This paper focuses on the design, modeling and basic control of a variable stiffness actuator to be used in combination with a regenerative electromechanical drive system. Due to the use of a flexible beam, the actuator has the ability to store and return elastic potential energy. Also, an ultracapacitor is used in the electromechanical drive, which allows electrical energy storage and return. Moreover, elastic and electrostatic energies can be exchanged, resulting in a highly efficient and lightweight design which will be beneficial for robotic prostheses, exoskeletons and other orthotic devices. The paper presents a model and calculation method for large beam deflections and the integrated electromechanical actuator model. A semiactive virtual control strategy is used to decouple the mechanical dynamics from the charge dynamics and achieve position control of the actuator. Simulation results are presented to illustrate the control system and the energy exchange features.

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