#8 TRANSDUCERS

      INTRODUCTION OF TRANSDUCERS

 A transducer is a device that convert one form of energy to other form. 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.

BLOCK DIAGRAM OF TRANSDUCERS

• 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.

CHARACTERISTICS OF TRANSDUCERS

1. Ruggedness 

2. Linearity 

3. Repeatability 

4. Accuracy 

5. High stabilityand reliability 

6. Speed of response 

7. Sensitivity 

8. Small size

TRANSDUCERS SELECTION FACTORS

1. Operating Principle: The transducer are many times selected on the basis of operating principle used by them. The operating principle used may be resistive, inductive, capacitive , optoelectronic, piezo electric etc. 

2. Sensitivity: The transducer must be sensitive enough to produce detectable output. 

3. Operating Range: The transducer should maintain the range requirement and have a good resolution over the entire range. 

4. Accuracy: High accuracy is assured. 

5. Cross sensitivity: It has to be taken into account when measuring mechanical quantities. There are situation where the actual quantity is being measured is in one plane and the transducer is subjected to variation in another plan. 

6. Errors: The transducer should maintain the expected input- output relationship as described by the transfer function so as to avoid errors.

7. Transient and frequency response : The transducer should meet the desired time domain specification like peak overshoot, rise time, setting time and small dynamic error. 

8. Loading Effects: The transducer should have a high input impedance and low output impedance to avoid loading effects. 

9. Environmental Compatibility: It should be assured that the transducer selected to work under specified environmental conditions maintains its input- output relationship and does not break down. 

10. Insensitivity to unwanted signals: The transducer should be minimally sensitive to unwanted signals and highly sensitive to desired signals.

CLASSIFICATION OF TRANSDUCERS

The transducers can be classified as:

I. Active and passive transducers. 

II. Analog and digital transducers. 

III. On the basis of transduction principle used. 

IV. Primary and secondary transducer 

V. Transducers and inverse 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. 

I.The active transducer are self generating devices which operate under the energy conversion principle. 

II.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. 

ACTIVE AND  PASSIVE TRANSDUCERS

Piezoelectric Transducer

CLASSIFICATION OF ACTIVE TRANSDUCERS

 Passive Transducers : 

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

II.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. 

III. ACTIVE AND PASSIVE TRANSDUCERS

CLASSIFICATION OF PASSIVE TRANSDUCERS

PRIMARY AND SECONDARY TRANSDUCERS 

Some transducers contain the mechanical as well as electrical device. The mechanical device converts the physical quantity to be measured into a mechanical signal. Such mechanical device are called as the primary transducers, because they deal with the physical quantity to be measured. 

The electrical device then convert this mechanical signal into a corresponding electrical signal. Such electrical device are known as secondary transducers.

CLASSIFICATION OF TRANSDUCERS According to Transduction Principle

CAPACITIVE TRANSDUCER: 

•In capacitive transduction transducers the measurand is converted to a change in the capacitance.

•A typical capacitor is comprised of two parallel plates of conducting material separated by an electrical insulating material called a dielectric. The plates and the dielectric may be either flattened or rolled. 

•The purpose of the dielectric is to help the two parallel plates maintain their stored electrical charges. 

• The relationship between the capacitance and the size of capacitor plate, amount of plate separation, and the dielectric is given by C =  ε0  εr A / d d is the separation distance of plates (m) C is the capacitance(F , Farad) ε0 : absolute permittivity of vacuum εr : relative permittivity  is the effective (overlapping)area of capacitor plates (m2)

CLASSIFICATION OF TRANSDUCERS According to Transduction Principle

PHOTO CONDUCTIVE TRANSDUCTION :

•In photoconductive transduction the measurand is converted to change in resistance of      semiconductor material by the change in light incident on the material.

CLASSIFICATION OF TRANSDUCERS 

Transducer and Inverse Transducer

TRANSDUCER:

•Transducers convert non electrical quantity to electrical quantity .

INVERSE TRANSDUCER:

• Inverse transducers convert electrical quantity to a non electrical quantity

PASSIVE TRANSDUCERS

 Resistive transducers : 

 Resistive transducers are those transducers in which the resistance change due to the change in      some physical phenomenon. 

 The resistance of a metal conductor is expressed by a simple equation. 

 R = ρL/A 

 Where R = resistance of conductor in Ω L = length of conductor in m A = cross sectional area of conductor in m2 ρ = resistivity of conductor material in Ω-m.

RESISTIVE TRANSDUCER

There are 4 type of resistive transducers.

1. Potentiometers (POT) 

2. Strain gauge 

3. Thermistors 

4. Resistance thermometer

POTENTIOMETER

 The potentiometer are used for voltage division. They consist of a resistive element provided with a sliding contact. The sliding contact is called as wiper. 

 The contact motion may be linear or rotational or combination of the two. The combinational potentiometer have their resistive element in helix form and are called helipots. 

RESISTANCE THERMOMETER

Resistance of metal increase with increases in temperature. Therefore metals are said to have a positive temperature coefficient of resistivity.  The platinum wire is wound in the form of spirals on an insulating material such as mica or ceramic. 

This assembly is then placed at the tip of probe 

This wire is in direct contact with the gas or liquid whose temperature is to be measured.

 The resistance of the platinum wire changes with the change in temperature of the gas or liquid 

 This type of sensor have a positive temperature coefficient of resistivity as they are made from metals they are also known as resistance temperature detector 

 Resistance thermometer are generally of probe type for immersion in medium whose temperature is to be measured or controlled.

THERMISTOR

•Thermistor is a contraction of a term “thermal resistor”. 

•Thermistor are temperature dependent resistors. They are made of semiconductor material which have negative temperature coefficient of resistivity i.e. their resistance decreases with increase of temperature. 

•Thermistor are widely used in application which involve measurement in the range of 0-60º Thermistor are composed of sintered mixture of metallic oxides such as magnese, nickle, cobalt, copper, iron and uranium

•The thermistor may be in the form of beads, rods and discs. 

•The thermistor provide a large change in resistance for small change in temperature. In some cases the resistance of themistor at room temperature may decreases as much as 6% for each 1ºC rise in temperature.

STRAIN GAUGE 

 The strain gauge is a passive, resistive transducer which converts the 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.

Half Bridge Strain Gauge Circuit 

 Unlike the Wheatstone bridge using a nullbalance detector and a human operator to maintain a state of balance, a strain gauge bridge circuit indicates measured strain by the degree of imbalance, and uses a precision voltmeter in the center of the bridge to provide an accurate measurement of that imbalance:

Strain Gauge

With no force applied to the test specimen, both strain gauges have equal resistance and the bridge circuit is balanced. However, when a downward force is applied to the free end of the specimen, it will bend downward, stretching gauge #1 and compressing gauge #2 at the same time:

Applications : Strain Gauge

 Strain gauges are used to measure force and small displacements. They are used for analyzing the dynamic strain of complex structures. They are used to measure tension, torque etc. 

 Types of strain gauges are: 

(a)Wire strain gauges 

(b)Foil strain gauges 

(c)Thin film 

(d)Semiconductor

Strain gauges 

Strain gauges come in many forms and types. 

Any material, combination of materials or physical configuration that changes its resistance due to strain constitutes a strain gauge. 

Will restrict our discussion to two types that account for most of the strain gauges in use today: 

 wire (or metal) strain gauges - resistive 

 semiconductor strain gauges.The resistive strain gauge Metallic strain gauge common form 

Strain gauges may also be used to measure multiple axis strains by simply using more than one gauge or by producing them in standard configurations. 

Thermocouples

➢ Two wires of different metal alloys. 

➢ Converts thermal energy into electrical energy. 

➢ Requires a temperature difference between measuring junction and reference junction. 

➢ Easy to use and obtain.

What are thermocouples?

 Thermocouples operate under the made by principle that a circuit connecting two dissimilar metals produces a measurable voltage (emf electromotive force) when a temperature gradient is imposed between one end and the other. 

 They are inexpensive, small, rugged and accurate when used with an understanding of their peculiarities.

.

.

.

.

.

.

.

.

If you want more books, slide, ppt's and notes,Request in the comment section.

Visit the site for more knowledge and share this site link to the needy one.

Thank You...