Post on 14-Apr-2018
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Robotic Systems(9b)
Dr Richard Crowder
School of Electronics and Computer Science
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Remote Centre Compliance - RCC
TheRCCsystem is a passive system, and relies on a springand link structure.
The system can correct for minor error in position, both
rotational and translational.
The design relies on the RCC and the object withstandingthe applied forces
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Remote Centre Compliance
Translation Rotation
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Remote Centre Compliance: Applications
Clearance Fits
Shafts onto bearings
Rivets into holes
Interference Fits
Bushings into housings
Shafts onto laminate stacks
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TheInstrumented RCC
Identical structure as the RCC, with the springs being replace byelastomer, and fitted with sensors.
The resultant movement is used to determine the tips motion throughthe robot, and hence the applied force.
Typical application: tip force is constant, the IRCC will remain in
constant position relative to the surface
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Peg in Hole Problem
Many robotic applications require the insertion of a peginto a hole.
With a fixed system, minor errors in the task model will
lead to a failure to complete.
Fine motion is required to accommodate these errors either by passive compliance or active force control.
Four stages to the process
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Stage 1 - Approach
All forces are zero
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Stage 2 Chamfer Crossing
Constrained motion
Trans(z )by chamfer
Trans(x) by the object
Rot(y) by the object
Z
X
Z
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Stage 3 One-point contact
Rotation around
the Y axis is possible
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Stage 4 Two point contact
Further rotation around the Y axis
To align the hole and peg axis.
At this stage the peg can either
jam or wedge in the hole
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Jamming and Wedging Jamming occurs when the applied forces and moments are incorrect and can be
resolved by the removal of forces
Wedging is geometric and hence the removal of external forceswill notresolve the issue.
For satisfactory insertion:
Initial position error must be less than the width of the chamfer or chamfercrossing will not occur.
To avoid wedging the orientation error must be
where R is the diameter of the hole, r the pegs diameter and thecoefficient of frictionThe forces must be within the jamming parallelogram
R
rR
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Jamming Parallelogram (information only)
Two point contact
One point contact
One point contact
Two point contact
12
1
1
1
z
x
f
f
xrf
M
r2
l
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Force Control
Distance not exactly known
Consider a robot
clearing a window
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Force Control
Distance not exactly known
Robot under position
Control applies enough
force to reach target
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Force Control
Distance not exactly known
Under force control,
Applies a defined force
to the end effector, will
stop if the object can
withstand the force
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Constrains
Natural Constraints
Derived from the compliance frames of the application
Artificial Constraints Derives from the control requirements
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Application of constraintsZ
XY 0v
0
0n0
0f0v
Natural
z
y
zx
yx
z
y
xz
xy
f
0n
0n0f0v
Artificial
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Force Sensor
Normally at least eight sensors
are required to fully resolve
all the forces
[F] = [D][S]
[F] is the force vector
[S} is the sensor vector
[D] is the calibration matrix (6n)
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Maltese-cross Force Sensor
W1 W2W7
W8
W3
W4
W5W6
Fz
Fx
Fy
x
y
z
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8
7
6
5
4
3
2
1
67656361
56
4844
38363432
2521
1713
0000
00000520
000000
0000000000
000000
=
ww
w
w
ww
w
w
cccc
cc
cc
cccccc
cc
FF
F
z
y
x
z
y
x
Solution
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Wrist Sensor
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Hybrid Position-Force Controller
Gain
Control
Control
Arm
ControlJ-1
JT
JT
I-S
I-S
S
S
J
J-1
Torque
ForwardKinematics
Position
Speed
Force