Design, Modeling And Control Of Shape Memory Alloy Based Poly Phase Motor

Abstract

In this thesis, a new Poly Phase Motor (rotary actuator) based on the Shape Memory Alloy (SMA) is presented. Details of Design, Modeling, Characterization, Realization and Control of Poly Phase SMA Motor are presented. Motor with 3 and 6 Phases, with appropriate Control circuit have been realized in laboratory and simulated results have been verified experimentally. In literature, broadly two types of Shape Memory Alloy based motors namely limited rotation motor and unlimited rotation motor are found. In the unlimited rotation type SMA based motor the SMA element is in the spring form. Hence, an attempt has been made in this research to develop an Unlimited Rotating type Balanced Poly Phase Motor based on SMA wire in series with a spring in each phase. By isolating SMA actuation and spring action a constant force by the SMA wire through out its range of operation is achieved. While designing the motor, similarity in function between Poly Phase SMA Motor and Stepper Motor was found. Hence, the Poly Phase Motor is characterized similar to that of a Stepper Motor. Functionally, the Poly Phase Motor can be used in stepping mode for generating incremental motion and servo mode for generating continuous motion. Various parameters of the motor have been defined. The motor can be actuated in either direction with different Phase sequencing methods, which are presented in this work. While explaining sequencing methods, effect of the thermal time constants has also been presented. The lumped thermal model is used for dynamic simulation of motor. The motor has been modeled with a new approach to the SMA wire Hysteresis model. This model is simple and useful for real time control applications. Model is implemented using Simulink and used for the simulation of the motor. Generalization of the motor concept is done and motor up to 16 Phases are studied and the simulation results done using MATLAB are discussed. It could be observed that the torque generated by the motor increases with increased number of phases while the torque ripple reduces. The motor torque ripple is better for motor with odd number of phases due to its construction. Two methods of achieving servo motion are presented. The first method is Micro Stepping, consisting of controlling single phase temperature with a position feedback. The second method is Antagonistic Control of temperatures of phases with position feedback. Both the above methods use PID Controller with optical encoder feedback for position sensing. Performance of the actuator with step, ramp and triangle inputs has been simulated using Simulink and verified experimentally for various loads and disturbances. Positional accuracy of 0.07% for the Step input and for the full rotation of 3600 is achieved. Vector Control of SMA Motor is presented. By this method Speed and the torque of the motor will be effectively controlled. Since the temperatures of the wires are controlled in this research, this method is named as Thermal Space Phasor or Vector Based Control. This method of rotation of motor is simulated using Simulink and verified experimentally. Here the current through the SMA is controlled so as to get near sinusoidal variation in temperature. This leads to a near Sinusoidal variation of force. It is shown that by controlling the temperature of phases Sinusoidally with a phase shift of 1200, the Resultant Force will be a constant over the Spatial angle of 3600 and its Velocity of rotation will be Constant. Open loop and closed loop control of the speed and torque is presented. While the motor rotates at fixed Speed and Torque in Open Loop Control, motor adopts to change in torque and velocity in Closed Loop control with reduced ripple. PID Controller is used for closed loop control. The presented rotary actuator and their experimental results set a new standard for SMA based new generation rotary actuators and control.