RU1783301C - Method of determining distance - Google Patents

Abstract

The invention relates to measuring distance by optical means, namely, to measure distances to stationary or slowly moving objects in control systems of industrial robots, positioning the working bodies of metal processing centers, in alignment operations and other technological operations. SUMMARY OF THE INVENTION: after recording the intensity of the interference band at the initial frequency, the measurements change the path length of the reference beam by a calibrated value, change the radiation frequency by

Description

q s

about

direction closer to the initial intensity, choosing the introduced changes in the path length of the reference beam and the frequency at which the difference in the path of the interfering beams is 5-10 times

the sensitivity threshold for measuring the distance does not exceed 1 / 4-1 / 8 of the radiation wavelength, and the distance is determined by the obtained ratio, which does not include the frequency change value, 1 ill.

The invention relates to measuring distances by optical means and can be used to measure distances to stationary or slowly moving objects, as well as in control systems for industrial robots, positioning the working bodies of metal processing centers, in alignment operations and other technological operations.

A known method for determining the distance using optical coherent radiation is to separate the laser radiation into the remote and reference beams, from which the remote beam is reflected from a controlled plane and the reference from a fixed mirror or reflector, combining the beams on a photodetector, recording the resulting radiation intensity from interference of the remote and reference beams and determining the distance by the formula

D (N + AN) A / 2,

where N is an integer of the order of interference;

AN is the fractional number of the order of interference;

A is the wavelength in the propagation medium.

The disadvantage of the interference method is the low accuracy of measuring the distance due to the inconstancy of the speed of propagation of optical radiation in the medium, and, consequently, of the wavelength A v / v, where v is the propagation speed; V is the oscillation frequency. In addition, at distances much greater than the radiation wavelength, measurement ambiguity arises due to errors in the determination of the interference order.

A known method for determining distances by laser radiation (2), based on the excitation between the mirrors installed at the ends of the measured distance, an integer number of standing waves generated by the radiation of a tunable frequency laser, measuring the difference in optical frequencies corresponding to integer numbers of waves at two adjacent frequencies , and determining the distances by the formula

D

at ST

 where v is the speed of light in the propagation medium;

Av is the difference of two adjacent resonant frequencies.

The disadvantage of this method is the influence of the variability of the velocity v, as well as the additional error in measuring the distance due to the dispersion of the refractive index

propagation medium, which determines the dependence of the speed of light in the propagation medium on the laser radiation frequency. At large distances, the difference frequency Av ~ v / 2D becomes comparable with the fluctuation instability of the tunable laser, which significantly reduces the accuracy of the distance measurements.

Closest to the invention is a method for measuring distances (3), based on the separation of coherent radiation into remote and reference beams, combining the beams reflected from the movable and stationary reflectors in the plane of the photodetector, smoothly changing the frequency of the radiation source and counting the number of interference fringes, whose movement relative to the photodetector is achieved by smoothly changing the frequency of the radiation source, and

the distance is determined by the formula:

D

V2-VI

Dt

where Dm is the change in the number of wavelengths overlapping the distance D;

vi and V2 are the start and end frequencies of the radiation source;

v is the speed of light in the propagation medium.

However, the known method has significant disadvantages. Thus, the measurement result directly depends on the value of the propagation velocity v of the radiation in the propagation medium, which varies widely depending on the temperature, pressure and humidity of the propagation medium. Since radiation is carried out at different wavelengths of coherent light beams, the dispersion of the speed of light distribution also affects the result of determining the distance due to the dependence of the refractive index of the propagation medium on the radiation wavelengths.

The purpose of the invention is to increase the accuracy of determining the distance by eliminating the influence of variability of the propagation speed of the radiation and dispersion of the refractive index of the propagation medium by compressing the frequency tuning range of the radiation source.

This goal is achieved by the fact that in the method of determining the distance to the object, in which optical coherent radiation is divided into remote and reference beams, a reflected remote beam from a stationary reflector is received, the reflected remote and reference beams are combined in one plane, measured field of view of the photodetector is the initial intensity of the interference band at the initial radiation frequency, the frequency of the radiation source is smoothly changed, the order of interference in the process is recorded e changes in frequency and processes the measurement results. New operations and conditions for their implementation have been introduced, namely, after recording the initial intensity of the interference band at the initial radiation frequency, the path length of the main beam is changed by the measured value; registering a second intensity value; compare it with the initial value of the intensity; change the radiation frequency in the direction of approaching the initial intensity with fixing the path length of the reference beam and the frequency at which the path difference of the interfering beams is 5.,. 10 times the threshold for measuring the distance and does not exceed 1/4 ... 1/8 radiation wavelengths; register the third value of the intensity, compare it with the previous one; restore the original path length of the reference beam and record the fourth value of the intensity, and the distance is determined from the expression

- („.- D- d-MgD -t

(n, - t) - (- nl.)

BEFORE.

where DO is the measured change in the length of the path of the reference beam;

u is the initial intensity of the interference band;

P2. P2 - the second intensity of the interference band, respectively, with a positive and negative value of DO; PZ, PZ - third of the intensity of the interference band, respectively, with a positive and negative change in frequency;

, um - the fourth is the intensity of the interference band, respectively, with a positive and negative value of the frequency change by ± D) the restored initial path length of the reference beam (DO 0).

The proposed method for measuring distances is carried out as follows.

The coherent optical radiation of the initial frequency is divided into reference and distance beams, from which the distance beam is directed to the reflector.

The reflected beams combine in the plane of the photodetector, forming interference fringes. The intensity of the interference strip, which is functionally related to the phase difference of the oscillations of the remote and reference beams, is converted into the voltage field of the photodetector into an electrical voltage

35

and 51 (1 + y1) -co52 C + b1, (1)

where Si is the normalized sensitivity of the photodetector;

yi AS / S - relative error

sensitivity reflecting a change in the slope of the calibration characteristic:

5i AU0 - absolute error of zero, reflecting the offset of the calibration characteristic;

10 is the intensity of optical coherent radiation;

p, the initial phase of the oscillations of the probe and reference beams.

The errors of the photoelectric converter y and di arise due to the time and temperature instability of the parameters of the photodetector and its zero drift and change its calibration characteristic,

The voltage (1) is converted to a digital code

n (1 + ya) Si (1 + yt) lox

X COS2 + 52) S3 (1

X5) x

ff1 - yes 2

+ 63.

(2)

where Em A is the integer part of the number A; Di is the quantization step of the analog signal (voltage); S3, US, 5z - sensitivity (5S 1 / Di) and errors of analog-to-digital conversion; 5c SvSa is the normalized sensitivity of the conversion of the radiation intensity to the code, knots, from - the total multiplicative (knots + knots) and additive () conversion errors.

By the nature of the manifestation of the error, and 3) are random functions of time containing slow non-stationary ($, 3i) and fast stationary ($. 3j) components. If the stationary components are easily minimized by time averaging, then the non-stationary components are difficult to detect and subsequently correct

To correct slowly varying errors $ id | the code u corresponding to the initial intensity proportional to the phase difference p - is subjected to functional transformation to linearize the code - phase difference of the interfering beams

Щ arc cos (n) 1/2 S3-lo (1 + И) + & 1/2

XЈL: LSЈ So о + and (w-tfi + d (3)

where So is the resulting sensitivity of the linearized analog-to-digital conversion of So V§j. | 0/2;

y, o - slowly varying resulting multiplicative and additive errors of the measurement transformation, taking into account the variability of the radiation intensity 10.

The intensity code (3), depending on the variable distance and frequency of the coherent radiation, varies according to a linearly periodic dependence, and each linear section of the conversion characteristic corresponds to a change in the phase difference of the interfering beams from O to 2 log

Щ 5- (1 + И ((2} + д So (1 + y) x

x (p + q) 2 + d,

(4)

where p is the integer number of phase cycles in 360 ° (2l);

q is the fractional number of the phase cycle. When the distance Dx is much greater than the radiation wavelength (A D Dx), the code u corresponds to one of the linear sections of the conversion characteristic. Within this section, taking into account the errors of conversion of the distance DI traveled by the remote beam, we have

m-So-d + iW,)) 15 - 2lr) + d

(5)

where vi is the initial frequency of the radiation source;

v is the propagation velocity of the radiation beams in the propagation medium;

D2 is the distance over which the reference radiation passes.

The initial intensity ni of the interference band is recorded. Next, the path length along which the reference beam passes is reduced by a calibrated quantity L D, which is selected from the condition

thirty

(5 ... 10) ... A,

(b)

where A is the wavelength of the optical radiation in front of propagation; AO0 is the minimum change in distance that can be detected by the interference method (sensitivity threshold).

The minimum measurable distance of the interference method depends on

level of phase fluctuations. Therefore, the value of the calibrated displacement DO can be expressed in terms of an additionally introduced shift

45

(5 ... 10) - Дсъ 27П г2ДД ... Д). v c zy

(7)

where Do is the standard deviation of the fluctuating phase difference.

The lower limit of inequality (6) is chosen in view of the possibility of detecting and recording the minimum change in the difference in the path of the interfering beams against the background of phase fluctuations. With a normal distribution of phase fluctuations, the maximum random error at a probability of 0.997 does not exceed the triple value of the mean-square deviations of the Duey value. Therefore, a factor of 5 ... 10 is sufficient for

reliable recording of the minimum change in the stroke difference Di - D2.

The upper limit of inequality (6) provides the conversion of the path difference Di - 02 (1/4 ... 1/8) A within one linear portion of the conversion characteristic, i.e. within one interference band.

As the difference in the path of the interfering beams decreases in accordance with (6), the intensity code changes within one linear portion of the characteristic and increases to a second value

P2

o (15

-2lr).

(8)

If the phase difference during the first transformation (5) turned out to be more than 270 °, i.e.

  D01 U2L,

where Du is the fractional part of the first phase difference, then a decrease in the reference path can cause not an increase, but a decrease in the code due to the transition to an adjacent linear section. The transition to the neighboring region of the characteristic indicates the exit of the recorded interference band from the field of view of the photodetector. To eliminate the ambiguity of the readings and to ensure operation within the same linear region of the characteristic, the intensities rn and n2 are compared. When registering the result ni, the path length of the reference beam is increased by DO and a reduced intensity code is recorded

n2 S0 (1+ y) - (arvi

l

-2lr).

.ri.2 (Di

If the registration result corresponds to (8), i.e. P2 ni, then gradually reduce the radiation frequency by the value Av, which is selected from the condition that the third intensity code approaches the original code ni

pz-3. (1) - (P - ° g + yP) -

2lr) + (5.

(YU)

where V2 -v - Dy is the second radiation frequency value. When registering the code pz # Hz, the frequency increment corresponds to condition (7), because work is carried out within the same linear section

(5 ... 10) AP 27aV2 (

-4p

(eleven)

Inequality (11) is satisfied automatically even in the absence of information about the approximate value of the measured distance, i.e. the magnitude of the difference in stroke Di - D2, in the process of a smooth change in frequency.

If the result of the second registration corresponds to relation (9), then according to the comparison result, the radiation frequency on the remote control is increased and the increased intensity code is recorded

ПЗ оО + уИ пИ01- - 0) - -2лр) + д, (12)

where V3 vi + DE is the third value of the radiation frequency.

Restore the original path length of the reference beam (TO 0) and fix the fourth intensity code

P4 50 (1 + d - () - 2lr) + 6.

(thirteen)

or depending on the sign of the frequency increment

45 p

USo (1 + yK2 -2 D rD2) - 2l) + d.

(14)

From the registered code values (5). (8), (9). (10). (12), (13) and (14) calculate the differences of the codes of the first (5) and fourth (13), (14) transformations, as well as the codes of the second (8), (9) and third (10), (12) transformations

ni - n4 ni1 - ni So (1 + y) x X (2.,. (15)

P2 - PZ 50 (1 + y) - (2l: -DUH ..2 (Di -D2 -AD) (16)

pz1 - nz1 - So (1 + y) - (2l: -Av x x2 (P1-B2-DV) (1)

Next, the ratio of the difference of the codes (15) to the difference of the difference codes (15) and (16) or (15) and (17) is determined depending on the sequence of operations performed

DI - Da (u -04) - (pa-ps) 3D

P1 - P4

. (eighteen)

P4 - u

(P4-P1) - (PZ-P2)

-P1-D2, 19)

Y5 (1U)

From the relations (18) and (19), the path difference between the remote and reference beams is determined

Di-da

P1 - P4

(П1-П4) (П2 Пз)

DO (20)

OR

Di-d2

n-w

(p4-p1) - (pz-p2)

AD. (21)

If we represent the path traveled by the distance beam in the form Di Doi + Dx, where Doi is the distance to the reference plane (Doi D2), where Dx is the definable distance, then the results of calculations (20) and (21) can be represented as

Dx

m - P4

(Щ -П4) - (П2 - Пз)

before

P4-P1

(nl, - u) - (pz - n2)

before

(22)

where AD is the calibrated change in the path length of the reference beam.

An additional change in the path length of the reference beam AO is carried out by displacing the stationary reflector, for example, by applying an electric voltage to the piezoceramic holder of this reflector. Calibration of this displacement is carried out according to the known difference of the distances traveled by the remote and reference beams. To do this, using a standard measure of length, for example, a plane-parallel end measure of 1 m with a relative error of 2-10, a known difference in the course of the interfering beams Do Di - D2 is established. The stationary reflector is then displaced by condition (6) using the applied voltage. The actual value of the change in the path length of the reference beam AO is determined from relations (20) or (21) depending on the sequence of operations (5), (8), (10) and (13) or (5), (9), (12) and (14)

DO (ПУ-П40) - (П20-ПЗО) 1 {23)

P / - P

then - P4o

fifteen

or

Dr (P40-P1o) - (pzO-P21

P40 - P10

(24)

5

0

5

0

5

0

5

where I drink, bastard, PZO, P40 or something, p2o. ISO, P4 ° codes of intensities of 4 measurements during calibration.

The voltage is fixed, which creates a displacement of the stationary reflector by a calibrated DO value, which is then used to create a calibrated change in the path length of the reference beam and determine the distance to the controlled object.

Thus, by measuring the four intensities of the interference band corresponding to the codes ni, P2, pz, P4 and the calibrated change in the path of the reference beam AD, the distance to the reflector of the object Ox is determined irrespective of the propagation velocity v and the slow conversion errors (y, o) of the difference phases of optical radiation in a digital code. Fast errors are reduced by repeated measurements and statistical processing of the measurement results.

Compared to method (3), the influence on the measurement result of the variability of the propagation velocity (v) is excluded. Accurate determination of the distance to the reflector of the controlled object can be carried out with high accuracy without knowing the approximate value of the distance. Due to small frequency changes

radiation (Av / v AD / D / 4 dfr "1))

the influence of the dispersion of the refractive index of the propagation medium on the measurement accuracy is excluded.

Thus, by method (3), the necessary change in frequency when measuring the distance Ox is determined by the expression

Av1 ъ -vi -Am. (25)

Here, Dm is the change in the number of wavelengths overlapping the distance Dx; v and vz are the start and end frequencies of the radiation source; v, s / n is the speed of light in the propagation medium, where c is the speed of light in vacuum, and n is the average refractive index of the propagation medium.

The minimum change in frequency occurs when Dm «1. Therefore, taking into account (25), we have

d. V C

min-shh

(26)

According to this method, to determine the same distance value according to inequality (11), the minimum. Change in the frequency of the radiation source is determined by the condition

2 do

.2 (P1-P2)

(5 ... 10) LR, (27)

where Dn is the standard deviation of the fluctuating phase difference, which determines the phase threshold of the interferometer sensitivity.

From the expression (27) it follows that

d., (5 ... 10) 4l67

(28)

The compression ratio of the frequency tuning range of the radiation source

Dumin 4JT

 5C (5 ... s) LRO

(29)

Modern double-beam interferometers have a sensitivity threshold for the optical phase difference of the order of 10 ... 10 radians. Given this value in this method, the frequency tuning range of the radiation source can be reduced by

Al

2r; s (3-l01 ... 1flP) paa

 (5 ... 10) (10-b ... 10-4)

The possibility of such a large compression of the frequency range, and therefore a small change in the radiation wavelength, allows the dispersion of the refractive index, for example, of the atmosphere, to be neglected.

The attached drawing shows a structural diagram of a range finder.,

The range finder is an example implementation of the method.

The range finder contains a laser 1. A laser radiation frequency frequency adjustment unit 2, a collimator 3, the output of which is optically coupled through a beam splitter 4 with corner reflectors 5 and 6. Reflector 5 is reference and can only be shifted by a calibrated movement ± DO, and reflector 6 installed on the object, the distance to which from the reference plane is 7 Dx

to be measured. A lens 3. an aperture 9 and a photodetector 10, to the output of which an analog-to-digital converter (ADC) 11. is connected to the data outputs via a microcomputer 12, is connected optically to a beam splitting cube 4. Digital-to-analog converters are connected to the data bus (DAC) 13 and 14, the analog outputs of which are connected to the control

the input of the generator 15 of electrical oscillations, and a piezoceramic sleeve (holder) 16 of barium titanate, acting respectively on the frequency adjustment unit 2 and the reflector 5, calculation results

tc output to digital indicator 17.

The range finder works as follows. The radiation of the laser 1 is separated by a cube 4 into the reference and remote beams. The reference beam is reflected from the reflector 5, and the distance from the reflector 6 associated with the controlled object. The reflected beams are combined with a lens 8, aperture 9 and a cube 4 in the plane of the photodetector 10, with which the intensity of the selected point of the interference fringe is converted into voltage and then into a code by means of ADC 11. This code is entered into microcomputer 12 and undergoes functional conversion. The result of the conversion (5) is stored in the random access memory (RAM) of the microcomputer 12.

According to the command of the computer 12 recorded in its read-only memory (ROM), a control voltage is applied to the holder 16 through the DAC 14, which moves the reflector 5 by the value ± DO, the corresponding result of the conversion (8) or (9) of the strip intensity after functional conversion to computers

12 RAM is stored. Then, by the command of the computer 12, a control action is generated on the oscillator 15 through the DAC 13, the frequency of which changes the frequency of the laser 1 by ± Du, which satisfies condition (11). The corresponding result of the conversion (10) or (12) of the band intensity after the functional conversion in the computer 12 is stored RAM. The next computer team 12

the initial position of the reflector 5 is restored, and the corresponding results (13) and (14) after the functional conversion are stored RAM. After performing the calculations according to formula (22), the result of measuring the distance to the reflector 6 of the controlled object is recorded by a digital indicator 17.

An algorithm for selecting the sign of the increment of the laser radiation frequency and moving the reference reflector from the operating conditions within one linear portion of the conversion characteristic is introduced into computer program 12; within one interference band. In computer memory 12, a calibrated displacement of the reference reflector 5 ± DO was also introduced as a constant.

The considered method was used in technological operations of adjusting precision mechanisms, in particular, ROM settings on Winchester drives. An LG-52-1 type laser was used as a coherent radiation source, an FD-271 photo diode array was used as a photodetector, and the frequency tuning unit was an ML-201 type acousto-optical modulator. The range of measured distances is 0.001-1 m, the measurement error is not more than +0.2 microns, when working in an environment with changing temperature and humidity.

Claims (1)

The claims The method for determining the distance to the object, in which the optical coherent radiation is divided into the distance and reference beams, receives the reflected distance beam from the object and the reference beam from the stationary reflector. combine the reflected distance and reference beams in one plane, measure the initial intensity of the interference strip at the initial radiation frequency in the field of view of the photodetector, smoothly change the frequency of the radiation source In this case, the order of interference is recorded in the process of changing the frequency, and the measurement results are processed, characterized in that, in order to increase the accuracy after recording the initial intensity of the interference band at the initial radiation frequency, the path length of the reference beam is changed by the measured value, the second intensity value is recorded , compare it with the initial value of the intensity, change the radiation frequency in the direction of approaching the initial intensity with fixing for the path of the reference beam and the frequency at which the difference in the path of the interfering beams is 5-10 times higher than the sensitivity threshold and does not exceed 1 / 4-1 / 8 of the radiation wavelength, a third intensity value is recorded, compared with the previous one, and the original reference path length is restored beam and record the fourth value of the intensity, and the distance is determined from the expression Dx ni-um Щ -П4) - (П2-Пз) D0 ni - m (P4-GZ) - (PZ-G) BEFORE, where DO is the measured change in the length of the path of the reference beam; u - initial intensity of the interference band; P2, the second intensity of the interference band, respectively, with positive and negative values of AO; pz, pz1 - third of the intensity of the interference band, respectively, with positive and negative values negative frequency change; L4, P4 - fourth is the intensity of the interference band, respectively, with positive and negative values of the frequency change during frequency shift by ± Av and the restored initial length of the reference beam (TO 0).