JP2006226987A - Photoelectric encoder and scale therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately and inexpensively detect the absolute position with reduced electric power consumption and high resolution, using one-track scale, without using two tracks. <P>SOLUTION: In this photoelectric encoder for acquiring an image of a bit pattern on the scale 10 by a detector 20 for detecting relative displacement between the scale and the detector, an ABS pattern on the scale 10 is expressed by a modulated pseudo-random number, and the accurate absolute position is detected by combining with an interpolation using an image correlation. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a photoelectric encoder that acquires an image of a bit pattern on a scale with a detector and detects a relative displacement between the scale and the detector, and to the scale, and more particularly, to a micrometer and an indicator. The present invention relates to a small-sized displacement measuring machine driven by a battery such as a photoelectric encoder suitable for use in a linear scale, and the scale.

  As described in Patent Documents 1 to 4, as shown in FIG. 1 (example of Patent Document 4), an ABS code 608 expressing a binary pattern for obtaining an absolute position, or a pseudo-random number sequence such as an M-sequence. A photoelectric absolute position detection (ABS) encoder that arranges a track having a position on a scale 607, detects an optical signal obtained by transmission or reflection by a detector 601, and decodes an absolute position from the binarized value It has been known. In the figure, 600 is a detection head, 601a and 601b are ABS code detectors at the end or front end in the X direction, 602 is a marker detector, 603 is a shift register 604, conversion circuits 605 and 606, and shift register control signal generation. A signal processing circuit including a circuit 610.

  However, in this apparatus, in order to detect the initial position without moving the scale 607 or the detection head 600, a detector having a long required bit length as in Patent Document 2 is required.

  As another method, there is a method in which tracks indicating absolute positions are arranged in parallel for necessary bits, and the same number of detectors are prepared at the same time.

  Further, in order to prevent 0 or 1 from becoming undefined at the boundary of the bit pattern, there are methods of increasing the number of detection points to 3 as in Patent Document 3 or moving the detector as in Patent Document 4. .

  Alternatively, in order to obtain a more accurate position, as shown in FIGS. 1 to 4, a track having an incremental code 609 is used together as shown in FIG. 1, and the interpolated incremental count and absolute position are synthesized. It is also done.

  Further, as in Patent Document 5, image correlation is also calculated using a two-dimensional image sensor.

JP-A-7-286861 JP 2000-234941 A Japanese Patent Application Laid-Open No. 2002-131088 JP 2002-168655 A (FIG. 6) JP 2002-230560 A

  However, in order to obtain high accuracy with a single track, the scale must be finely processed, resulting in high costs. In addition, reading of the absolute position code is likely to be inaccurate. Furthermore, as disclosed in Patent Documents 1 to 4, in the method using an incremental scale track together, it is necessary to read an incremental signal at all times, resulting in an increase in power consumption. Therefore, a battery-driven device requiring low power consumption. The mounting on will be limited. In addition, the cost increases as the incremental track is increased.

  Further, in the method of performing image correlation using a region corresponding to half of the image as in Patent Document 5, for example, even when only one-dimensional correlation is performed, the amount of calculation for image correlation calculation is enormous, However, in terms of power consumption, cost, and size, there are problems that it cannot be adopted for a battery-driven small displacement measuring machine.

  The present invention has been made to solve the above-mentioned conventional problems, and a first object is to provide a low-power consumption, low-cost, high-precision, high-resolution photoelectric encoder and its scale. To do.

  It is a second object of the present invention to make it possible to express a wider range of absolute positions and add error correction codes.

  The present invention provides a photoelectric encoder that acquires an image of a bit pattern on a scale with a detector and detects a relative displacement between the scale and the detector so that the bit pattern has a common part. The first problem is solved by modulation.

  Also, the absolute position can be detected by arranging the modulated bit pattern in a pseudo-random number sequence.

  Further, the second problem is solved by assigning a vector value to the bit pattern.

  The pseudo random number sequence is expressed by a plurality of bit patterns having the vector values.

  Further, image correlation is performed using a portion appearing in common in the modulated bit pattern as a reference image.

  Also, the decoding start position of the absolute position code can be accurately determined using the peak position of the image correlation value.

  Further, by using the image-correlated value, the extreme value of the correlation value can be estimated with high accuracy by the fitting by the least square method or the method described in Patent Document 5.

  Further, the amount of calculation is reduced by completing the image detection by performing the peak detection as much as possible.

  Further, by combining the obtained extreme value of the correlation value and the absolute position code, a highly accurate absolute position can be obtained.

  The present invention also provides a scale of a photoelectric encoder comprising a bit pattern modulated so as to have a common portion.

  Further, the present invention provides a scale of a photoelectric encoder, wherein the modulated bit pattern is arranged in a pseudo random number sequence.

  Further, the present invention provides a scale of a photoelectric encoder characterized in that a vector value is assigned to the bit pattern.

  Further, the present invention provides a scale of a photoelectric encoder characterized in that a pseudo random number sequence is expressed by a plurality of bit patterns having the vector values.

  According to the present invention, since the bit pattern on the scale is modulated so as to have a common part, the position of the bit pattern can be accurately known and becomes 0 or 1 at the boundary and the accuracy is lowered. There is nothing to do. Therefore, it is possible to accurately read a bit pattern without increasing the number of detection points, moving the detector, widening the detection range, and increasing the size of the detector. Absolute encoder for absolute position with low power consumption and low price can be supplied.

  In particular, when the modulated bit pattern is arranged in a pseudo-random number sequence, the absolute position can be detected with a single track.

  Further, when a vector value is assigned to the bit pattern to make it multi-valued, it is possible to express a wider range of absolute positions and add an error correction code.

  In addition, when image correlation is performed using a portion that appears in common in the modulated bit pattern as a reference image, a highly accurate encoder can be realized even with image correlation calculation with a small amount of calculation.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  As shown in FIG. 2, the first embodiment of the present invention includes a scale 10 in which the frame pattern shown in FIG. 3 has only tracks arranged in a pseudo-random sequence as shown in FIG. A detector 20 on which a lens 24 and a light receiving element array 26 are mounted, and a signal processing device 30 are provided.

  The signal processing device 30 removes noise output from the light receiving element array 26 and amplifies the noise filter / amplifier circuit 32, and converts an analog signal output from the noise filter / amplifier circuit 32 into a digital signal. An A / D converter 34, a signal correction circuit 36 for correcting the output signal of the A / D converter 34, a correlation calculation circuit 38 for performing a correlation calculation of the output of the signal correction circuit 36, and the correlation calculation circuit 38 The ABS calculation circuit 40 that calculates the absolute (ABS) position based on the output of, and the ABS position obtained by the ABS calculation circuit 40 and the interpolation position obtained by the correlation calculation circuit 38 are combined and output as an absolute position. And a position synthesis circuit 42.

  In such a configuration, the transmitted image of the scale 10 enlarged or equalized by the lens 24 is detected by the light receiving element array 26, and the detection signal is transmitted to the signal processing device 30, and the position is obtained by position calculation. . Although the example of FIG. 2 is shown as a transmission type, the light emitting element 22 may be arranged on the same side as the lens 24 and the light receiving element array 26 as a reflection type optical system.

  In the scale 10, a frame pattern as shown in FIG. 3 is arranged as an M-sequence number sequence of 9 bits, for example, along the moving direction of the scale as shown in FIG.

  As the frame pattern, as shown in FIG. 3, a frame pattern in which values of “1” and “0” are modulated as codes to indicate absolute positions can be used. The patterns “1” and “0” have a common image (in the example, near the start bit) with the same width. “1” and “0” having this pattern are arranged on the scale 10 as pseudorandom numbers in the length measuring direction. Examples of the pseudo random number include an M series and a gold series. Note that pseudo-random numbers other than the M series and the Gold series can also be used.

  The encoder has a light receiving element array 26 along the length measuring direction, and the light emitting element 22 illuminates the scale 10 so that a transmission or reflection image of the scale 10 can be obtained. The width of the light receiving element is determined in relation to the necessary resolution, and the number of light receiving elements is determined by the length measurement range and the width of the light receiving element.

  In the example of FIG. 4, a range A indicated by a broken line indicates a range where the image is actually captured, and the scale 10 is moved to the left side of the screen and the imaging range is moved to the right side. When the start bit of the frame on the left end side is detected and the frame on the right side is decoded in the order of a predetermined frame interval, a 9-bit code is obtained. The pattern on the scale decoded at this time changes from 10Eh → 01Dh → 1D1h as shown in FIG. 4 as the scale moves. These values are values that appear only once within the measurement range, and have a one-to-one relationship with the absolute position. Therefore, the absolute position can be known from this value. However, due to practical manufacturing problems of the light receiving element width and scale, it is not possible to obtain sub-micron position accuracy without using a considerably high magnification optical system.

  Therefore, in the present embodiment, image correlation is used in order to obtain high resolution on a scale having a single track shown in FIG. That is, as in Patent Document 5, the difference between the two images is digitized using the difference between the reference image and the captured real image or the sum of the absolute values of the products. However, in the present embodiment, as in Patent Document 5, images having a size about half the screen are not compared with each other, but as shown in FIG. 5, the image portion common to the frames “1” and “0” is used. A pattern having the minimum required size selected from A larger reference image can be expected to improve accuracy, but the amount of computation increases. The reference image also depends on the scale design. Depending on the system applied, the optimal design of the frame and reference image can be selected. In the present embodiment, only the image near the start bit of the scale shown in FIG. 3 is used as a reference image as shown in FIG.

  The reference image may be obtained from an actual representative image or may be artificially created. FIG. 6 shows another example of the reference image.

  The overall processing procedure will be described below with reference to the flowchart shown in FIG.

  First, in step 100, image data is acquired via the noise filter / amplifier circuit 32, the A / D converter 34, and the signal correction circuit 36.

  Next, at step 110, the correlation calculation circuit 38 performs correlation calculation as shown in FIG. Although FIG. 8 shows a difference method using a difference, a multiplication method using a product may be used.

  Next, in step 120, a peak appearing in an inverse convex shape is detected from the obtained image correlation value as shown in FIG. 9, and the correlation value in the vicinity of the peak is used to obtain FIG. 10 (see FIG. 12 of Patent Document 5). The true peak is interpolated as shown in FIG. For the interpolation calculation, the method described in Patent Document 5 or estimation by fitting a polynomial by the least square method can be used. Also, this peak detection allows the ABS code decoding start position to be determined with high accuracy. Note that the correlation calculation need not be performed over the entire screen. If the peak is detected and only the minimum necessary for interpolation is obtained, the calculation can be completed and the amount of calculation can be reduced. As shown in FIG. 3, since the start bit exists in both the patterns “1” and “0”, the range of image correlation greatly increases or cannot be detected depending on the value of the pseudo random number sequence. There is nothing to do.

  Next, in step 130, the ABS calculation circuit 40 acquires ABS information based on the peak position, and in step 140, the ABS data is decoded.

  Next, after performing an interpolation operation in step 150, in step 160, the position combining circuit 42 calculates the absolute position to be obtained by combining the ABS position value and the interpolation position value in consideration of the magnification of the optical system. can do.

  Hereinafter, a specific example of the composition calculation method will be described with reference to FIG.

First, a position X _ABS where decoding of M-sequence data is started is obtained. This position is not just a pixel number, but a value less than or equal to a subpixel is required.

X _ABS = X _p −S _pitch · n (1)
X _p : Peak detection position of interpolation calculation S _pitch : One frame length of scale, 120 μm in the embodiment
n: Number of frames from the interpolation calculation position to the ABS code start position

Then the addition by adding the ABS frame number P _ABS optical power S _opt from the origin position, a position X to obtain can be obtained by the following equation.

X = (P _ABS · S _pitch −X _ABS ) / S _opt + X ₀ (2)
P _ABS : Decoded ABS frame number X _ABS : Decoding start position of M series ABS code S _opt : Optical magnification X ₀ : Offset adjustment value

  By these methods, the amount of calculation can be greatly reduced as compared with the case where the total lengths are compared, and the power consumption can be greatly reduced.

  In the present embodiment, since peak detection is performed only in the central portion with less image distortion, high-precision measurement can be performed with a small calculation load. Note that it can be performed at two locations on both ends to make it less susceptible to misalignment or as a whole.

  Note that since the modulation bit pattern of the first embodiment expresses only a 1-bit signal in one frame, the absolute position range that can be expressed is based on the relationship between the imaging range and the minimum width of the scale due to manufacturing limitations. It was limited to several hundred mm. For example, when the frame length is 120 μm, the minimum mark width is 20 μm, and the imaging range is 1.44 mm (12 frames are captured at a time by the image sensor), the absolute position range that can be expressed is about 245 mm. Further, in applications such as a linear scale, in order to detect and correct bit reading errors due to dust and noise, more information must be added, making it difficult to apply to a linear scale.

  Next, a detailed description will be given of a second embodiment of the present invention in which the absolute position range that can be expressed is expanded to be applicable to a linear scale.

  In the present embodiment, the scale design is improved in the same apparatus configuration as that of the first embodiment.

  That is, as a code for indicating the absolute position, a code that assigns a vector value to one frame to be multi-valued and expresses all values of the vector in a plurality of frames is used. 12 shows a frame having a 2-bit value, FIG. 13 shows a frame having a 3-bit value, and FIG. 14 shows an example of a frame having a 4-bit value.

  In any dimension frame, each bit pattern has the same width and a common part. In the examples of FIGS. 12, 13, and 14, the left end of the bit pattern is a common image (referred to as a reference image) that changes from black to white. Also, the image other than the common part must not have the same part as the reference image. Then, as in the first embodiment, pseudo random numbers are arranged on the scale in the length measurement direction. At this time, the pseudo random number sequence is divided by the dimension of the vector to be used, and a frame corresponding to the vector value is assigned. And a scale pattern. As the pseudo random number, an M series, a gold series, or the like can be used. In addition, the scale design by 2nd Embodiment is not limited to what was shown in FIGS. 12-14, Many other modifications are possible. Also, the vector values are not limited to four (FIG. 12), eight (FIG. 13), and sixteen (FIG. 14) from the top as shown in the figure, and any combination is used for decoding, and the remaining spare part. Can be used for error correction. The bit length may be 5 bits or more.

  As an example, Table 1 shows the frame length L necessary for expressing each vector by the number of dimensions M when the minimum line width corresponding to the start bit is 20 μm and the other line widths are 20 μm × N.

  In each example, the absolute position range P that can be expressed is represented by the following relational expression.

P = L * 2 ^ (N-1) (3)
N = {ROUND (F / L) -1} * M (4)
* ROUND () means rounding L: 1 frame length N: Number of bits in the imaging range F: Imaging range of the entity when using an image sensor, etc. M: Number of vector dimensions (multi-valued number)

  For example, assuming that the vector dimension is M = 3, one frame length L = 160 μm, and imaging range F = 1.44 mm, the absolute position range P that can be expressed is expressed as follows when the absolute position is expressed by all bits: Become.

P = 0.160 * 2 ^ {(ROUND (1.44 / 0.16) -1) * 3-1}
= 1342177 [mm] (5)

  When the dimension M is 4, 24159191 [mm] is obtained by the same calculation. The results are shown in the absolute position range P that can be expressed in Table 1.

  From these results, it can be seen that even if the same optical system and mark minimum pitch width as in the first embodiment are used, the absolute position in a practically sufficient range can be shown by vectorization (multi-value).

  Further, by adding an error detection code to these bit information, it is possible to cope with a bit reading error due to dust or noise.

  FIG. 15 shows a method of designing a scale using this vectorization (multi-value) method. First, similarly to the first embodiment, a pseudo-random number sequence having a necessary length is created using an M sequence. As shown in FIG. 15, when 3-bit multi-value processing is performed, the obtained pseudo-random number sequence is divided every 3 bits, and a design corresponding to each division is assigned. If you assign, it ends.

  When decoding, as in the first embodiment, a reference image near the start bit is detected using image correlation, a bit pattern is extracted for each frame, a vector value for each bit pattern is extracted, and a pseudo-random number sequence Can be synthesized as follows. The calculation of the value below the sub-pixel by the image correlation can be performed as in the first embodiment.

  According to the second embodiment, since a wider range of ABS positions can be expressed than in the first embodiment, a wide range of absolute encoders such as a linear encoder can be supplied. Also, by adding an error correction code, a highly reliable encoder can be supplied.

  The encoder system can be applied not only to a one-dimensional light receiving element array but also to a general-purpose and inexpensive two-dimensional light receiving element matrix.

  In the above-described embodiment, the present invention is applied to an absolute encoder. However, the application target of the present invention is not limited to this, and can be applied to an incremental encoder as well.

The perspective view which shows the structure of an example of the conventional ABS encoder described in patent document 4 The front view which shows the whole structure of embodiment of the photoelectric encoder which concerns on this invention The top view which shows the frame pattern used in 1st Embodiment The top view which similarly shows the example of arrangement of the frame pattern on the scale A plan view showing the same reference image Similarly, a plan view showing a modification of the reference image Flow chart showing the processing procedure Similarly, a plan view showing the state of correlation calculation by the difference method A diagram showing an example of calculation results of image correlation values Diagram showing the state of interpolation The top view which similarly shows the mode of a composition of an ABS value and an interpolation value The top view which shows the frame pattern in the case of 2 bits used by 2nd Embodiment Similarly, a plan view showing a frame pattern in the case of 3 bits Similarly, a plan view showing a frame pattern in the case of 4 bits The top view which similarly shows the arrangement method of the frame pattern on the scale

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Scale 20 ... Detector 22 ... Light emitting element 24 ... Lens 26 ... Light receiving element array 30 ... Signal processing device 32 ... Noise filter / amplifier circuit 34 ... A / D converter 36 ... Signal correction circuit 38 ... Correlation calculation circuit 40 ... ABS calculation circuit 42... Position synthesis circuit

Claims (13)

In a photoelectric encoder that acquires an image of the bit pattern on the scale with a detector and detects the relative displacement between the scale and the detector,
A photoelectric encoder, wherein the bit pattern is modulated so as to have a common part.   The photoelectric encoder according to claim 1, wherein the modulated bit pattern is arranged in a pseudo-random number sequence.   The photoelectric encoder according to claim 1, wherein a vector value is assigned to the bit pattern.   4. The photoelectric encoder according to claim 3, wherein a pseudo-random number sequence is expressed by a plurality of bit patterns having the vector values.   5. The photoelectric encoder according to claim 2, wherein image correlation is performed using a portion appearing in common in the modulated bit pattern as a reference image. 6.   6. The photoelectric encoder according to claim 5, wherein the decoding start position of the absolute position code is determined using the peak position of the image correlation value.   The photoelectric encoder according to claim 6, wherein an extreme value of the correlation value is estimated using a value obtained by image correlation.   8. The photoelectric encoder according to claim 7, wherein image correlation is terminated by performing the peak detection as much as possible.   9. The photoelectric encoder according to claim 8, wherein the obtained extreme value and the absolute position code are synthesized to obtain a highly accurate absolute position.   A scale of a photoelectric encoder comprising a bit pattern modulated so as to have a common part.   The scale of the photoelectric encoder according to claim 10, wherein the modulated bit pattern is arranged in a pseudo-random number sequence.   The scale of the photoelectric encoder according to claim 10, wherein a vector value is assigned to the bit pattern.   The scale of the photoelectric encoder according to claim 12, wherein a pseudo-random number sequence is expressed by a plurality of bit patterns having the vector values.

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