Research And Application Of A Novel Charge Control Method For Piezoelectric Actuators Used In Atomic Force Microscope

In the micro actuating and precise positioning area, piezoelectric actuators (PZAs) have been widely used because of their advantages of high displacement resolution, large output force, and excellent dynamic response. However, the performances of the precise positioning system based on PZAs are largely limited because of the inherent properties of the piezoelectric material, such as the hysteresis and nonlinearity exhibited between the output displacement and applied voltage. Researchers have proposed many methods to overcome these problems. The most adopted methods are: feedforward control, feedback contro1and charge control. In this dissertation, two novel charge control methods for the PZAs, the switched capacitor charge pump (SCCP) method and a hybrid control method, are investigated theoretically and experimentally. Both methods are successfully implemented in a atomic force microscope (AFM) system.The novel switched capacitor charge pump method was carefully analyzed and successfully applied in an AFM for scanning. The working principle of the SCCP method is explained in detail. The influences of the chaging capacitor, the analog switch, and the operational amplifier to the circuit are analyzed. The influence of the switching frequency is also analyzed. In order to increase the switching frequency, an improved SCCP circuit with fast discharging is proposed. We build the experimental setup and employ an ordinary bluray disk as the scanning sample. The SCCP method is successfully implemented in an AFM to realize the traditional raster scanning. Imaging results demonstrate that the hysteresis and nonlinearity are reduced evidently under the SCCP drive, compared to the linear voltage drive. Facing with the low-frequency leakage problem of the SCCP method, a SCCP-based spiral scanning is proposed. The charge on the actuator is caculated carefully under the SCCP-based spiral scanning. Results show that the charge signal is a sine signal with linearly-varying amplitude, and the method is achievable. The scanning speeds of the traditional raster scanning and the spiral scanning are analyzed and compared. Results show that the scanning speed of the spiral scanning is1/2π times higher than that of the traditional raster scanning. At last, an image construction algorithm is developed.A hybrid control mehtod combining strain feedback control and charge control is firstly proposed in this dissertation. The hybrid control method is supposed to solve the low-frequency leakage of the charge control methods, including the SCCP method. The sensor-based feedback control is the most popular control strategy for the PZAs. It mainly uses a displacement sensor to detect the movements of the PZAs, and forms a closed-loop control. Because of the sensor-induced noise, high resolution and wide bandwidth can not be achieved simultaneously. Therefore the feedback control is mianly applied in the low frequency domain. The fundamental theory about charge control is that the output displacememnt of the PZAs is proportinal to the charges, but not the applied voltage, on the actuators. The charge amplifiers exhibit good linearity, high resolution, and fast response in the high frequency range. However, they usually become a voltage amplifier in the low frequency range due to the low-frequency leakage. Therefore the charge control method is normally used in the high frequency range. The proposed hybrid control method combines the advantages and avoids the disadvantages of the feedback control and charge control, by making the strain feedback control loop functioning in the low frequency range and the charge amplifying loop functioning in the high frequency range.The transfer function of the hybrid control method is analyzed in detail. It is theoretically proven that the frequency response of the strain feedback control and the charge control can cooperate with each other. The noise levels of the strain feedback loop and the charge amplifying loop are analyzed carefully. Results show that the achievable resolution is4.4nm for a bandwidth of10kHz under the strain feedback control alone; the achievable resolution is48.8pm for a bandwidth of10kHz under the charge control alone. Employing a piezoelectric stack which has a dimension of10mm x10mm x20mm as the experiment target, the hybrid control method is tested. Giving the piezoelectric stack a triangular excitation, the hybrid control method has stable gain in the whole frequency range from0.1Hz to50Hz, and the nonlinearity of the output displacements is less than2%. When the bandwidthes of the strain feedback loop and the charge amplifying loop are both set at1kHz, the theoretical output displacement noise of the strain amplifier is1.9nm while the measured equivalent displacment noise is2.5nm; the theoretical output displacement noise of the charge amplifier is25pm while the measured equivalent displacement noise is30pm. By the comparison of the hybrid control and feedback control, the hybrid control is proven having fast response.An improved hybrid control method is proposed and successfully implemented in an AFM for scanning. When the original hybrid method is applied to drive the piezoelectric positioner in the AFM, an existing problem is that the piezoelectric actuator is float-grounded, which may cause disturbation to the system. An improved hybrid control method based on "mirror control" is proposed to overcome this problem. A reference actuator and a target actuator are employed. The grounded target actuator is indirectly controlled by linearizing the charges on the float-grounded reference actuator. The linearized charges on the reference actuator results in linear displacement of the target actuator. The analysis of the transfer function proves the feasibility of the method. The improved hybrid method is also successfully implemented in a commercial AFM for scanning. Imaging results show that the hysteresis and nonlinearity are largely reduced by using this improved hybrid method, and the image quality is improved evidently.