What are two ways to measure force with non-electric?
- Spring (pressure down creates linear reading; Ex: Fish Scale)
- Hydraulic (More reliable and complex; Ex: Grip Dyanometer (Hollow chamber with oil get compress)
What is an electric way to measure force?
Strain Gauge
* Type of transducer (converts one energy form into a different energy form)
* When object is deformed it changes the conductivity and thus changes the voltage (conductivity = 1/resistance)
When a stretching force is applied the conductor becomes….
the narrower and longer (increasing electrical resistance), decreasing conductivity.
When a compression force is applies the conductor becomes…
broader and shorter (decreasing its electrical resistance), increasing conductivity
When a compression force is applies the conductor becomes…
broader and shorter (decreasing its electrical resistance), increasing conductivity
Types of Strain Gauges
- Wire Strain Gauge
- Piezoelectric
Each measure load
- Single: Tension/Compression
- Multiple: Tension/Compression/Torque
(Arranging the strain gauge in different configuration, allows for multiaxis transducer)
Properties of a good force transducer
7
- Low temperature sensitivity
- High electric stability/low electrical interference
- High linearity
- Low hysteresis effects
- Low “cross talk”
- High natural frequency
- Overload protection
Low sensitivity to temperature
Will measure in really hot or really cold with no change
High electrical stability/low electrical interference
Not sensitive to electrical issues in the environment
High linearity
- For a given force, gives same output signal of force
- No sensor is perfectly linear but the best are close
Low Hysteresis
- Loading and unloading. Direction dependent resonse. Want it close to the line.
- Input and output are not the same
- Ex: Memory foam pillow; more force in, less out
Low “cross talk”
- Put a force in correct direction.
- Not coming in at the correct direction results in improper alignment and an inability to record proper measurements (Get mix of AP and ML). Often needs to be ML or AP
What is natural frequency
- The way in which something vibrates
- Want it thick and dense enough to create really high frequency
- Want to avoid up and down
- Need to be aware of how you land on force plate as different locations may have a different frequency
Pros and Cons of Piezoelectric
Pro (Good Insulator - responds to dynamic loads, responds quick)
* High frequency response
* Greater linearity
* Lower hysteresis
* Less temperature sensitive
Con
* Expensive
* May be more susceptible to overload damage
How can treadmills measure force?
- Force plates on the bottom of the treadmills
- Super dynamic movements
Force Platform Calibration
- “Factory Calibrated”
- Check Accuracy via own weights in different locations
- 4 Transducers per platform (All together give the forces and moments)
– 3 Forces (Fz, Fy, Fx); 3 moments (Mz, My, Mx)
– Center of pressure
— X coordinate = My / Fz
— Y coordinate = Mx / Fz
Net Force
- Is applied at the center of pressure
- Use information of COP and Force Vectors into inverse dynamics and sway measure create muscle forces and angles.
What do we need to be careful about when taking Center of Pressure measurements?
- If you have too big of a force plate and person puts both feet while steping, the system uses them together rather than seperate. Therefore, you lose overall resolution of center of pressure as well.
How does information collection from a force become data?
- Force: Mechanical signal
- Transducer: Electrical signal
- Computer: Digital signal
Analog to Digital Conversion (changes electrical voltage signal and converts it to a digital signal)
* Analog - continuous
* Digital - discrete
Amplifer does summing before ADC; Summing: Add voltages together from the force plate
Signal Characteristics
- Frequency (f): number of waves
- Amplitude (A): height of wave (loudness)
- Offset (a0): Add constant amount and move in value
- Time Shift (t0): Lean or Lag; based on frequency and moves through time
Time vs Frequency Domains
Difference:
* High/Deep Slope is like landing on forceplate quickly. High Frequency.
* Shallow Slope is like landing on a force plate lightly. Low Frequency.
Real signals we want are in low fequency range BUT unique and interesting portions are in high frequency.
Sampling Frequency
- Number of samples per second
- General rule is to same as fast as possible
- Nyquist Theorem - 2x times the fastest frequency in movement (Prevents aliasing)
Aliasing
If you record too slow you have a slower sign wave. Results in undersamples and inaccurate signal.
Application: Noise and Filters
Reality: Noise somewhere in the system. Typically electrical at 60 Hz often overlay into signal.
Want to analyze at higher Hz to be able to properly seperate out the electrical noise.
