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# Shape memory alloy actuator with on-off controller

A shape memory alloy (SMA) is able to memorize and recover its original shape, after it has been deformed by heating over its transformation temperature. This heating is producing linear displacement in proposed actuator. To control the displacement we can control heating by controlling the power supplied to the shape memory alloy spring.

Power supplied here is controlled using on-off controller through a relay, which is used to turn on or turn off the power supply of the shape memory alloy actuator. The displacement is measured through laser displacement sensor and it converted into respective voltage by the displacement sensor itself.

This resulting voltage due to displacement is compared with a reference voltage(it is equivalent to the desired displacement). Then, if the displacement is more than the desired displacement value the controller turn off the relay using Schmitt trigger circuit. If the displacement falls below the desired displacement value of lower threshold the controller turn on the relay using schmitt trigger. We are trying to use small dead band to get better performance.

## Components of on-off controller

### Schmitt trigger consisting Op-amp

The Schmitt trigger is a comparator application which switches the output negative when the input passes upward through a positive reference voltage. It then uses negative feedback to prevent switching back to the other state until the input passes through a lower threshold voltage, thus stabilizing the switching against rapid triggering by noise as it passes the trigger point. The Schmitt trigger action uses a comparator to produce stable level-crossing switches in contrast to the action of a straight reference comparison. Schmitt trigger action is a double threshold comparator process. This dependence upon the output voltage gives the dual threshold. The two output states give the thresholds.

Schmitt trigger consisting Op-amp

$V_H=V_2=\dfrac{R_{123}V_{ref}}{R_2}+\dfrac{R_{123}V_{cc}}{R_2}$, $V_L=V_2'=\dfrac{R_{123}V_{ref}}{R_2}-\dfrac{R_{123}V_{cc}}{R_2}$