Natural Admittance Research Articles

An admittance shaping controller for exoskeleton assistance of the lower extremities

We present a method for lower-limb exoskeleton control that defines assistance as a desired dynamic response for the human leg. Wearing the exoskeleton can be seen as replacing the leg's natural admittance with the equivalent admittance of the coupled system. The control goal is to make the leg obey an admittance model defined by target values of natural frequency, peak magnitude and zero-frequency response. No estimation of muscle torques or motion intent is necessary. Instead, the controller scales up the coupled system's sensitivity transfer function by means of a compensator employing positive feedback. This approach increases the leg's mobility and makes the exoskeleton an active device capable of performing net positive work on the limb. Although positive feedback is usually considered destabilizing, here performance and robust stability are successfully achieved through a constrained optimization that maximizes the system's gain margins while ensuring the desired location of its dominant poles.

Closed-Form Solution to Controller Design for Human-Robot Interaction

This paper deals with controller design for gentle physical human-robot interaction. Two objectives are set up. The first is to establish an analytical framework in order to justify the good features of state of the art controller, recently designed by numerical search of parameter space. The second is to investigate the possibilities to improve the performance of such controller. Our method ensures “prescribed” admittance behavior of the robot, similar to natural admittance controller design but with both more realistic model of the robot and more realistic target admittance. Joining natural admittance approach with the concept of complementary stability allows reaping the benefits of both. Limited knowledge about the environment via structured uncertainty allows a very simple worst-case analysis using elementary tools such as Routh–Hurwitz stability criterion. Consequent relation within the parameters determines an allowed region in the parameter space, where the contact stability is guaranteed. Not surprisingly, on one border of this region, the system behaves exactly the same as when the state of the art controller is employed. In addition, unexpected stability regions are discovered, suggesting theoretical performance improvements.

Stable interaction control and coulomb friction compensation using natural admittance control

AbstractA new control system design formulation is presented for achieving high‐performance, guaranteed‐stable impedance control. While the bandwidth of the resulting controller is no higher than alternative techniques, the new formulation significantly improves performance when Coulomb friction is present in the system. The technique requires a careful choice of the target impedance. The resulting feedback compensators are causal and have stable poles, although they are often non‐minimum phase. General rules for controller synthesis are derived. Experimental performance results are presented for a two‐mass system with internal compliance and Coulomb friction. Results demonstrate that the technique is successful in rejecting internal friction force disturbances while maintaining a passive driving‐point impedance. © 1994 John Wiley & Sons, Inc.