On the Feasibility of MultiDegreeofFreedom Haptic Devices Using Passive Actuators Stability and transparency are key design requirements in haptic devices. Transparency can be significantly improved by replacing conventional electric motors with passive actuators such as brakes or dampers. Passive actuators can display a wide range of impedance and since they can only dissipate energy, stability is guaranteed. However, passive haptic devices suffer from a serious drawback; the direction of the force output is difficult to control. This issue was addressed extensively for planar manipulators but devices with higher degreesoffreedom (DOF) have not been examined. In this paper, we introduce a new analytical framework to evaluate the feasibility and performance of nonredundant passive haptic manipulators with any DOF. The method identifies different regions in the workspace where a force can be created or approximated, and regions where a passive system cannot create force at all for a given user input. The results indicate that the range of forces a passive device can display increases with the number of DOF. This framework can aid in the design of control methods for multiDOF passive haptic devices.

Relaxing the Conservatism of Passivity Condition for Impedance Controlled Series Elastic Actuators This paper proposes a practical and less conservative passivity analysis for series elastic actuators (SEAs) by introducing load port definition and shows that the achievable stiffness by the impedance control of SEA can be set higher than the inherent stiffness of SEA depending on the condition of the load dynamics. Since SEA can inherently measure or estimate a transmitted force thanks to its embedded spring element, impedance control is often exploited to render compliant behaviors related between the motion and the force. Although the stability of the SEA control system is of great importance, the conventional passivity analysis gives conservative criteria, and indeed limits the actual actuator performance. To tackle the conservatism of the conventional passivity in SEAs, we first explore the dynamic characteristics of SEA including load dynamics, which has been ignored for the sake of simplicity of the passivity analysis by excluding uncertain load dynamics. The inclusion of the load dynamics into the passivity analysis allows us to properly derive the less conservative limit of achievable stiffness by impedance control and the factors that determine the limit. The proposed analysis is verified by numerical simulations and applied to a passivity observer design for experimental validation on an actual SEA setup.