Patent ID: 8446475

Claim:
A shape memory alloy actuation apparatus comprising: a support structure; a movable element supported on the support structure by a suspension system which guides movement of the movable element; a shape memory alloy actuator connected between the support structure and the movable element such that contraction of the shape memory alloy actuator on heating drives said movement of the movable element relative to the support structure, the shape memory alloy actuator having a property that resistance varies with length along a curve on which the resistance decreases from a local maximum resistance to a local minimum resistance during contraction; and an endstop arranged to limit movement of the movable element to prevent extension of the shape memory alloy actuator in its unheated state beyond a maximum length which is at or below the length corresponding on said curve to a resistance below the resistance of the local maximum resistance by a predetermined amount of the difference between the resistance of the local maximum resistance and the resistance of the local minimum resistance.