Control of a shape memory alloy actuation apparatus

A method of determining the ambient temperature of an SMA actuation arrangement comprising an SMA actuator arranged on contraction caused by heating to drive movement of a movable element relative to a support structure, wherein the SMA actuator has 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 method comprising:

• supplying drive current through the SMA actuator;
• detecting a measure of the resistance of the SMA actuator and controlling the drive current on the basis of the measure of the resistance of the SMA actuator;
• and
• deriving a measure of an electrical characteristic of the SMA actuator that is representative of the ambient temperature, said electrical characteristic of the SMA actuator being the power of the drive current required to hold the resistance of the SMA actuator at a predetermined level relative to the maximum resistance and/or minimum resistance of the SMA actuator.

A control system for an SMA actuation arrangement comprising an SMA actuator arranged on contraction caused by heating to drive movement of a movable element relative to a support structure, the control system comprising:

• a current source operative to supply drive current through the SMA actuator to heat the SMA actuator;
• a detector circuit operative to detect a measure of the resistance of the SMA actuator;
• and
• a control unit operative to control the current source and to control the power of the drive current on the basis of the detected measure of the resistance of the SMA actuator, wherein the control unit is operative, when the SMA actuator is in an unheated state, to initially apply the drive current to cause the resistance of the SMA actuator to rise to a local maximum resistance,
• wherein the control unit is arranged to derive a measure of an electrical characteristic of the SMA actuator that is representative of the ambient temperature, said electrical characteristic of the SMA actuator being the difference between the resistance of the SMA actuator in the unheated state when the drive current is initially applied and the maximum resistance of the SMA actuator.

A method of controlling an SMA actuation arrangement comprising an SMA actuator arranged on contraction caused by heating to drive movement of a movable element relative to a support structure, the method comprising:

• supplying drive current through the SMA actuator to heat the SMA actuator;
• detecting a measure of the resistance of the SMA actuator,
• generating a closed-loop control signal for controlling the power of the drive current supplied by the current source on the basis of an error between the measure of the resistance of the SMA actuator detected by the detector circuit and a target value for the resistance of the SMA actuator,
• after generation of the control signal, adding an offset to the control signal which reduces the steady-state value of said error, the power of the drive current supplied by the current source being controlled in accordance with the control signal having the offset added thereto.

A control system for an SMA actuation arrangement comprising an SMA actuator arranged on contraction caused by heating to drive movement of a movable element relative to a support structure, wherein the SMA actuator has 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 operative to supply drive current through the SMA actuator to heat the SMA actuator;
• a detector circuit operative to detect a measure of the resistance of the SMA actuator;
• and
• a control unit operative to control the current source and to control the power of the drive current on the basis of the detected measure of the resistance of the SMA actuator,
• wherein the control unit is arranged to derive a measure of an electrical characteristic of the SMA actuator that is representative of the ambient temperature, said electrical characteristic of the SMA actuator being the power of the drive current required to hold the resistance of the SMA actuator at a predetermined level relative to the maximum resistance and/or minimum resistance of the SMA actuator.

A control system for an SMA actuation arrangement comprising an SMA actuator arranged on contraction caused by heating to drive movement of a movable element relative to a support structure, the control system comprising:

• a current source operative to supply drive current through the SMA actuator to heat the SMA actuator;
• a detector circuit operative to detect a measure of the resistance of the SMA actuator;
• and
• a control unit operative to generate a closed-loop control signal for controlling the power of the drive current supplied by the current source on the basis of an error between the measure of the resistance of the SMA actuator detected by the detector circuit and a target value for the resistance of the SMA actuator,
• wherein the control unit is operative, after generation of the control signal, to add an offset to the control signal which reduces the steady-state value of said error, the control signal having the offset added thereto being supplied to control the power of the drive current supplied by the current source.