BMEG 230 MT2 Flashcards
3D tracking methods
Passive relective markers
Active Markers
Markerless
Pros and Cons of Passive Reflective Markers
Pros:
* Accurate
* Capable of large frame rates
* Complex 3D motions
* No wires
Cons:
* Need multiple cameras
* Manual marker identification or need software
* Mis-identified markers
* Usually constrained to a lab setting, struggles
in uncontrolled lighting
Pros and Cons of Active Marker
Pros:
* Accurate (especially in uncontrolled lighting)
* High resolution
* Greater capture volume
* Complex 3D motions
* No need to identify markers
* Less likely to loss sight of marker
Cons:
* Wires can be intrusive
* May need multiple cameras
* Complex to setup
* “bit of a pain” – Pawel
Pros and Cons to Markerless tracking
Pros:
* Cheap and easy
* Unconstrained motion
* In the field data collection
* Can have various modalities of video
Cons:
* Not very accurate (yet)
* Mostly still in the research phase
* Usually need large training data sets
(i.e. machine learning)
* Can be constrained to single-plane and
simple motions
* Takes time/computing power to
process data
What do force plates measure?
GRFs - Fx,Fy,Fz
CoP
momets of forces
impulse
how do they measure this
- Convert energy from one form to another
- In our case: mechanical to electrical
- We cannot “measure” force, but we can measure strain as a
change in electrical resistance
Strain Gauge
- change their resistance as they are stretched or compressed
- to measure a desired change, there is a velocity drop acroos the strain gauge for a given level of force.
- uses differing sensors in different locations to determine CoP
Pizoelectric in force plates
*Deformation to cause charge is very
small (usually a few microns)
* High frequency response
* Can not measure static loads
* Excellent for dynamic and impact loads
CoP
All ground reaction forces
can be replaced by a single force acting
at a single point. This point is called
the center of pressure.
how is voltage used to determine CoP in force plates
the force plates outputs the voltages proportional to forces, then coverted while accounting for amplifier gains and plate sensitivites. The forces are then used to calculate where the CoP is
Degree of Freedom
an expression that describes the ability of an object to move in space.
How many degrees of freedom are there for:
human finger
human shoulder
2
3
why are degrees of freedom important
- Understanding Movement: Knowing the degrees of freedom helps us understand how joints and bodily systems move and what
limitations they may have.
2.Modeling and Simulation: In computational biomechanics, degrees of freedom are essential parameters for creating accurate
models of human movement.
3.Injury Prevention: By understanding the limitations of a joint’s degrees of freedom, healthcare professionals can better prescribe
treatments or preventative measures for potential injuries.
4.Prosthesis Design: Engineers use the concept to design more efficient and functional prosthetic limbs that mimic the natural degrees of freedom as closely as possible.
Global vs Local coordinate system
- For global the rotation is performed concerning the global coordinate system whilst local concerns the local.
- in global, the axes of rotation are fixed and do not change as the object rotates, in local the axes is fixed to the object.
- A global Z-axis rotation means that no matter the current orientation of the object, the rotation will be around the fixed global Z-axis. In local it will rotate around its own z axis
.
Definition of Inertia
the tendency of an object to maintain its state of rest or uniform motion, the resistance to change in motion of an object. The more mass the more inertia, the more resistance
moment of inertia
depends not only on the masses, but also on how far they are from the axis of rotation
Parallel axis theorem
if the mass moment of inertia about an axis passing through the center of gravity is known, the moment of inertia about a second, parallel axis is given by
I_pat = I_cg +M_t(R_cg)^2
I_cg = MOI about axis through cg
m_t = total mass of the object
R_cg = perpendicular distance between the axes
Momentum
tendency to resist changes in the existing state of motion, only applies to moving ibjects, while inertia applies to moving and stationary
force is a change in momentum and time
p=mv
L=Iw
impulse momentum principle
F * delta(t) = delta(p)
Energy
kinetic = (1/2)mv^2
potential = mgh
work energy principle
SUM(Fd)=(1/2)mv^2b-(1/2)mv^2a
What is the instantaneous centre of zero velocity
when a body is moving there is a point where its velocity is zero for a moment, that point is not always attached to the body, seems to spin about that point
how to find IC
velocity mag is proportional to relative distance, therefore we can use velocity being perp to relative distance. Draw lines perp to velocity, where they intersect is the IC.
v_a/r_a,ic = v_b/r_b,ic
Instrumented Treadmills
- Force Platforms are built either into (AMTI) or under (Bertec) the treadmill
- Use strain gage platforms
- Measure Plate Forces and Moments (Center of Pressure and Free Moment computed
from output)
types of force plates
Strain Gage:
* AMTI
* Bertec
* Kyowa-Dengyo
Piezo-Electric:
* Kistler
Standing Balance Control
- Muscle Stretch Reflex
- Role in balance
- Muscle tension adjustment
2.Vestibular System
- Location and function
- Orientation in space
3. Proprioceptive Feedback
- Sources (muscles, tendons, joints)
- Role in balance and posture
4.Teleceptive Inputs - Vision
- Vision in spatial navigation
- Environmental perception
Kinetic energy equation for rotation
E = (1/2)mv^2 + (1/2)Iw^2
rigid body force equations
F = ma
sum(Fx) = ma_x
sum(Fy) = ma_y
sum(Fn) = (w^2r)m
sum(Ft) = (alphar)m
angular momentum about the COM
sum(M)=I(alpha)
a body’s trajectory can be determined if
the applied forces and moments are known
if the moments is taken about a point different to COM
apply parallel axis theorem (I//) to determine the new I
If COM is accelerating
sum(M_newpoint) = I//alpha +m(R_new/com x a_com)
Definition of Dynamics
the study of how forces affect motion
Definition of Kinematics
the study of motion in isolation
2D planar video adavntages and disadvantages
Pros:
* Cheap and easy
* Various speeds
* Can see the video after
Cons:
* Only in one plane
* Can take a long time to process data
* Hard to align properly and distortion
correction is often required
Distance Callibration
s = actual length / digitized length
frame rate
- Fast Movements (e.g., running, jumping): Use a high frame rate, typically 120 fps or higher. This ensures fine-grained detail, reducing motion blur
- Slower Movements (e.g., walking, stretching): A lower frame rate like 60 fps may suffice, but higher frame rates provide more detailed data
Data triangulation
Enhanced Accuracy and Validation
Minimization of Occlusion and Blind Spots
Improved Depth of Analysis through Multidimensional Data
Enhanced Reliability and Reproducibility
Broader Context and Environmental Factors
Reduction of Measurement Bias and Artifacts
2D marker placement
3 DOFs
X, Y of the COM and one angle describing its orientation
3D marker placement
6 DOFs
X, Y, Z of COM and 3 angles describing its orientation
equations for attached bodies
R_b = R_a + R_b/a
V_b = V_a + W_ab x R_b/a
A_b = A_a + alpha_ab x R_b/a - W^2*R_b/a
unit vector
<R_k/a = R_k/a divided by |R_k/a|
Acceleration
A_b = A_a + A_b/a
A_b = A_a + (A_b/a)t + (A_b/a)n
when would V_a = 0
when there is rolling without slipping
