JPS61149822A - Correcting device for phase error - Google Patents

Abstract

PURPOSE:To attain real-time, high-precision phase correction by a simple data processing system through simple operation by inputting a known phase difference and a coefficient of correction to a storage device, and multiplying its output value by a sine wave signal and thus calculating an angle correcting value. CONSTITUTION:A sine wave detector 2 and a cosine wave detector 3 are connected to an encoder 1 and their detected values are inputted to an angle computing element 6 through A/D converters 4 and 5 respectively to calculate the angle value. A phase difference setting switch 7 for inputting the known phase difference, on the other hand, is provided and its set value and coefficient K of correction are stored in a storage device 8. The absolute value of the sine wave signal is similar to an angle measurement error curve, so the output value of the storage device 8 and the output value of the A/D converter 4 are inputted to a multiplier 9 to obtain the angle correcting value. This angle correcting value is subtracted from the output of the angle computing element 6 by a subtracter 10 to obtain a corrected angle signal, which is displayed on a display device 11.

Description

DETAILED DESCRIPTION OF THE INVENTION a. Technical Field The present invention relates to a phase error correction device that can sequentially and automatically correct phase errors included in angle measurement values of an optical encoder or the like.

b. Prior art and its problems An optical encoder consists of a disk having black and white patterns arranged at equal intervals on the circumference and a scanning disk rotating in opposition to the disk. The rotation angle is measured from a pair of sine wave signals and cosine wave signals. Normally, this angle measurement value includes angular errors, but the causes of these errors are diverse. The main causes are errors in the manufacturing of the disc, such as pitch errors in the disc scale, relative positional deviations between a pair of detection slits on the scanning disc, eccentricity of the disc, and problems in which the two discs are arranged in parallel. Mechanical errors such as no optical errors such as the spread angle and inclination angle of the light beam illuminating the disk. In the case of commercially available optical encoders, the angle is measured by dividing one pitch of the scale into four equal parts using photoelectrically detected positive and cosine waves.The accuracy is relatively rough, and the angle error in this case is possible. The extent of this is acceptable. On the other hand, in a precise encoder that guarantees angle measurement accuracy on the order of several seconds, the angle measurement error must be suppressed to less than this accuracy, and it is not easy to eliminate the cause of the error. Among the above error factors,
Errors in the manufacture of the disc and mechanical errors related to assembly and adjustment must be eliminated as much as possible, but they cannot be completely eliminated.
Therefore, the angle measurement accuracy of the encoder, that is, the degree to which the scale pitch can be varied, is limited by these errors.

As a conventionally known error reduction method, there is a method in which the illumination light beam of the disk is intentionally made obliquely incident, and the phase of the cosine wave signal relative to the photoelectrically detected sine wave signal is corrected so as to approach π/2. In addition, the scanning grating is arranged symmetrically on the disk diameter to perform so-called diagonal reading to eliminate the influence of disk eccentricity, and to average the signal amplitude by canceling out the DC component included in the scanning signal. A method of correcting the phase between both signals is also widely known.

However, there is a limit to such a phase correction method, and in the case of a precision encoder such as the one described above, the residual error will reduce the angle measurement accuracy.6 On the other hand, this phase error over the entire circumference of the scale disc are almost equal, measure this phase difference in advance,
It is conceivable to correct this using sine and cosine wave signals. The specific means for this is to perform Fourier analysis on both signals, find the DC component, amplitude, and phase difference of each signal from the zero-order and first-order components, and use this information as a pre-caulking correction constant for the goniometer. Store it in the electronic circuit.

It is sufficient to perform correction operations online during angle measurement. However, this correction operation is complicated, and the processing circuit is also somewhat complicated and expensive.

C0 target The present invention has been made in view of the above points.

It is an object of the present invention to provide a phase error correction device that can perform highly accurate phase correction in real time with a simple correction operation using an inexpensive data processing system.

d. Configuration of Embodiment FIG. 1 is a block diagram showing an embodiment of the present invention. A sine wave detector 2 provided in the optical encoder 1. The outputs of the cosine wave detector 3 are connected to A/D converters 4 and 5, respectively, and the outputs of these A/D converters 4.5 are connected to an angle calculator 6.
The configuration of 0 or more inputted to is found in conventionally known general goniometers, but the data processing system described below is the main part of this embodiment. The phase difference setting switch 7 is connected to a storage device 8 . The output of this storage device 8 and the output of the sine wave signal A/D converter 4 are both input to a multiplier 9. Further, the output of the multiplier 9 and the output of the angle calculator 6 are both connected to a subtracter 10, and the output of the subtracter 10 is connected to the display 11.
It is connected to the.

e. Effect of the Embodiment Next, a method for correcting the phase error between a sine wave signal and a cosine wave signal using an embodiment of the present invention having the above configuration will be described. However, in order to make it possible to correct such a phase error, one revolution (360') of the scale disc of the encoder must be
It is assumed that the phase error φ of the cosine wave signal b cos (θ+ψ) with respect to the sine wave signal a 5inO is approximately constant over the period, and that this phase error φ is pre-measured and known.

Note that a and b above are amplitudes, and θ is a phase angle.

The cause of this phase error φ mainly depends on the scale error in manufacturing the scale disk and scanning grating.

If the encoder is mechanically and optically adjusted with high precision, the phase error ψ will be almost the same for scale discs and scanning gratings from the same lot. This fact has been fully confirmed by experiment.

FIG. 2 is a diagram showing the behavior of the phase error ψ within one pitch (corresponding to a phase of 2π) of the scale disk. 6 When the cosine wave signal 13 lags behind the sine wave signal 12 by the phase error φ, Angle measurement error curve 14 for phase error φ within one pitch of marked line
exhibits a two-period sinusoidal change with peaks near π/2 and 3π/2, but the angular error Δ at 0 and π
θ is almost zero. As is clear from a simple calculation, the angle measurement error per phase error ψ=1° corresponds to about 0.27%. As mentioned above, if the phase error ψ is known, this angle measurement error curve 14 is digitized for each phase angle θ, and with this value Δθ, the actual angle measurement value θ'=tan""' (asi
nθ/bcos(θ+φ)], but it is extremely impractical to subdivide one phase angle θ and store a huge amount of correction values Δθ for all sampling points in the device. It is. Therefore, in the phase error correction device of the present invention, it is shown that the angle measurement error can be corrected using a sine wave signal that is actually sequentially measured in real time, and this is an essential feature of the present invention.

As this sine wave signal, one digitized by the A/D converter 4 shown in FIG. 1 is used.

FIG. 3 compares the sine wave signal and the angle measurement error curve.

The absolute value (solid line 16) of the sine wave signal 15 is expressed as the angle measurement error curve 1.
4, it is noticed that both waveforms are almost similar. Therefore, the next multiplication (correction value Δθ) = lasi
nθIX (known phase error φ) × (coefficient k) ・
When (1) is performed, it is found that this amount approximately matches the angle measurement error curve. Note that the sign of this correction value is selected to be the same as the sign of the phase error setting value.

Now, the total number of marking lines around the scale disk of the encoder is 1.
If there are 6200 lines, one pitch of the score line corresponds to an angle of 801. As mentioned above, the angle measurement error per phase error φ=1″ is approximately 0.27%, so the coefficient is 0.0027×80
'=0.22'. On the other hand -I(A/ of asinθ
D conversion value) 1 is a constant within 21 bits.

For example, the maximum value of this A/D conversion can be set to a constant 200.

Referring again to FIG. 1, the operation of one embodiment of the present invention will be explained.

The phase difference setting switch 7 is a 4-bit digit switch including the sign (±). The correspondence between the known phase difference ψ and the set bit value is set as or. Bit value 7 corresponds to the maximum phase difference ψwax that can be considered for the actual device.

This phase difference φ is read into the storage device 8 together with the above-mentioned constants when the power is turned on. The sequentially input sine wave signal is A/D
The memory bag is digitized with converter 4! ! 8 is input to the multiplier 9, and the multiplier 9 calculates the correction value shown in equation (1). On the other hand, in the angle calculator 6, the actual angle measurement value θ' including the phase error ψ=tan-1(asinθ/bco
s(θ+ψ)] is calculated. This angle measurement value θ' is subtracted in the subtracter 10 using the output of the multiplier 9, θ0=[(
Angle measurement value θ′)−(correction value Δ0)] is calculated. This θ.

is the true angle measurement value with the phase error corrected, and 00 is displayed on the display 11.

As mentioned above, the correction method of the phase error correction device of the present invention is an approximate solution method using a sine wave signal. Therefore,
Let us examine the residual errors that cannot be corrected even with this method.

FIG. 4 shows the angle measurement error curve and the residual error curve.

Now, from the angle measurement error curve 17 when one phase error is 20° in FIG. 4(a), the above numerical example (l pitch 80'
), the maximum angular error before correction is 4.41
corresponds to This error is calculated by coefficient k = 0.22'/deg
A residual error curve 18 is shown after correction using , but this residual error has small positive values and is largely biased toward negative values. This is a phenomenon caused by the fact that the cosine wave signal is deviated from the sine wave signal by ψ, as well as the approximation correction using the absolute value of the sine wave signal. Therefore, the coefficient k is set to 0.18' so that the residual error is balanced in both positive and negative directions.
/deg, i.e. the above coefficient k =0.22' /dec
, the residual error curve 19 in Figure 4(b)
get. From this residual error curve 19, the initial angle measurement error Δ
It can be seen that O=4.4' is reduced to ±11.

Assuming that a 5-second reading encoder is realized by dividing one pitch (go') of the marked line into 16 parts, the angle measurement accuracy is ±1'. Note that the phase error of 20° illustrated here is under the worst possible conditions for current equipment.
If consideration is given to the production of the scale disk and mechanical and optical considerations during assembly and adjustment, the residual error can be made even smaller.

f, Effect As described above, according to the phase error correction device according to the present invention,
With a relatively simple and inexpensive electronic circuit system, the calculation time is extremely short, and angle measurement value correction can be realized online.

Although only an optical encoder has been described in the above embodiment, the phase error correction device according to the present invention is applicable to a measuring device 1 that processes a pair of sine wave and cosine wave signals, such as a magnetic encoder, a counting scale using moiré fringes, etc. , and is widely applied to instruments for measuring interference fringes.

[Brief explanation of the drawing]

, Figure 1 is a block diagram showing one embodiment of the present invention, Figure 2 is a diagram showing changes in phase error φ within the l pitch of the scale disc, and Figure 3 is a diagram comparing the sine wave signal and the angle measurement error curve. Figure 4
Figures (a) and (b) are diagrams showing angle measurement error curves and residual error curves. 1... Encoder 2... Sine wave detector 3...
・Cosine wave detector 4, 5...A/D converter 6...
・Angle calculator 7...Phase difference setting switch 8...
・Storage device 19... Multiplier IO... Subtractor 1
1...Display device

Claims (1)

[Claims] A rotation angle measuring device using an optical encoder or the like, comprising: a phase difference setting switch that inputs a known phase difference; and an A/D converter that converts a sine wave signal generated as the encoder rotates into a digital signal; a storage device that stores the output of the phase difference setting switch and a correction coefficient; a multiplier that multiplies the output of the storage device and the output of the A/D converter to output an angle correction value; and an output of the multiplier. A phase error correction device consisting of a subtracter that corrects the angle measurement value.