Chap 2 KINEMATICS

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Chap 2KINEMATICS
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2.1 KINEMATIC CONVENTIONS
Absolute Spatial Reference System
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Total Description of a Body Segment in Space

• Position(x,y,z)of segment center of mass
• 2. Linear velocity

of segment center of mass
3. Linear acceleration 4. Angle of segment in two
planes, ?xy, ?yz 5. Angular velocity of segment
in two planes, ?xy, ?yz 6. Angular acceleration
of segment in two planes, ?xy, ?yz
of segment center of mass
feetlegstightstrunkheadupper armsforarms
and hands 12
12x15180 variances
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2.2 DIRECT MEASURMENT TECHNIQUE
Goniometers

• inexpensive, available immediately for recording
• relative angular data
• excessive length of time to fit and align
• encumbered by the straps and cables
• more complex goniometers required, if not hinge
  joints

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Accelerometers

• Fma
• strain gage, piezoresistive type
• vKan

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Accelerometers

• Problems of limb rotation
• Limitation
• - not change drastically
• - horizontal flexion of the forearm about a
  fixed elbow joint
• Available immediately for recording
• Relative to its position on the limb segment
• expensive,
• encumbered movement,
• easily broken,
• artifact in rapid movements

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A typical electric circuit of an accelerometer
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2.3 IMAGE MEASURMENT TECHNIQUE
Review of Basic Lens Optics

• Image size is related to object size by
  triangulation

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f-Stop Setting and Field of Focus

• f fraction of lens aperture opening
• Larger f, lower f-stop setting
• 22, 16, 11, 8, 5.6, 4, 2.8, 2

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Cinematography

• Consideration
• ASA rating
• Shutter factor
• Frame speed

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Multiple Exposures

• strobe light,
• strips, markers are placed on the subject

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2.4 DATA CONVERSION TECHNIQUE
Analog-to-Digital Converters
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Nature of Unprocessed Data
2.5 PROCESSING OF RAW KINEMATIC DATA

• Raw data many noises from many sources

• Raw data should be smoothed !

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Harmonic (Fourier) Analysis
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Sampling Theorem
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Signal vs Noise
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Smoothing and Fitting of Data

• Curve-Fitting Techniques
• The Trajectory signal has a predetermined shape.
• By fitting the assumed shape to a best fit with
  the raw data, a smooth signal will result.

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Digital Filtering
(a) Hypothetical frequency spectrum of a waveform
with a signal plus high-frequency noise
(b) Response of low-pass filter Xo(f)/Xi(f) to
attenuate noise
(c) Spectrum of the output waveform
High freq noise is severely attenuated, with only
minor distortion in the transition region fc
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Digital Filtering

• Signal spectrum vs Noise spectrum
• Cutoff frequency
• Recursive digital filter

Depend on type and order of the filter, sampling
frequency and the cutoff frequency
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Choice of Cutoff Frequency- Residual Analysis

• To carry out a harmonic analysis
• Residual analysis better

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Comparison of Smoothing Techniques

• Accelerations
• Accelerometer
• Film after digital filtering

Flex/Ext angle

• Accelerations
• 9th order
• Chevyshev
• polynomial
• accelerometer

• Accelerations
• FD method
• accelerometer

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Limb-segment Angles
2.6 CALCULATION OF ANGLES FROM SMOOTHED DATA
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Joint Angles

• 0 flexion when the knee is fully extended
• knee flexion, when the leg moves in a posterior
  direction relative to the thigh

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Velocity Calculation
2.7 CALCULATION OF VELOCITY ACCELERATION

• The line joining xi-1 to xi1 has the same
  slope as the line drawn tangent to the curve at
  xi

• Finite Difference Method

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Velocity Calculation
Finite Difference Method

• Forward Difference

• Backward Difference

• Central Difference

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Acceleration Calculation

• Central Difference

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