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EPHE 341 > Sensors and Data Acquisition > Flashcards

Sensors and Data Acquisition Flashcards

1
Q

What is a sensor

A

A device that measures a physical quantity and converts it into a signal which can be read by an observer of by an instrument

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2
Q

What is the signal most sensory convert to

A

voltage

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3
Q

What are 5 sensors used in biomechanics

A
  • Strain gauge
  • Force platform
  • Accelerometer
  • Potentiometer
  • Motion capture
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4
Q

What does a strain gauge do?

A

Converts forces along one axis to volts

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5
Q

What does a force platform do?

A

Converts force along three axes to volts

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6
Q

What does an accelerometer do

A

converts acceleration to volts

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7
Q

What does a potentiometer do

A

Converts angular displacement to volts

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8
Q

What does motion capture do

A

converts light to volts

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9
Q

A strain gage is really a…

A

specially designed resistive circuit that is attached to a malleable object

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10
Q

How does a strain gage work?

A

When a circuit is physically bent (in tension or compression) the resistance to the flow of electricity changes and therefore the voltage changes

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11
Q

In a strain gauge the chain in voltage is related to…

A

the physical change

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12
Q

If the strain gauge is in tension the area _____ and the resistance _____

A

narrows

increases

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13
Q

If the strain gauge is in compression the area _____ and the resistance _____

A

thickens

decreases

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14
Q

If a signal is analogue it means that there are…

A

No separations based on time between points in the signal

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15
Q

If a signal is digital it means that there are

A

Separations between points in the signal

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16
Q

What is sampling

A

the process of converting a continuous signal into a digital signal

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17
Q

most physical signals are inherently ______ but when we use sensors to quantify the data we must convert them to a _____ signal

A

continuous

Discrete digital

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18
Q

What is the sampling frequency

A

the number of samples in one second and is expressed in Hertz

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19
Q

What is calibration

A

The validation of specific measurement techniques and equipment
A comparison between measurements, one of a known magnitude and another of an unknown magnitude

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20
Q

How is a sensor calibrated

A

Apply various know forces to the device. Then we could graph the value of the force and the voltage that the sensor outputs related to this force. We get a slope. with the slope and the offset we can determine the force for any voltage output and the sensor is said to be calibrated

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21
Q

The offset is also known as the ____

A

y-intercept

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22
Q

If the relationship between applied force and voltage is non linear, we need…

A

more than just the slope and offset, we need the coefficients that describe the polynomial

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23
Q

The ____ can be used to convert from a measured voltage to a force in Newtons

A

Calibration equation

24
Q

What is hysteresis

A

The influence of the previous history of a system on its subsequent response to a given stimulus

25
Q

When do hysteresis occur

A

when a different force to voltage relationship exists when the force the force is incrementally increased compared to when it is incrementally decreased

26
Q

Why is hysteresis undesirable

A

Because it would require different calibration coefficients for loading and unloading

27
Q

What is drift

A

a gradual change in a transducer’s voltage output without an accompanying change in the applied force

28
Q

What can drift occur as the result of

A

a transducer warming up or as an effect of continued use

29
Q

If the drift is ___, the slope can be determined and the voltage output can be adjusted

A

linear

30
Q

What 3 things does a sensor normally include

A
  1. The physical sensing unit
  2. a power supply
  3. An operational amplified
31
Q

What is an operational amplifier

A

A dedicated circuit that amplifies the voltage from the sensor and can perform operations on the signal (from simple to complex math)

32
Q

How is data collected from a sensor

A

a break out box

33
Q

What is a break out box

A

A plastic or metal box with one or more rows of terminals (or connectors). A wire for each sensor connected to the breakout box is bunndled into one large cable that connects to the rest of the data acquisistion system

34
Q

What is multiplexing

A

Allows the breakout box to record from more than one sensor at a time.

35
Q

Each connection to a multiplexor is called a ____

A

channel

36
Q

How does a multiplexor work?

A

The DAQ draws the first sample from the first sensor; the multiplexor then switches the circuit to receive a sample from the second sensor, once all connected sensors have been read, the multiplexer switches back to the first sensor

37
Q

What are two problems with multiplexing

A
  1. The effective sampling rate is decreased (by a factors of the number of channels being multiplexed)
  2. Samples are draw sequentially, so there is a delay between each one. No simultaneous
38
Q

What is the sampling rate equation

A

number of samples divided by number of channels

39
Q

What is sampling skew

A

The sequential pattern of the samples

40
Q

Sampling skew increases as…

A

The number of channels being sample increases

41
Q

What is a gain

A

the number of times that a signal’s amplitude is increased

42
Q

How do you limit the amount of noise amplified

A

a preliminary amplifier located close to the sensor is often used

43
Q

What does an A/D do

A

Works to divide the analog signal into discrete values. The A/D tried to determine the number that most closely corresponds to the applied voltage

44
Q

What does the sample-and-hold circuit do

A

freezes the voltage signal while the A/D determines the correct numerical value. The voltage input must not change much or the number produced by the A/D will not be an accurate reflection of the voltage. The sample-and-hold circuit helps accomplish that

45
Q

What 3 things are AD converteers limited by

A
  • the range of voltages that they can read (window and amplification)
  • The number of magnitude options (Magnitude resolution or steps)
  • The number of time options (temporal resolution)
46
Q

If a voltage is applied beyond the upper and lower limits of an AD the digitized signal will be ___

A

truncated or clipped

47
Q

The magnitude resolution of an AD is usually expressed in units of -__

A

bits

48
Q

What is bits short for

A

binary digits

49
Q

an 8-bit A/D has _____ levels. this means that.

A
  1. That 8 digits of either 1 or 0 are required to represent 256 individual numbers
50
Q

An A/D’s range is divided into…

A

a specific number of equal steps

51
Q

How do you find the amplitude between each step

A

take the voltage range and divide it by the number of steps

52
Q

What is resolution

A

the number of steps that divides an A/’s range

53
Q

How does the amplifier facilitate sampling

A

by ensuring that the voltage input extends throughout as much of the range of the A/D as possible, this increases the resolution of the sampled signal by allowing the signal to be divided into more divisions

54
Q

What is the ideal gain of an amplifier

A

One that maximizes the range of the A/D but that doesn’t result in cliipping

55
Q

What occurs to data after it is converted to digital,

A

it is sent to a PC and is stored on a long-term memory device

56
Q

What is the aim of any software package used to process and analyze data

A

to perform many operations on digital signals very quickly

EPHE 341 (17 decks)

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