JP3223789B2 - Laser measuring instrument - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser length measuring device used for an apparatus for observing interference fringes from an output signal of a Michelson interferometer and for determining an optical path difference, and more particularly, to a laser length measuring device. The present invention relates to an improvement for removing a DC component from an interference signal.

[0002]

2. Description of the Related Art In a laser measuring device using the polarization interference method,
There has been proposed a system in which a three-phase signal is optically generated to remove a DC component of the signal and processed by an electric circuit. For example, Japanese Patent Publication No. 60-24404 (publication number: Showa 55-
Japanese Patent No. 500039) discloses an example of a DC component removal system in “Interferometer system”.

FIG. 3 shows an embodiment of an interferometer using the Michelson interferometer described in the above patent. In the figure, light emitted from a laser light source 1 (assumed to be already polarized and linearly polarized) is split by a beam splitter 2, transmitted light is turned back by a corner cube 3, and reflected light is turned back by a corner cube 5. Are combined on the beam splitter 2. However, the reflected light passes through the ８ wavelength plate twice, becomes circularly polarized light, and reaches the beam splitter 2.

The light multiplexed on the beam splitter 2 is converted into two orthogonal polarization components P by a polarization beam splitter 6.
And S, and the light of the P and S components is detected by photodetectors 7 and 8 which are photoelectric conversion elements, respectively. Light that has been turned back by the corner cube 3 and transmitted through the beam splitter 2 passes through the plane polarizer 6 and is detected by the photodetector 10. The interferograms detected by the photodetectors 7 and 8 have a phase difference of 90 °, and the interferograms detected by the photodetectors 8 and 10 have a phase difference of 180 °.

FIG. 4 shows an example of a circuit that counts interference fringes by removing a DC component from the outputs of these photodetectors. The outputs (including the DC component and the AC component) of the photodetectors 10, 8, 7 are supplied to amplifiers 21, 22, 23, respectively.
Amplified by The amplifiers 21 and 23 are connected to a differential amplifier 24, and the amplifiers 22 and 23 are connected to a differential amplifier 25. The differential amplifier is connected to an adder 28 via squarers 26 and 27, respectively, and the output of the adder 28 is connected via a square root 29 to one input of each of two ratio amplifiers 30 and 31. I have. The output of the square rooter 29 is a signal proportional to the peak amplitude of the input signal having a phase difference of 90 °.

The outputs of the differential amplifiers 24 and 25 are respectively applied to the other inputs of the ratio amplifiers 30 and 31. The two ratio amplifiers 30 and 31 output a phase difference of 90 ° as a function of the optical path difference in the interferometer. Two output signals which change sinusoidally with a constant amplitude are obtained. The fringe counting unit 32 counts interference fringes corresponding to the optical path difference from the two signals.

[0007]

However, in the above-mentioned interferometer system, the optical system becomes complicated because the interference light is divided into three and received, and precise balance adjustment between each signal is required. There was a problem.

In view of the foregoing, it is an object of the present invention to remove a DC component of an interference signal by measuring and correcting an intermediate value between a maximum value and a minimum value of the interference signal at every fixed displacement, thereby removing an AC component. It is an object of the present invention to realize a laser length measuring device having a simple configuration, which is capable of converting into only the interference fringes and counting the interference fringes.

[0009]

In order to achieve the above object, according to the present invention, there is provided a laser light source, a half mirror for splitting light from the laser light source and recombining the light from the laser light source, and a fixed arrangement of the laser light source. One reflecting means for reflecting one of the divided lights; another reflecting means movably disposed to reflect the other divided light; and a luminance according to a change in an optical path difference caused by the two reflecting means. And an interferometer that outputs two interference signals having a phase difference of 周期 period from each other and in which the leading and lag relations of the phases are inverted according to the moving direction of the movable reflecting means; and After two sets of photoelectric conversion elements for detecting signals and the interference signal detected by each of the photoelectric conversion elements are filtered to remove high-frequency components through a low-pass filter and subjected to analog-to-digital conversion,
The average value of the maximum value and the minimum value during the period of a certain number of interference signals is measured and digital-to-analog converted, and the difference between the digital-to-analog converted signal and the interference signal detected by each of the photoelectric conversion elements is measured. And two sets of signal processing circuits for removing the DC component of the interference signal detected by each of the photoelectric conversion elements, and comparing each output signal of the two sets of signal processing circuits with a predetermined value to generate a pulse. Convert to signal 2
A set of comparators, a calculating means for determining the moving direction of the movable reflecting means from the phase relationship between the two pulse signals output from the two sets of comparators, and the pulse signal based on the moving direction obtained by the calculating means. An up-down counter for adding or subtracting the number of the above-mentioned numbers, and a movement amount calculating means for converting the integrated value of the up-down counter by a wavelength value of the laser light to convert the integrated value into a movement amount of the movable reflecting means. Features.

[0010]

According to the photoelectric conversion element, two interfering light intensities shifted by a 1/4 period phase are periodically changed in accordance with the optical path length difference, and the lead / lag relationship is reversed by the expansion / contraction of the optical path length, ie, the moving direction of the reflection means. Is detected. The output of each photoelectric conversion element includes a DC component and an AC component. Since the interference fringe is counted when the output of the photoelectric conversion element exceeds a certain value, accurate counting cannot be performed if the DC component fluctuates. Generally, the values of the DC component and the AC component decrease as the distance to the reflection unit increases. In order to reliably perform interference fringe counting, it is desirable to remove a DC component from an interference signal and count a positive or negative output as only an AC component.

The signal processing circuit performs the DC component removal processing on each interference signal. First, when the power is turned on or during the reset operation, the wavelength of the laser light source is changed to generate an interference signal that changes by one or more fringes. In the meantime, the interference signal is digitally converted to obtain the maximum value and the minimum value, the average value is calculated and stored in the latch, and the value of the latch is converted into an analog signal, thereby outputting the DC component value in the initial state. The DC component is removed by subtracting this value from the interference signal with a differential amplifier.

After performing the above initial operation, the wavelength of the laser is stabilized at a constant value. Thereafter, for each interference signal generated due to the movement of the reflection means, each AC signal obtained in the above processing is compared with a predetermined value and converted into a pulse signal. The moving direction of
The number of pulse signals is integrated by an up / down counter depending on the direction. The displacement amount is obtained by multiplying the integrated value by the wavelength of the laser.

The DC component of the interference signal is measured by the above-described method while the integrated value of the pulse changes by a predetermined amount, and when the integrated value of the pulse changes by a predetermined amount, the latch value is updated to correspond to the change of the DC component. In this way, even if the reflection means moves and the value of the DC component changes, the DC component at that time is always removed to obtain an interference signal of only the AC component, and accurate interference fringe counting can be performed. it can. Further, since the value of the DC component is stored as a digital value, the value is maintained at a constant value even if the reflecting means does not move for a long time.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration diagram showing one embodiment of a laser length measuring device according to the present invention. The same parts as those in FIG. 3 are denoted by the same reference numerals, and the description of those parts will be omitted.

In FIG. 1, reference numeral 10 denotes a wavelength setting circuit for setting the wavelength of the laser light source 1 to a predetermined value. 11 is a switch, and 12 is an AC signal generator. By turning on the switch 11, the output of the AC signal generator 12 is added to the output of the wavelength value setting circuit 10, and the wavelength of the laser light source 1 can be modulated by the AC signal generator 12.

Reference numeral 40 denotes a signal processing circuit for receiving the output of the photoelectric conversion element 8, and reference numeral 50 denotes a signal processing circuit for receiving the output of the photoelectric conversion element 7. Since both are circuits having the same configuration, the signal processing circuit 40 will be described below, and the description of the signal processing circuit 50 will be omitted.

In the signal processing circuit 40, a current-voltage converter 41 converts the output current of the photoelectric conversion element 8 into a voltage signal. Reference numeral 42 denotes a low-pass filter, which smoothes a high-frequency component of the output of the current-voltage converter 41. Reference numeral 43 denotes an analog-to-digital converter (hereinafter, referred to as an A / D converter) for digitally converting the output of the low-pass filter 42. Reference numeral 44 denotes a level detection circuit which outputs an average value of the maximum value and the minimum value of the output of the A / D converter 43 as a digital value while being reset by the pulse output of the delay circuit 67.

Reference numeral 45 denotes a latch circuit, which latches the output of the level detection circuit 44 in accordance with a carry signal or a carry signal of an up / down counter 66. 46 is digital
It is an analog converter (hereinafter, referred to as a D / A converter), and converts the output value of the latch circuit 45 into an analog signal. 4
Reference numeral 7 denotes a differential amplifier, which outputs D / D
A voltage obtained by subtracting the output of the A converter 46 is output.

Reference numeral 61 denotes a comparator, which is a differential amplifier 4
7 is compared with a predetermined voltage. If the former value is larger, a digital signal of a HIGH level (hereinafter referred to as H level) is output, and if the latter value is larger, a digital signal of a LOW level (hereinafter referred to as L level) is output. . Reference numeral 63 denotes a moving direction discriminating means for judging the moving direction of the reflecting means 5 based on the phase relationship of which of the outputs of the comparators 61 and 62 rises first.

Numeral 64 denotes an up / down counter which adds or subtracts the number of rises from the L level to the H level in the output of the comparator 61 according to the output of the moving direction discriminating means 63. A multiplier 65 multiplies the output of the up / down counter 64 by the wavelength value of the laser light source 1 and outputs the amount of movement of the reflecting means 5.

Reference numeral 66 denotes an up / down counter, the operation of which is the same as that of the up / down counter 64. However, when the count value reaches a predetermined number, the latch circuits 45 and 55
And outputs a pulse signal to the delay circuit 67 to reset the count value of itself (the up / down counter 66). The delay circuit 67 includes an up / down counter 66
After a signal indicating that has reached a predetermined number is output, a pulse is output to the level detection circuit 44 with a certain delay, and the level detection circuit 44 is reset.

FIG. 2 shows the configuration of the level detection circuit 44.
In the figure, 101 and 103 are binary comparators, and 102 and 104 are latch circuits. Latch circuit 1
02 is reset to the minimum value by the output pulse of the delay circuit 67. The binary comparator 101 has A /
The output of the D converter 43 and the output of the latch circuit 102 are compared in magnitude, and when the former is larger, a pulse corresponding to a latch instruction is output to the latch circuit 102.

The latch circuit 104 is preset to a maximum value by the output pulse of the delay circuit 67. The binary comparator 103 compares the output of the A / D converter 43 with the output of the latch circuit 104 and outputs a pulse corresponding to a latch command to the latch circuit 103 when the former is smaller.

Reference numeral 105 denotes an adder, and the latch circuit 102
, And 104 (all of which are parallel digital signals). The average value of the outputs of the latch circuits 102 and 104 is obtained by using the digital value of the carry among the added values as the most significant bit and using the output value excluding the least significant bit. This value is further latched by the latch circuit 45.

The operation in such a configuration will be described below. The linearly polarized laser light emitted from the laser light source 1 is split by the half mirror 2, the reflected light is turned back by the fixed corner cube 3, the transmitted light is turned back by the movable corner cube 3, and the half mirror 2 (the beam splitter in FIG. ). However, the transmitted light is 1
The light passes through the / 8 wavelength plate 4 twice, becomes circularly polarized light, and reaches the half mirror 2.

A part of the light multiplexed on the half mirror 2 is further divided into orthogonal polarization components by a polarization beam splitter 6, and each intensity change is detected by a photodiode 8. Although both signals obtained here are composed of an AC component and a DC component, it is necessary to remove the DC component in order to count interference fringes.

In the signal processing circuit 40, after the signal obtained by the photoelectric conversion element 8 is converted into an electric signal by the current-voltage converter 41, the high-frequency component is smoothed by the low-pass filter 42, and the digital signal is converted by the A / D converter 43. After the conversion, an average value of the maximum value and the minimum value of the interference signal is obtained by the level detection circuit 44 while the reflecting means 5 makes a constant displacement. After this value is latched by the latch circuit 45, it is converted into an analog signal by the D / A converter 46, and the differential amplifier 47 obtains the difference from the output signal voltage of the current-voltage converter 41 including the DC component, thereby obtaining interference. Let the signal be only the AC component.

The signal processing circuit 50 also performs the same processing as described above on the signal obtained by the photoelectric conversion element 7 to obtain an interference signal having only an AC component. Comparators 61 and 62 compare the output values of differential amplifiers 47 and 57 with a predetermined voltage and output digital signals. The moving direction determining means 63 determines the moving direction of the reflecting means 5 from the phase relationship between the two digital signals, and the up / down counter 64 adds or subtracts the outputs of the comparators 61 and 62.

Finally, a multiplier 65 multiplies the output of the up / down counter 64 by the wavelength of the laser light source 1 to determine the amount of movement of the reflecting means 5. The data update of the level detection circuits 44 and 54 is performed when the count value of the up / down counter 66 reaches a predetermined number. Further, in the initial state, the wavelength of the laser light source 1 is modulated by the AC signal generator 12 to generate an interference signal, and the DC component is measured and set as an initial value.

The foregoing description of the invention merely illustrates preferred embodiments for purposes of explanation and illustration. Accordingly, the present invention is subject to many modifications, without departing from its essence,
Can be deformed. For example, the corner cube is not limited to this, and may be a mirror that reflects a laser beam, which is collectively referred to as a reflection unit here. Also, a beam splitter may be used as the half mirror,
The half mirror in the claims of the present invention also includes a beam splitter. The level detection circuits 44 and 54 can also be realized by using a microcomputer.

[0031]

As described above, according to the present invention, the following effects can be obtained. An interference signal including a DC component is digitized after passing through a low-pass filter, and the DC component is obtained from the maximum and minimum average values. Therefore, if the frequency of the interference signal is faster than the response of the A / D conversion circuit for digitizing, the interference signal becomes a constant DC after passing through the low-pass filter. What is necessary is just to output an average value of. When the frequency of the interference signal is lower than the band of the low-pass filter, a DC component can be obtained as an average value from the maximum and minimum values digitized by the A / D conversion circuit. This has the effect that even if the reflection means does not move for a long time, the DC component can be measured more reliably than the analog circuit holding circuit. In addition, since the low-pass filter is provided before the A / D converter, even if impulse-like noise is mixed in the interference signal, the interference signal is attenuated by the low-pass filter, so that an error hardly occurs. Furthermore, since the update of the DC component is performed not by the time but by the output of the up / down counter,
Regardless of the speed of the reflection means, the level changes each time a constant displacement is made, and a smooth AC component can be obtained. (Updating every fixed time, when the reflection means moves at a high speed, the The DC component greatly changes, and a step occurs in the AC signal). Further, according to the present invention, it is not necessary to add an optical system for detecting a DC component, the optical system can be made simple and small, and the balance of signals can be easily adjusted.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of a laser length measuring device according to the present invention.

FIG. 2 is a diagram showing an embodiment showing details of a level detection circuit;

FIG. 3 is a configuration diagram showing an example of a conventional interferometer using a Michelson interferometer.

FIG. 4 is a diagram illustrating an example of an interference fringe counting circuit used in the interferometer system of FIG. 3;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser light source 2 Half mirror 3 Fixed corner cube 4 1/8 wavelength plate 5 Movable corner cube 6 Polarization beam splitter 7,8 Photodiode 10 Wavelength setting circuit 11 Switch 12 AC signal generator 40,50 Signal processing circuit 41,51 Current Voltage converters 42, 52 Low-pass filters 43, 53 Analog / digital converters 44, 54 Level detectors 45, 55 Latch circuits 46, 56 Digital / analog converters 47, 57 Differential amplifiers 61, 62 Comparator 63 Moving direction discriminating means 64, 66 Up / down counter 65 Multiplier 67 Delay circuit 101, 103 Binary comparator 102, 104 Latch circuit 105 Adder

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01B 9/00-11/30 102

Claims (2)

(57) [Claims] 1. A laser light source, a half mirror for splitting light from the laser light source and multiplexing the light again, one reflecting means which is fixedly arranged and reflects the one of the divided light, and is movable. And the other reflecting means for reflecting the other divided light, and the luminance are changed in accordance with a change in the optical path difference between the two reflecting means, and have a phase difference of 1/4 cycle from each other. An interferometer that outputs two interference signals in which the phase advance / delay relationship between the phases is inverted depending on a possible moving direction of the reflection unit; two sets of photoelectric conversion elements that detect the two interference signals; and each of the photoelectric conversion elements After removing the high-frequency component from the interference signal detected in step 1 through a low-pass filter and converting it from analog to digital, measure the average of the maximum and minimum values during the period of a fixed number of interference signals and measure the digital value. The DC component of the interference signal detected by each photoelectric conversion element is removed by calculating the difference between the analog-converted signal and the interference signal detected by each photoelectric conversion element. A set of signal processing circuits, two sets of comparators for comparing each output signal of the two sets of signal processing circuits with a predetermined value and converting the output signals into pulse signals, and a phase of both pulse signals output from the two sets of comparators Calculating means for determining the moving direction of the movable reflecting means from the relationship; an up / down counter for adding or subtracting the number of the pulse signals according to the moving direction obtained by the calculating means; and integration of the up / down counter A moving amount calculating means for multiplying the moving value by a wavelength value of the laser light to convert the value into a moving amount of the movable reflecting means. The length measuring device 2. A laser light source driving circuit for changing a wavelength of the laser light source and changing each interference signal intensity over a predetermined period while the movable reflecting means is stopped within a length measurement range. The laser length measuring device according to claim 1, wherein: