Control of Rotatory Decoupled Antagonistic SMA Actuators

Abstract

For repeated actuation in shape memory alloy (SMA) actuators, a restoring force is needed to return to the initial starting position after activation. Therefore, SMA wires are often coupled with mechanical springs, which lengthen said wires again after activation through heating and resulting contraction. In more advanced SMA actuation systems a second SMA wire is used as an actively controllable restoring element instead of passively working spring forces. A disadvantage of these antagonistic SMA actuator systems is that after activation of the first SMA wire, the return movement cannot be carried out immediately by the antagonistic partner. This delay caused by the first SMA wire’s cooling time leads to longer cycle times. To compensate for this disadvantage, a decoupled antagonistic SMA actuator has been developed. This enables the actuator to move back to its initial position immediately, regardless of the state of the antagonistic SMA wire. This work deals with the construction as well as the control of two rotatory decoupled antagonistic SMA actuators. The first actuator enables a 90° rotational movement through 2 mm of SMA wire stroke via a gear drive. The second actuator contains a bistable element to enable two energy-free switching positions. This bistable element serves as output device of the actuator and an output stroke of 8 mm is realized by an SMA wire stroke of 1.9 mm.