The new areas of technical exploitation of robotics systems has recently set new trends for the robotic actuation by demanding more versatile systems which can cope with unpredictable interactions within not well defined environments and work in close vicinity with the human. Following these trends, this project focuses on the development of novel var1iable impedance actuation systems with embodied characteristics such as passive compliance and damping. Although the superior performance of compliant actuation prototypes with respect to safety and efficiency have been successfully demonstrated, the introduced compliance in the joint drive train forms a limiting factor to the tracking performance that can be achieved by the control system.
Fig. 1 – The first prototype of a Variable Physical Damping Actuator (VPDA). A mass spring system formed by the torsion spring and the outer link is used to emulate the joint compliance and induce suitable oscillations as required for the evaluation of the damper performance.
The compliance embedded in these actuators introduces dynamics which can induce oscillations with a frequency that depend on the stiffness of the joint and the inertia of the link. This complicates the dynamics of the system, especially if a multiple DOF system is considered resulting in a system which is difficult to control. In fact, the poles introduced by compliance define an upper bound for the bandwidth of the closed loop plant at about 1/3 of the natural frequency to guarantee enough damping in the controlled system. To cope with this, a Variable Physical Damping Actuator (VPDA) unit has been developed, Figs. 1, 2. Apart from facilitating the control by regulating the oscillations induced by passive compliance the VPDA unit can assist in managing the energy transfer from/to the compliant module. The introduction of the VPDA module is motivated by the fact that variable damping could be required to deal with configuration and/or load variations, furthermore such a system cannot deteriorate the inherent safety property of the compliant joints because it can be regulated and completely disconnected when required. The use of compact and lightweight piezoelectric actuators for the development of the VPDA facilitates the integration of this semi-active friction damping system within compliant actuators, such as the SEA presented in, with the final target of developing compact compliant actuators (CompAct™) with Variable Physical Damping, Fig. 2.
Fig. 2 – The CompAct™ actuator is essentially a series elastic actuator which has the ability of varying the physical damping in parallel to the joint compliance and comes from the integration of the VPDA, within the CompAct™ SEA.
The CompAct™ actuator shown in Fig. 2 has been formatted as in layouts in Fig. 3a for developing two actuation modules configurations which can be interconnected arbitrarily to develop compliant robots with variable physical damping, e.g. the CompAct™ arm.
- M. Laffranchi, N. G. Tsagarakis and D. G. Caldwell, “Antagonistic and Series Elastic Actuators: a Comparative Analysis on the Energy Consumption,” in International Conference on Intelligent Robots and Systems, St. Louis, 2009.
- M. Laffranchi, N. G. Tsagarakis and D. G. Caldwell, “A Variable Physical Damping Actuator (VPDA) for Compliant Robotic Joints,” in International Conference on Robotics and Automation, Anchorage, Alaska, 2010.
- N. G. Tsagarakis, M. Laffranchi, B. Vanderborght and D. G. Caldwell, “A Compact Soft Actuator for Small Scale Robotic Systems,” in International Conference on Robotics and Automation, Kobe, Japan, 2009.
- M. Laffranchi, N. G. Tsagarakis and D. G. Caldwell, “A Compact Compliant Actuator (CompActTM) with Variable Physical Damping,” in International Conference on Robotics and Automation (ICRA), Shanghai, China, 2011.