Research on high-speed piezoelectric ceramic drive power supply

Piezoelectric ceramics have a series of characteristics such as small size, high resolution, fast response, and large thrust. Piezoelectric ceramic drivers made of it are widely used in micro-displacement output devices, force generation devices, robots, impact motors, optical scanning and other fields. Therefore, the driving power supply technology of piezoelectric ceramics has become a very important research hotspot.

At present, the common piezoelectric ceramic devices in China are mainly based on static characteristics. Therefore, the dynamic characteristics of this type of piezoelectric ceramic drive power supply are not ideal, the AC load capacity is poor, and it is not suitable for application in dynamic fields. For example, the piezoelectric ceramic tube impact motor is based on the impact principle and uses a sawtooth wave to drive the piezoelectric ceramic tube, so that the piezoelectric motor produces positive and negative rotation. The wide frequency response range and high rise and fall rates are important dynamic characteristics that this type of piezoelectric ceramic drive power supply must meet. However, there is not much research on this type of drive power supply in China, and it is expensive. Therefore, it is necessary to design a piezoelectric ceramic drive power supply that meets the above requirements and is low-cost.

1 High voltage drive power supply principle and circuit design

The high-voltage drive power supply is mainly composed of three parts: a high-voltage DC power supply, a constant current source and a power amplifier circuit. The power amplifier circuit amplifies the sawtooth wave signal to drive the piezoelectric ceramic tube. In order to obtain a fast voltage drop rate and make the piezoelectric ceramic tube form an impact, a constant current source is required to help the piezoelectric ceramic of the capacitive load quickly discharge the charge.

1.1 High voltage DC power supply

The high-voltage DC power supply is shown in Figure 1. The industrial frequency 220 V AC is converted into dual 130 V AC by the transformer. After rectification by the rectifier bridge and filtering by the capacitor, 180 V DC is obtained as the working voltage of the drive circuit.

1.2 Constant current source circuit

The constant current source circuit is shown in Figure 2. The operational amplifier of this design circuit selects OP467, whose rise rate can reach 170 V/μs, and has an extremely wide response frequency, which can fully meet the requirements. When the input voltage at point A is VA, according to the virtual short principle, VA=VB, the input current of the amplifier's same-direction input terminal and the reverse input terminal are both 0, then VB=VC, so the current flowing through the field effect tube is constant I=VA/R3, at this time VGS≥3.5 V, and the field effect tube is turned on. If the input voltage VA has a voltage fluctuation +△V, the differential mode gain of the amplifier is close to infinity, so VG increases, VGS increases, the current flowing through the field effect tube increases, and VC increases; and because VB=VC, VB also increases, and eventually equals VA, ensuring the normal operation of the constant current source, on the contrary, when △V is negative, the same applies. The current of this constant current source circuit is I=50 mA, that is, the voltage VA=7.5 V. The purpose of this constant current source circuit is mainly to help the capacitive load piezoelectric ceramic discharge charge, so that the sawtooth wave driving the piezoelectric ceramic tube has a fast decline rate. When connected to the power amplifier circuit, the drain of the constant current source field effect tube is connected to point D of the power amplifier circuit in Figure 3.

1.3 Power amplifier circuit

The power amplifier circuit is shown in Figure 3. The op amp of this part of the circuit also selects OP467, which can ensure that the speed of the two parts of the circuit matches. The power amplifier stage uses the field effect tube IRF840, which has the characteristics of large current load capacity and fast switching speed (nanosecond level), so it is suitable for driving capacitive loads. This power amplifier circuit contains a floating ground (point D in Figure 3). After the power supply voltage of the constant current source circuit op amp passes through the DC/DC conversion module, the op amp reference point of the power amplifier circuit is separated from the ground.

When the circuit works in the linear region, if the voltage range of the input signal Uin is -10～0 V, it is equal to the voltage at point F, and current is generated on channel DF. R6 and R7 are voltage divider resistors. The ratio of R6 and R7 determines the multiple of the amplified voltage. Then the voltage Uout driving the piezoelectric ceramic is (Uin／R6)(R6+R7). Since the current I is constant, the resistance values ​​of R6 and R7 cannot be too small to ensure that they have sufficient current load capacity to drive the piezoelectric ceramic.

2 Design results

The input signal amplitude range of this design is -10～0 V, the output range is 0～350 V, and the discharge current is 50 mA. In the experimental test, with a capacitive load of 230×(1±0.1)pF as the standard, in the frequency range of 100 Hz～100 kHz, when a square wave signal with VPP=6 V biased at -3 V is input, the relationship between different amplification factors and frequency is measured as shown in Figure 4. It can be seen that after 60 kHz, the waveform has been severely distorted, so the use of high-voltage power supply drivers above 60 kHz should be avoided.

When there is no load and the square wave input is VPP=6 V, bias is -3 V, and f=50 kHz, the power supply output result is shown in Figure 5. In Figure 5, the output square wave is the waveform attenuated 10 times by the oscilloscope. It can be seen that the peak-to-peak value of the output square wave is 240 V, and the rising rate is higher than the falling rate. From the data shown in Figure 5, it can be calculated that the falling rate is about 48 V/μs and the rising rate is about 80 V/μs, which has obvious advantages over other dynamic piezoelectric ceramic drive power supplies. This parameter can meet the conditions required for the piezoelectric ceramic tube to impact the stator of the motor to form torsion.

3 Conclusion

The designed high-speed piezoelectric ceramic drive power supply has the characteristics of high operating frequency, good voltage following performance, simple structure and low price. It has obvious advantages over other circuits in dynamic performance and can provide good driving effect for some piezoelectric ceramic devices that need to utilize the impact principle.