Nonlinear combined SMC-SDRE control versus SMC and SDRE approaches for electrical flexible-joint robots based on optimal observer

This paper is concerned with the problem of designing and comparing three observer-based control schemes for Electrical Flexible-Joint Robots (EFJRs). Considering electrical subsystem of the actuator as well as its mechanical subsystem in the Flexible- Joint Robot’s (FJR’s) dynamic model leads to a more complete and realistic model. However, due to extremely nonlinear complicated multivariable nature of the system, controller design becomes a non-trivial challenge. Moreover, the manipulator works in the presence of uncertainty in the parameters and external disturbances in the workspace. To cope with these problems, a Sliding Mode Controller (SMC) and a State-Dependent Riccati Equation (SDRE)-based controller are designed both in conjunction with an SDRE-observer. However, SMC suffers from chattering and SDRE needs to improvement in robustness characteristic. Hence, in order to benefit from both of these methods and compensate their weaknesses, a combination of SDRE and SMC is applied to EFJR in such a way that SDRE is employed to design the sliding surface. Simulation results verified the superiority of the observer-based combined SDRE-SMC versus the other two methods in terms of error reduction, improved robustness and smooth small control input.