Characterization and Modeling of Opposing SMA-Wire System for Multifunctional, Resistance-Based Controls Applications

Shape Memory Alloy (SMA) actuator wires are often discussed in the context of multi-functional materials. This is because the temperature-induced phase transformation causes a significant (∼4%) contraction and a corresponding change in resistance. When a restoring force such as a pre-stretched spring is placed in series with an SMA wire, the contraction creates a repeatable actuation force, and the resistance provides measurement of the strain the wire, or structural (spring) deflection. Work has already been presented to demonstrate the stress, strain, and resistance characteristics of a single SMA actuator wire in series with a spring flexure under different pre-stresses and heating power input frequencies. Also, a method has been presented to linearly approximate the resistance vs strain characteristic, thus providing a direct mapping from SMA wire resistance to flexure deflection. This mapping method has been tested in the context of a simple PID controller used to position a flexure in series with a single SMA. This paper expands previous work by characterizing a system consisting of a spring flexure in series with two opposing SMA wires. Such an opposing SMA configuration is relevant to embedded SMA applications and gives the potential to increase cycling frequency by providing an active restoring force. The characterization results show that coupling SMA wires introduces alters the resistance vs strain characteristics of both wires because the second SMA wire essentially becomes a non-linear, hysteretic spring in series with the first. However, knowledge of the physics behind the complicated behavior enables sensible calibration schemes to be developed to accurately map resistance to strain for simultaneous sensing and actuating applications.