WO2017043249A1 - Procédé et appareil pour déterminer une position sur une échelle - Google Patents

Procédé et appareil pour déterminer une position sur une échelle Download PDF

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Publication number
WO2017043249A1
WO2017043249A1 PCT/JP2016/073694 JP2016073694W WO2017043249A1 WO 2017043249 A1 WO2017043249 A1 WO 2017043249A1 JP 2016073694 W JP2016073694 W JP 2016073694W WO 2017043249 A1 WO2017043249 A1 WO 2017043249A1
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Prior art keywords
signal
scale
threshold
marks
crossings
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PCT/JP2016/073694
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English (en)
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Amit K. AGRAWAL
Jay Thornton
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Mitsubishi Electric Corporation
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Publication of WO2017043249A1 publication Critical patent/WO2017043249A1/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
    • G01D5/244—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing characteristics of pulses or pulse trains; generating pulses or pulse trains
    • G01D5/245—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing characteristics of pulses or pulse trains; generating pulses or pulse trains using a variable number of pulses in a train
    • G01D5/2454—Encoders incorporating incremental and absolute signals
    • G01D5/2455—Encoders incorporating incremental and absolute signals with incremental and absolute tracks on the same encoder
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D18/00—Testing or calibrating apparatus or arrangements provided for in groups G01D1/00 - G01D15/00
    • G01D18/001—Calibrating encoders
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
    • G01D5/244—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing characteristics of pulses or pulse trains; generating pulses or pulse trains
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
    • G01D5/244—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing characteristics of pulses or pulse trains; generating pulses or pulse trains
    • G01D5/24428—Error prevention
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
    • G01D5/244—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing characteristics of pulses or pulse trains; generating pulses or pulse trains
    • G01D5/24457—Failure detection
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
    • G01D5/244—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing characteristics of pulses or pulse trains; generating pulses or pulse trains
    • G01D5/24471—Error correction
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
    • G01D5/244—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing characteristics of pulses or pulse trains; generating pulses or pulse trains
    • G01D5/24471—Error correction
    • G01D5/2448—Correction of gain, threshold, offset or phase control
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/347—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells using displacement encoding scales
    • G01D5/34707—Scales; Discs, e.g. fixation, fabrication, compensation
    • G01D5/34715—Scale reading or illumination devices
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/347—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells using displacement encoding scales
    • G01D5/34776—Absolute encoders with analogue or digital scales
    • G01D5/34792—Absolute encoders with analogue or digital scales with only digital scales or both digital and incremental scales

Definitions

  • the invention generally relates to position measurement devices, and in particular to measuring positions with absolute encoders.
  • Position estimation is an important task in industrial automation, and similar applications.
  • Devices such as numerically controlled (CNC) machines, drill bits, robot arms or laser cutters, and assembly lines need position measurements.
  • Feedback control often uses precision position measurements. It is desired to determine positions at high sampling rates to enable accurate feedback control.
  • Optical encoders are typically used to measure incremental or relative positions.
  • a scale having regularly spaced marks is used along with a readhead including sensors to estimate the relative position between the marks.
  • Incremental linear encoders can only measure the relative position within a period of the scale.
  • a relative position encoder counts the number of scale periods traversed to determine the absolute position.
  • An absolute position encoder can determine the absolute position directly. Absolute position encoders are preferred because they do not require a memory and power to store the current position. In addition, absolute encoders provide absolute position at start up, while relative position encoders typically need to locate a beginning point to determine a current position at start-up, which takes time and may not be possible for some applications.
  • a unique pattern of marks representing codes of one and zero bits, is used for each position.
  • a position change is determined when the bit pattern in the sensed code changes.
  • the resolution of the position estimate is the same as that of the pattern on the scale, and may be insufficient.
  • one method uses multiple scales aligned in the detection direction with periodic scale patterns including opaque and transparent marks.
  • One scale is mounted in the read head and the other is mounted on the moving part.
  • the scales are illuminated from one side and a photodiode senses the light that passes through the two scales in series to the other side.
  • the signal on the photodiode varies between a maximum value when the transparent marks in the two scales are aligned and a minimum value when the transparent marks on one scale are aligned with the opaque marks on the other.
  • a demodulation procedure can then determine the phase ⁇ of the signal, which is transformed into the relative position estimate.
  • the relative position can be recovered at a higher resolution than the scale resolution.
  • one of the scales can be replaced by a grating inside the readhead.
  • a small number of photodiodes in the readhead of linear encoders need precise radiometric calibration of the sensed signal. Often, non-linearity in the signals results in a bias, and sub-divisional ripple errors during the phase estimation.
  • One absolute linear encoder uses one scale, and a single readhead. It has two separate mechanisms for reading incremental and absolute positions.
  • the incremental positions are obtained using a filtering readhead technique, which utilizes a grating inside the readhead for generating fringes that are sensed in a photodiode array.
  • the absolute positions are sensed using a different mechanism, which uses an imaging lens and a detector, i.e., a linear image sensor.
  • the embodiments of the invention provide a method for determining high precision position estimates for absolute single track encoders.
  • the high precision of the method can achieve absolute accuracy within a micron.
  • the high speed of the method achieves rates of several KHz using a conventional digital signal processor (DSP).
  • DSP digital signal processor
  • Fig. 1 is a schematic of a scale and readhead according to embodiments of the invention.
  • Fig. 2 is a schematic of a sensed signal and a subsequence using the scale of
  • Fig. 3 is a schematic of decoding a bit sequence to obtain a coarse position according to embodiments of the invention.
  • Fig. 4A shows an ideal relative waveform that can be processed by embodiments of the invention.
  • Fig. 4B shows an ideal absolute waveform that can be processed by embodiments of the invention.
  • Fig. 4C shows a waveform with a fundamental carrier frequency that can be processed by embodiments of the invention.
  • Fig. 4D shows a waveform without a fundamental carrier frequency that can be processed by embodiments of the invention.
  • Fig. 5 is a schematic of a threshold-crossing point detected according to embodiments of the invention.
  • Fig. 6 is a schematic of the number of bits between every two threshold- crossings.
  • Fig. 7 is a schematic for fitting line to a rising edge of the waveform according to embodiments of the invention.
  • Fig. 8 is a schematic for fitting line to a falling edge of the waveform according to embodiments of the invention.
  • the embodiments of our invention provide a method and system for determining high precision position estimates for absolute single track linear encoders.
  • Fig. 1 shows a scale 100 of an absolute encoder for one embodiment of our invention.
  • the scale can include a sequence of light reflecting 101, and non-reflecting 102 marks. Each mark is B microns long, which defines a resolution of the scale.
  • a readhead 1 10 is mounted at some distance and parallel to the scale.
  • the readhead includes a sensor 11 1, a (LED) light source 1 12, and an optional lens.
  • the sensor can be a detector array of N sensors, e.g., N is 2048.
  • the array can be complementary metal-oxide-semiconductor (CMOS) or charge coupled device (CCD).
  • CMOS complementary metal-oxide-semiconductor
  • CCD charge coupled device
  • the readhead also includes a conventional digital signal processor 115 connected to sensor.
  • the marks can alternate between opaque and transparent or between reflective and transparent, depending on a relative position of the light source with respect to the readhead.
  • Every subsequence has a finite length and is unique, e.g., a de Bruijn sequence 103.
  • a &-ary de Bruijn sequence B(k, n) of order n is a cyclic sequence of a given alphabet with size k, for which every possible subsequence of length n in the alphabet appears as a sequence of consecutive characters exactly once.
  • a de Bruijn sequence is truncated from front or back, the resulting sequence also has the uniqueness property with the same n.
  • a 50000 bit long sequence is required.
  • This sequence can be truncated from the front or back to obtain a 50000 bit sequence. It should be noted that any non-periodic sequence with non-repeating subsequences can be used with this method.
  • the detector array requires a field of view (FOV) of at least n bits for decoding to be possible.
  • FOV field of view
  • the field of view is designed to be 1-2 mm to have the desired accuracy.
  • each bit of the sequence i.e., each mark of the scale
  • maps to at least two pixels in the linear detector array. This requires at least 16x2 32 pixels, which is well- within the number of pixels in conventional sensors.
  • the number of pixels per mark can be increased.
  • the marks on the example scale are arranged linearly. Other configurations of the marks on the scale are also possible, for example circular, oval, serpentine, and the like. The only requirement is that the marks are arranged sequentially for a particular code or non-periodic sequence.
  • Fig. 2 shows a sensed one-dimensional (ID) signal S 201, and a corresponding decoded sequence 202.
  • a look-up table of length 2" can be used to determine the position decoded sequence within the entire de Bruijn sequence.
  • Fig. 3 shows a de Bruijn sequence 301, a decode sequence, a result of code matching with a look-up table, and a coarse position P A 310 corresponding to one bit in the sequence.
  • the look-up table stores all possible subsequences of the non- periodic sequence, and their distance P A from the start 300 of the scale.
  • encoding schemes such as Manchester encoding
  • the de Bruijn sequence can be designed to enable fast position decoding with a smaller look-up table.
  • the recovered resolution of the position should be substantially higher than the scale resolution of B.
  • the accuracy requirement could be 0.5 micron, 40 times smaller than B (20 microns).
  • a ID representative signal of the scale is acquired.
  • the length of a block of pixels corresponding to each black or white mark on the scale is F, where F depends, optionally, on a lens magnification.
  • a mark is B microns in length, which corresponds to F pixels.
  • the intensity (amplitude) of the reflecting (or transparent) region of scale is large, e.g. 200 for a gray scale of 255 levels for an 8 pixel sensor, and the intensity of the non-reflecting region of the scale is be small, e.g. zero on the gray scale.
  • the signal of a relative scale corresponds to a square waveform as the sensor signal is high for F pixels, and then low for F pixels, etc.
  • the sensed signal is high for some integer multiple of F, low for some integer multiple of F, and so on.
  • the integer multiple depends on the underlying absolute code.
  • the multiple is always one.
  • Fig. 4C illustrated a method that can be used to ensure that the absolute scale has a fundamental carrier frequency.
  • Each absolute code bit is expanded into 2 bits using the substitution 0- ⁇ 00 and 1 - ⁇ 01. This scale is similar to the scale used by Gribble, see U.S. 20120072169.
  • An arctangent method is one known method for position estimation using an incremental scale.
  • the arctangent method is based on estimating a phase ⁇ of the signal using a demodulation technique.
  • the sensed signal is multiplied by a sine wave and a cosine wave of the fundamental carrier frequency.
  • the result is low pass filtered and averaged.
  • the arctangent of the ratio of two values is used to determine the phase of the sensed signal.
  • the phase can be converted to the
  • the position correction can be defined using a reference distance in sensor pixels D 501 of the signal with respect to the start of the signal 502, as shown in Fig. 5.
  • the distance D is approximately the distance from the start of the signal to the first edge, but each estimated edge position has some noise, so D will eventually be estimated using all the edges in the signal as described herein.
  • the position correction P is converted to microns using the
  • the coarse position P A is obtained by matching the underlying code sequence with the known general, non-periodic sequence.
  • the coarse position can be obtained using a pre-determined look-up table.
  • a threshold m can be subtracted from S and the zero-crossings of the resulting signal (or threshold-crossings) correspond to the edges in the original scale.
  • the threshold can be pre-determined, e.g., for 128 of the gray level, or estimated from sensed signal S, e.g., an average gray value of S.
  • the threshold can be fixed, or refined along with phase and frequency.
  • the signal can be filtered before detection of threshold-crossings to reduce the effect of noise as in conventional edge-detection techniques.
  • the initial value of m is estimated from the signal S. Because the gain of the signal S is unknown, a pre-determined value, such as 128, is incorrect. Therefore, the initial value of m is selected to be an average intensity (amplitude) of the signal S
  • N is the number of samples of the signal S.
  • Pixel positions for rising edges are determined such that the signal value S is less than m for the current pixel, and greater than m for the next pixel.
  • the pixels p correspond to the rising edges of the signal.
  • a line 701 is fitted to a rising edge, and a slope a and intercept b of the line are determined.
  • the first threshold-crossing z(l) 702 is the spatial location, in terms of pixels, corresponding to the intensity of m on the line 701 is
  • threshold-crossings are determined for the falling edges by locating pixels such that the signal value is greater than m for current pixel and less than m for the next pixel.
  • a line 801 is fitted to the falling edge, and the slope a and intercept b of the line is determined.
  • the second threshold-crossing z(2) 802 is the spatial location corresponding to the intensity value of m on the line
  • z(i) denotes the z th threshold-crossing.
  • the position correction, P is dependent on D, the offset between the P A and the system of edges.
  • a joint estimation of D, F and m is performed to refine the value of these variables. This estimation uses the idea that the difference between successive threshold-crossing dz ⁇ i) is an integer multiple of F
  • k(i) are all equal to 1 because edges occur after every F pixels.
  • the values k i) depend on the non- periodic sequence, and change with every position of the readhead as shown in Fig. 6.
  • the number of bits between every two successive threshold-crossings is represented by k(i).
  • F and m, k i) are determined using the coarse value of F and threshold-crossings
  • a linear system is formed to refine D, F and m and optimize the fit of this simple model to the observed edge locations that necessarily are perturbed by various sources of noise.
  • each threshold crossing is an integer multiple of F away from the first threshold-crossing D.
  • each threshold-crossing, (i) can be written in terms of D, F and m as
  • threshold-crossing is c(i).
  • the i threshold-crossing is c(i) times F from the first threshold-crossing
  • the position correction P can be determined.
  • the sequence k(i) provides the underlying code in the current signal, and can be used to determine the absolute position P A using the look-up table of the non-periodic sequence.
  • the final position P is P A + P t .
  • the method can iterate over the steps of threshold-crossing detection, and solving the linear system.
  • the refined m can re-determine the threshold-crossings, the slopes a ⁇ i), and intercepts b(i) of the fitted lines followed by the refinement of D, F and m, and so on.
  • m can be determined by averaging high intensity pixels and low intensity pixels separately, followed by taking their averages. Any other way of determining m using the signal S is within the scope of the invention.
  • edge detection methods such as the Sobel operator, Canny operator or any other edge detection method can be used to determine the edge locations in the signal, without the need for determining m.
  • the determined edge locations can be used to refine D and F by solving a K by two linear system
  • D the offset of the edge grid from the start of the signal is defined with respect to the first threshold-crossing.
  • D can be defined with respect to any threshold-crossing.
  • the threshold-crossing nearest the center of the signal can be used to describe D and to solve the linear system.
  • the threshold-crossing used to define D could change with the each new position.
  • the plane of the scale can be rotated with respect to the readhead.
  • the signal sensed from the scale can have a uniform or non-uniform scaling factor from one end of the sensor to the other end. This scaling factor can be incorporated into the above method by appropriately compensating the determined threshold-crossings.
  • Optical distortions such as radial distortion due to the lens, cause shifting of the threshold-crossings.
  • Such distortions can be handled by a calibration step, where the estimated threshold-crossings are appropriately shifted before solving the linear system to compensate for the radial distortion.
  • Optical distortions can also be handled by augmenting the linear system to have additional parameters.
  • the equation can be augmented to have terms dependent on square of c(i)
  • a linear system with five variables (m, D, F, a ] and a 2 ) can be constructed.
  • the parameters ⁇ and 2 model the deviation of threshold- crossings from the original linear model and can handle optical distortions in the captured image. Additional parameters depend on powers of c(i) or a ⁇ i) can be added depending on the specific application.
  • Thermal expansion of the scale leads to a change in F, i.e., pixels per-bit.
  • F i.e., pixels per-bit.
  • a varying expansion across the field of view shifts the threshold-crossings according to the expansion coefficient.
  • the shift in threshold-crossings can be determined during calibration.
  • threshold-crossings can be appropriately shifted for compensation, before solving the linear system.
  • Embodiments of the invention also apply to a relative scale to obtain the position correction ,.
  • the method can be used to obtain and the coarse position P A can be obtained using other known methods, such as using a second scale or a digital register that keeps a running count of the bits as the scale 100 passes by the sensor 1 10.
  • the invention is also applicable to single-track rotary encoders. If the non- periodic de Bruijn sequence is used, then other configurations of the scale can be used, for example, a circular, serpentine, or other arbitrary shapes that conform to the positions to be determined.
  • Prior art methods are typically based on demodulation techniques, and require a reference sine and cosine signal for demodulation in relative encoders, or a reference waveform depending on an underlying code for absolute encoder as in the related application. This invention does not require generating such a reference signal.
  • Some prior art methods use a two step process.
  • the fundamental frequency is estimated.
  • the reference signals are generated using the fundamental frequency.
  • the reference signals are used for demodulation or position decoding.
  • the errors in the first step lead to frequency mismatch between the sensed signal and the reference signal. This can lead to significant phase errors. Gribble attempts to address those errors within the framework of arctangent methods.
  • This invention does not require reference signals.
  • bit width, F, and edge offset, D are estimated jointly, thus significantly reducing the position errors in estimation of Pi.
  • the invention works independently of the gain of the sensed signal and can recover the position estimate without the knowledge of the gain of the sensed signal.

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  • Optical Transform (AREA)

Abstract

Selon l'invention, une position est déterminée en détectant un signal correspondant à une sous-séquence de marques dans une séquence non périodique des marques sur une échelle. Une position grossière P A est déterminée en mettant la sous-séquence en correspondance avec toutes les sous-séquences possibles de la séquence non périodique. Les passages par le seuil correspondant aux fronts montants du signal et les passages par le seuil correspondant aux fronts descendants du signal sont détectés. Une correction de position P i est déterminée à l'aide des passages par le seuil. La position grossière et la correction de position sont additionnées pour obtenir la position.
PCT/JP2016/073694 2015-09-10 2016-08-04 Procédé et appareil pour déterminer une position sur une échelle WO2017043249A1 (fr)

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US14/850,246 US20150377654A1 (en) 2012-02-07 2015-09-10 Method and System for Estimating Positions Using Absolute Encoders

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EP4336149A1 (fr) * 2022-09-08 2024-03-13 Renishaw PLC Dispositif et procédé de codage de position

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WO2013118423A1 (fr) * 2012-02-07 2013-08-15 Mitsubishi Electric Corporation Procédé et appareil de détermination de position
US20130253870A1 (en) * 2012-02-07 2013-09-26 Mitsubishi Electric Research Laboratories, Inc. Self-Calibrating Single Track Absolute Rotary Encoder
WO2014188894A1 (fr) * 2013-05-21 2014-11-27 Mitsubishi Electric Corporation Procédé d'auto-étalonnage d'un codeur rotatif

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US20120072169A1 (en) 2009-06-05 2012-03-22 Renishaw Plc Position measurement encoder and method of operation
WO2013118423A1 (fr) * 2012-02-07 2013-08-15 Mitsubishi Electric Corporation Procédé et appareil de détermination de position
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WO2014188894A1 (fr) * 2013-05-21 2014-11-27 Mitsubishi Electric Corporation Procédé d'auto-étalonnage d'un codeur rotatif

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP4336149A1 (fr) * 2022-09-08 2024-03-13 Renishaw PLC Dispositif et procédé de codage de position
WO2024052669A1 (fr) * 2022-09-08 2024-03-14 Renishaw Plc Appareil codeur

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