Impact of Kinematic Structure on the Force Displayability of Planar Passive Haptic Devices

Haptic devices containing passive actuators, such as controllable brakes or dampers, are an attractive alternative to their motor-driven counterparts due to intrinsic stability and improved impedance bandwidth. Passive actuators cannot generate energy and, therefore, the output force can only oppose the applied velocity. In the same way the kinematic structure of traditional manipulators is designed to maximize dexterity and manipulability, one must consider adapting the device topology to optimize force displayability when designing passive actuators. This article introduces a set of metrics to evaluate and compare the performance of 2-degree-of-freedom (DOF) serial and parallel passive haptic devices. These metrics consider the impact of the kinematic structure on the force displayability according to the directions of the device velocity and desired force. Applying these metrics to nine manipulators revealed that: 1.) serial manipulators can generate passive forces in all directions equally regardless of the link length ratio; 2.) the base link length of five-bar parallel manipulators strongly influences passive force displayability; 3.) five-bar parallel manipulators with the base link length of zero can generate the widest range of passive forces when all links have the same length. The novel performance metrics presented in this paper can aid in the design of 2-DOF passive haptic devices.