SMA Actuator Control via Resistance Feedback
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Solution Overview
Problem
The control of Shape Memory Alloy (SMA) actuation arrangements in miniature cameras is complicated due to the non-linear and hysteretic relationship between temperature and position, and the lack of a simple relationship between drive current and temperature, making precise position control challenging, especially in varying ambient temperatures.
Innovation Solution
A method to determine the ambient temperature of an SMA actuation arrangement by measuring the resistance of the SMA actuator and using this information to control the drive current, either through closed-loop or open-loop control, with the option to add an offset to reduce steady-state errors and extend the range of movement beyond local maximum and minimum resistances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If SMA actuator is used for precision actuation in miniature camera, then high power per unit mass and small component size are achieved, but precise position control becomes complicated due to non-linear and hysteretic relationship between temperature and position
Solution Approach 1:
The patent implements closed-loop control by continuously measuring the resistance of the SMA actuator and using this feedback to adjust the drive current. The control system compares the measured resistance with target resistance values corresponding to desired lens positions, and dynamically adjusts the heating current to minimize the error, thereby achieving precise position control despite the non-linear and hysteretic characteristics of the SMA material.
Solution Approach 2:
The patent changes the control parameter from direct temperature control to resistance-based control. By measuring the electrical resistance of the SMA actuator, which correlates with its phase state and length, the system indirectly controls the position. This parameter transformation simplifies the control approach by providing a measurable quantity that reflects the actuator state without requiring direct temperature measurement.
2Measurement precision
If drive current is increased to overcome steady-state errors in position control, then positioning accuracy is improved, but energy consumption increases
Solution Approach 1:
The closed-loop control system continuously monitors the actual position via resistance measurement and adjusts the drive current only to the extent necessary to correct position errors. This feedback mechanism eliminates the need for excessive drive current by precisely applying just enough heating power to maintain the desired position, thereby reducing overall energy consumption while maintaining high positioning accuracy.
Solution Approach 2:
The SMA actuator's inherent resistance change during phase transformation is utilized as a self-indicating feature for position feedback. The system leverages the natural electrical property changes of the material during actuation, eliminating the need for separate sensors or additional energy-intensive measurement systems, thus achieving accurate control with minimal energy expenditure.
3Measurement precision
If ambient temperature compensation is implemented to maintain control accuracy in varying temperatures, then positioning precision is improved, but device complexity increases
Solution Approach 1:
The system uses the SMA actuator's own resistance characteristics to inherently compensate for ambient temperature variations. Since the resistance of the SMA material changes predictably with temperature and phase state, the control system can distinguish between resistance changes due to ambient temperature and those due to actuator contraction, automatically adjusting control parameters to maintain accuracy without requiring external temperature sensors or complex compensation mechanisms.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows for precise control of the SMA actuator, reducing steady-state errors and increasing the range of movement, enabling accurate and efficient positioning of camera lens elements in miniature cameras despite varying ambient conditions.
Implementation Method 1
Actuation may be achieved by control of the temperature of the SMA actuator over an active temperature range in which the SMA actuator changes between martensite and austenite phases in which the stress and strain of the SMA actuator changes
Implementation Method 2
Another type of actuation arrangement which has been proposed uses SMA material as an actuator
Implementation Method 3
The temperature of the SMA actuator may be changed by selectively passing a drive current through the SMA actuator to heat it causing the phase change
Data Source
AI summary
An SMA actuation arrangement comprises: an SMA actuator arranged on contraction caused by heating to drive movement of a movable element relative to a support structure; a current source operative to supply drive current through the SMA actuator to heat the SMA actuator; and a detector circuit operative to detect a measure of the resistance of the SMA actuator. While 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; there is derived a measure of an electrical characteristic of the SMA actuator that is representative of the ambient temperature. While applying closed-loop control an offset is added to the control signal which reduces the steady-state value of an error between the measure of the resistance of the SMA actuator and a target value. While applying open-loop control, the control signal is generated taking into account the electrical characteristics of the SMA actuator measured during a calibration stage.