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