Sensors Flashcards
What is the purpose of a pre-amplifier?
Conditions the electrical signal for transmission to the acquisition system
What is the sensitivity of a sensor?
The transfer function (dy/dx)
What is the resolution of a sensor?
The smallest change of input that can be reliably detected (determined by noise)
What is the difference between range and dynamic range?
Range is two numbers, while dynamic range is the difference between the max.and min. numbers
What is hysteresis?
When the value of a parameter changes depending on the direction of change
What is characteristic error?
When there is a difference between the ideal and real transfer function e.g. d.c. offset, non-linearities
What is dynamic error?
Error due to transient behaviour, i.e. slow responses
What are four examples of light sensors and what electrical property do they transmit?
Photodiode (V)
Photodiode PMT (I)
Photoresistor (R)
Charge-coupled device (Q)
What are three examples of temperature sensors and what electrical property do they transmit?
Thermocouple (V)
IR thermocouple (I)
Positive/negative temperature coefficient materials (R)
What are two examples of force sensors and what electrical property do they transmit?
Piezoelectric (V, Q) Strain gauge (R)
What is the difference between accuracy and precision?
Accuracy is the closeness to the true value
Precision is the closeness of multiple observations
What is the difference between repeatability and reproducability?
Repeatability is the closeness of multiple observations over a short time interval
Reproducability is the closeness of multiple observations over a long time interval or with different operators/instruments/location
What is bias error?
The difference between true value and measurement average
What is the sensitivity error?
The difference between max. and min. sensitivity at a given sensor output
What is the purpose of calibration?
To eliminate or reduce bias in the measurement system
What are the three levels of standard in instrument calibration?
Primary standard - doesn’t need calibration
Secondary standard - calibrated against primary
Working standard - calibrated against secondary
What circuit is analogous to an angular potentiometer?
A voltage divider
What are the characteristics of PT100?
A platinum thermistor which has 100ohm resistance at O degrees
Low sensitivity, high reproducability
Stable calibration
Linear
What is n and p doping?
n - addition of electrons
p - addition of holes
Name 4 common semi-conductor materials
Silicon
Phosphorus
Arsenic
Boron
What are the semiconductor properties of silicon?
High temperature dependence
High sensitivity, low reproducability
Non-linearity
What does NTC stand for?
Negative temperature coefficient
Give two exapmles of materials with -ve alpha values
Graphite and silicon
How do photodiodes generate a current proportional to incident light?
Incident photons create electron-hole pairs in spatial charge region of semiconductor
Is photodiode performance linear?
Yes, over a wide range
How to photodiodes behave at different frequencies of light?
UV - photodiode too opaque, so photons absorbed at surface
Visible Light - Response
Near IR - Peak response
IR - Photons do not have enough energy to generate electron-hole pairs
What is the difference between a metal and a silicon strain gauge?
Silicon gauge factor 100x that of metal
How can temperature sensitivity of a strain gauge be compensated for?
- Use a dummy gauge not exposed to strain
- Use low thermal expansion coefficient material
- Use low thermal resistivity dependent materials
- Use bridge configuration
How can a wheatstone bridge be configured to compensate for temperature crosstalk?
Replace R3 with a strain gauge which doesn’t experience the strain, but does experience the temperature
How can a wheatstone bridge be modified to detect bending?
Replace R2 with a strain gauge
As R4 is extended R2 compresses, so bridge is unbalanced
How can a wheatstone bridge be modified to detect extension and compression?
Replace R1 with a strain gauge
As R4 is extended, R1 is also extended and so the bridge is unbalanced
