Download Kinesthetic Displays for Remote and Virtual Environments

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
• Fo = Fa - Ff(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