Offset correction device and position measurement device

Abstract

The present invention provides an offset correction device and a position measurement device. The offset correction device includes: an amplitude adjuster for adjusting the amplitude of a detection signal output from an encoder by adjusting the gain of the detection signal so that the amplitude of the detection signal is within a predetermined range; an offset corrector for correcting the offset of the amplitude center of the detection signal; and a memory for pre-storing the relationship between the gain and the offset, wherein the offset corrector refers to the relationship stored in the memory when the amplitude adjuster changes the gain, obtains an offset corresponding to the changed gain, and corrects the offset based on the obtained offset.

Description

Technical Field

[0001] A certain aspect of the embodiments described herein relates to an offset correction device and a position measurement device. Background Art

[0002] A method of using a two-phase sine wave signal as a position detection signal in an encoder to detect the relative position between a scale and a detector head is known (for example, see miniaturized linear encoder "FIBER SCALE", Mitutoyo, HiroatsuMORI, Hiroaki KAWADA, Tomotaka TAKAHASHI). In these encoders, the intensity of the detection signal may change due to a change in the relative posture between the scale and the detector head.

[0003] As a countermeasure to the above situation, automatic gain control (AGC) is performed to automatically adjust the gain of the detection signal so that the intensity of the detection signal is within a predetermined range. However, the amplitude center of the detection signal may be offset due to the AGC. As a countermeasure to the above situation, a technology for automatically adjusting the offset is disclosed (for example, see Japanese Patent Laid-Open No. 2014-25871). Summary of the invention

[0004] However, the above technique requires measurement points within a certain range in the cycle of the sine wave in order to perform correction. Therefore, there is a time lag from the timing of gain switching until the offset is corrected.

[0005] In one aspect of the present invention, an object is to provide an offset correction device and a position measurement device that reduce skew and correct offset.

[0006] According to aspects of the present invention, there is provided an offset correction device, comprising: an amplitude adjuster for adjusting the amplitude of a detection signal output from an encoder by adjusting the gain of the detection signal so that the amplitude of the detection signal is within a predetermined range; an offset corrector for correcting the offset of the amplitude center of the detection signal; and a memory for pre-storing the relationship between the gain and the offset, wherein the offset corrector refers to the relationship stored in the memory when the amplitude adjuster changes the gain, obtains an offset corresponding to the changed gain, and corrects the offset based on the obtained offset.

[0007] According to another aspect of the present invention, there is provided a position measuring device including: the above offset correction device; an encoder; and an arithmetic device for calculating a position based on a detection signal whose offset is corrected by the offset correction device. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1A shows the detector head, and Figure 1B Show the scale;

[0009] Figure 2 Calculation of the position is shown;

[0010] Figure 3 shows the signal strength of a detection signal obtained from a receiver coil in a case where the amount of gap between the detector head and the scale is changed from a reference position;

[0011] Figure 4 shows the signal strength when a predetermined threshold is set and AGC is performed;

[0012] Figure 5 Shows the change in the center value of the sine wave;

[0013] Figure 6 Shows the error of the detection signal;

[0014] Fig. 7A shows the structure of a position measuring device according to an embodiment, and Figure 7B showing a table stored in an offset storage unit;

[0015] Figure 8 Shows the precision error;

[0016] Fig. 9 showing the signal strength of the output signal in the case where a plurality of thresholds are set for each gain; and

[0017] Fig.10 A table stored in the offset storage unit is shown. DETAILED DESCRIPTION

[0018] Before describing the embodiments, an exemplary encoder will be described. Figure 1A A detector head 201 is shown. Figure 1B There is shown a scale 204. The detector head 201 and the scale 204 have a substantially flat plate shape, and are configured to be opposed to each other across a predetermined gap.

[0019] like Figure 1A As shown, the detector head 201 includes a transmitter coil 202 and a receiver coil 203. The transmitter coil 202 forms a rectangular coil. The receiver coil 203 is arranged inside the transmitter coil 202.

[0020] like Figure 1B As shown, in the scale 204, a plurality of coupling coils 205 are periodically arranged along the measuring axis. The coupling coils 205 are separated from each other and insulated from each other. Each coupling coil 205 is electromagnetically coupled to the transmitter coil 202, and is also electromagnetically coupled to the receiver coil 203.

[0021] A current passes through the transmitter coil 202, and an electromotive force generated in the receiver coil 203 via the coupling coil 205 is measured as a detection signal. The position in the measurement axis of the scale 204 can be measured by using this detection signal.

[0022] In this encoder, for example, two-phase sinusoidal wave signals (A-phase signal and B-phase signal) having a phase difference of 90 degrees from each other are used as detection signals. Figure 2 As shown in FIG. 1 , a Lissajous figure is drawn with two-phase (A-phase and B-phase) sine waves having a phase shift of 90 degrees from each other, and the position is calculated by the angle θ of the output point P with the center point of the Lissajous figure as a reference. In this encoder, a change in the relative posture between the detector head 201 and the scale 204 causes a change in the intensity of the two-phase sine wave signal and waveform interference, resulting in detection failure or reduced accuracy.

[0023] Figure 3 The signal strength of the detection signal obtained from the receiver coil 203 when the gap amount between the detector head 201 and the scale 204 changes from the reference position is shown. The signal strength of the detection signal refers to the amplitude. Figure 3 In FIG. 1 , the horizontal axis represents the change in the gap amount, and the vertical axis represents the signal strength. Figure 3 As shown, as the gap amount increases, the signal strength decreases, and as the gap amount decreases, the signal strength increases.

[0024] exist Figure 3 In the example of , the gap amount changes from the reference position by -0.2 mm, and the signal strength increases to outside the detection range (overrange). In this case, it is difficult to calculate the position normally. This means that in the case where the device body to which the encoder is attached slides and undergoes positional displacement, thereby changing the relative position between the detector head 201 and the scale 204, or in the case where the position of the device body changes from the initial installation state of the device body over time, the encoder cannot perform its original performance.

[0025] The use of automatic gain control (AGC) that automatically adjusts the gain of the detection signal according to the magnitude of the signal strength reduces the variation in signal strength due to posture changes during use of the encoder. Figure 4 The signal strength is shown when a predetermined threshold is set and AGC is performed. Figure 4 In FIG. 1 , the horizontal axis represents the gap amount, and the vertical axis represents the signal strength. Figure 4 As shown, by performing AGC, the signal strength falls within a predetermined range regardless of whether the gap amount is large or small. Therefore, an encoder that is independent of the sliding performance of the device on which the encoder is mounted and that is resistant to time changes is realized.

[0026] On the other hand, as described above, when the output position of the encoder is calculated based on the angle relative to the center point of the Lissajous figure, offset correction is required for adjusting the center value to zero by performing signal matching after the encoder is installed.

[0027] However, after the offset is corrected, during actual measurement, the signal strength may vary due to a change in the relative position between the detector head 201 and the scale 204. In the case where the gain is adjusted by AGC according to the change in signal strength, the center value of the sine wave obtained as the detection signal may also vary as shown in FIG. Figure 5 The change of the center value δ makes the angle θ of the output point P as shown in Figure 6 The change is shown as an angle θ', resulting in an error in the value of the detection signal. As the rate of change of the gain increases, the value of the error also increases.

[0028] As a countermeasure to the above situation, it is possible to consider using a technology for automatically correcting the offset of the detection signal output from the encoder. However, in the case where the correction requires a measurement point within a certain range in the cycle of a sine wave, there is a time lag from the timing of gain switching until the correction is applied. In addition, a calculation load for correction is generated, which has a negative impact on the throughput of the measurement. In addition, the increase in error after gain switching can be reduced, but it cannot be zero.

[0029] Therefore, in the following embodiments, an offset correction device and a position measurement device that can reduce a skew and correct an offset will be described.

[0030] First embodiment

[0031] Fig. 7A FIG. 2 shows a structure of a position measuring device 100 according to an embodiment. Fig. 7A As shown, the position measurement device 100 includes an encoder 10, an offset correction device 20 and an arithmetic device 30. The detection principle of the encoder 10 is not particularly limited as long as the encoder 10 outputs sinusoidal wave signals having a phase difference with each other. Examples of the encoder 10 include, but are not limited to, a photoelectric encoder and an electromagnetic induction encoder. The encoder 10 is a linear encoder or a rotary encoder, etc. In the present embodiment, the encoder 10 outputs a two-phase sinusoidal wave signal (A phase signal and B phase signal) having a phase difference of 90 degrees.

[0032] The offset correction device 20 includes two analog / digital (A / D) converters 21a and 21b, an amplitude adjustment unit 22, an offset correction unit 23, a signal strength calculation unit 24, an AGC unit 25, and an offset storage unit 26. Figure 7BAs shown, the offset storage unit 26 stores the relationship between the setting values ​​G1, G2, G3, ... of the gain and the corresponding offset amounts as a table. The relationship between the setting values ​​of the gain and the offset amounts can be obtained in advance through experiments or the like.

[0033] A / D converter 21a digitizes the A-phase signal output from encoder 10 by sampling it at a predetermined sampling cycle. A / D converter 21b digitizes the B-phase signal output from encoder 10 by sampling it at a predetermined sampling cycle.

[0034] The amplitude adjustment unit 22 adjusts the amplitude of the A-phase signal and the B-phase signal by multiplying the A-phase signal and the B-phase signal by a gain (multiplication factor). The offset correction unit 23 corrects the offset of the center value of the A-phase signal and the B-phase signal after the amplitude adjustment. For example, the initial gain is set by the user as the initial setting of the position measurement device 100. In addition, the offset to be used by the offset correction unit 23 is set by the user as the initial setting of the position measurement device 100.

[0035] The signal strength calculation unit 24 calculates the signal strength (amplitude) of the A-phase signal and the B-phase signal after the offset correction during the actual measurement after the initial setting. The AGC unit 25 performs AGC according to the calculation result of the signal strength calculation unit 24 so that the signal strength of the A-phase signal and the B-phase signal is within the detection range. More specifically, the AGC unit 25 increases the gain when the signal strength of the A-phase signal and the B-phase signal is less than the first threshold, and reduces the gain when the signal strength is greater than the second threshold. The AGC unit 25 sets the obtained gain to the amplitude adjustment unit 22. The amplitude adjustment unit 22 adjusts the amplitude of the A-phase signal and the B-phase signal using the set gain. In this case, the AGC unit 25 refers to the offset storage unit 26 to read the offset corresponding to the gain set to the amplitude adjustment unit 22, and sets the read offset to the offset correction unit 23. The offset correction unit 23 corrects the offset of the A-phase signal and the B-phase signal using the set offset.

[0036] Arithmetic device 30 calculates the position using A-phase signal and B-phase signal after amplitude adjustment by amplitude adjustment unit 22 and offset correction by offset correction unit 23. More specifically, in encoder 10, the displacement amount of the relative position between the detector head and the scale on the measuring axis is calculated.

[0037] In the present embodiment, the offset amount caused by gain switching is obtained in advance by experiments or the like, and the offset storage unit 26 stores the relationship between the gain and the offset amount. This enables the offset correction amount to be appropriately set in the case where the gain is switched by performing AGC. In this case, since only the offset amount needs to be read, the offset correction is substantially performed simultaneously with the gain switching. Therefore, the offset can be corrected while reducing the time lag.

[0038] In addition, even in the case where a time change occurs after the initial setting, the gain can be appropriately adjusted, and the offset can also be appropriately corrected. In addition, since the offset can be corrected only by reading the offset amount, the calculation load is reduced. As a result, the influence on the throughput of the position measurement is reduced. In addition, when the relationship between the gain and the offset amount is pre-stored in the offset storage unit 26, the offset after the gain switching can be made zero by obtaining the offset amount that causes the offset to be zero.

[0039] Figure 8 “No correction” in ∝ represents the accuracy error when the gap between the detector head and the scale in the encoder 10 changes by −0.2 mm and the offset correction using the relationship stored in the offset storage unit 26 is not performed. Figure 8 The "with correction" in represents the accuracy error when the gap between the detector head and the scale in the encoder 10 changes by -0.2 mm and an offset correction using the relationship stored in the offset storage unit 26 is performed. As the offset error caused by not performing the offset correction increases, the accuracy error increases. Figure 8 In the example of , the accuracy error in the case of "with correction" is reduced by about 80% compared to the accuracy error in the case of "without correction".

[0040] The amplitude adjustment unit 22 can change the threshold value in the case of performing AGC according to the value of the gain. For example, the amplitude adjustment unit 22 can use the gain setting value at the reference position in a wide range by setting the threshold value for switching the gain setting value to a different value according to each gain setting value. For example, Fig. 9 The signal strength of the output signal when the threshold is set for each gain is shown in . This structure enables the AGC to cope with large gap changes and enables the area in which the error due to offset change can be kept to a minimum without gain switching near the reference position to be widened.

[0041] The offset storage unit 26 may be non-rewritable or rewritable. For example, the relationship between the gain and the offset stored in the offset storage unit 26 may be rewritten from an external device through an interface of the position measurement device 100. In this structure, the information stored in the offset storage unit 26 may be updated as needed.

[0042] Second embodiment

[0043] In the first embodiment, the combination of gain and offset is stored in the offset storage unit 26, but this is not intended to impose any limitation. Fig.10 As shown, the rate of change of the offset amount caused by gain switching in the experiment is stored in the offset storage unit 26.

[0044] For example, the manufacturer can create the offset change rate for each gain in advance. After shipping, in the environment where the user installs the encoder, the signal is read once at an arbitrarily selected gain position, and the offset value (absolute amount) is calculated. When this value is imported into an entry in the correction table, the offset correction amount corresponding to other gains can be automatically calculated based on the set change rate. The user only needs to obtain the signal at an arbitrarily selected position. Therefore, an offset storage unit can be created that reduces the impact of environmental changes and posture changes between manufacturing and use, and reflects the use environment, thereby achieving more accurate measurements while minimizing the workload.

[0045] In the structure shown in FIG7 , a signal adjustment unit and an offset correction unit are commonly provided for the A-phase signal and the B-phase signal. On the other hand, the offset of the A-phase may become unequal to the offset of the B-phase. For example, the correction amount for a specific gain may be different, such as 50 for the A-phase and 80 for the B-phase. In the case of using the same correction amount, it may become difficult to improve the accuracy. However, when a common rate of change is stored and the corresponding offsets obtained for the A-phase and the B-phase at arbitrarily selected positions are multiplied by the magnification, the corresponding offset correction amounts for the A-phase and the B-phase are calculated, thereby enabling more accurate measurement.

[0046] For example, in actual use, when the offset of gain 1 is 100, by using Fig.10 The rate of change of the offset shown, Gain 2 is automatically calculated to be 110, and Gain 3 is automatically calculated to be 119.9. This calculation can be done for each phase.

[0047] In the present embodiment, the offset caused by switching the gain is obtained in advance by experiment, and the rate of change of the offset is stored in the offset storage unit 26. This makes it possible to appropriately set the offset correction amount in the case of switching the gain by performing AGC. This improves the measurement accuracy. Since the offset is corrected at the same time as the gain is switched, the time lag from gain switching to correction is reduced. In addition, when switching the gain, since only the rate of change of the offset needs to be read and multiplied, the calculation load is reduced. In addition, the user only needs to obtain the signal at one position arbitrarily selected in the correction operation. Therefore, the user's work burden can be minimized, and the influence of environmental changes and posture changes between manufacturing and use can be reduced.

[0048] In the above example, the amplitude adjustment unit 22 is used as an example of an amplitude adjustment unit for adjusting the amplitude of the detection signal output from the encoder by adjusting the gain of the detection signal so that the amplitude of the detection signal is within a predetermined range. The offset correction unit 23 is used as an example of an offset correction unit for correcting the offset of the amplitude center of the detection signal. The offset storage unit 26 is used as an example of a storage unit for pre-storing the relationship between the gain and the offset.

[0049] The present invention is not limited to the specifically disclosed embodiments and variations, but may include other embodiments and variations without departing from the scope of the present invention.

Claims

1. An offset correction device, comprising: an amplitude adjuster for adjusting the amplitude by adjusting a gain of a detection signal output from an encoder so that the amplitude of the detection signal is within a predetermined range, wherein the gain is adjusted by increasing the gain when the intensity of the detection signal is less than a first threshold value and decreasing the gain when the intensity of the detection signal is greater than a second threshold value; an offset corrector for correcting an offset of an amplitude center of the detection signal; and A memory for pre-storing the relationship between the gain and the offset, The encoder outputs sinusoidal wave signals with different phases as the detection signals. wherein the offset corrector refers to the relationship stored in the memory when the amplitude adjuster changes the gain, obtains an offset corresponding to the changed gain, and corrects the offset based on the obtained offset, and The amplitude adjuster changes the predetermined range according to the value of the gain by changing the first threshold and the second threshold according to the gain.

2. The offset correction device according to claim 1, wherein: The memory stores values ​​of the gain set in a stepwise manner, and stores an offset corresponding to each value of the gain.

3. The offset correction device according to claim 1 or 2, wherein: The memory stores the offset as a rate of change relative to an offset corresponding to a predetermined gain.

4. The offset correction device according to claim 1 or 2, wherein: The relationship between the gain and the offset stored in the memory is writable.

5. A position measuring device, comprising: The offset correction device according to claim 1 or 2; Encoder; as well as Arithmetic means for calculating a position based on the detection signal whose offset is corrected by the offset correcting means.

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