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Instrumentation > Force Measures > Flashcards

Force Measures Flashcards

1
Q

What are two ways to measure force with non-electric?

A
  • Spring (pressure down creates linear reading; Ex: Fish Scale)
  • Hydraulic (More reliable and complex; Ex: Grip Dyanometer (Hollow chamber with oil get compress)
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2
Q

What is an electric way to measure force?

A

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)

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3
Q

When a stretching force is applied the conductor becomes….

A

the narrower and longer (increasing electrical resistance), decreasing conductivity.

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4
Q

When a compression force is applies the conductor becomes…

A

broader and shorter (decreasing its electrical resistance), increasing conductivity

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4
Q

When a compression force is applies the conductor becomes…

A

broader and shorter (decreasing its electrical resistance), increasing conductivity

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5
Q

Types of Strain Gauges

A
  • 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)

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6
Q

Properties of a good force transducer

7

A
  • Low temperature sensitivity
  • High electric stability/low electrical interference
  • High linearity
  • Low hysteresis effects
  • Low “cross talk”
  • High natural frequency
  • Overload protection
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7
Q

Low sensitivity to temperature

A

Will measure in really hot or really cold with no change

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8
Q

High electrical stability/low electrical interference

A

Not sensitive to electrical issues in the environment

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9
Q

High linearity

A
  • For a given force, gives same output signal of force
  • No sensor is perfectly linear but the best are close
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10
Q

Low Hysteresis

A
  • 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
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11
Q

Low “cross talk”

A
  • 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
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12
Q

What is natural frequency

A
  • 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
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13
Q

Pros and Cons of Piezoelectric

A

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

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14
Q

How can treadmills measure force?

A
  • Force plates on the bottom of the treadmills
  • Super dynamic movements
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15
Q

Force Platform Calibration

A
  • “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
16
Q

Net Force

A
  • 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.
17
Q

What do we need to be careful about when taking Center of Pressure measurements?

A
  • 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.
18
Q

How does information collection from a force become data?

A
  • 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

19
Q

Signal Characteristics

A
  • 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
20
Q

Time vs Frequency Domains

A

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.

21
Q

Sampling Frequency

A
  • 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)
22
Q

Aliasing

A

If you record too slow you have a slower sign wave. Results in undersamples and inaccurate signal.

23
Q

Application: Noise and Filters

A

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.

24
Q

Analog to Digital Conversion

A
  • Computer processes digital slignals (electrical analog signal from plate is continuous)
  • A to D converter (defines the percision of the converter by the number of discrete intervals available to describe continuous signal); bits (8 or 12 is most common)
  • AD units is then converted to force
25
Q

8 bits = ____ intervals
12 bits = ____ intervals

A

8 bits = 256 intervals
12 bits = 4,096 intervals

26
Q

What happens to the gain (amplitude) when it is too high or too low?

A

Too high: Converter chops off the signal, too zoomed in.
Too low: Can barely see the graph, too zoomed out.

27
Q

What happens if the frequency is too high or too low?

A

Too high: leads to a sawtooth look
Too low: hard to see signals

28
Q

How do you determine the # of bits to use?

A
  • Depends on the question you are asking
  • 8 or 12 is generally good
29
Q

What must you do with all data?

A
  • Normalize it!
  • This allows you to compare between people but you must need the weight of the person to normalize. Heavier = more force.
30
Q

Selecting a Signal Conditioner

A

Amplitude range:
* Can you adjust the gain of your amplifier to properly fill the full range of your ADC?
* Will you frequently have trials that may excess this typical range?

Bits of ADC:
* 12 is adequate for most tasks depends on range
* 16-24 is gold standard

High sampling frequency:
* Depends on what you need to measure

Built-in noise filters vs software filters…
* Little flexibility, but cannot be forgotten

31
Q

GFR data collection: confounding factors

A
  • Participant alignment with platform
  • Running/walking speed (are they different speeds)
  • Jump height/height of drop (how someone lands)
  • Missing contact with the force platform during trial
  • Targeting (don’t want to change natural movement)
  • Double foot strike (only one foot at a time)
  • Type of foot strike: rear vs. fore foot (need to stay consistent)
  • Footwear type
32
Q

Application: Frequency

A

FF: Higher peak, lower slope early
RF: Lower peak, early transient peak, high slope early

In low frequency look similar. Gray is a little taller and is better represented similar to sin waves. High frequency is better shown in later frequency. If you sample too low these signals will look the same. Must be careful of data cutoff

Instrumentation (11 decks)

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