A method of measuring the displacement

 

(57) Abstract:

A method of measuring the movement relates to fiber-optic transmission systems in measurement technology and is designed to measure displacements of the object. Summarized in the measurement area, the modulated radiation at a frequency of1modulate and lower than1frequency 2and12. Emit the signal of the first harmonic of the modulation frequency 1. Then the signals of the first harmonic and second harmonic served on the block comparison, where they perform the comparison of the latter. The output signal of the block comparison serves to the input of the comparator where it is compared with a reference voltage. The output signal of the comparator is fed to the control input of the sampling device and storage, which includes the modulation signal with a frequency of2and the measured value of relative displacement of the end faces of fiber-optic channels is determined by the output signal of sample and hold. The invention eliminates the effect of multiplicative noise and increase the accuracy of the measurements. 5 Il.

The invention relates to fiber-optic transmission systems in measurement technology and can be used to measure premasticated the amount of movement of the object [1], at which measured value the effect on the mutual location of the receiving and transmitting ends of the fiber-optic channels or change the conditions of propagation of radiation between the fixed ends of the receiving and transmitting optical fiber channels. For this monochromatic radiation through the transmitting fibre channel fail in the measurement zone, where they form the flux enclosed in a cone of aperture of the light guide. Part of the radiation flux illuminating the input end of the receiving fiber-optic channel, derived them from the zone dimensions, and is applied to the photodetector, where the radiation is converted into a proportional electrical signal, which is used to determine the measured physical quantities. The physical basis of this method of measurement is the change in the intensity of radiation under the action of a measured parameter, which takes place from the end face of the transmitting optical fiber channel on the receiving end of a fiber link in accordance with the directivity, the light transmission fiber-optic channels, the influence of the measured values and different noise.

However, this method of measuring displacements have the military reduces the measurement accuracy.

Famous work [2], which presents a way to measure, for example, the moving object by using a Fabry-Perot interferometer, namely, that form a monochromatic radiation, by using the transmitting fibre channel fail him in the measurement zone, then through a host fibre channel down radiation to the photodetector, where transform it into a proportional electrical signal. It uses homodyne methods of measurement of various physical quantities, changing according to the harmonic law, which laid the basis for the study of harmonic components of the signal at the output of the homodyne system with further decoding and analysis of its envelope. Thus, to implement one of the methods described use the decomposition of the signal taken from the output of the measuring system in the spectrum. Set the value of the phase difference so that sin = 1. Then from the state of rest smoothly excite oscillations and find the first maximum amplitude of the harmonic component at the fundamental frequency of the investigated object 1. Then measure an unknown amplitude: to re-establish the value of = /2 +is hronicheskoi component at a frequency of1. Further in the formula to find the unknown value.

The main disadvantages are described in [2] method are necessary to calculate the arguments of the Bessel functions and units values of the phase difference in the measuring system, the limitation of the range of measurements associated with the scope of uniqueness of Bessel functions, as well as the assumption that when the required two units of the magnitude of the phase difference remains constant characteristics of laser radiation (frequency stability, the laser intensity noise) and environmental parameters. To implement these conditions in practice is extremely difficult.

Closest to the invention in its technical essence is a way of measuring displacements [3]. This way, selected as a prototype, is that form of monochromatic radiation, modulate its intensity and wavelength at a frequency of1according to the harmonic law and light through the transmitting fibre channel surface of the object to the measured distance that is experiencing interference phenomena, the result of which are non-linear distortion occurring in the optical system. On the which selects the signal of the second harmonic of the modulation frequency1and the magnitude of its amplitude is determined by the desired distance. In this case, the implementation of the method is based on the following physical phenomenon: the power and the wavelength of a semiconductor laser depends on its current pumping [4].

The disadvantages of this method are the relatively low accuracy of the movement, the noise immunity and complicated implementation. This is because, firstly, there is no accounting for multiplicative noise, secondly, although the second harmonic and is a periodic function of the phase difference, its amplitude nonlinear changes over the period. Therefore, the determination of the unknown quantity on the basis of the amplitude of the second harmonic is inaccurate due to the nonlinearity of the latter. Let us consider the interference, which, as is well known, are divided into multiplicative and additive. To reflect the radiation power in the optical channel P can be expressed as follows [4]:

P = f(t,z)P0+ A(t,z), (1)

where

f(t,z) is the expression for the multiplicative noise;

P0- source optical power;

A(t,z) is the expression for the additive noise;

t - time;

z - external influence.

Additive interference voznikali on the photodetector. Their suppression is relatively easy: to conduct a more thorough protection to sensitive components from external radiation. Multiplicative noise due to the following factors: the instability of radiation sources; heterogeneity transparent environment fiber optic tract associated with aging fiber, its microengine, temperature. To compensate for multiplicative noise requires a fundamental change in the design of the device, method for determining the desired value.

The disadvantages include the limitation on the range of measurements associated with a scope ambiguity function on the period.

The task of the invention is to eliminate these disadvantages, i.e., improving the reliability and accuracy of measurement of moving object.

This object is achieved by a method for motion measurement, namely, that form a monochromatic radiation, modulate its intensity and wavelength at a frequency of1according to the harmonic law, by the transmitting fibre channel modulated radiation down to the zone of measurement light input end face of the receiving fiber optic canal, located on rosstanoutinmo channel radiation is led to the photodetector and the device, emit the signal of the second harmonic of the modulation frequency1that is different from the known fact that the emission modulate and also at a lower frequency2and1>>2emit the signal of the first harmonic of the modulation frequency1then the signals of the first and second harmonics served on the block comparison, where they perform the comparison of the latter, the output signal of the block comparison serves to the input of the comparator where it is compared with a reference voltage, the output signal of the comparator is fed to the control input of the sampling device and storage (water economy Department), at the entrance of the water economy Department serves the modulation signal with a frequency of2and the measured value of relative displacement of the end faces of fiber-optic channels is determined by the output signal UHV.

The main features that distinguish the proposed method from the known, are an additional modulation of the radiation at the lower frequency2further allocation of the first harmonic of the modulation frequency1with the subsequent comparison of the signals of the first and second harmonics, which defines the novelty. From the above it follows that the proposed method meets the criterion of "inventive step".

This gives the advantage of WPI is th measurements by improved signal processing.

In Fig. 1 presents a functional diagram of a device that implements the proposed method, Fig.2 is a plot of the ratio between the amplitudes of the second harmonic to the first signal frequency modulation1from the measured distance between the receiving and transmitting ends of the fiber-optical channels; Fig.3 is a diagram of signal processing for the case of comparison by division; in Fig.4 is a diagram of signal processing for the case of comparison by subtracting Fig.5 is a graph of the depth of modulation signal at a frequency of 2depending on the base of the Fabry - Perot interferometer (IFP).

A device that implements the proposed method contains the emitter 1, made in the form of a semiconductor laser, which is a source of monochromatic radiation, the device modulation radiation 2 at a frequency of2the device modulation radiation 3 frequency1the forming device DC 4 connected to the emitter 1 sequentially transmitting fiber-optic channel 5, the input end of which is optically connected with the emitter 1 and the outlet end is located in the measurement area, the receiving fiber-optic channel 6, the input end of which is located in the measurement zone coaxially with the output end of the transmitting from the photodetector 7, and further devices emit the signal of the first harmonic 8 and the second harmonic 9 frequency modulation1, Comparer 10, where a comparison is made of the signals coming from devices 8 and 9, a further comparator 11 is connected with the block of the reference voltage 12, the output of comparator 11 is connected to the control input of the sampling device and the storage 13, an input connected with the device modulation radiation 2.

The forming device DC 4 outputs the emitter 1 to the operating point, the modulation device 2, and 3 change the pump current of the emitter 1, which in turn affects the intensity and spectral composition of the radiation of the latter [4]. In this case, the device 3 modulates the radiation harmonic law on the high frequency1(1 - 10 MHz), the device 2 modulates the radiation at the lower frequency2(1 - 1000 kHz). For simplicity, we assume that the modulation at a frequency of2happens sawtooth law - slow linear increase of the signal to a certain level the latter, then quickly drops to the initial level. The frequency value1whichever is greater for a given hardware implementation of schemes 7 - 10, and the frequency value2- from the condition that the response time of the circuits is teaching at frequencies2and1. Modulation of the pump current device 3 causes the modulation of the radiation, namely in addition to the permanent component of wavelength0an additional value of1similarly, the device 2 provides a Supplement to the wavelength0the value of 2. The value of1is chosen from the condition that the modulation of the radiation at a frequency of1will not lead to the emergence of the next order of interference, i.e.

< / BR>
where

m is the order of interference.

Modulation of the radiation at the frequency , in contrast, would lead to a shift of the order of the interference by one, that must match

< / BR>
Therefore, the amplitude modulation of the radiation by the sawtooth law i0is calculated based on the value of1which in turn directly determines for each specifically made IFP. The amplitude modulation of the radiation by the harmonic law, i should be much less than the value of i0, i.e., 1 < i0. Thus, the generated radiation is supplied by the transmitting fiber-optic channel 5 in the measurement zone. The output end of the transmitter 5 and the input receiving end 6 of the fiber-optic channels are mirrors And the rank difference of the rays . In addition, it depends on the parameters of the IFP and the supplied radiation. Under the influence of the measured physical parameter, in particular when moving the studied object, there is a change in the magnitude of the difference of the rays in the IFP, which linearly depends on the distance between the mirrors [4]

< / BR>
where loptthe distance h between the mirrors IFP taking into account the refractive index n and the angle of incidence of rays, lopt= hn;

- wavelength radiation.

The pump current of the emitter 1 is modulated as follows on the same period "saw":

I = I0+ i0t + icos(1t), (5)

where

I0- constant component of the pump current;

i0- small value in comparison with I0representing the current modulation sawtooth law with frequency2;

i - small value in comparison with I0representing the current modulation harmonic law with frequency1;

1- frequency modulation;

t - time.

Then the power of the laser will be determined in the first approximation [4]

P0= aI + b,

where

a is a constant of order 7,510-2W/a [4];

b is a constant of order - 2,510-3W [4].

the characteristic classical IFP has the following form [4]:

< / BR>
where

P0the radiation power at the input IFP;

- reflectance mirrors IFP.

But as a result of modulation of the pump current, the wavelength is variable and can be expressed as follows [4]:

=0+ kI, (9)

where

0the wavelength at a constant pump current I0;

k - d /dI 610-9m/A [4],

or (2) the wavelength is represented

=0+ ki0t + kicos(1t). (10)

The power of the optical radiation transmitted IFP (i.e., falling into the input end of the transmitting fiber-optic channel 5 and then on the photodetector 6), taking into account (4) and (7) can be represented in the form

< / BR>
The expression (11) describes the transfer characteristic of IPP as a function of time and frequency1modulation of the pump current. The graph of this function in the coordinates of the power (PIFP)) and time(1t) represents a curve that contains the number of extrema. The number and shape depend on the magnitude of the modulation of the pump current i, the parameters of the interferometer and emitter (a, b, k, ) and the main thing - from base IFP, i.e. the distance between the mirrors h. So, when changing h (all other parameters fixed) form of transfer is elsaelsa as the origin of the two following pulses from the edges (considered in the period), and then the process repeats.

Curve (11) is periodic, it is lawful to judge its decomposition into a Fourier series of harmonics. In this case, interested in the amplitudes of the first and second harmonic frequency1. For this purpose, the signal from the photodetector 7 is fed to the input of the device emitting the signal of the first harmonic 8 and the second harmonic 9 frequency modulation1the output signals of the devices 8 and 9 is fed to the input of block comparison 10. The Comparer 10 operates as follows: electronically divides the amplitude of the second harmonic to the amplitude of the first harmonic or adjusts the signals of the first and second harmonics at one level and then subtracts one from the other. Both options give you the opportunity to get rid of the multiplicative noise and therefore are of technical interest. Here the first option signal processing. In this case, the signal at the output of block 10 is

S = I2/I1, (12)

where

Iithe amplitude of the i-th harmonic frequency1decomposition of a signal (11).

The graph of the function S on the distance between the mirrors IFP is periodic, with period there is a weak minimum and a pronounced maximum (Fig.2). In the prototype searched her. First, the multiplicative noise, which is superimposed on the radiation at the output of IPP, to the same extent affect the magnitude of the second harmonic. Therefore, the use of the latter as an informative parameter obviously leads to a distortion of the desired movement by the amount of interference. Secondly, the amplitude of the second harmonic nonlinear changes over the period, therefore, the definition of the desired amplitude of the second harmonic is inaccurate.

In this way it is proposed to use the peak of the second relationship to the first harmonic as a signal that controls the comparator 11. The latter compares the output signal of the block 10 with support Uop, which is provided by the block 12, i.e. e can be configured for a certain pre-selected mode of operation. This may be related, for example, a gauge scale, thus avoiding the limitations of the measurement range. In addition, electronic division leads to the suppression of the multiplicative noise contained in the signal and its harmonics.

Consider, for example, a linear increase of size in IFP (measured move) (Fig. 3, a). The comparator 11 is activated according to the level of the incoming signal is in the front, is a control device for sampling and storage 13, which also receives the sawtooth signal frequency2from device 2 (Fig.3, g). The device 13 generates a signal in accordance with the measured parameter U () and stores it until the next control pulse from the comparator 11 (Fig.3, D.). The reference voltage Uopis selected so that the comparator 11 worked in the field, where the output signal Comparer 10 has the highest slope.

In the second case, the block comparison 10 subtracts the signals from one another, for example from the amplitude of the first signal depending on the amplitude of the second signal prior to their adjustment to the intersection. This is possible with the use of electronic amplifier, the gain of which kusis as follows: the signal amplitudes of the first and second harmonics become such that the graph of their difference crosses zero (the time axis) in its most steep section. The output signal of the block 10 is

S = I2- I1,

with each period of the graph of the function S crosses zero twice (Fig.4, b and C). It also allows you to set the mode of operation of the comparator using a zero reference voltage U2, I1and is influenced by them, but their difference does not currently have the interference at the point where it vanishes. In addition, in the case of subtracting the hardware diagram of the device easier compared to the case of division. E-the division requires the use of digital technology, which complicates the implementation of the method. When the analog division division accuracy does not exceed 0.5 - 1%.

Specifically, the method can be implemented as follows.

Will calculate the amplitudes of the currents pumping for GaAs semiconductor laser. We will use the numerical data given in [4].

The forming device DC 4 outputs the emitter 1 to the operating point, forming a constant current I0= 100 mA. For simplicity, we assume that the modulation current of the pumping device 2 at a frequency of 2happens sawtooth law, providing a Supplement to the wavelength0the value of2. This modulation of the radiation at a frequency of 2should lead to a shift of the order of the interference by one, which should correspond to (3) or

< / BR>
where

m is the order of interference.

We define the amplitude modulation of the radiation by the sawtooth law i0. Ukazatel of refraction of the medium between the mirrors IFP;

h is the distance between the mirrors IFP (base IFP);

- wavelength radiation.

Formulas (14) and (15) lead to the following expression:

< / BR>
The wavelength taking into account (2) can be represented

< / BR>
from here defined

< / BR>
For the calculation we will take k = 610-9m/A,0= 810-7M, n = 1. The expression (18) leads to the following results: to measure changes h base IFP, part of the order of several millimeters, it is advisable to set the value of i0about 10 mA. A plot of the depth of modulation signal at a frequency of2the distance between the mirrors IFP h shown in Fig.5. When the modulation depth is about 10%.

The amplitude modulation of the first voltage pumping device 3 must be less than the value of i0much more, in order to ensure condition (2). In this case, we can take i = 1 mA.

Thus, unlike the prototype, the proposed method allows to eliminate the influence of multiplicative noise by suppressing them with electronic comparison signals. Furthermore, the method allows for more accurate measurements of the displacements due to the fact that an informative signal proportional measure the modulation her. - M.: Energoatomizdat, 1989, S. 5-8.

2. Optical homodyne measurement methods. - The magazine "Foreign Radioelectronics", 1995, N6, S. 43 - 48.

3. Author's certificate N 1516775 the USSR, CL G 01 B 11/14, 1989 (prototype).

4. Butusov M M Fiber optics and instrumentation. M.: Mashinostroenie, 1987, 330 S.

Method for motion measurement, namely, that form a monochromatic radiation, modulate its intensity and wavelength at a frequency of 1according to the harmonic law, by the transmitting fibre channel modulated radiation down to the zone of measurement light input end face of the receiving fiber-optic channel at a distance from the output end of the transmitting fiber-optic channel, then using the receiving fibre channel radiation is led to the photodetector and the device emitting the signal of the second harmonic of the modulation frequency1, characterized in that the radiation modulate and also at a lower frequency2and 12emit the signal of the first harmonic of the modulation frequency1then the signals of the first and second harmonics served on the block comparison, where compare vyhodnoi signal of the comparator is fed to the control input of the sampling device and storage at the input of the sample and hold signal modulation with a frequency of2and the measured value of relative displacement of the end faces of fiber-optic channels is determined by the output signal of sample and hold.

 

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