CHAPTER 4

MEASURING

DEVICES (SENSOR & TRANSDUCER)

OUTLINE

Introduction

What is sensor and transducer?

Selecting Transducer

Types of transducer

Passive Transducer

Self Generating Transducer

For many years, a transducer is a source of

information.

The operation of the transducer defines the

reliability of the information.

In spite of a wide variety of different systems

containing transducer, they can be divided into

two big groups i.e measuring system and control

system.

INTRODUCTION

Component of instrumentation system

INTRODUCTION CONT’D

Sensor /

Transducer

Physical

Parameters

Electrical

Signal

Pressure

Temperature

Flow

Light Intensity

Sound

Position

Acceleration

Force

Strain

Current

Voltage

Sensor is a device that detects, or senses, a signal or

physical condition.

Most sensors are electrical or electronic, although other

types exist.

A sensor is a type of transducer.

Sensors are either direct indicating (e.g. a mercury

thermometer or electrical meter) or are paired with an

indicator (perhaps indirectly through an analog to digital

converter, a computer and a display) so that the value

sensed becomes human readable. Aside from other

applications, sensors are heavily used in medicine,

industry and robotics.

WHAT IS SENSOR?

Transducer is a device that provides a usable

output in response to a specific measured.

In other word, transducer is a device that

converts energy in one form to energy in

another.

Transducer that provide an electrical output are

frequently used as sensors.

The transducer is the most important portion of

the sensor, in fact some “sensor” are merely

transducer with packaging

WHAT IS TRANSDUCER?

There are four factors to be considered in

selecting a transducer in a system:

Operating range The transducer should maintain range requirements and good

resolution

Sensitivity The transducer must be sensitive enough to allow sufficient output

SELECTING TRANSDUCER

Ability to suite with the environment

condition such as pressure Do the temperature range of the transducer, its corrosive fluids, the

pressures, shocks, and interactions it is subject to, its size and

mounting restrictions make it in application

High accuracy to produce sufficient output The transducer may be subject to repeatability and calibration

errors as well as errors expected owing to sensitivity to other

stimuli

SELECTING TRANSDUCER

CONT’D

Transducer can be classified into

two types:

(i) Passive Transducer

(ii) Self-Generating Transducer

(Active)

TYPES OF TRANSDUCER

Require an external power and their output is

a measure of some variation such as resistance

or capacitance

Examples:

LVDT

POTENTIOMETER

STRAIN GAUGE

CAPACITIVE TRANSDUCER

PASSIVE TRANSDUCER

LVDT (Linear Variable Differential Transformer)

The linear variable differential transducer (LVDT) is

a type of electrical transformer used for measuring linear

displacement

The transformer has three solenoid coils placed end-to-

end around a tube.

The centre coil is the primary, and the two outer coils are

the secondary.

A cylindrical ferromagnetic core, attached to the object

whose position is to be measured, slides along the axis of

the tube.

LVDT

LVDT CONT’D

A reliable and accurate sensing

device that converts linear

position or motion to a

proportional electrical output.

Basic construction of LVDT as shown in figure below:

LVDT CONT’D

Primary Secondary

A

B

A

B

Displacement/

Figure 1

LVDT consists of :

• a transformer with a single

primary winding

• two secondary windings

connected in the series-

opposing manner

(berlawanan arah)

LVDT CONT’D

Primary Secondary

A

B

A

B

Displacemen

t/

Cor

eVo

ut

Core

positio

n

Relationship between

displacement and output

VOUT = VA – VB

The core displacement determine the output:

If the core at the center, VA=VB, VOUT=0

Core at the ‘upper’ A

VA max, VB min VOUT max & +ve

Core at the ‘lower’ B

VA min, VB max VOUT max & -ve

LVDT has the following data:

Vin= 6.3V, Vout= + 5.2V &

displacement range = + 0.5 in.

Calculate the displacement when Vo is +2.6V.

EXAMPLE 1

+5.2 V

+2.6V

0.5”?

Vout

Core position

An ac LVDT has the following data: input 6.3V,

output ± 5.2V, range ±0.50 in. Determine:

a) The plot of the output voltage versus core position

for a core movement going from +0.45 in to -0.03

in.( 4.68V, -3.12V)

b) The output voltage when the core is -0.25 in. from

center. (-2.6V)

EXAMPLE 2

Applications of LVDT:

Used for measuring

displacement and

position

Used as null detectors in

feedback positioning

systems in airplanes and

submarines

Used in machine tools as

an input system

LVDT CONT’D

Example: Measuring position

POTENTIOMETER

A potentiometer is a variable

resistor that functions as a

voltage divider

Electromechanical device

containing a resistance that is

contacted by movable

slider.

Motion of the slider results in

a resistance change

depending on the manner in

which the resistance wire is

wound.

VO

W

R1

R2

Vi

ℓ1

ℓ2

ℓT

RT

ℓT = Shaft StrokeW = Wiper

There are various type of potentiometer:

Low Power Types:

Liner potentiometers

Logarithmic potentiometers

High Power Types:

Rheostat

Digital Control:

Digitally controlled potentiometers (DCP)

POTENTIOMETER CONT’D

The output voltage under ideal condition:

POTENTIOMETER CONT’D

ViVo

T

2

R inal,input term at the Resistance

R , minaloutput ter at the Resistance

T

T

RR

11

T

T

RR

22

ℓT = Shaft StrokeW = Wiper

VO

W

R1

R2

Vi

ℓ1

ℓ2

ℓT

RT

POTENTIOMETER CONT’D

Theory of operation:

The potentiometer can be

used as a potential divider (or

voltage divider) to obtain a

manually adjustable output

voltage at the slider (wiper)

from a fixed input voltage

applied across the two ends of

the pot. This is the most

common use of pots

The voltage across RL is determined by the formula:

s

L

LL V

RRR

RRV .

||

||

21

2

EXAMPLE 3

A resistive positive displacement transducer with a shaft

stroke of 10cm is used in the circuit of figure below. The

total resistance of potentiometer is 500Ω and the applied

voltage Vi is 15V. If the wiper, W is 7.5cm from A, what

is the value of

(a) R2 (125Ω)

(b) Vo (3.75V)

POTENTIOMETER CONT’D

Transducers

Potentiometers are widely used as a part of displacement transducers

because of the simplicity of construction and because they can give

a large output signal

Audio control

One of the most common uses for modern low-power potentiometers

is as audio control devices. Both sliding pots( known as faders) and

rotary potentiometer ( called knob) are regularly used to adjust

loudness, frequency attenuation and other characteristics audio

signals

A strain gauge is a metal or semiconductor element whose resistance changes when under strain.

Strain gauge is a passive transducer that uses “electrical resistance variation” in wires to sense the strain produced by a force on the wires.

It can measures:

Weight

Pressure

Mechanical Force

Displacement

STRAIN GAUGE

STRAIN GAUGE

The function of strain gauge is to sense the strain

produces by force on the wires.

The strain gauge is generally uses as an arm of a bridge.

This is only applicable when temperature variation in

wire.

Types of strain gauges:

STRAIN GAUGE CONT’D

Wire gauge Foil gauge Semiconductor gauge

Considering the factors that influence the

resistance of the element a relationship between

changes in resistance and strain can be derived.

Resistance is related to length, l(m) and area of

cross-section of the resistor ,A(m2) and

resistivity, ρ(Ωm) of the material as

STRAIN GAUGE CONT’D

STRAIN GAUGE CONT’D

When external force are

applied to a stationary object,

stress and strain are the result.

Stress is defined as the object’s

internal forces.

For a uniform distribution of

internal resisting forces, stress

can be calculated by dividing

the applied force (F) by the unit

area (A): A

F

Where; F Force

AArea

N/m2

*Stress – tekanan

STRAIN GAUGE CONT’D

The effect of the applied stress is produce a strain.

Strain is a fractional change (∆L/L) in the dimensions of

an object as a result of mechanical stress (force/area).

Calculated by dividing the total deformation of the

original length by the original length (L).

L

L

Where; ∆L Change in length

L Original unstressed length

Unit-less

*Strain – regangan

STRAIN GAUGE CONT’D

The constant of proportionality between stress

and strain for a linear stress-strain curve is

known as Young’s Modulus, E.

E

E

Young’s modulus in kilograms per-square meter

The stress in kilograms per square meter

The strain (no units)

STRAIN GAUGE CONT’D This changes its resistance (R) in proportion to the strain

sensitivity of the wire's resistance. When a strain is introduced, the strain sensitivity, which is also called the Gauge Factor (GF), is given by:

RRGF

L

L

L

L

R

R

GF

= gauge factor (unit less)

= the initial resistance in ohms (without strain)

= the change in initial resistance in ohms

= the initial length in meters (without strain)

= the change in initial length in meters

= gauge factor (unit less)

= the initial resistance in ohms (without strain)

= the change in initial resistance in ohms

= the initial length in meters (without strain)

= the change in initial length in meters

= gauge factor (unit less)

= the initial resistance in ohms (without strain)

= the change in initial resistance in ohms

= the initial length in meters (without strain)

= the change in initial length in meters

A resistant strain gauge with a gauge factor of

2 is fastened to a steel member, which is

subjected to strain of 1x10-6. If the original

resistance value of the gauge is 130Ω,

calculate the change in resistance. (260µΩ)

EXAMPLE 4

SOLUTION

The capacitor consists of two parallel plates separated by

an air space or by a dielectric (insulating material).

The capacitance of the of the pair of the plates is measure

of the amount of charge that can be transferred before a

certain voltage is reached.

CAPACITIVE

TRANSDUCER

Plate 1

Plate 2

Dielectric

material

The basic construction of capacitor

CAPACITIVE TRANSDUCER CONT”D

d

kAC o

k = dielectric constant of the material in the gap

εo = the permittivity of free space

= 8.854 x 10-12 farad/meter

A = Plate area (m2)

d = the separation between plate (m)

Plate 1

Plate 2dd

width

Length

Schematic diagram

of parallel-plate

capacitor

CAPACITIVE TRANSDUCER CONT”D

There are three criteria/conditions that can change the

capacitor (variation of capacitance) :

(a) Changing the surface area

(b) Changing the dielectric constant

(c) Changing the spacing between plate

xDisplacement

x=0

d

kAC o

(a) Changing the surface area

CAPACITIVE TRANSDUCER CONT”D

If one plate of the parallel plate capacitor is displayed in a

direction parallel to the plate, the effective area of the plates

will change proportionally to the value of capacitance

Plate 1

Plate 2

Dielectric

material

C

A

CAPACITIVE TRANSDUCER CONT”D

(b) Changing the dielectric constant

The value of capacitance will increase when the dielectric

constant is increased

Plate 1

Plate 2

Dielectric

material

C

k

CAPACITIVE TRANSDUCER CONT”D

(c) Changing the spacing between plate

The value of capacitance will decrease when the spacing

between plate increased

Plate 1

Plate 2

Dielectric

materiald

C

d

εo = 8.854 x 10-12 Fm-1, kair = 1, kmaterial = 5

Two square metal plates, side 6 cm separated

by a gap of 1 mm.

Calculate the capacitance of the sensor when

the input displacement of x is:

(a) 0.0 cm (159.38pF)

(b) 3.0 cm (63.75pF)

EXAMPLE 5

SOLUTION