Discrete-time Integral Sliding Mode Control of a smart joint for minimally invasive surgeries

In this work, a shape memory alloy(SMA) actuator based joint (smart joint) is controlled using a discrete-time integral sliding mode (DISM) control to guide the motion of an active catheter. Controller is designed on the base of a simplified physical model of a single SMA actuator which eliminates the necessity of obtaining an accurate model. SMAs are nonlinear actuators and for this reason, a disturbance observer (DOB) is integrated in to the controller to compensate the model uncertainties and external disturbances to the system. A linearized model is used to design the controller. Bandwidth of SMA actuator is small (response frequency is less than 0.1Hz) and hardware communication frequency is 20Hz. Due to high sampling time (τ= 50ms) it is idealized to design a discrete-time controller, as switching frequency of the controller variable is then limited by τ-1. An experimental setup is designed to test the proposed controller with position feedback. In experimental results, DISM controller with DOB is shown to be robust against system model uncertainties and external disturbances. Different frequency responses are compared and it is shown that the response of 0.04 Hz can be achieved with rms tracking error of 0.0112 radians.