Control and low-level rendering issues in haptic interfaces

Tim SalcudeanUniversity of British Columbia

Department of Electrical and Computer Engineering

Vancouver, Canada

http://www.ece.ubc.ca/~tims

IMA Workshop 9: Haptics, Virtual Reality, and Human Computer Interaction, June 14-15, 2001

REFERENCES WILL BE INSERTED LATER -

PLS SEE References.pdf FOR A LIST

Outline

• Contributors: S.P. DiMaio, K. Hashtrudi-Zaad, M. R. Sirouspour, W.H. Zhu

• List of references will be provided

• introduction and motivation• network model of haptic interaction• stability, performance specifications• controller design approaches and challenges• modelling with hybrid systems• discussion

Introduction/motivation

Some haptic interfaces designed in our lab:• 3-DOF planar device• 5-DOF haptic pen• 6-DOF maglev interface

Introduction/motivationHaptic interface control important:• human hand characteristics difficult to match

– tradeoffs? Performance optimization?

• provides means for psychophysics studies• provides guidelines for design

Studies of:• specific rendering problems• general methodology for coupling user to dynamic

simulator

Network model of haptic interaction

Network model of haptic interaction

Network model of haptic interaction

Stability

Stability

Stability

Stability

Stability

Passivity condition:

Stability

Absolute stabilityconditions are satisfied.

Performance

Tradeoff example

With computational of transmission delay:

hybrid matrix becomes:

Absolute stability condition:

Controller design• Two channel architectures proposed (e.g. impedance display, admittance simulations).

• Four channel architectures allow better tradeoff exploration - Haptic interface behaves as a force or a position sensor

Controller design

• standard linear loop shaping with nominal stability, passivityat the environment• adaptive control• dual hybrid teleoperation• Lyapunov-based methods

• kinematic scaling, impedance shaping possible• velocity control mode possible• force sensing may be replaced by observers

Controller designExperiments with a 3-DOF planar haptic device:

Controller designImpedance simulation: - slave tracks position, force returned

Controller designFully transparent four-channel:- force feedforward, PD controllers for position correspondence

Controller designFour-channel with adaptive damping: - damping proportional to environment force amplitude

Controller designAbsolutely stable four-channel:- reduced force channels to satisfy absolute stability criterion

Adaptive Teleoperation Controller•Adaptive motion/force control of master and slave robots [Zhu ‘97]

,)~

( 2 LLFKXX fd

h, (master)e, (slave)

{

hpfehpee

phd

hfehphped

FKKFAXKXXK

X

FAKXXKXKX

~~~~1

~~~

where

fKpK

w~

-- motion scaling parameter-- force scaling parameter

-- filtered by a first-order filter w

[Anderson 89, Lawrence 92, Yokokohji 92, Colgate 93]

Adaptive Teleoperation Controller

• transparency:

LLXXK

LLXXK

ehp

ehp

2

2

AC

s

K

KZ

K

KZ

f

pe

f

ph

Assuming {e,h} 2nd order LTI systems + the usual on robots, we have:• stability:

Adaptive Teleoperation Controller

Video ...

Adaptive Teleoperation Controller

Hybrid system modelConsider stick-slip friction model:

In practice, implemented as:

Hybrid system model

• video...

Hybrid system model

Hybrid system model

Discussion / Challenges• teleoperation models and controllers applied to haptics• passivity/absolute stability allows modular software• controller analysis is now well understood for linear

and some nonlinear models

• controller synthesis difficult even for simple linear models• adaptive controllers complex and difficult to implement• stability/performance issues not understood for changing

environments

Discussion / Challenges• low level programming is challenging

– need model covering a wide range of behaviours in a (provably?) stable manner