High-order, sliding-mode-based precise control of direct-drive systems under heavy uncertainties

The high-performance control of direct-drive (DD) systems requires the full system dynamic effects to be taken into account owing to the eliminated gear mechanism. The actuator dynamics and structural flexibilities become of increased importance, particularly when highspeed, high-accuracy operation of lightweight structures is aimed for; however, in most related literature, these effects are neglected to avoid increased computational complexity at the expense of compromising the tracking accuracy of the system in the transient and steady state. As a solution to the problem, in this study, high-order, sliding-mode (HOSM) controllers (HOSMCs) are developed, which exploit the robustness properties of sliding-mode controllers (SMCs) while also increasing accuracy by reducing chattering effects. Different from standard HOSMCs, which are derived by artificially increasing the system order, the third-order HOSM (3-HOSM) control laws in this study are derived by including the actuator dynamics and structural flexibilities in the control design process. Two HOSMCs are developed for this purpose: one with a discontinuous input and one with a continuous input aimed at reducing chattering. The performance of the novel HOSMCs is tested by simulations for the precise position and tracking control of a one-degree-of-freedom (IDOF) DD weapon-positioning system as an example of a direct-drive system under heavy uncertainties and external disturbances. The improved accuracy obtained, particularly with the novel continuous-input 3-HOSMC, motivates the implementation of the schemes for demanding control applications under heavy uncertainties.