This paper aims to develop a controller for a robot manipulator equipped with series elastic actuators (SEAs) to precisely track the desired link position coping with deflections from the intrinsic compliance. The well-known time-delay estimation (TDE) technique is modified for devising the new controller. In this paper, we first report that the conventional use of a constant gain matrix for the TDE framework is insufficient for high accuracy tracking because the tracking accuracy is significantly deteriorated and the closed-loop stability may be threatened. Accordingly, the new controller employs link inertia information with dynamic coupling terms to define the gain for TDE and then employs terminal sliding mode (TSM) control to enhance robustness and convergence speed. Particularly, the modified TDE is applied in the two-staged manner which enables to compensate the complicated nonlinear dynamics terms in SEA dynamics. The TSM synergistically amalgamates the accuracy, robustness, and convergence in tracking. The proposed controller is numerically validated by comparative experiments with a SEA-driven manipulator.

This study proposes an actuator whose design relies on a coordinated approach to control and hardware design: impedance control is supplemented with the introduction of a spring-damper coupler between the actuator and reference (ground). The coupler between the actuator and reference has the effect of further reducing the impedance apparent to the environment. Unlike traditional series elastic actuation, where the deformation of a physical spring coupler introduced between an actuator and link is fed back for control, our coupler is located between actuator and reference and its deformation is not fed back for control. The proposed control further employs time-delay estimation to account for the system dynamics that include both the physical and virtual couplers and to realize accurate and robust control over end-effector position and contact forces. We present a simple method for estimating interaction forces and regulating the contact force without a priori knowledge of the environment. A numerical simulation demonstrates the efficacy of the proposed approach.

This paper focuses on the development of a dynamic model-free whole-body controller for a humanoid robot with high kinematic redundancy. The proposed controller is based on force-level operational-space control framework, which computes joint torques for the required forces of prioritized multiple tasks. While typical approaches based on this framework require to obtain an accurate robot dynamics model, which has been generally recognized as a major hurdle to overcome for implementation in real humanoid robots, the proposed controller incorporates adaptive sliding-mode and online dynamics estimation schemes; thus, it can be easily realized on a humanoid without identifying complex robot dynamic parameters. As a result, the gains of the proposed controller are adaptively adjusted to assure the control accuracy, when the humanoid robot changes its posture and undergoes uncertain disturbances. Experiments with a 23-DoFs humanoid under uncertain disturbances verify that the proposed controller can robustly perform multiple tasks with high accuracy.

The need for safe human-robot interaction in emerging robotic applications has recently driven the development of new range of actuation systems spanning from variable stiffness drives to variable damping or full variable impedance joints. Concerning the provision of variable physical damping in compliant actuators, the main objective is to improve the control of compliant joint. In a particular class of these variable physical damping actuators (VPDAs), the level of generated damping is realized by creating a friction force which is actively controlled by piezoelectric actuators (PEAs). Therefore, to effectively control the damping output, the accurate force control of the PEAs is required. However, difficulties to precisely control the generated piezoelectric force stem from its highly nonlinear behavior such as hysteresis and creep effect. This paper presents a novel practical force tracking controller for PEAs with unknown hysteresis behavior. The proposed control consists of two elements: terminal sliding-mode based desired dynamics injection and model-free compensation for nonlinear dynamics of PEAs, which allow fast convergence and extraordinary robustness to the closed-loop system. The stability of the overall system is proved in the sense of Lyapunov. On the real prototype of VPDA, the proposed control scheme is experimentally verified and analyzed by comparison to other controllers, demonstrating improved force tracking performance of PEAs in VPDA system.

An accurate and robust impedance control technique is developed based on internal model control structure and time-delay estimation: the former injects desired impedance and corrects dynamics estimation error; the latter estimates and compensates the nonlinear dynamics of robot manipulators. Owing to the simple structure, the proposed control is designed without requiring entire dynamics computation or complex algorithms. The accuracy and robustness of the proposed control are verified using a two degrees of freedom robot with stiff wall simulation. The proposed control realizes desired impedance accurately compared with other competent controllers throughout the task, i.e., free motion, constrained motion, and transition between these motions. Further, proposed control realizes four sets of desired impedance accurately. Thus, accuracy and robustness of proposed control is confirmed.

This paper presents stability analysis and experimental results of the nonlinear bang-bang impact controller for robotic manipulators. A stability condition has been derived based on the analysis in Ln ∞ space and its physical interpretation has been given. The analysis shows that the stability of nonlinear bang-bang impact control depends on sampling time and the accuracy of inertia estimation. Stability is enhanced with the decrease of changes in the Coriolis, centrifugal, and disturbance forces. The stability condition is verified by simulations and experiments. Experiments show that overall performance is comparable to or better than existing impact control techniques which employ more complicated control strategies.