Development of a shape memory alloy actuator. Improvement of output performance by the introduction of a σ-mechanism

摘要

The driving-force-generating principle of an actuator made of shape memory alloy (SMA) is based on the thermal elastic martensitic transformation, a kind of elastic phenomenon. As a result, conventional SMA actuators which use a circular pulley have tended to exhibit undesirable characteristics such as the maximum generable torque being changed depending on the rotating angle when the robot joint was driven by rotating motion transformed from expansion/contraction of the SMA, so that the servo system to support the torque under a certain load could not have an operable range wide enough for practical application. This paper intends to clarify these problems of SMA actuators and proposes a new joint mechanism using a σ-shaped non-circular pulley, called the σ-mechanism, for joint linkage to overcome the problems. This design enables the maximum generated torque to be kept uniform at all times by reducing the torque-arm-length in inverse proportion when the SMA tension increases corresponding to the rotating angle of the joint. Subsequently, a specifically designing algorithm for the proposed σ-mechanism is discussed. The validity of the new mechanism is demonstrated by an experimental model using an SMA actuator with a σpulley.