TRANSDUCERS
Transducer: A substance or device that converts input energy of one form into output energy of another. In short A transducer is any device which converts one form of energy into another.
Basic Concepts of Transducers
There are many variables which affect our everyday lives: speed of a car, velocity of the wind, temperature of the oven, light level in the room. In most situations these variables are continuously monitored. It is these variables that are the feedback used to control the speed of a car, oven temperatures, and light levels. The elements that sense these variables and convert them to a different output energy are transducers. This is an energy conversion – temperature (heat energy) to voltage (electrical energy).In summary, a transducer is a substance or a device that converts an input energy into a different output energy. Because of this broad definition, there are many devices that can be defined as transducers. Such devices come in many varieties converting several different types of energies. Following is a discussion of some of the more common types of transducers.
ELECTROMECHANICAL TRANSDUCERS
Electromechanical transducers are transducers that either convert electrical to mechanical energy or motion, or convert mechanical movement such as deformation or stress into electrical energy.
Electromechanical Transducers – (Some are also called actuators)
Strain gauge – Converts the deformation (strain) of an object into a change in electrical resistance
Galvanometer – Converts the electric current of a coil in a magnetic field into movement
Generator – Converts mechanical energy (motion) into electrical energy.
Motor – Converts electrical energy into mechanical energy (graphic below)
Micro accelerometer – Converts motion or mechanical energy to electrical energy
Micro gyroscope – Converts rotation or tilt (movement) to electrical energy
RESISTIVE TRANSDUCER:
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 heli pots.
• Fig shows a linear pot and a rotary pot.
• There are number of applications of the potentiometer or pots, of these one of the most common applications is the measurement of displacement. To measure the displacement of the body, this body, which is moving, is connected to the sliding element of the potentiometer.
• As the body moves, the position of the slider located on the potentiometer also changes so the resistance between the fixed point and the slider changes. Due to this the voltage Vo across these points also changes. The change in voltage or the resistance is proportional to the change in the displacement of the body. Thus the voltage change indicates the displacement of the body. The potentiometer can be used for the measurement of translational as well as well rotational displacement. Since these potentiometers work on the principle of resistance, they are also called as the resistive potentiometers.
INDUCTIVE TRANSDUCER:
• In inductive transduction, the measurand is converted into a change in the self inductance of a single coil. It is achieved by displacing the core of the coil that is attached to a mechanical sensing element
VARIABLE RELUCTANCE INDUCTIVE TRANSDUCER
• Fig shows a variable reluctance inductive transducer.
• As shown in fig the coil is wound on the ferromagnetic iron. The target and core are not in direct contact with each other. They are separated by an air gap.
• The displacement has to be measured is applied to the ferromagnetic core
• The reluctance of the magnetic path is found by the size of the air gap.
• The self inductance of coil is given by
• L = N2 / R = N2 / Ri + Ra
• N : number of turns, R : reluctance of coil, Ri : reluctance of iron path, Ra : reluctance of air gap, The reluctance of iron path is negligible
• L = N2 / Ra
• Ra = la / μoA
• Therefore L œ 1 / la i.e. self inductance of the coil is inversely proportional to the air gap la.
• When the target is near the core, the length is small. Hence the self inductance is large. But when the target is away from the core, the length is large. So reluctance is also large. This result in decrease in self inductance i.e. small self inductance.
• Thus inductance is function of the distance of the target from the core. Displacement changes with the length of the air gap, the self inductance is a function of the displace
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
A is the effective (overlapping) area of capacitor plates (m2)
PIEZOELECTRIC TRANSDUCER :
A piezoelectric transducer is a device that transforms one type of energy to another by taking advantage of the piezoelectric properties of certain crystals or other materials. When a piezoelectric material is subjected to stress or force, it generates an electrical potential or voltage proportional to the magnitude of the force. This makes this type of transducer ideal as a converter of mechanical energy or force into electric potential.
The high sensitivity of piezoelectric transducers makes them useful in microphones, where they convert sound pressure into electric voltage, in precision balances, in accelerometers and motion detectors, and as generators and detectors of ultrasound. They are also used in non-destructive testing, in the generation of high voltages, and in many other applications requiring the precise sensing of motion or force.
The piezoelectric effect also works in reverse, in that a voltage applied to a piezoelectric material will cause that material to bend, stretch, or otherwise deform. This deformation is usually very slight and proportional to the voltage applied, and so the reverse effect offers a method of precision movement on the micro scale. A transducer may, therefore, be used as an actuator for the exact adjustment of fine optical instruments, lasers, and atomic force microscopes.
PHOTOELECTRIC TRANSDUCER:
A transducer that generates an electric output corresponding to the incident light is called photoelectric transducer
A photoelectric sensor, or photo eye, is a device used to detect the distance, absence, or presence of an object by using a light transmitter, often infrared, and a photoelectric receiver. They are used extensively in industrial manufacturing. There are three different functional types: opposed (through beam), retro-reflective, and proximity-sensing (diffused).
Photoelectric transducers are divided into two groups according to their applications: 1) a photo-detector that handles light signals, and 2) a photovoltaic power system such as a solar battery that responds to light energy. In the former case, sensitivity and response speed are important, while in the latter case, energy conversion efficiency is important. Classified by their operating principles, photoelectric transducers can be divided into a photovoltaic type, typified by photodiodes and solar batteries, and a photo-conductive type, typified by photo-conductive cells and image pick-up tubes.
