JP5553669B2 - Optical absolute position measurement encoder - Google Patents

Description

  The present invention relates to an optical absolute position measuring encoder, and in particular, an optical absolute position capable of improving the detection accuracy of an absolute (ABS) pattern and reducing the influence of noise caused by dust on the scale. It relates to a length measuring encoder.

  Conventionally, an optical encoder is provided with a light emitting element, an optical system, and a light receiving element for detecting an origin on the detector side, and a predetermined value on the scale side, as illustrated in Patent Document 1, for detecting the origin. The origin pattern is arranged. According to this configuration, when the scale and the detector are relatively moved, when the origin pattern on the scale crosses the detection unit, for example, a delta function like that illustrated in FIG. 5 or FIG. The peak signal is generated, and the origin can be detected by setting an appropriate threshold value in advance.

  Further, as described in Patent Documents 2 and 3, a plurality of reference marks are arranged on the scale, and the arrangement pitch is characteristically shifted as unequal, thereby performing pseudo absolute position (ABS) detection. Optical encoders have also been put into practical use.

  In addition to the incremental (INC) pattern formed by the arrangement pitch Pi of light and dark equal intervals and the absolute (ABS) pattern in which the absolute position is expressed by a pseudo-random pattern in Patent Documents 4 and 5, the applicant has disclosed an absolute. Proposing a three-track optical absolute position measurement encoder with a position reference pattern that has a predetermined phase relationship with the pattern and is formed with a light and dark equidistant arrangement pitch Pr (> Pi) Yes.

  In the optical absolute position measurement type encoder based on the three tracks (ABS, position reference, INC) proposed in Patent Documents 4 and 5, when position information is extracted from the captured ABS pattern or position reference pattern, A technique using image correlation is used to detect coincidence. This method generates a single correlation peak on the correlation function obtained by calculating the "difference" or "product" between the reference pattern stored in itself and the detection pattern obtained from imaging. It is a method of letting.

JP-A-7-318371 (FIGS. 5 and 13) Japanese Patent Laid-Open No. 60-120216 (FIG. 1) JP 62-291507 A (FIGS. 2 and 3) JP 2008-261701 A JP 2009-2702 A

  However, for example, as shown in FIG. 1A, when the ABS pattern is arranged according to the random code sequence, there are a wide pattern, so the number of edges is small, and the width of the correlation peak as illustrated by a broken line in FIG. However, the detection accuracy is wide and insufficient. In addition, when the correlation peak is as shown by a broken line in FIG. 1 (d) when there is no dirt as shown in FIG. When dirt is attached as described above, the correlation peak becomes as shown by a solid line in FIG. 1D, and there is a problem that the possibility of erroneous detection increases due to the occurrence of an error peak due to dirt.

  The present invention has been made to solve the above-described conventional problems, and it is an object of the present invention to improve the detection accuracy of the ABS pattern and to make it less susceptible to noise caused by dust on the scale.

The present invention includes a relatively movable detector and the scale, the detector-side light-emitting element for absolute position detection, arranged optical system and the light receiving element, and is the scale side disposed a predetermined absolute pattern In the optical absolute position measuring encoder, the above-mentioned problem is solved by arranging the absolute patterns according to a random code sequence and dividing each absolute pattern in the length measuring direction at random or at equal intervals .

Here, a part of the edge position of the absolute pattern can be changed in the length measuring direction.

The present invention also includes a relatively movable detector and the scale, the light-emitting element for absolute position detection in the detector side, place the optical system and the light receiving element, a predetermined absolute patterns on the scale side In the arranged optical absolute position measuring encoder, the absolute pattern is arranged according to a random code sequence, and a part of the edge position of the absolute pattern is set to ± the line width of the pattern corresponding to one code of the random code sequence The problem is solved by changing in the length measuring direction within a range of 1/2 .

  According to the present invention, in detecting the coincidence of the ABS pattern by image correlation, as illustrated in FIG. 3B, the ABS pattern is divided to increase the edge position, and the edge position is shifted as necessary. As a result, a sharp correlation peak is generated as shown by a solid line in FIG. 2 to improve the detection accuracy of the ABS pattern, and as shown in FIG. 3E, it is hardly affected by noise due to dust on the scale. can do.

A figure explaining an example of the conventional ABS pattern and its problems A diagram illustrating an example of a conventional ABS pattern and other problems The figure explaining the effect | action of this invention Schematic showing the overall configuration of an embodiment of the present invention The top view which shows the structure of the scale of the said embodiment. Similarly, a plan view showing the configuration of the photodiode array Similarly, a circuit diagram showing a detailed configuration of the ABS photodiode array. The same figure explaining the operation (A) The top view which shows the example which has arrange | positioned the random code series on a scale, and (B) the example which shifted the position of the edge at random Diagram showing definition of edge position Diagram showing how to shift the edge position Schematic showing a modification of the embodiment

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

  FIG. 4 is a schematic diagram showing the overall configuration of the optical encoder according to the embodiment of the present invention. The optical encoder of this embodiment includes a light emitting element 11, a scale 12, a lens 13, a photodiode array 14, and a signal processing circuit 20.

  The light emitting element 11 is a light source that emits coherent light, for example, a laser diode.

  As shown in FIG. 5, the scale 12 includes an incremental track 301 composed of an incremental pattern 31 formed on a transparent glass substrate at an evenly spaced arrangement pitch Pi (for example, 40 μm), and a pseudo-random pattern (here, M An absolute track 302 composed of a general absolute pattern 32 expressing an absolute position by a series code), and an arrangement pitch Pr (<Pi) having a predetermined phase relationship with the absolute pattern 32 and at equal intervals between light and dark. A position reference track 303 formed by the position reference pattern 33 having a width Wr in the length measurement direction is formed.

  That is, the absolute pattern 32 represents the absolute position of the equally spaced pattern of the position reference pattern 33. For example, the arrangement pitch Pi of the incremental pattern 31 is set to 1 / integer of the arrangement pitch Pr of the position reference pattern 33. In the present embodiment, it is assumed that Pr = 4Pi as an example. As an example, when Pi = 40 μm, Pr = 160 μm is set.

  Since each of the incremental pattern 31 and the position reference pattern 33 is formed at an equally spaced arrangement pitch (Pi, Pr) over the entire length of the encoder, it is relatively easy to form accurately over the entire length of the encoder. Here, an encoder is assumed in which there is no position reference pattern 33 and only the incremental pattern 31 and the absolute pattern 32 exist. In such an encoder, for example, when the arrangement pitch of the incremental patterns 31 is 40 μm, the accuracy of the absolute pattern 32 must be less than ± 20 μm, which is less than half of the entire length of the scale 12.

  On the other hand, if the position reference pattern 33 having an arrangement pitch Pr larger than the arrangement pitch Pi of the incremental pattern 31 is formed as in the present embodiment, the position accuracy of the absolute pattern 32 is assured by the arrangement of the position reference pattern 33. It is sufficient to match the pitch Pr. Therefore, the tolerance of the position error of the absolute pattern 32 can be increased. For example, if the arrangement pitch Pr of the position reference pattern 33 is 160 μm, which is four times Pi, the position error of the absolute pattern 32 can be allowed to be ± 80 μm over the entire length of the scale 12. Therefore, the arrangement pitch Pi of the incremental pattern 31 can be determined without considering the accuracy of the absolute pattern 32, and the pitch of the incremental pattern 31 can be reduced to increase the accuracy of the encoder.

  The light emitting element 11 irradiates the scale 12, and the irradiation light transmitted through the scale 12 is projected onto the photodiode array 14 via the lens 13.

  As shown in FIG. 6, the photodiode array 14 composed of, for example, a CMOS, corresponds to the incremental track 301, the absolute track 302, and the position reference track 303, respectively, and includes an INC photodiode array 41, an ABS photodiode array 42, and A position reference photodiode array 43 is provided. Each of the photodiode arrays 41 to 43 is configured by arranging photodiodes at an arrangement pitch corresponding to the pitch of the corresponding patterns 31 to 33.

  The INC photodiode array 41 has, for example, four sets of photodiode arrays having different phases by 90 degrees, detects a light / dark signal based on the incremental pattern 31, and outputs a four-phase sine wave signal having a phase difference of 90 degrees. The ABS photodiode array 42 outputs a signal obtained by sweeping a light / dark signal based on the absolute pattern 32 in the length measuring direction. The position reference photodiode array 43 has a dimension WPDr in the length measurement direction so that at least one position reference pattern 33 can be detected (WPDi> Pr + Wr).

  The ABS photodiode array 42 and the position reference photodiode array 43 respectively sweep the light receiving elements 1 to N of the ABS photodiode array 42 or the position reference photodiode array 43 by a switching element 45 as shown in FIG. To output the obtained signal.

  The signal processing circuit 20 shown in FIG. 4 includes, for example, a noise filter / amplifier circuit 21, an A / D converter 22, a relative position detection circuit 23, preamplifiers 24 and 27, correlation calculation circuits 25 and 28, and an absolute position detection circuit 26. The reference position detection circuit 29 and the absolute position synthesis circuit 30 are provided.

  The noise filter / amplifier circuit 21 removes noise from the analog output signal (90-degree phase difference 4-phase signal) from the INC photodiode array 41 and then amplifies and outputs the signal.

  The A / D converter 22 converts the analog output signal output from the noise filter / amplifier circuit 21 into a digital signal. The relative position detection circuit 23 performs an arctan calculation of the amplitude of the obtained digital signal (90-degree phase difference signal), thereby outputting a relative position signal D1 indicating the relative movement amount / movement direction of the scale 12.

  The preamplifier 24 amplifies and outputs the output of the switching element 45 connected to the ABS photodiode array 42. The correlation calculation circuit 25 performs a correlation calculation with the design value, and the absolute position detection circuit 26 outputs an absolute position signal D2 indicating the absolute position of the scale 12 based on the calculation result of the correlation calculation circuit 28.

  The preamplifier 27 amplifies and outputs the output of the switching element 45 connected to the position reference photodiode array 43. The correlation calculation circuit 28 performs a correlation calculation with the design value, and the reference position detection circuit 29 outputs a position reference signal D3 indicating the reference position of the position reference pattern 33 based on the calculation result of the correlation calculation circuit 28.

  The absolute position synthesis circuit 30 calculates the fine absolute position of the scale 12 based on the absolute position signal D2, the relative position signal D1, and the position reference signal D3. The operation of the absolute position synthesis circuit 30 will be described with reference to FIG. The absolute position signal D2 has information on the absolute position of the scale 12. Since the absolute pattern 32 is formed with predetermined information with respect to the position reference pattern 33, the absolute position is obtained from the absolute position signal D 2, so that the scale 12 is positioned at what cycle of the period Pr of the position reference pattern 33. Can be identified ((1) in FIG. 8).

  When the cycle Pr of the position reference pattern 33 is specified, the signal amount of the position reference signal D3 is detected thereafter, and in what cycle of the incremental pattern 31 the scale 12 is positioned. Can be specified ((2) in FIG. 8). Since the incremental pattern 31 and the position reference pattern 33 are both formed by equally spaced light and dark patterns, even when the ratio of the arrangement pitch Pr and Pi is large, it is easy to keep the positional accuracy between them high. For this reason, the cycle of the position reference pattern 33 is determined, and the signal amount of the position reference signal D3 is further detected, thereby specifying the cycle of the incremental pattern 31 to which the scale 12 is positioned. it can. Thereafter, the absolute position of the scale 12 can be calculated and output by calculating the brightness of the relative position signal D1 obtained from the incremental pattern 31.

  As described above, according to the present embodiment, the absolute position of the scale 12 is detected in relation to the position reference pattern 33 based on the absolute position detection signal D2 obtained by the absolute pattern 32, and then the position reference A precise absolute position signal of the scale 12 can be obtained by the position reference signal D3 based on the pattern 33 and the relative position signal D1 based on the incremental pattern 31. The absolute pattern 32 does not require positional accuracy with respect to the finely formed incremental pattern 31, and it is sufficient if it is formed with a predetermined positional accuracy with respect to the position reference pattern 33 having a larger arrangement pitch. Therefore, according to the present embodiment, the pitch of the incremental pattern 31 can be reduced, and thus the encoder can be highly accurate.

  Further, the absolute pattern 32 is divided as shown in FIG. 3B with respect to the design value as shown in FIG. 3A, and the edge positions of some patterns are changed in the length measuring direction. ing. Therefore, as shown in FIG. 3D, even when dirt is attached, the occurrence of an error peak is suppressed as shown in FIG. The method of dividing the absolute pattern 32 may be either random or equally divided in the length measuring direction.

Further, as shown in FIG. 9A, when a pattern is arranged on a scale according to a random code sequence, if the line width of the pattern corresponding to one code of the random code sequence is W _bit , the random pattern arranged on the scale The pattern edge of the code sequence appears only at the position of W _bit × n (n is a positive integer).

Due to this feature, when performing the correlation calculation, the edge positions coincide with each W _bit , so that an error peak is likely to occur due to contamination, and there is a high possibility of erroneous detection.

Therefore, as illustrated in FIG. 9B, when the correlation calculation is performed by shifting the position of the scale pattern edge in the range of −W _bit / 2 to W _bit / 2, the edges match at positions other than the incorrect position. As a result, it is possible to suppress the occurrence of error peaks.

As shown in FIG. 10, the edge shift method is such that the pattern edge shift is Δp _i (i = 1, 2, 3... N), and the range of Δp _i is −W _bit / 2 <Δp _i <W _bit. As shown in FIG. 11A, it is shifted randomly as shown in FIG. 11A, shifted according to a normal distribution as shown in FIG. 11B, shifted according to a triangular wave as shown in FIG. As shown in (D), it can be shifted according to a sine wave.

  In the above embodiment, the present invention is applied to a transmissive encoder. However, the application target of the present invention is not limited to this, and a reflective optical system as in the modification shown in FIG. As an alternative, the light emitting element 11 may be arranged on the same side as the lens 13 and the photodiode array 14.

DESCRIPTION OF SYMBOLS 11 ... Light emitting element 12 ... Scale 13 ... Lens 14 ... Photodiode array 20 ... Signal processing circuit 21 ... Noise filter and amplification circuit 22 ... A / D converter 23 ... Relative position detection circuit 24, 27 ... Preamplifier 25, 28 ... Correlation Arithmetic circuit 26 ... Absolute position detection circuit 29 ... Reference position detection circuit 30 ... Absolute position synthesis circuit 31 ... Incremental (INS) pattern 32 ... Absolute (ABS) pattern 33 ... Position reference pattern 41 ... INC photodiode array 42 ... ABS photodiode Array 43 ... Position reference photodiode array 45 ... Switching element 301 ... Incremental (INC) track 302 ... Absolute (ABS) track 303 ... Position reference track

Claims (3)

With a relatively movable detector and scale,
Emitting element for absolute position detection in the detector side, the optical system and the light receiving element is arranged,
In the optical absolute position measurement type encoder in which a predetermined absolute pattern is arranged on the scale side,
An optical absolute position measuring encoder characterized in that the absolute patterns are arranged according to a random code sequence, and each absolute pattern is divided randomly or at equal intervals in the length measuring direction. 2. The optical absolute position length measuring encoder according to claim 1, wherein a part of the edge position of the absolute pattern is changed in the length measuring direction. With a relatively movable detector and scale,
Emitting element for absolute position detection in the detector side, the optical system and the light receiving element is arranged,
In the optical absolute position measurement type encoder in which a predetermined absolute pattern is arranged on the scale side,
The absolute pattern is arranged according to a random code sequence, and the edge position of a part of the absolute pattern is changed in the length measurement direction within a range of ± 1/2 of the line width of the pattern corresponding to one code of the random code sequence An optical absolute position measuring encoder characterized by the above.

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