SMA Actuator Resistance Control for Miniature Camera Lens Positioning
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Solution Overview
Problem
SMA actuators face challenges in providing accurate and repeatable control for miniature camera lenses due to hysteresis between applied current and actual position, temperature uncertainty, and creep effects, which affect the precision of positional control in compact devices.
Innovation Solution
The method involves monitoring the resistance of the SMA actuator to control its position, using a flyback technique to account for hysteresis and adjusting for creep by calculating a target resistance value based on detected resistance maxima, and applying conductive materials to reduce thermal dead-spots and mechanical stresses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If SMA actuator is used for miniature camera lens actuation, then compact size and high power per unit mass are achieved, but accurate and repeatable control is difficult due to hysteresis between applied current and actual position
Solution Approach 1:
The patent implements feedback control by monitoring the resistance of the SMA actuator and using this information to adjust the applied current. The control system continuously measures the actuator's resistance, which correlates with its temperature and phase state, and adjusts the heating current accordingly to achieve the desired lens position despite hysteresis effects.
Solution Approach 2:
The patent changes the control parameter from direct current control to resistance-based control. By monitoring resistance changes in the SMA actuator, the system indirectly tracks temperature and phase transitions, allowing for more accurate position control that compensates for the non-linear hysteresis behavior of the SMA material.
2Temperature
If temperature of SMA actuator is increased to induce contraction, then lens position control is achieved, but temperature uncertainty and cooling rate variations affect control accuracy
Solution Approach 1:
The patent uses electrical resistance as an intermediary parameter to indirectly measure temperature in the SMA actuator. Since resistance changes correlate with temperature and phase transitions in SMA materials, the control system uses resistance measurements as a proxy for temperature, avoiding the need for direct temperature sensors that would add complexity and measurement uncertainty.
Solution Approach 2:
The patent replaces direct temperature measurement (which would require thermal sensors) with electrical resistance measurement. This substitution allows for more precise and rapid monitoring of the SMA actuator's thermal state, as electrical measurements are faster and more accurate than thermal measurements, thereby improving control precision.
3Length of moving object
If SMA actuator contracts to drive lens movement, then focusing and zooming are achieved, but creep effects cause drift from target position over time
Solution Approach 1:
The patent maintains continuous monitoring and adjustment of the SMA actuator's resistance and applied current to compensate for creep effects. Rather than achieving the target position once and stopping, the control system continuously adapts the heating current based on real-time resistance measurements, ensuring the lens remains at the desired position despite gradual creep-induced drift over time.
4Power
If phase change in SMA material is utilized for actuation, then high power per unit mass is achieved, but hysteresis in phase transition temperature range complicates control
Solution Approach 1:
The patent uses feedback control based on resistance monitoring to manage the hysteresis in phase transition. By continuously measuring resistance and adjusting the applied current accordingly, the control system navigates the hysteresis loop in the phase transition, allowing reliable position control despite the complex temperature-dependent behavior of the SMA material.
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 enables accurate and precise control of miniature camera lenses by mitigating hysteresis and creep, ensuring reliable and repeatable actuation within compact devices.
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
Implementation Method 2
at high temperatures the SMA actuator transforms into the austenite phase which induces a deformation causing the SMA actuator to contract
Implementation Method 3
a first heating step of heating the SMA actuator from a state in which it is not contracted, whilst monitoring the resistance of the SMA actuator
Implementation Method 4
The temperature of the SMA actuator may be changed by selectively passing a current through the SMA actuator to heat it
Data Source
Figure 1~2
Figure 3
Figure 4
AI summary
A miniature camera lens actuation apparatus employs an SMA actuator comprising SMA wire to move a camera lens element. To provide autofocus, the SMA actuator is heated across its range of contraction and the resistance at which the focus is at an acceptable level is stored. To combat hysteresis, there is performed a flyback in which the SMA actuator is cooled, before heating the SMA actuator to the stored resistance. The stored resistance is adjusted to combat creep caused by non-linear heating of the SMA actuator. The SMA wire has conductive material extending along on a portion of the SMA wire which extends from a member connected to the SMA wire and being in electrical connection with the SMA wire in order to short out that portion of the SMA wire to reduce creep. Additional material is applied over the SMA wire and the member to reduce fatigue.