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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

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**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

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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

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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

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**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!)

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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

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**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)

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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 Microsoft Kinect Optical, camera-based measurement with markers is still the “gold standard” for human motion labs still very expensive

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**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

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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

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**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

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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

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**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

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**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

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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**

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

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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 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)

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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

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