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

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

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

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

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)

Simulation 2nd order linear system Soft surfaces Hard surfaces

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

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

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

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

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

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

Cross Modality Displays Keeps feedback as information Extra cognitive burden

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

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

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

Dynamics F o = F a - F f (x,v) - M d(v) Mass interferes with display velocity –Particularly complex with multidimensional systems

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

Examples - Salibury, JPL

Examples -- Utah

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