Download Humanoid body construction

Advanced Programming for 3D
Applications
CE00383-3
Introduction to Human
Motion
Lecture 2
Bob Hobbs
Staffordshire university
General Outline
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Human Skeleton
Muscle Groups
How Robots simulate humans
Kinematics
Gait
Locomotion
Human Dynamics
• Users described as participants
• basic interaction involves control of
camera (viewpoint)
– exploratory navigation / locomotion
– Walk through systems
• More advanced environment allow
interaction
– Touch , selection, manipulation
– referred to as direct manipulation
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Simulation of Body
• Body model is the description of the
interface
• eyes are viual interface, ears are audio interface
• geometric description drawn from egocentric point
of view
• description of hand and fingers forms basis of
grasping simulation for picking up objects (Boulic
1996)
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Simulation of Body
-• The
Building
the
body
more points represneting the body the
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more realistic the movement
Up to 90 points for motion-capture in animation
Standard for human skeleton (H-Anim 1999)
More typically head, Torso, Both hands
Inferred movement from limited points
Inverse kinematics problem - infinite possibilities
of movement in virtual environment, consistent
restraint
Elbow position in 4- Tracker system (Badler,
1993)
H-Anim
Humanoid
L Hip
L Knee
L Ankle
L Midtarsal
Sacroiliac
R Hip
R Knee
R Ankle
L Shoulder
L Elbow
L Wrist
R Shoulder
R Elbow
R Wrist
vl5
Skullbase
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R Midtarsal
Simulation Of body - tracking
the participant
• Choice of system depends on 5 factors
– accuracy, resolution, range, lag, update rate
• Many different tracking technologies
– Meyer 1992
– frequency and time
• ultrasonic time-of-flight measurement
• Pulsed Infra-red
• GPS
• Optical Gyroscopes
• Phase difference
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Simulation Of body - tracking
the participant
• Spatial Scan
• Outside-in
• Inside-out
• Inertial sensing
– mechanical gyroscope
– Accelerometer
• Mechanical Linkages
• Direct - Field Sensing
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Interaction with virtual Body
• Limitations mean reliance on metaphors
for
– object manipulation (grasping and moving)
– locomotion (movement)
• Limitations in haptics mean that restraint
on the virtual environment exists
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Muscles
• http://www.youtube.com/watch?v=T-
ozRNVhGVg&feature=PlayList&p=37A3DC
6AF2D7C881&index=5
• http://www.youtube.com/watch?v=pbTah
5NVOtU&NR=1
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The Musculotendinous Unit
• Tendon- spring-like elastic
component in series with
contractile component
(proteins)
F
• Parallel elastic component
x
(epimysium, perimysium,
endomysium, sarcolemma)
PEC: parallel elastic component
CC: contractile component
SEC: series elastic component
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II. Mechanics of Muscle Contraction
• Neural stimulation – impulse
• Mechanical response of a motor unit - twitch
t
F (t )  F0 e
T

t
T
T: twitch or contraction time, time for tension to reach maximum
F0: constant of a given motor unit
Averaged T values
Tricep brachii 44.5 ms
Biceps brachii 52.0 ms
Tibialis anterior 58.0 ms
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Soleus 74.0 ms
Medial Gastrocnemius 79.0 ms
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Summation and tetanic contraction
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(ms)
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Generation of muscle tetanus
100Hz
10 Hz
Note: muscle is controlled by frequency modulation from neural input
very important in functional electrical stimulation
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Wave summation & tetanization
Critical frequency
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Motor unit recruitment
All-or-nothing event
2 ways to increase tension:
- Stimulation rate
- Recruitment of more motor unit
Size principle
Smallest m.u. recruited first
Largest m.u. last
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Models
• Springs
• Joints
• Segments
• Muscles
Robots
• Springs
• Screws
• Metal parts
• Servos
• Rubber
simple, fast, easy to understand
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Robotic Basics
• Have moveable segments
• Connected with joints
• Robots spin wheels and pivot jointed segments
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with some sort of actuator
Some robots use electric motors and solenoids
as actuators some use a hydraulic system and
some use a pneumatic system (a system driven
by compressed gases).
Robots may use all these actuator types.
Robots usually have some sort of sensor
Actuators
• Electrical current
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drives actuators
controlling individual
joints
Directly to motors or
solenoids
To valves controlling
flow of fluids to
hydraulic or
pneumatic systems
Robot arm
• Simplest sort of robot
• Typical arm has 7
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segments, 6 joints
6DOF
Human arm 7DOF
Usually driven by Step
Motors
Main use is in
manufacturing
Robot Arm
• Fitted with end effector
• Usually interchangeable
• Artificial Hand , paint gun, welding rod
• Pressure sensor needed to prevent
crushing
• Programmed by incremental steps which
are then replicated ad infinitum
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Step Motor
• electromagnetic,
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rotary
actuator, that mechanically
converts
digital
pulse
inputs to incremental shaft
rotation.
The rotation not only has a
direct relation to the
number of input pulses,
but its speed is related to
the frequency of the
pulses.
Step Motor
Each pulse corresponds to an angular rotation
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Step Motors
• Between steps holds position w/o brake or
clutch
• Can be programmed to move a precise
number of steps and then hold position
• Possible to be bi-directional
• Rapid acceleration, deceleration and
reversal
• cf DC Servo motors
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Choosing the right motor
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Basic Types:
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Parameters to be considered
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Variable Reluctance,
Permanent Magnet,
Hybrid
Distance to be traversed.
Maximum time allowed for a traverse.
Desired detent (static) accuracy.
Desired dynamic accuracy (overshoot).
More parameters
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Settling time
Required step resolutiong
System friction
System inertia.
Speed/Torque characteristics of the motor: When
selecting a motor/drive, the capacity of the motor must
exceed the overall requirements of the load.
Torque-to-inertia Ratio
Torque Margin: Selecting a motor drive that provides
at least 50% margin above the minimum required
torque is ideal.
Frameworks, Chains (or
Skeletons)
• A lot of mechanical objects in the real
world consist of solid sections connected
by joints
• Obviously robot arm but also
– Creatures such as humans and animals.
– Car Suspension
– Ropes, string and Chains
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Frameworks, Chains (or
Skeletons)
• Sections and joints of robot arm are
known as a 'chain‘
• In creatures could be referred to as a
skeleton
• Moveable sections correspond to bones
• Attachments between bones are joints.
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Frameworks, Chains (or
Skeletons)
• Motions of chains can be specified in terms of
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translations and rotations.
Forward Kinematics - From the amounts of
rotation and bending of each joint in an arm, for
example, the position of the hand can be
calculated.
Inverse Kinematics - If the hand is moved, the
rotation and bending of the arm is calculated, in
accordance with the length and joint properties
of each section of the arm.
Joint Translation-Rotation
• We can use a transform
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(T) to transform each
point relative to the body
to a position in world
coordinates.
If we want to model both
linear and angular
(rotational) motion then
we need to use a 4x4
matrix to represent the
transform
What is Inverse Kinematics?
• Forward Kinematics
?
End Effector
Base
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What is Inverse Kinematics?
• Inverse Kinematics
End Effector
Base
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What does
looks like?
?
End Effector
Base
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Solution to
• Our example
Number of equation : 2
Unknown variables : 3
Infinite number of solutions !
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Redundancy
 System DOF > End Effector DOF
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Our example
System
DOF = 3
End Effector DOF = 2
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Redundancy
• A redundant system has infinite number of
solutions
• Human skeleton has 70 DOF
– Ultra-super redundant
• How to solve highly redundant system?
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Iterative solution
• Start at end effector
• Move each joint so that end gets
closer to target
• The angle of rotation for each joint is
found by taking the dot product of
the vectors from the joint to the
current point and from the joint to the
desired end point. Then taking the
arcsin of this dot product.
• To find the sign of this angle (ie
which direction to turn), take the
cross product of these vectors and
checking the sign of the Z element of
37 the vector.
Goal Potential Function
• “Distance” from the end effector to the goal
• Function of joint angles : G(q)
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Our Example
Goal
distance
End Effector
Base
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Ground reaction force (N)
Dynamics of the long jump
m1
k
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Nonlinear
spring-damper
element
m2
time (ms)
Energetic losses
may increase
performance!
Seyfarth et al. (1999) J. Biomech.
Joint Structures
• This allows two nodes to
be attached to each other
in a flexible way so that
forces in the plane of the
joint will be transmitted
through it, but forces
perpendicular to the joint
will cause it to bend. This
will provide IK like
capabilities
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Types of Joint
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•Name
•Symbol
•DOF
•Revolute joints
•R
•1
•Prismatic joints
•P
•1
•Helical joints
•-
•1
•Cylindrical joints
•RP
•2
•Spherical joints
•3R
•3
•Planar joints
•RRP
•3
Joint Structures
• In character animation, only 2 types of joint
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need to be considered. These are the "revolute"
and "prismatic" joints. All other types can be
based on these two.
1 degree of freedom:
– rotational joint - wheel.
– hinge - similar to rotational joint above but with limits
to motion (end stops)
• 2 degrees of freedom
– ball & socket joint
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Dynamics
• Forward Dynamics - The movements are
calculated from the forces, such as, force
= mass * acceleration.
• Inverse Dynamics - Constraints are
applied which specifies how objects
interact, for example, they may be linked
by a hinge joint or a ball joint, and from
this the forces can be calculated
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Forward Dynamics
1. If no forces act on a particle, the particle
retains its linear momentum.
2. The rate of change of the linear
momentum of a particle is equal to the
sum of all forces acting on it.
3. When two particles exert forces upon
each other, these forces are equal in
magnitude and opposite in direction.
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Forward Dynamics
• These laws can also be applied to rigid bodies
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by assuming that the forces are acting on the
centre of mass of the object.
Assuming that the mass is constant then the
second law becomes:
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force = mass * acceleration
• Euler extended these laws to include rotation.
So there are equivalent laws for rotation such
as:
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torque = inertia * angular acceleration.
What is a robot?
• Joseph Engelberger, a pioneer in industrial
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robotics, once remarked "I can't define a robot,
but I know one when I see one."
Many different machines called robots
Everybody has a different idea of what constitutes
a robot
Name from robota – forced labour
What relevance to us?
• VR models use robotic principles
• Avatars behave like robots
• Simulations of robots used to test real
robots
• May be used to control remote robotics
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Virtual Actors: Autonomous
or Guided
Guided Actors are Slaved to the Motions of a Human
Participant Using Body Tracking
– Optical, mechanical, . . .
– A.K.A. Avatar
• Autonomous Actors Are Controlled by Behavior Modeling
Programs, and Can
- Augment or replace human participants
- Serve as surrogate instructors
- Act as guides in complex synthetic worlds
• Hybrid Control Desirable
- VRLOCO uses interaction to invoke and control
locomotion behaviors
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The Weiss 6-Level Motor
Organization Hierarchy
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3. Muscle Group
- Coordinated action of
several muscles
- Motion at one joint
2. Muscle
- Muscle contraction
1. Motor Unit
- Neuron + muscle fibers
- Twitching, shivering
Organism Level
6. Motor
Behavior
5. Motor Organ
System
4. Motor Organ
3. Muscle
Group
2. Muscle
1. Motor Unit
Neuron Level
The Weiss 6-Level Motor
Organization Hierarchy
6. Motor Behavior
- Movement of the whole organism
- E.G., Goal-directed locomotion
- Task manager
5. Motor Organ System
- Coordinated action of several limbs
- E.g., Walking
- Motor programs, skills
4. Motor Organ
- Coordinated action of several joints
- E.G., Stepping motion of a limb
- Local motor programs
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Organism Level
6. Motor
Behavior
5. Motor Organ
System
4. Motor Organ
3. Muscle
Group
2. Muscle
1. Motor Unit
Neuron Level
Motion and Reaction
• Sensorymotor level
- Levels 1 - 5
- Peripheral and proprioceptive feedback associated with
reflex arcs
- Motor programs and reflexes coordinate and control
motion
- Executes behaviors
• Reactive level
– Level 6 and higher
- Perception triggers and modulates behavior
- Organism responds to environmental stimuli to select and
compose behaviors
- Selects behaviors
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Organization of a Virtual
Actor
Organism Level
Level 6 and above
Reactive level
Levels 1-5
Sensorymotor level
6. Motor
Behavior
5. Motor Organ
System
4. Motor Organ
3. Muscle Group
2. Muscle
1. Motor Unit
Neuron Level
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Virtual Actor
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Abstraction and Interaction
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Representation and
Abstraction
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Finite State Machines for
Walking
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Control and Abstraction
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Avatars
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Kinematic
Chains
• Solid links connected at movable
joints
• Fixed end: base
• Movable end: tip or end effector
• One degree of freedom (DOF) per
joint
• Open chain: one fixed end, one
movable end
• Closed chain: both ends fixed
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Forward and Inverse
Kinematics
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Kinematic Redundancy
• End-effector has 6 DoFs
- (x, y, z) position
- ( , , ) orientation
• Non-redundant linkage has < = 6 joints (DoFs)
• Redundant linkage has > 6 joints (DoFs)
- Human arm has 7 DoFs
» Shoulder 3
» Elbow 1
» Forearm 1
» Wrist 2
- Redundancy enables multiple solutions
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Inverse Kinematics (IK)
• Non-redundant Linkages
- Analytical solutions
• Redundant Linkages
- Many techniques
» Pseudo-inverse (Jacobian)
» Gradient
» Others
• IK Commonly Found in Animation Packages
- 3D Studio Max
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Static Balance
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Weight
• Bend:
– Non-weight-bearing motion
– Traverse subtree rooted at rotating joint
• Pivot
– Weight-bearing motion
– Traverse entire tree starting at root EXCEPT for
subtree rooted at rotating joint
• Critical Element of Realism
– Is the character supported by its legs, or are the
legs dangling in space as the character is
translated along?
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Bend
Non-weight-bearing motion
– traverse subtree
rooted at rotating joint
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Pivot
Weight-bearing motion
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– traverse entire tree starting
at root EXCEPT for subtree
rooted at rotating joint
Gait Parameters
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• Gait Pattern
– Sequence of lifting and placing feet
• Gait Cycle
– One repetition Of the gait pattern
• Period
– Duration of one gait cycle
• Relative Phase of Leg I
– Fraction of gait cycle before leg I is lifted
• Duty Factor
– Fraction of gait cycle period a given leg spends on ground
• Swing Time
– Time a leg spends In the air
• Stance Time
– Time a leg spends On the ground
• Stroke
– Distance body travels during a leg's stance time
Finite State Machines for
Walking
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Locomotion
• Tracker has a limited range
• Must use locomotion metaphor to move
greater distances
• Locomotion is on an even plane , virtual
terrain may not be even
• Collision detection can be employed to
raise or lower the participant accordingly
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Directions of locomotion
Fly in direction of aim
Fly in direction of pointing
Fly in direction of gaze
Fly in direction of torso
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