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  • Position control of shape memory alloy actuators

    Deshraj Meena and Shovan Bhaumik

    Shape memory alloy based actuator is highly nonlinear in nature because of its complex electro-thermo-mechanical properties which correspond to its hysteresis behavior; and this behavior introduces delay and leads to inaccuracy in motion control of shape memory alloy based actuator.

    It is evident from the above discussion that controllers designed for shape memory alloy based actuators need to be robust with respect to parameter variations of system and environment. So the development of controller to accurately control the SMA integrated system is one major area of research.

    To get a complete picture of the shape memory alloy based actuator behavior we are designing a linear Shape Memory Alloy based actuator. We implement on-off controller to control the position of SMA.

    Working principle

    Current is passed to heat the shape memory alloy. When SMA is heated it started changing its shape. Here we took shape memory spring. When the spring is heated by the passing of current, it starts changing it's length. A push plate is attached with the spring. The elongation is determined by the laser displacement sensor. The sensor generates voltage propostional to the position of the actuator. The generated voltage is compared against set point and on-off control is implemented through relay.

    Laser Displacement Sensors

    To control the position or displacement current position/displacement should be measured. This could be done by any position sensor. We used laser displacement sensor for that. Laser displacement sensors are comprised of a light-emitting element and a position sensitive detector (PSD) and detect targets using triangulation. A semiconductor laser is used as the light emitting element. A lens focuses the beam on the target. The target reects the beam back through the lens where it is focused on the position sensitive detector (PSD), forming a beam spot. The beam spot moves as the target moves. Displacement can be determined by detecting the movement of the beam spot.

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    Position control of SMA actuator

    On-Off controller

    Schmitt trigger