Design of driving circuit for multi-channel PZT based on single chip microcomputer

1 Introduction

In the adaptive optical synthetic aperture imaging system, when the original signal phase information of a certain aperture channel changes due to factors such as atmosphere and carrier vibration, the redundant information will reflect the change information of the two channels. The information used for redundant interval correction is extracted through the optical system, and the real-time correction of the phase is completed through computer feedback control of the driving voltage drum. We introduced the piezoelectric ceramic tube PZT in this feedback system for feedback control. Under the action of an external electric field, the piezoelectric ceramic material (PZT) with the inverse piezoelectric effect will deform, and the optical fiber on the PZT tube will also change in length with the radial displacement of the PZT tube, thereby changing the phase of the light wave. The use of any PZT is inseparable from the corresponding drive circuit. Whether the PZT can work normally and effectively depends on the performance of its drive circuit. For the system with dynamic feedback control of PZT, whether the PZT drive circuit can be linearly amplified is our most concerned issue. Based on the previous PZT drive circuit structure, this paper successfully realizes the design of multi-channel PZT drive circuit by using single-chip microcomputer C8051F005 and new D/A converter AD5308.

2 PZT drive voltage requirements

The optical fiber phase modulation realized by the PZT tube is to apply voltage to the PZT tube to produce piezoelectric effect, so that the outer diameter circumference of the PZT tube changes, driving the length and refractive index of the optical fiber wrapped around the PZT tube to change, thereby changing the phase of the light wave transmitted in the optical fiber. The phase change caused by the phase change is expressed as follows:






3 System hardware design

According to the experimental requirements, this system uses AD5308 with 8-bit digital-to-analog conversion accuracy as the digital-to-analog conversion chip, uses C8051F005 microcontroller to control AD5308, and sends each data to the corresponding D/A conversion channel through the serial communication port to achieve the purpose of converting phase difference information into corresponding analog voltage. The main hardware circuit connection of the system is shown in Figure 1. The single-chip microcomputer C8051F005 is connected to two AD5308 chips through SCL (clock line), SDL (data line), SYINC1 (chip select line of the first AD5308) and SYIN2 (chip select line of the second AD5308). The data line SDA transmits 12 data in time-sharing mode, and the minimum data refresh rate is 3HZ. The clock line SCL provides the clock signal for the latter. The single-chip microcomputer selects one of the AD5308 chips to work through chip selects SYNC1 and SYNC2. In this system, the P0.0 pin of C8051F005 is connected to SDA, the P0.1 pin is connected to SCL, the P1.3 pin is connected to SYNC1, and the P1.4 pin is connected to SYNC2. We choose the REF195 chip as the reference voltage source. At the same time, it provides a 5V reference voltage for the two AD5308 chips. The LDAC pin of AD5308 is grounded. The microcontroller generates the clock signal of D/A and sends the data to AD5308 through the serial port. After the AD5308 converts and outputs, it provides the voltage signal required to drive PZT.

4. Software Implementation of the System

The data transmission mode of AD5308 is word transmission. The output voltage range depends on the D4 and D5 bits in the control word. The D4 bit controls channels A, B, C, and D, and the D5 bit controls channels E, F, G, and H. If D4 and D5 are 0, the output is 0V-REF V. If they are 1, the output is 0V-2REF V (REF V is the reference voltage). This experiment requires that each voltage output is 0-4V, and the reference voltage REF V is 5V. Therefore, we set D4D5=00. The SYNC pin is an enable pin, which is level-triggered and is valid at low level. The LDAC pin signal starts the D/A conversion of 8-channel data, which is valid at low level. The serial data transmission timing of AD5308 is shown in Figure 2.

As can be seen from the figure, when the SYNC signal is low, data starts to be written at the falling edge of the clock signal SCLK. After the 16th SCLK falling edge, SYNC must be set to a high level to stop data transmission. If SYNC is set to a high level before the 16th pulse falling edge arrives, data transmission fails. After that, the data in the shift register will automatically enter the selected DAC register. The data in the DAC register starts to be converted and updated under the LDAC control signal. When the microcontroller writes 16-bit data to AD5308, the high bit comes first and the low bit comes later.

Data writing method

Set the MSB (D15 bit) to 0, indicating that data is written, D14D13D12=000, indicating the channel DACA address, 001, indicating the DACB channel address, and so on, D14D13D12=111, indicating the channel DACH address, and D11-D4 indicates 8 bits of data to be converted. The lower four bits are all set to 0. For example, writing data 0011 0101 0001 0000 means writing data 0101 0001 to DAC D channel.

Control word writing method

Setting MSB (D15 bit) to 1 means that the control word is written. D14D13=00 means gain and reference voltage selection mode. 01: LDAC working mode; 10: power saving mode; 11: AD5308 reset mode. In gain and reference voltage mode, according to formula (6), we can write control word 1000 0000 0000 0011, which means using REF195 as reference voltage with a gain range of 0-5V; in LDAC mode, writing control word 1010 0000 0000 0000 means continuously updating DAC register. In reset mode, writing control word 1111 0000 0000 0000 means resetting all registers and control bits. This system does not use the energy-saving mode.

Main program design

First, initialize the C8051F005 microcontroller, including crystal oscillator initialization, port initialization, define the I/O interface and cross switch that control AD5308, then initialize AD5308, load each control word, and finally write data to each conversion channel. The AD5308 initialization flow chart is shown in Figure 3, and the main program flow is shown in Figure 4.

5 Experimental results

The observation results show that the signal frequency of each channel is about 12.35 Hz, and the output voltage amplitude ranges from 4.88V to 5V. The 3 Hz scanning speed and 0 to 4V voltage requirements of the experiment are met. An 8-bit D/A converter is selected with an output accuracy of 0.02V, so the phase difference correction accuracy is 0.01 radians, which meets the experimental accuracy requirements. Figure 5 shows the sawtooth wave signal obtained by the digital-to-analog conversion of the DAC E channel observed on the oscilloscope.

6 Conclusion

This paper uses C51 language to write a D/A conversion control program for 12 phase data. The serial data transmission method and the application of 8-channel AD5308 digital-to-analog converter greatly simplify the system hardware circuit, making the software programming relatively simple, which can meet the application of controlling multiple PZTs to realize fiber phase modulation. The author's innovation point: using the new digital-to-analog converter AD5308 with 8 channels, using data serial transmission method, in the adaptive optical synthetic aperture imaging phase real-time correction system, the driving voltage required by 12 PZTs is provided in time-sharing. The real-time correction of phase is successfully completed. The economic benefits generated by this project: more than 5 million yuan.