Design of high-resolution piezoelectric ceramic drive power supply based on ARM

　　0 Introduction

　　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. They 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.

　　1 System structure of piezoelectric drive power supply

　　1.1 Classification of piezoelectric drive power supplies

　　With the development of piezoelectric ceramic micro-displacement positioning technology, various driving power supplies dedicated to piezoelectric ceramic micro-displacement mechanisms have emerged. At present, there are two main forms of driving power supplies: charge-controlled and DC-amplified. The charge-controlled driving power supply has zero-point drift and poor low-frequency characteristics, which limit its application. The DC-amplified driving power supply has the characteristics of good static performance, high integration, and simple structure. Therefore, the design principle of this article adopts the DC-amplified piezoelectric driving power supply. The principle of the DC-amplified power supply is shown in Figure 1.

　　1.2 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 0~100 V 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.

　　2 Low voltage circuit design based on ARM

　　2.1 Introduction to ARM Controller

　　The ARM controller in the piezoelectric ceramic drive power supply mainly provides two functions: as a communication device, it provides a general input/output interface; as a controller, it runs related control algorithms and generates control signals or waveforms to achieve static positioning operations of PZT. In response to the above requirements, this design uses LPC2131 as the main controller. LPC2131 is a microcontroller based on the 32-bit ARM7TDMI-S-CPU that supports real-time simulation and tracking, produced by Philips, with a main frequency of up to 60 MHz; LPC2131 has 8 KB on-chip static RAM and 32 KB embedded high-speed FLASH memory; it has two general UART interfaces, I2C interface and one SPI interface. Because LPC2131 has high data processing capabilities and rich interface resources, it can be used as a control chip for piezoelectric drive power supply.

　　2.2 D/A Circuit Design

　　Since the piezoelectric drive power supply requires an output voltage range of 0~100 V and a resolution of millivolts, the resolution of the D/A must reach sub-millivolts. This design uses AD5781 as the D/A device. AD5781 is an SPI interface 18-bit high-precision converter with an output voltage range of -10~10 V, providing ±0.5 LSB INL, ±0.5 LSB DNL and 7.5 nV/ Hz noise spectrum density. In addition, AD5781 also has extremely low temperature drift (0.05 ppm/℃). Therefore, this D/A converter chip is particularly suitable for the acquisition and control of precision analog data. The D/A circuit design is shown in Figure 2.

　　In the hardware circuit design, due to the precision architecture adopted by AD5781, forced detection buffering of its voltage reference input is required to ensure the specified linearity. Therefore, the amplifier selected for buffering the reference input should have low noise, low temperature drift and low input bias current characteristics. Here, AD8676 is selected. AD8676 is an ultra-precision, 36 V, 2.8 nV/ Hz dual-channel operational amplifier with 0.6 μV/℃ low offset drift and 2 nA input bias current, so it can provide a precise voltage reference for AD5781. The CLR and LDAC pin levels of AD5781 are pulled down through pull-down resistors to set AD5781 to DAC binary register coding format and configure the output to be updated on the rising edge of SYNC.

　　In the software design on the ARM side, in addition to correctly configuring the relevant registers of AD5781, the clock phase, clock polarity and communication mode of SPI should also be correctly configured. The correct SPI interface timing configuration diagram is shown in Figure 3. [page]

　　3 High-voltage linear amplifier circuit design

　　The piezoelectric drive power supply in this paper adopts the DC amplification principle and obtains a 0~100 V continuously adjustable DC voltage to drive the piezoelectric ceramic through a high-voltage linear amplifier circuit. The amplifier circuit determines the resolution and linearity of the power supply output voltage and is the key to the entire power supply.

　　3.1 Classical 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 8 mV, the temperature drift is -63 V/°C, the conversion rate is 350 V/μs, the input impedance is 108 Ω, the output impedance is 44 Ω, and the common mode rejection ratio is 118 dB. The linear amplifier circuit design based on PA78 is shown in Figure 4. PA78 is configured as a forward amplifier with a gain of Gain=1+ R2 R1, and the output voltage range is 0~100 V.

　　If the voltage on both input terminals of the op amp is 0 V, the output voltage should also be equal to 0 V. 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.

　　3.2 Improvement of amplifier circuit

　　Here, PA78 is regarded as the controlled object G (S), and the offset voltage and temperature drift are regarded as the disturbance N (S). In this way, improving the output voltage accuracy of the amplifier is transformed into a controller design problem to reduce the steady-state error of the control system. There are two commonly used correction methods in the design of the controller: series correction and feedback correction. Generally speaking, the number of components required for feedback correction is small and the circuit is simple. However, in the high-voltage amplifier circuit, the feedback signal is provided by the output stage of PA78. The power of the feedback signal is high, which brings inconvenience to the component selection and circuit design, so the feedback correction method is not used in the linear amplifier circuit. In the series correction method, the input of the active device does not contain the high-voltage feedback signal, so this design adopts the series correction method and uses the analog PI (proportional-integral) controller G1 (S) for correction, as shown in Figure 5.

　　The proportional reaction input signal e(t) and its integral, namely:

　　From equation (2), it can be observed that the PI controller is equivalent to adding an open-loop pole located at the origin in the control system. The existence of the open-loop pole can improve the type of the system. Due to the improvement of the type of the system, the step disturbance steady-state error of the system can be reduced (for linear amplifier circuits, offset voltage and temperature drift can be regarded as step disturbances). At the same time, the PI controller also adds an open-loop zero point located in the left half plane of the complex plane. The addition of the complex zero point can increase the damping degree of the system, thereby improving the dynamic performance of the system and alleviating the adverse effects of sacrificing dynamic performance in exchange for steady-state performance on the system.

　　An active analog PI controller is used in the design of the amplifier circuit, and the improved linear amplifier circuit is shown in Figure 6. The amplifier of the PI controller uses AD8676, which has an input offset voltage of less than 50 μV (at full temperature range) and a voltage noise of ≤0.04 μV (PP) @0.1~10 Hz. Therefore, it is suitable for series correction to improve the steady-state performance of the system and reduce the output voltage drift.

　　3.3 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 is stray capacitance 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 6, C5 and C6 are used to compensate for the stray capacitance at the reverse end. From the perspective of system function, it constitutes 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 6, R4, C1 and R5, C2 form an RC compensation network.

　　In addition, the function 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.

　　The parameters of the PI controller are set to KP=10, KI=0.02; the lead correction compensation capacitors are 12 pF and 220 pF respectively; the RC compensation network is R=10 kΩ, C=22 pF. The amplitude-frequency characteristic and phase-frequency characteristic curves are obtained by simulation using the Spice model of the linear amplifier circuit as shown in Figure 7. It can be observed from the figure that the bandwidth of the amplifier system can reach 100 kHz, thus ensuring good dynamic characteristics of the system. At the same time, the phase margin γ>60° makes the system have higher stability (since the load reactance characteristics of PZT are generally capacitive, it is necessary to leave a larger phase margin). [page]

　　4. Driving power supply experimental results

　　The voltage regulator of the piezoelectric ceramic drive power supply used in the experiment adopts the 4NIC-X56ACDC DC power supply of Changfeng Chaoyang Power Supply Company, with an output voltage accuracy of ≤1%, a voltage regulation rate of ≤0.5%, and a voltage ripple of ≤1 mV (RMS), 10 mV (PP). The measuring equipment adopts KEITHLEY 2000 6 1/2Multimeter.

　　First, the DAC output resolution is measured. The ARM controller outputs a step signal lasting 5 s. At the same time, the voltage signal is measured at the DAC output. The measurement results are partially displayed in Figure 8. Figure 8 shows that the output voltage resolution of AD5781 can reach 3.89e-5 V, that is, 38.9 μV.

　　Noise is unavoidable in analog circuits, and for piezoelectric drivers, the level of noise limits the output resolution of the driver.

　　Figures 8 and 9 show the test noise of the classic amplifier circuit and the improved amplifier circuit respectively. It can be seen from the figure that the output noise of the piezoelectric drive power supply is reduced from 1.82 mV (RMS) to 0.43 mV (RMS) by using the PI controller and the phase compensation element.

　　Figure 10 shows the output resolution of the amplifier circuit. The resolution of the amplifier circuit determines the positioning accuracy of the PZT. To achieve nanometer-level positioning accuracy, the resolution of the driving power supply needs to reach the millivolt level. In Figure 10, the resolution of the output voltage can reach 1.44 mV.

　　Finally, the linearity curve of the driving power supply voltage is given. Linearity can truly reflect the degree of deviation of the output value relative to the input true value.

　　The linearity curve is shown in Figure 11. The fitting straight line Yfit=9.846Vin+0.024 2 is obtained, and the maximum nonlinear error is 0.024%, which can meet the requirements of precise positioning.

　　5 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 100 kHz. 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 0.43 mV, the maximum output nonlinear error is lower than 0.024%, and the resolution can reach 1.44 mV, which can meet the needs of static positioning control in high-resolution micro-displacement positioning systems.