2. Temperature
??
mercury thermometers
thermoelements
physical property used?
Seebeck effect:
emf in a circuit connecting elements
maintained at different temp
RTD (resistance temperature detector)
Thermistors (thermally sensitive resistors)
resistors: dR/dT != 0
semiconductors: dR/dT != 0
Applications:
chemical processes, food processing, pasteurization,
many manufacturing processes, …
inaccurate, but very sensitive
platinum: linear over large range
3. Position/Displacement
variable
(potentiometer)
variable capacitance
physical property used?
variable inductance
(resolver)
capacitors: dC/dx != 0
Applications: ??
Faraday’s law, induced emf in coils
+ -
A
R
Length of resistor = L
Total resistance = R
Loop Resistance (function of position, x) = Rx/L
L
x
+ -
A
R
Length of resistor = L
Total resistance = R
Loop Resistance (function of position, x) = Rx/L
L
x
very sensitive (sub-micron range)
4. Velocity
dR/dx
Linear Velocity
physical property used?
Applications: ?
Faraday’s law
+ -
A
R
Length of resistor = L
Total resistance = R
Loop Resistance (function of position, x) = Rx/L
L
x
+ -
A
R
Length of resistor = L
Total resistance = R
Loop Resistance (function of position, x) = Rx/L
L
x
potentiometer
Linear Velocity
Angular Velocity
tachometer
commutator
V01
N S
commutator
V01
N S
N S
5. Derivatives of velocity
acceleration: a = dV/dt
(accelerometer)
physical property used?
jerk: da/dt
Applications:
Machine tools, Crash sensors, …
+ -
A
R
L
x
+ -
A
R
L
x
spring
spring loaded potentiometer
+- A
R
L
x
+- A
R
L
x spring electronic
differentiator
6. Presence detection
Faraday’s law
moving magnet type
physical property used?
optical sensor
applications
inductive Faraday’s law
Photoelectric effect
oscillator
induced
emf
reverse
emf
damped
current
current
sensor
Photoconductive cells
cadmium selenide, cadmium sulfide:
conductivity incident light
Photovoltaic cells
Transmitter
Receiver
beam
Reflector
object to
be sensed
Transmitter Receiver
beam
Transmitter
Receiver
beam
object to
be sensed
(a) Retroflective Arrangement
(b) Opposed (Through-Beam) Arrangement
(c) Diffuse Arrangement
object to
be sensed
Transmitter
Receiver
beam
Reflector
object to
be sensed
Transmitter
Transmitter
Receiver
Receiver
beam
Reflector
Reflector
object to
be sensed
object to
be sensed
Transmitter
Transmitter Receiver
Receiver
beam
Transmitter
Transmitter
Receiver
Receiver
beam
object to
be sensed
object to
be sensed
(a) Retroflective Arrangement
(b) Opposed (Through-Beam) Arrangement
(c) Diffuse Arrangement
object to
be sensed
object to
be sensed
7. Other sensors
ammeter, glavanometer
current
voltage
physical property used?
Flow rates of fluids
Mass
voltmeter
resistance ohmmeter
multimeter
P1
P2
Bernoulli's principle
??
Force/Pressure
dynamometers
piezo-electric
piezoelectric effect
Newton’s law
quartz: watches
Barium titanate, lead zirconate:
ultrasonic sensors, microphones
8. 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
9. Sensor Characteristics: Dynamic
0.5
1.0
Mp
Td
Tp Ts
Sensor
Response
Time
2 4 6 8 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
10. Sensor Characteristics: Statistical
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Low Accuracy, Low Precision
Low Accuracy, High Precision
High Accuracy, High Precision
ACCURACY: the difference between the ACTUAL and the MEASURED value
measure: (mean readings – actual value)
PRECISION: the variations of the MEASURED value
measure: (standard deviation of readings)
