large machine compensation- improving … machine compensation-improving accuracy, reducing...
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Large Machine Compensation-Improving Accuracy, reducing
compensation time
Rob FlynnElectroimpact, Inc
CMSC 2011
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Today’s Scope
• Who is doing Volumetric Compensation?
• What are the Benefits – time and accuracy
• Universal implementation tips– Targeting
– Triggering
– CNC software maintenance tools
• Uncertainty Analysis for better results– Case studies
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Large machine benefits
• Volumetric comp works with any size
• Benefits may be more obvious for large machine
• Applicable to standard machines as well as bespoke
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Large versus small
• Large machine – may not be able to grind large parts
• May not be able to machine parts in single setup
• Tracker error a much smaller % of volume
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Large machines?
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Volumetric Comp Developers
• API-Boeing-Cincinnati MAG-Siemens-DOD team VALMT
• Electroimpact – Todd Rudberg
• Renishaw• Precision Technologies (UK) - University of
Huddersfield
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Few players but more coming…
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VALMT
• US research project, Volumetric Accuracy for Large Machine Tools (VALMT)
• API, Boeing, Siemens, MAG Cincinnati and DOD
• Siemens VCS is the control technology
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Electroimpact Volumetric Comp
• Developed by Todd Rudberg
• Successfully Used with up to 7 axis machines
• Method predicates resultant accuracy prior to final check – validates metrology
• Used on 25+ large machines
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CNC Suppliers with volumetric comp features
• Siemens
• FANUC
• Fagor Automation
• Others?
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Volumetric Compensation – Metrology Software of Interest
• New River Kinematics (started with Robots)
• Metrologic
• Verisurf
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What are we compensating?
• Only repeatable errors.
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Some machine error sources
• Consider an X axis – rack and pinion drive with encoder feedback: leaves several types of error– Pinion-rack backlash (not corrected with
compensation)
– Gearbox hysteresis
– Rack pitch error (correctable with single axis compensation)
– Transient errors such as structure deflection
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Secondary errors
• As the machine moves vertically (Y axis), axis non-perpendicularity causes Z and X error
• As machine moves in X, 2 axis bed height variations cause the toolpoint to move in X, Y and Z.
• Additional X for X, X for Y, X for Z
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Mechanical Errors
• Generally accepted 43 mechanical error sources for a 5 axis machine
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CAN WE COMP ERRORS?
• SINGLE AXIS ERRORS CAN BE COMPENSATED
• AXIS FOR AXIS ERRORS – MAY OR MAY NOT BE COMPENSATED, DEPENDING ON CNC
• MAY RUN OUT OF OVERLAY TABLES• SOME ERRORS MAY REQUIRE MECHANICAL
COMPENSATION
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Why Volumetric Comp?
• Eliminates much or all mechanical compensation
• Potential for improved accuracy
• Provides complete kinematic model of machine behavior
• Much faster process for many machines
• Eliminates axis by axis comp tables in CNC
• Enables more complex machine benefits
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OTHER BENEFITS
• ENABLES SHARED COORDINATE FRAME FOR CELL
• ENABLES SHARED PART PROGRAM FOR MULTIPLE MACHINES (E.G. LEFT, RIGHT, OR REDESIGNED MACHINE)
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Is Volumetric Method Valid?
• Test using random point check!
• Over 25 machines of similar scale successfully compensated with this process
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Not a substitute…
• Volumetric compensation is not a substitute for making a good machine. Still must have accurate parallel surfaces for linear bearing rails, etc.
• Must be stiff
• Must be very highly repeatable
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Solver behavior
• If compensation data is bad for any reason (e.g. tracker is bumped during session or machine is not repeatable in an axis), solver cannot resolve a complete solution.
• Bad data = NO SOLUTION
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Solver error
• Solver solutions are not perfect, due to tracker error
• More stations get you less tracker uncertainty
• More measurement accuracy gets you better solver solutions
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Tools to aid volumetric comp
• FRS – Foundation Reference System
• Improved triggering methods
• CNC Compensation screen (with CNC maintenance tools)
• Uncertainty analysis tool (e.g. SA, Metrologic)
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FRS aka Control Network
Foundation Reference System
•Permanent monuments
•Accurately valued
•Validated with independent measurements
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WHY A CONTROL NETWORK?
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Why a control network?
• Multiple stations can be bundled to improve accuracy for compensation…and a better solver solution!
• FRS is another tool to discover problems– Foundation cracking– Foundation curing shrinkage– Foundation shifting
• Tomorrow’s problem – an FRS may help answer hard questions in the future
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CNC Compensation Screen
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Triggering methods
• SA-measure stable point
• Swifty – dry contact to PC USB input
• X-Keys-serial tool for dry contact to serial input
• Closed loop method
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SWIFTY
• CONVERTS DRY RELAY CONTACT TO A USB INPUT
• COMES WITH INTERFACE SOFTWARE AND DRIVERS
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SWIFTY IMPLEMENTATION
CNC
RELAY
TWO WIRE CABLE TO SWIFTY
SWIFTY USB DEVICE
TRACKER PC
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Pneumatic trigger?
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Locating the tracker-considerations
• VERIFY THAT MACHINE MOTION DOES NOT INDUCE MOVEMENT OF TRACKER DUE TO FOUNDATION DEFLECTIONS
• CHECK LINES OF SIGHT FOR TARGETS/MEASUREMENT LOCATIONS
• MINIMIZE SHOT LENGTHS• FOR MULTIPLE STATIONS, TRY TO ACHIEVE
LARGE ANGLES FROM TRACKER TO MOST OF THE TARGETS
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UNCERTAINTY ANALYSIS – WHAT FOR?
• DETERMINE UNCERTAINTY FOR POINTS OF INTEREST
• QUICKLY EVALUATE ALTERNATE TRACKER LOCATIONS
• EVALUATE VALUE OF ADDITIONAL STATIONS FOR REDUCING UNCERTAINTY
IMPROVED ACCURACY FOR LESS WORK
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UNCERTAINTY ANALYSIS EXAMPLES
• CASE 1 – AFP MACHINE
• CASE 2 – MYSTERY MACHINE
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CASE 1
• MOVING COLUMN AFP MACHINE
• 19.5M x 6.4M x 4.2M ( 64‘by 21' by 14‘) WORK ENVELOPE
• 1000 POINTS USED IN COMPENSATION
• STATIONS USED: 1
• How much would we benefit from the use of additional stations?
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SINGLE STATION UNCERTAINTIES
• AVERAGE UNCERTAINTY 0.064mm
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CONFIGURATION “A”
• TWO STATIONS
• AVERAGE UNCERTAINTY: 0.051mm
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CONFIGURATION “B”
• TWO STATIONS
• AVERAGE UNCERTAINTY: 0.031
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CONFIGURATION “C”
• 3 STATIONS
• AVERAGE UNCERTAINTY 0.030
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CONFIGURATION “D”
• 3 STATIONS
• AVERAGE UNCERTAINTY 0.026
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RESULT OF SHIFTING STATION LOCATION
• 52% REDUCTION IN AVERAGE UNCERTAINTY IF WE ADD A SECOND STATION – IN THE RIGHT PLACE.
• ADDING A THIRD STATION DROPS UNCERTAINTY ONLY TO 59% REDUCTION FROM ORIGINAL.
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Uncertainty Results
Point 148 254 375 442 Average*
Single station .100 .083 .064 .075 .064
2 STATIONS “A” .044 .084 .037 .070 .051
2 STATIONS “B” .063 .059 .048 .054 .031
3 STATIONS “C” .029 .035 .023 .027 .030
3 STATIONS “D” .041 .047 .031 .038 .026
AVERAGE UNCERTAINTY IS FOR ALL POINTS IN THE GROUP, NOT JUST THE SAMPLES SHOWN
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Case 1 Volumetric Compensation Results
• Max radial error of < 0.2mm (0.008”) within the work envelope of 19.5M x 6.4M x 4.2M ( 64‘ by 21' by 14‘) (excluding measurement error)
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CASE 2
• VERTICAL DRILLING/FASTENING MACHINE
• WORK ENVELOPE 34M X 6M X 1M
• 700 POINTS USED IN COMPENSATION
• 2 STATIONS
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SINGLE STATION
• AVERAGE UNCERTAINTY: 0.11
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TWO STATIONS
• AVERAGE UNCERTAINTY 0.047
• N288 REVEALS WEAKNESS OF COLINEAR POINTS AND STATIONS
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Uncertainty Results
Point 288 223 82 287 145 Average*
SINGLE STATION
.26 .13 .13 .04 .04 .11
2 STATIONS .14 .04 .04 .03 .03 .05
. .
. .
. .
*AVERAGE UNCERTAINTY IS FOR ALL POINTS IN THE GROUP, NOT JUST THE SAMPLE POINTS SHOWN
ADDDING A SECOND STATION YIELDS A 55% DROP IN UNCERTAINTY OR DELTA OF 0.06MM (0.002”).
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Compensation case studies conclusion:
• Uncertainty analysis illuminates compensation metrology:– Quantifies benefits of adding a particular station
– Enables meaningful comparison of alternate metrology plans
– Allows quantitative comparison of alternate stations
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Summary
• A few companies are doing 5+ axis volumetric comp
• Benefits include reduced time to comp, accuracy
• Active Target and smarter triggering can help
• Use uncertainty analysis for better results
• Eyes open –always new developments!
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VNC server + VNC Client + wireless router + Ipod Touch = full remote
control of your tracker
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Thank you!
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