Transducers

TRANSDUCERS

TRANSDUCERSA transducer is a device that convert one form of energy to other. It converts the measurand to a usable electrical signal. In other word it is a device that is capable of converting the physical quantity into a proportional electrical quantity such as voltage or current.The physical quantity

may be temperature, pressure etc.

The electrical signal is then used for either displaying or controlling that physical quantity.

Transducer contains two parts that are closely related to each other i.e. the sensing element and transduction element. The sensing element is called as the sensor. It is device producing measurable response to change in physical

conditions. The transduction element convert the sensor output to suitable electrical form.

CLASSIFICATION OF TRANSDUCERS

ACTIVE AND PASSIVE TRANSDUCERS

Active transducers:

These transducers do not need any external source of power for their

operation. Therefore they are also called as self generating type transducers.

The active transducer are self generating devices which operate under the energy conversion principle. As the output of active transducers we get an equivalent electrical output signal e.g. temperature or strain to electric potential, without any external source of energy being used.

CLASSIFICATION OF TRANSDUCERS

ACTIVE AND PASSIVE TRANSDUCERS

Passive transducers:

These transducers need external source of power for their operation. So they are not self generating type transducers.

A DC power supply or an audio frequency generator is used as an external power source. These transducers produce the

output signal in the form of variation in resistance, capacitance, inductance or some other electrical parameter in

response to the quantity to be measured.

ANALOG AND DIGITAL TRANSDUCERSAnalog transducer: The output of these

transducers is in the analog form that means it is a continues function of time. e.g. LVDT,

thermocouple etc.Digital transducer: The output of these

transducers is in the digital form that means discrete in time. e.g.PRIMARY AND SECONDARY TRANSDUCERS

Primary Transducer: There are transducers which converts the physical quantity to be measured into a mechanical signal known as primary

transducer. Secondary Transducer: There are transducers which convert the mechanical signal into a corresponding electrical signal known as

secondary transducers

CLASSIFICATION OF TRANSDUCERS

Capacitive transduction: In these transducers ,the measurand is converted to a change in the capacitance.Electromagnetic transduction: In these transducers, the measurand is converted to a voltage induced in a conductor by change in the magnetic flux.Inductive induction: In it, the measurand is converted into a change in the self inductance of a single coil.Piezoelectric induction: In it, the measurand is converted into a change in electrostatic charge or voltage v generated by crystals when mechanically stressed.Photovoltaic transduction: In it, the measurand is converted into a voltage generated when the junction between two metals is illuminated.Photoconductive transduction: In it, the measurand is converted into a change in the resistance of semiconductor material by the change in the light incident on the material.

CLASSIFICATION BASED ON TRANSDUCTION PRINCIPLE

CLASSIFICATION OF TRANSDUCERS

LVDTLinear Variable Differential Transducer

LVDT is an inductive transducer, which converts the physical motion into change in inductance. It is a passive differential inductive transducer.

Cutaway view of an LVDT. Current is driven through the primary coil at A, causing an induction current to be generated through the secondary coils at B.

LVDTCONSTRUCTION

The LVDT has three solenoidal coils placed end-to-end around a tube. The centre coil is the

primary, and the other two coils are the top and bottom secondaries. A cylindrical core, attached to the object whose position is to be measured,

slides along the axis of the tube.

CONSTRUCTION OF LVDT

An alternating current drives the primary.It causes a voltage to be induced in each secondary proportional to the length of the core linking to the

secondary. The two secondary windings are connected in series opposition. Hence voltage induced into these windings are of

opposite polarities.The core provides a low reluctance path for magnetic flux

linking the coils.

LVDTCONSTRUCTION

LVDTWORKING

Assume core is exactly at the centre. Then flux linked to both the secondaries will be equal, but with opposite

polarities. The output voltage of LVDT is therefore zero. This position is called as the null position.

If the core is displaced from its null position towards secondary-1, then flux linked to secondary-1 increases and flux linked to secondary-2 decreases. Therefore induced

voltage is negative.

Similarly, if core is displaced towards secondary-2, then output voltage will be positive.

Thus the magnitude of output signal is made to vary linearly with the mechanical displacement.

THERMOCOUPLE

THERMOCOUPLE

Thermocouple is an active type temperature sensitive device

which operates under the energy conversion principle.

The operation of thermocouples is based on a phenomenon known as seeback effect. It states

that a current flows in a closed circuit made of two dissimilar metals if the junctions of the two metals are kept at different

temperatures. Due to this current flow, an EMF

proportional to the temperature difference is produced.

THERMOCOUPLE

BASIC THERMOCOUPLE CIRCUITIn measurement, one of the two junctions can be

maintained at a known temperature (reference or cold junction) while other junction is called as the hot or sensing junction. When temperature difference

exists between the sensing junction and the reference junction, and EMF us produced causing current in the circuit. The millivolt meter is used to measure the

generated EMF.

THERMOCOUPLE

EMF vs. TEMPERATURE CHARACTERSTICS FOR THERMOCOUPLE ALLOYS.LIMITATIONS:

The temperature characteristics are slightly non linear.Thermally generated EMF is small, so amplification is

required.

APPLICATIONS:In measurement of surface temperature.

For the applications which need wide temperature range (upto 1100OC).

STRAIN GAUGEStrain gauge is a passive resistance transducer

which converts mechanical elongation and compression into a resistance change. This change in resistance takes place due to variation in length and cross sectional area of the gauge wire, when an external force acts on it.

The working of strain gauge is based on the fact that when stress is applied on the metal

conductor, its resistance changes due to change in length and cross sectional area of the conductor. Resistance of conductor is also changed due to change in resistivity of the conductor. This property is called the piezo-

resistive effect.

STRAIN GAUGEWIRE STRAIN GAUGEIn bonded wire strain gauge

fine resistance wire of diameter of 25 microns or less is wound back and forth in a grid with as many loops as possible, laid side by side. The grid is cemented to the

base which may be a thin sheet of paper or bakelite or a

sheet of teflon. Two connecting leads are

soldered or welded to the ends of the grid. The strain gauge

is fixed with a special adhesive to the structure whose strain is to be

measured.

STRAIN GAUGE METAL FOIL STRAIN GAUGE

The bonded metal foil strain gauges are formed by rolling out a foil of resistive material and then cutting

away parts of the foil by photoetching process to create the required grid pattern. The metal foil strain gauges use similar

materials to wire strain gauges. The metals and alloys used for foils are

constantan (Ni and Cu), nickel, platinum.

The bonded metal foil gauges have a greater heat dissipation capacity in comparison to the wire wound gauges. This is because they have a large surface area for the same volume.

TYPICAL FOIL STRAIN GAUGE

STRAIN GAUGESEMI CONDUCTOR STRAIN GAUGE

Semiconductor gauges are used in applications where a high gauge factor is desired. A high gauge factor means relatively higher change in resistance that can be measured with good accuracy. The resistance of semiconductor gauges changes as strain is

applied to it. The semiconductor gauges depend for their action upon the piezo-resistive effect i.e. change in value of

resistance due to change in resistivity.Silicon and germanium are used as resistive materials for

semiconductor gauges.These gauges are produced by using semiconductor wafer having a thickness of 0.05 mm and bonding them on desired insulating

substrates like Teflon. Gold leads are used for making contacts.

PIEZO ELECTRIC TRANSDUCER

PIEZOELECTRIC DISK GENERATES A VOLTAGE WHEN DEFORMED

It is an active transducer. Some materials possess a property called piezoelectricity. These materials will generate an

electrostatic charge or voltage when mechanical stresses are applied across them. Materials possessing this property are

called as piezoelectric materials. For example, quartz, different ceramic materials. Natural quartz is the most suitable device as it has a higher resistivity and less sensitive to temperature and

moreover exhibits good linearity over a wide range of stress

level.

PIEZO ELECTRIC TRANSDUCERAdvantages:

They do not require external source.Their frequency response is good.Small in size and light in weight.

Disadvantages:The output is affected by temp.

variations.Problem of leakage of charge.

Applications:For the measurement of force and

pressure.Used in high frequency accelerometers.

In underwater detection system.

Also known as optoelectronic transducers which comes under the category Passive transducers . They usually works when a material is illuminated. They are classified as:1. Photo emissive2. Photoconductive3. Photovoltaic

PHOTO ELECTRIC TRANSDUCER

Photo emissive transducers: These operate on the principle that radiation falling on a cathode causes electrons to be emitted from the cathode surface. The commonly used material is Caesium antimony.

These are also called phototubes. Phototubes are of 3 types:Vacuum

Gas filledPhoto Multiplier

PHOTO ELECTRIC TRANSDUCERPhotoconductive Transducers: In it, the measurand

is converted into a change in the resistance of semiconductor material by the change in the light incident on the material. These are also known as photo cells. When it is kept in dark, then its resistance is called dark resistance. It is very high. It can be photo diode . The photoconductive

material used is cadmium sulphide, cadmium selenite or cadmium sulphonide.

They are also called as LDR(Light dependent Resistor) as the incident light varies the

resistance)Another example of it is phototransistor.

PHOTO ELECTRIC TRANSDUCER

Photovoltaic Transducer: In it the measurand is converted into a voltage generated when the junction between two metals is illuminated. It is also called as Solar cell. Materials used for it are Silicon and Selenium. It converts solar energy into radiant power. Gold doped germanium cells work as photovoltaic devices in the infrared region. It is used in spacecraft, applications like data processing multiple photovoltaic devices.