Unit-2- Transducers

Introduction to Transducers - Classification - Primary - Secondary and Tertiary - Mechanical - Bellows - Bourdon’s Tube - Springs - Proving Rings - Diaphragm - Monometer - Bimetals - Electrical- Resistance - Inductance and Capacitance - Strain Gauges and its Orientation for Measurement - Vibration and Acceleration Measurement - Advantages and Limitation. Measurement of Force – Torque -

Power - Temperature and Flow.

TRANSDUCER

• "A transducer is a device which converts the energy from one form to another", Most of the transducers either convert electrical energy into mechanical displacement and for convert some non-electrical physical quantity (e.g.. force, sound, temperature etc.) to an electrical signal.
• A transducer performs the following functions in an electronic instrumentation system:
• Detects or senses the presence, magnitude and changes in physical quantity being measured.
• Provides a proportional electrical output signal.
• A transducer can be broadly defined as a device which converts an non-electrical quantity into an electrical quantity.

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CLASSIFICATION OF TRANSDUCERS

Active transducers. They are also known as self-generating type transducers. These transducers develop their own voltage or current. The energy required for production of an output signal is obtained from the physical phenomenon being measured.

Examples: Thermocouples and thermopiles, piezoelectric pick-up, photovoltaic cell.

Passive transducers. They are known as externally-powered transducer. These transducers derive the power required for the energy conversion from an external power source. However, they may absorb some energy from the physical phenomenon under study.

Examples: Resistance thermometers and thermistors, potentiometric devices, differential transformer photoemission cell etc.

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CLASSIFICATION BASED ON THE TYPE OF OUTPUT:�

Analogue transducers: These transducers convert the input physical phenomenon into an analogous output which is a continuous function of time.

Examples. Strain gauge, a thermocouple, a thermistor or an LVDT (linear voltage differential transformer)

Digital transducers: These transducers convert the input physical phenomenon into an electrical output which may be in form of pulse.

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Classification based on electrical principle involved

• Variable-resistance type:

(i) Strain and pressure gauges

(ii) Thermistors, resistance thermometers

(iii) Photoconductive cell etc.

• Variable-inductance type:

(i) Linear variable transformer (LVDT)

(ii) Reluctance pick-up

(iii) Eddy current gauge.

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• Variable-capacitance type:

(i) Capacitor microphone

(ii) Pressure gauge

(iii) Dielectric gauge.

• Voltage-generating type:

(i) Thermocouple

(ii) Photovoltaic cell

(iii) Rotational motion tachometer

(iv) Piezoelectric pick-up.

• Voltage-divider type:

(i) Potentiometer position censor

(ii) Pressure-actuated voltage divider.

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BELLOWS PRESSURE GUAGE

• The bellows is a sealed, collapsible chamber that expands and contracts in response to changes in pressure. It's usually made of a flexible material that can withstand the pressure of the fluid being measured.
• Connected to the bellows is a linkage mechanism that translates the movement of the bellows into rotary or linear motion.
• The rotary or linear motion from the linkage mechanism is then used to move an indicator on a calibrated scale. This indicator points to a pressure value on the scale, allowing the user to read the pressure directly.

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Advantages

• Moderate cost.
• Suitable for absolute pressure as well as gauge pressure.
• It can be used for measuring low and moderate pressure.

Disadvantages

• It requires ambient temperature compensation.
• It is not suitable for high pressure measurement.

Selection of Material

• Strength,
• Pressure range.
• Hysteresis,
• Corrosiveness due to environmental impacts, and
• Ease of fabrication etc.

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Bourdon Pressure Gauge

• When the fluid (either liquid or air or gas), whose pressure is to be measured, is made to flow through bourdon tube, its original elliptical cross section tends to become circular.
• This makes the tube straighten itself out with an increase in radius of curvature. This causes the free end of the tube to move/displace.
• This displacement of free end of tube creates angular movement in rack and pinion through mechanical linkage.
• The pointer, which is pivoted on the pinion, moves over a calibrated scale, which directly indicates pressure in terms of N/rn2 or 'm' head of mercury.

Bourdon Gauge- Other Types

Advantages

• Low cost.
• Simple construction.
• Available in wide variety of range
• High accuracy as compared with cost

Disadvantages

• Low spring gradient.
• Highly sensitive to shock, vibration and Hysteresis

Factors for Material selection

• Elastic characteristics suitable for pressure measurement.
• Range of measurement.
• Pressure of medium.
• Temperature of medium.
• Corrosive resistance of medium

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Proving Ring

• Proving ring is a circular ring of rectangular section. It may be carrying tensile force or compressive force. The proving ring is made up of steel, which is used for calibration of material testing machines.
• The deflection of the ring is measured with the help of micrometer. One end of the micrometer is connected to vibrating read for getting vibratory motion. The micrometer wheel is moved forward, until vibration is observed.

DIAPHRAGM-TYPE PRESSURE GAUGE

• To amplify the motion that a diaphragm capsule produces, several capsules are connected end to end.
• Diaphragm type pressure gauges are used to measure gauge, absolute, or differential pressure.
• They are normally used to measure low pressures of 25 mm of Hg, but they can also be manufactured to measure higher pressures in the range of 0 to 7 kPa.
• They can also be built for use in vacuum services.

MANOMETER

• A manometer is a device used for measuring the pressure of a fluid (liquid or gas) within a closed system.
• It consists of a U-shaped tube partially filled with a liquid, usually mercury or water, and is used to compare the pressure of the fluid being measured with the pressure of the surrounding atmosphere or another reference point.
• The principle behind a manometer is based on the fact that the pressure exerted by a fluid at a particular point is transmitted equally in all directions.
• Creating a height difference between the two columns of liquid. The height difference is proportional to the pressure difference between the fluid being measured and the reference point.

Advantages

• Direct Measurement
• Simple and Inexpensive
• Real-Time Monitoring
• Applicable to Various Fluids
• No External Power Required

Limitations

• Limited Measurement Range
• Reading Errors
• Bulkiness
• Temperature Sensitivity
• Slow Response Time

Bimetallic strip Thermometer

• Two pieces of metal with different co-efficients of thermal expansions are bonded together to form a bimetallic strip. Metal B would be chosen to expand faster than metal A
• This causes the strip to bend upward, touching the contact so that current can flow.
• Bimetallic strip is heated to the temperature higher than the bonding temperature, it will start to bend in one direction. If it is heated to a temperature lower than the bonding temperature, it will bend in the other direction.

• The bimetallic thermometer uses the bimetallic strip which converts the temperature into the mechanical displacement.
• The working of the bimetallic strip depends on the thermal expansion property of the metal.
• The Seebeck effect, also known as the thermoelectric effect, is a phenomenon in which a temperature difference between two different metals or semiconductors in a closed circuit generates a voltage difference. This voltage difference leads to the flow of electric current when the circuit is closed.

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Advantages

• The thermometer is simple in construction, robust and less expensive.

Disadvantages

• The thermometer gives the less accurate result while measuring the low temperature.

Applications of Bimetallic Thermometer

• The bimetallic thermometer is used in household devices likes oven, air conditioner, and in industrial apparatus like refineries, hot wires, heater, tempering tanks etc. for measuring the temperature.

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Electrical Transducer

An electrical transducer is a sensing device by which the physical, mechanical or optical quantity to be measured is transformed directly by a suitable mechanism into an electrical voltage/current proportional to the input measured.

Advantages of Electrical Transducers

• Electrical amplification and attenuation can be easily done.
• Mass-inertia effects are minimized.
• Effects of friction are minimized.
• The output can be indicated and recorded remotely at a distance from the sensing medium.
• The output can be modified to meet the requirements of the indicating or controlling unit

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Resistive Transducers

• A resistive transducer, also known as a resistive sensor, is a type of sensor or transducer that converts a physical quantity (such as pressure, temperature, or strain) into a change in resistance. This change in resistance can then be measured and correlated to the value of the physical quantity being sensed
• Potentiometer
• Strain Guage
• Thermistor
• Pressure sensor

• A resistive transducer is an electronic device that is capable of measuring various physical quantities like temperature, pressure, vibration, force, etc.
• These physical quantities are otherwise extremely difficult to measure as they can change easily.
• However, using this transducer, you can easily calculate the values of these quantities.
• The resistance of this transducer changes concerning the change in the physical quantities.

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POTENTIOMETER�

• Potentiometer consist of a resistance element with a sliding contact which can be moved over a length of the element.
• Element can be used for linear or rotary displacement, displacement is converted into potential difference

Temperature Detection Sensors - Resistance Temperature Detector (RTD)�

• A Resistance Temperature Detector (also known as a Resistance Thermometer or RTD) is an electronic device used to determine the temperature by measuring the resistance of an electrical wire.
• it has good linear characteristics over a wide range of temperatures
• In RTD devices; Copper, Nickel and Platinum are widely used metals. These three metals are having different resistance variations with respective to the temperature variations. That is called resistance-temperature characteristics.

Inductive Transducer

• A transducer that works on the principle of electromagnetic induction or transduction mechanism is called an inductive transducer.
• A self-inductance or mutual inductance is varied to measure required physical quantities like displacement (rotary or linear), force, pressure, velocity, torque, acceleration, etc. These physical quantities are noted as measurands. 

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Linear Variable Differential Transformer (LVDT)

• The LVDT is a position-to-electrical sensor whose output is proportional to the position of a movable magnetic core.
• The core moves linearly inside a transformer consisting of a center primary coil and two outer secondary coils wound on a cylindrical form.

CAPACITIVE TRANSDUCER

• A capacitor is formed by two plates, which can store an electric charge. The stored charge generates a potential difference between the plates and may be maintained using an external voltage.

STRAIN GAUGE

• Strain gauge is a metal wire foil or semiconductor strip which is wafer like and can be struck onto the surfaces like a postage stamp.
• When subjected to strain by an applied load, its resistance changes, fractional change in resistance is proportional to the load applied.

Accelerometer

• In a piezoelectric accelerometer, a piezoelectric material (usually a crystal or ceramic) generates an electric charge when subjected to mechanical stress or acceleration.
• The device consists of a seismic mass attached to the piezoelectric material. When the accelerometer experiences acceleration, the mass compresses or stretches the piezoelectric material, generating an electric charge proportional to the applied force.
• This charge can then be measured and converted into an acceleration value.

• An accelerometer is a device that measures acceleration forces acting on an object. It is commonly used in various applications such as
• smartphones,
• vehicles,
• aircraft,
• industrial equipment
• to detect changes in motion, tilt, vibration, and other forms of acceleration.

Thermocouple

• The thermocouple working principle is based on the Seeback Effect. This effect states that when a closed circuit is formed by jointing two dissimilar metals at two junctions, and junctions are maintained at different temperatures then an electromotive force (e.m.f.) is induced in this closed circuit.

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Thermistor

Flow Sensors