Inverted Pendulum-Type SMA Actuator

The SMA technology is particularly attractive for miniaturized actuators. Miniaturized SMA actuators often make use of flexures for guiding the moving parts, thereby avoiding problems with friction and backlash and at the same time reducing the number of parts. However, such flexures will introduce an additional stiffness into the mechanism, which increases the stress in the SMA wires and tends to limit the achievable actuator stroke. This paper investigates the use of an Inverted Pendulum (IP)-type kinematics in SMA actuators to overcome this problem. An inverted pendulum is inherently instable, and this property can be balanced against the mechanical stiffness of a flexure joint. The impact of the kinematic parameters on actuator stroke is analysed analytically. Using a model-based approach, an experimental setup of an IP-type SMA actuator was implemented to verify the predictions. For appropriate values of joint stiffness, actuator strokes of more than 14deg and a good adjustability of the angular position were achieved.