The authors have previously proposed the use of finite element method (FEM) for the modeling of coordinate measuring machine probes. Whilst the modeling results have been published previously, this paper presents the detailed experimental validation to compare the FEM and experimental results. The comparison shows that the agreement is generally good with probing contacts at lower latitudes near the equator of the reference sphere. The differences between the modeling and experimental results become large at higher latitudes. This is believed to be mainly caused by the sliding effects which occur during probing contact in the experiments

Jones, BE

Salleh, MR

Wei, P

Yang, QP

English

Mohd Rizal Salleh

Qing Ping Yang

Peng Wei

Barry Jones

Crossref

Experimental Validation of FEA Modeling of Touch Trigger Probes

Brunel University Research Archive

Instrumentation and MeasurementTechnology Conference - IMTC 2007Warsaw, Poland, May 1-3, 2007Experimental Validation of FEA Modeling of Touch Trigger ProbesMohd Rizal Salleh, Qing Ping Yang, Peng Wei, Barry JonesSchool of Engineering and Design, Brunel University, Middlesex UB8 3PH, UKPhone:+44 1895 265854, Fax:+44 1895 269763, Email: emstppygbrunel.ac.ukAbstract -The authors have previously proposed the useof Finite Element Method (FEM) for the modeling ofcoordinate measuring machine probes. Whilst the modelingresults have been published previously, this paper presentsthe detailed experimental validation to compare the FEM andexperimental results. The comparison shows that theagreement is generally good with probing contacts at lowerlatitudes near the equator of the reference sphere. Thediferences between the modeling and experimental resultsbecome large at higher latitudes. This is believed to bemainly caused by the sliding effects which occur duringprobing contact in the experiments.Keywords - Coordinate Measuring Machines (CMMs), TouchTrigger Probe, Pre-travel Variation, Finite Element Method (FEM),Model ValidationI. INTRODUCTIONProbe lobing and, in particular, probe pre-travel variationsis the highly repeatable errors that have been studied by manyresearchers because of their great significance in CMMmeasurements [1] - [8]. They are usually characterized by thekey influencing parameters or factors, e.g. probe styluslength, probing angle, probe orientation, probe stiffness, etc.There are generally four methods used by various researchgroups. The early investigations were mainly performed byexperiments where the relationships between the probe errorsand the influencing variables are determined. This method isstraightforward in principle, however, it is very timeconsuming due to the nature of 3D measurements. From theexperimental data, neural network models can also be readilydeveloped. Although neural network is very powerful forerror prediction and correction, it experiences the sameproblem since large amounts of data are required to train theneural networks [2] - [3]. Another obvious approach isanalytical mathematical models, which have been developedfor some probes and their operations [4] - [6]. Analyticalmathematical model is a simple, thus preferred modelingmethod, but it is limited to probes with simple structure.The authors have proposed the use of finite elementmodeling (FEM) as a general approach to probe modeling[7]. The initial results and subsequent refinements have beenreported in [8], together with the discussions of theapplications of the FEM results, e.g. for the evaluation ofprobe measurement uncertainty. As theoretical models, FEMresults have to be experimentally validated. Whilst theinformal validation has been described in [8], this paperpresents the detailed experimental validation. Thecomparison between modeling and experimental results undervarious conditions is presented.(a)/n~~~/I/A(b)Fig. 1: (a) Probing angles and (b) Probe orientationII. PROPOSED METHODOLOGYIn order to compare the modeling and experimental results,they should ideally be obtained from the same probe. Sinceour FEM was based upon a touch triggering probe, which hadbeen mechanically disassembled, the experiments wereperformed with a different probe but of the same type anddesign. Further, the modeling and experiments wereperformed under the similar conditions with the sameparameter values. The comparison mainly considers theinfluences of the key parameters, i.e. probe stylus length,approaching direction, probing latitude, probe orientation.The experimental setup was based upon the measurementof a reference sphere, which simplifies the collection of the1-4244-0589-0/07/$20.00 ©2007 IEEE 1experimental data under the influences of the aboveparameters. The modeling was performed with the followingconfigurations: three stylus lengths: 1=7 mm, 20 mm and 50mm, with the total length including stylus holder is 17 mm,30 mm and 60 mm, respectively. The details of probe modelhave been explained in [7] and [8] including the dimensionsand materials properties. However, for validation purpose, 20mm and 60 mm styli were selected for experimentalvalidation.The influence of stylus length was studied together withthose of probe orientations A and probing angles, and 0,which representing longitude and latitude, respectively. Theyare defined in Fig. 1. 0 has an interval of 150, and has aninterval of 300, and probe orientations A at 0 and 450.The contact force at different probing angles was generatedfrom the mathematical modeling of probe in [4], which variesslightly at different probing angles. The error resultsgenerated from FEM are then directly compared withexperimental results to validate the modeling.Experimental DesignThe experiments were performed using a Renishaw TP2-5W touch trigger probe with a Mitutoyo FN503 CMM. Dueto their significant influences on the probe performance, thefour parameters mentioned above were selected, i.e. styluslength 1, probe orientation A, and approaching directions 0and y.In each experiment, six latitudes with 36 probing points ateach latitude and one point at top of the reference ball withthe total 217 points were measured, as shown in Fig. 2. Theinterval between points: y, is 300 and half of the referencesphere was equally divided by 0 at 150 between latitude. Eachset of experiment used different combination of y, 0, L and A.All measurements were repeated three times. Renishaw'sprobe head, PH9, was used in experiment to set the probeangle A. It was first set at vertical i.e. A=00, the equallydistributed 217 points were measured at the six latitudes andat top of reference sphere. The probe body and stylusassembly was then rotated to 450 and similar procedure wasapplied to measure half of reference sphere.7.43 }Fig. 2: Experimental sampling plan of 0Other probing characteristics were kept constant with astylus diameter of 3 mm, stylus tip diameter 4 mm.Minimum trigger force was set to 7 - 8 gf. Operatingtemperature was 20 ± 2°C at 4000 relative humidity.Reference ball diameter was 19.9960 ± 0.0005 mm.Both FEA and experimental probe errors were computedusing the measured coordinate points on the surface ofreference sphere. The distance of a measured point relative tothe centre of the reference ball is defined as r1:i= - X )2 + (yi - y) + (zi - zc (1)where i = 2, 3 .......n; (xc, Ye, zc) is the center of referenceball. The best fit least square circle, Rj, for j= 1, 2 ..... ,6(latitude number) is obtained by averaging ri, and hence1inR = Irnn i=l(2)The pre-travel at ith latitude, 4, becomes3i Ri r (3)III. RESULTSFig. 3: Comparison of the modeling and experimental errorresults (with 20 mm and 60 mm styli)In general, the probe error patterns reflected the internalmechanical construction of the probe with three lobingscorresponding to the tripod legs that support the stylus holderand stylus stem. Furthermore, stylus length significantlyinfluences the size of probe lobing as shown in Fig.3. It issignificantly increased and varied when longer styli wereused. The lobing is proportional to stylus length. Fig. 3shows that the agreement between experimental and FEAmodeling results of 20 mm stylus was generally good withsmall differences. However, the differences become largerfor 60 mm stylus. The standard deviations (SDs) ofexperimental and FEA results for 20 mm stylus are 3.05ptmand 2.39ptm, respectively. The SDs for 60 mm stylus are 3.07ptm for experiment and 10.89 ptm for FEA modeling.260.0050.00-40.00 -30.00 -.0 0-10.000.00 0-10.00 1-20 X 4-0 3-20 .00 - Probing IndexX Exp. 20mmnu FEA 20 nmA Exp. 60mmnun FEA 60 nmFurther validation was performed on 20 mm stylus tovalidate the model in accordance with probe orientation andprobing direction.Fig. 4 and 5 show the results of modeling and experimentswhen both were performed at A=00 and 450, respectively. Itcan be seen that the errors patterns were similar with a smallO (<300) and the differences becomes larger when 0> 45°.They generally show how stylus configuration, contact pointsand approach directions influenced the probe lobing.180+ FEA result -n - Experiment0 = 000340 3jkoo-- -_10 20330 2 30/<>320/ IxA40310/ -/<T\\ 150300 5060290 ~ ' ~70280 L 81270 9'260 lo-q1250 iio~11240 ~~~~~~~~~~120230 /130220 1'l40210 15020 9 7016180-.-FEA -- Experimentt0 =450'40\' ts\/-/3 2120230 y 130220\ L 140210 150200190 170160180FEA -w - Expernmert0= 150220 1\\ /140210 1~--5020019-0o' 170'160180-.--FEA -- - Experiment0 = 600310 \50300 60290 70,80 8070~~~~q 902500110230* Y130220 1'I40180FA-U - Experiffeert0= 300310 5000 - 6070100908011040 120230 130220" 1>I40210 1502019 170,160180--FEA -U - Exper'iment0 = 750Fig. 4: Validation ofFEA model with experimental results at A=0003403o - ° 20320/< -\|< 40310 ' \503007 \60290. .70280. 80270 90260\ 100250\ -* 11024O0\ \ 120220\ '-14020019-q 170-160180+ FEA result Experinent0 = 0010 2330 3320 \40310A d \\5-300/4,290 /~~280270260250240 --220Xt '5210 150200 190 - 70 1180EA -> - Expenrinm50>\60\\70_ 80- d 90- 100110/1201301400= 1500320,< X40310/ >50300/ - 60290 f 70280 /_ J 80270 I- I- -F 90260 100250 110240\',V /120230'< C130220'<K / YI 402102O 170160180*-FEA -u - Experimerlt0 = 3003300/, \290280 A270260'\250-240\~230K /220 /140210 1~~~~502019& io16180FEA -U - Expen'ment0 =450\60\70- 80- 90/ 100,/110230' /- IIY130220OK--~140210 150--q180FEA -U - Expen'ment0 = 6003;3 1 0/300/290 /280 1- _270 1- -I260 \250\240\<2302:034Q-3"O /.JO3 0'~ 0-30ooO0W°\40 IdO %A' _/ '/-4 \50\607080100110-N 120Y13020 K>140210o ~ ~ '1502 0019&- -11ro160180---- FEA -U - Expen'ment0 = 750Fig 5: Validation ofFEA model with experimental results at A=45°The large differences at the higher latitudes are bbe caused by the sliding effect during the probingthe experiments. The sliding effect was not consideFEA modeling, which assumed that all conperpendicular to the probe surface. Fig. 6 indicateprobing contacts without sliding effect will sigincrease the errors at higher latitudes. Accordi]operating principle of the actual probe, when thcontact is at higher latitudes, even though the coimade perpendicular to the surface, the sliding wioccur, which will significantly reduce the lobing oreffect. Further discussion on this can be found in [4E 3530*° 25> 2015-~10;c-o 10`JVC* 0//l'- - it/U-no000--00 150 300 450 600-*-- ENperiment -U - FEAielieved tocontact inred in theitacts are-s that the,nificantlyng to thee probingntacts arebelieved to be mainly caused by the sliding effects whichoccur during probing contact in the experiments.The proposed methodology of using FEA for probemodeling has been successfully demonstrated. It can be usedin the further studies of various probe characteristics and toimprove the design and performance ofCMM probes.REFERENCESill always [1] John A. Bosch, "Coordinate Measuring Machines and Systems",pre-travel Marcel Dekker Inc., 1995.and [6]. [2] Q. Yang, C. Butler, P. Baird, "Error compensation of touch triggerprobes", Measurement Vol. 18, No. 1, 1996, p47 - 57.[3] Y. Shen, S. Moon, "Mapping of probe pretravel in dimensionalmeasurements using neural networks computational technique"Computers in Industry 34, 1997, p295 - 306.[4] W. Tyler Estler, et al, "Error Compensation for Touch Trigger Probes",Precision Engineering 19, 1996, p85 - 97.* [5] A. Wozniak and M Dobosz, "Metrological Feasibilities ofCMM TouchTrigger Probes. Part I: 3D Theoretical Model of Probe Pre-travel",Measurement 34, pp 273-286, 2003.[6] Y. Shen and X. Zhang, "A pre-travel model for touch trigger probeswith straight styli, Part I: Vertical probes", Proceedings of NorthAmerican Manufacturing Research Conference, Vol. 24, SME, AnnArbor, Michigan, 1996, p127 - 132[7] M. R. Salleh, Q. Ping Yang, Barry J., "Finite Element Modeling andSimulation of Probe System", Instrumentation and MeasurementTechnology Conference, 2005. IMTC 2005 Vol. 2, 16 - 19 May 2005,750 p849- 852.[8] M. R. Salleh, Q. Ping Yang, Barry J., "Evaluation of Touch TriggerLatitude Probe Measurement Uncertainty Using FEA" Instrumentation andMeasurement Technology Conference, 2006. IMTC 2006, 24 - 27April 2006, p833 - 836.Fig. 6: Probe errors at different latitudes (A=0°)IV. CONCLUSIONSThe FEA modeling results have been systematicallycompared and validated against the experimental results. Theagreement is generally good with shorter stylus length andprobing contacts at lower latitudes near the equator of thereference sphere. The differences between the modeling andexperimental results become large at higher latitudes. This is440

A pre-travel model for touch trigger probes with straight styli, Part I: Vertical probes&quot;,

Coordinate Measuring Machines and Systems&quot;, pre-travel Marcel DekkerInc.,

Error Compensation for Touch Trigger Probes&quot;,

Evaluation of Touch Trigger Latitude Probe Measurement Uncertainty Using FEA&quot;

Finite Element Modeling and Simulation of Probe System&quot;,

Mapping of probe pretravel in dimensional measurements using neural networks computational technique&quot;

Experimental validation of FEA modelling of touch trigger probes

Experimental validation of FEA modelling of touch trigger probes

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