An Enhancement in Control Laws of Super Twisting Sliding Mode Servo Drive Controller Using Hyperbolic Tangent Function and Arc Tangent Smoothing Function

Super Twisting Sliding Mode (ST-SMC) controller belongs to a class of controller known as Sliding Mode Control (SMC). SMC is widely known as a robust controller and has been shown in literature to be an effective medium for excellent control performance especially regarding disturbance force rejection. However, the control performance of SMC is often affected by chattering phenomenon thus reducing the applicability of SMC as position controller of choice in machine tools application where chattering induced vibration cannot be tolerated. ST-SMC is constituted as a higher order SMC. This paper explores control performances of ST-SMC in term of chattering reduction by introducing two types of switching functions in the control laws of the controller; namely, a hyperbolic tangent function (HST-SMC) and an arc tangent function (Arc-ST-SMC). The control performances are analysed based on reduction in magnitude of the tracking error (RMSE) and reduction in magnitude component of the chattering elements observed in frequency domain. The optimized ST-SMC produced the best tracking performance but chattering effect is still persistent. In comparison, HST-SMC produced a comparable tracking performance to ST-SMC with minimal difference of only 12.5% (RMSE). HST-SMC offers a fair trade-off between tracking accuracy and chattering attenuation. On the other hand, arc-ST-SMC produced the most reduction in chattering.