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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 1

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

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

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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 – www.zebris.de www.zebris.de 4

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Active marker systems Markers are LEDs – 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 5 Optotrak Codamotion (no lenses!)

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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 6

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3D measurement requires at least two (2D) cameras lens x z y image plane 3-D space v u camera model: DLT (direct linear transformation) a1…a11 are calibration constants (different for each camera) 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

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

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

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

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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 end

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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

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Angle measurement Two markers on a body segment segment angle Joint angle = difference between two segment angles Winter, 3 rd Edition, Fig. 2.31 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:

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Some real data What is the knee angle at time = 2959.594329? 1: RGTRO right greater trochanter 2,3: RLEK right lateral epicondyle of the knee 4: RLM right lateral malleolus theta21 = atan2(y1-y2, x1-x2); theta43 = atan2(y3-y4, x3-x4); theta_knee = theta21 – theta43; theta21 = atan2(0.90533-0.51603, -0.19465-0.01730) theta43 = atan2(0.51603-0.12862, 0.01730--0.09302) theta_knee = theta21 - theta43 X Y

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Force plate 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 – http://www.kwon3d.com/theory/grf.html http://www.kwon3d.com/theory/grf.html AMTI (a)Fxyz, Mxyz (b)forces acting on foot (c)forces in load cells (d)force and torque acting at center of pressure (COP) Equivalent force systems: (b) = (c) = (d) F x,M x F y,M y F z,M z

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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:

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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

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

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Simple force plate Vertical force only Three points of support (no static indeterminacy) Gives accurate COP in certain conditions (Z sensor * F x << M y and Z sensor * F y << M x ) FORCE Z sensor

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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

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

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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 http://youtu.be/cBBlTYMulsE

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