Sensors

Sensing device TransducerProcessInterface

Control/ReportingInterface

sensor

Sensing device TransducerProcessInterface

Control/ReportingInterface

sensor

-measure and report the state of some variable which characterizes the process

uses a physical/chemical property converts it into useful signal

Temperature

?? mercury thermometers

thermoelements A

B

C

T2

T2

T1

V

Fe

Al

A

B

C

T2

T2

T1

V

Fe

Al

physical property used?

Seebeck effect:emf in a circuit connecting elementsmaintained 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

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 = LTotal resistance = RLoop Resistance (function of position, x) = Rx/L

L

x

+ -A

R

Length of resistor = LTotal resistance = RLoop Resistance (function of position, x) = Rx/L

L

x

very sensitive (sub-micron range)

~

Stator

Rotor

AC supply

V01 = KVref sin

V02 = KVref cos

Vref

~~

Stator

Rotor

AC supply

V01 = KVref sin

V02 = KVref cos

Vref

Velocity

dR/dxLinear Velocity

physical property used?

Applications: ?

Faraday’s law

+ -A

R

Length of resistor = LTotal resistance = RLoop Resistance (function of position, x) = Rx/L

L

x

+ -A

R

Length of resistor = LTotal resistance = RLoop Resistance (function of position, x) = Rx/L

L

x

potentiometer

Linear Velocity

~

Stator

Rotor

AC supply

V01 = KVref sin

V02 = KVref cos

Vref

~~

Stator

Rotor

AC supply

V01 = KVref sin

V02 = KVref cos

Vref

Angular Velocity

tachometer

commutator

V01

N S

commutator

V01

N SN S

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

+- AR

Lx

+- AR

Lx spring electronicdifferentiator

Presence detection

Faraday’s law moving magnet type

physical property used?

optical sensor

applications

inductive Faraday’s law

Photoelectric effect

oscillatorinduced

emfreverse

emfdampedcurrent

currentsensor

Photoconductive cells cadmium selenide, cadmium sulfide:

conductivity incident light

Photovoltaic cells

Transmitter

Receiver

beam

Reflector

object to be sensed

Transmitter Receiverbeam

Transmitter

Receiver

beamobject 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

TransmitterTransmitter

ReceiverReceiver

beam

ReflectorReflector

object to be sensedobject to

be sensed

TransmitterTransmitter ReceiverReceiverbeam

TransmitterTransmitter

ReceiverReceiver

beamobject to

be sensedobject to

be sensed

(a) Retroflective Arrangement

(b) Opposed (Through-Beam) Arrangement

(c) Diffuse Arrangement

object to be sensedobject to

be sensed

Other sensors

ammeter, glavanometercurrent

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

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

0.5

1.0

Mp

Td Tp Ts

Se

nso

r R

esp

on

se

Time2 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

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)