JPH01185476A - Laser length measuring apparatus - Google Patents

Abstract

PURPOSE:To make it possible to perform real time measurement at a high speed, by providing a PLL circuit, by which one of a reference beat signal or a measured beat signal is multiplied. CONSTITUTION:This apparatus is composed of the following parts: a PLL circuit 20, by which a reference beat signal REF is multiplied; a latch circuit 8, which is synchronized Pl1 with a measured beat signal MEAS and holds the contents of a frequency dividing counter of the circuit 20; a latch circuit 9, which holds previous sampling data Dt-1 in the circuit 8; an up/down counter 11, which judges the change exceeding the multiplying number of the circuit 20 and the changing direction from the data of the circuits 8 and 9 and performs addition and subtraction in corresponding with the changing direction; and the like. In this way, the REF is multiplied in the circuit 20. The contents of the counter 4 are sampled when the MEAS rises up or falls down. Since the contents indi cate the data of the phase theta of the signal, the phase difference can be measured intactly in real time. Since the circuit 20 follows the frequency fluctuation of the signal resolution is not changed by the fluctuation but determined by the frequency dividing ratio of the counter 4. Thus the resolution can be made constant.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a laser length measuring device that measures the length of two light waves having slightly different frequencies by terodyne interference.

[Conventional technology]

The principle of optical heterodyne interferometry using a two-frequency laser,
Displacement measurement using the time measurement method will be explained with reference to FIG.

Laser light emitted from a three-frequency laser light source is laser light f, , f with orthogonal polarization planes and slightly different frequencies.
Consists of 2. Laser light fl.

The signal f2 is split by a beam splitter BS, and mixed and detected by a photoelectric converter PD to become a reference signal 1b. The other beam split by the beam splitter BS is split into a laser beam f1 and a laser beam f2 by a polarizing beam splitter PBS. The laser beam f is reflected by the reference mirror M, and the laser beam f2 is reflected by the measurement mirror M2, and is superimposed again by the polarization beam splitter PBS, and mixed and detected by the photoelectric converter PD2 to become the measurement signal Ib2. If we take the reference optical path length, the reference signal 1
b, , the measurement signal b2 is expressed by the following equation.

I b, =A cosωt ---
(1) I b2 = B cos(ωt+φ)---(2
) If the amount of displacement of the measurement optical path length 12 is Δ1, the amount of phase change Δφ of the measurement signal is as shown in the following equation.

Therefore, the phase difference φ between the reference signal Ib and the measurement signal Ib2 is measured, and the displacement amount Δl can be measured from the amount of change Δφ.

The reference signals Ib, , and the measurement signals 1b2 are supplied to the comparators 21.
22, the reference logic signal REF is converted into logic signals REF and MEAS, respectively, and the period T1 of the reference logic signal REF is measured by a period counting circuit 26 using a clock signal sent from a clock generator 28, and sent to a phase calculator 27.

The reference logic signal REF and the measurement logic signal MEAS are each sent to rising pulse generators 23 and 24, the rising pulse signals of both signals are sent to the time difference counting circuit 5, and the time T2 between both rising pulse signals is measured by the clock signal. and sent to the phase calculator 27. The phase calculator 27 calculates the phase difference φ from the period data T1 and the time difference data T2 using the following equation.

T1≦T2, and as it is, changes of 2π or more cannot be measured, so when the phase difference φ changes from 0 to 2π, 2π is added, and when the phase difference φ changes from 2π to 0, 2π is subtracted. Furthermore, the initial phase difference is set to φ0, and the amount of change Δl is measured using the following equation.

[Problem to be solved by the invention]

In the above-mentioned time measurement method, it is necessary to measure the period and time difference of the signal and perform arithmetic processing, which makes the circuit configuration complicated.Also, since the arithmetic processing takes time, the data sampling time becomes long. If the moving speed of the measurement mirror is fast and the phase change is severe, the direction of the phase change cannot be determined, making measurement impossible.

An object of the present invention is to provide a laser length measuring device that is capable of high-speed real-time measurement using phase measurement that does not require complicated circuitry or arithmetic processing.

(Means for Solving the Problems) The laser length measuring device of the present invention includes: a laser light source that outputs laser light of three frequencies; an optical system that generates a reference beat signal using the laser light; an optical system that generates a measurement beat signal whose phase changes with respect to the reference beat signal; a PLL circuit that multiplies either the reference beat signal or the measurement beat signal; a first sampling circuit that holds the contents of the frequency counter; a second sampling circuit that holds the previous sampling data of the first sampling circuit; and a multiplication of the PLL circuit from the data of the first and second sampling circuits. an up/down counter that discriminates between changes and the direction of change, and performs addition and subtraction according to the direction of change; and a displacement calculation circuit that calculates the amount of displacement from the data of the first sampling circuit and the data of the up/down counter. It has

[For production]

The content of the frequency division counter in the PLL circuit represents the phase information of the signal, and the content of the frequency division counter is sampled in synchronization with the rising edge of the other signal, and up/down is performed from the previous sampling data and the current sampling data. By the counter performing addition or subtraction, a phase difference of 2π or more can be easily determined.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the laser length measuring device of the present invention, and FIG. 2 is a timing chart of signals of each part thereof. Note that the detection process of the reference signal REF and measurement signal MEAS is the same as that in FIG. 3, and will therefore be omitted.

Phase comparator 1, low pass filter 2, voltage controlled oscillator 3
and the frequency division counter 4 (frequency division ratio N=24) constitute a PLL circuit, manually input the reference signal REF (frequency fo), and obtain the clock signal CLK of the frequency f0, which is obtained by multiplying the frequency f of the reference signal REF by 24. . The highest output f of the frequency division counter 4.

is synchronized with the reference signal REF, so the frequency division counter 4
The output is nothing but the phase of the reference signal REF divided into 24. The latch pulse generator 5 outputs the measurement signal MEAS.
is input, and a latch pulse P synchronized with the clock signal CLK is generated at the falling edge of the signal. The latch pulse P passes through a delay circuit 6 having an appropriate delay time and is input to the latch circuit 8 as a latch pulse pH.The latch circuit 8 holds the contents of the frequency division counter 4 by the latch pulse pH. It was held by the latch circuit 8. The data is obtained by measuring phase difference information between the reference signal REF and the measurement signal 2π MEAS with high resolution. The upper two bits of the output data of the latch circuit 8 are input to the latch circuit 9 and latched by the latch pulse P1. Therefore, the latch circuit 9 holds the previous sampling data. The up/down pulse generator 10 compares the previous sampling data Dt-1 and the current sampling data Dt, and determines that D is 00,
When Dt is 11, down palless, Dt-1 is 11,
When Dt is 00, an up pulse is output. In the timing chart of FIG. 2, time 1. , 12.1°, 14.1
5 and the latch pulse P is output, but when the latch pulse Pt is output at the time Dt=. Since 0O1Dt becomes 11, a down pulse is output. up/
Down counter 11 counts up/down pulses. That is, the data of the latch circuit 8 is data of 0 to 2π, and any change beyond this is determined by the up/down pulse generator 10 and counted by the up/down counter 11. Since the output of the up/down counter 11 has a slight time delay compared to the output of the latch circuit 8, the latch pulse pt is held in the latch circuit 12 by the signal P12 passed through the delay circuit 7 with an appropriate delay time. .

The output data φ of the latch circuit 12 obtained in this way
DATA is obtained by measuring the phase difference between the reference signal REF and the measurement signal 2π MEAS with a resolution of /24 and corresponding to changes in the phase difference of 2π or more. The arithmetic circuit 14 receives initial phase data φ. The amount of change from DATA is determined, and the amount of displacement Δl is determined using the following equation.

λΔ! =(φDATA−φ.DATA) ×−ball,
--- (7) In the conventional technology, the measured period data T
1 and the time difference data T2, it is necessary to calculate the phase difference φ using equation (5). Also, the phase difference φ is from 0 to 2π
, it is necessary to subtract and add 2π when changing from 2π to O, and further, by calculating equation (6), the displacement Δ! was looking for. In this embodiment, the measured data φDATA is the phase difference itself, and the displacement amount ΔI can be obtained only by calculating the equation (7).

Note that the error detector 13 detects the upper two bits d2. of the previous data. d3 and the upper 2 bits of the current data D2. D3 is compared, and if the relationship between the two data is the following four, an error signal ERROR is issued.

That is, the phase 2π is divided into four quadrants, and if there is a change in two quadrants between the previous data and the current data, an error is detected.

〔Effect of the invention〕

As explained above, the present invention multiplies a reference signal or a measurement signal by a PLL circuit, and utilizes the fact that the content of the frequency division counter of the PLL circuit represents the phase information of the signal.
By sampling the contents of the frequency division counter at the rise or fall of the other signal, the period and time difference can be measured as in conventional time measurement methods, without the need for complex circuits and processing to perform calculations. It becomes possible to measure the phase difference in real time, and in the time measurement method, the resolution is determined by the clock frequency relative to the signal frequency, so if the signal frequency fluctuates, the measurement resolution changes, but in the present invention, the signal Since the PLL follows frequency fluctuations, there is no change in resolution due to fluctuations in signal frequency, and the resolution is determined by the frequency division ratio of the frequency division counter and can be kept constant.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the laser length measuring device of the present invention, FIG. 2 is a timing chart of signals of each part thereof, and FIG. 3 is a block diagram of a conventional example. 1... Phase comparator, 2... Low pass filter, 3... Voltage controlled oscillator, 4... Frequency division counter, 5---- --Latch pulse generator, 6.7...Delay circuit, 8, 9, 12--Latch circuit, 10-----Up/down pulse generator, 11-
...Up/down counter, 1%...
Error detector, 14... Arithmetic circuit.

Claims (1)

[Claims] 1) A laser length measuring device that measures length by optical heterodyne interference between two light waves with slightly different frequencies, comprising: a laser light source that outputs laser light of two frequencies; an optical system that generates a reference beat signal; an optical system that generates a measurement beat signal whose phase changes with respect to the reference beat signal due to movement of the object to be measured; and a PLL that multiplies either the reference beat signal or the measurement beat signal. a first sampling circuit that holds the contents of a frequency division counter of the PLL circuit in synchronization with the other beat signal; a second sampling circuit that holds the previous sampling data of the first sampling circuit; an up/down counter that determines a change exceeding the multiplier of the PLL circuit and the direction of change from the data of the first and second sampling circuits, and performs addition/subtraction according to the direction of change; A laser length measuring device comprising a displacement calculation circuit that calculates a displacement amount from data of the up/down counter.