Parametric Contact Modeling

1
Parametric Contact ModelingTestTrack
Simulations

• PCM Overview
• Current Models
• TestTrack Simulations
• Observations

2
PCM Overview

• Develop Non-Coulomb Friction Contact Models to
  Capture Adhesion
• Integrate Models into Simulation Environment and
  Test/Validate
• Use Models to help foot/robot design, trajectory
  planning, control, etc.

3
Current PCM Models

• Orientation Based Adhesion
• Force-Threshold Adhesion
• Anisotropic Friction

4
Orientation Based Adhesion
Global Adhesion when Angle between Foot Wall
within Limit
Relative Adhesion when Change in Angle between
Foot Wall within Limit Always make initial
contact
Foot becomes PINNED
Contact! Foot is FREE
Contact! Foot is PINNED
Remains PINNED
Foot becomes FREE
Remains PINNED
Foot becomes FREE
Remains FREE
5
Force-Threshold Based Adhesion

• 5 Spring/Dampers in X, -X, Y, -Y, Z
• 6th Constraint either
• Wall (DEs Unilateral Constraint)
• Spring/Damper in Z
• Set K, B, and Limits in X, Y, and Z

Y
Z
g
X
6
Anistropic Friction

• Coefficient of Friction is a Function of
  Direction of Velocity
• Specify Mu in 4 Directions
• Actual Mu is Linear Interpolation Based on Angle
  of Velocity Vector in XY Plane

• E.G.
• Mu X 0
• Mu X 0
• Mu Y 0
• Mu Y 2.0

7
TestTrack Simulation

• DE Simulation of Stanford TestTrack with BDI
  Fourbar

QUICKTIME MOVIE
8
TestTrack Simulation

• Trajectory - Validate Models and Simulation
• Early, Nominal, Late Attachment/Detachment
• Ankle Compliance - Begin to Explore Foot Design
• Yaw Only
• Pivot at Tip of Coupler

Y
X
Foot
Z
Fourbar
2cm
Motors Frame
Coupler
Rotational Spring/Damper
9
TestTrack ResultsX Lateral, Pull-In
Attachment
E N
L
E N L
Detachment
10
Compared to Experiments
Attachment
E N
L
E N L
Detachment
11
TestTrack ResultsY Fore/Aft, Pull-Up
Attachment
E N
L
E N L
Detachment
12
Compared to Experiments
Attachment
E N
L
E N L
Detachment
13
TestTrack ResultsZ Normal, Pull-Out
Attachment
E N
L
E N L
Detachment
14
Compared to Experiments
Attachment
E N
L
E N L
Detachment
15
General Observations

• Point of Contact wrt Coupler Link is Very
  Important
• Without Compliance, certain Foot configurations
  would cause motors to stall against Lateral (X)
  constraints
• Deviations of Straight-Line Path can result
  in unwanted Push-Off forces
• Next Models should include Rotational
  Effects/Constraints