Sensors
-measure and report
the state of some variable which characterizes the process
sensor
Process
Interface
Sensing device
uses a physical/chemical property
Transducer
Control/Reporting
Interface
converts it into useful signal
Temperature
physical property used?
mercury thermometers
thermoelements
RTD (resistance temperature detector)
??
Seebeck effect:
emf in a circuit connecting elements
maintained at different temp
resistors: dR/dT != 0
platinum: linear over large range
Thermistors (thermally sensitive resistors)
semiconductors: dR/dT != 0
inaccurate, but very sensitive
Applications:
chemical processes, food processing, pasteurization,
many manufacturing processes, …
Position/Displacement
physical property used?
L
variable 
(potentiometer)
x
R
+ -
variable capacitance
Length of resistor = L
Total resistance = R
Loop Resistance (function of position, x) = Rx/L
A
capacitors: dC/dx != 0
very sensitive (sub-micron range)
variable inductance
(resolver)
Applications: ??
Faraday’s law, induced emf in coils
Velocity
physical property used?
L
x
Linear Velocity
Length of resistor = L
Total resistance = R
Loop Resistance (function of position, x) = Rx/L
potentiometer
R
+ -
dR/dx
A
Linear Velocity
commutator
Angular Velocity
N
S
tachometer
V01
Applications: ?
Faraday’s law
Derivatives of velocity
physical property used?
acceleration: a = dV/dt
(accelerometer)
spring
L
x
spring loaded potentiometer
R
A
+ -
jerk: da/dt
L
x
+-
R
spring
A
electronic
differentiator
Applications:
Machine tools, Crash sensors, …
Presence detection
physical property used?
moving magnet type
applications
Faraday’s law
inductive
Faraday’s law
oscillator
induced
emf
reverse
emf
damped
current
current
sensor
(a) Retroflective Arrangement
optical sensor
Transmitter
beam
cadmium selenide, cadmium sulfide:
Photoconductive cells
Reflector
Receiver
conductivity  incident light
object to
be sensed
(b) Opposed (Through-Beam) Arrangement
Transmitter
Photovoltaic cells
beam
Receiver
Photoelectric effect
object to
be sensed
(c) Diffuse Arrangement
Transmitter
beam
object to
be sensed
Receiver
Other sensors
physical property used?
current
ammeter, glavanometer
voltage
voltmeter
resistance ohmmeter
multimeter
Flow rates of fluids
P2
P1
Bernoulli's principle
Mass
??
Force/Pressure
dynamometers
piezo-electric
Newton’s law
quartz: watches
Barium titanate, lead zirconate:
ultrasonic sensors, microphones
piezoelectric effect
Sensor Characteristics: Static
Sensitivity: change in output / change in the input
Resolution: smallest amount of change in the input that can be detected and accurately indicated
Linearity: (of calibration curve)
(i) plot static output versus static input
(ii) measure linearity of graph
Drift: deviation in output value when the sensor is kept at constant input level for long time
Special cases: Zero Drift, Full-scale drift
Range: (upper limit – lower limit) of output (or input)
Repeatability: deviation in repeated measurements of same object, from same direction
Reproducibility: repeatability over long time lapses between measurements
Sensor Characteristics: Dynamic
Sensor Response
Mp
1.0
0.5
Td
2
Tp
4
6
Ts
8
Time
10
Rise time: time to pass between 10% to 90% of the steady state value (SSV) of response
Delay time: time it takes to reach 50% of SSV for the first time
Peak time: time it takes to reach the maximum reading for the first time
Settling time: time taken to settle down to within, e.g. ± 1%, the steady state value (SSV)
Percentage overshoot: (peak value - SSV) / SSV
Steady-state error: deviation of the actual SSV from the desired value
Sensor Characteristics: Statistical
ACCURACY: the difference between the ACTUAL and the MEASURED value
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measure: (mean readings – actual value)
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PRECISION: the variations of the MEASURED value
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Low Accuracy, Low Precision
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Low Accuracy, High Precision
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High Accuracy, High Precision
measure: (standard deviation of readings)