Transducers Saneeju M Salu

Transducer Any device that converts energy in one form to energy in another from Device which converts one physical quantity or condition to another Physical quantity – heat , intensity of light, flow rate, liquid level, humidity etc Transducer Physical Quantity Electrical Quantity (Current/ Voltage)

Classification of transducers Based on principle of transduction Active & passive Analog & digital Based on Electrical Principle

Based on principle used Mechanical – strain guage, Thermal – thermistors,thermocouples Magnetic – LVDT Optical – photoconductive, solar Acoustical - microphone Chemical - pH Nuclear – ionization chamber Biological – ECG, EEG

Passive & Active transducer Device which derive power reqd. for transduction from auxiliary power source - externally powered Eg : resistive, inductive, capacitive Without power they will not work Active They do not require external power source Eg: solar cell

Analog & Digital transducer convert I/p quantity into an analog o/p Analog o/p- a continuous fn. Of time Eg. Strain gauge, L VDT, thermocouple Digital Converts I/p into an electrical o/p in the form of pulses

Based on Electrical Principle Variable – resistance type Strain & pressure gauge 2) thermistors 3. Photoconductive cell 4) chemical conductive meter b) Variable – inductance type Linear Variable Differential Transformer (LVDT) Eddy current gauge c) Variable – capacitance type Capacitor microphone 2) dielectric gauge 3)pressure gauge d) Voltage – divider type Potentiometer position sensor 2) pressure actuated voltage divider e) Voltage generating type 1) Solar cell 2) Thermocouple

Tachometer A tachometer is a sensor device for measuring the rotation speed of an object such as the engine shaft in a car. This device indicates the revolutions per minute (RPM) performed by the object.

Electromagnetic Flowmeter The principle of operation for the magnetic flowmeter is based on the Faraday’s Law of Electromagnetic Induction.

Flowmeter When a conductive liquid flows through the magnetic field, a small voltage (u) is induced. This voltage is proportional to the velocity of the flow and is accurately measured by two stainless steel electrodes mounted opposite each other inside the metering pipe

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER(LVDT) AN LVDT transducer comprises a coil former on to which three coils are wound. The primary coil is excited with an AC current, the secondary coils are wound such that when a ferrite core is in the central linear position, an equal voltage is induced in to each coil. The secondary are connected in opposite so that in the central position the outputs of the secondary cancels each other out.

LVDT contd… The excitation is applied to the primary winding and the armature assists the induction of current in to secondary coils. When the core is exactly at the center of the coil then the flux linked to both the secondary winding will be equal. Due to equal flux linkage the secondary induced voltages (eo1 & eo2) are equal but they have opposite polarities. Output voltage eo is therefore zero. This position is called “null position”

Now if the core is displaced from its null position toward sec1 then flux linked to sec1 increases and flux linked to sec2 decreases. Therefore eo1 > eo2 and the output voltage of LVDT eo will be positive Similarly if the core is displaced toward sec2 then the eo2 > eo1 and the output voltage of LVDT eo will be negative.

LVDT Advantages Applications It gives higher o/p voltage for small changes in core position Temp range-265oC to 600oC It is available in radiation resistant design for operation in nuclear reactors Applications In accelerometers Jet engine control Measurement of roll position Measurement of material thickness in hot strip

Piezoelectric Transducer In piezoelectric induction the measure and is converted into a change in electrostatic charge q or voltage V generated by crystals when mechanically it is stressed as shown in fig

Strain Gauge A strain gauge (or strain gage) is a device used to measure strain on an object

Applications It is rugged, easy to use In ac applications, it is capable ofmeasuring variations upto 100KHz It has a excellent linearity It is less sensitive to shock and undesired vibrations It is available in multielement

Light dependent resistor (LDR)

Photojunction devices phototransistor photodiode

Photovoltaic Solar Cells Can convert about 20% of light power into electricity Voltage is low (diode drop, ~0.6V)

Resistance Temperature Detectors (RTDs)

Thermistor A thermistor is a type of resistor whose resistance is dependent on temperature, more so than in standard resistors thermistor is generally a ceramic or polymer

the relationship between resistance and temperature is linear R=KT R- Change in resistance T- Change in temperature K- temperature coefficient of resistance Thermistors can be classified into two types, If  is positive, the resistance increases with increasing temperature, and the device is called apositive temperature coefficient (PTC) thermistor, or posistor. If  is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor

Attitude and wind velocity Advantages It is chemically stable Used in nuclear environment Arranged in series-parallel arrangement for increased power handling capacity Used for measuring temp even at all places inaccessible to other thermometers Applications Measuring temp Thermal conductivity Attitude and wind velocity

Thermocouple A thermocouple is a device consisting of two dissimilar conductors or semiconductors that contact each other at one or more points. A thermocouple produces a voltage when the temperature of one of the contact points differs from the temperature of another, in a process known as the thermoelectric effect. Thermocouples are a widely used type of temperature sensor for measurement and control

Types Type E: (chromel – constantan) has a high output (68 µV/°C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic. Wide range is −50 °C to +740 °C and Narrow range is −110 °C to +140 °C. Type J : (iron – constantan) has a more restricted range (−40 °C to +750 °C) than type K, but higher sensitivity of about 50 µV/°C. TheCurie point of the iron (770 °C) causes a smooth change in the characteristic, which determines the upper temperature limit. Type K : (chromel – alumel) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C. It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C / -330 °F to +2460 °F range Type M: (Ni/Mo 82%/18% – Ni/Co 99.2%/0.8%, by weight) are used in vacuum furnaces for the same reasons as with type C (described below). Upper temperature is limited to 1400 °C. It is less commonly used than other types. Type N: (Nicrosil – Nisil) thermocouples are suitable for use between −270 °C and +1300 °C owing to its stability and oxidation resistance. Sensitivity is about 39 µV/°C at 900 °C, slightly lower compared to type K. Type T (copper – constantan) : thermocouples are suited for measurements in the −200 to 350 °C range Chromel – gold/iron alloy thermocouples Tungsten/rhenium alloy thermocouples Platinum/rhodium alloy thermocouples

Microphone A microphone is a type of transducer. Early microphones were invented for communication purposes. Later modifications were made to design as the microphone was used more in entertainment industry.

Overview of a Microphone Diaphragm—part of microphone which receives the vibration from sound waves. Thickness and material of diaphragm are changed depending on the sound waves you wish to pick up. How it works? Electrical circuit is used to change these detected vibrations into an electrical signal that “images” the sound with an output voltage or current.

Characteristics of Microphone  

Requisites of a good microphone High sensitivity High SNR Flat frequency response over most of the audible frequency range Very low distortion It should have correct o/p impedance to match the with the line impedance The directivity of the microphone should be such as to meet the requirement of application

Different Types of Microphones Carbon Ribbon Moving Coil Crystal Ceramic Capacitor Electret

Carbon Microphone—Historical Design Thomas Alva Edison and Sir Emile Berliner filled patents for Carbon Button Microphone in 1877. Edison's design became the accepted design and patent. This is a popular microphone for telephones as well as early radio and speeches.

Carbon microphone—Design Diagram

Ribbon Microphone Diaphragm: Thin piece of metal foil suspended in a magnetic field. Vibrations in ribbon produce a small voltage which is then stepped up by a transformer. Diaphragm is very easily damaged by wind or loud incoming sounds.

Ribbon Microphone— Design Diagram

Moving Coil Microphone

Crystal Microphone

Ceramic Microphone

Capacitor Microphone

The Electret Microphone It is similar to capacitor microphone that it does not require A polarization voltage for its capacitor

Loudspeaker It is a transducer which convert electrical signal to sound wave

Loudspeaker

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