Piezoelectric ceramic actuator drive power supply based on LabView8.5 and PA96

Piezoelectric ceramic actuators are new micro-displacement devices developed in recent years. They have the characteristics of small size, large thrust, high precision and displacement resolution, and fast frequency response. It is noiseless and heat-free during use, and is an ideal micro-displacement device. It has been widely used in aerospace, precision measurement, robotics, and precision machining. The performance of the driving power supply has a great influence on the piezoelectric ceramic actuator. In recent years, the domestic research and development of static piezoelectric ceramic driving power supplies has made certain progress, but most piezoelectric ceramic driving power supplies are composed of discrete devices with complex structures and are prone to self-oscillation, which will affect the stability of the power supply. The driving power supply using a high-voltage op amp can achieve a resolution of mV level and a small output ripple, which not only improves the circuit integration, but also enhances reliability. Therefore, it can be used to drive piezoelectric ceramic actuators.

Piezoelectric ceramic actuator driving power supply
1 Requirements for driving power supply of piezoelectric ceramic actuators
The driving power supply of piezoelectric ceramic actuators should have the following characteristics: (1) The response speed of the displacement output of the piezoelectric ceramic actuator to the external driving control voltage mainly depends on the size of the driving current of the driving power supply. Therefore, the driving power supply should have a large driving current, which should generally not be less than 150mA; (2) The output control voltage of the driving power supply is continuously adjustable. For the domestic piezoelectric ceramic actuator PTBS200 series, the output voltage of the driving power supply is required to be DC 0~200V, which is continuously adjustable; (3) In order to meet the requirements of high-frequency response, the driving power supply should have a circuit for rapid discharge of capacitive loads; (4) Since piezoelectric ceramic actuators are mainly used in the field of micro-nanotechnology, the driving power supply should have good stability, and its output ripple voltage should be controlled within a very small range; (5) In order to achieve automatic control of displacement, the driving power supply is preferably controlled by a computer.
The external circuit of the piezoelectric ceramic actuator is a capacitive load, and has hysteresis and creep phenomena. The driving power supply can generally be divided into charge-controlled type and voltage-controlled type. The charge-controlled driving power supply is based on the principle of capacitor charging ( for the applied voltage, each piezoelectric ceramic piece is equivalent to a parallel plate capacitor ), which can improve the hysteresis and creep of piezoelectric ceramics. The voltage-controlled driving power supply mainly has the following two forms: one is a switching driving power supply based on the principle of DC/DC converter , which is small in size and high in efficiency, but has a large power output ripple and a narrow frequency response range; the other is a DC amplifier power supply with a wide frequency response range. From the development trend, its application prospects are broad.

2 Design of driving power supply
According to the requirements of the piezoelectric ceramic actuator for its driving power supply, the power supply in this design adopts a DC amplifier circuit. The circuit principle block diagram is shown in Figure 1. The entire power supply circuit is mainly composed of several parts such as a computer and a data acquisition card, an operational amplifier circuit and a high-voltage circuit. The high-voltage circuit provides a 220V DC voltage. The computer controls the data acquisition card through LabView8.5 to generate a certain output waveform and obtain a continuously adjustable control voltage of 0 to 5V; the amplifier circuit realizes the linear amplification and power amplification of the voltage, outputs a continuously adjustable DC voltage of 0 to 200V, and determines the resolution and stability of the power supply output voltage, which is the key to the entire power supply.

Figure 1 Power supply block diagram

① High voltage circuit

Figure 2 Schematic diagram of high voltage circuit

Since the stability of the DC voltage directly affects the stability of the driving power supply, a high-voltage circuit with a 220V output voltage is used (see Figure 2). The main part is a full-bridge rectifier power supply circuit that converts the AC 220V mains power into a +220V DC voltage.

② Waveform generation circuit
A good input waveform is one of the keys to the power supply, which is related to the expansion and contraction changes of piezoelectric ceramics. The frequency and amplitude of the input waveform signal are variable, the signal waveform is good, and the distortion is small, which can not only eliminate the hysteresis and creep characteristics of the piezoelectric ceramics themselves, but also obtain a wider range of applications. Due to the high voltage accuracy requirements, NI's multifunctional data acquisition card 6221 with 16-bit analog input and output is used to convert digital quantities into analog quantities. The output voltage is 0~5V, and the voltage resolution reaches 5/216, which is approximately equal to 0.076mV. Using LabView 8.5 programming can realize arbitrary waveforms and slow-changing DC outputs, which is flexible and can meet various needs.

③High voltage amplifier circuit

Figure 3 Schematic diagram of high voltage amplifier circuit

The high-voltage operational amplifier PA96 and the high-precision operational amplifier OP07 produced by the American APEX company are connected in series to form a series negative feedback amplifier circuit, as shown in Figure 3. PA96 is a high-voltage, large-bandwidth MOSFET operational amplifier with an output current of 1.5A and an output voltage of nearly 300V. The safe operating area (SOA) has no secondary breakdown restrictions. By selecting a suitable current-limiting resistor , the safe operating curve under any load can be observed. The maximum offset voltage of PA96 is 5mV. For the piezoelectric ceramic drive power supply with a required resolution of less than 10mV, its input characteristics cannot meet the requirements. In the linear amplification part of the power supply, a composite amplifier connected in series with PA96 and OP07 is used, so that the input offset voltage is controlled by the preamplifier OP07. Since the input voltage of the composite amplifier is 0~5V and the output voltage is required to be 0~200V, the amplification factor of the composite amplifier is required to be 40. However, excessive gain will affect the stability of the operational amplifier, so the closed-loop amplification factor of PA96 is selected as 31, and the amplification factor provided by PA96 and OP07 in series is 40. According to the distribution requirements of the amplification factor, we can get:
R1=3kΩ, R2=117Ω, R3=180Ω, R4=6kΩ.
Since it forms a negative feedback circuit, the output resistance is very small (mΩ level), so it has a strong load capacity. R5 in Figure 3 is a current limiting resistor, and its value is obtained by the formula IL=0.68V/R5=125mA, which is 5.4Ω here.

3 Phase compensation and output protection
Self-excitation is a major factor affecting the stability of the power supply. When the open-loop gain of the integrated operational amplifier is a certain value, the circuit will oscillate due to excessive phase shift, so the integrated operational amplifier should be phase compensated. Usually, compensation components are connected externally at the input and output ends for phase compensation. Phase compensation can not only improve the stability of the operational amplifier, but also expand the bandwidth. The closed-loop gain of the first stage of PA96 in the circuit is 31, from which its gain is calculated to be 30. According to the data of PA96, the phase compensation capacitor value CC=10pF is determined, and the closed-loop bandwidth is about 1MHz. The feedback capacitors C1 and C6 of the circuit in Figure 3 are used to improve the stability of the amplifier circuit at high frequencies. The diodes D1 and D2 at the input of OP07 provide differential mode and common mode protection to prevent transient overvoltage, and the output diodes D3 and D4 can protect against transient overvoltage to prevent transient overvoltage from damaging the output of OP07. The diodes D5, D6, D7, and D8 at the input of the PA96 amplifier clamp the voltages at the positive and negative input terminals of the amplifier within the specified range, thus protecting the operational amplifier.

Experimental results and conclusions of the driving power supply
The output characteristics of the voltage-stabilized power supply were tested using a high-precision FLUKE 8580A voltmeter at 0-200V. The measured data were compared with the input voltage (see Table 1), and the linear regression method was used for analysis. The linear correlation coefficient of the output of the voltage-stabilized power supply was r≈1, indicating that the output linearity of the voltage-stabilized power supply is good. The driving power supply works continuously under load, and the output voltage drift is less than 10mV. The output ripple voltage of the driving power supply is less than 10mV. This ripple voltage has little effect on the displacement accuracy of piezoelectric ceramics in the micro-positioning system.