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Constrained Body Dynamics. Chapter 4 in: Mirtich Impulse-based Dynamic Simulation of Rigid Body Systems Ph.D. dissertation, Berkeley, 1996. Preliminaries. Links numbered 0 to n Fixed base: link 0; Outermost like: link n Joints numbered 1 to n Link i has inboard joint, Joint i

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Constrained body dynamics

Constrained Body Dynamics

Chapter 4 in:

Mirtich

Impulse-based Dynamic Simulation of Rigid Body Systems

Ph.D. dissertation, Berkeley, 1996


Preliminaries
Preliminaries

  • Links numbered 0 to n

  • Fixed base: link 0; Outermost like: link n

  • Joints numbered 1 to n

  • Link i has inboard joint, Joint i

  • Each joint has 1 DoF

  • Vector of joint positions: q=(q1,q2,…qn)T

2

n

1


The problem
The Problem

  • Given:

    • the positions q and velocities of the n joints of a serial linkage,

    • the external forces acting on the linkage,

    • and the forces and torques being applied by the joint actuators

  • Find: The resulting accelerations of the joints:


First determine equations that give absolute motion of all links
First Determine equations that give absolute motion of all links

Given: the joint positions q, velocities and accelerations

Compute: for each link the linear and angular velocity and acceleration relative to an inertial frame


Notation
Notation links

Linear velocity of link i

Linear acceleration of link i

Angular velocity of link i

Angular acceleration of link i


Joint variables
Joint variables links

joint position

joint velocity

Unit vector in direction of the axis of joint i

vector from origin of Fi-1 to origin of Fi

vector from axis of joint i to origin of Fi


Basic terms
Basic terms links

  • Fi – body frame of link i

  • Origin at center of mass

  • Axes aligned with principle axes of inertia

ui

Frames at center of mass

Fi

ri

di

Fi-1

Axis of articulation

State vector


From base outward
From base outward links

  • Velocities and accelerations of link i are completely determined by:

    • the velocities and accelerations of link i-1

    • and the motion of joint i


First determine velocities and accelerations
First – determine velocities and accelerations links

From velocity and acceleration of previous link, determine total velocity and acceleration of current link

To be computed

Computed from base outward

From local joint


Compute outward
Compute outward links

Angular velocity of link i =

angular velocity of link i-1 plus

angular velocity induced by rotation at joint i

Linear velocity:


Compute outward1
Compute outward links

Angular acceleration propagation

Linear acceleration propagation

Rewritten, using

and

(from previous slide)

(relative velocity)


Define w rel and v rel and their time derivatives
Define w linksrel and vrel and their time derivatives

Joint acceleration vector

Joint velocity vector

Axis times parametric velocity

Axis times parametric acceleration

prismatic

revolute


Velocity propagation formulae
Velocity propagation formulae links

(revolute)

linear

angular


Time derivatives of v rel and w rel
Time derivatives of v linksrel and wrel

(revolute)

Joint acceleration vector

Change in joint velocity vector

From joint acceleration vector

From change in joint velocity vector

From change in change in vector from joint to CoM


Derivation of
Derivation of links

(revolute)


Propagation formulae
Propagation formulae links

(revolute)

linear

angular


First step in forward dynamics
First step in forward dynamics links

  • Use known dynamic state (q, q-dot)

  • Compute absolute linear and angular velocities

  • Acceleration propagation equations involve unknown accelerations

But first – need to introduce notation to facilitate equation writing

Spatial Algebra


Spatial algebra
Spatial Algebra links

Spatial velocity

Spatial acceleration


Spatial transform matrix
Spatial Transform Matrix links

r – offset vector

R– rotation

(cross product operator)


Spatial algebra1
Spatial Algebra links

Spatial transpose

Spatial force

Spatial joint axis

Spatial inner product

(used in later)


Computeseriallinkvelocities
ComputeSerialLinkVelocities links

(revolute)

For i = 1 to N do

Rrotation matrix from frame i-1 to i

rradius vector from frame i-1 to frame i (in frame i coordinates)

Specific to revolute joints

end


Spatial formulation of acceleration propagation
Spatial formulation of acceleration propagation links

(revolute)

Previously:

Want to put in form:


Spatial coriolis force
Spatial Coriolis force links

(revolute)

These are the terms involving


Featherstone algorithm
Featherstone algorithm links

Spatial acceleration of link i

Spatial force exerted on link i through its inboard joint

Spatial force exerted on link i through its outboard joint

All expressed in frame i

Forces expressed as acting on center of mass of link i


Serial linkage articulated motion
Serial linkage articulated motion links

Spatial articulated inertia of link i

Spatial articulated zero acceleration force of link I

(independent of joint accelerations)

Develop equations by induction


Base case
Base Case links

Consider last link of linkage (link n)

Force/torque applied by inboard joint + gravity = inertia*accelerations of link

Newton-Euler equations of motion


Using spatial notation
Using spatial notation links

Link n

Inboard joint


Inductive case
Inductive case links

Link i-1

outboard joint

Inboard joint


Inductive case1
Inductive case links

The effect of joint I on link i-1 is equal and opposite to its effect on link i

Substituting…


Inductive case2
Inductive case links

Invoking induction on the definition of


Inductive case3
Inductive case links

Express ai in terms of ai-1 and rearrange

Need to eliminate from the right side of the equation


Inductive case4
Inductive case links

Magnitude of torque exerted by revolute joint actuator is Qi

A force f and a torque applied to link i at the inboard joint give rise to a spatial inboard force (resolved in the body frame) of

u

d

f

t

Moment of force

Moment of force


Inductive case5
Inductive case links

previously

and

Premultiply both sides by

substitute for Qi , and solve



And form i z terms
And form I & Z terms links

To get into form:


Ready to put into code
Ready to put into code links

Using

  • Loop from inside out to compute velocities previously developed (repeated on next slide)

  • Loop from inside out to initialization of I, Z, and c variables

  • Loop from outside in to propagate I, Z and c updates

  • Loop from inside out to compute q-dot-dot using I, Z, c


Computeseriallinkvelocities1
ComputeSerialLinkVelocities links

(revolute)

For i = 1 to N do

Rrotation matrix from frame i-1 to i

rradius vector from frame i-1 to frame i (in frame i coordinates)

Specific to revolute joints

end


Initseriallinks
InitSerialLinks links

(revolute)

For i = 1 to N do

end


Serialforwarddynamics
SerialForwardDynamics links

Call compSerialLinkVelocities

For i = n to 2 do

Call initSerialLinks

For i = 1 to n do


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