Absolute Accuracy Presentation

Precision Robot Calibration Using Kinematically Placed Inclinometers Alec Robertson 1 Patrick Willoughby 2 Alexander Slocum 2 1 ABB Robotics, Vsters, Sweden 2 MIT, Cambridge, USA ASPE Annual Meeting, November 10 15, 2001 ABB Robotics MIT PERG Industrial Robots Precision Engineering Project Overview Functional requirement Calibrate the robot home position by leveling align each axis to be perpendicular or parallel to gravity vector

Project aims Determine the accuracy of the existing robot leveling system Design and construct a new leveling system with significantly improved accuracy Provide a simple to use software automation tool to allow rapid and consistent calibrations Project phases Prototype 1 Mount 2 sensors on an improved mounting bracket Automate with PC based software Prototype 2 Design and build a simple concept unit using kinematic couplings Automate with controller based software Prototype 3 Design and build accurate customer demonstration unit Product Integrate into the existing ABB product line

Existing System Current leveling process Problems Mount Wyler ZeroTronic inclinometers on sensor plates Place mounting plates on robot structure Place reference sensor plate on robot foot Place leveling sensor plate on mounting plates on robot structure Move (jog) robot manually until leveling sensor matches reference sensor Update home position in controller Poor process repeatability Difficult to accurately place mounting plates Large user-dependent errors from manual jogging of robot axis 4,6 0 axis 3 0 axis 5 0 Z Y reference

sensor 1 Key hand leveling sensor X axis 2 90 unit 2 Wyler spread foot 0 12 Ideal tool0 Actual tool0 Design Requirements Analysis of existing system Recalibration errors

At robot flange 1.0mm At 350mm tool 1.5mm Error breakdown Design goals Improve process repeatability No separate mounting plates or components High accuracy mounting unit Compact and compatible with existing robot variants Absolute measurement not relative Software process automation Recalibration error 0.2mm Prototype 1 Proof of concept Software automation Mount sensors on single structure Set reference levels on robot foot Place on existing mounting plates on wrist, lower arm, upper arm and flange PC based software automation of controller and sensors Calculate and apply axis adjustments Wrist (axes 34)

Isometric View Front sensor Bottom View Control holes Flange (axes 56) Lower arm (axis 2) g Side Sensor Under body (axis 1) Foot (reference) Prototype 1 unit Software automation IRB6400R-2.5m-150kg Prototype 2 Cube construction Mount magnets and tooling balls on separate steel plates

Assemble plates into cube and mount sensors Use kinematic couplings for high accuracy placement on robot structure Plate construction Machine separate grooved plates Bolt onto existing mounting locations in robot structure Use hardened steel plates to maximize magnet preload force and minimize long-term wear Axis 2 mount Foot/Wrist mount Prototype 2 Axis 1 mounting Axis 2, wrist mounting Measure rotation of axes parallel to the gravity vector Use dynamic kinematic coupling to ensure accurate measurement Magnet preload holds unit in place and allows accurate re-alignment

within coupling geometry during robot motion Axis 1 mount Dynamic kinematic coupling Use standard, stationary groove kinematic couplings Flange mounting Axes 5 and 6 calibration highly sensitive to cube orientation Press-fit groove plate into robot flange as an alternative to adaptor plates Flange mount Prototype 2 Test results Tested at BMW Germany on an IRB6400R-2.5m-150kg robot Average recalibration error = 0.4mm Prototype 3 Conceptual Design Mount sensors in a single-piece open cube structure

Magnet bolted to front and bottom surfaces of cube structure Smaller grooves and thinner plates for reduced profile Open cube structure and low profile groove plates Final Design Solid cube structure requires only three precision machined surfaces Size: 404060mm. Weight: 500g Full software automation implemented in robot controller Recalibration accuracy = 0.2mm Product Options Upgrade kit for existing robots Bolt groove plates onto existing mounting locations Machine grooves directly into flange 1 4 2 3 1.

Foot 2. Axis 1 3. Wrist 4. Flange 5. Axis 2 5 Integrated product for new robots Standard calibration method for new heavy duty robot IRB7600 Machine grooves directly into robot structure

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