Research on simulation system with the wide range and high-precision

laser energy characteristics Dong ke-yan, Lou yan, He Jing-yi, Tong shou-feng, Jiang Hui-lin

Changchun University of Science and Technology, Changchun 130022, Jilin, China

ABSTRACT

The Hardware-in-the-loop(HWIL) simulation test is one of the important parts for the development and performance testing of semi-active laser-guided weapons. In order to obtain accurate results, the confidence level of the target environment should be provided for a high-seeker during the HWIL simulation test of semi-active laser-guided weapons, and one of the important simulation parameters is the laser energy characteristic. In this paper, based on the semi-active laser-guided weapon guidance principles, an important parameter of simulation of confidence which affects energy characteristics in performance test of HWIL simulation was analyzed. According to the principle of receiving the same energy by using HWIL simulation and in practical application, HWIL energy characteristics simulation systems with the crystal absorption structure was designed. And on this basis, the problems of optimal design of the optical system were also analyzed. The measured results show that the dynamic attenuation range of the system energy is greater than 50dB, the dynamic attenuation stability is less than 5%, and the maximum energy changing rate driven by the servo motor is greater than 20dB/s.

Key words: Semi-active laser guidance; Hardware-in-the-loop simulation; Crystal absorbent; Energy characteristic

1. INTRODUCTION

In the research and development of high precision semi-active laser-guided weapon, HWIL simulation test is an important and even an indispensable part. In order to get laser energy characteristics at a high confidence level simulation, scholars have conducted a lot of research work[1-4]. At present, the main methods used are mechanically adjustable diaphragm method, the photoelectric technical dimming method and the absorption gradient attenuators dimming. But all of the three ways have disadvantages during the working process. For example, disadvantage of mechanically adjustable diaphragm method and photoelectric dimming method is energy changes in a small dynamic range, bad linearity, requirement of closed-loop compensation, The absorption gradient attenuators dimming method often requires a multi-stage cascade and thus is prone to speckle. In this paper we researched and analyzed about the energy characteristics of semi-active laser-guided weapons during work, and developed an energy attenuation system based on the neutral gray glass according to optical properties of the parallel plate and Lambert's law. To make the prism group move in relative motion, linear brakes of this system is driven by the coil motor. Thus, we completed simulation of the laser energy characteristics, and make an indoor test on a prototype system.

2. ANALYZE OF LASER ENERGY PARAMETERS Semi-active laser guided weapon consists of two parts, the missile body and the laser target designator. During the work of the laser guidance system, the laser target designator launches a laser beam with specific coding information,

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Propagating through the atmosphere, the beam points the target with certain incident angle, and the formed laser spot carries certain information; Visual axis of the seeker with the target have a definite angle, whether the seeker can detect the spot, on the one hand depends on the internal parameters of the seeker; on the other hand depends on the energy characteristics of the laser target designator. The main factors to affect the energy characteristics of the laser target designator are the energy range, the rate of change and control accuracy.

2.1 Energy range

In HWIL simulation tests, the dynamic range of energy is based on semi-active laser-guidance type and the specific working environment. Suppose that P is laser pulse power of the target radiated by the laser target designator,

LS is

Laser beam cross-sectional area at the target, α is the angle between illuminated surface of target and the beam cross-section, ρ is the average reflectance of the target surface. Then, the reflected optical power per unit area is[5-7]

cos LrP P Sρ α= （1）

Suppose that S is entrance pupil Area of the Seeker, S is the target area in FOV(field of view) of the receiving optical system, The distance between the seeker and the target is 1R , and assume that the target can be a uniform diffuse reflection, the atmospheric transmittance between the indicator and target is 1τ , atmospheric transmittance between the target and the seeker is 2τ , so the echo laser power received by the seeker is

1 2 21

ere

SPP S

Rτ τ

π= ⋅ ⋅ ⋅ ⋅ （2）

If the irradiation spot is entirely in the field of view, take (1) into (2):

1 22

1

cos ee

L

SPP S

S Rτ τ ρ απ

= ⋅ ⋅ ⋅ （3）

and then with cos LS Sα =

1 22

1

ee

PSP

Rρτ τ

π= ⋅ （4）

Power density of at the pupil of the seeker is

1 22

1

e

e

P PS R

τ τ ρπ

= （5）

We know from the above equation, the dynamic range of energy at the entrance pupil of the seeker depends on the power of the laser target designator, the average reflectance of the target, the seeker and the target distance and atmospheric transmittance. Under the simulation conditions: visibility is 20000m, 8000m, 2000m respectively, the distance of indicator from the target is 4000m, the single pulse energy is 100mJ, the target reflection angle is 20°, and the average reflectance is 0.3, the curves of the seeker receiving energy with the distance of the missile and target are shown in Figure 1. We can see from figure 1, when guided weapons are flying to target from 5000m to 50m, the maximum energy density range is only caused by the link distance and is about 105.

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Fig.1 Energy density at entrance pupil vs distance for different visibility scale

2.2 Change of energy density

In the stage of approaching to the target, the rate of energy change is sharply. Assume that laser-guided weapons fly forward to target at 0.65Ma. When the seeker is far from the target, the rate of the energy density changes slowly; but the energy density rate of change almost reaches 20dB/s when it is closer to target, as shown in Fig1. In the course of a Semi-active laser-guided weapon’s operation, if we do not take into account of atmospheric effects, detector will receive an increasing laser power when the guided missile is close to the target, with power being quadratic versus the distance.

2.3 Energy regulation accuracy

The impact of atmospheric factors on the received power of the seeker is not only reflect the average attenuation of the laser energy, but also reflect serious random fluctuations. The jitter variance of received power caused by atmospheric turbulence and flicker effect depends on the receiving aperture, the refractive index structure constant, and the link distance. The specific expression is:

2 2 7 6 11 61.23I nC k Lσ = ⋅ ⋅ ⋅ (6)

For the semi-active laser-guided weapons, in weak turbulence conditions, fluctuation of energy range caused by light intensity flashes is from 0.5dB to 3dB .Figure 2 shows the received power of the laser beam by a CCD camera due to the effect of atmospheric turbulence, from the curve of the time domain it can be found a certain amount of jitter range exists.

Fig.2 Effect of atmospheric turbulence flashing on received power

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3. WORKING PRINCIPLE OF SIMULATION SYSTEM BASED ON CCRYSTAL ABSORBERS

According to Lambert's law, for a beam through the material whose thickness is l , Light intensity I meet the following formula

0lI I eα= （7）

Where 0I is light intensity before entering the crystal, α is the absorption coefficient.

the simulation system is composed of two prisms which has the exact same wedge angle. these two prisms make up a parallel plate, as shown in Figure 3. The laser beam passes through the larger prism which can moves precisely along the axis, and the optical path of the laser beam changes continuously, thereby the light intensity changes continuously[8].

Small prism

Big prism

Moving directionIncident laser beam

Effluent laser beam Fig.3 Principle of energy character simulation system

4.SPECIFIC DESIGN EXAMPLES AND ANALYSIS

4.1 Optical design

According to the requirement of the rate of energy change in HWIL simulation tests and based on the absorption properties of neutral gray glass at 1064nm, we designed a small wedge angle of 18.8° prism group. The prism group include a bigger one and a smaller one. When the bigger prism moves 1mm, the laser energy changes approximate 1dB. To achieve the dynamic range greater than 50dB attenuation, the long side of the big prism is 65mm. In addition, taking into account the actual system is installed to adjust the direction of the light path, joined a turning prism into the energy attenuation system. A laser beam with 3mm diameter passes successively through the turning prism, smaller prism and bigger prism, and finally is coupled into the 400μm power optical fiber. The optical system shown in Figure 4:

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Fig.4 Energy characteristics of analog systems

4.2 Control unit design

A DPL laser is used as the system light source, the wavelength is 1064nm, and the single pulse energy is greater than 20mJ; the parameters of a precision translation stage are: travel is 100mm, screw lead 4mm motor, control accuracy of better than ±5μm, maximum operating speed is 70mm/s. In the process of laser-guided weapons’ work, the energy characteristics of the seeker at the change will follow with its motion parameters and environmental parameters. The motion parameters and environmental parameters were analyzed and solved by the host computer, then the resolving result was sent into the motion control unit, which drove the linear movement of the linear slider brake, as shown in Figure 5. Linear slider brake was equipped with a high-precision grating, a closed-loop control on the motor and the grating was formed to ensure the accuracy of the operation of the linear slider, and the relative motion between the prisms. Figure 6 is the photo of the simulation system for the energy characteristics.

Fig.5 Hardware of laser energy adjust cell Fig.6 Energy characteristics of analog systems

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The simulation systems on energy characteristics sends laser out and it is probed by a power meter at a Vertical angle. The linear slider brake is controlled by the host computer. From the test curves in Figure 7, we find the device meet the design requirements.

Fig.7 Prism decay curve in the actual test

5.2 Test of energy change

According to the rate of change of energy density in the semi-active laser guided weapons requirements and absorption properties of the crystals of this system, the expected curve of the energy changes in the solver to the motion controller target trajectory is shown in Figure 8:

Fig.8 Target expect trajectory

Servo driver is controlled by the host computer, and the target expect trajectory will be discretized into 4000 discrete step response points, and sends the appropriate instructions to the servo drive every 2ms, which is shown in Figure 9. According to the position data from grating feedback, we can get the target response curve as shown in Figure 10.

5.1 Test of energy attenuation characteristics by the crystal

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Fig.9 Expect the target trajectory after discrete Fig.10 Actual target trajectory

Fig.11 Maximum dynamic error between the actual target trajectory and desired trajectory

As shown in Figure 11, the actual response curve of the trajectory and discrete expected target trajectories fit well. The accuracy of servo control system for steady-state is 2μm, the maximum dynamic error is 0.2mm, in other word, the system accuracy of energy regulation and control is 4.7% (0.2dB), which meets the requirements of the energy rate of change of the laser target simulation systems and control accuracy.

6. CONCLUSIONS

According to the requirements of energy characteristics in simulation and test system for laser semi-active guided weapons, we developed an energy characteristics simulation systems based on the absorption of the crystal structure. The test results show that the dynamic range of energy regulation is more than 50dB, the maximum rate of energy change is more than 20dB/s and energy control precision is less than 5%. Under the premise of ensuring energy simulation accuracy, the system improves the scope of the energy simulation, and avoids many of the inadequacies of the traditional energy regulation.

REFERENCES

1 Wang Kuang-biao, Latest development and trends of semi-active laser guidance technology [J]. Infrared and Laser Engineering, 2008, 37(Supplement):275-279

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2 MPodgaetsky V. An investigation for attenuation and scattering of short-duration laser pulses in a strongly scattering medium [J]. SPIE. 1999, 3735: X

3 Su Jian-gang, FU Meng-yin, et al. Research on hardware-in-the-loop simulation technology for line of sight of laser guided weapons[J]. Journal of System Simulation, 2007, 19(8): 1717-1720

4 Shan Jia-yuan, Liu Zao-zhen, Li Zhong-wu, et al. The hardware-in-the-loop simulation system for the guidance system laser guided weapon[J].Computer Emulation, 2002, 19(2):15-17

5 Yao Mei, Zhang Le, Li Ying-hua, et al. Analysis of laser guided weapon countmeasure hardware In the loop simulation test influenced by atmospheric scattering [J]. Infrared and Laser Engineering, 2007, 36(Supplement):385-388

6 J. Dubois, F. Reid. Detecting laser sources on the battlefield [J]. SPIE. 2007, 6796, 67962F-1 7 Li Hua, Qin Shi-qiao , Yao Mei, et al. Test and model to extra laser energy in the lab on laser2guided weapon

simulation[J]. Journal of Optoelectronics Laser, 2009, 20(3):374-377 8 Guan Ying-zi, Kang Wei-min. Design and implementation of a precision stepless dimmer [J] Opto-Electronic

Engineering, 2003, 30(4):56-58

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