Week 6 - Force Measurement Flashcards
What is a force transducer?
- Force transducers give an electrical signal proportional to an applied force
- They detect the force to be measured and convert (transduce) it into an electrical signal proportional to the force applied.
- Most common piezoelectric or piezoresistive
Explain the difference between Piezoelectric and Piezoresistive transducers
- Piezoelectric: Work on the principle of development of an electrical charge on quartz crystal when force applied
- Piezoresistive: Strain gauges made of materials whose resistance changes when deformed
Outline the advantages and disadvantages of piezoelectric and piezoresistve transducers.
- Piezoelectric: Less affect by temp, good for body movements, more difficult to install, widely adjusted range (to even feeling pulse in feet). Deformation changes electrical charge.
- Piezoresistive: Temperature compensation effect, frequent calibration needed, easier to install, strength of electrical signal (resistance) changes in relation to deformation
______ is the property of a material to generate a voltage when mechanical force is applied to it. In contrast, the________ effect is the property of a material’s resistivity to change when subjected to a mechanical force
Piezoelectric, piezoresistive
In what form is the signal collected using force transducers and what must be done to it to allow a numerical value of force to be calculated?
- Must convert physiological (analogue) signal to a digital signal (A-D)
- Software performs this and also allows a numerical value of force to be calculated
Should the sampling theorem be considered when collecting force data? Explain your answer.
Yes, sampling rate must be at least twice the frequency of the highest frequency of oscillation (sampling theorem)
Several operational characteristics of force transducers can influence the accuracy and reliability of the system. Outline and explain several points of consideration regarding these characteristics. (9)
- Adequate system sensitivity
- Low force detection threshold
- High linearity
- Low hysteresis
- Low cross talk
- Elimination of cable interference
- Electrical inductance and temp and humidity variations.
- Large enough area for movement to be recorded
- No outside vibrations (Can be filtered out)
Explain the term “ecological validity” and provide examples in relation to measuring ground reaction forces.
- changes in movement pattern due to being measured –> e.g., targeting platform, lab settings vs real environment, different surfaces etc
- realism with which a design of evaluation setup matches the user’s real work context
Injury, or the susceptibility to injury is closely linked with which biomechanical variable that can be determined using a force measurement system? Can you explain why?
Load rate or rate of force development loading is closely linked to injury (much steeper force slope, reaches peak way quicker)
Other than force platforms, what force measuring devices are often used in human movement analysis?
Accelerometer, pressure mat, force measurement (PEDAR), inverse dynamics
Describe pressure mat and what it does
- In quiet standing on a force platform, these forces are under the motor control of the ankle muscles
- COP is the neuromuscular response to imbalances of the body’s COG
- Pressure mat is simply a force platform that has the appropriate software to calculate the previous equations and thus calculate the COP at a given point in time
Explain accelerometer
- Device that measures acceleration
- Most have strain gauges that measure reaction forces within a given acceleration
- For acceleration to be accurately measured the acceleration vector must act at right angles to the face of the force transducer
Explain force measurement (PEDAR)
- shoe inserts that have multiple compressible elements that have electrical current passed through them.
- When compressed, the force is transduced to an electrical current
Describe inverse dynamics
- Indirect method of measuring joint force
- We can calculate acceleration and if we know the mass of the system then we can calculate the force required to move a system of that mass across a given distance In-shoe
Why would we measure force in running?
- Find different patterns in foot load - rear, mid, fore foot strikers
- Effect of different surface or shoes
- Used for developing running shoes and investigating sport surfaces
