UNIT 7
Electrical Transducers
7.1.1. Introduction
Electrical Transducers are used to convert energy of one kind into energy of another kind.
For example, a microphone (input device) converts sound waves into electrical signals for the amplifier to amplify (a process), and a loudspeaker (output device) converts these electrical signals back into sound waves and an example of this type of simple Input / Output (I/O) system is given below.
7.1.2. Parameters for Selection of Transducers
The following are the parameters for selection of parameters for Selection of transducers-
Sensitivity:
Transducer must be sensitive enough to give a detectable output but not to give an output.
Operating range:
A transducer should have good resolution over its entire range of operation.
Accuracy:
High degree of accuracy is needed.
Error:
The transducer should maintain expected input output relationship by its transfer function to avoid error.
Loading effect:
Transducer should have high input impedance and low output impedance to avoid loading effect.
Insensitivity to unwanted signals:
A good transducer should be insensitive to unwanted signals (noise).
7.1.3. Wire type strain gauge
A strain gauge works on the principle of electrical conductance and its dependence on the conductor’s geometry. Whenever a conductor is stretched within the limits of its elasticity, it doesn’t break but, gets narrower and longer. Similarly, when it is compressed, it gets shorter and broader, ultimately changing its resistance.
We know, resistance is directly dependent on the length and the cross-sectional area of the conductor given by:
R= L/A
Where,
R = Resistance
L = Length
A = Cross-Sectional Area
The change in the shape and size of the conductor also changes its length and the cross-sectional area which eventually affects its resistance.
7.1.4. Load cell
A load cell is a force transducer that converts a load or force acting on it into an electronic signal. This electronic signal can be a voltage change, current change or frequency change depending on the type of load cell and circuitry used.
There are different kinds of load cells-
- Resistive load cells
- Capacitive load cells
7.1.5. LVDT
An LVDT (linear variable differential transformer) is an electromechanical sensor used to convert mechanical motion or vibrations, specifically rectilinear motion, into a variable electrical current, voltage or electric signals, and the reverse. Actuating mechanisms used primarily for automatic control systems or as mechanical motion sensors in measurement technologies. The classification of electromechanical transducers includes conversion principles or types of output signals.
A linear displacement transducer is essentially a miniature transformer having one primary winding; two symmetrically wound secondary coils, and an armature core that is free to move along its linear axis in precision bearing guides. A push rod connects the monitored component to the armature core, such that the displacement of that component moves the core off-centre.
7.2.1. Thermocouple
The thermocouple is a temperature measuring device used for measuring the temperature at one particular point. Thermocouple is a type of sensor used for measuring the temperature in the form of an electric current or the EMF.
The thermocouple consists two wires of different metals which are joined together at the ends.
There are many types of thermocouples, each with its own unique characteristics in terms of temperature range, durability, vibration resistance, chemical resistance, and application compatibility. Type J, K, T, & E are “Base Metal” thermocouples, the most common types of thermocouples. Type R, S, and B thermocouples are “Noble Metal” thermocouples, which are used in high temperature applications (see thermocouple temperature ranges for details).
Thermocouples are used in many industrial, scientific, and OEM applications. They can be found in nearly all industrial markets: Power Generation, Oil/Gas, Pharmaceutical, BioTech, Cement, Paper & Pulp, etc. Thermocouples are also used in everyday appliances like stoves, furnaces, and toasters.
Thermocouples are typically selected because of their low cost, high temperature limits, wide temperature ranges, and durable nature.
7.2.2. Thermistor
A type of resistor whose electrical resistance varies with changes in temperature is called a thermistor which are accurate, cheap, and robust way to measure temperature.
Thermistors act as a passive component in a circuit.
Uses of Thermistors
Thermistors have a variety of applications. They are widely used as a way to measure temperature as a thermistor thermometer in many different liquid and ambient air environments. Some of the most common uses of thermistors include:
- Digital thermometers (thermostats)
- Automotive applications (to measure oil and coolant temperatures in cars & trucks)
- Household appliances (like microwaves, fridges, and ovens)
- Circuit protection (i.e. surge protection)
- Rechargeable batteries (ensure the correct battery temperature is maintained)
- To measure the thermal conductivity of electrical materials
- Useful in many basic electronic circuits (e.g. As part of a beginner Arduino starter kit)
- Temperature compensation (i.e. maintain resistance to compensate for effects caused by changes in temperature in another part of the circuit)
- Used in wheatstone bridge circuits
7.2.3. Reluctance Pulse Pickup
The variable reluctance sensor is strain-based, wherein a magnetic circuit is formed, and the parameter input causes mechanical deflection of the spring member as a function of pressure, force, or acceleration. In other words, A variable reluctance sensor (commonly called a VR sensor) is a transducer that measures changes in magnetic reluctance.
7.2.4. Photoelectric Pickup
A photoelectric sensor is equipment used to find the distance, absence, or presence of an object by using a light transmitter, generally infrared, and a photoelectric receiver.
7.2.5. LDR
A Light Dependent Resistor (LDR) is also called a photoconductor, is basically a photocell that works on the principle of photoconductivity. The passive component is generally a resistor whose resistance value decreases when the intensity of light decreases. Its applications include camera light meters, street lights, and clock radios.
7.2.6. Solar Cell
An electrical device that converts light energy into electrical energy through the photovoltaic effect is a solar cell. A solar cell is generally a p-n junction diode.
The materials used in Solar Cell-
- Silicon.
- GaAs.
- CdTe.
- CuInSe2
References:
1. Electrical Technology (Volume I & 2), B L Thereja, 22nd edition, S Chand & Company Ltd
2. Basic Electrical Engineering, V K Mehta, Revised edition, S Chand & Company Ltd
3. Basic Electronics Solid State, B L Thereja, Revised edition, S Chand & Company Ltd
4. Digital Principles and Applications, Albert Malvino , Donald Leach, Tata McGraw Hills Publication
5. Principles of Electronic Devices and Circuits (Analog and Digital), B. L. Theraja , R. S. Sedha , S. Chand publication
