Transcript Concepts for a Kinematically Coupled Robot Baseplate System

High-Accuracy, Repeatable Wrist Interface
Pat Willoughby ([email protected])
Prof. Alexander Slocum, Advisor
MIT Precision Engineering Research Group
August 15, 2001
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Overview of Common Coupling Methods
Pinned Joints
Flexural Kin. Couplings
Elastic Averaging
Kinematic Couplings
No Unique Position
Kinematic Constraint
Non-Deterministic
Kinematic Constraint
Quasi-Kinematic Couplings
Near Kinematic Constraint
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Exact Constraint (Kinematic) Design
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Exact constraint means a component has an equal number of constrained points to
number of degrees of freedom
If component is over constrained, clearance and high tolerances required to prevent
premature failure or assembly incompatibility
Kinematic design means that the motion is exactly constrained and geometric equations
can be written to describe its motion
Kinematic Couplings constrain components exactly, commonly providing repeatability
of ¼ micron or on the order of parts’ surface finish
Managing Hertz contact stresses is the key to successful kinematic coupling design
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Prototype Coupling Designs – Canoe Ball
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Prototype Coupling Designs – Three Pin
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Schematic of Three Pin Coupling Design
Preload Bolt and
Third Pin
Two “Pins” on Arm Plate
Two “Holes”
On Wrist Plate
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Prototype Wrist Plate Mounting
Tests at ABB Robotics Vasteras, July/August 2001:
 Test static (bolted) and dynamic (5-point
path) repeatability of canoe ball and threepin wrist prototypes
 Test variety of preloads (canoe balls)
 Replacement in two orientations (45 and
90 degrees to ground)
 Measure tool point motion using Leica
LTD500 Laser Tracker
 Repeatability of robot path +
measurement system approximately 20
microns
Average Repeatability for Each Case
0.4500
0.4000
0.3500
0.3000
0.2500
0.2000
0.1500
0.1000
0.0500
0.0000
Case 1
Case 2
Case 3
Case 4
Case 5
Case 6
Case 7
Case 8
Case 9
Case 10
Case 11
Case 12
Case 13
Case 14
Average Repeatability (mm)
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Repeatability Performance
Repeatabilty for Best Cases
0.1200
 Canoe balls vs.
Normal Wrist @
45 deg = 35%
reduction
 Canoe balls vs.
Normal Wrist @
90 deg = 64%
reduction
 Three-pin vs.
Normal Wrist @
45 deg = 44%
reduction
Repeatability (mm)
0.1000
0.0800
0.0600
0.0400
0.0200
0.0000
Normal Wrist
Canoe Balls with Canoe Balls with Three Pin, Before Canoe Balls with
Proper Preloading, Proper Preloading, Damage, Dynamic Proper Preloading,
Static 45 deg
Dynamic 5 Point 5 Point Positions
Static 90 deg
Position
Positions
Position
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Recommended Next Steps
 Test three pin coupling in lab setting for ideal case
repeatability
 Adapt canoe ball design to fit into space of wrist
 Suggest production designs for different concepts
 Investigate:
• Three pin coupling in 90 degree position
• Effect of friction reduction using TiN coated
elements or lubrication
• Coupling design independent of mounting
position
• Applicability quasi-kinematic couplings
 Evaluate long-term dynamic performance
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High-Accuracy, Quick-Change, Robot
Factory Interface
John Hart ([email protected])
Prof. Alexander Slocum, Advisor
MIT Precision Engineering Research Group
August 15, 2001
MIT Confidential
Project Goals
Design, test, and demonstrate production feasibility of a modular
robot baseplate with kinematic couplings as locators:
 A repeatable, rapidly exchangeable interface between the foot
(three balls/contactors) and floor plate (three grooves/targets)
 Calibrate robots at ABB to a master baseplate
 Install production baseplates at the customer site and calibrated
the kinematic couplings directly to in-cell tooling
 Install robot according to refined mounting process with
gradual, patterned preload to mounting bolts
 TCP-to-tooling relationship is a deterministic frame
transformation
 Base calibration data handling is merged with ABB software,
enabling 0.1 mm TCP error contribution from repeatability
and exchangeability error of kinematic couplings
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Prototype Coupling Designs
Design 3-point kinematic coupling mounts for the 6400R foot:
Canoe Ball
 Six “point” contacts
 0.5 m radius ball surface
 20 mm diameter elastic
Hertzian contact
Three-Pin
 Three line + three surface contacts
 In-plane preload overcomes
friction to deterministically seat
pins
 Vertical bolt preload engages
horizontal contact surfaces
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Prototype Coupling Designs
Groove/Cylinder
 Twelve line contacts
 Aluminum cylinders
 Apply bolt preload (elastic
deflection of cylinders) for
dynamic stability
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Prototype Base Mounting
Tests at ABB Robotics Vasteras, July/August 2001:
 Static (bolted) and dynamic (5-point path)
repeatability of canoe ball and three-pin
interfaces
 Static (manipulator rest only) repeatability
of groove/cylinder interface
 Test both basic (air wrench) and refined
(torque wrench, greased bolts) mounting
processes
 Measure tool point motion using Leica
LTD500 Laser Tracker
 Repeatability of robot path +
measurement system approximately 20
microns
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Repeatability Performance
0.45
Repeatability [mm]
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
BMW Three-pin: Three-pin:
base (diff. basic
refined
robots) mounting mounting
Canoe
balls:
basic
mounting
Design
Canoe Aluminum Aluminum
balls:
cylinders: cylinders:
refined
(static)
(static)
mounting
basic
refined
mounting mounting
 Three-pin vs. BMW base
= 83% reduction
 Canoe balls vs. BMW
base = 85% reduction
 Cylinders vs. BMW base
= 92% reduction
 Refined mounting vs.
basic mounting = 5070% reduction
 8-bolt blue pallet
repeatability (not shown)
= 1.63 mm
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Exchangeability Performance
Simulate exchangeability error from manufacturing variation:
 Calibrate interfaces by measuring
contacts and calculating interface
error transformation
 Model direct measurement of pins +
contacts, and offset measurement of
canoe balls
 Exchangeability is error between
calculated and true interface
transformation, given chosen level of
calibration and manufacturing
tolerances (low, med, high)
 250-trial Monte Carlo simulation in
MATLAB at each calibration level
Three-pin exchangeability:
0 = no interface calibration
3 = full (x,y,z) of pins and contact
surfaces
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Total Mechanical Accuracy
“Quick-Change” Accuracy = Repeatability + Exchangeability
(measured)
Canoe balls
Three-pin
Groove/cylinder
0.22 mm =
0.12 mm =
=
0.06
0.07
0.03**
(simulated)
+
+
+
0.16*
0.05
(Incomplete)
 Interface calibration decouples accuracy from manufacturing tolerances of mounting
plates and couplings (if direct measurement of contacts)
 Results show repeatability is highly f(mounting process) – this may present a
performance limit for factory mountings; interface should be micron-repeatable under
perfect conditions
Totally, a near-deterministic prediction of robot interface accuracy
*driven by error of offset position
measurement
**static only
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Recommended Next Steps
 Test groove/cylinder interface with preload +
motion
 Test traditional quasi-kinematic couplings
 Evaluate long-term dynamic performance
 Design production three-pin adaptation to BMW
2-pin base
 Evaluate canoe ball 4-point mounting for
Voyager?
 Build kinematic coupling “Expert System” –
combine test results, simulation results, etc. into
design tool that gives minimum cost design
recommendation and tolerance budget as
f(accuracy requirement)
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