On the Feasibility of Multi-Degree-of-Freedom 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 dis-sipate 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 degrees-of-freedom (DOF) have not been examined. In this paper, we introduce a new analytical framework to evaluate the feasibility and performance of non-redundant 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 multi-DOF passive haptic devices.