Calibration of CMO-Stereo-Microscopes in a Micro Robot System

CalibrationofCMO-Stereo-Microscop esinaMicroRob otSystemGaudenz DanuserandOlaf K ublerImage Science Lab oratorySwissFederal Institute of Technology, CH-8092Z urichphone:+411632f5281,5282gfax:+4116321199email:fdanuser,kublerg@vision.ee.ethz.chKEYWORDS:LightMicroscopy,StereoVision,Calibration, BundleAdjustment,Image Distortion,MicroRob ot VisionABSTRACTMicro-Rob oticshasb ecomeanactive eldofresearchinthelastyears.Onema jorproblemthisnewtechnologyishowtobuildasensorsystemthatcancontroltherob otmotion within therequiredaccuracy.Our approach is based on microscopic vision as it has the advantage that the rob ot and the target ob jects canb e observedand p ositioned within a larger eld of view.The strong requirements for accuracy and reliabilitydemand a thorough sensor calibration. A versatile photogrammetric approach for three dimensional calibrationof a Stereo Light Microscop e with a Common Main Ob jective (CMO) lens is prop osed.We particularly addressthe problem of how to set up calibration standards with submicron precision and how to comp ensate the imagedistortion caused by the CMO lens.Numerical results based on simulated data are discussed.1INTRODUCTIONA newchallengeof scienceand technology in thelow micrometer scaleisthehandling of ob jectsas individ-uals, not as ensemble memb ers.Anticipating an emerging imp ortance of micro-technology, the SwissFederalInstituteof Technology starteda pro jectthataims at thesemi automated manipulation of micro-parts withsub-micrometer precision.The ob jectives of a rst b enchmark are to selectively grasp diamond mono-crystalswith the dimension of some tens of micrometers, to transp ort them through a working space of ab out 1cm3andto place them in correct orientation with a three-dimensional (3D) precision of ab out 1minto dimples etchedona silicon wafer.To ful ll thistaskwearedeveloping asixdegreeof freedom(6DoF) rob otsystem.The rstexp erienceswiththissystemwill leadusinto awide eldof p otentialapplications in micro-machining,micro-mechanics and micro-surgery.Theminiaturizationofassemblybyrob otsisaccompaniedincreasingdemandsonprecisionanddevicecontrol.Classical \macroscopic" rob ots areusually working in an \op en-lo op" mo de.This means, that afterde ningacertaintaskorsequenceoftasks(thiscanb eachievede.g.by xedworkingprogram,sceneanalyzing system or a human op erator), the rob ot is able to carry out the exp ected functions using its ownlocalsensors.Based on a thorough calibration, the various sensor informations can b e transformed into an absoluteworld co ordinateframe.This approach is basedon temp oral and random errorsthat areb elow thesp eci edworking precision.Both assumptions are not valid in the case of micro-rob otics.Therefore, it is imp erative touseglobalsensorswhich are able to measure the geometrical relation b etweenthe target ob ject and the rob otto ols within a larger eld of view (FoV). An ideal global sensorwould provide6D (p osition and orientation)data of theentireworking spaceon the requiredprecision level and in real-time.Unfortunately, thistyp e ofsensor do es not exist.To minimize the trade-o b etween the size of the FoV, the sp eed and the precision we prop ose to employ stereolight microscopic vision.Stereomicroscop esdeliver3D data of themicro-world and, compared toany othersensor system, have by far the largest FoV p er time unit.The price for this is the high computation cost, as thedata for the rob ot control must b e extracted from stereo image sequences.A second problem is that o cclusionsmay cause data drop-outs during the grasping pro cedure of the rob ot.For that reason we decided to add some