Transducer Overview

Transducer Overview

a transducer  has following parts:

1. Input device - receiving the quantity which going to be measured

2. Signal conditioning and processing device -Amplification, Attenuation, Filtration

(Modulation/modification into some other acceptable form)

3. an output device - the signal is converted to some other form that may be displayed for its analysis.

the transducer can be defined in theoretical way as "a device which converts one form of energy into some other form"

Apart from mechanical forces some physical parameters are also used for the transducer purpose like: heat, intensity of light, flow rate, liquid level, and humidity and Ph values

SOME EXAMPLES OF TRANSDUCERS

Photoconductor :

a transducer which receives light falling upon it and converts it intocorresponding electrical from.

Thermocouple :

a device which convert heat energy into electrical one i.e. performs function of a transducer.

Piezoelectric crystal :

a crystal which produces electrical signal according to change of pressure imposed upon it which is generally used for timing purpose in computers and watches.

Strain gauge :

uses an metal element whose resistance changes according to change in its dimensions so that current flowing through it get similar modulation thus producing an electrical signal which is analogous to changes carried in it.

ELECTRICAL TRANSDUCER

Electrical measurement techniques have been used for measurement of electrical quantities but now with beginning of the new era of electronics it is also getting used for measurements of non-electrical quantities and booming the industries. Here an electrical signal means current or voltage or a frequency which is produced by some electrical effects like resistance, capacitance, inductance etc.  .

Advantages of electrical transducer

1. The electrical signals can be amplified or attenuated easily with static devices.

2. Mass and inertia effect s are minimized i.e. while dealing with electrical signal we encounter moving electrons which are very small sizes so in their case mass of inertia remains very ineffective on the overall result obtained at output so electronic measurement instrument are inert to mass of inertia.

3. As it involves no moving parts in it processing system so problem of friction is also null.

4. Only a very small power is required for the purpose of controlling.

5. As the output is in form of electrical quantity i.e. current, voltage or frequency so it is always easier to manipulate and process them for the purpose of measurement.

6. there are many fields like radio monitoring and space research and development s are using telemetry for its data transmission, measurement, processing, controlling, monitoring or measurement itself. As in the modern technology the remote sensing, recording and indication has become the indispensable and important part.

7. With use of  IC it is possible to miniaturize the measurement devices to an extent that can never be achieved with use of physical means.

 8. To deal with the modern digital computers it has become very essential to use the electronic means for Using the output from input device Transmit the signal To process them

BASIC PARTS OF TRANSDUCER

1. Sensing element

2. Transduction element 

Where it is considered to perform the function of conversion of  non-electrical quantity into analogous Electrical  signal. In addition to these two basic elements it may also contain some other important parts for making it work and to maintain its accuracy, versatility and reliability which can: amplifier stages, processing parts, power supply, calibrating and reference source and some mechanical mounting features.

Sending  element

This is the part of the transducer which relates it to external conditions i.e. it exhibits change in its response with respect to the physical conditions or physical phenomenon to it is exposed to. Any change in physical condition leads to similar response obtained from sensing element.

Transduction  element

 The response from sensing element it converted to an analogous electrical signal with the help of transduction stage.

TRANSDUCERS ON THE BASIS OF TRANSDUCTION FORM

The principle used for transduction can be used for its classification because it may use resistive, inductive or capacitive property for the purpose. Depending upon these principals some of the example of each one of them is:

Resistive type

 All the transducers falling in this category uses their property of change in resistance due to different external conditions to which they are exposed to.

Potentiometer device:

an external applied force causes the position of a slider to change its position upon the variable resistance wire.

Resistance strain gauge :

the external applied force causes an elongation or compression is the resistor or a semiconductor which leads to variation in its resistance.

Pirani gauge or hot wire meter :

heat transfer through the gas may take place my many means in convection one type which is used in this transduction for cooling of a heating element which definitely leads to corresponding change in its resistance.

Resistance thermometer :

it uses the property of the metals by virtue of which its resistance change with change in its temperature. In this method those metals are used which are having a large positive temperature coefficient which fulfill to purposes one is to allow it to sent a large rage of temperature and second to obtain a maximum possible accuracy because temperature to résistance variation is not perfectly linear but have a little curve nature which can be ignored in the case of metals having large value of temperature coefficient.

Thermistor :

These are transducer which used certain metal oxides which have negative temperature coefficient or resistance i.e. which show decrease in resistance when its temperature is increased.

Resistance  hygrometer :

it is a very useful and innovative type of transducer because it senses content of humidity in environment. It uses a strip which exhibits a change in its resistance according to the moisture content of its surroundings

Photo-conductive cell :

as the name suggests it shows change in its resistance according to the intensity of the incident light.

Capacitive type

 The second form of transduction is capacitive which uses change in capacitance of the materialused in it.

Variable capacitance pressure gauge :

in this type distance between two parallel plates isvaried by application of external force.

Capacitor microphone :

here an arrangement is formed in which there is a fixed plate and a movable diaphragm which move after the incidence of sound waves upon it which caused its capacitance to vary.

Dielectric gauge :

capacitance of a capacitor also depends upon the material placed in between two places (generally a dielectric) so any change in the dielectric causes similar change in capacitance of concerned capacitor.

Inductance type

 In the same ways this is the category which brings inductance in use for sensing external condition.

Magnetic circuit transducer :

like the electric circuit there can also be a magnetic circuit conducting the magnetic field through it and any change in this magnetic circuit makes the self inductance or mutual inductance to vary in the same fashion.

Reluctance transducer :

reluctance in magnetic circuit is analogous to the resistance in an electric circuit. Reluctance of an magnetic circuit depends upon the medium the magnetic flux passes through thus any change in the medium will lead to a corresponding change in the reluctance in it. Iron core in inductors being the medium of passage of magnetic flux in it affect its reluctance if is position is changes i.e. any change in position of the core leads to corresponding change in output obtained from it.

Differential transformer :

in these types a transformer contains a single primary winding whiletwo secondary winding generating their own voltages. The external applied force is subjected tothe magnetic core which causes change in its positioning which ultimately leads to deference invalues of voltages generated by them i.e. a deferential voltage is generated from the twosecondary windings on the account of the positioning of the magnetic core.

Eddy current gauge :

eddy current is the current induced in the secondary winding on theaccount of the variation in the magnetic field in magnetic core due to primary winding. Andinductance of a coil is changes by the proximity of the eddy current palates.

Magnetostriction gauge :

there are certain materials which shows change their magnetic property when any change is taken to its dimensions which may be due to external pressure or anmechanical stress.

CLASSIFICATION OF TRANSDUCERS

1. Primary and secondary transducer 

2. Passive and active transducer 

3. Analogue and digital transducer

4. Transducer and inverse transducer 

5. And on the basis of transduction form used

Primary and secondary transducers

Generally in most of the transducers the whole assembly is constructed of two parts

1. Mechanical device and

2. Electrical device

Where mechanical device is generally exposed to the external physical condition and converts it into corresponding physical signal which is further fed to the next section i.e. electrical device that performs the function of conversion of this physical signal into corresponding electrical signal. In this way the first section viz. Mechanical device acts as primary transducer while the second one viz. electrical device acts as a Secondary transducer. 

It can be understood with the help of an example of a load cell which is an arrangement for measurement of applied force. It is a small column with the resistance wire placed in it such that the change in the width of the column leads to development of a stain in the resistance wire and its resistance gets changed which is further detected and measured by some means. Here, as soon as a force is applied to the column it causes a mechanical displacement in the resistance wire gauge and develops a strain this constructs the mechanical device or a primary transducer the signal obtained from this section is a kind of mechanical signal.  Now the strain or change in length is converted into change in resistance of the wire this constructs the secondary transducer which converts mechanical change into change in resistance i.e. performs the electrical part. 

Another example can be taken of a Bourdon's tube used for pressure measurement it consists of  pressure tube and a linear variable differential transformer (LVDT). The pressure, generally exerted by some gas, is applied to the tube which is generally fixed with a movable arc which shows some deflection according to the pressure applied, this deflection causes displacement in the position of the core of LVDT, where LVDT is differential transformer whose output voltage from its two secondary winding depends upon the position of its core. Thus change in pressure in tube is converted into corresponding displacement in core of LVDT and this displacement is further converted into a corresponding change in reluctance. In this way the tube can be called primary transducer and LVDT can be called as secondary transducer. In both the case a mechanical signal is involved in between both the stages which serve as intermediate signal.

Passive and active transducer

For production of its output every transducer requires some power which it can take from physical mean upon it is working or from some external source. Thus depending upon source of  power used by transducer it can be classified into two categories i.e. passive transducer andactive transducer.

Passive transducer

 These does not generates the power from physical mean that is why they depends upon someauxiliary power source for producing its output. But it some cases they may drive some part of their power from physical quantity. This is the reason that they are also known as"externally powered transducers" generally all the resistive, capacitive and inductive transducers are passive transducer.

POT can be taken as a best example to study a passive transducer it measures the displacement for which requires a potentiometer as it is resistive type transducer and change in positioning of the slier over the potentiometer wire determines the displacement for which it totally depends upon an external power source, as long as it is powered by some external agency it produces some output otherwise it cannot produces nay output.

Active transducer

 Unlike passive transducers they drive their power for transduction from the physical quantity getting measured and generate own current and voltage. The common examples of this type are:

tachogenerators, thermocouple, photovoltaic cell and piezoelectric crystal. To understand the basic function of an active transducer let us take an example of piezoelectric crystal used in accelerometer it measures the acceleration of the body upon which it is placed. Here a piezoelectric crustal is places in between two metallic plates and some fixed weight isalso place on the top of upper metal plat while the lower plate is fixed to some base. The crystal has a property that is produces it own voltage when some force is applied upon it whose magnitude depends upon quantity of force applied. When base is subjected to acceleration, as the weight upon the upper plate is fixed, so a corresponding force will be applied by the weight upon crystal and an equivalent voltage will be generated by crystal which can be, further, used for displaying the output. In this way the whole system generates its own power and does not depends upon any auxiliary power source.

Analogue and digital transducer

The output generated by transducer can also be used to classify them i.e. whether the output is acontinuous function of time, like in analogue signals, or a discrete type like digital signal.

Analogue transducer

 As the name suggests these receives the input and produces output in the form of analogue signalwhich is a continuous function of time. Some very common examples of this category are:

strain gauge, LVDAT (linear variable differential transformer), thermocouple, Thermistor

Digital transducer

 This category of transducer contains those transducers which generate output in form of pulses which are not continuous function of time but discrete in nature. As it is easier to process and transmit the data when it is function of only two variables i.e. 0 & 1for example in case of an glass plate it can have two states opaque and transparent; and in case of conducting material the two state are easily possible viz. conducting and non-conducting.