Kistler portable triaxial Force Plate 1
Transducers Transducer - any device that converts one form of energy into another Sensors convert physical quantities into electrical signals electrical signals are convenient to use, record and analyze Electrodes are the interface between biopotentials and electronic equipment 2
Sensors Sensors - measure physiological and biomechanical quantities measurement: input sensor e.g., accelerometer, blood pressure/flow monitor, thermistor, gas concentration, force plate feedback: actuator (output sensor) e.g., strain gauge, piezoelectric crystal, treadmill speed & grade 3
Sensor properties AIM: to accurately select and convert physiological/physical quantities to electrical signals, with a minimal effect on the environment being measured REQUIREMENTS: adequate frequency response amplitude & phase linearity low hysteresis 4
Frequency response The frequency bandwidth is the range of frequencies that can be picked up by a sensor e.g., hearing bandwidth is 20Hz-20kHz the bandwidth is equivalent to how fast the sensor can respond to changes in the measured quantity An inadequate frequency bandwidth results in amplitude and phase distortion 5
Sensor response The time/frequency response of a sensor can distort the output signal Transfer function used to model sensor Sensor Measured Quantity Sensor Input Sensor Output 6
Amplitude linearity Sensor Output Device calibration Linear calibration Measured Quantity Sensor output Measured quantity Performance expressed as a relative error e.g., ± 1% 7
Phase linearity The time relationship between the frequency components of a complex waveform must remain the same To ensure that time delays caused by the sensor are the same for all frequency components, the phase shift for each component must be proportional to the frequency c.f., φ=ωt 8
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Hysteresis Sensor Output Measured Quantity e.g., thermostat controlling an airconditioner Sensor output should be independent of the direction of change of the input Performance expressed as a relative error 10 e.g., ± 1%
Kistler 9826AA Wide measuring range Fz= 0-10 kn Force threshold Fz <250 mn Size: 60cm x 40cm x 3.5cm Mass <18 kg allows flexible, portable use can be used on any flat surface Solid aluminium block high stiffness, very small deformation
Limitations Has to be mounted on level, non-slip surface solid rigid surface with minimal vibrations Has to be kept at constant temperature It is not waterproof Cable connection needs to be treated gently! Need mains power for the DAQ unit Low natural frequency (z) ~200Hz
Force Sensors quartz multi-component force sensors with built-in charge amplifiers triaxial force sensors made up of 3 quartz rings calibration will not change with time
Piezo-electric Sensors x Amplifier Crystal Piezo-electric materials (i.e., quartz, ceramics) generate a charge when their shape is deformed act like an electric sponge discovered in 1880 by Curie brothers Charge amplifier voltage stress = force/area 14
Piezo-electric Frequency Response Mechanical resonance Output voltage Input force Usable range fc Low natural frequencies not high enough to measure high frequency impacts accurately (z) ~200Hz (x,y) ~350Hz 15
Output Signals 4 x triaxial force sensors 8 x channels built-in charge amplifiers
Coordinate Systems
Application Gait Analysis Gait is the movement pattern of an individual who is walking, running etc. Force plate measures ground reaction forces as the individual steps on and off the plate Gait analysis in orthopedics, prosthetics, rehabilitation and sports Helps improve running patterns, identify sport injuries and with posture-related problems
Other Applications Balance studies in neurology, psychology, drug tests and orthopedics Ergonomics, as well as in the development of shoes and playing surfaces Explosive events such as jumps, impacts and push-offs in sports Coefficients of friction
Measurements forces (GRF) moments centre of pressure (COP) excellent accuracy, free moment COM displacement, velocity and acceleration impulse, work and power force development (explosivity) N/s coefficient of friction frequency analysis
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LAB vertical force test using body weight center of pressure gait analysis balance with eyes open and eyes closed compare shoes vs. bare feet Vertical jumps several hops and countermovement