CN1424591A - Adaptive variable-speed scanning laser imager - Google Patents

Abstract

An adaptive variable-speed scanning laser imaging device is used to acquire a three-dimensional image of a target, and belongs to the technical field of earth observation imaging. It is mainly used in 3D imaging of ground targets based on scanning lasers, and can also be applied to 3D laser imaging and tomographic scanning of close-range targets. It mainly uses a closed loop to realize the variable-speed scanning of the scanning mirror driven by the scanning motor, and the variable speed is based on the ups and downs of the ground target. This closed-loop loop is realized by several links of electrical signal, mechanical rotation, and laser detection. The closed-loop circuit consists of four parts, which are optical components and laser detection parts, variable-speed stepping motors, distance measurement and fluctuation prediction circuits (including prediction algorithms), and adaptive variable-speed drive signal generation circuits. Through their organic combination, And combined with other parts in the system, the adaptive variable-speed scanning laser imaging is finally realized.

Description

Adaptive Variable Speed Scanning Laser Imaging Device

Technical field:

The invention relates to a device for acquiring a three-dimensional image of a target, belonging to the technical field of earth observation and imaging. It is mainly used in 3D imaging of ground targets based on scanning lasers, and can also be applied to 3D laser imaging and tomographic scanning of close-range targets.

Background technique:

The three-dimensional imaging technology for earth observation usually adopts microwave synthetic aperture radar (SAR) imaging, optical imaging, laser imaging, stereo photography, imaging spectrum, etc. as the main means, and is equipped with GPS positioning system and attitude measurement device to determine the position and attitude of the flying platform .

Direct laser imaging technology using high-brightness, high-direction and coherent lasers for earth detection can constitute a three-dimensional system for direct laser imaging for earth observation. Using laser as active irradiation not only uses laser ranging, but also detects the target reflection intensity information carried by laser echo pulses. By detecting the target distance, reflection intensity and waveform feature information carried by laser echo pulses, it is possible to obtain every Pixel high-resolution distance data and grayscale images. Combining GPS and INS can directly obtain the three-dimensional image information of ground objects, and its image and three-dimensional coordinates are completely matched without additional ground control points. Further, it can also achieve three-dimensional recognition and classification of targets, and realize high-resolution, high-efficiency, accurate, active and direct three-dimensional imaging of the ground. Laser remote sensing imaging can obtain high spatial resolution, and can obtain uniform high-resolution digital elevation maps without interpolation operations, and the efficiency of information processing will be relatively high. Laser remote sensing imaging uses laser light for active irradiation. The active remote sensing method is less affected by the environment, climate, target illumination and contrast, and can work around the clock. It is especially suitable for engineering applications and military reconnaissance. With the development of laser and its detection technology, the research and application in this area will become more and more extensive.

Earth observation laser imaging is developed from airborne laser altimetry, and has a development history of more than 20 to 30 years. The early implementation of off-machine point altimetry has poor accuracy. Later, it developed to airborne scanning altimetry, which is the mainstream of development and application at present. Some research has been carried out in this area, and several tests or application systems have been completed. At present, almost all airborne laser altimetry + spectral imaging systems or airborne direct laser three-dimensional imaging systems adopt the scanning detection method of laser beams, and control the regularity of laser beams through the movement of plane rotating mirrors, plane pendulum mirrors, polygonal mirrors, etc. Scan the ground and detect the laser echo, and the scanning methods include linear scanning and conical scanning. Most of them use diode-pumped solid-state lasers as pulsed laser radiation sources, which require the laser to have a high pulse repetition rate.

In the scanning mode, if the repetition frequency of the laser is high enough, it can achieve very high-density ground sampling, and then generate a three-dimensional image of the ground target by processing the laser echo signal to achieve the purpose of scanning laser imaging for earth observation.

The current laser repetition rate reaches tens to more than one hundred kilohertz, the ground sampling point interval is at least less than 1 meter, and the distance measurement resolution is about 10 centimeters. . Typical systems include GLRS, SHOALS, AOL/ATM, RASCAL and ABS systems in the United States, LARSEN 500 and ALTM 1025 series in Canada, TopoSys and ALS40 in Germany, LADS and WRELADSII in Australia, and China's National 863 Program supports ASLRIS linear and conical Scan both systems.

The composition of a general earth observation scanning laser imaging information acquisition system is shown in Figure 1. The scanning laser imaging optical machine head 3 is controlled by the system synchronization signal, and the laser 2 outputs pulsed laser light to the scanning mirror in the optical machine head 3. , at the same time, the scanning motor drives the coaxial scanning mirror to rotate, and the scanning mirror deflects the output laser beam of the laser 2 and shoots it to the ground target, and the back-reflected laser signal of the ground target is turned to the telescope through the scanning mirror , so that it is received by the laser detector located at the focal point of the telescope, and after processing, information such as the distance of the target and other information can be obtained. The information is first sent to the data acquisition and processing circuit 7, and then sent to the data recording and monitor 6 for storage and real-time monitoring and display after processing. The navigation device 1 in the figure is installed in the cockpit of the aircraft and is used for aircraft navigation to ensure that it flies according to a relatively straight line. The attitude measurement gyroscope 5 is used to obtain the attitude of the optical machine head 3, and the attitude measurement is performed by the attitude measurement circuit 10. The attitude measurement gyroscope 5 and the optical machine head 3 are hard-connected and installed on a base 4 together. The GPS receiver 9 and the antenna 13 provide the spatial three-dimensional coordinates of the optical machine head 3, and its data, attitude measurement data, and laser measurement data are collected and processed by the data acquisition and processing circuit 7 together. The overall driving circuit 8 provides synchronous timing and various driving signals of the whole system. The power source 12 and the power converter 11 provide various power sources required by the whole system through the power conversion of the work site.

Through research, it can be clearly seen that the effect of scanning laser imaging in the above system is directly related to the ground sampling interval of the laser. The smaller the sampling interval, the better the laser imaging effect. The size of the ground sampling interval (reflecting the size of the ground resolution) is inversely proportional to the pulse laser repetition rate, and proportional to the scanning speed and the flight speed of the platform. At present, in order to reduce the sampling interval on the ground, it is necessary to increase the pulse laser repetition rate and reduce the flight speed and scanning speed. However, the pulse repetition rate of the laser is limited by various technical conditions and it is difficult to achieve an ideal high level, and the reduction of the flight speed of the sensor platform is not realistic. Therefore, on the surface, it is necessary to reduce the scanning speed. However, the reduction of the scanning speed It is manifested that within a certain period of time when the platform is running, the number of scan lines decreases, resulting in an increase in the ground sampling interval in the flight direction, so it is not advisable. Then, how to obtain a satisfactory ground sampling interval has always been a technical problem to be solved in the field of scanning laser imaging.

It can be obtained from the Nyquist sampling law that the sampling rate that the sampling result can truly reflect the characteristics of the target is more than twice the rate of change of the target. Therefore, in order to obtain a better laser imaging effect, it is necessary to apply an appropriate sampling rate. Too high a sampling rate can get good results, but it will cause waste, and sometimes it is difficult to achieve. If the sampling rate is too low, the three-dimensional feature information of the target will be lost. Therefore, through the prediction and processing of the fluctuation of the ground target, real-time adjustment of the sampling rate will obtain better laser imaging effect under the condition of limited resources. Under the premise of a certain laser repetition rate and a certain flight speed, adaptively changing the scanning rate will be the most effective way to obtain a better three-dimensional imaging effect.

Invention content:

From the above, in the earth observation scanning laser imaging, how to sample the ground with different fluctuations of the ground target at different laser scanning rates, so that more samples are taken on the ground with frequent fluctuations, and less samples are taken on flat ground It is the technical problem to be solved by the present invention. Therefore, the purpose of the invention is to provide an adaptive variable-speed scanning laser imaging device to achieve non-uniform ground sampling, to make up for the defects of constant-speed sampling in current scanning laser imaging, and to achieve the purpose of obtaining the best laser imaging effect of ground targets.

The present invention is realized in the following way: in the general earth observation scanning laser imaging information acquisition system, the optical machine head is redesigned to form the self-adaptive variable speed scanning laser imaging device of the present invention. The scanning laser of the conventional system samples the ground at a constant speed, which is realized by the scanning motor in the head of the optical machine driving the scanning mirror to rotate at a certain speed. In the present invention, the scanning motor is replaced by a stepping motor, and its rotation speed is adjustable at any time, that is, real-time variable-speed scanning. The control signal of the variable-speed scanning comes from an adaptive variable-speed drive signal generation circuit, and the variable-speed drive signal is controlled by the prediction result of the ground fluctuation given by the distance measurement and fluctuation prediction circuit. Therefore, the feature of the present invention is exactly: the variable-speed scanning of scanning mirror is realized by a closed-loop loop, and this closed-loop loop realizes closed-loop by several links of electric signal, mechanical rotation, and laser detection. The closed-loop circuit consists of four parts, which are optical components and laser detection parts (multiple components), variable-speed stepping motors, distance measurement and fluctuation prediction circuits (including prediction algorithms), and adaptive variable-speed drive signal generation circuits. Their organic combination, as well as the combination with other parts in the system, finally realizes the adaptive variable-speed scanning laser imaging.

Compared with the prior art, the present invention has outstanding substantive features and significant progress. The present invention uses a stepping motor to replace the existing scanning motor, and the speed-controllable scanning stepping motor drives the coaxial scanning mirror to rotate. The scanning mirror deflects the output laser beam of the laser and shoots it to the ground target, and the back-reflected laser signal of the ground target is turned to the telescope through the scanning mirror, so that it is received by the laser detector at the focus of the telescope, and processed After that, the distance and other information of the target can be obtained; by processing several such distance information before a moment, the prediction of the fluctuation of the ground target in the future can be made, and the prediction result is used as a follow-up point of a scanning line or the next scanning line The control parameter of the scan rate; through the closed-loop control of this parameter, the rotation rate of the stepping motor will be adjusted to achieve variable speed rotation, and a closed-loop loop will realize the variable-speed scanning of the scanning mirror, and finally realize adaptive variable-speed scanning laser imaging, making In the case of constant laser repetition rate, intensive sampling is performed on areas with large ground undulations, and the area with small ground undulations is quickly scanned. The equivalent scanning rate will not change greatly, but the actual 3D laser imaging The effect has been significantly improved.

Description of drawings:

Figure 1 is a block diagram of an existing scanning laser imaging information acquisition system.

Fig. 2 is a schematic diagram of the head structure of the scanning laser imaging optical machine of the present invention.

Fig. 3 is a block diagram of the distance measurement and fluctuation prediction circuit in the optical machine head of the present invention.

Fig. 4 is a flow chart of the prediction processing program adopted by the prediction calculation circuit in Fig. 3 of the present invention.

Fig. 5 is a schematic diagram of the relationship between the current point of ground sampling and the previous m and n points in the prediction algorithm in Fig. 4 of the present invention.

Fig. 6 is an adaptive variable speed drive signal generation circuit in the optical machine head of the present invention.

Detailed ways:

A better embodiment of the present invention is given below according to Fig. 2 to Fig. 6 . Please refer to shown in Fig. 2, the optical machine head 3 among the present invention is placed in the system shown in Fig. 1, forms scanning laser imaging information acquisition system together with other components, and it is installed on the mounting base 4 of flight platform .

As shown in FIG. 2 , the optical machine head 3 receives the laser beam emitted by the laser 2 and divides it into a sampling laser beam and a detection laser beam by a beam splitter 32 thereof. Wherein, the sampling laser beam outputs a laser sampling pulse signal to an input end of the distance measurement and fluctuation situation prediction circuit 38 through the output end 331 of the laser emission sampler 33; Straight mirror 34, cube prism 35, scanning mirror 36 of scanning ground target 14, primary mirror 371 of telescope 37, secondary mirror 372 of telescope 37, and the laser detector 373 that is positioned at the focal point of telescope 37 form the rear of ground target 14 The reflected laser signal is input to the laser echo detection and processing circuit 37', and then the laser echo pulse signal is fed into the other input end of the distance measurement and fluctuation situation prediction circuit 38 from its output terminal 371', and then the The distance measurement and fluctuating situation prediction circuit 38 feeds the 8bits rate change index signal to the adaptive variable speed drive signal generation circuit 39, and finally provides the drive signal to the variable speed scanning stepper motor 31, making it a scanning mirror connected coaxially with it 36 self-adaptive variable-speed scanning to realize the system's self-adaptive variable-speed scanning laser imaging. In Fig. 2, it can also be seen that the input end of the distance measurement and fluctuation situation prediction circuit 38 is also connected with a manual intervention information input part 38 ', and a synchronous encoder 30 is connected with the variable-speed scanning stepper motor 31.

In a nutshell, the variable-speed scanning stepper motor 31 is an executive component in the present invention, and is an actuator for adaptive variable-speed scanning. It drives the coaxial scanning mirror 36 to rotate, and it can be realized by selecting a common stepping motor. The selection of its load capacity is determined by the moment of inertia and starting moment of the scanning mirror; the selection of its rotation rate is determined by the laser scanning speed on the ground; the selection of the step interval is determined by the system's requirements for the minimum scanning sampling interval. For example, if the scanning speed is required to be 60 lines per second, and the scanning sampling interval is required to be 0.63mrad, then the minimum stepping interval of the stepping motor is required to be 0.63mrad (0.036°), and the scanning motor speed is 60 revolutions/second. The frequency of the driving pulse is 600 kHz. Changing the scan rate means changing the frequency of the drive pulse. Applying a variable-frequency drive signal to the stepping motor 31 can realize variable-speed scanning. The variable-frequency drive signal is provided by the adaptive variable-speed drive signal generation circuit 39 involved in the present invention.

See Figure 3 for the block diagram of the distance measurement and fluctuation prediction circuit. First, the main wave pulse conversion circuit 381 and the echo pulse conversion circuit 382 respectively carry out level and phase conversion on the pulses sent by the laser emission sampler 33 and the laser echo detection and processing circuit 37', and then they enter the range gate together. The generation circuit 383 generates a range gate, and the gate signal is sent to the distance measurement circuit 385 . In the distance measurement circuit 385, the 250MHz oscillating pulse signal input by the clock oscillator 384 counts and measures the gate width. At the same time, the accumulated integration method is used for more precise measurement. The common measurement results are the emission of the main wave and the laser echo. The time delay between them can be converted into the distance of the target. The distance data will be sent to the distance data buffer 386. The buffer 386 is a RAM memory, which buffers all the data of several scanning lines before the current sampling point, and its quantity should meet the quantity requirement of the prediction processing circuit. At the same time, under the control of the program, the prediction and processing single-chip microcomputer 387 calls the previous distance data for processing, calculates the undulations of the ground through the previous distance data, and then predicts the subsequent ground undulations, and then gives the scanning rate of the next step The change index is sent to the adaptive variable speed drive signal generating circuit 39 in FIG. 2 . The rate change index is an 8-bit binary number, and the rate change can be divided into 256 levels at this time.

The prediction processing circuit is realized by a digital signal processing microcontroller (TMS320C30-DSP) 387. The flowchart of the prediction program 100 is shown in FIG. 4 , and its data source is the distance data buffer 406 . The basic method is: select the m-point distance data (step 101) of this scanning line before the current moment, the distance data (step 102) of n points before the column (along the track direction) where the current moment is located, and the ground points corresponding to these two parts of data Arranged in a cross shape, crossing the current point, basically reflecting the ground undulations around the current point. Then calculate the previous ground relief condition (step 103), the ground relief condition is the spatial frequency and amplitude of the ground height fluctuation. By performing a linear prediction operation on them (step 107), the ground relief around the current sampling point can be deduced. Then, a change index of the ground fluctuation is given, which will be used as the basis for adjusting the speed of the variable speed control signal and sent to the adaptive variable speed drive signal generation circuit. Then, the sampling scanning rate of the subsequent points in this row and the adjacent points in the same column of the subsequent scanning row will be controlled according to the predicted result. In the forecasting process, in order to control the forecasting accuracy, it is very important to calculate the error between the last forecasted result and the measured result (steps 108, 104, 105), and generate sample point number control words m and n, as The basis for currently selecting the number of predicted sample points (step 106). The number of predicted sample points is the number m of the ground point distance data before the current point in the scan line and the number n of the previous ground point distance data in the column where the current point is located. m and n affect the prediction error. The relationship between the current point of the prediction algorithm and its previous m and n points is shown in Figure 5.

The composition of the adaptive variable speed drive signal generating circuit 39 is shown in FIG. 6 . It accepts the 8bits rate change index data sent by the distance measurement and fluctuation situation prediction circuit 38, and sends it to the rate change index latch 390 for latching. This data is supplied to the preset terminal of the frequency divider 392 which can preset the count, thereby changing the frequency division ratio of the output signal frequency of the oscillation circuit 391. Since the change of the frequency division ratio changes with the integer rule of 1-256, the frequency conversion interval of the frequency division output changes in multiples, so it is necessary to use a frequency conversion signal synthesis circuit 393 to combine the frequency-divided signal and the undivided signal according to certain rules Synthesis is carried out so that the frequency change proceeds according to the arithmetic difference law. Therefore, the output pulse frequency of the variable frequency signal synthesis circuit 393 can be changed at equal intervals. After the output signal passes through the multiphase signal generation circuit 394, several variable frequency signals (depending on the requirements of the stepping motor 31) with different phases are output to the variable frequency drive amplifier. Circuit 395. The circuit 395 amplifies the power of the signal to improve its driving capability and then sends it to the stepping motor 31 . Because the driving signal of the stepper motor 31 is a pulse train, its pulse interval size determines the rotation rate, so the key of the drive signal frequency control circuit for driving the stepper motor is exactly to change the pulse interval in real time. The circuit is realized by a frequency division circuit that can preset the frequency division ratio, and the frequency division ratio is the rate change index sent by the previous stage. Since the frequency of the pulse train output by the oscillating circuit is relatively high, which is about tens of times of the actual required frequency of the motor 31, its time interval is many times smaller than the minimum interval of the stepping motor 31 driving signal. At this time, after the rate change index is sent, the time interval between the current output pulse and the last pulse of the drive signal frequency control circuit can be changed immediately, and the variable speed control signal can be output.

Claims (4)

1, a kind of adaptive rate scan laser imaging device, comprise the ray machine head (3) on the mounting base that is installed in flying platform, it contains the beam splitting chip (32) that a laser beam with laser instrument (2) emission light inlet drive head unit (3) is divided into sampling laser beam and exploring laser light bundle, and this sampling laser beam forms emission laser sample-pulse signal after a Laser emission sampler (33); Behind the scanning mirror (36) and telescope (37) of this exploring laser light bundle via the laser alignment mirror (34) that connects with light path successively, block prism (35), scanning terrain object (14), form the retroreflection laser signal of terrain object (14), survey and treatment circuit (37 ') output echo pulse signal through a return laser beam again; It is characterized in that:

A. also have one to become a variable-ratio of coaxial connection to scan stepper motor (31) with the scanning mirror (36) of this scanning terrain object (14);

B. be provided with the range observation and the fluctuating situation prediction circuit (38) of a described emission laser sample-pulse signal of reception and echo pulse signal, its output speed variability index signal is after an adaptive rate drive signal generation circuit (39) forms a closed loop structure with this variable-ratio scanning stepper motor (31) with electricity-machine connection.

2, adaptive rate scan laser imaging device according to claim 1, it is characterized in that described range observation and fluctuating situation prediction circuit (38) comprise main wave impulse translation circuit (381) and the echo-pulse translation circuit (382) of accepting described emission laser sample-pulse signal and echo pulse signal respectively, and after connect the range gate generative circuit (383) that they also generate the range gate signal; This range gate generative circuit (383) is connected with a range observation circuit (385) respectively with a clock oscillator (384) and exports terrain object (14) range data signal, this range data signal is sent into a range data buffer (386) that connects and form the closed loop circuit structure successively with circuit, the prediction processing single-chip microcomputer (387) and an address control unit (388) of operation predictor (100), and by this prediction processing single-chip microcomputer (38) output speed variability index signal.

3, adaptive rate scan laser imaging device according to claim 1 and 2 is characterized in that said rate variation exponential signal, and its length is 8bit.

4, adaptive rate scan laser imaging device according to claim 1, it is characterized in that said adaptive rate drive signal generation circuit (39) comprises the rate variation index latch (390) of acceptance from the rate variation exponential signal of described range observation and fluctuating situation prediction circuit (38) output, this latch (390) with an oscillatory circuit (391) but be connected respectively a preset count mark frequently device (392) preset end and clock signal input terminal, connect a frequency variation signal combiner circuit (393) that connects with circuit successively behind its output terminal, an one polyphase signa generative circuit (394) and a frequency conversion drive amplifying circuit (395), this oscillatory circuit (391) also connects this frequency variation signal combiner circuit (393).

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