Biomimetic Robots for Robust Operation in Unstructured Environments

1
Biomimetic Robots for Robust Operation in
Unstructured Environments

• M. Cutkosky and T. KennyStanford University
• R. Full and H. KazerooniU.C. Berkeley
• R. HoweHarvard University
• R. Shadmehr
• Johns Hopkins University

Site visit -- Stanford University, Sept. 2, 1990
http//cdr.stanford.edu/touch/biomimetics
2
Main ideas
BioMimetic Robotics MURI Berkeley-Harvard Hopkins
-Stanford

• Study insects to understand role of passive
  impedance (structure and control), study humans
  to understand adaptation and learning(Full,
  Howe,Shadmehr)
• Use novel layered prototyping methods to create
  compliant biomimetic structures with embedded
  sensors and actuators (Cutkosky, Full, Kenny)
• Develop biomimetic actuation and control schemes
  that exploit preflexes and reflexes for robust
  locomotion and manipulation (Full, Cutkosky,
  Howe, Kazerooni, Shadmehr)

3
Issues in studying, designing and building
biomimetic robots (and the basic outline for
todays site visit)
2.
1.
Biomimetic Robots
MURI
3.
4
Guiding questions
What passive properties are found in Nature?
Preflexes Muscle and Exoskeleton Impedance
Measurements (Berkeley Bio.)
What properties in mechanical design?
Biological implications for Robotics Basic
Compliant Mechanisms for Locomotion
(Stanford) Variable compliance joints (Harvard,
Stanford) Fast runner with biomimetic trajectory
(Berkeley ME)
Fabrication
How should properties be varied for changing
tasks, conditions ?
Matching ideal impedance for unstructured dynamic
tasks (Harvard)
5
Guiding questions
What strategies are used in insect locomotion and
what are their implications?
MURI
Low-Level Control
Insect locomotion studies (Berkeley Bio) New
measurement capabilities (Stanford)
What motor control adaptation strategies do
people use and how can they be applied to robots?
Fabrication
Compliance Learning and Strategies for
Unstructured Environments (Harvard Johns
Hopkins) Implications for biomimetic robots
(Harvard, Stanford)
6
Guiding questions
High-Level Control
Low-Level Control
How do we build robust biomimetic structures and
systems?
Shape deposition manufacturing of integrated
parts, with embedded actuators and sensors
(Stanford)
How do we build-in tailored compliance and
damping?
Structures with functionally graded material
properties (Stanford)
Effects of Compliance in simple running machine
(Stanford, Berkeley ME)
7
930-1100 Low Level Biomimetic Control

• Results on measurements of muscles, exoskeleton,
  compliance, damping (Full 30)
• Implications for biomimetic robots (Bailey
  20min)
• Matching leg trajectory and scaling (Kazerooni
  15)
• Matching impedance to dynamic task (Matsuoka 15)

8
What passive properties are found in Nature?
Preflexes Muscle and Exoskeleton Impedance
Measurements (Berkeley Bio.)
What properties in mechanical design?
Biological implications for Robotics Basic
Compliant Mechanisms for Locomotion
(Stanford) Variable compliance joints (Harvard,
Stanford) Fast runner with biomimetic trajectory
(Berkeley ME)
Fabrication
How should properties be varied for changing
tasks, conditions ?
Matching ideal impedance for unstructured dynamic
tasks (Harvard)
9
MURI Year One Meeting 1999
Lower Level Control
Professor Robert J. Full Daniel Dudek Dr. Kenneth
Meijer
University of California at Berkeley Department
of Integrative Biology rjfull_at_socrates.berkeley.ed
u http//polypedal.berkeley.edu
10
Lower Level Control
aero- , hydro, terra-dynamic
Higher
Sensors
Environment
Centers
Open-loop
Mechanical
Feedforward
Behavior
System
Controller
(CPG)
(Actuators, limbs)
Feedback
Closed-loop
Controller
Adaptive
Sensors
Controller
11
Chain of Reflexes
Cruse Controller Inspired by Stick Insects
12
Rough Terrain
Fractal Surface Variation -
3 times the height of the center of mass
13
Control Challenge
Precise
Control
Novel
Slow
Mechanical
Dynamic
Static
Neural
Feedforward
Feedforward
14
PolyPEDAL Control
Control algorithms embedded in the form of
animal itself.
Control results from properties of parts and
their morphology.
Musculoskeletal units, leg segments and legs do
computations on their own.
15
Lower Level Control
aero- , hydro, terra-dynamic
Higher
Sensors
Environment
Centers
Open-loop
Mechanical
Feedforward
Behavior
System
Controller
(CPG)
(Actuators, limbs)
Feedback
Closed-loop
Controller
Adaptive
Sensors
Controller
16
Contribution to Control
Mechanical System
Preflex
Feedforward
Motor program acting through moment arms
Intrinsic musculo-skeletal properties
Predictive
Rapid acting
Passive Dynamic Self-stabilization
17
MURI Interactions
Rapid Prototyping
Stanford
Muscles and
Motor Control
Learning
Locomotion UC Berkeley
Johns Hopkins
MURI
Robot Leg Mechanisms
Manipulation
Harvard
UC Berkeley
Sensors / MEMS
Stanford
18
Manufactured Legs
What properties should legs possess? Why? Act as
springs to store and return energy? How? Act to
reject disturbances?
19
Road Map
1. System Impedance 2. Leg Impedance
3. Muscle
Impedance
20
Spring-Mass Systems
Legged
SIX-
21
Virtual Leg Stiffness
F
mg
TROTTERS

RUNNERS
Dx
HOPPERS
x
100
Blickhan and Full, 1993
Human
Quail
Dog
Cockroach
10
k
Hare
rel,leg
Kangaroo
Crab
1
0.01
0.001
0.1
1
10
100
Mass (kg)
22
Sagittal Plane Model
ORGANISM
Spring Loaded Inverted Pendulum
m
k
b
Leg Springs ?
Multi-Leg
23
Road Map
1. System Impedance 2. Leg Impedance
3. Muscle
Impedance
24
Leg as Spring Damper
?x
Force
Stiffness, k Damping coefficient, c
Restorative Forcesand Perturbation Damping
For an Oscillating System Force force due to
force due to force due to mass
stiffness damping
.
..
Force kx cx mx
25
Experimental Setup
Oscillate Leg At Multiple Frequencies To
Determine k and c
Servo Motor
Roach leg
Length and Force recording
26
Leg Oscillation Experiments
Small Deflection at 12 Hz
Force (N)
Displacement (mm)
Time (s)
27
Leg Is Spring and Damper
Small Deflection at 12 Hz
Slope Impedance
Force (N)
Displacement (mm)
28
Effect of Frequency
Impedance Increases with Frequency
k25 Hz gt k0.08 Hz
Force (N)
Displacement (mm)
29
Impedance
Impedance of Metathoracic Limb of Cockroach
Preferred Stride Frequency 12 Hz
Impedance (N/m)
Frequency (Hz)
30
Leg Model
Standard Linear Solid

• At high frequenciesForce a (k1k2)(displacement
  )
• At low frequenciesForce a k2(displacement)

c
k1
k2
31
Frequency vs Speed
Cockroach
20
Natural Frequency?
Impedance Constant

15
Impedance Increases
Alter Leg Spring Angle Take Longer Strides
Stride frequency (Hz)
10
5
0
0
0.2
0.4
0.6
Speed (m/sec)
32
Impedance
Large Deflection
Non-linear
k24 Hz gt k0.25 Hz
33
Perturbation Rejection
Perturbation
RestorativeForce4x Body Mass
34
Discoveries
1. Insect leg behaves like a spring and damper
system. 2. Strain energy is stored in the leg and
returned. 3. Force displacement relationship
shows hysteresis with significant energy
dissipation (50 or more).
35
Discoveries
4. Leg impedance increases with frequency up to
12 Hz, the preferred speed of the animal. 5. Leg
impedance remains constant at frequencies above
12 Hz. 6. The legs natural frequency is near the
frequency used by the animal at its preferred
speed.
36
Discoveries
7. Insect leg could simplify control by rejecting
perturbations. For a deflection of only one mm,
the leg produces a force of 0.75-4x body mass.
37
Road Map
1. System Impedance 2. Leg Impedance
3. Muscle
Impedance
38
MURI Interactions
Rapid Prototyping
Stanford
Muscles and
Motor Control
Learning
Locomotion UC Berkeley
Johns Hopkins
MURI
Robot Leg Mechanisms
Manipulation
Harvard
UC Berkeley
Sensors / MEMS
Stanford
39
Manufactured Legs
What properties should actuators possess?
Why? Act as springs to store and return energy?
How? Act to reject disturbances? Power generation?
40
Horizontal Plane Model
Muscle-Apodeme Damped Springs ?
41
Muscle Lever
Control
Stimulation
Stimulation
- pattern
- magnitude
- phase
Servo and
Strain
Force
- pattern
Transducer
- magnitude
Frequency
42
Workloop Technique
43
Muscle Capacity
179
Powerspace
177c
Powerspace
Power
2 Muscle Action Potentials
3 Muscle Action Potentials
(W/kg)
100
0.0
Damper
80
Damper
Spring
60
Stimulation phase ()
-100.0

40
Motor

in vivo
in vivo
Spring
20
conditions
conditions
-200.0
0
20
4
6
8
10
12
14
5
10
15
Muscle Strain
44
Musculo-skeletal Model
Preflexes
Intrinsic musculo-skeletal properties
Force
Insect Leg
Velocity
Brown and Loeb, 1999
45
Perturbation Experiments
Passive Muscle Stiffness Significant
46
Effect of Step Length Increase
Passive resistance is significant in muscle 177c
Stimulated (Twitch)
(n 4)
Force increase (mN)
Relaxed
Step size ()
47
Oscillatory Perturbations
Ecomplex (DForce/Area)/strain Eviscous/Eelastic
tan(phase angle)
48
Visco-elastic Properties
Passive Muscle Impedance increases with frequency
in muscle 179 Impedance independent of frequency
in muscle 177c Significant viscous damping in
both muscles.
Ecomplex (N/m2)
tan(phase angle)
Frequency (Hz)
Frequency (Hz)
49
Effect of Length
Passive Muscle Impedance increases with
length Contribution viscous damping decreases
with length
50
Perturbation experiments
Impedance during workloop.
51
Multiple Muscle System
Anatomically similar muscles provide impedance
during different phases of the locomotion cycle!
Muscle strain ()
52
Discoveries

• 1. Passive muscle can reject perturbations.
• 2. Preflexes comprise passive (fixed) and active
  components (adjustable).
• 3. Passive muscle acts like a visco-elastic
  actuator.
• (Viscous damping is responsible for a significant
  part of total force response to perturbation.)
• 4. Impedance of anatomically similar muscles is
  distributed over the locomotion cycle.

53
Impact on Deliverables
1. Energy storage 2. Reject perturbations 3.
Simplify control 4. Penetrate new environments 5.
Increase robustness
54
Guiding questions
What passive properties are found in Nature?
Preflexes Muscle and Exoskeleton Impedance
Measurements (Berkeley Bio.)
What properties in mechanical design?
Basic Compliant Mechanisms for Locomotion Biologic
al implications for Robotics (Stanford) Variable
compliance joints (Harvard, Stanford) Fast runner
with biomimetic trajectory (Berkeley ME)
Fabrication
How should properties be varied for changing
tasks, conditions ?
Matching ideal impedance for unstructured dynamic
tasks (Harvard)
55
Locomotion Biomimetic Ideology

• Goal
• Navigate rough terrain with simple, robust,
  compliant robots
• Mindset shaped by Biology
• Tunable, passive mechanical properties
• Purpose-specific geometry
• Simple control scheme
• Robust components

56
Variable Compliance? Interpreting Biological
Findings

• Idea
• Desired reaction forces depend on the environment
  and locomotion speed
• How do we represent these findings?
• Not traditional spring or damper elements
• Energy spent per cycle independent of frequency
  (area enclosed by curve is the energy spent)
• Results suggest hysteretic damping

57
Variable Impedance New Design Direction

• Whats the difference between compliance and
  impedance?
• Impedance refers to the relationship dF/dx
• Stiffness refers to particular impedance
  relationship, namely dF/dx k
• Hysteretic Damping
• Characteristic of some heterogeneous materials
• Loading and unloading create different
  stress-strain paths
• Stress-strain curve is independent of frequency
• Design Implications
• Compliance is mainly a function of displacement
• Damping has a significant frequency dependant term

58
Variable Impedance Design Approach

• Traditional Robotic Compliance
• Actuator powered
• Proportional feedback control - variable
  compliance
• Complex
• multiple control laws with different objectives
  must work together
• Low bandwidth - controller delays

59
Variable Impedance Design Approach

• Different Approach
• Compliant member powered
• Adjustable geometry - variable impedance
• Simple
• mechanical properties are more predictable
• separate from control law
• intrinsic low level stability
• Biology is telling us what mechanical properties
  we really need

60
Sprawl 1.0 Legged Testbed

• Capture the essential locomoting elements in a
  low DOF robot
• Explore the roles of compliance and damping in
  locomotion
• Identify areas which can be improved by SDM

61
Sprawl 1.0 Biomimetic, not just a copy

• Fulls research highlights certain important
  locomoting components
• Power-producing thrust muscles
• Supporting/repositioning hip joints

62
Sprawl 1.0 Thrusting
Very Low Friction Pneumatic Piston

• Fulls research on power-producing muscles
  177a,c,d,e (Ahn, Meijer)
• Thrust production - Decoupled, compliant system

63
Sprawl 1.0 Repositioning/Supporting

• Fulls research on Trochanter-Femur joint (Dudek)
• Repositioning/Supporting - Decoupled, compliant
  system

64
Sprawl 1.0 Findings

• Good design and passive mechanical properties
  take burden off control
• Compliance and damping
• Simple alternating tripod locomotion scheme
• Built-in posture control
• Low bandwidth geometry changes
• Walking, stopping, turning, and running
• Need for robust components
• Traditional components are not robust - poster
  child for SDM

65
Sprawl 1.0 Future Work

• Suggestions from Full
• Change location of center of mass
• Increase gait frequency
• Dynamically control middle leg set points
• Weaken front leg force
• Work in Progress
• Add compliant springs in parallel with constant
  force pistons
• Replace RC servo hip actuators with more
  biomimetic components