Measuring Techniques of Force

Static Force

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Dynamic Force

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Capacitor Sensors

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Conductor Sensors

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Piezoelectric Sensors

Non-conducting crystal that generates an electrical charge when subjected to mechanical strain
-strain results in electrical charges on surface of quartz
-charges temporarily stored in the element
-charge dissipates with time due to leakage
-electrical circuits overcome problem
-allow dynamic and quasi-static measurements
ADVANTAGE
-can measure very LARGE forces (hoof of galloping horse) and human microvibrations (force on ground from heartbeat)

Strain Gauge Sensors

Electrical Resistance Strain Gauge (Force Transducer)
-stress to a structure changes its geometry (i.e. stretched wire-->cross sectional area is reduced and increased resistance)
-calibrate change in resistance of stretched structure
-transducers are arranged to maximize electric output (Wheatstone Bridge)
Tension: cross sectional area and resistance increases
Compression: cross sectional area and resistance decreases
Strains of up to at least 10% can be measured with proper installation of gauge onto object

Transducers

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Voltage

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Current

-I
-Current (amps)
-# electrons passing through wire
-rate of water flowing through the hose

Resistance

-R
-ohms
-Friction of electrons in wire
-friction inside the hose
-resistance increases with a narrower hose

Strain Gauge

Strain= change in length/original length
-guage needs to be highly flexible
-little load must be needed to deform gauge
-no load can be transferred to gauge

Gauge Factor

GF=(change in resistance/resistance of the undeformed gauge)/strain

Quality of Strain Gauge

Relationship of interest is applied force input to registered voltage output

Implantable Force Transducer

curved spring with foil strain gauges inserted directly into a longitudinal slit in tendon/ligament
ligament in tension--> exerts compressive load on sensor

Buckle Transducer

Goes around tendon
transverse force results in beam deforming

Force Plates

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3D Force Vector

Described by nine quantities
-3 forces(Fx,Fy,Fz)
-3 moments(Mx,My,Mz)
-3 spatial coordinates (x,y,z)
-Outputted moments and forces are orthogonal (90 degrees to one another)
-Only interested in 6 quantities when analyzing movement
-3GRF components (Fx,Fy,Fz)
-COP location of GRF (x,y)
-Free Moment (Tz)

Plate Reference System

-generally in center of plate slightly beneath level of top surface
-x,y,z is location of resultant force vector with respect to the PRS origin
-3 moment components described about the PRS origin

Free Moment

Reaction to a twisting moment applied by subject about a vertical axis (z-axis) located at the COP coordinates
-no free moment about x or y axis

AMTI and Bertec Force Plates

-Strain Guage
Output: 3 forces and 3 moments signals expressed about PRF origin
-use output signals to calculate COP and Tz
-Less expensive
-Good Static Capabilities

Kistler Force Plates

12 piezoelectric force sensors
-output from amp is 8 forces
2 in x direction
2 in y direction
4 in z direction
-output used to calculate resultant 3 forces and 3 moments
-can also calculate COP and Tz
-High Frequency response and high sensitivity
-need special electronics to enable static force measurements
-high range (horse vs heartbeat)

Impact Forces

Forces that result from a collision of two objects reaching their maximum earlier than 50 milliseconds after the first contact of the two objects

Active Forces

Forces generated by movement that is entirely controlled by muscular activity