JPH02203206A - Interference fringe measuring instrument - Google Patents

Abstract

PURPOSE:To determine a threshold level with a small error automatically at all times by converting two light signals which are pi/2 out of phase with each other into electric signals, and processing their outputs and outputting two rectangular waves. CONSTITUTION:When a body to be measured moves, the two light signals which are pi/2 out of phase with each other are converted by optoelectric elements 1A and 1B into electric signals, which are digitized by Ad converters 2A and 2B are inputted to an arithmetic processing unit 3. The unit 3 uses tens of signal data which are inputted previously and approximates them to a circle by a method of the least squares to find its center coordinates (a,b). Then this is used as the threshold level to process the signals which are inputted currently. Then the output is inputted to a counter circuit 4 and the variation of the value is counted.

Description

[Detailed Description of the Invention] [Summary] The purpose of this invention is to optimally determine the threshold level when processing a detection signal regarding the counting of interference fringes in a laser length measuring device, and the purpose of this invention is to optimally determine the threshold level when processing a detection signal. An AD converter performs AD conversion on each of the output signals of a photoelectric element that converts two optical signals into electrical signals, and the outputs of the two AD converters are input and processed to generate two rectangular waves with different π/2 radian phases. and a counter circuit that inputs the two rectangular waves having different phases of π/2 radians and counts the number of moving interference fringes. The two signals (X, Y) are expressed using the XY coordinate system, and the data of multiple signals obtained in the past (X l, Y l+ X !, Y t
, ... 11, ), the center coordinate values a and b are used as the threshold level of each signal to perform signal processing and interference fringe counting.

[Industrial Application Field] The present invention relates to an interference fringe counting device in a laser length measuring device.

A laser length measuring device using the interference fringe counting method uses two signals with a phase difference of π/2 radians to precisely measure the distance traveled by counting the number of interference fringes that occur when the object is moved. An offset occurs in the output voltage of the photoelectric element to be detected, and this value must be used as a threshold level for signal processing and interference fringes are counted, but the offset varies and must be processed accordingly. becomes.

[Conventional technology]

As shown in Figure 4, a laser length measuring device using the interference fringe counting method splits the laser beam into two beams using a beam splitter, one of which is reflected by a fixed reference mirror, and the other by a movable measuring mirror. When the two reflected beams are reflected and put into the beam splitter again and the combined beam is photometered, interference fringes are observed due to the displacement of the optical path difference as the measurement mirror moves. By counting the number of moving interference fringes, the moving distance of the measuring mirror can be measured. However, by simply counting the number of interference fringes that have moved, it is not possible to determine in which direction the interference fringes have moved.

Therefore, we extract waves whose phase differs by π/2 radians and observe the movement of interference fringes for these waves.

In fact, the method of detecting two waves with a phase difference of π/2 is to use the interference fringes created within the interference light as shown in Figure 4.
/2 The method of detecting fringes at positions A and B with a phase shift (general Michelson interferometer) or the method of detecting fringes at positions A and B with a phase difference of π/2, such as the interferometer devised by the NFL (National Physical Laboratory in the UK), There is a method of optically creating two interference lights.

FIG. 5 shows an NFL interferometer. The laser beam emitted from the stabilized laser has a polarization plane of 45 mm with respect to the horizontal plane.
It passes through a 1/2 wavelength plate (the 1/2 wavelength plate is provided to prevent reflections) and is split into two parts by a beam splitter (B, S, ), and one is directly connected to the measuring mirror. Then, the other beam passes through the 1/8 wavelength plate, is reflected by the reference mirror, enters the beam splitter again, and interferes. The 1/8 wavelength plate has an axis, and only light on a plane parallel to the axis produces a 1/8 wavelength phase delay. Therefore, the phase of the light reflected by the reference mirror is delayed by 90 degrees by the vertically polarized wave. The light that has interfered with the beam splitter enters the polarizing beam splitter, is split into two by the polarization plane, passes through a filter and a polarizing plate, and is received by the photodiode A and photodiode B, respectively.

As a result, optical signals having a phase difference of 90 degrees are detected.

→ Photoelectric elements such as photodiodes are used to detect interference fringes, but the signals detected by these photoelectric elements (for example, a sine wave when the object to be measured moves at a constant speed) are An offset occurs, resulting in a sine wave with direct current superimposed on it. Therefore, when counting interference fringes (sine waves) by dividing them into 1/4 (counting with an accuracy of 1/4 of the period of the sine wave), the offset is determined as the threshold level and the signal is processed. conduct.

That is, as shown in FIG. 6, the electrical signal detected by photodiode A and the electrical signal detected by photodiode B are both amplified by an operational amplifier (OP) and input into a comparator, and are then output to a threshold level. The signals are compared with a reference voltage, converted into rectangular waves having different phases by π/2 (90 degrees), and input into a counter. π/2 for counter
Count two square waves with different phases, 1/4 of the period
Outputs the number and direction of movement of interference fringes with an accuracy of .

[Problem to be solved by the invention]

The offset described above has the following problems.

■The signal intensity changes over time due to changes in laser light intensity, fluctuations in the optical path, and external light, so the offset is not constant (temporal displacement).

■The transmittance and reflectance of the beam splitter used in the optical system are not the same, or the intensity varies depending on the position within the laser beam (Gaussian distribution), so the intensities of the two signals are not the same. That offset is also not the same (
(Positional displacement) As a solution to these problems, a signal with no offset can be obtained by taking three signals whose phases differ by π/2 and extracting two signals from these by difference. Therefore, the threshold level can be set to zero. However, this is only possible if the offsets and amplitudes of the three signals are equal at the same time.

Regarding (2), it is possible to determine a fixed threshold by considering each offset separately, but it is not possible to deal with the displacement of the offset with respect to time.

These two solutions do not satisfy both problems at the same time. To solve these two problems, it is necessary to separately determine threshold levels corresponding to the time displacements of the offsets for the two signals. One way to do this is to find the maximum and minimum values for each signal separately from dozens of previously detected outputs, and set the intermediate value as the threshold level. , there is a problem that it is not effective if the maximum output and minimum output are not included in the output range.

The problem to be solved by the present invention is to provide an interference fringe counting device that eliminates such conventional problems.

[Means to solve the problem]

FIG. 1 is a block diagram showing the configuration of the present invention.

In the figure, 1^, IB is a photoelectric element, and the phase is π/
Converts two optical signals that differ by 2 radians into electrical signals.

2A, 2B (A/D) are analog/digital converters (hereinafter abbreviated as AD converters), and optical elements I
The output analog signals of A and 4B are each converted into digital signals.

/2 outputs two different square waves.

4 is a counter circuit which receives two rectangular waves whose phases differ by π/2 and counts the number of moving interference fringes with an accuracy of π/4.

[For production]

Now, let X and Y be the outputs of two signals whose phases differ by π/2 when the object to be measured is moving. At this time, when (X, Y) is expressed in the XY coordinate system, the phases of X and Y differ by π/2, so as shown in Figure 2,
It turns out that it becomes a circle.

Therefore, since the several dozen signal outputs (XY) detected previously also form a part of the circle, if we find the center of this circle, we get
It is possible to determine the offset, or threshold level, of each of the signals X and Y.

To find the center of the circle, approximate the dozens of previously detected signal output data (X, Y) to a circle using the least squares method.
Find the coordinates of its center.

The signals detected by photoelectric elements IA and IB are AD
It is converted into a digital value by converters 2^ and 2B and input to the arithmetic processing unit 3. The arithmetic processing device 3 approximates a circle using the least squares method using several dozen previously input signal data (X, Y), and determines its center coordinates (a, b).

The two currently input signals are processed using the values of the center coordinates a and b as threshold levels. That is,
Currently input signal X! ! Compared to a, the signal Y is b
If it is larger, it is output as 1, and if it is smaller, it is output as 0. The counter circuit 4 receives this output as an input and counts changes in its value.

Approximation of a circle by the least squares method using N sets of X and Y data is performed as follows, as is known.

The approximate circle is (X-a)"+(Y-b)"=r".

X' = (X-a)", Y' = (Y-b)"
Then, Y'=-X'10r'', and the sum of squares of residuals Q for all data is
'+r"-Y')"=Σ(r"-(X-a)"
-(y-b)'')'', aQ/8a=O, aQ
By finding a, b, and r that satisfy /ab-0, δQ/F3r=0, the X coordinate, Y coordinate, and radius of the center of the circle can be obtained, respectively.

(N books ΣX”Y+N rate ΣY3− ΣxiΣY−ΣX Σ
Y") 2 ([(ΣX) "-N*Σx21 (ΣX ΣY-N books ΣXY)"-(N*ΣY"X+N*
ΣX3* [(E Y) ”-N* ΣY”]-ΣY2
ΣX−ΣV ΣX") ((ΣY)" -8 pieces Σ
Y"1 (ΣX ΣY - N pieces Σxy) 2) Equation (1) % expression %: ] ... - Equation (2) According to this method, even if the movement of the object to be measured is minute, that is, each signal Even if the maximum output and minimum output are not included in the output range of (even if the signal period does not change by one cycle)
, since the circle can be reproduced, the offset of each signal can always be found.

〔Example〕

FIG. 3 is a flowchart showing the operation of one embodiment of the present invention.

This embodiment is applied to the NPL interferometer shown in FIG. 5, from which two signals X and Y whose interference fringe phases differ by π/2 are obtained. In this embodiment, the configuration after the two photodiodes is the same as shown in FIG.

Hereinafter, the operation of the microcomputer (arithmetic processing unit) will be explained according to the processing steps of the flowchart in FIG.

■Since some data (Xi, Yi) are required to obtain the first threshold level, first of all, by moving the measurement mirror of the optical system, n pieces of data ( Xi, Yt) (i-1-n) is obtained. These data are AD converted each time and input into the microcomputer.

■ ΣX, ΣY, ΣX", ΣXY, IY", ΣX3 for n pieces of data (Xi, Yi) (i = l to n).
ΣX2Y ΣXY”, ΣY3 are calculated.

(2) Using equations (1) and (2) described in the previous section, determine the center coordinates (a, b) of the circle using the values calculated in (2).

■The threshold level for the latest n-th data (Xn, Yn) is ab, and Xn, a, Y
n and b are compared, and if each is larger than a5b, 1,
If it is smaller, it is output as 0.

The n+1th data (Xn
+L Yn+1) is input, the threshold levels a' and b' of X and Y are determined for the latest n data (i=2 to n+1) in the same manner as steps ■ to ■. Latest n+1 data (Xn+1.Yn+1)
, and outputs 1 or 0, respectively.

The n+2nd data (X
n+2. When Y n+2) is input, the threshold levels a,
b” and the latest n+2-th data (X n+2.
Y n+2) and outputs 1 or 0, respectively.

■ Below, n+3. n+4. A threshold level is determined using the latest n pieces of data for the data inputted as -.-, and the latest data is processed based on this.

The two signals outputted from the microcomputer through the above processing are counted by a counter circuit for changes in their values, and outputted as the number of moving interference fringes.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, the threshold level can always be automatically determined with little error in response to changes in signal strength over time or differences in the strength of two signals, and each time. This has the significant industrial effect of eliminating the need for adjusting the threshold level.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the configuration of the present invention, Fig. 2 is a diagram showing the principle of the invention, Fig. 3 is a flowchart showing the operation of an embodiment of the present invention, and Fig. 4 is general Michelson interference. FIG. 5 is a diagram showing the configuration of an NFL interferometer, and FIG. 6 is a diagram showing the configuration of a conventional example. In the figure, 1A and 1B are photoelectric elements (photodiodes), 2A and 2
B is an AD converter, 3 is an arithmetic processing unit (microcomputer), and 4 is a counter circuit. Fig. 1 Fig. 2 Flowchart showing the operation of an embodiment of the present invention Fig. Fig. 2 Fig. 2 Fig. 2 Fig. 2 Fig. 1 Fig. 2 Fig. 2 Fig. 2 Fig. 2 Fig. 1 Fig. 1 Fig. 2 Fig. 1 Fig. 1 Fig. 1 Fig. 1 Fig. 2 Fig. 1 Fig. 1 Fig. 1 Fig. 2 Fig. 2 Fig.

Claims (1)

[Claims] In an interference fringe counting type laser length measuring device that measures the moving distance of an object by counting the number of moving interference fringes in two signals whose phases differ by π/2 radians, A photoelectric element (1
Analog/digital converters (2A, 2B) that convert the output signals of A, 1B) into analog/digital, respectively, and the outputs of the two analog/digital converters (2A, 2B) are input and processed and converted to π/2. an arithmetic processing device (3) that outputs two rectangular waves with different radian phases; and a counter circuit (4) that inputs the two rectangular waves with different π/2 radian phases and counts the number of moving interference fringes. and the values of the signals that differ by π/2 radians of the phase are respectively
, Y, and the two signals (X, Y) with the XY coordinate system
is expressed as data of multiple signals obtained in the past (X_1,
Y_1, X_2, Y_2,...,
b), and then the two obtained signals (X_n_+_1, Y_n_+
_1), the interference fringe counting device is configured to perform signal processing using values a and b of the center coordinates as threshold levels of the respective signals, and perform interference fringe counting.