High-speed Actuator For High-speed Scanning Probe Microscope

This dissertation studies the high-speed actuator for high-speed scanning probe microscope (SPM). SPMs are important scientific instruments, which are widely used in physics, chemistry, biology, material science and many other fields. However, SPM suffers from its imaging speed. According to the working principle of SPM, images are obtained by mechanically scanning the sample, therefore, the imaging speed is limited by a few factors, such as the resonance frequency of the cantilever, the response time of the optical system, the speed of the data acquisition and processing, and the bandwidth of the scanner. Among all these factors, the bandwidth of the scanner is becoming the foremost limitation.A scanner consists of actuators and mounting structure. High-speed actuation of an actuator will generate large inertial force and induce the structure to vibrate with large amplitude when the frequency reaches to the resonance frequency of the structure. As a result, the bandwidth of the scanner will be limited by the resonance frequency of the structure, rather than the actuator.Symmetric actuator structure, such as a dual-mounted actuator structure is widely used to reduce the inertial force from high-speed actuators and to alleviate the vibration of the base in a scanning probe microscope. However, the inertial force resulting from an unbalanced weight should be considered and is conventionally performed by adding a dummy mass to balance the weight, which is inconvenient.In this dissertation, the inertial force resulting from an unbalanced weight is balanced by adjusting the amplitude ratio of the voltages applied to two actuators in a dual-mounted actuator structure. The experimental results show that the base vibration is well suppressed even if an unbalanced weight exists. The vibration suppression effect of the proposed approach is comparable to the effect generated when using the traditional method.An asymmetric actuator structure generating negligible influence on the supporting base for high performance scanning probe microscopes is proposed. The actuator structure consists of two piezostacks, one is used for actuating while the other is for counterbalancing. In contrast with balanced structure, the two piezostacks are installed at the same side of the supporting base. The effectiveness of the structure is proved by some experiments with the actuators fixed to the free end of a cantilever. Experimental results show almost all of the vibration modes of the cantilever are suppressed effectively at a wide frequency range of90Hz-10kHz.A dual-stage piezoelectric stack is presented. It consists of three parts, which are used for high-speed actuation, vibration isolation, and long-range actuation. This dual-stage piezoelectric stack is a modified piezoelectric stack and appears as a single stack. The first resonance frequency of the original piezoelectric stack is40kHz. The1-dB cutoff frequency of the original piezoelectric stack is approximately12kHz, and the travel range is16micrometers. The1-dB cutoff frequency of the dual-stage piezoelectric stack is approximately85kHz, and the travel range varies approximately from1.2micrometers to12.8micrometers. The main contribution of this design is the use of a vibration isolation actuator to prevent coupling between high-speed and long-range actuators.Material of high-speed actuator is discussed. Nonlinearity of piezoelectric ceramics has an influence on inertial force balancing. Driving piezoelectric actuators with charge rather than voltage can significantly reduce the nonlinearity and improve the effect of balancing.1-3piezoelectric composite is discussed for an alternative material of high-speed actuator. When the base is constrained, normal material is not free to contract laterally which introduces a counteractive stress and consequently, a reduction in range and vibration of the supporting surface. The problem becomes significant in high-speed actuators with a small length compared to their lateral dimensions. However,1-3piezoelectric composite do not have this problem, because the elongation of1-3composite results in negligible lateral contraction. Therefore,1-3piezoelectric composite is an promising material for high-speed actuator.