This paper features a novel sliding mode controller for robotic arms using nonlinear model-based switching functions. The new controller is experimentally validated on a 7-DOF exoskeleton arm used for upper-limb rehabilitation applications. The proposed approach features a novel concept using model-based switching functions in the sliding mode controller, which leads to considerable simplifications on the torque control inputs. Compared to conventional linear switching functions, model-based switching functions show substantial control performance improvements on the torque inputs, such as transient constraints reduction and enhanced robustness, while maintaining a very good tracking performance. Moreover, model-based switching functions design ensures a complete decoupling of chattering effect between joint axes. Furthermore, this approach can be combined with existing chattering reduction techniques to ensure proper control of chattering levels on the torque inputs. These advantages make the practical implementation of the model-based switching functions approach particularly desirable for wearable robotics, where smooth movements and high accuracy are important requirements for patients' comfort and security.

This paper proposes a robust nonlinear current controller that deals with the problem of the stator current control of a six-phase induction motor drive. The current control is performed by using a state-space representation of the system, explicitly considering the unmeasurable states, uncertainties and external disturbances. To estimate these latter effectively, a time delay estimation technique is used. The proposed control architecture consists of inner and outer loops. The inner current control loop is based on a robust discrete-time sliding mode controller combined with a time delay estimation method. As said before, the objective of the time delay estimation is to reconstruct the unmeasurable states and uncertainties, while the sliding mode aims is to suppress the estimation error, to ensure robustness and finite-time convergence of the stator currents to their desired references. The outer loop is based on a proportional-integral controller to control the speed. The stability of the current closed-loop system is proven by establishing sufficient conditions on the switching gains. Experimental work has been conducted to verify the performance and the effectiveness of the proposed robust control scheme for a six-phase induction motor drive. The results obtained have shown that the proposed method allows good performances in terms of current tracking, in their corresponding planes.

In this paper, the problem of high-accuracy stator currents tracking of a six-phase induction motor in the presence of perturbations and unmeasurable rotor currents is tackled. Since the good features offered by sliding mode theory motivate the community of researchers on control, a time delay estimation based discrete-time super-twisting controller is proposed. First of all, an outer loop is performed to regulate the speed and to construct the desired stator currents. Then, the inner loop, based on an indirect rotor field-oriented control, is performed based on the proposed method. The proposed structure allows an accurate and simple estimation of uncertainties and rotor currents, a good tracking, a fast convergence of the currents tracking error to a neighbour of zero. The design procedure and the stability analysis are detailed for the current closed-loop system. Experimental work was carried out on a six-phase induction motor to demonstrate the effectiveness of the developed discrete approach. In addition, the performances obtained are compared to the ones obtained using the discrete-time sliding mode with time delay estimation. The results obtained highlighted the satisfactory stator currents tracking performance in steady-state and transient conditions and under different sampling times, parameters mismatch and with load and no-load conditions.