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Control of Adjustable Compliant Actuators

Philips Chair for Medical Information Technology, RWTH Aachen University, Pauwelsstrasse 20, Aachen 52074, Germany
Author to whom correspondence should be addressed.
Machines 2014, 2(2), 134-157;
Received: 28 February 2014 / Revised: 6 May 2014 / Accepted: 15 May 2014 / Published: 20 May 2014
(This article belongs to the Special Issue Advances in Control Engineering)
PDF [2233 KB, uploaded 20 May 2014]


Adjustable compliance or variable stiffness actuators comprise an additional element to elastically decouple the actuator from the load and are increasingly applied to human-centered robotic systems. The advantages of such actuators are of paramount importance in rehabilitation robotics, where requirements demand safe interaction between the therapy system and the patient. Compliant actuator systems enable the minimization of large contact forces arising, for example, from muscular spasticity and have the ability to periodically store and release energy in cyclic movements. In order to overcome the loss of bandwidth introduced by the elastic element and to guarantee a higher range in force/torque generation, new actuator designs consider variable or nonlinear stiffness elements, respectively. These components cannot only be adapted to the walking speed or the patient condition, but also entail additional challenges for feedback control. This paper introduces a novel design method for an impedance-based controller that fulfills the control objectives and compares the performance and robustness to a classical cascaded control approach. The new procedure is developed using a non-standard positive-real Η2 controller design and is applied to a loop-shaping approach. Robust norm optimal controllers are designed with regard to the passivity of the actuator load-impedance transfer function and the servo control problem. Classical cascaded and positive-real Η2 controller designs are validated and compared in simulations and in a test bench using a passive elastic element of varying stiffness. View Full-Text
Keywords: compliant actuator; adjustable stiffness; impedance control; rehabilitation robotics; robust control compliant actuator; adjustable stiffness; impedance control; rehabilitation robotics; robust control

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This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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MDPI and ACS Style

Misgeld, B.J.; Gerlach-Hahn, K.; Rüschen, D.; Pomprapa, A.; Leonhardt, S. Control of Adjustable Compliant Actuators. Machines 2014, 2, 134-157.

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