RU2610905C2 - Laser device for measuring velocity and/or displacement of small-size objects in limited-access places - Google Patents

Abstract

FIELD: physics, instrumentation technology.

SUBSTANCE: invention relates to laser Doppler measuring devices. A laser device for measuring velocity and/or displacement of small-size objects in limited-access places includes a single-frequency semiconductor laser with wavelength of 1064 nm and an oscillation line width of not more than 3 MHz. The device also includes an optical isolator, a fibre amplifier with a pumping laser diode laser, an optical radiation splitter performing the function of a beam splitter. The splitter is connected to a connector with a corner chip which performs the function of a weakly reflective mirror, a collimator with a beam diameter of 0.8-1.2 mm and an optical receiver. The output of the receiver is connected to the input of an oscilloscope which is connected to a computer via a USB interface.

EFFECT: invention provides a long (about 100 m) coherence length.

2 cl, 7 dwg

Description

FIELD OF THE INVENTION

The claimed invention relates to the field of measuring equipment, namely to laser Doppler speed and / or displacement meters and can be used, for example, for non-contact investigation of the dynamics of small-sized fast-moving objects in hard-to-reach places in a wide range of speeds, in particular for measuring speed and / or displacement bullets of small-caliber small arms or large-caliber projectile, in the study of fast-moving detonation and explosive processes occurring in nale of the weapon barrel.

State of the art

In a wide range of tasks of scientific research (meteorology, studying the dynamics of detonation processes, anemometry, etc.) and production (for example, in the steel rolling industry and the military-industrial complex), accurate non-contact measurements of the linear velocity of a moving body are required. One of the well-proven methods is speed measurement based on the Doppler effect (A. Anikin, A. Fedoseysky, “Laser Doppler meters in the hot rolled metering system”, “METALLURGY DEVELOPMENT”, 2007).

Laser Doppler velocity meters (LDIS) are based, as the name implies, on the Doppler effect, which manifests itself in a change in the frequency of the recorded oscillations when the emitter or receiver moves. In practice, the double Doppler effect is usually used when the object is moving, and the receiver and emitter remain stationary. One of the LDIS methods used in practice is the use of two coherent rays incident on the object under study at an angle to each other (“Laser Doppler speed and length meter”, A. V. Hurhesova, EM Fedorov, III Scientific and Practical Conference “ Information-measuring equipment and technologies "May 3-5, 2012).

However, in cases where it is necessary to study fast-moving objects, access to which for one reason or another is complicated, for example, in the case of the study of detonation or ballistic processes occurring in the barrel channel of a firearm or large-caliber weapon, in particular when studying dynamic processes (bullet movement structural elements) that occur during the shot, the most suitable means of measurement are laser speed meters with autocollimation schemes (homodyne or heterodyne), allowing measurements of both small and high speeds of moving objects in hard-to-reach places, for example, inside the bore of a small arms or any other type of weapon (A.L. Pavlov, Yu.N. Pyrkov, VB Tsvetkov “Autocollimation Doppler speed meter ”,“ Applied Photonics ”No. 2/2014, pp. 71-83).

The prior art laser Doppler speed meter, including a semiconductor laser with a sinusoidally changing frequency, interferometer, photodetector (patent GB 2183956 A, publ. 06/10/1987).

The disadvantage of the meter is that a laser source with a sinusoidally changing frequency is used as a master oscillator, which allows you to record a slight longitudinal movement of the object (vibration), but when measuring fast-moving objects, measurements are made with a large error. In addition, the absence of a laser amplifier in the circuit leads to the fact that when the device is in operation it is not possible to control the power of optical radiation, which in the case of a poorly reflecting surface of the measured object or when measuring high speeds of the object leads to too low a level of useful electrical signal. In addition, the device has a complex circuit.

A laser Doppler speed meter is also known, including a two-frequency semiconductor laser, at the opposite ends of which there is a reflective and partially reflective mirror, an optical divider, an interferometer, a photo detector connected to an oscilloscope (patent US 3864041, publ. 04.02.1975).

The disadvantage of the meter is that a dual-frequency laser is used as the master oscillator, which complicates the design of the device to ensure the transition of the laser from gigahertz frequency ranges to megahertz. In addition, the absence of a laser amplifier in the circuit leads to the fact that when the device is in operation it is not possible to control the power of optical radiation, which in the case of a poorly reflecting surface of the measured object or when measuring high speeds of the object leads to too low a level of useful electrical signal.

Also known is a laser Doppler speed meter, including a master laser, a Mach-Zander interferometer, a photodetector (patent US 8144334 B2, publ. 03/27/2012).

The disadvantage of the meter is the small measurable range of speeds and displacements, which makes it possible to record a slight longitudinal movement of the object, i.e. the device is actually a vibrometer.

Information confirming the implementation of the invention

The problem to which the claimed invention is directed is to create a laser Doppler speed and / or displacement meter for studying the dynamics of motion of small-sized fast-moving objects in places with limited access in a wide range of speeds, in particular for studying the dynamics of the movement of a bullet in the channel of a gun barrel.

The technical result achieved by the implementation of the claimed invention is to expand the arsenal of technical means of laser speed meters of moving objects by measuring the speed and / or movement of small objects in places with limited access, in particular for measuring movement dynamics, for example, speed and / or movement bullets in the barrel bore of small arms in the range from 0.1 to 180 m / s, as well as for studying other detonation processes occurring in the barrel bore at the time of the shot. According to the results of studies that can be carried out during the testing process, it is possible, in particular, to evaluate the technical characteristics of the weapon, for example, the effectiveness of the weapon, including oscillatory processes at the time of the shot, or design flaws that can be taken into account at the stage of design and / or manufacture of weapons, affect.

The claimed technical result is achieved due to the fact that the laser meter for speed and / or movement of small objects in places with limited access includes a single-frequency semiconductor laser with a wavelength of 1064 nm and a generation line width of not more than 3 MHz, an optical isolator, a fiber amplifier with a laser pump diode , an optical radiation divider, which performs the function of a beam splitter plate, to which a connector with an angled chip is connected, which performs the function of a weakly reflecting mirror, a collimator with a diameter m 0.8-1.2 mm beam and an optical receiver, whose output is connected to the input of the oscilloscope, connected via USB-interface with a computer.

A distinctive feature of the claimed invention is the use as a laser radiation source of a single-frequency semiconductor laser with a wavelength of 1064 nm and a generation line width of not more than 3 MHz, which provides a large coherence length (about 100 m), allowing measurements of the movement of the investigated object in the range of movement up to 100 m and in the speed range from 0.1 to 180 m / s. The restriction on the measured range of displacements is caused by a decrease in the contrast of the interferogram by about 2.7 times with a difference in the optical lengths of the working and supporting arms of the interferometer equal to the length of the coherence of the source (a decrease in contrast leads to a significant deterioration in the signal-to-noise ratio), therefore, when using a source with a line width up to 3 MHz (which is equivalent to a radiation coherence length of about 100 m) it is possible to measure for movements plus 50 m and minus 50 m relative to the support arm (50 = 100/2; dividing by 2 you Vano double passage of the light path in the forward and reverse directions), giving a total range of positions of the measuring object 100 m.

The implementation of the invention

FIG. 1 is a schematic diagram of a laser speed meter and / or moving small objects in places with limited access;

FIG. 2 - fragment interferogram shot;

FIG. 3 - spectrogram shot;

FIG. 4- graph of the dependence of speed and acceleration of a bullet on time;

FIG. 5 is a graph of the velocity and acceleration of a bullet versus distance;

FIG. 6 - combined graphs of various parameters of the shot from time to time;

FIG. 7 is a graph of a module for the speed of oil droplets and bullets.

A laser meter for the speed and / or movement of small objects in places with limited access (Fig. 1) includes a laser radiation source 1 connected by an optical fiber to an optical isolator 2, a fiber amplifier 3 with a laser pump diode 4, an optical divider 5, acting as a beam splitter for the separation of optical radiation in a 1: 1 ratio, connected by an optical fiber to the FC / APC 6 connector, which serves as a low-reflecting mirror, and a collimator 7 with a beam diameter of 0.8-1.2 mm, and an optical ISRC optical receiver 8. The output 8 is connected to the input of the oscilloscope 9, connected through a USB-interface with the personal computer 10. In the experiment, when measuring the speed of the bullet 12 into the barrel 11 of air rifle also used protective plexiglass measuring elements 13. [pos. 1 + pos. 2 + pos. 3 + pos. 4], pos. 5, pos. 6, [pos. 7 + pos. 12], pos. 8 are an optical fiber analog of a Michelson interferometer. The source of 1 laser radiation is a semiconductor single-frequency laser stabilized using a Bragg grating, operating at a current of 120 mA, with a radiation power of about 20 mW and a wavelength of 1064 nm with a generation line width of not more than 3 MHz, which ensures a long coherence length and, therefore, provides the ability to measure the dynamics of the object in the range of movement up to 100 m and in the speed range from 0.1 to 180 m / s. The design of the laser 1 with an amplifier [pos. 2 + pos. 3 + pos. 4] is disclosed in the publication (see AI Trikshev, AS Kurkov, VB Tsvetkov “Single-frequency hybrid laser with an output power of up to 3 W at a wavelength of 1064 nm,“ Quantum Electronics ”, No. 5/2012 , pp. 417-419). Optical isolator 2 transmits radiation from the laser 1 in only one direction and is used so that reflected radiation traveling in the opposite direction does not adversely affect the laser diode 4. THORLABS® IO-F devices can be used as optical isolator 2 -780, IO-F-780ARC or similar. The use of fiber amplifier 3 in the proposed scheme does not improve the contrast of the received signal, but increases its amplitude, which allows in some cases to improve the signal-to-noise ratio. Connector 6 with an angled cleavage serves as a low-reflectivity mirror to increase the contrast of the interference pattern received by receiver 8. The need for such a mirror arises from a very small fraction of the radiation returning back to the collimator 7 after scattering from the object under study and passing through the collimator 7. According to estimates, this fraction radiation is about 10 ^-3 -10 ^-4 of the radiated power of the laser 1. As a connector 6 can be used devices standard FC / APC with a cleaving angle of 8 °. To create a beam with low divergence in the measured volume, an optical system 7 is used, for example, a fiber collimator with a gradient lens, which in the preferred embodiment creates a beam with a diameter of 0.8-1.2 mm, which is caused by the need to measure the speed and / or movement of objects in places with limited access, for example, in the barrel bore of small-caliber rifles having a small diameter (4.5 mm), as well as to reduce the influence of speckle patterns on the measurement results, which with an increase in the diameter of the beam lead to reducing the contrast of the interference pattern. As an optical radiation detector (detector) 8, in the particular case of the invention, a PROM-364M-34368 series receiver (Telaz company) with a fiber input (for example, see hltp: //www.telas.m/product/ pr364m.htm). The signal from the receiver 8 is fed to a digital oscilloscope 9, for example, model B-424, connected to a personal computer via a USB interface. Oscilloscope 9 is capable of saving 16776704 samples in one measurement, which at the maximum data acquisition rate (one sample every 2 ns) gives a measurement duration of about 33.6 ms. The oscillograms are saved in text format using the specialized software “Oscilloscope” included in the set of the used oscilloscope, after which they are processed on a personal computer using standard mathematical packages.

The device operates as follows.

To confirm the implementation of the invention and achieve the claimed technical result, as an example, a speed measurement and the movement of a bullet in the barrel of a firearm were carried out.

The measurements were carried out using a spring-piston air rifle IL-61 and lead bullets weighing 0.5 grams. The IZH-61 rifle operates on the basis of a piston, which is cocked by a lever into a firing position, while accumulating potential energy in a spring attached to this piston. At the time of the shot, the piston is released by the trigger, which leads to its movement inside the glass and, consequently, to pressure buildup. Through a special hole, compressed air enters the barrel, which leads to acceleration of the bullet. The rifle was fixed on the optical table, the protective plexiglass 12 was fixed at an angle of about 60 ° degrees relative to the barrel 11 of the rifle. Next, a shot is made from a rifle by pressing the trigger with the simultaneous supply of a clock pulse to the synchronization input of the oscilloscope 9 using a special sensor. Oscilloscope 9 in the write once mode upon arrival of the clock pulse records 16776704 samples with the acquisition period dt = 2ns. The waveform is a signal from the optical receiver 8, i.e. essentially a finished interferogram.

Processing received data

The measurement result is an oscillogram / interferogram of radiation reflected from the object (bullet) being studied, a small portion of which is shown in FIG. 2. In the selected configuration, it was a time-equidistant sequence of 16776704 pairs of values (time and voltage). Due to the Doppler effect, beats with a frequency directly proportional to the speed of the measured object (bullet) will be observed in this waveform, therefore, it is necessary to calculate the spectral components of the waveform at different points in time. The most convenient format for presenting such results is a spectrogram, i.e. diagram showing the dependence of the spectral density of the signal power on time. Traditionally, a three-dimensional spectrogram is represented as a two-dimensional image, in which the spectral density is displayed in the form of color, or the intensity of the points in this image.

In calculating the velocity and spectrogram at some instant t, we used the fast Fourier transform (FFT). At the selected time, the block size N is selected (the number of samples in the vicinity of the selected time), according to which the FFT function will be calculated. Since FFT is used in the calculations, N must be equal to the power of two.

In addition, block overlap of N / 2 was used, i.e. the second half of the block was used as the first half of the next block. Thus, we obtain the following spectrogram characteristics: frequency resolution δƒ = 1 / (dt * N), maximum measurable frequency ƒ _max = 1 / (2 * dt) and time resolution δt = dt * N / 2. Thus, it is seen that the resolution in time and frequency are interrelated and an increase in one of the parameters will inevitably lead to a decrease in the second. For this reason, it is necessary to choose the optimal block size N depending on the characteristic acceleration of the measured object. This block size can be determined based on the relation a = λ / (N * dt) ² , where a is the characteristic acceleration. Having calculated the FFT for different time instants dt, we obtain the spectrogram shown in FIG. 3, i.e. three-dimensional graph of the spectral density of the interferogram as a function of frequency and time. Then, algorithmically or manually, we select a useful signal, i.e. the most likely graph of frequency versus time. Then according to the formula: Δƒ = ƒ-ƒʺ = 2ν / λ, where

Δƒ - change in frequency, MHz,

λ is the wavelength of the laser radiation source, 1064 nm,

ν is the speed of the investigated object, m / s,

recalculate the obtained graph of frequency versus time into the graph of velocity versus time shown in FIG. 4. After that, according to the obtained speed graph, using numerical methods of differentiation and integration, we calculate the acceleration and distance (Fig. 4). Direct calculations were performed using standard mathematical packages.

Analysis of the experimental results

In FIG. Figure 3 shows a characteristic spectrogram of a shot in which the useful signal is highlighted by a solid line. With increasing frequency, the intensity of the useful signal decreases, which is probably due to the limited speed of the used receiver 8. For the zero point in time, the start of the piston’s movement is the rifle, which is indistinguishable on this frequency scale of the spectrogram, for this reason the spectrogram is presented from the beginning of the bullet’s movement. It can be seen from the graph that the duration of the bullet acceleration is slightly less than 5 ms from the time instant of 6.5 ms (when the speed starts to increase sharply) to the point in time when the acceleration ends. During the previous stage, lasting about 6.5 ms (see Fig. 6 and Fig. 7), based on the low speeds of movement and knowledge of the design features of the rifle, we can conclude that there is an initial acceleration of the piston of the rifle, which causes the barrel to move (together with a bullet) due to recoil. Thus, the duration of the shot from the moment the piston begins to move until the bullet collides with the protective plexiglass 12 is about 11.2 ms.

After extracting the useful signal, a graph of speed (solid line) and acceleration (dashed line) obtained by the method of numerical differentiation from time, shown in FIG. 4. Using numerical integration, for example, by the Simpson method, it is possible to plot the acceleration (dashed line) and velocity (solid line) of the bullet versus the distance traveled by the bullet in the barrel channel, shown in FIG. 5. The graph shows that the total distance traveled by the bullet was 48 cm, which is in good agreement with the barrel length of the rifle, which is 45 cm, and the distance to the protective glass 10, which is about 3 cm.

Also, in addition to the direct movement of the bullet, as a result of measurements, additional phenomena occurring in the barrel channel were recorded. The first and most obvious is the "hump" (see Fig. 6, dotted line) preceding the main acceleration of the bullet. The maximum speed corresponds to about 32 cm / s, the duration is slightly more than 6 ms. The most reliable explanation for this phenomenon is the return of the rifle associated with the operation of the piston. The rifle during the shot, moving as a solid object, carries with it a bullet, i.e. the “hump” observed on the graph displays the backward movement, after which the bullet itself begins to accelerate due to gas pressure, which leads to a sharp drop in speed and the transition to the acceleration pattern discussed in the previous section. For additional verification of this phenomenon, experiments were carried out with the removal of the interferogram during an empty shot, as well as from the end of the rifle barrel with a loaded bullet. The results of the processed spectrograms, combined with the result of a conventional measurement of the movement of a bullet, are presented in FIG. 6. It is taken into account that due to the recoil of the piston, the movement actually occurs in the opposite direction. The graph (Fig. 6) shows that with a single shot, the barrel speed is higher than with a charged shot, which can be explained by a higher return caused by the movement of the piston, because there is no bullet preventing the free exit of gases. If we compare the measurement of the velocity of the end of the barrel with the measurement of the velocity of the bullet itself, we can see that the graphs are quite similar, up to about 6.5 ms, i.e. until the moment when the pressure in the barrel becomes sufficient to overcome the frictional force of the bullet on the barrel and the beginning of its acceleration. In addition, in the time interval from 9 ms to 12 ms, the oscillations of the barrel are clearly visible, which are presumably caused by the impact of the piston on the bottom of the glass. Another phenomenon that was able to measure is, presumably, the flight of droplets of oil, carried away by the air flow in front of the bullet. A graph of this phenomenon is shown in FIG. 7. To the right is the signal from the bullet during the main acceleration, below the “hump" of the piston recoil (on this graph, unlike the previous ones, the speed module is presented), which was discussed in detail earlier. Unlike the signal corresponding to the pool (solid line), the signal from the oil droplets is noticeably broader. This fact on the graph is expressed as two dashed lines indicating the range of speeds of the oil droplets at some point in time. Thus, it is shown that the inventive device allows the simultaneous measurement of various processes taking place in the barrel of a rifle.

In addition to directly measuring the dependence of the speed of the bullet in the barrel on time, additional phenomena were identified and measured, such as: the movement of the rifle body at the initial moments of the shot during normal and idle shots, the oscillations of the rifle barrel in the moments preceding the departure of the bullet, the speed of the oil droplets ahead of the bullet.

According to the results of studies that can be carried out during the testing process, it is possible, in particular, to evaluate the technical characteristics of weapons or design flaws that can be taken into account at the stage of design and / or manufacture of weapons. The applicability of the claimed device for the tasks of remote measurement of the speed of small objects in hard-to-reach places is shown.

Claims (2)

1. Laser meter for the speed and / or movement of small objects in places with limited access, including a single-frequency semiconductor laser with a wavelength of 1064 nm and a generation line width of not more than 3 MHz, an optical isolator, a fiber amplifier with a laser pump diode, an optical radiation divider, the function of a beam splitter plate, to which a connector with an angled chip is connected, which performs the function of a weakly reflecting mirror, a collimator with a beam diameter of 0.8-1.2 mm, and an optical receiver, the output of which is connected chen to the input of the oscilloscope connected via USB-interface with the computer. 2. The laser meter according to claim 2, characterized in that the optical radiation divider is configured to divide radiation 1: 1.

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