SU1758433A1 - Displacement measuring device - Google Patents

Abstract

This invention relates to a measurement technique. The purpose of the invention is to improve accuracy and speed. For this, a device for measuring displacements, including a laser 1, a collimator 4, a beam splitter 8, a moving corner 11 and a fixed 9 reflectors and a recording unit with photoreceivers of the reference 12 and measuring 13 channels, two formers 15, 16 connected to them, a reversible counter 21 and the display unit 22, is provided with an additional photo-receiver 14 of the measuring channel, a third driver 17 connected to it, a comparator 18, a multiplexer 19 and a switch 20, entering the registration unit. The laser 1 is made ringed with a non-reciprocal element 2 in the cavity and a mixer 3 of counterpropagating waves at one of the outlets and optically coupled to the photodetector 12 of the reference channel. The collimator 4 is designed as a system of a short-focus lens 5, the focus of which is aligned with the center of the radiation spot from the surface of the output mirror of the laser 1, and two long-focus lenses 6, 7, whose focus is aligned with the images of the center of the spot in the focus lens of the short-focus lens 5 and the fixed reflector is made in the form of two angular reflectors 9, 10 rigidly connected to each other 9, 10, or ill. (L 00 00

Description

The invention relates to a measurement technique and can be used to measure linear displacements.

A device for measuring displacements is known, comprising an optically coupled single-frequency laser, an electro-optical converter, a beam splitter, a movable and fixed reflector, an I / 4 plate, a photodetector, and a recording unit. In the known device, an electro-optical converter consisting of a niobate lithium crystal and an I / 4 plate converts a single-frequency laser radiation into a two-frequency one with a frequency difference determined by the frequency voltage feeding the niobate lithium crystal. By means of a beam splitter, radiation with one frequency is directed to a mobile one, and radiation with a different frequency to a fixed reflector. After the reflections, the radiations are combined in the beam splitter, forming a traveling interference pattern, which is recorded by the photodetector. Fluctuations of the amplitude and phase of the voltage supplying the niobate lithium crystal have a significant effect on the accuracy of measurements in the known device. To reduce this effect and, thus, improve the measurement accuracy, it is necessary to filter the output signal of the photodetector, but this, in turn, reduces the speed limits of the interferometer measurements.

Thus, the disadvantages of the known device are low accuracy and speed.

A device for measuring displacements is known which, by a combination of essential features, is closest to the proposed one and adopted as prototype.

The known device comprises a dual-frequency laser, a collimator, a beam splitter, a splitter, a moving and stationary angular reflectors and polarization filters mounted in front of them, which are successively installed in the laser beam of the laser; a photodetector optically coupled to the laser via a side beam splitter, and a photoreceiver optically coupled to the laser via a beam splitter and corner reflectors; polarization filters installed in front of the photodetectors, two imagers connected to the photodetectors and a reverse counter with a display unit connected to the outputs of the imagers. As a laser, we use a Zeeman two-frequency laser whose ionizer is isotropic, and the active medium is placed in an axial magnetic field.

The known device works in the following manner;

The Zeeman laser emits radiation in the form of two orthogonally polarized circular polarization modes with frequencies v and V2. A collimator converts radiation into a wide beam of the desired diameter with a plane wave front. The by-pass beam divider transfers a small part of the radiation to the reference channel. In the measuring channel, using a beam splitter and polarization radiation filters with frequencies are separated and fed: one to the moving corner reflector, the other to the fixed corner reflector. Reflecting from the reflectors and again uniting in the beam splitter, the radiation interferes and passes through the polarization filter to the photodetector of the measuring channel. At the same time, the interference pattern on the photodetector oscillates with a frequency (v - vi + ± AXt)), where A v (t) is an addition to the frequency difference of the laser radiation, is proportional to the speed of movement of the moving corner reflector. The photodetector of the reference channel registers the initial frequency difference between the two laser modes (reference frequency) (v - -V2). The shapers convert the signals arriving at them from the photodetectors into two pulsed ones sequentially; one with the following frequency (v -V2 + Ar (t)), the other with the reference following frequency (vi -). Two of these generated sequences are fed to the summing and subtracting inputs of a reversible counter, which performs the algebraic summation of the frequencies of these sequences. If at the same time in the imaging unit each new counting pulse is formed when the movable reflector is moved to I / 2 (where I is the radiation wavelength), that is, one pulse from one interference band, then the measured length L is determined by the number of pulses registered by the reversing pulse in accordance with the expression MY II. The measurement result is displayed on the display unit.

The speed of the known device is determined by two factors, the frequency splitting of the two laser modes (vi -vz) and the passband of the recording unit. When the reflector moves, the frequency of the wanted signal A (t) is added to the reference frequency, which is proportional to the speed of movement of the movable reflector. When moving the reflector to one side

the resulting signal should have a frequency (v-i - V2 + А К)), and when moving in the opposite direction - frequency - I vi - vz Ai (t). Thus, on the one hand, the frequency of the useful signal Av (t) cannot be more (v- -), and on the other hand, the high-frequency signal with the frequency (vi - V2 + Ai {(t)) must fall within the bandwidth of the block registration.

In the known device, the accuracy of displacement measurements is equal to A / 2, which corresponds to one interference band. To improve the accuracy of measurements, the known device is equipped with an interpolator that produces counting pulses when the movable reflector moves by the value A / 2n, where n is the interpolation coefficient, or a recording system based on a cumulative phase meter, which allows recording the phase of the signal within one interference band. The recording of a small fraction of the interference band is possible only when the phase of the signal depends linearly on the measured displacement. This is true only when there is an ideal separation of the radiation of two frequencies in the two arms of the interferometer. In the known device, the separation of two frequencies with circular polarization occurs in a polarization manner. With the aid of A / 4 plates, the radiations with circular polarizations are converted into radiations with linear orthogonal polarizations. Next, the two frequencies are separated using crossed polys. The non-ideality of plates and poloid leads to an incomplete separation of radiation with frequencies v wi. As a result, a desired signal with a frequency (v - vi ± t ± v (t)) is superimposed with a parasitic signal with a frequency (y -V2.} Of small amplitude, depending on the quality of the polarization elements. This, in turn, leads to that the dependence of the phase of the interference signal on the movement ceases to be linear, and this limits the ability to measure movements within one interference band and, thereby, reduces the accuracy of the known device.

Thus, the disadvantages of the known device are low speed and accuracy.

The purpose of the invention is to increase speed and accuracy.

. This goal is achieved in order. that a device for measuring displacements, including a laser, a collimator, a beam splitter,

movable angular and fixed reflectors and a registration unit with photoreceivers of the reference and measuring channels and two formers connected to it, a reversible counter and a display unit, are provided with an additional photoreceiver of the measuring channel entering the registration unit, the third driver connected to it, a comparator , the multiplexer and the switch, the outputs of the formers of the measuring channels are connected to the inputs of the comparator and the multiplexer, the ESCHOUT of the multiplexer and the output of the driver of the reference through the switch is connected to the inputs of the reversible counter, and the comparator output is connected to the control inputs of the multiplexer, the laser is made ringed with a non-reciprocal element in the resonator and

0 by the mixer of counterpropagating waves at one of the outlets and optically connected with the photodetector of the reference channel, the collimator is designed as a system of a short-focus lens, the focus of which is combined with

5 by the center of radiation on the surface of the laser output mirror, and two long-focus lenses, whose foci are aligned with the images of the center of the spot in the output focal plane of the focus-focus lens, and the fixed reflector is made in the form of two rigidly connected two corner reflectors.

The proposed technical solution makes it possible to ensure the formation and automatic switching of two measuring channels, direct and specular, in an instantaneous dependence on the direction of movement of the moving vehicle.

0 reflector, as well as complete filtering of the radiation of two frequencies from each other, thereby reducing the frequencies of the signals at the inputs of the processing unit by reducing the initial frequency difference and eliminating the non-linear dependence of the phase

interference signal from the measured displacement to increase speed and accuracy.

FIG. 1 is a block diagram of the device in question; in fig. 2 - optical scheme of the collimator; in fig. 3 is a variant of the comparator block diagram.

A device for measuring displacements (Fig. 1) consists of a ring laser 1

5 with a non-reciprocating element 2 inside the resonator and a mixer 3 of counterpropagating waves, installed at the output of laser 1 collimator 4, comprising a short-focus 5 and two long-focus 6, 7 lenses, a splitter 8, two coupled fixed 9.10

corner reflectors, movable 11 corner reflectors and registration unit. The recording unit contains three photodetectors. One photoreceiver 12 is optically coupled to laser 1 via mixer 3, the other two photoreceivers 13, 14 are optically coupled to laser 1 via beam splitter 8 and reflectors 9-11. To the outputs of the photodetectors 12-14 connected, respectively, the shapers 15-17. The outputs of the drivers 16, 17 are connected to a comparator 18 and a two-input multiplexer 19, the control input of which is connected to the output of the comparator 18. The outputs of the driver 15 and multiplexer 19 of the subkeys are switch 20, the control input of which is connected to the output of the comparator 18. K the outputs of the switch 20 is connected reversible counter 21 with block 22 of the display,

A device for measuring displacements works as follows (FIG. 1)

In the ring laser 1, two opposite-direction waves are generated. At the output of laser 1, a single wave propagates along path 23, and an opposite wave — along path 24. Non-reciprocal element 2, installed in laser resonator 1, creates frequency splitting of counterpropagating waves. Both beams 23 and 24 with frequencies v and V2 are converted by the collimator 4 into two parallel wide beams are planar wavefronts and arrive at the beam splitter 5, where they are separated. Part of the radiation with frequency v goes to the fixed reflector 9, and part goes to the movable reflector 11. Part of the radiation at frequency V2 goes to the fixed reflector 10, and part goes to the movable reflector 11. Part of the radiation at frequency u goes to the fixed reflector 10, and part - on a movable reflector. Reflected from the reflectors 9-11, the beams are combined in the beam splitter 8 and interfere to form two measuring channels 25 and 26. At the same time, the beam with frequency (vi ± D v (t)), reflected from the moving reflector 11, is combined with beam with a frequency of V2, reflected from the fixed from

razhatel 10, forming a measuring channel

25. A beam with a frequency (yr ± A v (t)) reflected from the movable reflector 11 is combined with a beam with a frequency reflected from the fixed reflector 9, forming the second measuring channel 26. The radiation beam with a frequency ( vi - V2) at the outlet of the mixer 3 forms a support channel. The radiation of the reference and two measuring channels arrive at the photodetectors 12-14, Block

0 5 0

5 0 5 0 5

0

registration performs the conversion of signals from photodetectors 12-14 into countable pulse sequences, the procedure of selecting two measurement channels of one, the signal frequency of which is greater than the other, cross-switching of the selected measurement channel and the reference channel between the inputs of the reversing counter 21, algebraic pulse counting corresponding to the desired movement, and the display of measurement results in a convenient form by the unit 22.

Collimator 4 consists of short-focus 5 and two long-focus 6, 7 (Fig. 2) lenses. Lens 5 is set so that its focus is aligned with the center of the radiation spot on the output mirror of the laser 1. Two beams 23 and 24 of the laser 1, passing through the lens 5, are focused, forming two images of the center of the radiation spot on the output focal plane of the lens 5 laser output mirror. The focal points of objectives 6 and 7 are combined with these images in combination. At the output of collimator 4, the laser radiation 1 consists of two wide parallel beams with flat wave fronts.

In the recording unit, the signals from the photodetectors 12-14 are converted by the formers 15-17 into countable pulse sequences. The pulse frequency at the output of the imaging unit 15 is equal to the reference frequency (v –vzj, and the following frequencies at the outputs of the formers 16, 17 are equal to vy, wvq, respectively. When the corner reflector 11 moves in the direction to the beam splitter 8, the frequency gain Av (t) is proportional to the speed The displacement of the reflector 11 is positive, when the reflector 11 is removed from the beam splitter 8. Av (t) is negative. Consider the case when the reflector 11 approaches the beam splitter 8. In channel 26, the oscillation frequency of the interference field is equal to ultrasonic IV2 + A v (i ) - v I, and in the second channel 25 it is equal to lvs2i + Av (t) -t $ l. Denote v -V2 -v0 and without loss of generality, let us assume that VQ O. Then

Va, VQ + Ai (t)

D) - VQ, when I A v (t) I v0 |

B

Iv0 - Af (t), with I Ai (t) I 10 This shows that in this mode, firstly, VA V3 and, secondly, at high speed of movement of the reflector 11, creating Av (t) additive greater, the operative frequency and, the value of 1% does not correspond to the measured displacement, since it passes through zero and changes sign, and the driver generates a counting sequence proportional to the absolute value of the frequency at its input (without taking the sign into account). Thus, the true information about the measured displacement is carried only by channel 25, the signal frequency in which 4 is in absolute magnitude greater than that in channel 26. When the reflector 11 is removed from the splitter 8, channels 25 and 26 are swapped:

V3 V0 + Av (t)

v0 - Dg (t.), when I A v (t) I v0 No. D DH) - v0, with I Ai (t) I 10

In this case, the correct information about the measured movement contains only channel 26 with a frequency V3. Thus, it follows from (1) and (2) that in order to have true information about the movement of the reflector 11, regardless of its speed, it is necessary at each time point register the signal of only one channel, the frequency of the signal in which is higher than in the other.

The comparator 18 and multiplexer 19 provide a comparison of the frequencies of the signals of the two measuring channels 25 and 26 and the supply to the input of the switch 20 only one signal whose frequency is equal to max (, 4). The switch 20, in turn, provides the signals of the reference and selected measuring channels to the summing and subtracting inputs of the reversible counter 21, depending on the direction of movement of the reflector 11.

When the two signals from the outputs of the drivers 16, 17 are applied to its outputs, the comparator 18 produces a logical signal depending on the sign of the difference in the frequency of the pulse sequences in channels 25, 26. For example, if the frequency in channel 25 is higher, then a logical unit is generated, otherwise logical zero. If the frequencies of the sequences coincide (the reflector is stationary), then the logical signal is determined by the history of the movement of the reflector 11 and does not affect the operation of the device, since it makes no difference which of the two equal frequencies is chosen. Comparator 18 may have various options for a specific implementation. One embodiment is shown in FIG. 3. The comparator consists of a 4-bit binary reversible counter 30, the summing input of which is fed through the key 31 and a signal from the driver 16, and to the subtractive input through the key 32 - from the driver 17. The control inputs of the keys 31, 32 are connected to the outputs 33 , 34 positive and negative transfer (overflow) of the counter 30, which are also connected to the R and S inputs of the RS flip-flop 35, The output of the flip-flop 35 is the output of the frequency comparator 18.

A two-input multiplexer 19, to the control input of which a signal is supplied from the comparator 18, provides for connecting the output to one of the inputs to which signals from the drivers 16, 17 are connected.

Two signals are supplied to the inputs of the switch 20: one from the driver 15 of the reference signal, the other from the multiplexer 19 output. To the control input 29 of the switch

5 20, a control logic signal is output from the output of the comparator 18. When a logical unit is applied to the control input 29, the switch 20 connects the output of the multiplexer 19 with a summing input

0 27 of the reversing counter 21, and the output of the generator 15 - with the subtractive input 28. If the input 29 is a logical zero, that output of the multiplexer 19 is connected to the subtracting 28, and the output of the generator 15 is connected to the counter 21. The reverse counter 21 integrates the difference between the frequencies of the reference and measuring pulse sequences, taking into account the direction of movement of the reflector 11. In

0 in the first mode, this difference is equal to (V4 - v0) Av, and in the second mode (Кз - VQ) Д. The state of the counter is reflected by the display unit 22.

In principle, any two-frequency laser can be used as a radiation source in a given device. However, since this device allows the removal of the limit on the speed of movement of the reflector 11 associated with

0 by the splitting frequency of the laser radiation, there is no need for a large frequency difference (v - vz). On the contrary, it is advisable to choose the splitting of frequencies to be minimal, since this reduces the requirements for

5, the bandwidth of the processing unit, since the frequency (vi -) D v (t)} is recorded. On the other hand, in order to preserve the main advantage of interferometers with signal transfer 0, the discrimination of low-frequency interference associated with a number of factors (acoustic and mechanical vibrations, turbulence of the medium in which measurements are made, laser power fluctuations and etc.), it is necessary that the frequency of the splitting () does not fall into the low-frequency noise band. Low-frequency fluctuations usually occupy a band from zero to ten, hundreds of Hz. Thus,

It is most advisable to have a frequency splitting of laser radiation of several units, tens of kHz. This allows the use of a ring laser with a non-reciprocal element inside the resonator, providing the required frequency splitting as a radiation source. Unlike the Zeeman laser, the production of ring lasers is established, since they are widely used as precision gyroscopes, goniometers, etc. In addition, the ring laser allows one to remove restrictions on the accuracy of measuring the fractional part of the interference band, due to the incomplete separation of radiations of two frequencies v and V2 in the reference and measuring arms of the interferometer.

Unlike a linear laser, where the field in the resonator forms a standing wave, in the ring laser there are two traveling waves propagating in opposite directions. If a non-reciprocal element is installed inside the resonator, in which the optical path lengths for opposite directions are unequal, then, firstly, the two traveling waves will have different frequencies, and secondly, the coupling of the counterpropagating waves will overlap the Bennet dips and only weak b of counterpropagating waves through backscattering. As a result, the laser output already has two beams of radiation with different frequencies v and vz propagating at some angle to one another, and no additional optical elements, as in a Zeeman laser, are required to separate beams with different frequencies. Since there is only one frequency in each beam, the phase of the interference signal within a single linear band depends on the measured displacement. This makes it possible to record an arbitrarily small fraction of the band, while increasing the measurement accuracy.

To implement the device, the following element base can be applied. As a laser 1 can be used any ring laser with non-reciprocal element 2 in the resonator and the mixer 3 of the opposing waves. The elements included in the optical design (pos. 4-11 in Fig. 1) are of a wide release in the optical industry. Electronic registration circuit (pos. 12-22) can be implemented on photodiodes FD 256 or

LFD-2M, KT316, KT361 transistors and 500, 155, 555 series microcircuits.

Thus, the proposed solution allows the formation and automatic switching of two measuring channels in an instantaneous dependence on the direction of movement of the movable reflector, as well as complete filtering of the two frequency emissions from each other and thus

reducing the purity of the signals at the inputs of the processing unit by reducing the initial frequency difference and eliminating the nonlinear dependence of the phase of the interference signal on the measured displacement,

improve speed and accuracy.

Claims (1)

Apparatus of the Invention A device for measuring displacements comprising a laser, a collimator, a beam splitter, a movable angular and fixed reflectors, and a recording unit with photodetectors of the reference and measuring channels and two formers connected to them, A reversible counter and display unit, characterized in that, in order to increase accuracy and speed, it is provided with an additional photo-receiver of the measuring channel included in the registration unit, a third driver connected to it, a comparator, a multiplexer and a switch, the outputs of the driver of measuring channels are connected to the inputs the comparator and the multiplexer, the output of the multiplexer and the output of the reference circuit driver are connected to the inputs of the reversible counter via a switch, and the output of the comparator is for prison to the control inputs of the multiplexer and the switch, the laser is made ringed with a nonreciprocal element in the resonator and a mixer of counterpropagating waves at one of the outputs and optically connected with the photodetector of the reference channel, the collimator is made in as a system of a short-focus object, whose focus is aligned with the center of the spot of radiation on the surface of the laser output mirror, and two long-focus lenses, whose foci are combined with images of the center of the spot in the output focal plane of the short-focus lens, and the stationary corner reflector is made in the form of two corner reflectors rigidly connected to each other. R Compiled by S.Sinitsa Editor A.Makovska Tehred M.MorgentalKorrektor A.Vorovich Order 2990 Circulation Subscription VNIIPI State Committee for Inventions and Discoveries at the State Committee on Science and Technology of the USSR 113035, Moscow, Zh-35, 4/5 Raushsk nab.