A globally stable state feedback controller for flexible joint robots

The paper addresses the problem of controlling the joints of a flexible joint robot with a state feedback controller and proposes a gradual way of extending such a controller towards the complete decoupling of the robot dynamics. The global asymptotic stability for the state feedback controller with gravity compensation is proven, followed by some theoretical remarks on its passivity properties. By proper parameterization, the proposed controller structure can implement a position, a stiffness or a torque controller. Experimental results on the DLR lightweight robots validate the method.     

Stability analysis and robust control for fixed-scale teleoperation

In this article we present a new theorem that reduces the stability problem of teleoperators from a structured singular value problem to a maximum singular value problem. A control scheme realizing an ideal response for fixed-scale teleoperation is proposed and its stability is proven analytically. Next, the ideal control scheme is extended to control schemes with a higher stability robustness. Also, for these more complicated schemes, stability analyses are performed making use of the new theorem and stability conditions are derived. Experiments on a 1-d.o.f. setup are conducted to confirm the validity of the proposed control schemes.     

Initial experiments on a leg mechanism with a flexible geared joint and footpad

In the present work, a leg mechanism together with a flexible joint system is designed and constructed. The leg mechanism is expected to walk on uneven terrain and be subject to certain impact or contact forces. The effect of the designed flexible gear system and footpad on shock absorption is a main concern. The joint torque of the motor gear and the contact force of the foot are analyzed with different control schemes. The control performance of the leg movement by these methods is also presented. The proposed control methodology could next be applied to a newly designed two-link leg mechanism. It is hoped that the present study can be extended to an exoskeleton leg mechanism that is being developed.     

Performance analysis of an electrohydraulic impedance controller for robotic interaction control

Performance of an electrohydraulic impedance controller, developed in the physical domain, for controlling the contact forces during robotic interaction tasks is analyzed in this paper. The impedance controller is capable of varying the mechanical impedance of the manipulator during an interaction task and thus controls the interaction force. An electrohydraulic servo actuator, appended to a 1-d.o.f. manipulator arm, acts as the controller in the physical domain. The controller performance depends on the physical parameters such as size of the actuator, bulk modulus of the oil, etc., of the hydraulic system. The influence of these physical parameters on the controller performance is analyzed through frequency analysis and acceptable ranges of values for these parameters are determined. Controller sensitivity, stability of the constrained manipulator system and its frequency response are analyzed theoretically and results are presented. The limitation of application of the controller for practical applications is also analyzed and the results are presented.     

Design, analysis and experimental validation of a robust nonlinear path following controller for marine surface vessels

The problem of path following for marine surface vessels using the rudder angle is addressed in this paper. A four-degree-of-freedom nonlinear surface vessel model, together with the Serret-Frenet equations, is introduced to describe the ship dynamics and path following error dynamics. While similar models have been used and reported in the literature for path following control algorithm development, the novelty of the approach presented in this work lies in the following aspects. (a) The back-stepping nonlinear controller design is based on feedback dominance, instead of feedback linearization and nonlinearity cancelation; (b) additional design parameters are employed in the Lyapunov function that lead to simplification of the controller in the design procedure and normalization of different variables in the Lyapunov function to improve the controller performance; (c) relying on feedback dominance and the introduction of the additional parameters in the Lyapunov function, the resulting controller is almost linear, with very benign nonlinearities allowing for analysis and evaluation; and (d) the performance of the nonlinear controller, in terms of path following, is analyzed for robustness in the presence of model uncertainties. The simulation results are presented to verify and illustrate the analytic development and the effectiveness of the resulting control against rudder saturation and rate limits, and delays in the control execution, as well as measurement noises. Furthermore, the control design is validated by experimental results conducted in a tank using a model ship.