Haptics Laboratory

Gianni Campion, Andrew H. Gosline, and Vincent Hayward

Haptics Laboratory, Center for Intelligent MachinesMcGill University

Montréal, Québec, Canada{champ,andrewg,hayward}@cim.mcgill.ca

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/Vcc determines max update rate.

Haptics Laboratory

Eddy Current Brake Physics

Induced current density:

|J| = R|B|

Power dissipated:

Pd = 0.25D2dB2R22

Resistive torque:

d= 0.25D2dB2R2[Lee and Park, 1999]

d

RD

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.

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Rendering Results - Wall

• Manipulandum thrust and held against virtual wall by elastic band .

• Dampers on during wall penetration.

• Limit cycles quenched or reduced.

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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

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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