Identification of Human Grasp Dynamics and the Effects of Displacement Quantization and Zero-Order Hold on the Limit Cycle Behavior of Haptic Knobs Doctoral Dissertation Defense Christopher J. Hasser November 19, 2001
Dec 22, 2015
Identification of Human Grasp Dynamics and the Effects of Displacement Quantization and
Zero-Order Hold on the Limit Cycle Behavior of Haptic Knobs
Doctoral Dissertation Defense
Christopher J. Hasser
November 19, 2001
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Reading Committee
J. Kenneth Salisbury
Mark R. CutkoskyJ. Christian Gerdes
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Acknowledgements
• Stanford faculty and staff
• Immersion Corporation
• Haptic research community
• Fellow students
• Family
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Haptic
Greek origin – “of or pertaining to the sense of touch”
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Common Haptic System Architecture
Illustration: Immersion Corporation
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Haptic Knobs
Illustrations: BMW/ Immersion Corporation
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Nissan Concept
Haptic Scroll Wheel in Nissan Concept Car
Close-up of Haptic Scroll Wheel
Illustrations: Nissan/ Immersion Corporation
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• Often occur during contact with a virtual barrier
• Distracting, unacceptable user experience• Relevant factors:
– Zero-order hold delays– Displacement signal– Velocity signal– Physical damping– Virtual barrier stiffness
Limit Cycle Oscillations
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Goal
Understand the effect of displacement quantization on limit cycle oscillations in sampled data haptic systems.
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Approach
1. Identify the dynamics of the human hand grasping a haptic knob
2. Model and simulate the effects of displacement quantization
3. Analyze using nonlinear control theory
4. Empirically confirm simulation and theory
5. Discuss effect origins and design implications
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Why Simulate?
• Easily observable, repeatable conditions
• Precise control over experiment parameters
• Physically impossible configurations
• Analysis of hardware yet to be constructed
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EE Student to EE Professor:
“But how do you *get* the plant model?”
EE Professor:
“You hire a mechanical engineer.”
Why System Identificaton?
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Why System Identificaton?
• Simulation requires a plant model
• Two choices for obtaining model:– Analytic construction– System identification
• System identification most attractive for complex human hand under well-constrained conditions
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Apparatus
Design and drawing: B. Schena
• For system ID and simulation verification
• 25 mm brushed DC motor
• Knob with grip force load cell
• 640,000 count per revolution optical encoder
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Pinch Grasp
• Nine subjects – five male, four female• Subject squeezed knob slowly• 20 ms torque pulse applied when grip force reached
threshold
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Second-Order Lumped Parameter Model
finger finger, knob, & motor rotor
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Torque, Acceleration, Velocity, and Displacement
Input Torque (upper left), Acceleration (upper right)Velocity (lower left), and Displacement (lower right)
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Torque Contributions and Model Check
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Model Performance
Pulse (Step) Responses for Various Grip Forces
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Results Across All Subjects
Moment of Inertia (J), Damping (B), Stiffness (K), and Damping Ratio (ζ)
J
K
B
ζ
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Fourth-Order Model
Block Diagramfinger fingerpad/knob/motor
• Fourth-order model explains moment of inertia variation at high grip forces
• Low grip forces are the most interesting for studying chatter
• Details in dissertation
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Other Grasp Postures
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1. Identify the dynamics of the human hand grasping a haptic knob
2. Model and simulate the effects of displacement quantization
3. Analyze using nonlinear control theory
4. Empirically confirm simulation and theory
5. Discuss effect origins and design implications
Approach
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Finger/Manipulandum/Wall Model
Gillespie's Model of a Finger/Manipulandum Contacting a Virtual Wall (from Gillespie, 1996)
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Block Diagram
Gillespie and Cutkosky, 1996
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Energy Leaks
Plot of modeled manipulandum position and control effort (from Gillespie and Cutkosky, 1996).
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Encoder Quantization
Continuous-Time Simulation with Encoder Displacement Quantization
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Simulation with Hand Stiffness and Damping
Simulation of Hand Lightly Pressing Knob Against Stiff Virtual Wall, with Lines Fitted to Steady State Peaks and Troughs to Measure Limit Cycle Magnitude (2000 Hz, 8192 encoder counts/revolution)
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Simulation with Hand Stiffness and Damping
Oscillation Magnitude as a Function of Sample Rate and Displacement Resolution (Log Magnitude for Growth Rate)
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Simulation with Hand Stiffness and Damping
Peak-to-Peak Oscillation Magnitude, Expressed in Units of Encoder Counts
Unsaturated Saturated
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Oscillation Frequency
Oscillation Frequency as a Function of Sample Rate and Displacement Resolution
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Summary of Simulation Results
• Displacement quantization possesses no inherent energy leak
• Limit cycle magnitude scales directly with displacement quantization and ZOH delay
• Limit cycle frequency relatively unaffected by displacement quantization but sharply affected by ZOH delay
• For great majority of cases, limit cycle oscillations are smaller than ±1 encoder count
33System ID Simulation Theory Hardware Discussion
1. Identify the dynamics of the human hand grasping a haptic knob
2. Model and simulate the effects of displacement quantization
3. Analyze using nonlinear control theory
4. Empirically confirm simulation and theory
5. Discuss effect origins and design implications
Approach
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Describing Function Analysis
Assumptions:• Single nonlinear element• Nonlinear element is time-invariant• Linear component has low-pass properties• Nonlinearity is odd
Describing Function: The ratio of the fundamental component of the nonlinear element to the input sinusoid
Slotine & Li, 1991 Slotine & Li, 1991
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Describing Function Analysis
Nyquist PlotRelay nonlinearity
Slotine & Li, 1991
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Describing Function Analysis
Nyquist Plot with Describing Function at Various Phase Delays
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DFA Results-- Amplitude --
Oscillation Magnitude as a Function of Sample Rate and Displacement Resolution
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DFA Compared to Simulation-- Amplitude --
Oscillation Magnitude as a Function of Sample Rate and Displacement Resolution
Oscillation Magnitude as a Function of Sample Rate and Displacement Resolution
DFA Simulation
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• Mean: -54%• Std. Dev.:
±15%• Range:
-75% to -17%
Difference Between DFA and Simulation Magnitudes as a Percentage of Simulation Magnitudes
DFA Compared to Simulation-- Amplitude --
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DFA Results-- Frequency --
Oscillation Frequency as a Function of Sample Rate and Displacement Resolution
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DFA Compared to Simulation-- Frequency --
Oscillation Frequency as a Function of Sample Rate and Displacement Resolution
Oscillation Frequency as a Function of Sample Rate and Displacement Resolution
DFA Simulation
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• Mean: 4%• Std. Dev.:
±14%• Range:
-21% to +30%
Difference Between DFA and Simulation Frequencies as a Percentage of Simulation Frequencies
DFA Compared to Simulation-- Frequency --
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Summary of Describing Function Results
• Relay nonlinearity with phase delay provides good approximation of quantized displacement with ZOH delay
• DFA does excellent job of predicting magnitude and frequency sensitivities
• DFA underestimates simulated oscillation magnitude, but provides close prediction of simulated oscillation frequency
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1. Identify the dynamics of the human hand grasping a haptic knob
2. Model and simulate the effects of displacement quantization
3. Analyze using nonlinear control theory
4. Empirically confirm simulation and theory
5. Discuss effect origins and design implications
Approach
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Hardware Testing
Limit Cycle Oscillations for Various Encoder Resolutions and Sample Rates
Wor
seni
ngE
ncod
erR
esol
utio
n
WorseningSample Rate
455 Hz 1 kHz 2 kHz 5 kHz
256 cts/rev
512 cts/rev
1024 cts/rev
2048 cts/rev
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Hardware Testing- Amplitude Results -
Oscillation Magnitude as a Function of Sample Rate and Displacement Resolution
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Hardware Testing - Frequency Results -
Oscillation Frequency as a Function of Sample Rate and Displacement Resolution
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Hardware Tests Compared to Simulation (Frequency)
Oscillation Frequency as a Function of Sample Rate and Displacement Resolution
Oscillation Frequency as a Function of Sample Rate and Displacement Resolution
Hardware Simulation
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Summary of Hardware Testing Results
• Simulations, approximation, and analysis provide reasonable predictions of amplitude sensitivities
• Hardware oscillation frequencies deviate from simulation and analytic predictions
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1. Identify the dynamics of the human hand grasping a haptic knob
2. Model and simulate the effects of displacement quantization
3. Analyze using nonlinear control theory
4. Empirically confirm simulation and theory
5. Discuss effect origins and design implications
Approach
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Displacement Quantization Effect Explained
Illustration of Barrier Penetration and Resultant Torque Outputs for a Traditional ZOH System and a ZOH System with Displacement Quantization
resolutionsample rate
Oscillation Magnitude
2
1
)()(t
t
errorleak dtttTE
)()(1
kkTEN
kerrorleak
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Amplitude Approximation
Simulation Results Predictions
Hardware Results Predictions
)sin( tAt
tCA
For limit cycles of form:
Approximate amplitude:
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Potential Limit Cycle Mitigation Approaches
• Increase displacement resolution
• Physical damping & friction
• Electromechanical damping
• Virtual damping using velocity sensor
• Corrective torque pulses
• Phase estimation damping
• Velocity-adaptive low-pass filtering
Goal: Decrease amplitude without increasing frequency
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Design Implications
• ZOH and displacement quantization effects interact – they are not independent
• Avoid increasing oscillation frequency
• Increasing sample rate is often not the answer
• Pick the highest acceptable sample rate and then work to maximize position resolution
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Design Implications (cont.)
• Other factors in addition to chatter discourage low-resolution displacement sensing
• Potential but speculative role for oscillation mitigation schemes
• Supports approaches such as nonlinear springs with increasing stiffness
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Design Implications
Notional Optimization Surface
QF = max(logmagnorm, freqnorm, .45)
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Conclusions
• Human hand grasping a haptic knob can be modeled as a second-order system– Stiffness and damping increase with grip force
– Model breaks down for high grip forces
• Displacement quantization increases magnitude of limit cycle oscillations by exacerbating effect of delays in control law updating
• Described design implications for displacement resolution and sample rate selection
• Two tools: – Simple approximation (magnitude)
– Describing function analysis (magnitude & frequency)
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Questions?