CHAPTER 6 CHAPTER 6

U N I M A P

SENSORS AND SENSORS AND TRANDUCERSTRANDUCERS

6.1 INTRODUCTION6.1 INTRODUCTION

To introduce the basic concepts in measurement systems

To define sensor terminology

To identify sensor applications

To present the need for microsensors

U N I M A P

OBJECTIVESOBJECTIVES

U N I M A P

6.2 MEASUREMENT SYSTEMS6.2 MEASUREMENT SYSTEMS

Input Signal = Measurand Sensor (Input Tranducer)

Chemical Quantity

Eg. displacement, presure

Eg. Gas concentration

Physical Quantity

Sensor : a device that converts a non-electrical physical

or chemical quantity into an electrical signal.

U N I M A P

Detect Modify

Display

Record

Transmit

System Boundary

Input Signal

Output Signal

Fig. 6-1: Functional block diagram of a measurement

6.2 MEASUREMENT SYSTEMS (cont……)

6.2 MEASUREMENT SYSTEMS (cont……)

U N I M A P

6.2 MEASUREMENT SYSTEMS (cont…)

6.2 MEASUREMENT SYSTEMS (cont…)

Sensor Processor Actuator

(input transducer) (output transducer)

System Boundary

Input Signal

Output Signal

Fig. 6-2: Basic components of a measurement or information-processing system

Processor : a device that modifies the electrical signal coming from the sensor without changing the form of the energy that describes the signal.

Actuator or output transducer : a device that converts an electrical signal into a physical or chemical quantity.

U N I M A P

6.3 CLASSIFICATION OF SENSING DEVICES6.3 CLASSIFICATION OF SENSING DEVICES

Form of Signal

Measurands

Thermal

Radiation

Mechanical

Magnetic

Chemical

Temperature, heat, heat flow, entropy, heat capacity.

Gamma rays, X-rays, ultra-violet, visible, infra red,

micro-waves, radio waves.

Displacement, velocity, acceleration, force, torque,

pressure, mass, flow, acoustic wavelength and

amplitude.

Magnetic field, flux, magnetic moment, magnetisati-

on, magnetic permeability.

Humidity, pH level and ions, concentration of gases,

vapours and odours, toxic and flammable materials,

pollutants.

Table 6-1: Classification of sensors by signal form.

U N I M A P

Form of Signal

Measurands

Biological

Electrical

Sugars, proteins, hormones, antigens.

Charge, current, voltage, resistance, conductance, capacitance, inductance, dielectric permittivity, polarisation, frequency.

Table 6-1: Classification of sensors by signal form.

6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

U N I M A P

Table 6-2: Classification of the human senses .

Human

Sense

Signal Measurand

Sensing Device

Analogue Device

Sight

Hearing

Smell

Radiant

Mechanical

Chemical

Intensity and

wavelength

of light

Intensity and

frequency of

sound

Odorants

Rods and

cones in

retina

Cochlea in

inner ear

Olfactory receptor cells in nose

Photographic

film, photodiode,

Phototransistor

Microphone

Electronic nose

6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

U N I M A P

Table 6-2: Classification of the human senses .

Human

Sense

Signal Measurand

Sensing Device

Analogue Device

Touch

Taste

Mechanical

Biological

Pressure,

force

Proteins

Nerves

Taste buds in tongue

Potentiometers

and LVDTs

(simple touch),

optical gauging

and tactical

arrays (complex

touch)

6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

U N I M A P

Table 6-3: Classification of some common actuators.

Function

Actuator Signal Principle

Display

Transmit

Light emitting

diode

Visual display unit

Liquid crystal

display

Loudspeaker

Aerial

Electric motor

Radiant

Radiant

Radiant

Mechanical

Radiant

Mechanical

Current generation of

photons

Fluorescent screen

Transmittance of

polarised molecular

Crystals

Generation of sound

Generation of radio

wave

Generation of motion

6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

U N I M A P

Table 6-3: Classification of some common actuators.

Function

Actuator Signal Principle

Record Thermal printer

Magnetic recording

head

Laser

Thermal

Magnetic

Radiant

Ink is melted

Magnetisation of thin

films on computer

disc

Ablation of material

on optical disc

6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

U N I M A P

6.4 Ideal Sensor Characteristics and Practical Limitations6.4 Ideal Sensor Characteristics and Practical Limitations

InputSENSOR

Output

x (t) y (t)

System

SENSOROutput

x (t) y (t) + yd

External drive xd

Input

(a) Self-exciting (b) Modulating

Fig. 6-3: Basic representation of self-exciting and modulating sensor systems

A sensor in its simplest form may be regarded as a system with an input x (t) and output y (t).

U N I M A P

6.4 Ideal Sensor Characteristics and Practical Limitations (cont……)

6.4 Ideal Sensor Characteristics and Practical Limitations (cont……)A self-exciting sensor has its output energy supplied entirely by the input signal x (t).

The general equation that describes a self-exciting sensor system is

))(()( txFty

where F(x(t)) is the characteristic relationship that describes the behavior of a self-exciting sensor.

(6.1)

U N I M A P

Eg : A thermocouple → input signal = the difference in junction temperatures ΔT(t) and the output = e.m.f Φ(t) in volts.

))(()( tTFt

In the case of the modulating sensor, the system equation can be written more explicitly as

))(()( dxtxFty

where the external supply signal xd(t) should ideally be stationary and noise free.

6.4 Ideal Sensor Characteristics and Practical Limitations (cont……)

6.4 Ideal Sensor Characteristics and Practical Limitations (cont……)

(6.2)

U N I M A P

Fig. 6-4: Ideal input-output relationship of self-exciting and modulating sensors.

Slope, S

ym

xm-xd

yd

Sensor output

Sensor input0

xd ≠ 0

xd = 0

The ideal sensor not only has a linear output signal y(t) but it should instantaneously follow the input signal x(t), whence

)(.)( txSty The slope S is usually referred to as the sensitivity.

6.4 Ideal Sensor Characteristics and Practical Limitations (cont……)

6.4 Ideal Sensor Characteristics and Practical Limitations (cont……)

(6.3)

U N I M A P

General properties of a good sensor are:

1) Optimum measurement accuracy

2) Good durability

3) Ease of calibration and reconditioning

4) High sensitivity

5) Good reproducibility

6) Long term stability

7) Fast response

8) Continuous operation

9) Insensitivity to electrical and other environmental

interference

10) Low fabrication, operation and maintenance cost