ARM high-resolution piezoelectric ceramic drive power supply design

The piezoelectric ceramic actuator (PZT) is the core of the micro-displacement platform. Its main principle is to use the inverse piezoelectric effect of piezoelectric ceramics to produce deformation, thereby driving the actuator to undergo micro-displacement. Piezoelectric ceramic actuators have the advantages of high resolution, fast response frequency, large thrust and small size, and have been widely used in aerospace, robotics, micro-electromechanical systems, precision machining and bioengineering. However, the application of piezoelectric ceramic actuators is inseparable from piezoelectric ceramic drive power supplies with good performance. To achieve nano-level positioning applications, the output voltage of the piezoelectric ceramic drive power supply needs to be continuously adjustable within a certain range, and the voltage resolution needs to reach the millivolt level. Therefore, piezoelectric ceramic drive power supply technology has become a key technology in piezoelectric micro-displacement platforms.

System structure of DC amplified piezoelectric drive power supply

The driving power circuit is mainly composed of a microprocessor, a D/A conversion circuit and a linear amplifier circuit. The microprocessor controls the D/A to generate a high-precision, continuously adjustable DC voltage (0~10 V), and the amplifier circuit performs linear amplification and power amplification on the DC voltage output by the D/A to control the PZT drive precision positioning platform.

In this design, LPC2131 is used as the microprocessor to generate control signals and waveforms; the 18-bit voltage output DA chip AD5781 is used as the main chip of the D/A conversion circuit to generate a continuously adjustable DC low-voltage signal; the power amplifier PA78 of APEX is used as the power amplifier device to output a 0~100V high-voltage signal to drive PZT. In order to realize the application of high-resolution piezoelectric actuators, the design index of the piezoelectric drive power supply resolution reaches the order of 1 mV.

Classic linear amplifier circuit design

The amplifier circuit uses the high-voltage operational amplifier PA78 produced by the American APEX company as the main chip. The input offset voltage of PA78 is 8mV, the temperature drift is -63V/°C, the conversion rate is 350 V/μs, the input impedance is 108Ω, the output impedance is 44Ω, and the common mode rejection ratio is 118 dB. The linear amplifier circuit design based on PA78 is shown in Figure 2. PA78 is configured as a forward amplifier with a gain of Gain=1+ R2 R1, and the output voltage range is 0~100V.

If the voltage on both input terminals of the op amp is 0V, the output voltage should also be equal to 0V. But in fact, due to the manufacturing process of the amplifier, it is inevitable that the non-inverting and inverting input terminals are mismatched, so that there is always some voltage at the output terminal, which is called offset voltage. The offset voltage changes with the change of temperature. This phenomenon is called temperature drift (temperature drift), and the size of the temperature drift changes with time. The offset voltage and temperature drift of PA78 are 8 mV and -63 V/°C respectively, and the offset voltage and temperature drift are random, which makes PA78 unable to be used for voltage output with millivolt resolution, and the amplifier circuit needs to be improved.

Phase compensation

From an engineering perspective, the existence of interference sources will change the stability of the system and cause the system to oscillate. Therefore, the way to ensure that the control system has a certain degree of anti-interference is to make the system have a certain stability margin, that is, phase margin.

Since there are stray capacitances in the actual circuit, the capacitance to ground at the reverse input of the amplifier has a great influence on the stability of the system. As shown in Figure 3, C5 and C6 are used to compensate for the stray capacitance at the reverse end. From the perspective of system function, it constitutes a lead correction, increases the open-loop cutoff frequency of the open-loop system, and increases the system bandwidth to improve the response speed.

PA78 has two pairs of phase compensation pins, which are used to compensate the zero poles inside the amplifier through an external RC network. From the data sheet of PA78, it can be seen that the zero poles inside PA78 are located in the high frequency band. According to the requirements of the control system's anti-noise ability, the RC network is configured to make the amplitude characteristic curve of the high frequency band decay rapidly, thereby improving the system's anti-interference ability. In Figure 3, R4, C1 and R5, C2 form an RC compensation network.

In addition, the role of C3 in the circuit is to prevent interference caused by vibration on the falling edge of the output signal; R10 acts as a bias resistor, injecting the power supply current into the output stage of the amplifier to improve the driving capability of PA78.

in conclusion

This paper designs a high-resolution piezoelectric ceramic drive power solution based on ARM. This solution adopts the principle of DC amplification and has the characteristics of low circuit noise, high resolution and low output nonlinearity. At the same time, the bandwidth of the drive power can reach 100kHz. The above characteristics enable the piezoelectric drive power supply of this solution to be applied to the needs of nano-level static positioning. Due to its high cost performance and simple structure, it has high practical value. The experimental results also show that the output voltage noise of the power supply designed in this solution is lower than 0.43mV, the maximum output nonlinear error is lower than 0.024%, and the resolution can reach 1.44mV, which can meet the needs of static positioning control in high-resolution micro-displacement positioning systems.