1. Measurements and Signal processing (part 2) MCE 493/593 & ECE 492/592 Prosthesis Design and Control September 30, 2014
Antonie J. (Ton) van den Bogert
Mechanical Engineering
Cleveland State University

2. Today Laboratory techniques for human motion Lab tour
Camera-based motion capture
Force plates & instrumented treadmills
Balance testing
Strength testing
Lab tour
7:20 PM
FH 269

3. History of motion capture
Muybridge, 1870s
multiple cameras, 2D
Marey, 1870s
strobe lights as markers
Braune & Fischer, 1895
strobe lights, 3D

4. Distance-based measurement
Measure distance to three (or more) sources
solve XYZ from 3 nonlinear equations with 3 unknowns
GPS
resolution insufficient
for human motion
Ultrasound

5. Active marker systems Markers are LEDs Camera
flashing sequentially
Camera
projects marker on image plane or line
Most common: three 1-D cameras in one box
high resolution
high frame rate
markers must be seen from box
Optotrak
Codamotion (no lenses!)

6. Passive marker systems
All markers visible
2D cameras
16 mm film, analog video
manually digitized
Digital video cameras
reflective markers
infrared strobe lights
high contrast, thresholding
2D marker centroid coordinates
combined into XYZ of markers
Vicon, Motion Analysis, Qualisys

7. 3D measurement requires at least two (2D) cameras
lens x z y
image plane
3-D space v u
Two cameras:
u,v are measured in each camera
Solve x,y,z from 4 equations
More cameras:
better accuracy
less chance of marker loss
camera model:
DLT (direct linear transformation)
a1…a11 are calibration constants
(different for each camera)

8. Capture Lab
at Electronic Arts:
132 Vicon cameras
Fenn Hall 269:
10 Motion Analysis cameras

9. Recent developments Markerless motion capture
Improved IMU data processing
IMU combined with range sensor
Microsoft Kinect
Optical, camera-based measurement with markers is still the “gold standard” for human motion labs
still very expensive

10. Camera-based motion capture in 2D
markers
assumed to
stay
in XY plane y v u x
lens
camera image plane
parallel to XY plane
Camera model:
Camera parameters:
S: scale factor (meters per pixel)
θ: angle between X-axis and U-axis
uO,vO: image coordinates of XY origin
determined by imaging a rod
of known length, one end at origin,
aligned with X-axis

11. Matlab code for measuring U,V from video
movie = VideoReader(‘testfile.avi'); % load the video file nframes = movie.NumberOfFrames; height = movie.Height; npoints = 10; % how many points must be measured in each frame uvdata = []; % make a matrix to store the data % display each frame and measure U and V of all points for i = 1:nframes d = read(movie,i); % extract frame i from the movie image(d); % put the image on the screen disp(['Frame ',num2str(i),':']); disp(['Click on ',num2str(npoints),' points']); disp('Click to the left of the image to stop.') g = ginput(npoints); % collect data until user has clicked on all points if (min(g(:,1)) < 0) % if any point had a negative U-coordinate, stop break end disp('Done') g(:,2) = height - g(:,2); % invert V coordinates so V-axis will point upward uvdata = [uvdata ; reshape(g’, 1, 2*npoints)]; % add a row to the data matrix

12. 16 mm film at 50 frames per second
Clinical Orthopaedics and Related Research, 1983
Techniques used:
16 mm film at 50 frames per second
camera car alongside walking subject
markers on wall behind subject for calibration
Numonics Digitizer & microcomputer
IBM 370 for processing
about 2 mm random error in coordinates
5 Hz low pass filter

13. Angle measurement
Two markers on a body segment  segment angle Joint angle = difference between two segment angles
Matlab:
theta21 = atan2(y1-y2, x1-x2);
theta43 = atan2(y3-y4, x3-x4);
theta_knee = theta21 – theta43;
atan would give results between –π/2 and π/2, requires extra “if-then” logic
atan2 function gives results between –π and π, can represent full range of rotation
use “unwrap” function on time series if angle jumps between –π and π
If you use Excel:
Winter, 3rd Edition, Fig. 2.31

14. Some real data Y X What is the knee angle at time = 2959.594329?
1: RGTRO
right greater trochanter Y
2,3: RLEK
right lateral epicondyle
of the knee
4: RLM
right lateral malleolus X
theta21 = atan2(y1-y2, x1-x2);
theta43 = atan2(y3-y4, x3-x4);
theta_knee = theta21 – theta43;
What is the knee angle at time = ?
theta21 = atan2( , )
theta43 = atan2( , )
theta_knee = theta21 - theta43

15. Force plate Measures ground reaction forces
AMTI
Measures ground reaction forces
rigid plate supported by four (or three) 3D force sensors
main vendors: Kistler, AMTI, Bertec
measures 6 variables: resultant 3D force (Fx,Fy,Fz) and moment (Mx,My,Mz) on the axes of the force plate
also available as instrumented treadmill
Fxyz, Mxyz
forces acting on foot
forces in load cells
force and torque acting at center of pressure (COP)
Equivalent force systems:
(b) = (c) = (d)
Fz,Mz
Fx,Mx
Fy,My

16. Resultant 3D force and moment from four load cells
3D force F, applied at r, is equivalent to a 3D force F applied at the origin, plus a 3D moment M = r x F
Resultant of all four:

17. COP (center of pressure) representation
3D force F is assumed at COP rather than origin
Definition of COP (x,y)
z=0 and Mx=My=0 at COP (zero moment point)
Remaining moment Tz about vertical axis
“free moment”
still 6 variables

18. DIY GRF measurement (and save $50,000)
Brodt et al. (2013) Instrumented foot bar for Pilates exercise
XXIV ISB Congress, Natal, Brazil

19. Simple force plate Vertical force only
Zsensor
Vertical force only
Three points of support (no static indeterminacy)
Gives accurate COP in certain conditions
(Zsensor * Fx << My and Zsensor * Fy << Mx)

20. Instrumented treadmills
Treadmill frame sits on three or four 3-axis load cells
must be stiff and light
Separate belts for left and right
Very good for clinical research
each step is a measurement
speed can be controlled or self-paced
weight support is possible
Prosthetics research
controlled speed
prosthetic device can be tethered
to power supply and computer
ADAL treadmill at Cleveland VA
Medical Center

21. Strength testing Maximal isometric torque Cybex Kincom
Speed dependent torque
motor and
torque sensor
force from
leg
Isometric test: constant joint angle
Isokinetic test: constant joint angular velocity
lengthening
(eccentric)
muscle shortening
(concentric)

22. Balance testing (clinical)
Platform with controlled rotation
Built-in force plate (vertical force only?)
COP calculation
screening for risk of falling
balance training
knee injuries
concussion testing
Biodex SD
$12,500