What are Electrical Transducers?
Any device which is able to convert one form of energy into another form is said to be a transducer. For example- a speaker can be called as a transducer as it converts electrical signal to pressure waves (sound). An electrical transducer is a device which is capable of converting any physical quantity into a proportional electrical quantity such as voltage or electric current.
There are of many different types of electrical transducer, they can be classified based on various criteria as:
Types of Transducer based on Quantity to be Measured
- Temperature transducers
- Pressure transducers
- Displacement transducers
- Flow transducers
- and so on.
Types of Transducer based on the Principle of Operation
- Photo-volatic transducers
- Chemical transducers
- Mutual Induction transducers
- Electromagnetic transducers
- Hall effect transducers
- Photo-conductor transducers
- Piezoelectric transducers
- Capacitive transducers
- Registive transducers
- Inductive transducers
- and so on.
Types of Transducer based on the output form
1. Analog or digital: Analog transducers convert the input quantity into an analog output which is a continuous function of time. Digital transducers convert the input quantity into an electrical output which is discrete function of time.
2.Transducer or Inverse transducer: Transducers convert non-electrical quantity to electrical quantity. Inverse transducers convert electrical quantity to a non electrical quantity.
Transducer may be classified as Active or Passive based on whether an external power source is required or not.
- Active transducers are those which do not require any power source for their operation. They work on the energy conversion principle. They produce an electrical signal proportional to the input (physical quantity). For example, a thermocouple is an active transducer.
- Transducers which require an external power source for their operation is called as a passive transducer. They produce an output signal in the form of some variation in resistance, capacitance or any other electrical parameter, which than has to be converted to an equivalent current or voltage signal. For example, a photocell (LDR) is a passive transducer which will vary the resistance of the cell when light falls on it.
Transducer also may be classified as secondary and primary.
⦁ 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.
Advantages of an Electrical transducer:
- Electrical signal obtained from electrical transducer can be easily processed (mainly amplified) and brought to a level suitable for output device which may be an indicator or recorder.
- The electrical systems can be controlled with a very small level of power.
- The electrical output can be easily used, transmitted and processed for the purpose of measurement.
- With the advent of IC technology, the electronic systems have become extremely small in size, requiring small space for their operation.
- No moving mechanical parts are involved in the electrical systems. Therefore there is no question of mechanical wear and tear and no possibility of mechanical failure.
- Friction effect is minimized.
- The output can be indicated and recorded remotely from the sensing element.
- Power requirement is very low for controlling the electrical or electronic system.
- An amplifier may be used for amplifying the electrical signal according to the requirement.
- Mass-inertia effect are also minimized, because in case of electrical or electronics signals the inertia effect is due to the mass of the electrons, which can be negligible.
Disadvantages of an Electrical Transducer:
The electrical transducer is sometimes less reliable than mechanical type because of the ageing and drift of the active components. Also the sensing elements and the associated signal processing circuitry are comparatively expensive. With the use of better materials, improved technology and circuitry, the range of accuracy and stability have been increased for electrical transducers. Using negative feedback technique, the accuracy of measurement and the stability of the system are improved, but all at the expense of increased circuit complexity, more space and obviously more cost.
Basic Requirements of a Transducer:
The main objective of a transducer is to react only for the measurement under specified limits for which it is designed. It is, therefore, necessary to know the relationship between the input and output quantities and it should be fixed. A transducer should have the following basic requirements:
1. Linearity: Its input vs output characteristics should be linear and it should produce these characteristics in balanced way.
2. Ruggedness: A transducer should be capable of withstanding overload and some safety arrangements must be provided with it for overload protection.
3. Repeatability: The device should reproduce the same output signal when the same input signal is applied again and again under unchanged environmental conditions such as temperature, pressure, humidity etc.
4. High Reliability and Stability: The transducer should give minimum error in measurement for temperature variations, vibrations and other various changes in surroundings.
5. High Output Signal Quality: The quality of output signal should be good, like the ratio of the signal to the noise should be high and the amplitude of the output signal should be enough.
6. No Hysteresis: It should not give any hysteresis during measurement while input signal is varied from its low value to high value and vice versa.
7. Residual Reformation: There should not be any deformation on removal of input signal after long period of use.
Overview of Commonly Used Electrical Transducers:
1) Potentiometers: They convert the change in displacement into change in the resistance, which can be measured easily.
2) Bridge circuits: These convert the physical quantity to be measured into the voltage.
3) Wheatstone bridge: It converts the displacement produced by the physical quantity to the current in the circuit.
4) Capacitive sensors or Variable Capacitance Transducers: These comprise of the two parallel plates between which there is dielectric material like air. The change in distance between the two plates produced by the displacement results in change in capacitance, which can be easily measured.
5) Resistive sensors or Variable Resistance Transducers: There is change in the resistance of these sensors when certain physical quantity is applied to it. It is most commonly used in resistance thermometers or thermistors for measurement of temperature.
6) Magnetic sensors: The input given to these sensors is in the form of displacement and the output obtained is in the form of change in inductance or reluctance and production of the eddy currents
7)Piezoelectric transducers: When force is applied to these transducers, they produce voltage that can be measured easily. They are used for measurement of pressure, acceleration and force.
8) Strain gauges: When strain gauges are strained or stretched there is change in their resistance. They consist of the long wire and are able to detect very small displacements produced by the applied force or pressure.
9) Photo electric transducers: When the light is applied to these transducers they produce voltage.
10) Linear variable differential transformer (LVDT): LVDT is the transformer consisting of the primary and the secondary coil. It converts the displacement into the change in resistance.
11) Antenna: is the most basic transducer and can be made from a simple piece of wire. It converts electromagnetic energy into electricity when it receives signals and does the opposite when it transmits.
12) Accelerometer: which converts the change in position of mass into an electrical signal. Accelerometers measure the force of acceleration and deceleration. They are used in car airbags, stability control, hard drives.
13) Geiger counter: detects radiation levels by using a transducer called a Geiger-Muller tube.
14) Microphone and Speaker: Microphones convert sound pressure waves into electrical current, while speaker convert electrical current into sound pressure waves.
15)Thermocouples: convert temperature differences into electrical signals. They are widely used in temperature measurement applications.