NEUR3101 Lecture Notes - Lecture 19: Discrete Event Simulation, Median Nerve
The control of grasp stability entails both the prevention of accidental slips and the
prevention of excessive fingertip forces
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The slip ratio is the grip:load force ratio at which grip force is at critically low level so that
slip and possible loss of the grasped objects may occur. It is determined by friction
between the object and the skin. More slippery objects require stronger grip force and
thus slip ratio is larger
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The safety margin against slips is the difference between the grip:load force applied by a
subject and the corresponding slip ratio
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Control of the precision grip
Grip force and load force are scaled in parallel to maintain adequate safety margin
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Load force increase rate is scaled proportional to the load required for the object's lift-off.
Therefore the lift-off time is the same regardless of the object's weight and forces
required
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Grip force and load force increase slows down towards the time of lift-off to avoid
overshoots
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Adjustments to object properties
Load-grip force coupling
Reactive feedback control
Robot applies load force (unpredictable movement)
When a subject is instructed to hold onto an object to which a robot is applying a
sinusoidal load force, the grip force of the fingers is high to prevent slippage and lags
behind the increases in load force (phase shift)
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Robot applies unpredictable perturbation: reactive feedback control
Predictive feedforward control
Human subject control load force (planned movement)
When the subject actively moves object up and down, producing a similar load force, the
load force can be anticipated and thus the grip force is exactly as high as needed at every
point in time and tracks the load force without delay
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Human subject makes movement: predictive feedforward control
The grip force is modulated in anticipation of changes in load force generated by
active movements, regardless of the grip type and muscles involved
○
The tight temporal coupling between grip force and load force during object
transport reflects a general control strategy that is not specific to any particular grip
or mode of transport (motor equivalence)
○
Grip controller intelligently incorporates movements of the arm as well as whole
body (e.g. jumping)
○
In all grips, grip force is modulated in phase with fluctuations in load force that are
induced by the arm movement indicating anticipatory feedforward control strategy. Note
that feedback control would inevitably cause a phase shift
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Load-grip force coupling: a general control strategy
Tactile sensory information is essential for fine object manipulation
Fast adapting (FA) type I&II receptors detect dynamic changes○
Vibroreceptors 30HZ and 250 Hz
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Slowly adapting (SA) type I&II receptors; signal static stimulus features○
Stretch and Pressure
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Pain, temperature
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The role of fingertip tactile receptors in control of hand's ability to handle objects
Tungsten microneurography electrode inserted in a fascicle of human Median nerve
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Human microneurography recording single afferent activity
Sensorimotor control of dexterous manipulation
Monday, 11 June 2018 3:49 AM
Lecture 19 Page 1
Document Summary
The control of grasp stability entails both the prevention of accidental slips and the prevention of excessive fingertip forces. The slip ratio is the grip:load force ratio at which grip force is at critically low level so that slip and possible loss of the grasped objects may occur. It is determined by friction between the object and the skin. More slippery objects require stronger grip force and thus slip ratio is larger. The safety margin against slips is the difference between the grip:load force applied by a subject and the corresponding slip ratio. Grip force and load force are scaled in parallel to maintain adequate safety margin. Load force increase rate is scaled proportional to the load required for the object"s lift-off. Therefore the lift-off time is the same regardless of the object"s weight and forces required. Grip force and load force increase slows down towards the time of lift-off to avoid overshoots.
