Motion Capture - PowerPoint by DTAvkE99

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									  Motion Capture
    Essentials

D. Gordon E. Robertson, PhD
Outline

1. Select appropriate marker set and type of markers
2. Determine number of cameras and their locations
3. Select appropriate location, number and types of force
   measuring devices (force platforms, force transducers)
4. Other demands: EMGs, netting, speed guns, standardized
   footwear and equipment?




  Biomechanics Laboratory, School of Human Kinetics
1. Marker Sets

• Motion capture requires selection of a suitable marker set
• Types:
   – individual markers
   – clusters of markers (includes wands) on rigid structures
   – virtual markers (uses wands to identify body locations)
• Static marker set
   – this set is necessary to establish joint centres and their 3D
     relationships with the dynamic marker set
   – virtual marks can be included
• Dynamic marker set
   – this may be a reduced set that with a combination of
     individual markers and/or clusters


   Biomechanics Laboratory, School of Human Kinetics
Static Marker Sets in static
               subject
                       posture with
• Helen Hayes marker set was a early simplified system but is not
                       marker clusters
  appropriate for full body 3D analyses
• Vicon’s Plug-in-gait marker set is a more robust full body marker
  set but requires several anthropometric measurements (e.g.,
  mass, height, leg length, knee width, ankle width, shoulder
  offset, elbow width, wrist width, & hand thickness)
• uOttawa marker set is a modified version of Plug-in-gait. It uses
  hip, iliac crest, medial limb, and ear markers, but no ASIS or
  PSIS markers. It also requires fewer anthropometric
  measurements (e.g., mass, height, hip offset, & shoulder offset)
• clusters may also be used for some or all segments instead of
  non-joint markers



   Biomechanics Laboratory, School of Human Kinetics
Plug-in-gait
Marker Set
•No hip markers
•No medial markers at
knees, ankles, elbows,
and wrists
•Head markers not over                            avoid placing markers
ears (location of cofg)
                                                  linearly on a segment
•Pelvis markers (ASIS &
PSIS) are horizontally
located
•Only one markers on
foot
•Potentially arm & leg
markers are linear


    Biomechanics Laboratory, School of Human Kinetics
uOttawa
Marker Set
•Hip markers
•Medial markers at
knees, ankles, elbows,
and wrists
•Head markers over ears
•No ASIS or PSIS
markers
•Two markers for each
foot, hallux optional
•Potentially arm & leg
markers are linear




    Biomechanics Laboratory, School of Human Kinetics
Clusters
•Usually 3 or 4 markers
•4th marker can be offset
•cluster must be well
fixated to the skin
•can be difficult to
automatically digitize
•can impede motion




     Biomechanics Laboratory, School of Human Kinetics
Dynamic Marker Sets

• this marker set that is similar to the static but with extraneous
  markers removed
   – medial markers
   – when clusters are used individual markers can be removed




   Biomechanics Laboratory, School of Human Kinetics
2. Camera Setup

• Number of cameras depends on complexity of task and
  complexity of the model
• 2 to 4 cameras may be sufficient for analyzing planar type
  motions of one side of the body
• 6 or more cameras for full-body models of small volumes
• 10 or more for large volumes such as walkways, cricket pitches



                    7 cameras,
                   10cameras
                     6 cameras
                   212 cameras
                     overhead




   Biomechanics Laboratory, School of Human Kinetics
3. Force Platforms
                                                    plates in 4 stairs
•       two side-by-side on kinetic and 1 in ground
                two for later
  Usually necessary side-by-side analyses
•               and one offset
        moveable platform
  One plate or multiple plates. How many?
• Imbedded into floor or into stairway
• Size:
   – small (40x60): good for starts or static jumps, easy to miss
     for running studies
   – large (60x90): good for running studies but can result in
     multiple feet on same plate (prevents inverse dynamics)
• Location:
                       mounts for up to movable
   – side-by-side, separated, alternating,6
                               large plates



    Biomechanics Laboratory, School of Human Kinetics
4. Other Equipment


• VO2 measurements

• EMGs of leg muscles

• torque resistance
  measured by
  ergometer


  Biomechanics Laboratory, School of Human Kinetics
Example: Single Markers, Side Kick

• Single markers one
  either side of each joint
  and one extra mid
  segment
• Three force platforms
• All joints can be
  analyzed since there
  are no closed loops




   Biomechanics Laboratory, School of Human Kinetics
Example: Clusters, Olympic Lifting

• Clusters identify limb
  segments
• Two force platforms
• Only ankle, knee, hip,
  and pelvis kinetics can
  be analyzed
• Arms kinetics are not
  possible because of the
  bar connection




   Biomechanics Laboratory, School of Human Kinetics
  Vicon
Workstation

  Trajectory
  Digitization
Steps

1.    Calibration
2.    Reconstruction
3.    Labeling
4.    Autolabeling
5.    Defragmentation
6.    Gap filling




     Biomechanics Laboratory, School of Human Kinetics
1. Calibration

• Static
   – origin and axes are defined




• Dynamic
   – calibration wand is used to calibrate volume



• Accuracy
   – usually < 1 mm



   Biomechanics Laboratory, School of Human Kinetics
2. Reconstruction

• Multiple 2D views to one 3D




  Biomechanics Laboratory, School of Human Kinetics
3. Labeling

• select the static trial
• label each
  trajectory
• save for
  autolabelling




   Biomechanics Laboratory, School of Human Kinetics
4. Autolabelling

• load dynamic trial(s)
• apply autolabelling




  Biomechanics Laboratory, School of Human Kinetics
5. Defragmentation

• join line fragments with same name




   Biomechanics Laboratory, School of Human Kinetics
6. Gap Filling

• fill gaps with interpolating spline




   Biomechanics Laboratory, School of Human Kinetics
Done
Ready for Kinematic/Kinetic Analyses




 Biomechanics Laboratory, School of Human Kinetics
   Visual3D


Inverse Dynamics and
 Joint Power Analysis
Marker and Force Data

• marker data come in a .C3D file created by Vicon, Motion
  Analysis, Qualysis or similar motion capture system
• SIMI and APAS may also be used but data must be placed in a
  ASCII (.FSV) file by, for example, BioProc2/3
• force data come with the .C3D file but may also be collected and
  placed in an ASCII (.FSV) file by BioProc2/3
• data may be streamed directly to Visual3D using newest version
  of Visual3D and compatible motion capture system, e.g., Nexus




   Biomechanics Laboratory, School of Human Kinetics
Creating the Model

• first a static trial is loaded to create the model (you can use a
  movement trial)
• a previous model may be used for a similar marker set
• model is customized for each participant (mass and height etc.)
• ideally markers are located at each joint but these markers do
  not have to be included in the movement (dynamic) trials
• should save the model file (.MDH) separately for use with other
  trials or experimental conditions of the same person
• new models must be created for each new marker set




   Biomechanics Laboratory, School of Human Kinetics
Model Building Screen

•   create segments



•   create landmarks



•   enter body mass
    and height




    Biomechanics Laboratory, School of Human Kinetics
Movement Trials

• one or more movement trials may be loaded into same
  workspace (.CMO), essential if trials are to be averaged
• model is applied to each trial
• trial may be viewed with bones or geometrical solids for each
  segment
• ground reaction forces should be checked
• errors in force platform locations and parameters may be
  checked and changed in necessary




   Biomechanics Laboratory, School of Human Kinetics
Workspace Screen

•   static trials



•   movement trials



•   tags to identify
    experimental
    conditions




    Biomechanics Laboratory, School of Human Kinetics
Event Labeling

• if events were not created by motion capture software they can
  be added automatically or manually with Visual3D
• special events used for reports can also be added (e.g., BEGIN,
  END, HIT)
• events should be checked for accuracy using GUI




   Biomechanics Laboratory, School of Human Kinetics
Signal and Event Processing Screen

•   event button



•   signal tree



•   body at current
    event

•   signal histories
    with events
    marked




    Biomechanics Laboratory, School of Human Kinetics
Data Processing Pipeline

• a pipeline may be applied to process the data for inverse
  dynamics, energy, or power analyses
• a script may be used to perform these operations repeatedly
• a typical script (.V3S) includes interpolation and data filtering
• special forces may be added such as moving force platforms,
  transducer forces, or pedal forces




   Biomechanics Laboratory, School of Human Kinetics
Pipeline Form

• commands     • pipeline    • options     • import signals   • opn/save scripts




  Biomechanics Laboratory, School of Human Kinetics
Graphical Reports

• reports, including graphs, of the various kinematics and kinetics
  are created next
• special tags may be used to display only certain types of trials,
  e.g., right vs. left leg starts, barefoot vs. shod gait, loaded vs.
  unloaded lifts
• these tags are created in the workspace area
• basic reports (.RGT) may be loaded or created and saved for
  later repeated use
• graphs may be exported for presentation




   Biomechanics Laboratory, School of Human Kinetics
Reporting Screen

•   contents of pages



•   report pages




•   graphical or
    tabular reports

•   select page
    viewed here



    Biomechanics Laboratory, School of Human Kinetics
Export Data to MatLab or BioProc3

• kinematic or kinetic data may be exported in .C3D or ASCII files
• BioProc3 has capability of computing total, internal and external
  work, angular impulses and work done by individual bursts of
  power
• script is used to create ASCII file for BioProc3
• MatLab can compute foot clearance




   Biomechanics Laboratory, School of Human Kinetics
BioProc3: External/Internal/Total Work

• external work




• internal work




• total work




   Biomechanics Laboratory, School of Human Kinetics
BioProc3: Work Done in Bursts

• work done

• which moment
  of force

• peak power

• peak moment

• mean power

• total work done



  Biomechanics Laboratory, School of Human Kinetics
Questions?       Comments?
www.health.uottawa.ca/biomech/watbiom
                                www.humankinetics
   Finis


Muchas Gracias

								
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