Different Types of Transducers | Characteristics, Classification, Applications


In this tutorial, we will learn about Transducers, Different Types of Transducers, their characteristics and a few important applications of transducers.

You might have heard of the terms like Sensors and Transducers, often frequently, sometimes confusingly interchangeably. There are different views and definitions of sensors and transducers. According to one set of definitions, a Sensor is an element that senses changes in input energy and produces an output in the same or different form of energy.

Coming to a Transducer, it uses the principle of Transduction to convert the measurand into a usable output. Based on these definitions, a Piezoelectric Crystal is the Sensor whereas a Piezoelectric Crystal with electrodes and some sort of input/output mechanism attached to it makes it a Transducer.

What are Transducers?

Before understanding what a transducer is or diving into the different types of Transducers, consider the following setup of a measuring system. In this block diagram of a simple measuring system, there are three basic elements:

  • Sensor
  • Signal Conditioning Unit
  • Data Representing Device

Different Types of Transducers Block Diagram

Sensor

A Sensor is a device that is used to detect changes in any physical quantity like Temperature, Speed, Flow, Level, Pressure, etc. Any changes in the input quantity will be detected by a Sensor and reflected as changes in output quantity.

Both the input and output quantities of a Sensor are Physical i.e., non-electrical in nature.

Signal Conditioning Unit

The non-electrical output quantity of the Sensor makes it inconvenient to further process it. Hence, the Signal Conditioning Unit is used to convert the physical output (or non-electrical output) of the sensor to an electrical quantity.

Some of the best known Signal conditioning units are:

  • Analog to Digital Converters
  • Amplifiers
  • Filters
  • Rectifiers
  • Modulators

Data Representation Device

A Data representation device is used to present the measured output to the observer. This can be anything like

  • A Scale
  • An LCD Display
  • A Signal Recorder

Transducer

In the above example, consider a Strain Gauge as the Sensor. Any changes in the strain will reflect as changes in its resistance. Now, in order to convert this change in resistance into equivalent voltages, you can use a simple Wheatstone Bridge circuit, which acts as the Signal Conditioning Unit.

The combination of Strain Gauge (Sensor) and Wheatstone Bridge (Signal Conditioning Unit) is Known as a Transducer.

Generally speaking, a Transducer is a device that converts one form of energy into another by the principle of Transduction. Usually, a signal in one form of energy is converted to a signal in another form by a Transducer.

From the above example, a Transducer is a device that converts a Physical Quantity into an Electrical Quantity.

Sensors and Actuators

From the above definition, actually, both Sensors, devices that responds to a physical quantity with a signal and Actuators, devices that respond to signals with physical movement (or similar action) can be considered as Transducers.

For example, a Microphone is a Sensor, which converts sound waves into electrical signals and a Loudspeaker is an Actuator, which converts electrical signals into audio signals.

Both Microphone and Loudspeaker are Transducers in the sense that a microphone converts sound energy into electrical energy and a loud speaker converts electrical energy into sound energy.

Classification of Transducers

There are several ways in which you can classify transducers that include but not limited to the role of the transducer, structure of the transducer or the phenomena of their working.

It is easy to classify transducers as Input Transducers or Output Transducers, if they are treated as simple signal converters. Input Transducers measure non-electrical quantities and convert them into electrical quantities.

Output Transducers on the other hand, work in the opposite way i.e. their input signals are electrical and their output signals are non-electrical or physical like force, displacement, torque, pressure etc.

Depending on the principle of operation, transducers can also be classified into mechanical, thermal, electrical, etc.

Let us see the classification of transducers based on the following three ways:

  • Physical Effect
  • Physical Quantity
  • Source of Energy

Classification based on Physical Effect

The first classification of Transducers is based on the physical effect engaged to convert the physical quantity to electrical quantity. An example, is the change in resistance (physical quantity) of a copper element in proportion to the change in temperature.

The following physical effects are generally used:

  • Variation in Resistance
  • Variation in Inductance
  • Variation in Capacitance
  • Hall Effect
  • Piezoelectric Effect

Classification based on Physical Quantity

The second classification of Transducers is based on the physical quantity converted i.e. the end use of the transducer after the conversion. For example, a Pressure Transducer is a transducer that converts pressure into electrical signal.

Following is small list of transducers classified based on the physical quantity and corresponding examples

  • Temperature Transducer – Thermocouple
  • Pressure Transducer – Bourdon Gauge
  • Displacement Transducer – LVDT (Linear Variable Differential Transformer)
  • Level Transducer – Torque Tube
  • Flow Transducer – Flow Meter
  • Force Transducer – Dynamometer
  • Acceleration Transducer – accelerometer

Classification based on Source of Energy

Transducers are also classified based on the source of energy. Under this category, there usually two types of transducers:

  • Active Transducers
  • Passive Transducers

Active Transducers

In Active Transducers, the energy from the input is used as a control signal in the process of transferring energy from power supply to proportional output.

Different Types of Transducers Active Transducers

For example, a Strain Gauge is an Active Transducer, in which the strain is converted into resistance. But since the energy from the strained element is very small, the energy for the output is provided by an external power supply.

Passive Transducers

In Passive Transducers, the energy from the input is directly converted into the output. For example, a Thermocouple is a passive transducer, where the heat energy, which is absorbed from input, is converted into electrical signals (voltage).

Different Types of Transducers Passive Transducers

Characteristics of Transducers

The performance characteristics of a Transducer are key in selecting the best suitable transducer for a particular design. So, it is very important to know the characteristics of transducers for proper selection.

Performance characteristics of transducers can be further classified into two types:

  • Static Characteristics
  • Dynamic Characteristics

Static Characteristics

The static characteristics of a transducer is a set of performance criteria that are established through static calibration i.e. description of the quality of measurement by essentially maintaining the measured quantities as constant values of varying very slowly.

Following is a list of some of the important static characteristics of transducers.

  • Sensitivity
  • Linearity
  • Resolution
  • Precision (Accuracy)
  • Span and Range
  • Threshold
  • Drift
  • Stability
  • Responsiveness
  • Repeatability
  • Input Impedance and Output Impedance

Dynamic Characteristics

The dynamic characteristics of transducers relate to its performance when the measured quantity is a function of time i.e. it varies rapidly with respect to time.

While static characteristics relate to the performance of a transducer when the measured quantity is essentially constant, the dynamic characteristics relate to dynamic inputs, which means that they are dependent on its own parameters as well as the nature of the input signal.

The following are some dynamic characteristics that may be considered in selection of a transducer.

  • Dynamic Error
  • Fidelity
  • Speed of Response
  • Bandwidth

Overall, both static and dynamic characteristics of a Transducer determine its performance and indicate how effectively it can accept desired input signals and reject unwanted inputs.

Different Types of Transducers

Basically, the two different types of Transducers are Mechanical Transducers and Electrical Transducers. Mechanical Transducers are those which responds to changes in physical quantities or condition with mechanical quantity. If the physical quantity is converted to an electrical quantity, then the transducers are Electrical Transducers.

Mechanical Transducers

As mentioned earlier, mechanical transducers are a set of primary sensing elements that respond to changes in a physical quantity with a mechanical output. As an example, a Bimetallic Strip is a mechanical Transducer, which reacts to changes in temperature and responds with mechanical displacement. The mechanical transducers are differentiated from electrical transducers as their output signals are mechanical.

The output mechanical quantity can be anything like displacement, force (or torque), pressure and strain. For any measuring quantity, there can be both mechanical and electrical transducers.

For example, we have seen Bimetallic Strip, which is a mechanical transducer and is used to react to changes in temperature. In contrast, a Resistance Thermometer, also reacts to changes in temperature, but the response is a change in resistance of the element. Hence, it is an electrical transducer.

The following table shows a small list of mechanical transducers for measuring different quantities and responds with mechanical signal.

Quantity to be Measured Mechanical Transducer Type of Output Signal (Mechanical)
Temperature Bimetallic Strip Displacement and Force
Fluid Expansion Displacement and Force
Pressure Ring Balance Manometer Displacement
Metallic Diaphragms Displacement and Strain
Capsules and Bellows Displacement
Membranes Displacement
Force Spring Balance Displacement and Strain
Hydraulic Load Cell Pressure
Column Load Cell Displacement and Strain
Torque Dynamometer Force and Strain
Gyroscope Displacement
Spiral Springs Displacement
Torsion Bar Displacement and Strain
Flow Rate Flow Obstruction Element Strain and Pressure
Pitot Tube Pressure
Liquid Level Manometer Displacement
Float Elements Displacement, Force and Strain

Electrical Transducers

As mentioned earlier, electrical transducers are those that respond to changes in physical quantities with electrical outputs. Electrical Transducers are further divided into Passive Electrical Transducers and Active Electrical Transducers.

The following table lists out a few electrical transducers (both passive and active).

Passive Electrical Transducers Resistive Transducers Resistance Thermometers
Resistive Displacement Transducers
Resistive Strain Transducers
Resistive Pressure Transducers
Resistive Moisture Transducers
Capacitive Transducers Capacitive Moisture Transducers
Capacitive Displacement Transducers
Capacitive Thickness Transducers
Inductive Transducers Inductive Displacement Transducers
Inductive Thickness Transducers
Eddy-Current Inductive Transducers
Moving core Inductive Transducers
Active Electrical Transducers Photoelectric Transducers Photoconductive Transducers
Photoemissive Transducers
Photovoltaic Force Transducers
Piezoelectric Transducers Piezoelectric Strain Transducers
Piezoelectric Acceleration Transducers
Piezoelectric Pressure Transducers
Piezoelectric Torque Transducers
Piezoelectric Force Transducers
Magnetostrictive Transducers Magnetostrictive Acceleration Transducers
Magnetostrictive Force Transducers
Magnetostrictive Torsion Transducers
Electromechanical Transducers Tachometers
Electrodynamic Pressure Transducers
Electrodynamic Vibration Transducers
Electromagnetic Flowmeters
Ionization Transducers Ionization Vacuum Gauge
Ionization Displacement Transducers
Nuclear Radiation Transducers
Radioactive Vacuum Gauge
Radioactive Level Gauge
Radioactive Thickness Gauge
Electrochemical Transducers
Hall-Effect Transducers
Thermoelectric Transducers

Applications of Transducers

Electromagnetic

  • Antennas
  • Hall-Effect Sensors
  • Disk Read and Write Heads
  • Magnetic Cartridges

Electromechanical

  • Accelerometers
  • Pressure Sensors
  • Galvanometers
  • LVDT
  • Load Cells
  • Potentiometers
  • MEMS
  • Linear and Rotary Motors
  • Air Flow Sensors

Electrochemical

  • Hydrogen Sensors
  • Oxygen Sensors
  • pH Meters

Electroacoustic

  • Speakers (Loudspeakers, earphones)
  • Microphones
  • Ultrasonic Transceivers
  • Piezoelectric Crystals
  • Sonar
  • Tactile Transducers

Photoelectric

  • LED
  • Photodiodes
  • Photovoltaic Cells
  • Laser Diodes
  • Photoresistors (LDR)
  • Phototransistors
  • Incandescent and Fluorescent Lamps

Thermoelectric

  • Thermistors
  • Thermocouples
  • RTD (Resistance Temperature Detectors)

Radioacoustic

  • Radio Transmitters and Receivers
  • G-M Tube (Geiger-Muller Tube)

Conclusion

A brief introduction to Transducers, Different Types of Transducers, Characteristics of Transducers, Different Classification and Applications of Transducers.



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