Tutorial 3
Contents
1. Coordinate System T Tubaria ono Fis kew Ekoa Fieis Froo a Dgo g A gml a l erpi r DE iwiohopece Sten EE Higa and Everd Pracernng fE Heei Balding JE Aecortrg S Aa Tere Capra BB Virtaala Hots CM Ceduna aio FIFI Teme ee ot D Aea Tima juhiga Arie T De Ea Frera hiom QJ FFREEMDHENT QO BHETIEHH EATE O HETAC 0 FEADER MFOR HATOR QO Paes TEF F T ri red i pi E 11i Lia Dii Fa Te fe h dle amp a1 E I I Wh al Rar hep pee Fi NOTE We often get questions from users claiming that the ANALOG signals for the force platform exist but the GRFORCE signal is zero Check to see that the Fz Channel of the ANALOG signals is negative If not then the ANALOG scale factor in the C3D file has probably been stored incorrectly by the motion capture software The scale factor will need to be changed see later section Modifying the Force Platform Parameters 1333 Force Platform data are perhaps the most confusing aspects of the C3D file format All of the required Force Platform parameters for the type of Force Platform must be defined correctly Some of the parameters are specified by the Force Platform manufacturer these values are provided in the Manufacturers User Manual Note that some of these parameters can not be transferred directly in the C3D file The C3D format assumes consistency between units throughout the file If the Motion Capture data has been collected in m2. define a baseline which is then subtracted from every frame of data The ORIGIN mm shows a 21 mm Z component The manufacturer manual will show a positive z component The c3d format assumes that this value will be a vector from the ORIGIN to the center of the top surface of the platform in the force platform coordinate system which means that the value in the C3D file should be negative Corner 1 Corner 2 Corner 3 and Corner 4 are the locations of the force platform corners specified in the laboratory coordinate system This information is used to transform the signals from the force platform coordinate system to the laboratory coordinate system In principle the force platform can be located in any orientation in the 3D data collection volume e g the force platform does not have to be on the floor The Analog Channels column shows the analog channel assignment number in the order the ANALOG channels appear in the c3d file 3 Click Done or Cancel when finished examining data Thresholding the Force Platform Signals 1334 When force platform data exists Visual3D attempts to assign the signal to a segment that comes into contact with it If there is a low level of background noise in the signal it will appear that something must be in contact with the force platform at all times To ensure that only sensible real contacts are used a threshold values is specified such that any ground reaction force signal below this value i
3. C Motion Online Documentation Visual3D Tutorial Force Platforms Force Platform Data 1331 Force Platforms provide critical data for biomechanical analysis because they measure the interaction between the subject and the environment If the subject is in contact with the floor the joint forces moments and powers have no physical meaning unless the reaction force data are included in the analysis NOTE The Analog signals that are collected from the force platform are NOT the force signals The force signals must be computed from these Analog signals using several parameters that specify the type of computations required different force platforms require different computations For more information see the Online Documentation Graphing a Force Platform Signal 1332 1 If the graph area at the right is still occupied right click anywhere on the graph area and select Remove all graphs 2 Inthe Data Tree select the ANALOG folder then ORIGINAL Right click FZ1 select Graph Z and New Graph This will open a graph of the Fz ANALOG channel of force platform1 3 In the Data Tree select the GRFORCE folder FP1 Z component and New Graph This will open a graph of Z component of the Ground Reaction Force signal Note that the two signals are related to each other but they are not identical The ANALOG signal is stored in the coordinate system of the force platform The GRFORCE signal is stored in the Laboratory Motion Capture
4. enter of Pressure Distance to Segment to specify a minimum distance 2m default This option allows you to verify that a particular signal has been caused by a segment Copyright 2002 2004 C Motion Inc
5. illimeters ALL distance measures in the file must be consistent This means that the corner locations and Force Platform ORIGIN values must be in millimeters and the moment channels of the force platform must be expressed in Newton millimeters To examine or modify the parameters 1 In the File menu select Modify Force Platform Parameters Modify Force Platform Parameters Select Force Plationm bo Hodiy FFT Denx Fence Piaam Ziso Hastina Inderva fi 10 Upwdabe Cb Fie p CO ante lt j aion Maii Seabed Anaka CaMati janak Analog _ n ee eee i 0 a ot ft f ft tt OR aia a 13 EE _ _ _ O N N E E S A m peg TT TF CTT PF PT PF PF TO PUP CUE 1 BSED El a Farinas Pan ns nn ns ne nn Wn A by p aeaa oaoaraa PD P ep E eap O fT fF Ff T fF fF ANTI Sip Corner SSE CR SRO TO ae RES NE SSE AE SUT nS Pa Uc STO PEEEEEEEEEEE F Le p z m p a a a a OCM Kater Center of Pienuie Comedian F actors bth onde Pohrana odlicen AMT Step2 Comers r Hisli Caria of Fierce Coercion Fach ca m Trsnalation Point Uribe Y Rotstion Degess Z a D irek 2 Click on the Get Current C3D Parameters button to populate the dialog box The Force Platform Zero Baseline Interval indicates that the first 10 frames of data will be averaged to
6. s assumed to be zero This ensures clean contacts with the force platform 1 Select the Properties menu 2 Click on the Modify Minimum Force Platform Value A dialog box will open to allow you to set a higher or lower threshold than the default Taari cme wua i Fol x Fis Siew Bice Peds Be ee OF Cais i LS Gor k ed dk i Esih ID ar Mowe Seemierty i Comp Body Lae Cuar ais Ipen DE hapan Ee Ti E Manche i o te ipp Target S oa Teen Cpe w Yoh Merci Hek Oo tance NE dep na ne gt DI ALDA kiip maTi mai OF DD e FE 05 orma Une mee ARIE PLATAE ORIEIH emim D cH Li CHE Cj cenn D Eini EE Seeing Teye i ijel ayasi Like meee Q EWG he atio inah S aj Aea Parte aa O ci z E p EE let the Himen Farce Cla om zalga Eii ee ed de n OHO Fodh PP Demim C Sree drien eee Body Hean Poe Aae Vale EMG Line cube Apyarp European oaa Cakes g EEU O Exar Dj cmne D zws Q ra O ma Fr FYE Enter Gh rerem gra an atu vou eak foe So ure bee fhe Fee pear HITE Her vedo ee ph Ae bo he aeae eike Mep rad g be Bakka Aa ha L wine ka pay coped a es D eta ea Plato H rrem heed 1U Dari Hren TS ale F piska bha vae bs dii D Ras Time jiki area FUEL TA Pere MPA T Dee m rera be Q EHETE HEHA TE I METRIC 5 FEADER IMFORMAT OA OQ FAE TEFA 8 You can also choose one of the other options for modifying force platforms Select Modify FP C
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