Shape memory alloy actuation apparatus

A control system for a camera lens actuation apparatus comprising an SMA actuator arranged on heating to drive movement of a camera lens element along the optical axis to change the focus of the camera lens element on an image sensor, the SMA 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, the control system comprising:

• a current source operable to pass current through the SMA actuator to heat the SMA actuator;
• a detector circuit operable to measure the resistance of the SMA actuator;
• and
• a controller operative to control the current source and to vary the power of the current using the measured resistance of the SMA actuator as a feedback signal to drive the measured resistance to a target value, wherein
• the controller is operative to perform a preliminary calibration operation in which the controller controls the current source to heat the SMA actuator, monitors the measured resistance of the SMA actuator to detect the local maximum resistance and the minimum local resistance, and derives and stores a resistance range magnitude equal to or less than the difference between the resistance value of the detected local maximum resistance and the resistance value of the detected local minimum resistance;
• the controller is operative to perform, more frequently than the preliminary calibration operation, a focus detection operation in which:
• the controller controls the current source to heat the SMA actuator and monitors the measured resistance of the SMA actuator to detect the local maximum resistance;
• the controller derives an upper target resistance value at or predetermined decrement below the resistance value of the detected local maximum resistance, and a lower target resistance value equal to the upper target resistance value less the stored resistance range magnitude;
• and
• the controller controls the current source to heat the SMA actuator so that the measured resistance varies from the upper target resistance value to the lower target resistance value, monitors the quality of focus of the image signal output by the image sensor, and stores the resistance value at which the quality of focus is at an acceptable level.

A control system for a shape memory alloy actuation apparatus comprising an SMA actuator arranged on heating to drive movement of a movable element, the SMA 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, the control system comprising:

• a current source operable to pass current through the SMA actuator to heat the SMA actuator;
• a detector circuit operable to measure the resistance of the SMA actuator;
• and
• a controller operative to control the current source and to vary the power of the current using the measured resistance of the SMA actuator as a feedback signal to drive the measured resistance to a target value,
• wherein the controller is operative to monitor the measured resistance to detect the local minimum resistance and, in response to detection of the local resistance minimum, to immediately control the current source to reduce the power of the current.

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 the local maximum resistance.