Novel actuation techniques for piezoelectric tube actuators

Piezoelectric tube actuators are widely used in applications such as fibre optics alignment, endoscopy imaging and scanning probe microscopy. Piezoelectric tubes are thin-walled cylinders of radially poled piezoelectric ceramics. In almost all applications, the tube is fixed at one end and free at the other. A conventional tube in atomic force microscopy consists of quartered outer electrodes, which cover two-thirds of the length for lateral actuation. The remaining one-third of the length is covered by a circumferential electrode for vertical actuation. The inner surface is covered by a continuous electrode grounded at all times. For lateral actuation (bending) along the X or Y-direction, two outer quartered electrodes on opposite sides are driven by voltages of equal magnitude but opposite polarity. Voltage applied to the top circumferential electrode produces vertical actuation. The simplest way to increase the scan range of a piezoelectric tube actuator is to increase its length. However, this increases the physical size and reduces the resonance frequency. This thesis describes a new method for increasing the vertical scan range by driving the internal electrode rather than grounding it. This approach eliminates the need for a circumferential Z-electrode, which is typically one-third of the tube length, thereby allowing longer quadrant electrodes for larger lateral scan range. Since the proposed technique does not change the physical size of the tube, it is ideal for compact applications. Experimental results show a 62% increase in lateral scan range and an 86% increase in vertical scan range with negligible increase in cross-coupling. Analytical modelling shows that driving the internal electrode does not interfere with the lateral scan range. This thesis also proposes to implement similar technique for an eight-electrode tube actuator to compensate for angular (tilting) and vertical cross-coupling. The conventional quartered electrodes are split into two vertical segments of equal length to create a total of eight electrodes. The tilting and vertical cross-coupling due to the lower segments is compensated by the upper segments giving a sigmoid shape to the tube during lateral motion. Finite element simulations and experimental results confirm a 96% decrease in tilt angle and 43% reduction in vertical cross-coupling. However, the trade-off encountered with this method is a 44% decrease in lateral scan range.