JPH0416713A - Optical linear encoder - Google Patents

Abstract

PURPOSE:To improve the measuring accuracy of a displacement by correcting the phase difference between a movable diffraction grating and a fixed diffraction grating electrically. CONSTITUTION:The light projected from a light source 3 is cast to a first diffraction grating 1 and a second diffraction grating 2. The light passing through the gratings 1, 2 enters a photodetector 4 to be converted to electric signals. The grating 1 is formed of a plurality of slits which are arranged with a predetermined pitch P, and is made movable. The grating 2 is fixed, and constituted of (n) rows of slits S1-Sn with the same pitch P. The (n)th slit Sn has the same phase as the first slit S1. A phase difference measuring part 5 measures the phase difference between the signal from the reference slit S1 and the signal from the phase correction slit Sn. A phase correcting part 6 corrects the phase of the signal detected by the photodetector 4 on the basis of the phase difference measured by the measuring part 5. In this manner, the measuring accuracy is improved by making correction corresponding to the phase difference of signals.

Description

[Detailed Description of the Invention] [Summary] The present invention provides a fixed diffraction grating with a plurality of rows of measurement slits arranged with the slit positions shifted by a predetermined distance in the direction of relative movement of the diffraction grating, and A phase correction slit is provided which has the same phase as the reference slit in the center and is placed a predetermined distance from the reference slit in a direction perpendicular to the direction of relative movement of the diffraction grating. The phase difference with the signal is measured, and the phase of the signal from the measurement slit is corrected based on the measured phase difference.

This makes it possible to remove the phase shift caused by the in-plane rotation between the diffraction gratings from the signal for displacement measurement, thereby further improving the measurement accuracy of displacement measurement.

[Industrial Application Field] The present invention relates to an optical linear encoder that measures relative displacement between a movable diffraction grating and a fixed diffraction grating by detecting light transmitted through diffraction gratings arranged opposite to each other.

[Conventional technology]

Conventionally, optical linear encoders have two rows of phase-shifted slits in a fixed diffraction grating placed opposite to a movable diffraction grating, and measure the displacement of the movable diffraction grating from changes in the intensity of light passing through these slits. It has been known.

FIG. 7 is a diagram showing the structure of such a conventional optical linear encoder. In the figure, light emitted from a light source 11 is converted into parallel light by a collimator lens 12, and enters a movable diffraction grating 13 and a fixed diffraction grating 14 disposed close to the movable diffraction grating 13. This movable diffraction grating 13 is provided with a plurality of slits at a pitch P, as shown in FIG. 8(a). In addition, the fixed diffraction grating 14 is provided with one
A plurality of slits are provided at a pitch P shifted by /4P pitch. Furthermore, at a position facing the light source 11 with the movable diffraction grating 13 and the fixed diffraction grating 14 in between, there is a photodetector 15 that detects the light transmitted through the upper slit of the fixed diffraction grating 14 and converts it into an electrical signal. A photodetector 16 is arranged to detect the light transmitted through the lower slit and convert it into an electrical signal.

In the optical linear encoder with the above configuration, if the movable diffraction grating 13 moves in the X direction, the photodetector 15
The intensity of the optical signal detected is maximum when the slits of the movable diffraction grating 13 and the slits of the fixed diffraction grating 14 coincide, and is minimum when the slit positions of both are deviated by 180°. Here, since the upper slit and the lower slit of the fixed diffraction grating 14 are arranged with a P/4 phase shift, the detection signals of the photodetectors 15 and 16 are as shown in FIG. 9(a).
, the waveform has a 90° phase difference as shown in ('b). The signal waveforms obtained by binarizing these detection signals have waveforms as shown in FIGS. 9(C) and (5). Further, the signal waveform obtained by detecting the difference between the two binarized signal waveforms becomes the waveform shown in FIG. 8(e). This edge detection signal is a signal generated at P/4 intervals, so by counting the number of pulses of the edge detection signal, the movable diffraction grating 1
The displacement ΔX of 3 can be obtained as ΔX=P/4 X (number of pulses).

[Problem to be solved by the invention]

As described above, in order to be able to calculate the displacement ΔX from the edge detection signals of the signals detected by the photodetectors 15 and I6, it is assumed that the slits of the movable diffraction grating 13 and the fixed diffraction grating 14 are parallel. Become.

However, it is difficult to arrange the slits of the two diffraction gratings completely parallel, and in reality they are arranged with some kind of angular deviation. This angular deviation between the two is called the in-plane rotation angle. If the in-plane rotation angle is not zero, the phase difference between the light passing through the upper slit of the fixed diffraction grating 14 and the light passing through the lower slit will not be exactly P/4.
For example, as shown in FIG. 10, there is a problem in that the phase of the edge detection signal is shifted by the in-plane rotation angle, resulting in an error in displacement measurement.

Therefore, conventionally, when arranging the movable diffraction grating 13 and the fixed diffraction grating 14, an adjustment mechanism was provided to suppress in-plane rotation.

An object of the present invention is to enable electrical correction of a phase shift due to in-plane rotation between a movable diffraction grating and a fixed diffraction grating.

[Means to solve the problem] FIGS. 1(a) and 1(b) are diagrams explaining the principle of the present invention.

Detecting the light from the light source 3 which is incident through the first diffraction grating 1 and the second diffraction grating 2 which are arranged oppositely, the first diffraction grating 1
In the optical linear encoder that measures the relative displacement between the second diffraction grating 2 and the second diffraction grating 2, the second diffraction grating 2 has a plurality of slits arranged at the same pitch and with the slit positions shifted by a predetermined distance in the relative movement direction of the diffraction grating. Column measurement slit 31
, 32..., and a phase correction which is in the same phase as the reference slit (Sz) in the plurality of rows of slits, and which is heated and placed at a predetermined distance from the reference slit s+ in a direction perpendicular to the direction of relative movement of the diffraction grating. slit Sn. This second diffraction grating 2 is, for example, a fixed diffraction grating, and the first
Diffraction grating 1 is a movable diffraction grating.

The second diffraction grating 2 shown in FIG. 1(b) is composed of n slit rows, but there is no particular need for n slit rows.
It is sufficient to have at least a reference slit for phase correction, a correction slit, and one or more displacement measurement slits.

The photodetector 4 detects the light incident through the first diffraction grating 1 and the second diffraction grating 2 and converts it into an electrical signal.

The phase difference measurement unit 5 is a circuit that measures the phase difference between the signal from the reference slit SI and the signal from the phase correction slit Sn.

Further, the phase correction section 6 calculates a plurality of columns of measurement sulins) S+, based on the phase difference measured by the phase difference measurement section 5.
Sz... and corrects the phase of the signal detected by the photodetector 4.

[For production]

Since the reference slit Sl and the phase correction slit position are arranged as described above, when the in-plane rotation angle between the two diffraction gratings is zero, the signals from both slits measured by the phase difference measuring section 5 The phase difference between the two diffraction gratings becomes zero, and when the in-plane rotation angle between the two diffraction gratings is not zero, the phase shift caused by the in-plane rotation angle is measured by the phase difference measurement unit 5 as the phase difference between the signals from both slits. Ru. That is, by measuring the phase difference between the reference slit SL and the phase correction slit Sa, the amount of phase shift due to in-plane rotation between the two diffraction gratings can be determined.

Therefore, the signal phase from the measuring ring (S+, Sz, etc.) for measuring the relative displacement between the diffraction gratings is calculated from the phase difference of each signal calculated from the phase difference measured by the phase measuring section 5. By correcting this, the phase shift due to in-plane rotation can be removed from the measurement signal, and measurement accuracy can be further improved.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a configuration diagram of an optical linear encoder according to an embodiment of the present invention. In the figure, light emitted from a semiconductor laser 11 serving as a light source is converted into parallel light by a collimator lens 12, and is irradiated onto a movable diffraction grating 13 and a fixed diffraction grating 17.

The light that has passed through the movable diffraction grating 13 and the fixed diffraction grating 17 passes through a magnifying lens 18, enters a detector array 19 consisting of a plurality of photodetectors, and is converted into an electrical signal.

The movable diffraction grating 13 consists of a plurality of slits 13a arranged at a constant pitch P, as shown in FIG. 3(a), and is movable in the X direction. Further, the fixed diffraction grating 17 is composed of n slit rows S1 to Sn with the same pitch P, as shown in FIG. They are arranged to be shifted by a predetermined distance (for example, P/n) in the moving direction X, and the nth slit Sn (phase correction slit) is in the same phase as the first slit S (reference slit). . In this embodiment, the fixed diffraction grating 17 is provided with n slit rows, and the slits are arranged with a phase difference in order to increase the position detection resolution during displacement measurement. The number and arrangement of
They may be arranged as appropriate depending on the position detection resolution, pitch P, etc. required for measurement.

The detector array 19 includes slits 51 to 51S.

It consists of n photodetectors that detect the transmitted light of each of them, and outputs the detection signal of each photodetector to a signal selection section 20 and a phase difference measurement section 21, which will be described later.

Next, the displacement measuring operation when moving the movable diffraction grating 13 will be explained with reference to the circuit configuration diagram of the displacement measuring section in FIG. 4 and the signal waveform diagram in FIG. 5.

The displacement measurement section in FIG. 4 is a circuit that measures the displacement of the movable diffraction grating 13 by removing the amount of phase shift caused by in-plane rotation of the diffraction grating from the displacement measurement signal detected by the detector array 19. In the same figure, the signal selection section 20 is a circuit that sequentially selects n detection signals (moiré signals) detected by the director array 19 under the control of the encoder control section 22 and outputs them to the phase difference correction section 23. It is.

The phase difference measurement unit 21 is a circuit that measures the phase difference between the first slit St and the n-th slits S and l of the fixed diffraction grating 17.

In this embodiment, the slit S1 and the slit S1 are arranged so that they have the same phase, so when the detector array 19 detects the light that has passed through these two slits, the phase difference between the detection signals is zero. It should be. However, since the two diffraction gratings actually have a certain in-plane rotation, the detection signals from both slits are out of phase by the phase shift caused by the in-plane rotation. Therefore, by measuring the phase difference between the signals from both slits, it is possible to determine the amount of phase shift caused by the in-plane rotation of the two diffraction gratings.

In the phase difference measuring section 21, for example, as shown in FIG. The signal y2 from the slit Sn is taken in, and the absolute value ΔI of the difference in signal strength between the two is calculated. Here, between the signal strength difference Δ■ and the phase difference Φ,
Since there is a relationship as shown in FIG. 6, if the intensity difference ΔI is determined, the phase difference Φ between both signals can be calculated.

The phase difference correction section 23 is a circuit that corrects the phase of the signal output from the signal selection section 20 based on the phase difference measured by the phase difference measurement section 21, and outputs the corrected signal as an encoded signal.

Now, if the phase difference between the signal from the slit S1 and the signal from the slit Sn measured by the phase difference measurement unit 21 is Φ, the amount of phase shift due to in-plane rotation of the signal of the k-th slit S is Φh = ((k-1)/(n-1))×
It can be expressed as Φ.

Therefore, by determining whether the phase shift at that time is positive or negative, and subtracting or adding the amount of phase shift Φk to the signal from the second slit Sk, the measured signal can be calculated from the diffraction grating. The influence of phase shift due to in-plane rotation can be removed. This eliminates the need for a mechanical adjustment mechanism for adjusting the in-plane rotation of the diffraction grating. Further, since the phase shift is corrected electrically, there is no change over time, and the phase shift does not increase due to the shift between the diffraction gratings due to mechanical vibration or the like.

〔Effect of the invention〕

According to the present invention, the amount of phase shift caused by in-plane rotation of the diffraction grating can be corrected from the phase difference of the signal from the phase correction slit etc. provided in the second diffraction grating. This makes it possible to improve the accuracy of displacement measurement.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the principle of the present invention. Figure 2 is a configuration diagram of an optical linear encoder according to an embodiment of the present invention. Figure 3 is a structural diagram of a diffraction grating. Figure 4 is a displacement measuring section. 5 is a diagram showing the detection signal waveform of the photodetector, FIG. 6 is an explanatory diagram of the relationship between the detection signal strength and the phase difference, and FIG. 7 is a conventional optical linear encoder. 8 is a structural diagram of the diffraction grating of FIG. 7, FIG. 9 is a waveform diagram of a photodetection signal, and FIG. 10 is an explanatory diagram of phase shift due to in-plane rotation. ■...First diffraction grating, 2...Second diffraction grating, 3...Light source, 4...Photodetector, 5...Phase difference measurement section, 6...Phase correction section.

Claims (1)

[Scope of Claims] An optical system that detects light from a light source that enters through a first diffraction grating and a second diffraction grating that are arranged opposite each other to measure the relative displacement between the first diffraction grating and the second diffraction grating. In a linear encoder, a plurality of rows of measurement slits are arranged at the same pitch and the slit positions are shifted by a predetermined distance in the direction of relative movement of the diffraction grating, and a reference slit among the plurality of rows of slits is in the same phase, and a second diffraction grating comprising a phase correction slit arranged at a predetermined distance from a reference slit in a direction perpendicular to the direction of relative movement of the diffraction grating, and detecting light incident through the first diffraction grating and the second diffraction grating. phase difference measurement that measures the phase difference between a signal that enters through the reference slit and is detected by the photodetector, and a signal that enters through the phase correction slit and is detected by the photodetector. and a phase correction section that corrects the phase of the signal that is incident through the plurality of rows of measurement slits of the second diffraction grating and detected by the photodetector, based on the phase difference measured by the phase difference measurement section. An optical linear encoder comprising: