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MIT Confidential
High-Accuracy, Repeatable Wrist Interface
Pat Willoughby ([email protected])Prof. Alexander Slocum, Advisor
MIT Precision Engineering Research Group
August 15, 2001
MIT Confidential
Overview of Common Coupling Methods
Elastic Averaging
Non-Deterministic
Pinned Joints
No Unique Position
Kinematic Couplings
Kinematic Constraint
Flexural Kin. Couplings
Kinematic Constraint
Quasi-Kinematic Couplings
Near Kinematic Constraint
MIT Confidential
Exact Constraint (Kinematic) Design• 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
MIT Confidential
Prototype Coupling Designs – Canoe Ball
MIT Confidential
Prototype Coupling Designs – Three Pin
MIT Confidential
Schematic of Three Pin Coupling Design
Two “Pins” on Arm Plate
Two “Holes” On Wrist Plate
Preload Bolt and Third Pin
MIT Confidential
Prototype Wrist Plate MountingTests at ABB Robotics Vasteras, July/August 2001: Test static (bolted) and dynamic
(5-point path) repeatability of canoe ball and three-pin 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
0.0000
0.0500
0.1000
0.1500
0.2000
0.2500
0.3000
0.3500
0.4000
0.4500
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)
MIT Confidential
Repeatability Performance
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
Repeatabilty for Best Cases
0.0000
0.0200
0.0400
0.0600
0.0800
0.1000
0.1200
Normal Wrist Canoe Balls withProper Preloading,
Static 45 degPosition
Canoe Balls withProper Preloading,Dynamic 5 Point
Positions
Three Pin, BeforeDamage, Dynamic5 Point Positions
Canoe Balls withProper Preloading,
Static 90 degPosition
Re
pe
ata
bil
ity
(m
m)
MIT Confidential
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
MIT Confidential
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
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
Project Goals
MIT Confidential
Design 3-point kinematic coupling mounts for the 6400R foot:
Prototype Coupling Designs
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
MIT Confidential
Prototype Coupling Designs
Groove/Cylinder Twelve line contacts Aluminum cylinders Apply bolt preload
(elastic deflection of cylinders) for dynamic stability
MIT Confidential
Prototype Base MountingTests 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
MIT Confidential
Repeatability Performance
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
BMWbase (diff.
robots)
Three-pin:basic
mounting
Three-pin:refined
mounting
Canoeballs:basic
mounting
Canoeballs:
refinedmounting
Aluminumcylinders:
(static)basic
mounting
Aluminumcylinders:
(static)refined
mountingDesign
Rep
eata
bilit
y [m
m]
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 = 50-70% reduction
8-bolt blue pallet repeatability (not shown) = 1.63 mm
MIT Confidential
Exchangeability PerformanceSimulate 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 calibration3 = full (x,y,z) of pins and
contact surfaces
MIT Confidential
Total Mechanical Accuracy“Quick-Change” Accuracy = Repeatability +
Exchangeability
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
Canoe ballsThree-pinGroove/cylinder
(measured) (simulated)
0.22 mm = 0.06 + 0.16*
0.12 mm = 0.07 + 0.05 - = 0.03** + (Incomplete)
*driven by error of offset position measurement**static only
MIT Confidential
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)