Online detection of laser alarm based on adjustable simulated laser pulse

introduction

Laser warning devices[1] can quickly detect and identify laser threats from the enemy and issue sound and light warnings. They are an important laser countermeasure equipment and are widely used in large weapons such as tanks, armored vehicles and ships[2]. To ensure their reliability in actual combat, our army conducts batch inspections of various types of laser warning devices in service every year. The inspection method is mainly the traditional diffuse reflection target plate method, which requires the warning probe to be removed and the azimuth and elevation coverage inspections to be performed in the inspection room with specific equipment. The inspection efficiency is low and very cumbersome. In order to meet the needs of future wars, our army's equipment department urgently needs an efficient laser warning device performance inspection device.

This article introduces an online detector for laser alarms based on adjustable analog laser pulse technology. Its basic structure is a laser tube hemispherical positioning system with a single-chip microcomputer as the core. Due to the precise and flexible position control and the adjustable wavelength, power and modulation frequency of the laser, the detection of parameters such as the azimuth coverage, pitch coverage, angle resolution, detection wavelength and minimum detectable power of the alarm can be realized. The detector is easy to operate. There is no need to disassemble the alarm probe. It only needs to place the hemispherical cover equipped with the positioning system on the probe and keep the probe at its center. The operator can control the detection system through the human-computer interaction interface, and can quickly and intuitively grasp the performance parameters of the alarm under test, greatly improving the detection efficiency.

Diffuse reflection target plate detection method and its shortcomings

At present, the detection of laser alarms generally adopts the diffuse reflection target plate detection method, which includes two parts: azimuth coverage test and pitch coverage test, as shown in Figure 1. The left and right sides of the central dotted line are schematic diagrams of azimuth coverage test and pitch coverage test respectively. In the azimuth coverage test, the laser beam forms a light spot A on the diffuse reflection target plate. When the turntable drives the alarm to rotate on the horizontal plane, the image point of the light spot on the CCD device in the alarm moves horizontally to form an azimuth output signal. If the signal is linearly related to the rotation angle of the turntable and is not interrupted (i.e., missed report), the azimuth coverage of the alarm is determined to be qualified. In the pitch coverage test, the vertical movement of the laser causes the vertical distance H from the light spot B to the alarm to change, so that the image point of the light spot on the CCD moves vertically to form a pitch output signal. If the pitch signal satisfies the triangular relationship formed by H and the horizontal distance L between the alarm and the target plate, the pitch coverage of the alarm is considered to be qualified.

The principle of the diffuse reflection target plate detection method is simple, but there are some problems. First, the test is generally carried out indoors, so the alarm needs to be removed from the weapon and then installed again after the test, which is cumbersome. When there are a large number of alarms to be tested, the workload of the operator will become very large, inefficient and prone to errors. Secondly, in the pitch test, the spot height must be infinitely high to achieve 90o incidence, and the height of the test target plate and the indoor space are always limited, so this method has a detection blind spot.

Aiming at the shortcomings of the diffuse reflection target plate test method, this paper proposes an online detector for laser warning devices based on adjustable simulated laser pulse technology. [page]

Adjustable analog laser pulse technology

From the perspective of the laser alarm in actual use, the equal-power laser threat sources A1, A2, A3, ... within its alarm range can be equivalent to points A`1, A`2, A`3, ... on a sphere with the alarm O as the center and a radius of R (located at the intersection of rays OA1, OA2, OA3, ... and the sphere respectively), and the equivalent luminous power of A`1, A`2, A`3, ... is inversely proportional to the square of the distance between A1, A2, A3, ... and the alarm (under the condition that the receiving aperture remains unchanged, the attenuation of optical power is proportional to the square of the distance). Based on this principle, in terms of mechanical structure, if the light source can be controlled to move on a sphere with the alarm as the center, the change in its two-dimensional incident angle can be used to detect the azimuth coverage and elevation coverage of the alarm, and the minimum center angle that causes the azimuth and elevation output to change represents the angular resolution of the alarm. In terms of circuit structure, by adjusting the luminous power of the light source to be slightly lower than the alarm threshold, the minimum detectable power of the alarm can be detected. By adjusting the output wavelength of the light source, the sensitivity of the alarm to input light of different wavelengths can be detected. In addition, the light source can also be modulated to detect the response of the alarm to pulsed light. This is the principle of the adjustable laser simulated pulse technology proposed in this article, which enables complete detection of the performance of the alarm.

Mechanical structure analysis of the detector

Based on the above principle, we designed a mechanical structure as shown in Figure 2, which can realize the controllable movement of the simulated light source on the hemispherical surface.

As shown in Figure 2, the outer layer of the structure is a semi-spherical cover, which is used to shield the interference of external stray light; the lower part is a circular disk-type base, on which a ring rack is fixed; the ring-shaped horizontal guide rail is located on the base, and under the drive of the stepper motor 1, it provides azimuth adjustment for the laser tube; the semi-circular vertical guide rail with a rack is connected to the horizontal guide rail, and the distance between its two ends is equal to the diameter of the horizontal guide rail. The stepper motor 2 drives the laser tube to move on the vertical plane, providing the laser tube with pitch angle adjustment. The structure is compact as a whole, light in weight (made of fiberglass), easy to install and transport, and the alarm does not need to be removed from the equipment when in use, and there is no problem of detection blind spots.

The stepper motor uses a conventional 2-phase hybrid stepper motor with a step angle of 1.8o. The gear ratio between the output shaft gear of motor 1 and the horizontal guide rack is 1:22.3, and the gear ratio between the output shaft gear of motor 2 and the vertical guide rack is 1:12.5 (180o coverage), so the minimum adjustment of the azimuth angle is 0.08o, and the minimum adjustment of the pitch angle is 0.072o, which meets the requirements.

Detector circuit

The control circuit of the alarm detector designed in this paper is shown in Figure 3. The core of the control circuit is the 8-bit Atmel AVR microcontroller ATmega16[3], which has the characteristics of rich internal resources, fast speed, low price and high stability, and is very suitable for portable control equipment. In the figure, IC2 is a 12-bit serial digital-to-analog converter LTC1456 with SPI interface, which can be directly connected to the SPI interface of ATmega16, simplifying the software and hardware design. LD1 and LD2 are laser diodes with output wavelengths of 1.06mm and 1.54mm[4] respectively. As the analog light source of the detector, they are driven by stepper motors M1 and M2 and run on the guide rail shown in Figure 2. In order to achieve the adjustable output power of the laser tube, its current must be controlled, so a digitally controlled current source composed of IC2, op amp IC3, field effect transistor Q and resistor R4 is designed[5]. Its working principle is: after the microcontroller writes the control word to IC2 according to the required laser output power, the VOUT terminal immediately outputs the corresponding voltage; when the op amp is working, the inverting and inverting input terminals are "virtually shorted", which will inevitably cause the source of Q to output a current value, so that the voltage on R4 is equal to the voltage of the inverting input terminal of IC3, thereby realizing the digital control of the current. The gate current of the field effect tube is zero, and its source and drain currents are equal, so the control accuracy is high, avoiding the problem that the control accuracy is affected by the base shunt when using bipolar transistors. R4 uses a precision wire-wound resistor with a small temperature coefficient, which reduces the influence of temperature on the current control accuracy and has a strong overload resistance. Since the output power-current curves of the two laser tubes are different, the microcontroller must also be "notified" when switching the laser tube to maintain the correct current control law. Therefore, a linkage switch composed of S1_1 and S1_2 is designed. The microcontroller determines the laser tube currently connected to Q by detecting the logic level on PD4 to adjust the control coefficient. LTC1456 has an asynchronous reset terminal, which provides convenience for laser modulation: by setting the timer interrupt of the microcontroller to make its overflow frequency twice the modulation frequency to be set, IC2 can be written and reset alternately. [page]

DR1 and DR2 are 2-phase stepper motor drivers. The number of steps of the stepper motor is equal to the number of pulses received by the driver control terminal P, and the D terminal is the direction control terminal; therefore, the laser tube can be controlled to run to any point on the hemispherical surface shown in Figure 2. Due to the inertia of the stepper motor rotor, the pulse frequency input to the P terminal at the start cannot be too high to prevent the motor from losing step and causing positioning control errors. As analyzed above, the adjustment of the azimuth and pitch angles is already fine enough, so there is no need to subdivide the stepper motor, saving costs.

According to user requirements, the detector should be able to communicate with a PC to achieve stronger data processing and operation functions. For this purpose, a level converter with IC4 MAX232 as the core is designed. The chip can generate a -10V negative voltage output under a 5V operating voltage, thereby achieving level compatibility with the RS-232 serial port of the PC, and data and command words can be transmitted.

In the absence of a PC, the detector implements functional operations through a 3×3 keyboard. Its working principle is as follows: After the microcontroller is powered on, PB0~PB2 are configured as low level, and PD5~PD7 are configured as high level (AVR microcontroller I/O port is push-pull output, strong pull-up), at this time, the output of the 3-input AND gate IC5 is high level. When a key is pressed, a low level appears at the input end of IC5, so the output falls, causing INT0 to interrupt; after entering the interrupt service program, PB0~PB2 are first configured as weak pull-up input mode, then PD5 is set low, and then the logic level of PB0~PB2 is read. Since the internal pull-up resistor of the microcontroller (tens of KW) is much larger than R1~R3 ​​(2KW), if K3, K6 or K9 is pressed, it is inevitable that a logic 0 can be read at PB0, PB1 or PB2, thereby determining the pressed key number. If no low level is found on PB0~PB2, it means that the pressed key is not in this list. Restore the high level of PD5, set PD6 and PD7 low in turn, and repeat the above process until the pressed key is found.

LCD 1602 liquid crystal is used to display menus, operation prompts and test results.

Conclusion

Laser warning device is an important modern weapon equipment. The adjustable simulated laser pulse technology proposed in this paper can test the performance parameters of the warning device by controlling the position of the light source on the hemispherical surface, the luminous power, the wavelength and the modulation frequency. The laser warning device online detector developed based on this technology has the characteristics of compact mechanical structure, mature and reliable circuit, precise control, easy use and complete functions, which greatly improves the efficiency compared with traditional methods.