Sensors and Instrumentation PDF Fall 2024-25

Fall 2024-25 Sensors and Instrumentation Instructor: Dr. Moganapriya C Email: [email protected] 1 2 QUIZ- 29.07.2024 4  Classification of Transducers Transducers can be classified...

Fall 2024-25 Sensors and Instrumentation Instructor: Dr. Moganapriya C Email: [email protected] 1 2 QUIZ- 29.07.2024 4  Classification of Transducers Transducers can be classified based on General features Type of output Electric principles involved 5 Classification of Transducers 6  Classification of Transducers Based on the general features, transducers are classified into active transducer and passive transducers Active transducer: no external power source is needed for the energy conversion. Examples of active transducers are thermocouple, photovoltaic cell, thermopiles, etc. Passive transducers are called as extremely powered transducers. The power required for the energy conversion will be obtained from an external power source. Examples of passive transducers are RTD, thermistors, etc. 7 Classification of Transducers 8 Classification of Transducers 9  Classification of Transducers Based on the type of output Analogue transducers: These transducers convert the input physical phenomena into an analogous output which is a continuous function of a time. Examples: Thermocouple, LVDT, thermistor etc.. Digital transducers: These transducers convert the physical phenomenon into an electrical output which may be in the form of pulse. 10  Classification of Transducers Based on the electrical principle involved Variable resistance type: strain and pressure gauges, thermistors resistance thermometers photovoltaic cell Variable inductance type: LVDT, Eddy current gauges Variable capacitance type: capacitor microphone, pressure gauge, dielectric gauge. 11 Classification of Transducers 12  Resistive Transducers The transducer whose resistance varies because of the environment effects - resistive transducer or resistance transducer The resistive transducer is used for measuring the physical quantities like temperature, pressure, displacement, vibration 13  Resistive Transducers Change in the value of resistance with change in conductor can be used for the measurement of translational and rotary displacement. Strain gauge is one of the examples for resistance transducer which measure displacement force and pressure. The types of resistance transducers are Linear motion potentiometer and Rotary motion potentiometer 14  Resistive Transducers Consider the equation given below ρ - resistivity of the material in ohm meter l - length of the conductor in meter A - area of the cross section in metre square The resistive transducer is designed by considering the variation of length, area and resistivity of the material 15 Resistive Transducers 16  Resistive Transducers Linear motion potential meter or Rotary motion potential meter converts motion of the moving shaft or rotating shaft into change in resistance Full scale of the Rotary device varies from 10 º to 60 º Measurements: pressure, force, acceleration, liquid level Power rating of potentiometer: 5W, 21ºC, 50 mm diameter, 100- ohm to10 kilo ohm in steps of 100 ohm Materials used: wire wound potentiometer and non-wire wound potentiometer 17  Resistive Transducers Advantages: high output, less expensive, available in different sizes & shapes, simple to operate, fast response, high electrical efficiency Drawbacks: limited life due to wear and tear, noisy output 18  Resistive Transducers Applications: Potentiometer Strain gauges Resistance thermometer Thermistor 19  Capacitive Transducers The capacitive transducer is used for measuring the displacement, pressure and other physical quantities It is a passive transducer, and it requires external power for operation The capacitive transducer works on the principle of variable capacitance The capacitance of the transducer changes because of change in overlapping area of plates, change in distance between the plates and dielectric constant 20  Capacitive Transducers The capacitive transducer consists of two parallel metal plates These plates are separated by dielectric medium such as air, material, gas or liquid In normal capacitor, the distance between the plates are fixed but in capacitive transducer, the distance between them are varied The input quantity causes the change of capacitance which is directly measured by the capacitive transducer 21 Capacitive Transducers 22  Capacitive Transducers The change in capacitance occurs because of the change in physical variables like displacement, force, pressure etc. Capacitance of the transducer also changes by the variation of their dielectric constant which is used for the measurement of the liquid or gas level It is mainly used for the measurement of the linear displacement 23 Transducer using the change in the Area of Plates 24 Transducer using the change in distance between the plates 25  Capacitive Transducers Advantage of Capacitive Transducer It requires an external force for operation and hence very useful for small systems. Capacitive transducer is very sensitive. It gives good frequency response because of which it is used for the dynamic study. The transducer has high input impedance hence they have a small loading effect. It requires small output power for operation. 26  Capacitive Transducers Disadvantages of capacitive Transducer The metallic parts of the transducers require insulation. The frame of the capacitor requires earthing for reducing the effect of the stray magnetic field. Sometimes the transducer shows the nonlinear behaviors because of the edge effect which is controlled by using the guard ring. The cable connecting across the transducer causes an error. 27  Piezoelectric transducer Piezoelectric transducer is an electroacoustic transducer use for conversion of pressure or mechanical stress into an alternating electrical force. It is used for measuring the physical quantity like force, pressure, stress, etc., which is directly not possible to measure. The piezoelectric transducer uses the piezoelectric material which has a special property, i.e. the material induces voltage when the pressure or stress applied to it. The material which shows such property is known as the electro-resistive element. 28  Piezoelectric transducer The word piezoelectric means the electricity produces by the pressure. The Quartz is the examples of the natural piezoelectric crystals, whereas the Rochelle salts, ammonium dehydration, phosphate, lithium sulphate, dipotassium tartrate are the examples of the man-made crystals. The ceramic material is also used for piezoelectric transducer. The ceramic material does not have the piezoelectric property. The property is developed on it by special polarizing treatment. The material has the capability of working at low voltages, and also it can operate at the temperature more than 3000ºC 29  Piezoelectric Effect EMF develops because of the displacement of the charges. The effect is changeable, i.e. if the varying potential applies to a piezoelectric transducer, it will change the dimension of the material or deform it. This effect is known as the piezoelectric effect. The pressure is applied to the crystals with the help of the force summing devices for examples the stress is applied through mechanical pressure gauges and pressure sensors, etc. The deformation induces the EMF which determines the value of applied pressure. 30 Piezoelectric Effect 31  Piezoelectric Effect g - voltage sensitivity of the crystals 32 Piezoelectric Effect 33  Piezoelectric Effect Properties of Piezo Electric-Crystal The piezoelectric material has high stability. It is available in various shapes and sizes. The piezoelectric material has output insensitive to temperature and humidity. Uses of Piezoelectric Crystal The piezoelectric material has high stability and hence it is used for stabilizing the electronic oscillator. The ultrasonic generators use the piezoelectric material. This generator is used in SONAR for underwater detection and in industrials apparatus for cleaning. It is used in microphones and speakers for converting the electric signal into sound. The piezoelectric material is used in electric lighter. 34  L V D Ts Linear Variable Differential Transformer Transformer: AC Input / AC Output Differential: Natural Null Point in Middle Variable: Movable Core, Fixed Coil Linear: Measures Linear Position  Linear Displacement - Inductive Methods (Linear Variable Differential Transformers LVDTs) LVDTs are accurate transducers which are often used in industrial and scientific applications to measure very small displacements 36  Linear Displacement - Inductive Methods (Linear Variable Differential Transformers LVDTs) An LVDT consists of a central primary coil wound over the whole length of the transducer and two outer secondary coils A magnetic core can move freely through the coil 37  Linear Displacement - Inductive Methods (Linear Variable Differential Transformers LVDTs) The primary windings are energized with a constant amplitude AC signal (1 – 10kHz) This produces an alternating magnetic field which induces a signal into the secondary windings The strength of the signal is dependent on the position of the core in the coils When the core is placed in the center of the coil the output will be zero Moving the coil in either direction causes the signal to increase The output signal is proportional to the displacement 38  Linear Displacement - Inductive Methods (Linear Variable Differential Transformers LVDTs) Vo=V1-V2 V1 V2 -x LVDTs are devices to measure displacement by modifying spatial distribution of an alternating magnetic field. V1 > V2 Vi Vi Vo Oscillating excitation voltage-50 Hz to 25 kHz  Linear Displacement - Inductive Methods (Linear Variable Differential Transformers LVDTs) Vo=V1-V2 V1 V2 X=0 V2 = V1 Vi Vo Vi  Linear Displacement - Inductive Methods (Linear Variable Differential Transformers LVDTs) Vo=V1-V2 V1 V2 +x V2 > V1 Vi Vi Vo So, the direction of displacement can be determined from the relative phase of the signal. Linear Displacement - Inductive Methods (Linear Variable Differential Transformers LVDTs) 42  Linear Displacement - Inductive Methods (Linear Variable Differential Transformers LVDTs) 43  LVDT output This 180 degree phase shift can be used to determine the direction of the core from the null point by means of appropriate circuitry. As this diagram shows, the polarity of the output signal represents the core's positional relationship to the null point.  LVDT output The diagram shows also that the output of an LVDT is very linear over its specified range of core motion, but that the sensor can be used over an extended range with some reduction in output linearity.  LVDT output Gaurav's Farewell | Canva  Pros and Cons - LVDTs Range: ±2.5nm - ±10cm Advantages: Good resolution Disadvantages: Needs shielding to prevent interference from magnetic sources 47  Why LVDT? Friction-Free Operation One of the most important features of an LVDT is its friction-free operation. In normal use, there is no mechanical contact between the LVDT's core and its coil assembly. There is no rubbing, dragging, or other source of friction. This feature is particularly useful in materials testing, vibration displacement measurements, and high-resolution dimensional gaging systems.  Linear Displacement – Digital Methods It is also possible to measure displacement using digital methods As a binary system only uses 0’s and 1’s we can represent this using transparent and opaque areas on a glass scale or conducting and non conducting areas on a metal scale Each position will produce a unique code which represents a specific displacement 49  Photo resistive- LDR The resistance of the LDR changes as the amount of light (intensity) falling on it changes 50  Photo resistive- LDR A light-dependent resistor (LDR) is a special type of resistor. Its resistance changes as the intensity of the light falling on it changes. When the light levels are low, the resistance of a LDR is high. This is because an LDR is made of a semiconductor material where the outer electrons are bound weakly to the atoms. When bright light shines on an LDR, the resistance is much lower. The light energy is transferred to the outer electrons which can then break free from the atoms. They are then free to flow through the LDR. 51 Photo resistive- LDR 52 Photo resistive- LDR 53  Hall effect transducer The Hall effect is the production of a voltage difference (the Hall voltage) across a current carrying conductor (in presence of magnetic field), perpendicular to both current and the magnetic field. The hall effect element is a type of transducer used for measuring the magnetic field by converting it into an emf. The direct measurement of the magnetic field is not possible. The transducer converts the magnetic field into an electric quantity which is easily measured by the analogue and digital meters.  Hall effect transducer The Hall effect was discovered in 1879 by Edwin Herbert Hall while working on his doctoral degree at the Johns Hopkins University in Baltimore, Maryland, USA.  Hall effect transducer The principle of hall effect transducer is that if the current carrying strip of the conductor is placed in a transverse magnetic field, then the EMF develops on the edge of the conductor. The magnitude of the develop voltage depends on the density of flux, and this property of a conductor is called the Hall effect. The Hall effect element is mainly used for magnetic measurement and for sensing the current. The metal and the semiconductor has the property of hall effect which depends on the densities and the mobility of the electrons. Hall effect transducer  Hall effect transducer When the magnetic field is applied to the strip, the output voltage develops across the output leads 3 and 4. The develops voltage is directly proportional to the strength of the material. The output voltage is, hall-effect-equation-1 hall-effect-equation-2 The I is the current in ampere and the B is the flux densities in Wb/m2  Hall effect transducer The current and magnetic field strength both can be measured with the help of the output voltages. The hall effect EMF is very small in conductors because of which it is difficult to measure. But semiconductors like germanium produces large EMF which is easily measured by the moving coil instrument.  Hall Voltage Current  Hall effect transducer Precautions 1. Hall Voltage should be measured very carefully and accurately. 2. Distance between pole pieces of Electromagnet should not be changed during the whole experiment. 3. Current passing through semiconductor slab should be strictly within permissible limit.  Hall effect transducer Applications Hall effect devices produce a very low signal level and thus require amplification. In early 20th century vacuum tube amplifiers were expensive and unreliable. But with the development of the low cost integrated circuit the Hall effect sensor became suitable for mass application.  Hall effect transducer Current Sensor 250px-HallEffCurrentSense When electrons flow through a conductor, a magnetic field is produced. Thus, it is possible to create a non- contacting current sensor. This has several advantages: 1.No additional resistance (a shunt) can be inserted in the primary circuit. 2.Also, the voltage present on the line to be sensed is not transmitted to the sensor, which enhances the safety of measuring equipment. Hall effect current sensor with internal integrated circuit amplifier.  Hall effect transducer Electric Motor Control Some types of brushless DC electric motors use Hall effect sensors to detect the position of the rotor and feed that information to the motor controller. This allows for more precise motor control.  Hall effect transducer Magnetometer Smart phones like iPhone 3GS are equipped with magnetic compass. These compass measure Earth‘s magnetic field using 3-axis magnetometer. These magnetometer are sensors based on Hall Effect. These sensors produce a voltage proportional to the applied magnetic field and also sense polarity.  What is a Photovoltaic Cell? A photovoltaic cell is a type of PN junction diode which harnesses light energy into electricity. They generally work in a reverse bias condition. It is analogous to a solar cell since they belong to similar working principles  What is a Photovoltaic Cell? The cell consists of each a P-type and an N-type material and a PN junction diode sandwiched in between. This layer is responsible for trapping solar energy which converts into electricity. The N-type layer is also known as the first layer or the emitter layer. The P-type layer is the base layer and the intermediate layer between the two is the PN junction diode. The surface of the cell is covered by an anti-reflective material which traps the light energy and avoids any loss of energy. The bottom layer, the last one may completely be covered by the material in which the conductor is made up of.  What is a Photovoltaic Cell? A photovoltaic cell works on the same principle as that of the diode, which is to allow the flow of electric current to flow in a single direction and resist the reversal of the same current, i.e, causing only forward bias current. When light is incident on the surface of a cell, it consists of photons which are absorbed by the semiconductor and electron- hole pairs are liberated to produce an external DC supply. If the incident energy (hv) is greater than the energy gap of that semiconductor material, these electron-hole pairs are generated at the depletion region of a diode. When this photon from external radiation hits the diode, these electron-hole pairs disrupt the neutrality of the conductor. If an external current path has been provided, then the electrons flowing through the P-side travel towards the N-side, eventually generating a DC current and the magnitude of this electromotive force generated is directly proportional to the intensity of the incident radiation.  What is a Photovoltaic Cell? https://www.youtube.com/watch?v=X0OZ6tpZ3Mc  Thermoelectric transducer A thermocouple is made up of two dissimilar metals, joined together at one end, that produce a voltage (expressed in millivolts) with a change in temperature. The junction of the two metals, called the sensing junction, is connected to extension wires. Any two dissimilar metals may be used to make a thermocouple.  Thermoelectric transducer When two dissimilar metals are connected together, a small voltage called a thermo-junction voltage is generated at the junction. This is called the Peltier effect. If the temperature of the junction changes, it causes voltage to change, which can be measured by the input circuits of an electronic controller. The output is a voltage proportional to the temperature difference between the junction and the free ends. This is called the Thompson effect. Both of these effects can be combined to measure temperature. By holding one junction at a known temperature (reference junction) and measuring the voltage, the temperature at the sensing junction can be deduced. The voltage generated is directly proportional to the temperature difference. The combined effect is known as the thermo-junction effect or the Seebeck effect. Thermoelectric transducer  Thermoelectric transducer In practical operation, wires A and B are connected to a digital voltmeter (DVM), digital multimeter (DMM), digital data acquisition system, or some other voltage measuring device. If the measuring device has very high input impedance, the voltage produced by the thermo-junction can be measured accurately. However, the main problem with thermocouple temperature measurement is that wires A and B must connect to the leads of the voltmeter, which are generally made of copper. If neither wire A nor wire B is itself copper, connecting to the DVM creates two more thermo-junctions! (Thermocouple metals are typically not the same as those of the DVM leads.) These additional thermo-junctions also produce a thermo-junctive voltage, which can create an error when trying to measure the voltage from the sensing junction. Thermoelectric transducer  Thermoelectric transducer One simple solution is to add a thermo-junction, called a reference junction, by inserting an additional length of metal A wire into the circuit. The reference junction consists of metals A and B as indicated on the sketch.  Thermoelectric transducer With this arrangement, there are still two additional thermocouple junctions formed where the compensated thermocouple is connected to the voltmeter (DVM). The two junctions to the DVM are now both between metal A and copper. These two junctions are placed close together , and at the same temperature, so that their thermo-junction voltages are identical, and cancel each other out. Meanwhile, the new reference junction is placed in a location where the reference temperature TR is known accurately, typically in an ice-water bath with a fixed temperature of T R = 0°C. If the sensing junction is also at 0°C (Ts = 0 oC), the voltage generated by the sensing junction will be equal and opposite of that generated by the reference junction. Hence, Vo = 0 when Ts = 0°C. However, if the sensing junction temperature is not equal to TR, Vo will be non-zero. In summary, Vo is a unique function of the sensor temperature Ts and the two metals used for the thermocouple. Thus, for known reference temperature and known thermocouple wire materials, output voltage Vo can be used to measure temperature. This is the fundamental concept of thermocouple usage.  Thermoelectric transducer The most important factor to be considered when selecting a pair of materials is the “thermoelectric difference” between the two materials. A significant difference between the two materials will result in better thermocouple performance. Thermoelectric transducer Thermoelectric transducer