Optical Coordinate Measuring Techniques for the Determination and Visualization of 3D Displacements in Crash Investigations

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1 03-B55 ptical Coodinate Measuing Techniques fo the Detemination and Visualization of 3D Displacements in Cash Investigations Dik Behing, Jan Thesing GM mbh Holge Becke, Robet Zobel Volkswagen AG Copyight 2001 Society of Automotive Enginees, Inc. ABSTRACT Volkswagen Accident Reseach The measuement of 3D coodinates using optical techniques is well known fo moe than 50 yeas. Today, moden photogammetic systems ae based on handheld digital cameas and ae used to identify the location of any cicula make o featue on the object's suface. The ease of use and the accuate and automated deivation of 3D coodinates fom 2D digital images helped to establish a poweful tool fo position contol, assembly checks and evese engineeing. Documentation of the accident scene Analysis of vehicle damage Figue 1: Accident eseach at Volkswagen AG Evaluation of injuies A new application is the analysis of eal vehicle cashes. The location of hundeds of makes on the damaged vehicle can easily be detemined in vehicle body position. These coodinates ae being compaed to the undefomed geomety and povide heby 3D infomation on any displacement. Using evese engineeing techniques, sufaces ae ceated fom the 3D points and thus a 3D model of the cashed vehicle is available fo an easy visualization of the defomation. INTRDUCTIN Volkswagen pusues accident eseach. Numeous accident statistics and local accident suveys have been analyzed statistically to gain knowledge of the accident event. The goal is to impove vehicle safety. The main taget of the accident eseach is the detemination of the defomation duing the couse of the accident. Compaison of passenge injuies and defomation duing all phases of an accident allows to daw conclusions which could lead to an optimization of vehicle safety. FRMULATIN F THE RBLEM In ode to detemine the enegy absoption of an accident vehicle, detailed infomation concening the vehicle stuctue and the stuctual defomation duing the couse of the accident ae equied. Fo this eason, vehicles fo cash tests ae measued befoe and afte the test at pe-defined distinct points. Typically, coodinate measuing machines ae applied fo this task. The elationship between the loading of the cash test dummies and thei contacts with the vehicle inteio duing the single cash phases can then be estimated. The enegy absoption duing the couse of a cash can be estimated based upon the change of the points position in conjunction with othe measuements. Besides the enegy, these measuements also povide infomation on intusions into the passenge compatment and thei isks of injuies fo the occupants. The measuement expenditue gows with the inceasing demand fo the accuacy of the evaluation. In standad cash tests, typically not moe than 100 measuing points ae measued. The entie measuement pocedue is time consuming, since just the obligatoy tanspot of a cashed vehicle to the evaluation cente adds a lot of exta time.

2 Fo eal accidents, the detemination of the actual enegy absoption by way of this method is not possible. The accident analyst lacks pecise infomation about the situation befoe the accident. Usually the acquisition of the accident vehicle can not be ealized because of the costs. Theefoe the analyst has to use diffeent methods to measue the defomation and thus the absobed enegy of eal accident vehicles. An impotant facto fo the utilization of these methods is the exact measuement of the defomations. Examples of typically applied tools ae shown in figue 2. It is obvious that the expected accuacy is faily low. In addition, the actual way of detemining the defomation enegy does not help to impove the measuement esults. Typically, this enegy will be deived fom a compaison of the damage based upon pictues. ften it is even necessay to compae diffeent geneations of the same model, to gathe additional infomation. Volkswagen Accident Reseach can substitute the in-house metology equipment. Thus the equiements fo such a system ae as follows: otability, Flexibility, Easy to use, High numbe of measue points. TICAL CRDINATE MEASURING SYSTEM TRIT The photogammetic measuing device TRIT is based upon the pinciple of tiangulation. Multiple images ae taken fom the desied object using a handheld camea. Knowing the pojection equations of the used optical elements, 3D coodinates can be calculated fom as many object points as needed. HTGRAMMETRIC DETERMINATIN F 3D CRDINATES Basics of photogammety Taditional Measuing Device Measuing tools Figue 2: Taditional measuing tools fo the measuing of accident vehicles Fo the goals of accident eseach such a pocedue is not sufficient. It is impotant that good infomation of the defomation enegy can be detemined and in addition, a detailed desciption and measuement of the defomations is equied fo the evaluation of the injuy mechanisms. Moden design pocedues enable the manufactue to efe to its CAD data sets as thei pio condition and to have them set in compaison to the post cash situation. REQUIREMENTS FR THE MEASURING SYSTEM Using photogammety, the position of a point in 3D space can be detemined by tiangulating multiple bundles of obsevation ays. If the spatial oientation of each bundle is known in the object coodinate system, the intesection of the ays delives the desied 3D object coodinate, as shown in figue 3. Camea model The camea model is used to descibe the pojection of an object onto the image plane of a camea. This model is based upon a pinhole pespective device with the camea lens as the pinhole. In the following, light waves ae descibed as staight lines as typical fo the incoheent metology. An object point ( X, Y, Z ) and its obsevation p ( x, y ) in the image plane B as well as the pojection cente ( X, Y, Z ) ae on one pojection line, see figue 4 with the image plane being mioed fo bette illustation. The acceptance of accident vehicle analysis always depends on the quality and the accuacy of the deived measuements. The taditional cash analysis takes place in a laboatoy envionment. Tactile coodinate measuing machines ae used to detemine the defomed contou of a cashed vehicle. This wok typically delives esults with a sufficient accuacy but is time consuming and hadwae intensive. Fo the analysis of accident vehicles outside a laboatoy envionment, a measuement system is needed which

3 X - suppot coodinate systems, Y Z - bject coodinates of the, Y, Z X, pojection cente X, Y, Z - bject coodinates of the obseved object point The elationship between object and image coodinates can be descibed mathematically accoding to the collinea assumption x y c X = Z Y x + y H H dx + dy Figue 3: Coodinate detemination of object points i by tiangulating bundles of obsevation ays fom diffeent image planes B i with X Y Z X = R Y Z X Y Z X, Y, Z - suppot coodinates of the object point dx, dy - lens distotions R - otation matix The camea paametes like the pinciple distance c, the coodinates of the pinciple point ( x, y H H ) and the elements to descibe the lens distotions ( dx, dy ) ae called inne oientations. The values fo the pojection cente ( X, Y, Z ) and the otation matix R, which depends upon the camea position in the global coodinate system, establish the oute oientation. The othonomal otation matix is used fo the tansfomation of global coodinates into suppot coodinates. Figue 4: Cental pojection B - Image plane c - pinciple distance x, y - image coodinate system x, y - image coodinates of the H H pinciple point x, y - image coodinates of the pojected object point X, Y, Z - object coodinate system Bundle adjustment R = The bundle adjustment is used to detemine the unknown object coodinates and additionally the paametes of the detecting aangement. Fo this, multiple obsevations fom diffeent diections ae equied, with a patially ovelapping image aea. Futhemoe it is necessay that all desied object points exist in moe than one obsevation. A minimum of two obsevations of one object point is equied to ceate enough equations fo the deivation of its position

4 The image coodinates togethe with the appopiate poject cente define bundles of pojection ays as illustated in figue 3. The goal of the bundle adjustment is to detemine the unknown paamete in such a way that the collinea assumptions ae met as good as possible. The equation can typically not be solved exactly, as moe obsevations than unknown paametes exist and the obsevations have eos elated to the detection pocess. An iteative pocess is needed fo the appoximation of these system of equations. If all paametes of the detection setup ae unknown, seven additional degees of feedom exist in the equation system. Thee of them descibe a tanslation, thee a otation and one the scale. These degees of feedom, which ae called additional obsevations have to be esticted. ADDITINAL BSERVATINS Fo the tansfomation of the calculated point coodinates X i, Yi, Zi fom the model coodinate ' ' ' system to X i, Yi, Zi of the efeence coodinate system, esticted additional obsevations ae needed. Fo a definition of the efeence coodinate system without contadiction, the intoduction of the following additional obsevations is typical. Definition of a plane Restiction of the Z -coodinate object points system. ' ' 1 2, ' 3 Z Z Z of thee ' ' ' 1, 2, 3, in the efeence coodinate Definition of a diection ' ' Restiction of the Y -coodinate Y, Y 1 2 of two object ' ' points, 1 2 in the efeence coodinate system. Definition of a coodinate Restiction of the X -coodinate in the efeence coodinate system ' 1 ' X 1 of one object point Fo the use of these obsevations fo the adjustment calculation, the detection setup has to be tansfomed fom the model coodinate system into the efeence coodinate system which is defined by the additional obsevations. In case of a definition of the position of the efeence coodinate system without contadiction, the tansfomation takes place as follows. Scaling The scaling can be deived based upon the elationship between the measued distance S in the model coodinate system and the pe-defined distance S in the efeence coodinate system Tansfomation of the thee fixed Z -coodinates Tansfomation of the two fixed Y -coodinates Tansfomation of the one fixed X -coodinate CIRCULAR MARKERS To chaacteize the image points in the camea image, so called efeence tagets ae used, which ae applied onto the object s suface o the suounding aea. The following citeia should be taken in consideation fo the choice of these makes: High accuacy concening detemination of the position Ability to be identified automatically Easy poduction and handling In photogammety, typically cicula makes ae used as they meet these citeia. Identification of cicula makes Fo the coodinate detemination of the efeence tagets using the bundle adjustment, the cicula makes have to be identified uniquely in the diffeent images, to make sue that they can be elated to each othe. The goal is to have an automated identification to eliminate a time consuming and eoneous manual pocedue. Ring coding of cicula makes Fo the unique identification of the cicula makes a special ing coding was developed. The code is next to the cicle and coesponds to a binay coding. Thus, a otation invaiant identification of the efeence tagets is ealized. Identification of non-coded cicula makes The disadvantage of the ing code is the lage aea which they ae coveing in compaison to the non-coded cicles. It is then necessay to have less coded efeence tagets and to identify the emaining non-coded tagets based upon thei elationship to the coded ones. Mateial of the efeence tagets The mateial of cicula makes is an impotant facto fo the ease of use of such a photogammetic system. It is typical to use a eto eflective mateial. These tagets

5 can be easily identified in the images as the they appea bight and the est of the image black. Fo the actual accident eseach, eto eflective tagets cause some disadvantages as in the black images, details like scas o blood maks ae not ecognizable. The TRIT photogammety system also utilizes plane white tagets. Thei advantage is that they can easily be poduced on a standad lase pinte and in addition, the image itself is still visible. Thus the image can still be used fo some futhe pocessing and even colo images can be used. Using some advanced seach algoithms, the time to identify plain white tagets is now on the same level as fo eto eflective tagets and the identification accuacy even exceeds that of the eto eflective tagets. MEASUREMENT RCEDURE The steps of a photogammetic measuement ae: Example fo the evaluation of the 5 test lengths Figue 5: Test lengths Example fo additional evaluations Measue Line 1. Applying the non-coded makes at the equied measuement positions 2. Applying some coded makes 3. ositioning of scale length 4. Taking images 5. Automatic pocessing of all images and automated calculation of the 3D coodinates of all makes 6. Tansfomation of the esults into a distinct coodinate position. VERIFICATIN F ACCURACY When a photogammetic system is applied fo any measuement task, the use has to be sue the efficiency of the used system is adequate. The VDI/VDE guideline 2634 pat 1 intoduces a standadized way of detemining the length measuing deviation as stated in the IS Inspection and monitoing of photogammetic measuement systems ae pefomed by measuing calibated test-specimen. Hee one-dimensional length standads ae used which ae calibated with a coodinate measuing machine. The length measuing deviation has to be tested in the complete measuement volume. Fo this, seven diffeent measuement lines of the test-specimen ae evaluated. Along these measuing lines, the specimen should be positioned. Five test lengths have to be measued along one measuement line as shown in figue 5. The longest test length has to be as long as the measuement volume. The specimen should be positioned as shown in figue 6. The deviation between the calibated and the measued length is detemined and leads to the length measuing deviation of the photogammetic setup. Figue 6: Recommendations fo the positioning of the test specimen duing the detemination of the length measuing deviation DETERMINATIN F 3D DISLACEMENTS IN ACCIDENT INVESTIGATINS MEASUREMENT Fo the measuement of accident vehicles, the position of measuement tagets of an undefomed vehicle is defined. A map of tagets will be ceated which allows a pecise positioning of the tagets on the defomed vehicle. To ceate such a efeence setup, a vehicle which is completely equipped with tagets is measued on a coodinate measuing machine. Thus the position of the efeence points in the vehicle coodinate position ae detemined. This efeence will be used to position the measuement esults in the TRIT softwae into the vehicle coodinate system. Fo the measuement of a vehicle, some coded efeence tagets ae positioned on the vehicle. It is impotant that these tagets ae positioned caefully as they ae used fo the coodinate tansfomation. The defomed aeas ae maked with non-coded efeence tagets to make these aeas available fo the late analysis. Additional coded tagets ae positioned aound the vehicle on the floo to suppot the photogammetic calculation. These tagets have no impotance fo the evaluation of the defomation. Two scale bas ae used fo the scaling of the setup.

6 in the undefomed aeas will be evaluated to detemine the fou tagets which lead to smallest deviation to the efeence data. They ae used to ceate a basic tansfomation into a suppot coodinate system. The esult is a positioning of the measuement data with an acceptable accuacy. To futhe impove the tansfomation, all coded efeence makes in undefomed aeas ae used to minimize the sum of single positioning deviations. The final esult of the tansfomation steps is a good positioning of the point cloud into the vehicle coodinate system, figue 9. Figue 7: hotogammetic pepaation of a cash vehicle The next step is the ecoding of the vehicle using a digital camea. The camea is positioned in a bell shape aound the vehicle. The images ae pocessed in the photogammetic softwae TRIT. All images ae ead into the C and the position of the efeence tagets in the images and then in 3D space will be detemined automatically. FURTHER RCESSING AND ANALYSIS F THE MEASUREMENT RESULTS The coectly scaled point cloud is now located in a andomly chosen coodinate position, see figue 8. Fo the futhe pocessing it has to be tansfomed into the vehicle coodinate position. Figue 9: Cloud of 3D points in vehicle coodinate system VISUALIZATIN F CRASH RESULTS The photogammetic measued 3D points ae positioned in the vehicle coodinate system. Now they can be compaed to the CAD model of the vehicle. Simply impoting the 3D coodinates into a CAD viewe allows to quickly visualize the cash esults, figue 10. This helps the cash analyst to estimate the degee of defomation. Figue 8: Cloud of 3D points in a andom coodinate system This tansfomation takes place using the efeence tagets that ae placed at pe-measued positiones with the coodinate measuing machine. The following steps ae necessay fo the tansfomation: At fist, the coded tagets on the vehicle have to be examined whethe they ae positioned in a defomed o undefomed aea of the vehicle. The quality of the tansfomation with the tagets Figue 10: Diect compaison of 3D coodinates of the cash vehicle with the CAD model. In a second step, the 3D coodinates can be used to ceate a suface model of the defomed vehicle. The goal is a shaded view of the accident vehicle to bette visualize the measuement esults, see figue 11.

7 fo the detemination of the actual cash angle, in vehicle-to-vehicle cashes. The Finite Element Method can also be applied to the defomation model to evaluate cash enegies. CNCLUSIN The descibed method fo the detemination of 3D coodinates fo accident econstuction delives a measuement accuacy that can hadly be achieved by tools nowadays in use. In addition most of the known methods ae vey time consuming. In compaison: the subject photogammety discussed hee is able to pefom an evaluation of a complete vehicle with up to 5000 measuement points in less than two hous. Figue 11: Suface model of the defomed vehicle based upon the photogammetically measued 3D points. The suface data can be compaed with the vehicle CAD data as shown in figue 12 and 13. A clea advantage of the photogammetic method is the ability to futhe pocess the once photogaphed vehicles. Wheneve needed, the measuement can be efined and extended. It is also possible to establish a data base of accident vehicles to allow a compaable examination. Evaluating the diection of the measuement point displacement caused by the defomation, allows to detemine the diection of the impact s foces. These analysis methods ae new and possible, only with the help of the subject photogammety. This method is most beneficial fo the evaluation of eal life taffic accidents. Thus the base of infomation is extended to futhe impove vehicle safety. REFERENCES Figue 12: CAD data and suface model of the defomed vehicle. 1. Holge Becke, hotogammetic measuement as a mobile tool fo defomation analysis in eal wold accidents, VDI, Casten Reich, Vollständige optische Fomefassung duch photogammetische Veknüpfung von Teilflächen, Thesis, Cuvillie, Göttingen, VDI/VDE-Gesellschaft Mess-und Automatisieungstechnik (GMA), ptische 3D-Meßsysteme Systeme mit punktfömige Antastung, VDI/VDE 2634, Beuth GmbH, Belin, Thomas Luhmann, Nahbeeichsphotogammetie, Wichmann, Heidelbeg, CNTACT Figue 13: CAD data and suface model of the defomed vehicle. Now by having available a suface model of the defomation, a lot of diffeent ways of evaluating the cash ae possible. Sections though the suface data and the CAD data can be used to view and measue cetain aeas. These sections can subsequently be used GM mbh Dik Behing, d.behing@gom.com Jan Thesing, j.thesing@gom.com Mittelweg 7-8, Baunschweig, Gemany hone: ; Fax: Volkswagen AG Holge Becke, holge.becke@volkswagen.de Robet Zobel, obet.zobel@volkswagen.de K-EFF/G, Beline Ring 2, Wolfsbug, Gemany hone: ; Fax:

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