UBC Theses and Dissertations
Mechanism emulation with a magnetically levitated input/output device Vlaar, Tim
The emulation of stiff walls and stick-slip friction has been developed. A high performance, six degree-of-freedom , fine motion, magnetically levitated input/output device was used in the implementation. For the stiff wall emulation, two control algorithms were developed from the commonly used mass-spring-damper model: an observer-based design, and a braking pulse design. The mass-spring-damper model is limited in its achievable stiffness due to instability. Both control methods increased perceived stiffness substantially (stably) for one degree-offreedom. The braking pulse method for stiff wall emulation was extended to a six degree-of-freedom case in which a small cube is constrained within a larger cube. The contact forces and torques between the mobile inner cube and outer constraining cube are felt by the operator. A further extension was to a larger workspace which was developed by mounting the fine motion device on the end-effector of a coarse motion, six degree-of-freedom, elbow manipulator with a spherical wrist. The manipulator was controlled in rate mode by the fine motion device. Experiments have been designed to test the position at which a subject feels the stiff wall, and the completion time for a subject to move from an unconstrained environment to the stiff wall. For stick-slip friction, a modified Karnopp model was implemented using a Position Derivative (PD) controller within the stick friction threshold. Even though the PD controller allows some motion during the stick phase, the haptic feedback provided is remarkably similar to stick-slip friction.
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