Kinesthetic Displays for Remote and Virtual Environments

1. Kinesthetic Displays for Remote and Virtual Environments B. Hannaford and S. Venema Summarized by Geb Thomas

2. The Sense of Touch • Kinesthetic sense • movement or force in muscles and joints • Tactile sense • Nerves in skin for shapes and textures

3. History • Direct Mechanical Systems (Goertz) • Then remote, position controlled robots (which worked poorly)

4. Characteristics of Kinesthetic Channel • Two roles • Body position sense • Sensing and controlling contact with external environment • Bidirectional flow of energy • Rate of change of energy is: • Power = force*velocity

5. Position/Force Simultaneity • Force Feedback • Sense velocity (and/or position); apply force • Can’t control both force and velocity • To sense force, one would have to sense force in 3 directions (x, y, z) and 3 torques (roll, pitch, yaw)

6. Simulation • 2nd order linear system • Soft surfaces • Hard surfaces

7. Hard Surfaces • Update rate determines realism • Bandwidth depends on the operator and the contacted object. • At least in Audio frequencies • From other references, 1kHz is cited as a reasonable value

8. Physiology • Muscle is not a pure force generator or velocity generator. • Muscle spindles transduce muscle stretch and rate of stretch • Nonlinear • Principle source of body position information • Can be artificially stimulated with vibration • Golgi tendon organs encode muscle force • “Efferent copy” also encodes muscle force

9. Reference Frame • Vision and hearing are global reference frames • Kinesthetic sensation is perceived with respect to limbs -- body reference frame • Kinesthetic sensation is localized to the specific object

10. Contact modeling • Bond-graph method of Network theory • f1-z1(vp) - z2(vp) - f2 = 0 => • fz-z1(vp) - f2 + z2(vp) = fp • Operator and display are equally important • Can only control either force or velocity

11. Force feedback display • Sense velocity, apply force • uninhibited movement • accurately reproduce force • provide large forces for hard surfaces • high bandwidth • Same requirements for robot manipulators for contact force control

12. Displacement Feedback Displays • Sense force, impose controlled movement • Rigid enough to block movement • accurately reproduce displacements • provide for free movement • high bandwidth • Similar to robot manipulators for accurate trajectory following • More expensive because force sensors are expensive, position actuators are not available

13. Cross Modality Displays • Keeps feedback as information • Extra cognitive burden

14. Brakes • Exploratory • Constrain velocity to zero • Impossible to simulate contact with a surface not aligned with the main axes

15. Design Issues • Kinematics • Must be in constant contact with a moving operator • Share a common ground • Denavit-Hartenberg notation • Degrees of Freedom • Range of motion • Complexity

16. Singularity Analysis • One or more joints is at a motion limit • Workspace boundary singularity • Two or more joint axes become parallel • Workspace interior singularity • Jacobian matrix relates joint velocities (theta) to Cartesian velocities (V) • V = J(theta)* d(theta) • At singularities J become undefined and • d(theta) = J-1(theta)*V makes angle velocities go towards infinity

17. Dynamics • Fo = Fa - Ff(x,v) - M d(v) • Mass interferes with display velocity • Particularly complex with multidimensional systems

18. More Dynamics • Friction • Absorbs output force • Three types of friction • Static friction, resists onset of motion • Colomb, constant resistance to motion • Viscous, resists motion in proportion to velocity • Particularly important for slow motions • Stiffness • How the mechanism deforms under loads

19. Examples - Salibury, JPL

20. Examples -- Utah

21. Future Challenges • Hard Contact problem • Real-time dynamic modeling • Mechanism design