Sensors and Data Acquisition

Question 1

What is a sensor

Answer 1

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

Question 2

What is the signal most sensory convert to

Answer 2

voltage

Question 3

What are 5 sensors used in biomechanics

Answer 3

• Strain gauge
• Force platform
• Accelerometer
• Potentiometer
• Motion capture

Question 4

What does a strain gauge do?

Answer 4

Converts forces along one axis to volts

Question 5

What does a force platform do?

Answer 5

Converts force along three axes to volts

Question 6

What does an accelerometer do

Answer 6

converts acceleration to volts

Question 7

What does a potentiometer do

Answer 7

Converts angular displacement to volts

Question 8

What does motion capture do

Answer 8

converts light to volts

Question 9

A strain gage is really a…

Answer 9

specially designed resistive circuit that is attached to a malleable object

Question 10

How does a strain gage work?

Answer 10

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

Question 11

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

Answer 11

the physical change

Question 12

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

Answer 12

narrows

increases

Question 13

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

Answer 13

thickens

decreases

Question 14

If a signal is analogue it means that there are…

Answer 14

No separations based on time between points in the signal

Question 15

If a signal is digital it means that there are

Answer 15

Separations between points in the signal

Question 16

What is sampling

Answer 16

the process of converting a continuous signal into a digital signal

Question 17

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

Answer 17

continuous

Discrete digital

Question 18

What is the sampling frequency

Answer 18

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

Question 19

What is calibration

Answer 19

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

Question 20

How is a sensor calibrated

Answer 20

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

Question 21

The offset is also known as the ____

Answer 21

y-intercept

Question 22

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

Answer 22

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

Question 23

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

Answer 23

Calibration equation

Question 24

What is hysteresis

Answer 24

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

Question 25

When do hysteresis occur

Answer 25

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

Question 26

Why is hysteresis undesirable

Answer 26

Because it would require different calibration coefficients for loading and unloading

Question 27

What is drift

Answer 27

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

Question 28

What can drift occur as the result of

Answer 28

a transducer warming up or as an effect of continued use

Question 29

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

Answer 29

linear

Question 30

What 3 things does a sensor normally include

Answer 30

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

Question 31

What is an operational amplifier

Answer 31

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

Question 32

How is data collected from a sensor

Answer 32

a break out box

Question 33

What is a break out box

Answer 33

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

Question 34

What is multiplexing

Answer 34

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

Question 35

Each connection to a multiplexor is called a ____

Answer 35

channel

Question 36

How does a multiplexor work?

Answer 36

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

Question 37

What are two problems with multiplexing

Answer 37

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

Question 38

What is the sampling rate equation

Answer 38

number of samples divided by number of channels

Question 39

What is sampling skew

Answer 39

The sequential pattern of the samples

Question 40

Sampling skew increases as...

Answer 40

The number of channels being sample increases

Question 41

What is a gain

Answer 41

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

Question 42

How do you limit the amount of noise amplified

Answer 42

a preliminary amplifier located close to the sensor is often used

Question 43

What does an A/D do

Answer 43

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

Question 44

What does the sample-and-hold circuit do

Answer 44

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

Question 45

What 3 things are AD converteers limited by

Answer 45

- 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)

Question 46

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

Answer 46

truncated or clipped

Question 47

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

Answer 47

bits

Question 48

What is bits short for

Answer 48

binary digits

Question 49

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

Answer 49

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

Question 50

An A/D's range is divided into...

Answer 50

a specific number of equal steps

Question 51

How do you find the amplitude between each step

Answer 51

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

Question 52

What is resolution

Answer 52

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

Question 53

How does the amplifier facilitate sampling

Answer 53

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

Question 54

What is the ideal gain of an amplifier

Answer 54

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

Question 55

What occurs to data after it is converted to digital,

Answer 55

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

Question 56

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

Answer 56

to perform many operations on digital signals very quickly