JPH03115922A - Signal processing circuit and displacement measuring instrument equipped with the same - Google Patents

Abstract

PURPOSE:To improve the measurement accuracy by detecting the difference between a 1st and a 2nd signal whose phases are different from each other by 180 deg. and almost eliminating the DC component of a 3rd signal corresponding to the difference. CONSTITUTION:Photocurrent signals from photodetector 14 and 15 are converted into voltage signals whose phases are different from each other by 180 deg. by operational amplifiers 41 and 42 and the phase difference is detected by an operational amplifier 43 to send an output V0 from an output terminal 44. At this time, a variable resistor 61 is added to the amplifier 42 and variations of the DC components of sine wave signals from the elements 14 and 15 and a DC component attending on the rotation are canceled.

Description

[Detailed Description of the Invention] [Technical Field] The present invention relates to a signal processing circuit, and is particularly used in a displacement measuring device that measures the amount of movement (translational movement or rotation) and movement speed of an object to be examined. This invention relates to signal processing circuits.

[Prior art]

2. Description of the Related Art In recent years, precision machines such as NC machine tools and semiconductor printing equipment require precision measuring instruments that can measure the position of an object to be inspected with a resolution of 1 μm or less (submicron).

A measuring instrument that can measure displacement with submicron resolution uses a coherent beam of light such as a laser, and forms interference light by interfering with a pair of diffracted lights from a moving object.
Rotary encoders and linear encoders that photoelectrically convert the interference light are well known.

This type of encoder is disclosed in, for example, Japanese Patent Application Laid-Open No. 62-193922 and Japanese Patent Application Laid-open No. 6219392 by the applicant.
3, etc., and the devices in these publications split the laser beam from the laser into two, and each laser beam is split into two parts on a diffraction grating connected to a rotating object or a translational object. It is input to.

A pair of positive and negative diffraction lights of the same order generated from the diffraction grating are made to interfere with each other to form interference light, and the interference light is detected by a light receiving means and converted into an electric signal, thereby controlling the movement of the diffraction grating. is required with high precision.

The light-receiving means is usually composed of a pair or two or more light-receiving elements. For example, interference light beams having phases different by 80 degrees are made incident on the pair of light-receiving elements, and the electricity from these light-receiving elements is detected. The signal is processed to determine the amount of movement of the diffraction grating. The circuit for performing this processing includes a differential amplifier, which detects the difference between the signals from these light-receiving elements that are 180 degrees out of phase with each other, and inputs the detected signal to a comparator. It is configured to calculate the amount of movement by converting it into pulses and counting the pulses. In this processing circuit, since the DC component of the signal from each light receiving element is subtracted, the measurement accuracy is less likely to deteriorate due to changes in laser emission intensity or the like. However, since the level of the DC component of the signal output from each photodetector is different from each other, in conventional circuits, the signal from the differential amplifier contains a DC component, and due to the influence of this DC component, the measurement Further improvements in accuracy were difficult to achieve.

[Summary of the invention]

A first object of the present invention is to provide an improved signal processing circuit that can solve the above problems.

A second object of the present invention is to provide a displacement measuring device equipped with an improved signal processing circuit that can solve the above problems.

In order to achieve the above-mentioned first object, the signal processing circuit of the present invention includes a generating means for generating first and second signals having a phase difference of 180' from each other, and a first signal from the generating means and a second signal.
and subtraction means for detecting a difference between the signals and outputting a third signal according to the detected difference, wherein the generation means and/or subtraction means reduce the DC component of the third signal from the subtraction means to approximately zero. By having such a configuration, it can be used in a displacement measuring device such as a linear encoder or a rotary encoder to further improve the measuring accuracy of the device.

In addition, in order to achieve the above second object, the displacement measuring device of the present invention reads scales formed on a movable scale that rotates or translates, and has a phase difference of 180° from each other according to the displacement of the scale. It has a scale reading means for outputting a first and a second signal, and a subtraction means for detecting a difference between the first and second signals from the reading means and outputting a third signal according to the detected difference, In the displacement measuring device that measures the displacement of a scale based on the third signal, the reading means and/or the subtracting means includes an adjusting means for reducing the DC component of the third signal from the subtracting means to approximately zero. It is characterized by having By providing such an adjustment means, measurement can be performed based on the third signal in which the DC component is nearly zero, thereby improving measurement accuracy.

Some features and specific embodiments of the invention are described in the Examples below.

FIG. 1 is a block diagram showing an example of a displacement measuring device, and FIGS. 2 and 3 specifically illustrate the device shown in FIG. 1, and show a displacement measuring device equipped with the present signal processing circuit.

In Figs. 1 and 2, 1 is a semiconductor laser 2, a collimator lens that converts the light beam emitted from the laser l into a parallel light beam, 3 is an optical component made by bonding two trapezoidal prisms 31°3□, and 4 is a collimator lens that converts the light beam emitted from the laser l into a parallel light beam. The optical component 3 is a first light splitting means.
This means 4 consists of a prism 3□ and a 3° bonded surface, and this means 4 performs light division with the same function as a polarizing beam splitter. 5 and 7 are reflecting mirrors as first optical means, 6 is a rotating disk as a rotating object to be measured, a radiation grating (a diffraction grating type scale) is provided at 2, and 8 and IO are 174 wavelength plates. Reference numerals 9 and 11 are reflecting means, which are comprised of, for example, a cat's eye optical system.

12 is a quarter wavelength plate, 13 is a polarizing beam splitter-1
4 and 15 are light receiving elements. Also, ○ indicates radiation grating 6
The center, M, , M2 indicates an arbitrary point on the radiation grating 6.

A block in which each element from the collimator lens 2 to the quarter-wave plate 12 is combined into one block will be referred to as a diffracted light interference means 21, as shown in FIG. Incidentally, the diffracted light interference means 21 shown in FIG. 2 is merely an example, and it goes without saying that various known forms of examples may be adopted.

In this embodiment, a laser beam emitted from a laser 1 is converted into a parallel beam by a collimating lens 2, and this beam is converted into a parallel beam by a trapezoidal prism 3. After being reflected on the slope of the beam, the beam is directed toward the optical component 3 so as to be incident on the light splitting surface 4 at a predetermined angle. The luminous flux incident on the light splitting surface 4 is approximately 1:1
It is divided into two linearly polarized beams, a reflected beam and a transmitted beam, at a ratio of . Incidentally, the light beam from the laser 1 is composed of linearly polarized light polarized in a direction of 45° with respect to the orthogonal polarization plane of the light splitting plane 4.

The two beams split by the light splitting plane 4 are each reflected twice within the trapezoidal prisms 30 and 3 degrees, exit from the optical component 3, and are directed to predetermined positions M1 and M1 of the radiation grating 6 by the reflecting mirrors 5 and 7. M
2 at a predetermined angle of incidence. Of the transmitted diffracted light diffracted by the radiation grating 6, the +1st-order diffracted light is transferred to a quarter-wave plate 8,
10, the light is reflected by the reflecting means 9.degree. 11, and the same optical path is caused to travel backward. The +1st-order diffracted light is then re-injected into substantially the same positions M1 and M2 of the radiation grating 6, and the +1st-order diffracted light re-diffracted here is reflected by the reflecting mirror 5°7, and the same optical path is reversed. The light is reflected and guided to the light splitting surface 4. Each luminous flux (±first-order diffracted light) here is transmitted to the reflecting means 9.
] Since the light beams pass through the quarter-wave plate twice before and after being reflected by the radiation grating 6, their polarization directions differ by 90 degrees from those before entering the radiation grating 6. Therefore, the light beam that was reflected earlier on the light splitting surface 4, which also serves as the second optical means, is transmitted this time, and the light beam that was transmitted earlier is now reflected and overlaps, and the optical component 3 Internal reflection and 1/4 wavelength plate 12
incident on the Each light beam passing through the quarter-wave plate 12 becomes circularly polarized light in opposite directions.

This light beam is split by a polarizing beam splitter 13 into two beams with orthogonal polarization directions, and a light receiving element 14,
15. Then, the light receiving element 14. 15 is
Each corresponding interference light is photoelectrically converted and an electrical signal is output.

In this example, when the rotating object to be measured rotates by one pitch of the radiation grating 6, the phase of the +1st order re-diffracted light is 2π
only changes. Similarly, the phase of the 11th order diffracted light re-diffracted by the radiation grating 6 changes by 12π. As a result, the light receiving element 14 as a whole. From 15 (2-(-2)
=4 sinusoidal signals are obtained. In this embodiment, the amount of rotation of the radiation grating 6 is measured by detecting each sine wave signal at this time.

For example, if the pitch of the radiation grating 6 is 3.2 μm, when the rotating object rotates by 3.2 μm, the light receiving element 14
．． Since four sine waveforms can be obtained from 15, the resolution per sine waveform is 1/1 of the pitch of one diffraction grating.
3.2/4 of 4=0.8 μm is obtained.

In this embodiment, the light consisting of the ±1st-order diffracted light from the quarter-wave plate 12 is split by the polarizing beam splitter 13 into two interference lights whose polarization directions are different from each other by 90°. 14. From 15, the phases are 1 to each other.
A pair of sine wave signals shifted by 80° are output.

If it is necessary to obtain a signal with a different phase to detect the angle or direction of rotation of the rotating object to be measured, use a light splitting means or polarizing plate as appropriate to extract the interference light with the desired phase. Good.

Next, the signal processing circuit provided in the apparatus shown in FIG. 2 will be explained. FIG. 3 is a circuit diagram showing an example of a signal processing circuit. In FIG. 3, 14. 15 is Figure 1 and 2
The light-receiving elements are shown with the same reference numerals in the figure, and here a photodiode is used. 41.42° 43 is an operational amplifier, 51 to 55 are resistors, 61 is a variable resistor, C,, C
2 is a capacitor. Further, 44 is an output terminal of this circuit.

The photocurrent (signal) obtained by the photodetector 14.15 is
Operational amplifiers 41 and 42 convert the signals into voltage signals having a phase difference of 180 degrees, and an operational amplifier 43 (subtraction means) detects and amplifies the difference between these voltage signals. The capacitors C, , C2 are intended to stabilize the operation of the circuit in a high frequency region and prevent abnormal oscillations.

Now, the photocurrent (signal) obtained by the photodetector 14.15 is '+4+'15, and each resistance value is R+ (resistor 51),
R2 (variable resistor 61), R3 (resistor 52.54)
, R4 (resistor 53.55), the output voltage V obtained at the output terminal 44. is Vo ”= (I14R+ ll5R2) R4/R
3-(1).

■! 41 11! Ideally, l is a constant sine wave signal, but in reality it is a complex signal for the following reasons. First, if there are variations in the pitch or line width of the radiation grating 6, or if the edges of the grating are disturbed, the intensity of light passing therethrough will change. Secondly,
light-receiving element passing through a path other than the original optical path for forming interference light], 4. , 15 (referred to as a ghost component) exists. Furthermore, these vary slightly with the rotation of the rotating object to be measured.

Taking these into account, when rotating at a constant speed, '
+4+115 is generally expressed by the following formula.

1,, =a+bsin ωt・
= (2) 116 ” <Z a−βbsin (IJ
t - (3) where α and β are constants, a,
b is a coefficient that varies slightly depending on rotation. In general, α and β take close values, but the light receiving element 14. ±1 up to 15
It differs somewhat due to the difference in the optical path length of the next diffracted light and the difference in the mutual polarization direction. Further, ω is the angular frequency of the signal, and t is the time. Substituting these into equation (1), the output terminal 44
Output voltage V output to. is...(4). The feature of this embodiment is that an adjusting means is provided so that R1-αR20 is obtained here. This can be achieved by adding a variable resistor 61 to the operational amplifier 42.
This is achieved by making it possible to adjust to = Rl /α.

By the action of this variable resistor 61, the two light receiving elements 1
The DC component of each sine wave signal obtained by 4.15 and the fluctuation of the DC component due to rotation can be canceled, and the DC component of the output signal from the output terminal 44 is made almost zero, making the offset extremely stable. A sine wave signal can be obtained.

FIG. 4 shows an example of the waveform. FIGS. 4(A) and 4(B) are examples of the signal engineer 14+IIS, respectively. Figure 4 (C
) is subtracted using the same conventional ratio (i.e. R2=R
2) is the output waveform from the terminal 44, and some offset fluctuation remains. On the other hand, FIG. 4(D) shows the output waveform from the terminal 44 in this embodiment. Fourth
As shown in Figure (D), in this embodiment, a signal having a waveform with positive and negative symmetry and a stable offset can be obtained from the terminal 44.

Therefore, if the signal from the output terminal 44 of this embodiment is used for measurement, the amount of rotation of the rotating object to be tested can be accurately measured.

In the above embodiment, the variable resistor 61 is used to adjust the ratio of each voltage corresponding to the signal 114+III+ input to the operational amplifier 43, but the invention is not limited to this. For example, 61 is set to a resistance value. As the resistor of R8, resistor 5
4 or resistor 55 as a variable resistor and resistor 61
The resistor 54 or 55 can also be a variable resistor.

In addition, the circuit configuration is not limited to the above embodiment,
Any configuration is sufficient as long as it can substantially cancel the difference in the DC components of the signal f14+11B.

In addition, by using various optical parts such as light splitters, polarizing plates, polarizing beam splitters, and wavelength plates,
Not only the signal corresponding to , but also the signal of any phase angle, e.g.
Even if a means for extracting a 0° signal is also used, this does not depart from the spirit of the invention. Furthermore, a signal with an arbitrary phase angle, for example, 90°, and a signal with a phase of 1
A means is provided for extracting another signal shifted by 80', for example, a signal at 270°, and is synthesized using the idea of the above embodiment,
It is also possible to create a stable signal with an offset.

In this case, one more circuit shown in Fig. 3 is added to process signals with phase angles of 90° and 270°, and output terminal 44
Therefore, in the above embodiment, it is possible to output a signal whose phase is 90° different from the signal output from the output terminal 44, and by using the signals with a phase angle of 0° and a phase angle of 90°, the rotational moving object to be tested can be detected. It is possible to determine the direction of rotation and improve resolution by interpolating signals.

Furthermore, the same concept as in the above embodiment can also be used for an optical encoder that combines a conventional fixed scale and a moving scale. Here, we have used a rotary encoder as an example, but it goes without saying that it can also be applied to linear encoders, and it can also be applied not only to optical encoders but also to other types of encoders such as magnetic encoders.

〔Effect of the invention〕

As described above, the present invention includes an adjusting means that adjusts the circuit so that the DC component of the signal from the subtracting means that detects the difference between signals whose phases are different from each other by 180 degrees becomes almost zero.
A signal with a stable offset can be obtained from the subtraction means. Therefore, it is also possible to provide a displacement measuring device that can accurately measure the displacement (translational movement or rotation) of the object to be tested.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of a displacement measuring device. 2 and 3 are explanatory diagrams of a displacement measuring device equipped with the present signal processing circuit, specifically showing the device of FIG. 1. FIG. 4(A) to 4(D) are waveform diagrams showing signal waveforms in the circuit shown in FIG. 3 and signal waveforms in the conventional circuit. l... Laser 6... Radiation grating (scale) 14, 15... Light receiving element 21... Diffracted light interference means 41, 42.43... Operational amplifier 51, 52. 53. 54...Resistor 61...
Variable resistor 44...output terminal O

Claims (3)

[Claims] (1) A generating means for generating first and second signals having phases different from each other by 180 degrees, and a first signal and a second signal from the generating means.
subtraction means for detecting a difference between the signals and outputting a third signal according to the detected difference, wherein the generation means and/or the subtraction means reduce the direct current component of the third signal from the subtraction means to approximately zero. A signal processing circuit having adjustment means for (2) A scale reading means for reading the scale formed on the movable scale and outputting first and second signals having a phase difference of 180 degrees from each other according to the displacement of the scale; A displacement measuring device that detects a difference between two signals and outputs a third signal according to the detected difference, and measures displacement of a scale based on the third signal, the reading means and/or Alternatively, a displacement measuring device characterized in that the subtracting means has an adjusting means for making the DC component of the third signal from the subtracting means substantially zero. (3) The scale reading means includes a first light-receiving element and a second light-receiving element that receive a first light beam and a second light beam having a phase difference of 180 degrees from each other from the scale, and a signal from the first light-receiving element. The adjustment means includes a first operational amplifier that amplifies and outputs the first signal, and a second operational amplifier that amplifies the signal from the second light receiving element and outputs the second signal.
The displacement measuring device according to claim 2, characterized in that the second operational amplifier includes a variable resistor.