JP2001311630A - Optical encoder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize the reception signal of an optical encoder. SOLUTION: This device is provided with a scale 10 where a diffraction grating for forming interference fringes is provided, and photodetectors 16 and 18 that are formed in a grating shape. The two photodetectors 16 and 18 are arranged with a phase difference of 90 deg. and output a signal corresponding to the quantity of each received light. A reference signal arithmetic part 24 obtains a reference signal that is the root sum square of a signal corresponding to the quantity of light. The light intensity of a light source 12 is controlled by a light source control part 28. The AC component of the output signal of the photodetector indicates the shading of interference fringes, and the waveform of the output signal is made constant by controlling the light intensity of the light source based on it. Also, the reference signal calculates the absolute value of each AC component of a signal corresponding to the quantity of received light, and further the absolute values are added.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical encoder for performing position measurement based on an interference fringe formed by diffraction of light passing through a grating, and more particularly to control of the light amount of a light source. .

[0002]

2. Description of the Related Art An optical encoder using a diffraction grating is known as a device for measuring the amount of movement of an object. The optical encoder includes a light source, a first grating that receives light from the light source and forms interference fringes by diffraction, and a second grating that moves relative to the interference fringes and has the same period as the period of the interference fringes.
It has a lattice. Further, a light receiving element for detecting light passing through the second grating is arranged. The amount of light passing through the second grating, that is, the amount of light reaching the light receiving element, changes according to the position of the second grating relative to the interference fringes. By detecting this change, the number of interference fringes passing through the second grating is counted. can do. If one grid is fixed and the other is placed on the object to be measured, the movement amount of the target object can be measured by counting the number of passed interference fringes.

Further, in order to accurately measure the position of one interference fringe within one cycle, two second gratings and two light receiving elements are provided. The two second gratings are arranged such that the phases with respect to one cycle of the interference fringes are different from each other. Preferably, two second gratings are arranged with a shift of 1/4 period of the interference fringes, that is, with a phase shift of 90 °. Thus, when the first and second gratings are relatively moved, the output waveforms of the light receiving elements arranged corresponding to the two second gratings have a phase difference of 90 °. Therefore, the arc tangent of these outputs is the phase angle.

[0004] It is to be noted that a similar output can be obtained by forming the light receiving element in the same lattice shape as the second grating instead of the second grating and the light receiving element.

[0005] As a light source used in such an optical encoder, an LED (light emitting diode), an LD (laser diode) or the like is used. The light output of these elements is
It fluctuates depending on the applied voltage and the ambient temperature. Further, in the manufacturing process, variations occur in the line width and the film thickness of the lattice. Due to these fluctuations and variations, the amount of received light and the contrast of interference fringes fluctuate, and there is a problem in that the accuracy of measurement decreases.

[0006] To solve this problem, Japanese Utility Model Application
Japanese Patent No. 46023 discloses a method of receiving light from a light source without passing through a diffraction grating and controlling the light intensity of the light source so that the received light intensity is constant. Also,
JP-A-59-210322 and JP-A-60-5
No. 8513 discloses a method of adding outputs of a plurality of second gratings arranged at different phases and controlling the light intensity of a light source based on the added value.

[0007]

The apparatus disclosed in the former publication, that is, the apparatus disclosed in Japanese Utility Model Laid-Open Publication No. Sho 60-46023, can compensate for fluctuations and variations in the light intensity of the light source. However, since light passing through the grating is not used, it is not possible to compensate for variations in grating line width, changes in transmittance, and the like. Further, in the device disclosed in the latter publication, in addition to the fluctuation caused by the light source itself, the fluctuation of the total light amount due to the grating line width and the transmittance can be compensated. However, when this control is performed, the contrast of interference fringes may change. This is because, for example, when the grid line width changes, the total light quantity changes in proportion, whereas the contrast of the interference fringes does not change in proportion. This problem is caused by a change in the relative attitude of the two diffraction gratings, in addition to the change in the line width. The difference between the total amount of light of the light receiving element is small when the two diffraction gratings are arranged in parallel and when the two diffraction gratings are arranged obliquely. On the other hand, the parallel arrangement and the oblique arrangement involve a change in the distance between the gratings, which greatly affects the contrast of interference fringes. Therefore, the problem caused by the attitude of the diffraction grating cannot be solved even by controlling the light intensity of the light source based on the total light amount.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and has been made to stabilize the contrast of interference fringes.
An object of the present invention is to provide an optical encoder capable of obtaining good measurement accuracy.

[0009]

In order to solve the above-mentioned problems, an optical encoder according to the present invention is arranged with a predetermined phase difference in an arrangement direction of interference fringes formed by diffraction of a first grating. Reference signal calculation means for calculating reference signals having two second gratings and further calculating the amplitude of each AC component of the received light amount signal obtained by these second gratings, and the reference signal being constant Light amount control means for controlling the light amount of the light source as described above.

The AC component of the received light signal represents the contrast of the interference fringes. By controlling the light intensity of the light source based on this, the waveform of the received light signal can be made constant.

The reference signal may be obtained by calculating the absolute values of the AC components of the two received light quantity signals and adding them. The circuit for calculating and adding the absolute value can be configured relatively easily.

The reference signal may be obtained by squaring the AC components of the two received light amount signals and taking the square root of the sum of the squared components.

[0013]

Embodiments of the present invention (hereinafter referred to as embodiments) will be described below with reference to the drawings. FIG. 1 shows a schematic configuration of an optical encoder according to the present embodiment. The scale 10 on which the grid is formed is fixed to the moving measurement object, and moves in the grid arrangement direction indicated by the arrow in the figure. The scale 10 receives light from the light source 12 and forms interference fringes behind. That is, scale 1
The grating formed at 0 functions as the first grating in the present invention. The light receiving element array 14 is fixedly arranged at a position where interference fringes are formed by diffraction of the scale 10. On the light receiving element array 14, light receiving elements are arranged in a lattice. Light receiving element in the upper half in the figure (hereinafter A-phase light receiving element) 1
The 6 and lower half light receiving elements (B-phase light receiving elements) 18 are each formed with a grating having a pitch equal to the pitch of the interference fringes, and are arranged with a quarter pitch, that is, a phase of 90 °. When the A-phase and B-phase light receiving elements 16 and 18 receive light, they generate a voltage corresponding to the amount of light. Therefore,
A-phase and B-phase light receiving elements 16 and 1 with movement of scale 10
8 outputs a fluctuating voltage signal. Thus, the A-phase and B-phase light receiving elements 16 and 18 formed in a lattice shape also function as the second lattice having the A-phase and B-phase lattices in the present invention.

The output signals of the A-phase and B-phase light receiving elements 16 and 18 are sent to an arithmetic section 20. The calculation unit 20 includes a position calculation unit 22 that calculates the amount of movement of the scale 10 to obtain the position of the measurement target, and a reference signal calculation unit 24 that calculates a signal serving as a reference for controlling the light intensity of the light source 12. Including. The position calculation unit 22 first calculates the A-phase or B-phase output signal
The peak of the signal is counted to obtain the number of interference fringes that have passed by diffraction. If the pitch of the interference fringes is multiplied by the number counted,
The amount of movement of the scale 10 is roughly known. The amount of movement of the interference fringe or less is calculated by calculating the phase angle based on the AC components of the A-phase and B-phase output signals.

The reference signal calculation unit 24 calculates the square values of the AC components of the A-phase and B-phase output signals, adds these values, and generates a square-rooted reference signal. The reference signal is sent to the light source control unit 28, and the light source control unit 28 controls the light source 12 so that the reference signal becomes constant.
To control the light intensity.

FIG. 2 shows an example of a circuit configuration for realizing the functions of the reference signal calculator 24 and the light source controller 28. In this example, the A-phase and B-phase light receiving elements 1
Reference numerals 6 and 18 denote light-receiving elements having opposite phases, a-phase and a ′, respectively.
Phase, b-phase and b'-phase, and the A-phase and B-phase output signals are obtained as the difference between these phases. These output signals VA and VB are composed of AC components Va and Vb and a DC component Vref.
Can be divided into That is,

## EQU1 ## VA = Va + Vref VB = Vb + Vref

In the circuit shown in FIG.
Is configured as a circuit that calculates the square root of the sum of squares of the AC components of the A-phase and B-phase output signals. The AC components Va and Vb of the A-phase and B-phase output signals are sent to squaring circuits 36 and 38, respectively, where the squared signals Va ² and Vb ² are calculated. The square signals Va ² and Vb ² are sent to an adding circuit 40, where they are added, and further sent to a square root calculating circuit 42. The square root calculation circuit 42 is a general-purpose square root circuit 44
And an amplifier circuit 46 for adjusting the output signal of the adder circuit 40 to a voltage adapted to the input voltage of the general-purpose circuit.

The output of the square root calculation circuit 42 is the reference signal Vt.
And transmitted to the light source control unit 28. Light source control unit 28
Then, the light intensity of the light source is controlled so that the reference signal Vt becomes constant. The reference signal Vt is

Vt = (Va ² + Vb ² ) ^1/2 , and the light source 12 is controlled so that the signal Vt becomes constant.

FIG. 3 is a diagram showing the A-phase signal VA and the B-phase signal VB when the control of this embodiment is performed. The A-phase signal VA is plotted on the horizontal axis, and the B-phase signal VB is plotted on the vertical axis. VA-VB
Represents a coordinate system. When the circuit shown in FIG. 2 is controlled to keep the square root of the sum of squares constant, the output signals VA and VB of the A-phase and the B-phase have a radius Vt about the point O as shown by the broken line in FIG. , The so-called Lissajous circle. FIG.
The phase angle θ shown in

Equation 3 sin θ = Vb / Vt cos θ = Va / Vt The movement amount s of one or less pitch (one cycle) of the interference fringes is obtained from the phase angle θ.

S = (θ / 2π) p

Since the light intensity of the light source is controlled by the circuit shown in FIG. 2 so that the reference signal Vt is constant, the radius of the circle shown in FIG. 3 is always constant. This indicates that the waveform of the AC component of the received signal of each phase is constant, and a stable phase angle θ can always be obtained.

The circuit shown in FIG. 2 has squaring circuits 36 and 38 and a square root calculating circuit 42, and the circuit scale may be large. An example in which the circuit size of the reference signal calculation unit 24 can be reduced will be described below.

FIG. 4 shows another example of a circuit configuration for realizing the functions of the reference signal calculator 24 and the light source controller 28. Light source 12, A-phase and B-phase light receiving elements 16, 1
8, and the configuration of the difference circuit for obtaining the A-phase and B-phase output signals is the same as that shown in FIG. Therefore, A
The phase and B-phase output signals VA and VB are the same as those in the circuit shown in FIG.

The output signals VA and VB are calculated by an absolute value calculating circuit 3.
0,32. In the absolute value calculating circuits 30 and 32, the absolute values | Va | of the AC components Va and Vb of the output signal are calculated.
| Vb | is calculated and sent to the adding circuit 34. The adding circuit 34 calculates the reference signal Vs by adding the absolute values | Va | and | Vb | and sends it to the light source control unit 28. The light source control unit 28 controls the light intensity of the light source so that the reference signal Vs becomes constant. That is,

The brightness of the light source changes periodically so that Vs = | Va | + | Vb | = (constant).

A point P (VA, V) representing the position of the scale 10
B) draws a square locus indicated by a solid line in FIG. 3 as the scale 10 moves. When the scale 10 moves by one pitch of the interference fringes, the point P goes around a square locus. The angle θ between the line OP connecting the center O of the square diagonal and the point P and the horizontal axis is the phase angle, and represents the movement amount s of the scale 10 which is equal to or less than one pitch (one period) of interference fringes. . Assuming that one pitch of the interference fringes is p, the moving amount s of one cycle or less is

S = (θ / 2π) p

According to the circuit configuration of FIG. 4, since the light source control circuit operates so as to keep the reference signal Vs constant, the size of the square shown in FIG. 3 is always constant. The fact that the square size shown in FIG. 3 is constant indicates that the waveform of the AC component of the received signal of each phase is constant, and a stable phase angle θ can always be obtained.

As described above, due to the functions of the reference signal calculation unit and the light source control unit illustrated in FIGS. 2 and 4, in this embodiment, the light source becomes dark due to a decrease in the efficiency of the light source due to a change with time or the like. Even if the scale becomes dirty and the amount of light to the light receiving element array 14 decreases, the power supplied to the light source 12 is controlled to increase the amount of light, and the size of the circle or square in FIG. Is maintained. Further, even when an inclination occurs between the scale 10 and the light receiving element array 14, the size of the circle or square in FIG. 3 is maintained. The efficiency of the light source decreases, the scale becomes dirty, and the scale and the light receiving element array 14
When the light amount control of the present embodiment is not performed, such a phenomenon that the angle becomes oblique acts to reduce the circular locus indicated by the broken line in FIG.

The circuit shown in FIG. 4 is composed of an absolute value circuit and an addition circuit, and does not include a square circuit and a square root circuit as in the circuit of FIG.
The element circuit scale can be reduced. On the other hand, since the light source repeats blinking, it is necessary to have good responsiveness, and it is necessary to select a light source having an appropriate performance.

The circuit of FIG. 2 includes a squaring circuit and a square root circuit, and if only the reference signal calculation section is viewed, the circuit scale becomes large. However, other circuits such as a circuit for obtaining the phase angle θ based on the A-phase and B-phase output signals VA and VB may be based on the assumption that the output signals VA and VB are sine waves. In this case, in order to cope with the blinking of the light source as in the circuit of FIG. 2, the configuration of other circuits must be modified. Therefore, when the influence on other circuit configurations is large, it may be advantageous to employ the circuit of FIG.

In this embodiment, the absolute value calculation circuit 3
0, 32, the addition circuit 34, the squaring circuits 36, 38, the square root calculation circuit 42, and the like are configured by analog circuits.
It is also possible to digitally convert the phase signal and the B-phase signal, and to process the operation of each of the circuits by digital operation.

In the case of digital arithmetic processing, even if control based on a circular Lissajous figure, ie, control using the reference signal Vt, is performed, the circuit scale does not become larger than control based on a square figure. However, in this case, it is conceivable that the calculation processing time is extended and the control response speed is reduced. If this control delay becomes a problem, D
Equipped with an SP (Digital Signal Processor) chip to increase the operation speed.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a schematic configuration of an optical encoder according to an embodiment.

FIG. 2 is a diagram illustrating an example of a reception signal processing circuit and a light source control circuit.

FIG. 3 is a diagram illustrating a received signal obtained in the present embodiment.

FIG. 4 is a diagram illustrating another example of a reception signal processing circuit and a light source control circuit.

[Description of Signs] 10 scale (first grating), 12 light sources, 14 light receiving element array, 16 A-phase light receiving element (A-phase grating), 18
B-phase light receiving element (B-phase grating), 22 position calculation unit, 24
Reference signal calculation unit, 28 light source control unit, 30, 32 absolute value calculation circuit, 34 addition circuit, 36, 38 square circuit,
40 addition circuit, 42 square root calculation circuit.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2F065 AA02 AA09 BB13 BB15 BB18 CC21 DD09 FF16 FF18 FF48 FF51 GG01 GG12 HH05 HH13 JJ02 JJ05 JJ09 JJ25 LL41 MM03 NN02 QQ00 QQ27 QQ28 Q103BA06BA02BA032 EA15 EB01 EB16 EB33 ED02 FA00 FA12

Claims (5)

[Claims] A first grating that diffracts light from a light source to form interference fringes; and a second grating that moves relative to the interference fringes in the direction in which the interference fringes are arranged. A second grating having an A-phase grating and a B-phase grating arranged with a phase difference; and receiving light from a light source that changes according to the relative positions of the interference fringes and the A-phase grating and the B-phase grating, respectively. A-phase light-receiving element and B-phase light-receiving element for outputting a signal corresponding to the amount of received light; Reference signal calculating means for generating a reference signal representing the amplitude of the AC component from an AC component of each output signal of the A-phase and B-phase light receiving elements; and a light amount of a light source such that the reference signal has a predetermined value. Light type control means for performing control, optical type Encoder. 2. The optical encoder according to claim 1, wherein the reference signal calculating means calculates an absolute value of an AC component of each of the output signals, and further calculates a reference signal obtained by adding these values. An optical encoder. 3. The optical encoder according to claim 1, wherein the reference signal calculating means squares an AC component of each of the output signals and further calculates a reference signal obtained by taking a square root of the sum of the AC components. An optical encoder to be calculated. 4. The optical encoder according to claim 1, wherein the A-phase grating and the A-phase light receiving element are provided.
An optical encoder, wherein the B-phase grating and the B-phase light receiving element are formed by forming the light receiving elements in a lattice shape. 5. The optical encoder according to claim 1, wherein the A-phase grating and the B-phase grating have a width of 9 mm.
An optical encoder arranged with a phase difference of 0 °.