RU2821597C1 - Method of laser beam guidance to object - Google Patents

Abstract

FIELD: optical-mechanical instrument making.

SUBSTANCE: invention relates to optical-mechanical instrument making, to aiming a laser beam at objects. Method of laser beam guidance to object includes preliminary guidance of laser beam according to target designation data by coarse channel and processing of error of coarse channel by accurate channel according to measurements of coordinates of observed object by television receiving device, optical axis of which is aligned with optical axis of laser, and 4-quadrant photodetector, optically interfaced with output telescope of guidance system. Signal measured at the output of the television receiving device is fed to the input of the low-pass filter with a cutoff frequency, from the output of the low-pass filter, the signal is fed to the input of the accurate channel, using this signal, directing a laser beam to the object, measuring the coordinate of the laser radiation reflected from object by 4-quadrant photodetector, from output of 4-quadrant photodetector, the signal is transmitted to the input of the high-pass filter with a cutoff frequency ω_cut, equal to low-pass filter cutoff frequency, and is supplied to the input of the accurate channel in total with the signal from the output of the low-pass filter.

EFFECT: higher accuracy of guidance.

1 cl, 3 dwg

Description

The invention relates to optical-mechanical instrumentation, in particular to devices for pointing a laser beam at objects in space, holding the beam at a selected area of the object, and can be used to work in precise guidance channels of two-channel guidance systems of laser complexes.

It is known that in order to achieve high accuracy of pointing a laser beam at a spatial object, a two-channel design of the laser complex guidance system is used, when rough pointing is carried out by turning the structure of the optical-mechanical path, and precise pointing is carried out by rotating in limited angles individual elements of the optical-mechanical path, for example, the secondary focusing telescope mirrors [1] based on control signals from receiving devices.

The disadvantage of such a scheme when aiming the beam at dynamic extended objects is the difficulty of identifying them from a television image and the impossibility of accurately holding the beam on a given area of the object. This is caused, on the one hand, by the presence of a wide range of input and disturbing influences typical of laser complexes, which can lead to “blurred” images, and on the other hand, by the peculiarity of television receiving devices themselves for solving the above problems, since they have a limited frame rate , as well as additional delays in the processing and transmission of the television signal, which affects the stability of the servo drives of the laser beam guidance system and leads to interference at the input of the precise channel.

The closest in technical essence to the proposed method is the method implemented in the IRATS laser ranging system [2], which consists in preliminary pointing the optical axis of a television camera to a given point in space with the main mirror of the guidance system, which has an azimuth-angle mount. When a target is detected, the laser transmitter is turned on, the optical axis of which is aligned with the optical axis of the television camera, and the active location mode begins. Deviation of the target from the optical axis of the receiving channel causes a displacement of the spot of reflected radiation from the center of the 4-area photodetector, optically coupled with the output telescope; signals from this photodetector control precision drives - compensators, the load of which is mirrors installed sequentially along the location beam in the optical -mechanical path.

The disadvantage of this method is the inability to accurately hold the aimed beam on a section of the object selected from the television image, which is caused by delays in the signal of the television receiving device due to the limited frame rate, transport delays in transmitting the television signal to the processing device and the delay in calculating coordinates. These factors (delays) affect the stability of the servo drives of the laser beam guidance system and lead to interference at the input of the precise channel caused by phase distortions of the signal at the output of the television receiving device.

The objective of the invention is to create a method for aiming a laser beam at an object with increased accuracy when aiming at specified areas of highly dynamic objects due to the exclusion from the precise channel control signal of a part of the spectrum at the output of a television receiving device with an unacceptable phase shift of the signal according to accuracy conditions and the replacement of this part of the spectrum with a generated signal with output of a 4-quadrant photodetector without phase shifts.

The problem is solved by the fact that in the known method of pointing a laser beam, including preliminary pointing of the laser beam according to target designation data with a coarse channel and processing the error of the coarse channel with a fine channel according to measurement data of the coordinates of the observed object by a television receiving device, the optical axis of which is aligned with the optical axis of the laser, and Using a 4-quadrant photodetector device, optically coupled with the output telescope of the guidance system, the signal measured at the output of the television receiver is fed to the input of a low-pass filter with a cutoff frequency determined by the formula:

ω _av = ϕ _add / τ,

where ω _cf is the cutoff frequency of the low-pass filter,

ϕ _additional - phase shift at the output of a television receiving device relative to the signal at its input, permissible from the conditions of stability of the guidance system and the magnitude of the acceptable level of interference;

τ is the delay time of the signal at the output of the television receiving device,

From the output of the low-pass filter, the signal is fed to the input of the precise channel, a laser beam is directed at the object using this signal, the coordinate of the laser beam reflected from the object is measured by a 4-quadrant photodetector, from the output of which the signal is fed to the input of the high-pass filter with a cutoff frequency , equal to the cutoff frequency of the low-pass filter, and then feed it to the input of the precise channel in total with the signal from the output of the low-pass filter.

The proposed method of aiming a laser beam at an object is carried out in accordance with the optical-kinematic diagram of the laser complex guidance system (Fig. 1) and the structural diagram of the guidance system along one coordinate (Fig. 2) as follows. Preliminary guidance of the laser beam according to target designation data on the coordinates of the object 1 is carried out by drives of the coarse channel 2 with the transfer function W _g using an azimuth motor 3 and an elevation motor 4. The beam of the guided laser 5 passes through a dichroic mirror 6, which separates the visible radiation going to the television by wavelength the receiving device 7 (TVPU) and the guided laser beam, and sequentially hits the mirrors 8 and 9 of the precision channel 10 with the transfer function W _t containing engines 11 and 12, and through the forming lens telescope 13 goes into space. The radiation of the guided laser reflected by the object is received by the 4-quadrant photodetector device 14 (PDE), since its optical axis is optically coupled with the output telescope of the guidance system. The optical axis of the laser is preliminarily aligned with the center of the field of view of the television receiving device, and the field of view of the television receiving device overlaps the field of target designation errors. The output of the TVPU is connected to the input of the low-pass filter 15 (LPF), and the output of the FPU is connected to the input of the high-pass filter 16 (HPF). The signals from the output of both filters are summed in adder 17 and fed to the input of precise channel 10.

The system for pointing a laser beam at an object in accordance with the structural and optical-kinematic schemes works according to the proposed method as follows. After the coarse channel has processed external target designation signals (coordinate ϕ _tsu in Fig. 2), the television receiving device detects object 1 and transmits the coordinates of the object to the input of the fine channel, which, using motors 11 and 12, turns the mirrors of the fine channel 8 and 9, shifts the center of the field view of the television receiver to the area of the object selected from the television image. In this case, the aimed beam also moves in the direction of the selected area of the object. When a laser beam hits an object, a photoreceiving device 9 - a 4-quadrant photodetector - structurally connected to the elevation axis and having an optical axis adjusted parallel to the optical axis of the telescope, measures the coordinate of the laser beam reflected from the object. Since the laser radiation reflected from a section of an object significantly exceeds in power the radiation caused by backscattering in the atmosphere when the beam passes along the path to the object, it is isolated in the photodetector device by threshold signal processing. The electrical signal about coordinate information at the output of the photodetector appears without delay, that is, it has no lag. But since the signal from the television receiving device arrives with a delay proportional to the frequency of the signal, using the entire spectrum of this signal is impractical, since an additional phase shift of the signal appears in the control loop of the guidance system, which worsens the stability of the entire system when using a television receiving device, and for a pure link delays can be written [3]:

ϕ = -ωτ, (1),

where ϕ is the phase shift of the signal,

ω - current signal frequency,

τ is the delay time of the signal at the output of the television receiving device.

The delay of a harmonic signal at the output of a television receiving device (electrical signal) with a harmonic effect at its input (optical signal) arriving at the sensitive matrix of the television receiving device is illustrated in Fig. 3, from which it is clear that in addition to phase distortions of the signal, leading to a deterioration in the stability of the closed-loop system, the output sinusoid has sections where the signal has the opposite sign with respect to the input - in Fig. 3, the area under these sections of the sinusoid is shaded. Such a signal for the guidance system is false, which leads to an increase in the guidance error. Note that for higher-frequency harmonics at the input, the phase shift at the output can reach up to 180 degrees, which means that at these frequencies the entire signal interferes with the guidance system, increasing its error. At the same time, refusing to use a television receiver in these tasks is impossible due to the need to have an image of the object. The low-pass filter 15 at the output of the television receiving device 7 cuts off the part of the spectrum where the phase shift leads to exceeding the permissible level of interference based on the technical requirements for the guidance system. The cutoff frequency of such a filter (modulus) from expression (1) will have the value:

ω _av = ϕ _add / τ (2),

where ω _cf is the cutoff frequency of the low-pass filter,

ϕ _additional - phase shift (delay) at the output of a television receiving device relative to the signal at its input, permissible from the conditions of system stability and from the value of the acceptable level of interference. This phase shift also includes a component inherent to low-pass filters, which is minimized by the choice of low-pass filter parameters and configuration.

So, for example, if the permissible phase shift at the output of a television receiving device relative to the signal at its input ϕ _add should not exceed 0.5 rad, and the signal delay (measured or calculated) τ = 15 ms, then the cutoff frequency of the filter from (2) :

ω _av = ϕ _additional / τ = 0.5 / 15⋅10 ^-3 1/sec = ~ 30 1/sec.

The high-frequency part of the spectrum of the input action of the laser guidance system (usually up to several tens of hertz) is recorded by a 4-area photodetector 14, the low-frequency part of the signal spectrum of which is cut off by a high-pass filter 16, the cutoff frequency of which is equal to the cutoff frequency of the low-pass filter, determined from expression (2) . This filter provides phase advance, which has a positive effect on the stability characteristics of the system as a whole. After measuring the coordinates of the radiation reflected from the object using the FPU, the total signal from the output of both filters is supplied to the input of the precise channel of the guidance system, which makes it possible to direct the beam at the object using the image and stabilize its position on the selected area of the object with high accuracy based on the signal from the 4-area photodetector .

As a result, the coordinate of the induced beam ϕ _out , as shown in Fig. 2, will be the sum of the output coordinate of the coarse channel ϕ _g and the output coordinate of the fine channel ϕ _t , which, in turn, is obtained after processing two parts of the spectrum of the input influence that do not have phase distortions higher than permissible.

The proposed method of pointing a laser beam at an object allows increasing accuracy when pointing a laser beam at specified areas of highly dynamic objects due to the formation and addition of two control signals for the precise channel of the guidance system, one of which transmits coordinate information on the television image of the object and contains the low-frequency part of the input spectrum of the precise channel with insignificant phase distortions, and the other - high-frequency, without phase distortions with information about the “jitter” of the object in the field of view of the receiving devices.

Information sources:

1. Automatic systems and devices for aiming laser beams / Ed. V.B. Suitcase. - M.: Fizmatlit, 2016. - P. 36 - 44.

2. Protopopov V.V., Ustinov N.D. Infrared laser ranging systems. - M.: Voenizdat, 1987. - P. 85 - 86. - prototype.

3. Yurevich E.I. Theory of automatic control. - L.: Energy, 1975. - P. 50.

Claims (6)

A method of pointing a laser beam at an object, including preliminary pointing of a laser beam according to target designation data with a coarse channel and processing of a coarse channel error with a fine channel according to measurement data of the coordinates of the observed object by a television receiving device, the optical axis of which is aligned with the optical axis of the laser, and a 4-quadrant photodetector device , optically coupled with the output telescope of the guidance system, characterized in that the signal measured at the output of the television receiving device is fed to the input of a low-pass filter with a cutoff frequency determined by the formula: ω _av = ϕ _add / τ where ω _cf is the cutoff frequency of the low-pass filter; ϕ _additional - phase shift at the output of a television receiving device relative to the signal at its input, permissible from the conditions of stability of the guidance system and the magnitude of the acceptable level of interference; τ is the delay time of the signal at the output of the television receiving device, From the output of the low-pass filter, the signal is fed to the input of the precise channel, a laser beam is directed at the object using this signal, the coordinate of the laser beam reflected from the object is measured by a 4-quadrant photodetector, from the output of which the signal is fed to the input of the high-pass filter with a cutoff frequency , equal to the cutoff frequency of the low-pass filter, and then feed it to the input of the precise channel in total with the signal from the output of the low-pass filter.