A target precision tracking system based on piezoelectric ceramics

Abstract: In the experiment, we hope to receive a light spot from one kilometer away at the center of the imaging system. However, due to the influence of atmospheric turbulence, the light spot shakes near the center of the imaging system. Target tracking is to keep the light spot in the center of the imaging system by changing the angle of the tilt mirror. To this end, a position-sensitive sensor is used to collect the position of the light spot, and a microprocessor processes the data to obtain the offset of the light spot. Finally, the piezoelectric ceramic crystal is driven to change the tilt mirror angle.

introduction

When the laser is transmitted in the atmosphere, the interaction with the atmospheric turbulence causes the fluctuation of the amplitude and phase of the light wave. Its jitter frequency is mainly low-frequency components. The response frequency of the piezoelectric ceramic crystal is above 1000Hz, which can meet the needs of eliminating the influence of the light spot jitter caused by atmospheric turbulence. In the optical tracking system, the traditional device used for target tracking is CCD. Since the amount of data collected by CCD is large, the requirements for the subsequent data processing unit are very high, and processing a large amount of data increases the complexity and processing time of processing. This tracking system uses PSD position-sensitive sensors to collect light spot position information, and only four-way signals are output, requiring only five addition operations, one subtraction operation and one division operation, greatly reducing the amount of calculation. In addition, the microprocessor of this system samples the dsPIC33F series microcontroller , which has a 40M instruction cycle. Its internal addition and subtraction operations are single instruction cycles, and division only requires 19 instruction cycles, which greatly improves the calculation speed.

1. Composition and principle of the correction system

The overall block diagram of the correction system is shown in Figure 1. The light beam from one kilometer away is received by the laser radar system, reflected by the tilt mirror, and split by the spectroscope. One part enters the imaging system, and the other part enters the PSD position-sensitive sensor. The position-sensitive sensor collects the position of the light spot to form four current signals. After current-voltage conversion and amplification, the microcontroller performs A/D conversion and calculates the offset of the light spot. The voltage required to drive the piezoelectric ceramic is calculated based on the offset of the light spot. Finally, the driving voltage value is converted to D/A, and the high-voltage driver drives the PZT (piezoelectric ceramic crystal) to change the angle of the tilt mirror, so that the light spot is always in the center of the imaging system.

Figure 1 Tracking system block diagram

2. Overall system design

2.1 Position-sensitive sensor system

The position-sensitive sensor is composed of Si photoelectric diodes , and the output signal is a current signal. The current size is related to the position of the light spot and the intensity of the light. Its primary circuit must be a current-voltage conversion circuit. The relationship between the four output signals and the light spot position is:

Among them, i1, i2, i3, i4 are four output signals. The above formula uses division operation to calculate x and y, eliminating the influence of light intensity change on position, thereby obtaining a position signal that is independent of light intensity.

2.2 Single chip microcomputer Control system

This system uses dsPIC33F series microcontrollers to implement 12-bit high-speed A/D conversion, PID control, communication with D/A converters and communication with computers.

The A/D part of the series of single-chip microcomputers adopts successive comparison A/D conversion, with a maximum of 32 conversion channels, which can realize automatic channel selection mode sampling and have 16 result buffers . In this system, we use a sampling rate of 125K to sample four analog signals. When all 16 result buffers are full, an interrupt is generated and the average value of each signal is taken four times. Channel-by-channel sampling prolongs the sampling time of each signal, and the method of taking the average value of four samples can reduce the sampling error on the one hand, and play a filtering role on the other hand.

The target precision tracking system needs to achieve fast response, which requires our algorithm to achieve fast convergence. We use the PID (proportional-integral-differential) incremental algorithm to achieve fast convergence of the system. Among them, the P term is the proportional term. When the error is large, the coefficient of P is also large, which can achieve fast adjustment; when the error is small, the coefficient of P is also small, which can achieve small adjustments. As time goes by, the P term is conducive to reducing the total error of the system. But there is always a static error. The I term is the integral term. Integrating the error can achieve the accuracy adjustment of the error, so that the static error accumulates to a certain value and is multiplied by the gain factor of the I term before output, eliminating the influence of the static error. The D term is the differential term, which is used to achieve fast adjustment. It responds to the rate of change of the error signal.

The incremental algorithm is derived as follows:

Its incremental form is:

The coefficient of the differential term. From (2), it can be seen that since the PID output is related to the historical state, the calculation workload is very large and the deviation signal needs to be accumulated. However, the incremental PID algorithm, that is, the algorithm (2), is used, and the output is the increment of the error, which can reduce the calculation workload.

The conversion adopts SPI communication mode. The D/A converter is TLV5638 , which is a dual-output D/A converter. Its maximum output voltage is twice the reference voltage, and its saturation voltage is power supply voltage VDD -0.4v, which means that the reference voltage should not be greater than VDD -0.4v. In addition, D/A conversion must be performed at the falling edge of the chip select signal CS. The determination of the op amp offset and PID coefficient is controlled by the computer. The communication between the MAX232 serial port and the computer is introduced in many materials, so I will not repeat it here.

[page]

2.3 Requirements for high voltage circuits

Piezoelectric ceramics use the inverse piezoelectric effect or electrostrictive effect to produce deformation under the action of an external electric field. The driving power supply of the piezoelectric ceramic actuator should have the characteristics of large output current and small wave. We use the Darlington tube to form an active filter circuit, which can achieve small wave and large current output. In the rectifier circuit, high-voltage large capacitors are required, and a discharge circuit is required, but the discharge time is too long when using a large resistor ; when using a small resistor, the current on the resistor is too large during long operation, resulting in excessive heating of the resistor. For this reason, we use a two-stage discharge method to solve the above problems of long discharge time or excessive power consumption of the resistor. The schematic diagram of the two-stage discharge circuit is shown in Figure 2. When the capacitor voltage is very high, the comparator U1 outputs a low level, Q1 is cut off, and the capacitor can only discharge through the large resistors R1 and R2. When the capacitor voltage is lower than a certain critical value, U1 outputs a high level, and the capacitor discharges through the small resistor R3.

Figure 2 Two-stage discharge circuit

Figure 3 shows the high-voltage drive circuit and discharge circuit. The input signal output by the D/A conversion is compared with the feedback signal. If the input D/A conversion signal is large, U1 outputs a low level and is cut off, and the power supply charges the piezoelectric ceramic crystal. Among them, T2 and T2 form a Darlington tube. U2 forms a comparison amplifier circuit, and the output voltage is compared with the voltage of the piezoelectric ceramic. If the voltage of the piezoelectric ceramic crystal is high, U2 outputs a high level and is turned on, and the piezoelectric ceramic crystal is discharged. Otherwise, the piezoelectric ceramic crystal is charged.

2.4 Piezoelectric ceramic micro-displacement device

Under the action of an external electric field, the positive and negative charge centers of piezoelectric ceramics (PZT) produce relative displacement, which causes the piezoelectric body to deform, showing that the piezoelectric ceramic has a certain degree of expansion and contraction. The expansion and contraction ability of piezoelectric ceramics (PZT) can be used to control the angle of the tilt mirror. The principle is shown in Figure 4:

Figure 4 Schematic diagram of the tilt mirror system

Among them, O is the fulcrum, A and B are piezoelectric ceramics (PZT). The big circle represents the tilt mirror. The balance point of the piezoelectric ceramics (PZT) is the driver working at a working voltage of 100V. In this way, when the driving voltage increases, the tilt mirror moves in one direction; when the driving voltage decreases, the tilt mirror moves in the opposite direction.

The piezoelectric ceramic (PZT) of this system can stretch 30μm at 200V voltage, that is, the front-to-back change range of the piezoelectric ceramic (PZT) is 15μm. The length of OA and OB is 5cm. Calculation shows that the front-to-back change range of the tilt mirror is 0.3mrad. And because the magnification of the telescope is 10 times,

Therefore, this system can adjust the light spot with a jitter within 3 mrad, which can fully meet our requirements.

3. Test results and conclusions

The test results show that this system can achieve the correction of atmospheric disturbances within 40Hz, and has a good correction effect. Below we give the specific analysis results: the data was collected from 3 to 4 pm, and the atmospheric coherence length at that time was between 5.5-7. Figure 5 Spot jitter. The circled line in the figure reflects the spot jitter. From the figure, we can see that the range of the spot jitter is relatively large, and it contains high-frequency components and low-frequency components. The line without a circle reflects the spot jitter image after tracking. It can be seen from the figure that the jitter range of the spot is very small, and it is mainly composed of high-frequency components, and also contains certain low-frequency components. This is because: on the one hand, the one-kilometer spot jitter caused by atmospheric turbulence is measured in micro-arcs, and the spot jitter is very small, exceeding the resolution of PSD. On the other hand, the spot jitter is caused by mechanical jitter.

Figure 5 Light spot jitter

The next step is to improve the mechanical performance of the system, increase the resolution of the system and further increase the bandwidth of the system.