Haptics Laboratory Gianni Campion, Andrew H. Gosline, and Vincent Hayward Haptics Laboratory, Center for Intelligent Machines McGill University Montréal, Québec, Canada Initial Results using Eddy Current Brakes as Fast Turn-on, Programmable, Physical Dampers for Haptic Rendering

Haptics Laboratory Motivation Physical damping is required for passivity [Colgate and Schenkel, 1994] Sources of physical damping: 1.Dry Friction 2.Viscosity 3.Electromagnetic Dissipation is accidental byproduct of design. Difficult to model and not controllable.

Haptics Laboratory Prior Work Shunting a DC motor creates electrical damping. [Mehling and Colgate, 2005] –Frequency dependent, but not programmable, and limited to back EMF of DC motor. Magnetorheological (MR) particle brakes are programmable –Nonlinear, slow to actuate, and suffer from hysteresis [An and Kwon, 2004], [Gogola and Goldfarb, 1999], [Arcy, 1996]

Haptics Laboratory Proposal Eddy current brakes are: 1.Controllable 2.Fast turn-on 3.Linear 4.Friction free 5.Inexpensive Add eddy current brake to each driven joint Create multi DOF hybrid device

Haptics Laboratory Eddy Current Brakes Move a conductor through a magnetic field, get dissipative resistance. Currents generated according to the Lorentz Force Law. Energy is dissipated by the Joule Effect. Do not use contact. Inductance/V cc determines max update rate.

Haptics Laboratory Eddy Current Brake Physics Induced current density: |J| =  R  |B| Power dissipated: P d = 0.25  D 2 dB 2 R 2  2 Resistive torque:  d = 0.25  D 2 dB 2 R 2  [Lee and Park, 1999] dd R D

Haptics Laboratory Prototype Haptic Device Concentric aluminum blade added to each base joint of Pantograph. Toroidal electromagnet cores machined from iron and wrapped with 24g enamel coated magnet wire. Magnets driven in current mode by AMC 20A20 PWM servoamplifiers can achieve approx 500Hz on/off freq.

Haptics Laboratory Rendering Results - Wall Manipulandum thrust and held against virtual wall by elastic band. Dampers on during wall penetration. Limit cycles quenched or reduced.

Haptics Laboratory Rendering Results - Friction Friction model by Hayward and Armstrong, 2000, is prone to limit cycles in elastic stuck region. Dampers used in stuck state. Limit cycles quenched

Haptics Laboratory Limitations Damper blades add considerable inertia to the device. Considerable power is required to generate damping torque. Large ‘C’ shaped magnets flex and release under electromagnetic force, generating vibrations that are both audible and palpable. Damping is not homogeneous through the workspace.

Haptics Laboratory Conclusions Physical damping from eddy current brakes can stabilize renderings of virtual walls and friction that were unstable without it. Future Work Optimize design for both electromagnetic and dynamic performance Verify linearity of damping Further explore programmable damping in future control/passivity experiments