Transducer

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1. Transducers

2. Definition:   The  device which converts the one form of energy into another is known as the transducer. The process of conversion is known as transduction. The  conversion is done by sensing and transducing the physical quantities like temperature , pressure, sound, etc . The  electrical transducer  converts the  mechanical energy  into an  electric signal . The electrical signal may be voltage, current and frequency. The production of the signal depends on the resistive inductive and capacitive effects of the physical input.

3. Parts of Transducer The transducer consists two important parts. Sensing Element Transduction Element The transducer has many other parts like amplifiers, signal processing equipment, power supplies calibrating and reference sources, etc. Sensing or Detector Element – It is the part of the transducers which give the response to the physical sensation. The response of the sensing element depends on the physical phenomenon. Transduction Element – The transduction element converts the output of the sensing element into an electrical signal. This element is also called the secondary transducer.

4. In this block diagram of a simple measuring system, there are three basic elements:

5. 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 .

6. Some of the best known Signal conditioning units are: Analog to Digital Converters Amplifiers Filters Rectifiers Modulators D ata 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

7. 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

8. 1. 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

9. 2. 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

10. 3. 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.

11. 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).

12. 1. 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

13. 2. 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.

14. 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 . 1. 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 .

15. 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.

16. 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

17. 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.

18. Applications of Transducers 1 . Electromagnetic Antennas Hall-Effect Sensors Disk Read and Write Heads Magnetic Cartridges 2. Electromechanical Accelerometers Pressure Sensors Galvanometers LVDT Load Cells Potentiometers MEMS Linear and Rotary Motors Air Flow Sensors 3. Electrochemical Hydrogen Sensors Oxygen Sensors pH Meters

19. 5. Electroacoustic Speakers (Loudspeakers, earphones) Microphones Ultrasonic Transceivers Piezoelectric Crystals Sonar Tactile Transducers 6. Photoelectric LED Photodiodes Photovoltaic Cells Laser Diodes Photoresistors (LDR) Phototransistors Incandescent and Fluorescent Lamps

20. 7. Thermoelectric Thermistors Thermocouples RTD (Resistance Temperature Detectors) 8. Radioacoustic Radio Transmitters and Receivers G-M Tube (Geiger-Muller Tube)

21. Factors Influencing the Choice of Transducer The choice of the transducers used for measuring the physical quantity depends on the following factors. Operating Principle  – The transducers are selected by their operating principles. The operating principle may be resistive, inductive, capacitive, optoelectronic, piezoelectric, etc. Sensitivity  – The sensitivity of the transducer is enough for inducing the detectable output. Operating Range  – The transducer must have wide operating ranges so that it does not break during the working. Accuracy   – The transducers gives accuracy after calibration. It has a small value for repeatability which is essentials for the industrial applications. Cross Sensitivity  – The transducers gives variable measured value for the different planes because of the sensitivity. Hence, for the accurate measurement, the cross sensitivity is essential.

22. Errors   – The errors are avoided by taking the input output relations which is obtained by the transfer function. Loading Effect  – The transducers have high input impedance and low output impedance for avoiding the errors. Environmental Compatibility  – The transducers should be able to work in any specified environments like in a corrosive environment. It should be able to work under high pressure and shocks. Insensitivity to Unwanted Signals  – The transducer should be sensitive enough for ignoring the unwanted and high sensitive signals. Usage and Ruggedness   – The durability, size and weight of the transducer must be known before selecting it. Stability and Reliability  – The stability of the transducers should be high enough for the operation. And their reliability should be good in case of failure of the transducer. Static characteristic  – The transducer should have a high linearity and resolution, but it has low hysteresis. The transducer is always free from the load and temperature.

23. Applications of Transducer The following are the application of the transducers. The transducer measures the load on the engines. It is used as a sensor for knowing the engine knock. The transducers measure the pressure of the gas and liquid by converting it into an electrical signal. It converts the temperature of the devices into an electrical signal or mechanical work. The transducer is used in the ultrasound machine. It receives the sound waves of the patient by emitting their sound waves and pass the signal to the CPU. The transducer is used in the speaker for converting the electrical signal into acoustic sound. It is used in the antenna for converting the electromagnetic waves into an electrical signal.

24. Displacement transducer What is Potentiometer? The potentiometer is the electrical type of transducer or sensor and it is of resistive type because it works on the principle of  change of resistance of the wire with its length . It is defined as a three-terminal resistor having either sliding or rotating contact that forms an adjustable voltage divider. In order to use the potentiometer as a rheostat or variable resistor, it should have only two terminals with one end and the wiper.

25. Potentiometer Working Principle The potentiometer is also called as pots and it one of the most commonly used device for measuring the displacement of the body . The potentiometer consists of L which is a long resistive wire and a battery of known  EMF  V whose voltage is known as driver cell voltage. Assume a primary circuit arrangement by connecting the two ends of L to the battery terminals. One end of the primary circuit is connected to the cell whose EMF E is to be measured and the other end is connected to galvanometer G. This circuit is assumed to be a secondary circuit.

26. The working principle depends on the potential across any portion of the wire which is directly proportional to the length of the wire that has a uniform cross-sectional area and current flow is constant.

27. Types of Potentiometer There are basically two types of potentiometers that can be further divided depending on the movement of the wiper. Following is the table when the wiper is moving in a circular path which is known as rotary potentiometers. Type Description Applications Concentric pot Two potentiometers are adjusted individually with the help of concentric shafts Found in old car radios that were used to control volume and tone Single-turn pot Approximate rotation is 270 degrees in a single rotation Used where a single turn is enough to control the resolution Servo pot Used to adjust a servo motor automatically Used in remote-controls to control the volume of an audio equipment

28. Application of Potentiometer Audio control: Both linear, and rotary potentiometers, are used to control audio equipment for changing the loudness and other audio related signals. Television: They are used to control the picture brightness, colour response and contrast. Motion control: In order to create a closed-loop control, potentiometers are used as position feedback devices known as a servomechanism. Transducers: As these give large output signals, they find applications in designing of displacement transducers.

29. LVDT The LVDT sensor converts the linear (or rectilinear / straight-line) movement of the object the LVDT is coupled to, into a variable corresponding electrical signal proportional to that movement . LVDT works under the principle of mutual induction, and the displacement which is a non-electrical energy is converted into an electrical energy. That movement can be from as little as 0-0.5mm up to 0-1000mm in laboratory, industrial and submersible environments.  The LVDT or Linear Variable Differential Transformer is a well established electromechanical transducer design.  This design has been used throughout many decades for the accurate measurement of displacement and within closed loops for the control of positioning.

30. In its simplest form, the LVDTs design consists of a cylindrical array of a primary and secondary winding with a separate cylindrical core which passes through the centre .

31. LVDT consists of a cylindrical former where it is surrounded by one primary winding in the centre of the former and the two secondary windings at the sides . The number of turns in both the secondary windings are equal, but they are connected in series opposition to each other, i.e., if the left secondary windings is in the clockwise direction, the right secondary windings will be in the anti-clockwise direction, hence the net output voltages will be the difference in voltages between the two secondary coil. The two secondary coil is represented as S1 and S2. Esteem iron core is placed in the centre of the cylindrical former which can move in to and fro motion as shown in the figure. The AC excitation voltage is 5 to 12V and the operating frequency is given by 50 to 400 HZ.

32. The primary windings (P) are energised with a constant amplitude A.C. supply at a frequency of 1 to 10 kHz.  This produces an alternating magnetic field in the centre of the transducer which induces a signal into the secondary windings (S & S ) depending on the position of the core. Movement of the core within this area causes the secondary signal to change . As the two secondary windings are positioned and connected in a set arrangement (push-pull mode), when the core is positioned at the centre , a zero signal is derived.

33. Working of LVDT : Let’s study the working of LVDT by splitting the cases into 3 based on the iron core position inside the insulated former. Case 1: On applying an external force which is the displacement, if the core reminds in the null position itself without providing any movement then the voltage induced in both the secondary windings are equal which results in net output is equal to zero i.e., Esec1-Esec2=0

34. Case 2: When an external force is appilied and if the steel iron core tends to move in the left hand side direction then the emf voltage induced in the secondary coil is greater when compared to the emf induced in the secondary coil 2. Therefore the net output will be Esec1-Esec2 Case 3: When an external force is applied and if the steel iron core moves in the right hand side direction then the emf induced in the secondary coil 2 is greater when compared to the emf voltage induced in the secondary coil 1. therefore the net output voltage will be Esec2-Esec1

35. Advantages Of LVDT No Physical Contact Between the Core and the Coils Long Operating Life Theoretically Infinite Resolution Easy Modification Low Power Consumption High Accuracy Fast Response Applications of LVDT: LVDT is used to measure displacement ranging from fraction millimeter to centimeter. Acting as a secondary transducer, LVDT can be used as a device to measure force, weight and pressure, etc..