Different types of transducer and their application in Engineering

Transducer over view

Technically a transducer

be thought to an electronic instrument built up of a number of components capable of performing some kind of measurement and recording the result. And device which is capable of making some measurement should have following parts which can be listed below as:
1. Input device
2. Signal conditioning and processing device
3. And an output device

In these three parts the input devices performs the function of receiving the quantity which going to be measured, called measurand, and converts it into some other analogous form generally electrical from and hands it over to the second stage i.e. signal processing section where some operations are performed upon this input signal like:

• Amplification
• Attenuation
• Filtration
• Modulation/modification into some other acceptable form

Thus, obtained signal is then fed to the output section where the signal is again converted to some other form that may be displayed for its analysis.
The quantity to be measured is generally non-electrical in nature which cannot be manipulated of analysed by electrical instrument so for their analysis they have to be converted into some electrical form thus the device used for this purpose is called transducer in this way the transducer can be defined in theoretical way as" a device which converts one form of energy into some other form".

By the industrial view transducer which capable of converting mechanical force into equivalent electrical from construct a larger group and used primarily at a larger scale. Therefore, this class of transducer becomes a greater concern of instrumentation engineers and technologists. Apart from mechanical forces some physical parameters are also used for the transducer purpose like: heat, intensity of light, flow rate, liquid level, humidity and Ph values

Some examples of transducers

Photoconductor : a transducer which receives light falling upon it and converts it into corresponding 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. The working steps of such transducer can be listed as:
• Detector is taken under used in these transducer which extracts the physical quantity and convents it into a displacement

• This displacement is used to actuate an electrical transducer which fives the output in electrical form.

• This electrical signal is further fed to next section in which some standard method of electrical measurement is used.

• The result gives the magnitude of physical quantity being measured.

Here an electrical signal means current or voltage or a frequency which is produced by some electrical effects like resistance, capacitance, inductance etc.

As is discussed above a transducer can be defined to be a device that converts one from of energy into another form but here it is to be mentioned that there may be some restrictions from this definition in some places as it is not a universal definition. So, in order to a have proper definition of transducer we can define it as device which concerts a physical quantity or a physical condition into an electrical signal and in this way a transducer is also called as pick up

Advantages of electrical transducer

By the time being there are a number of transducers which transform a variety of physical quantity or condition into electrical form whose advantages are:

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. With the increasing trend of technologies there are many fields like radio monitoring and space research and development s are now-a-days using telemetry for its data transmission and measurement systems and because of their advancement and requirement it is not possible and practical for them to acquire physical means for fulfilling their purpose so they are left with the only alternative to use electronic system be it 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. Another very important and unique feature of the electronic systems is their ability of occupying a very small size i.e. their integration which leads the devices to be miniaturized keeping the quality of operation intact. 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 as any physical measurement system cannot withstand with them.

Basic parts of transducer

When it is thought to be a device that may be named as detector transducer stage then it comprises of two basic element operational in it i.e.
1. Sensing element
2. Transduction element

Where it is considered to perform the function of conversion of non-electrical quantity into analogous

. 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.

Classification of transducers

When it comes to the classification of transducers there are five bases upon which transducers can be classified in two various categories these can be listed as:
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

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 resistance 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 hygeometer: 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.

• Transmit the signal
• To process them as any physical measurement system cannot withstand with them.

Resistance thermometer 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 material used in it. Variable capacitance pressure gauge : in this type distance between two parallel plates is varied 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 affects 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 while two secondary winding generating their own voltages. The external applied force is subjected to the magnetic core which causes change in its positioning which ultimately leads to deference in values of voltages generated by them i.e. a deferential voltage is generated from the two secondary 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 the account of the variation in the magnetic field in magnetic core due to primary winding. And inductance 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 an mechanical stress.

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 and active transducer. Passive transducer These does not generates the power from physical mean that is why they depends upon some auxiliary 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 is also 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 a continuous 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 signal which 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 & 1 for 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. Considering an moving object leaving behind a mark having only two possibilities of appearance then a binary number can be generated for it position at any point by scanning the pattern across the scale.

Transducer and inverse transducer

A transducer has been discussed above as a device which converts a non-electrical quantity into electrical quantity. On the other hand an inverse transducer can be defined as a device which converts n electrical quantity into some non-electrical quantity i.e. just reverse of a transducer. It takes some electrical input and produces a weak non-electrical output the best suitable example of a piezoelectric crystal. It exhibits the analogous property that its dimensions are changes on application of external voltage upon it that is it taken under use in low power miniaturised speakers embedded with piezoelectric crystal, when electric audio signal is applied it produces same change in its dimensions causing production of sound wave upon its surface. Thus it can be used for production of sound from audio signals e.g. piezoelectric buzzer. From the point of view of industrial used there are a large number of devices that are to be controlled in an efficient way to ensure their satisfactory working. Controlling is possible only in the case when the present state of the system or device is known which is done with help of some or a group of measurement systems. In this way both the type of conversion are used for satisfactory working of the industrial systems.