robot dynamics – the action of a manipulator when forced
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Robot Dynamics – The Action of a Manipulator When Forced. ME 4135 Fall 2012 R. R. Lindeke, Ph. D. We will examine two approaches to this problem. Euler – Lagrange Approach: - PowerPoint PPT PresentationTRANSCRIPT
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ME 4135Fall 2012R. R. Lindeke, Ph. D.
Robot Dynamics – The Action of a Manipulator When Forced
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We will examine two approaches to this problem
Euler – Lagrange Approach:– Develops a “Lagrangian Function” which relates Kinetic
and Potential Energy of the manipulator thus dealing with the manipulator “As a Whole” in building force/torque equations
Newton – Euler Approach:– This approach tries to separate the effects of each link
by writing down its motion as a linear and angular motion. But due to the highly coupled motions it requires a forward recursion through the manipulator for building velocity and acceleration models followed by a backward recursion for force and torque
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Euler – Lagrange approach
Employs a Denavit-Hartenberg structural analysis to define “Generalized Coordinates” as common structural models.
It provides good insight into controller design related to STATE SPACE
It provides a closed form interpretation of the various components in the dynamic model:– Inertia– Gravitational Effects– Friction (joint/link/driver)– Coriolis Forces relating motion of one link to coupling effects of
other link motion– Centrifugal Forces that cause the link to ‘fly away’ due to coupling
to neighboring links
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Newton-Euler Approach
A computationally ‘more efficient’ approach to force/torque determination
It starts at the “Base Space” and moves forward toward the “End Space” computing trajectory, velocity and acceleration
Using this forward velocity type information it computes forces and moments starting at the “End Space” and moving back to the “Base Space”
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We’ll start with the E-L method by Defining the Manipulator Lagrangian:
( , ) ( , ) ( )
( , )
( )
L q q T q q U qhereT q q
U q
Kinetic energy of themanipulator
Potential energy of the manipulator
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Generalized Equation of Motion/ Force of the Manipulator:
1, ,i i ni i
i
dF L q q L q qdt q q
is a link of the manipulator
Fi is the Generalized Force acting on Link i
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Starting Generalized Equation Solution
We begin with focus on the Kinetic energy term (the hard one!)
Remembering from physics: K. Energy = ½ mV2
Lets define, for the Center of Mass of a Link ‘K’:
k
k
as Linear Velocity as Angular Velocity
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Rewriting the Kinetic Energy Term:
1
,2
T Tn K K K K K K
K
m DT q q
mK is Link Mass DK is a 3x3 Inertial Tensor of Link K about its center of mass
expressed WRT the base frame – this tensor characterizes mass distribution of a rigid object
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Focusing on DK: Looking at a(ny) link
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For this (any) Link: DC is its Inertial Tensor About it Center of Mass
In General:
2 2
2 2
2 2
V V V
C KV V V
V V V
y z dV xy dV xz dV
D m xy dV x z dV yz dV
xz dV yz dV x y dV
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Defining the terms:
The Diagonal terms are the “Moments of Inertia” of the link
The three distinct off diagonal terms are the Products of Inertia
If the axes used to define the pose of the center of mass are aligned with the x and z axes of the link defining frames (i-1 & i) then the products of inertia are zero and the diagonal terms form the “Principal Moments of Inertia”
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Continuing after this simplification:
2 2
2 2
2 2
0 0
0 0
0 0
V
C KV
V
y z dV
D m x z dV
x y dV
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If the Link is a Rectangular Rod (of uniform mass):
2 2
2 2
2 2
0 012
0 012
0 012
C K
b c
a cD m
a b
This is a reasonable approximation for many arms!
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If the Link is a Thin Cylindrical Shell of Radius r and length L:
2
2 2
2 2
0 0
0 02 12
0 0 2 12
C K
r
r LD m
r L
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Some General Link Shape Moments of Inertia:
From: P.J. McKerrow, Introduction to Robotics, Addison-Wesley, 1991.
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We must now Transform each link’s Dc
Dc must be defined in the Base Space To add to the Lagrangian Solution for kinetic energy (we will call it DK):
Where: DK = [R0K*Dc *(R0
K)T] Here R0
K is the rotational sub-matrix defining the Link frame K (at the end of the link!) to the base space – thinking back to the DH ideas
TK K KD
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Defining the Kinetic Energy due to Rotation (contains DK)
0 0
. .2
. .2
TK K K
TT K KK C K
DK E
R D RK E
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Completing our models of Kinetic Energy:
Remembering:
1
,2
T Tn K K K K K K
K
m DT q q
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Velocity terms are from Jacobians: We will define the velocity terms as parts of a “slightly” –
(really mightily) – modified Jacobian Matrix:
AK is linear velocity effect BK is angular velocity effect I is 1 for revolute, 0 for prismatic
joint types
1
1
1 0 1
0 ( )( )
( )0
KK
KK K
K K
c c A qq qJ qB qZ Z
Velocity Contributions of all links beyond K are ignored (this could be up to 5 columns!)
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Focusing on in the modified jacobian
This is a generalized coordinate of the center of mass of a link
It is given by: 1 0 ( ):
,0,0,1
K KK
K
c H T q chere
c
K
is a vector from frame k(at the end of link K) to the Center of Mass of Link K
land is: 2
Kc
A Matrix that essentially strips off the bottom row of the solution
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Re-Writing K. Energy for the ARM:
1
,2
TK K K Kn K K
K
A q m A q B q D B qT q q
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Factoring out the Joint Velocity Terms
1
,2
T TT K K T K Kn K K
K
q A m A q q B D B qT q q
1,2
n T TK K K KK K
T K
A m A B D BT q q q q
Simplifies to:
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Building an Equation for Potential Energy:
1
1
( ) ( )
( ) ( )
( ) ( )
nT
K KK
n
K KK
T
U q m g c q
g
c q m c q
U q g c q
is acceleration due to gravity andIntroducing a new term:
leads to:
This is a weighted sum of the centers of mass of the links of the manipulator
Generalized coordinate of centers of mass (from earlier)
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Finally: The Manipulator Lagrangian:
( , ) ( , ) ( )L q q T q q U q
1, ( )2
n T TK K K KK K
T TK
A m A B D BL q q q q g c q
Which means:
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Introducing a ‘Simplifying’ Term D(q):
1
{ }n T TK K K K
K KK
D q A q m A q B q D q B q
1, [ ] ( )2T TL q q q D q q g c q
Then:
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Considering “Generalized Forces” in robotics:
We say that a generalized force is an residual force acting on a arm after kinetic and potential energy are removed!?!*!
The generalized forces are connected to “Virtual Work” through “Virtual Displacements” (instantaneous infinitesimal displacements of the joints q),
Thus we say that the Virtual Displacement is a Displacement that is done without the physical constraints of time
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Generalized Forces on a Manipulator
We will consider in detail two (of the readily identified three):
Actuator Force (torque) →
Frictional Effects →
Tool Forces →
1TW q
2TW b q q
0ToolF in generalwill be taken
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Examining Friction – in detail
Defining a Generalized Coefficient of Friction for a link:
( )Kqv d s d
k K K K K Kb q b q SGN b b b e
C. Viscous Friction
C. Dynamic Friction
C. Static Friction
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Combining these components of Virtual Work:
1 2
TW W W b q q
F b q
leads to the manipulator Generalized Force:
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Building a General L-E Dynamic Model
Remembering:
1, ,i i ni i
dF L q q L q qdt q q
i
is a link of manipulator
Starting with this term
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Partial of Lagrangian w.r.t. joint velocity
, ,
i
L q q T q qq q
1
n
ij jj
D q q
It can be ‘shown’ that this term equals:
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Completing the 1st Term:
1
, n
ij jji
L q qd d D q qdt q dx
This is found to equal:
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Completing this 1st term of the L-E Dynamic Model:
1 1 1
n n nij
ij j k jj k j k
D qD q q q q
q
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Looking at the 2nd Term:
, ,i i i
L q q T q q U qq q q
31 1
1
( )
( )2
n nkj
k j nk j ji
k j kik j i
D qq q
qg m A q
This term can be shown to be:
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Before Summarizing the L-E Dynamical Model we introduce:
A Velocity Coupling Matrix (4x4)
A ‘Gravity’ Loading Vector (nx1)
1 1 , ,2ikj ij kj
k i
C q D q D q i j k nq q
for
3
1
nj
i k j kik j i
h q g m A q
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The L-E (Torque) Dynamical Model:
1 1 1
n n ni
i ij j kj k j i ij k j
D q q C q q q h q b q
Inertial Forces
Coriolis & Centrifugal
Forces
Gravitational Forces Frictional
Forces