The Study Of Control Method On Shape Memory Alloy Actuator

In the research and application of smart aircraft, a small and light actuator with high drive efficiency and control precision plays a key role. The traditional actuator has low power weight ratio, and it needs to work with speed variator. Both of that result in a complicated transmission system which it's difficult to apply to the aircraft directly. With the merits such as high power weight ratio, drive and sense in one and large strain, the shape memory alloy makes itself uniquely in the field of application of smart materials. However, it has low frequency and accuracy in control. On the basis of a SMA actuator for cross-section deformation mechanism in aircraft, the study of control methods of SMA will help us gain a deeper understanding of its features and make a more comprehensive promotion for its application.Firstly, this thesis introduces the characteristics and some practical applications of smart materials as the driving elements and analyzes the development trend of smart materials. Focusing on the material and mechanical characteristics of shape memory alloys it analyzes their advantages and disadvantages as driving elements. And then it selects the Ti-Ni alloy wire as the driving element. After describing some examples of SMA rotary actuators, according to the requirements of the smart aircraft on the actuator, with larger angle and torque outputs a smaller and lighter SMA rotary actuator based on compliant mechanisms is designed.Secondly, it studies and analyzes the heating methods and cooling methods of the SMA rotary actuator. While the drive frequency is an important standard to measure the actuator, the temperature is an important indicator in driving SMA. The study shows that: It can heat up SMA wires quickly by direct electric heating. The higher the current is, the faster the heating rate is. It can cool down SMA wires quickly by cold wind cooling. The greater the wind speed and the lower the wind temperature are, the faster the cooling rate is.Finally, it studies the control system of the SMA rotary actuator. Based on the relationship of resistance and strain of SMA wire, a method for controlling the actuator with the feedback of SMA resistance is proposed. The designs of hardware circuit are found on the requirements of the actuator: a high-power SMA drive control circuit for the actuator based on resistance measurement of SMA wires according to the characteristic that the heating speed is faster while the current is higher, a power drive circuit for thermal electrical refrigerator based on temperature measurement by thermal resistance and a main control circuit based on the AVR microcontroller. To obtain fast and precise control of the actuator, an integral separate type PI controller is designed according to the characteristic of the strain of SMA. According to the function of hardware circuit and the way of information transfer, a control and processing program structure is completed. This whole control system is verified and experimental analyzed on SMA rotary actuator. Experimental results show that: the system can precisely measure the SMA wire resistance while providing high enough heating current; the actuator can achieve closed-loop control under the PI controller; the system has a better driving frequency in cooperation with the refrigeration equipment. It provides an important reference for fast and precise control of SMA actuator.