BEKG 1233 Instrumentation and Measurement

7.1 Transducer

DEFINITION >>;

A device that receives energy from one system and transmits it to another, often in a different form

THERE ARE BASICALLY TWO TYPES OF TRANSDUCERS

ELECTRICAL TRANSDUCER

Converts the input measured into an electrical voltage/current.

MECHANICAL TRANSDUCER

Converts the input measured into a mechanical energy.

7.1.1 ELECTRICAL TRANSDUCER

DEFINITION > A device that converts a physical, mechanical or optical quantity into a proportional voltage or current quantity.

PARAMETERS NEEDED FOR THE ELECTRICAL TRANSDUCERS ARE

1) Linearity

Linear relationship between a physical parameter and the resulting electrical signal.

2) Sensitivity

Defined as the electrical output per unit change in the physical parameter (eg. V/0C)
High sensitivity is desirable.

3) Dynamic Range

Transducer must have a wide operating range.

4) Repeatability

The input/output relationship for a transducer should be predictable over a long period of time.

5) Physical Size

The transducer must have minimal weight and volume to minimize the disturbance to the existing conditions/environment.

CLASSIFIED INTO TWO MAJOR CATEGORIES

Active Transducers

Generates an electrical signal directly in response to the physical parameter (does not require external power to operate).
Example : piezo-electric sensor & photo cells.

Passive Transducers

Requires external power to operate.
Example : Strain gauges & thermistors.

An Electrical Transducers are used mostly to measure non-electrical quantities. A transducer which converts a non-electrical quantity into an analog electrical signal consists of two parts

The sensing element

Responds to a change in a physical phenomenon.

The transduction element.

Transforms the output of a sensing element to an electrical output.

The different electrical phenomena employed in the transduction elements of transducers are as follows:

+ Resistive + Photo-emissive

+ Inductive + Photo-resistive

+ Capacitive + Potentiometric

+ Electromagnetic + Thermo-electric

+ Piezo-electric + Frequency generating

7.1.2 RESISTIVE POSITION TRANSDUCER

Operates under a principle of resistance change by the physical movement under measurement.
The basic operation of this transducer uses resistive element with a sliding contact or wiper linked to the object being measured.

The output voltage of the circuit is given in the figure

The output voltage is proportional to R₂ .
If the resistance of the transducer is distributed uniformly along the length of the wiper movement, the resistance will be linear.

7.1.3 STRAIN GAUGES

Senses the strain produced by a force on the wires.
If a metal conductor is stretched or compressed, its resistance changes since both the length and diameter of the conductor changes.
The resistivity of the conductor when subjected to strain, this is called the piezo-resistive effect. Therefore, resistance strain gauges are also known as piezo resistive gauges.

THE BASIC OF STRAIN GAUGE EXPLAINED

When a gauge is subjected to a positive stress, THE length increaseS while the cross-sectional area decreases. Since the resistance of a conductor is directly proportional to its length and inversely proportional to its area of cross-section, the resistance of the gauge increases with positive strain where:

There are three main types of strain gauge:

1) Wire strain gauges

2) Foil strain gauge

3) Semiconductor strain gauge

1) WIRE STRAIN GAUGES

The measurement of sensitivity of a material to strain is called gauge factor whereas given by:

K = GF = gauge factor

ΔR = changes in initial resistance (Ω)

R = initial resistance

Δl = changes in length

l = initial length (m)

2) FOIL STRAIN GAUGE

It uses a metal foil as the sensing element but has the same materials as the wire strain gauge

3) SEMICONDUCTOR STRAIN GAUGE

Its smaller in size and has a very high gauge factor (50 times wire strain gauge), though it is very sensitive to changes in temperature. Its linearity is poor and is more expensive.

THE UNDERSTANDING OF HOW STRAIN GAUGES WORK

7.1.4 CAPACITIVE TRANSDUCER

Operates by a linear change in capacitance.

The capacitance increases if the effective area of the plate is increased and the material has a high dielectric constant. It is made of a fixed plate called Stator and a movable plate called the Rotor. The capacitance of the transducer is changing as the rotor changes its position relative to the stator.

Another example is capacitive pressure transducer. This sensor is designed to measure pressure in vacuum. A metallic diaphragm will move to the left when pressure is applied to the chamber and to the right when vacuum is applied. The capacitive transducer is simple to construct, inexpensive and effective for HF variations.

7.1.5 INDUCTIVE TRANSDUCER

Inductive transducers are either of the self generating or the passive type.

> The self generating type utilizes the basic electrical generator principle, ie. a motion between a conductor and magnetic field induces a voltage in the conductor. A tachometer directly converts speed or velocity into an electrical signal.

> The passive type inductive transducers utilizes the change in inductance. An inductive electromechanical transducer converts physical motion into a change in inductance.

THE BASIC OF INDUCTIVE TRANSDUCERS EXPLAINED

As the number of turns are changed, the self inductance and the output also changes. A variation of permeability causes the change in self inductance as the iron core inside is winding, its permeability and the inductance is increased.

This is a variable reluctance transducer. It consists of a coil wound on a ferromagnetic core. The displacement is applied to a ferromagnetic target. The core and the target are separated by an air gap. The self-inductance of the coil is inversely proportional to the length of the air gap. When the target is near the core, the length is small thus increases the self-inductance.

7.1.6 LVDT

Linear Variable Differential Transformer is the passive inductive transformer.
A movable soft iron slides within the hollow part of the transformer thus affects the magnetic coupling between the primary and the secondary windings.
The frequency of the ac voltage applied to the primary winding ranges from 50 Hz to 20 kHz.

The output voltage of an LVDT is given by :

E₀ = E_S1 – E_S2

Three possible position of the soft iron core:

-Normal position, E_S1 = E_S2, thus E_O = 0V

-Left position, E_S1 > E_S2, thus E_O = E_S1 - E_S2

-Right position, E_S2 > E_S1, thus E_O = E_S2 – E_S1

The amount of voltage change in either secondary winding is proportional to the amount of movement of the core.

Indication of the amount of linear motion.

By noting which output is increasing or decreasing, the direction of the motion can be determined.

Advantage: Produces higher output voltage for small changes.

AN EXPERIMENTS DESCRIBING THE APPLICATIONS OF LVDT

7.1.7 PIEZOELECTRIC TRANSDUCER

Piezoelectric transducer consists of crystal material such as Quartz, Rochelle salt and Barium titanite which produces an electromotive force when they are placed under stress.

Since it has a high frequency (HF) response, its principal use is in HF accelerometers. When an alternating voltage is applied to a crystal, it will vibrate at its natural resonance frequency.

Advantage: Needs no external power source. Its self generating.

Disadvantage: Can not measure static condition since it is a dynamic responding sensor.

<< THIS VIDEO EXPLAINS THE APPLICATIONS OF PIEZOELECTRIC TRANSDUCER.

THIS IS A DIY VIDEO OF PIEZOELECTRIC TRANSDUCER USING PRODUCTS AT HOME >>

7.1.8 TEMPERATURE TRANSDUCER

Temperature is one of the most widely measured and controlled variable in industry.

Most of the temperature transducers are of Resistance Temperature Detectors (RTD), Thermistors and Thermocouples. RTD and Thermistor are passive devices whose resistance changes with temperature, so they need an electrical supply to give a voltage output. Thermocouples are active transducers and are based on the principle of generation of thermoelectricity.

Tutorials & Solutions for 7.1

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