a method diat may be used for low resolution encoders. This method is fast but requires a larger lookup table and it reduces die useful resolution by a factor of two. The example in Figure 16C employs 9 bits to represent KLIN1 and provides an 8 bit output to represent COR1. C. AN ENCODER THAT COMBINES i YCLE/REV. AND N CYCLES/REV. TO INCREASE THE RESOLUTION OF THE ENCODER N TIMES.

1. USE THE PERIPHERY OF THE DISC TO GET A CYCLE/REV. AND A

CIRCULAR ROW OF N SLOTS TO GET N CYCLES/REV. Figure 17 shows an encoder disc diat combines the rotary disc shown in Figure 1 with a second track formed by row of 32 slots inside die minimum radius of die disc. The edges of die slots are formed by 64 equally spaced radial lines, i.e., die edges are spaced at 5.625 degree intervals. These slots modulate die light between two source-sensor pairs that are aligned wid slits 125 and 126 to produce a two phase analor ignal with 32 cycles per revolution of the disc. The maximum radius of die slit is slightly less th n the maximum radius of die slot. The minimum radius of the slit is slightly greater dian die minimum radius of the slot. The edges of the slit are radial lines and the angular widdi of die slit is 5/6 the angular width of die slot. In this example, the angular widdi of the slit is 4.6875 degrees.

These dimensions product a cyclic analog signal widi a maximum and minimum value mat is constant over an angle equal to 1/6 of the angular widdi of die slot or .9375 degrees. This angle multiplied by 32 is 30 degrees. This results in an analog signal diat has the same shape as die two phase signals shown in Figure 3, but repeated 32 times per revolution.

Phase A32 is generated by a source-sensor pair aligned widi slit 125. Slit 125 is shown located in the center of one of die slots. This position of die disc has been arbitrarily selected as the zero position for the X32 output. In this view, positive rotation is counter-clockwise. To be consistent with paragraph Al, the center of slit 126 which generates phase B32 must be aligned widi die clockwise edge of one of the odier slots. For the best accuracy it is preferable to select the nearest slot consistent with the physical size of the sources and sensors.

The source-sensor pairs aligned widi slit 127 generate phase Al and die source-sensor pairs aligned widi slit 128 generate phase B 1 of the one cycle per revolution signal. Slits 127 and 128 have an angular spacing of 90 degrees. At the position of die disc shown in Figure 17, phase Al output is about equal to die phase Bl output and referring to Figure 3, diese values correspond to die disc having a rotary position of about 135 degrees widi respect to the zero position defined in paragraph 1. 2. FOR EXAMPLE, COMBINE A32, B32, Al AND Bl TO OBTAIN LINEAR

OUTPUT THAT REPEATS ONCE PER REVOLUTION WITH 32 TIMES THE

RESOLUTION.

FIGURE 18 uses the mediod shown in Figure 7B to obtain KLIN1 from phase Al and phase

Bl and to obtain KLIN32 from phase A32 and phase B32. KLIN1 is corrected to obtain COR 1 and KLIN32 is corrected to obtain COR32 using die method shown in Figure 16A or 16B or 16C as required by the size of the error to be corrected.

CORl is multiplied by 32 and die result is separated into die most significant digits 32COR1A

(which have a range of values from 0 to 31) and the least significant digits 32COR1B (which have die same range of values as COR32). In this example, CORl and COR32 have a range of values of 0 to 9999. The range of values selected for convenience in computing and is limited by die resolution of the ADC and by die requirements of the application. Usually die maximum value is 1 less an some power of 2, for example 511, 1023 or 4095.

32COR1B is subtracted from COR32 to obtain DIF. If DIF is negative, 32COR1A is increased by 1. If the result is 32, substitute 0. The result identifies which of die 32 slots generated phase A32 and phase B32. In diis example, die result is die two most significant decimal digits of a six decimal digit number NX identifying die position of the disc.

The value NX was computed using die value COR32 which is die corrected value of KLIN32. The correction using Table 1 is complete only if all 32 slots are identical. In general, the slots are not identical and a further improvement in accuracy may be obtained using a diird lookup table that employs the most significant digits of NX as a table address. The example in Figure 18 uses die diree most significant decimal digits of NX, die values 0 to 319, to locate die correction terms OF and DDOF. The corrected output N equals NX - (OF + DDOF*NXB). In diis example, NXB is me three least significant decimal digits of NX. D. AN ENCODER THAT COMBINES N CYCLES/REV. AND N-l CYCLES/REV. TO INCREASE THE RESOLUTION OF THE ENCODER N TIMES.

1. USE A CIRCULAR TRACK OF N SLOTS AND A SECOND CIRCULAR TRACK

OF N-l (OR N+ 1) SLOTS TO GET THE ABSOLUTE VALUE OF N CYCLES/REV.

Figure 19 shows an encoder disc widi a circular periphery, with a first track formed by a row of 49 slots inside die minimum radius of die disc and widi a second track formed by a row of 50 slots inside die row of 49 slots. The edges of die outer row of slots are formed by 98 equally spaced radial lines, and die edges of d e inner row of slots are formed by 100 equally spaced lines, i.e., die edges of die inner rows are spaced at 3.6 degree intervals. The inner row of slots modulate die light between two source-sensor pairs that are aligned widi slits A50 and B50 to produce a two phase analog signal with 50 cycles per revolution of the disc. The outer row of slots modulate die light between two additional source-sensor pairs that are aligned widi slits A49 and B49 to produce a two phase analog signal widi 49 cycles per revolution. The maximum radius of each slit is slightly less dian die maximum radius of die associated row of slots. The minimum radius of each slit is slightly greater than die minimum radius of die associated row of slots. The edges of die slits are radial lines and die angular widdi of the slits is 5/6 the angular width of the associated slot. In this example, the angular widdi of die slits A50 and B50 is 3.0 degrees.

For die inner track, these dimensions produce a cyclic analog signal widi a maximum and minimum value diat is constant over an angle equal to 1/6 of the angular widdi of die slot or .6 degrees. This angle multiplied by 50 is 30 degrees. This results in an analog signal diat has die same shape as die two phase signals shown in Figure 3, but repeated 50 times per revolution. For the outer track, die angular dimensions are increased by die ratio of 50/49. The result is a two phase signal of die same shape but repeated 49 times per revolution.

Phase A50 is generated by a source-sensor pair aligned widi slit A50. Slit A50 is shown located in die center of one of the inner row of slots. This position of the disc has been arbitrarily selected as die zero position for the X50 output. In diis view, positive rotation is counter-clockwise. To be consistent with paragraph A 1 , the center of slit B50 which generates phase B50 must be aligned with the clockwise edge of one of die odier slots. For the best accuracy, it is preferable to select the nearest slot consistent with die physical size of die sources and sensors.

Phase A49 is generated by a source-sensor pair aligned widi slit A49. Slit A49 is shown located in die center of one of the outer row of slots. This position of the disc is also the zero position for the X49 output and the center of slit B49 which generates phase B49 must be aligned widi the clockwise edge of one of d e odier slots in die outer track. The source-sensor pairs aligned widi slit B49 generate phase B49 of die 49 cycle per revolution signal.

2. FOR EXAMPLE, COMBINE A50, B50, A49 AND B 49 TO OBTAIN A LINEAR

OUTPUT THAT REPEATS ONCE PER REVOLUTION WITH 50 TIMES THE RESOLUTION OF THE 50 SLOT TRACK.

FIGURE 20 uses die method shown in Figure 7B to obtain KLIN49 from phase A49 and phase B49 and to obtain KLIN50 from phase A50 and phase B50. KLIN49 is corrected to obtain COR49 and KLIN50 is corrected to obtain COR50 using die mediod shown in Figure 16A or 16B as required by die size of the error to be corrected. To obtain CORl, COR50 is subtracted from COR49. If the result is negative, a constant

(10,000 in this example) is added such that CORl is always positive.

CORl is multiplied by 50 and die result is separated into die most significant digits 50COR1A (which have a range of values from 0 to 49) and the least significant digits 50COR1B (which have die same rage of values as COR50). In this example, COR49 and COR50 have a ran^*r. of values of 0 to 9999. The range of values is selected for convenience in computing and is Anited by the resolution of the ADC and by die requirements of the application. Usually die maximum value is 1 less an some power of 2, for example 511, 1023 or 4095.

50COR1B is subtracted from COR50 to obtain DIF. If DIF is negative, 50COR1A is increased by 1. If die result is 50, substitute 0. The result identifies which of die 50 slots (0-49) generated phase A50 and phase B50. In diis example, the result is the two most significant decimal digits of a six decimal - Ait number NX identifying die position of die disc.

The value NX was computed using die value COR50 which is the corrected value of KLIN50. The correction using Table 1 is complete only if all 50 slots are identical. In general, the slots are not identical and a further improvement in accuracy may be obtained using a diird lookup table diat employs die most significant digits of NX as a table address. The example in Figure 18 uses the diree most significant decimal digits of NX, die values 0 to 499, to locate the correction terms OF and DDOF. The corrected output N equals NX - (OF + DDOF*NXB). In diis example, NXB is die diree least significant decimal digits of NX.

Claims

WHAT IS CLAIMED:

1. An absolute encoder comprising: a measuring scale with one or more tracks, a plurality of analog sensors per track, each sensor having an output modulated by the corresponding track to generate a plurality of cyclic nonsinusoidal multiphase analog signals from the sensors indicative of die relative position of the sensors and die measuring scale, analog to digital conversion means to produce digital values proportional to the analog signals, means to conditionally add or subtract the digital values of die analog signals to obtain a single output that increases linearly in proportion to the position of the sensors relative to the measuring scale.

2. The encoder of claim 1, in which each of the analog signals from one or more tracks has a maximum value that is constant for at least 1/25 of its cycle and has a minimum value mat is constant for at least 1/25 of its cycle.

3. The encoder of claim 1, in which die analog signals from one or more tracks are a two phase signal with phasing equal to 1/4 of its cycle and with minimum and maximum values iat are constant for about 1/12 of its cycle.

4. The encoder of claim 1 , in which the analog signals from one or more tracks are a three phase signal widi phasing equal to 1/6 of its cycle and wid minimum and maximum values diat are constant for about 1/6 of its cycle.

5. The encoder of claim 1 , in which die analog signals from one or more tracks are a three phase signal with phasing equal to 1/3 of its cycle and widi minimum and maximum values at are constant for about 1/6 of its cycle.

6. The encoder of claim 1, in which the analog signals from one or more tracks are a four phase signal widi phasing equal to 1/8 of its cycle and widi minimum and maximum values at are constant for about 1/4 of its cycle.

7. An absolute encoder comprising: a measuring scale with one or more tracks, two or more analog sensors per track, each sensor having an output modulated by die corresponding track to generate cyclic analog signals from the sensors indicative of die relative position of the sensors and die measuring scale wherein the sum of die absolute values of die slopes of all die analog signals from any one track approximates a constant, analog to digital conversion means to produce digital values proportional to the analog signals. means to conditionally add or subtract the digital values of die analog signals to obtain a single output that increases linearly in proportion to the position of die sensors relative to the measuring scale.

8. The encoder of claim 7, in which each of die analog signals from one or more tracks has a maximum value at is constant for at least 1/25 of its cycle and has a minimum value mat is constant for at least 1/25 of its cycle.

9. The encoder of claim 7, in which die analog signals from one or more tracks are a two phase signal with phasing equal to 1/4 of its cycle and widi minimum and maximum values diat are constant for about 1/12 of its cycle.

10. The encoder of claim 7, in which the analog signals from one or more tracks are a three phase signal with phasing equal to 1/6 of its cycle and widi minimum and maximum values mat are constant for about 1/6 of its cycle.

11. The encoder of claim 7, in which the analog signals from one or more tracks are a three phase signal with phasing equal to 1/3 of its cycle and widi minimum and maximum values that are constant for about 1/6 of its cycle.

12. The encoder of claim 7, in which die analog signals from one or more tracks are a four phase signal with phasing equal to 1/8 of its cycle and wid minimum and maximum values mat are constant for about 1/4 of its cycle.