Patent ID: 8448435

Claim:
A method of controlling an SMA actuation apparatus comprising an SMA actuator arranged on contraction caused by heating to drive movement of a movable element relative to a support structure on which the movable element is supported, the method comprising supplying a drive current through the SMA actuator to heat the SMA actuator, the power of the drive current being controlled on the basis of a closed-loop control signal for driving the resistance of the SMA actuator to a target value by performing the steps of: detecting a measure of the resistance of the SMA actuator; deriving an estimate of at least one characteristic temperature of the SMA actuation apparatus on the basis of the power of the drive current, using a thermal model that relates the at least one characteristic temperature of the SMA actuation apparatus to the power of the drive current; deriving a resistance offset on the basis of the estimate of at least one characteristic temperature of the SMA actuation apparatus, using a resistance model that relates a resistance offset that compensates for the variation of the resistance of the movable element with the at least one characteristic temperature of the SMA actuation apparatus to the at least one characteristic temperature of the SMA actuation apparatus; and generating the closed-loop control signal for controlling the power of the drive current on the basis of the error between the measure of the resistance of the SMA actuator and the target value of the resistance of the SMA actuator, adjusted by the resistance offset.