SENSORS AND TRANSDUCERS

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1 Electrical Measurements International Program Department of Electrical Engineering UNIVERSITAS INDONESIA ANDRITTO ABDUL GHAFFAR ANDHIKA ADIEL INSANI Lecturer : Ir. Chairul Hudaya, ST, M.Eng., Ph.D., IPM SENSORS AND TRANSDUCERS

2 INTRODUCTION SENSOR A device that measures a physical quantity and converts it into signal which can be read by an observer or by an instrument. For example, a mercury thermometer that converts measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube.

3 INTRODUCTION TRANSDUCERS Is a device usually electrical, electronic, electro-mechanical, electromagnetic, photonic or photovoltaic that converts one type of energy or physical to another (generally electrical or mechanical) for various measurement purposes including measurement or information transfer. For example, pressure censors.

4 ELECTRICAL TRANSDUCE RS

5 ELECTRICAL TRANSDUCER DEFINITION The transducers which convert one form energy to electrical energy for measurement purposes. The quantities which can t be measured directly such as pressure, temperature, displacement, fluid, humidity etc., are required to be sensed and changed into electrical signal first.

6 ELECTRICAL TRANSDUCER ADVANTAGES Power requirement is very low for controlling the electronic system. Amplifier may be used for amplifying the electrical signal. Less friction effect. Less mass-inertia effect, because the inertia effect of the signals is due to the mass of electron which can be neglected. The output can be indicated and recorded remotely from sensing element

7 ELECTRICAL TRANSDUCER BASIC REQUIREMENTS Linearity: The input vs output should be linear and balance. Ruggedness: Transducer should be capable to protect itself from overload. Repeatability: Transducers should reproduce the same output signal when the same input signal is applied again under unchanged environmental conditions, e.g. temperature, pressure, etc.

8 ELECTRICAL TRANSDUCER BASIC REQUIREMENTS (CONT) High reliability and stabilty: Transducers should give minimum error in measurement for various change in surroundings. High output signal quality: The ratio of the signal to the noise should be high and the amplitude of the output signal should be enough. No hysteresis: Transducers should not give any hysteresis during measurement while input signal is varied from its low value to high value and vice versa. Residual reformation: There should not be any deformation on removal of input signal after long period of use.

9 LINEAR VARIABLE DIFFERENTIAL TRANSFORMER

10 LINEAR VARIABLE DIFFERENTIAL TRANSFORMERS DEFINITION Linear Variable Differential Transformer (LVDT) is the inductive transducer that used to translate the linear motion into electrical signals.

LINEAR VARIABLE DIFFERENTIAL TRANSFORMERS CONSTRUCTION It consists of a single primary windinds and two secondary windings wound on a cylindrical former.

11 LINEAR VARIABLE DIFFERENTIAL TRANSFORMERS CONSTRUCTION It consists of a single primary windinds and two secondary windings wound on a cylindrical former. The two secondaries have equal number of turns and placed on either side of the primary winding. A sinusoidal voltage of amplitude 3 to 15 Volt and frequency 50 to 20K Hz is used to excite the primary. The primary winding is connected to an AC source. A movable soft iron core is placed inside the former. The displacement to be measured is applied to the arm attached to the soft iron core.

12 LINEAR VARIABLE DIFFERENTIAL TRANSFORMERS OPERATION

13 STRAIN GAUGES

14 STRAIN GAUGES BASICS The strain gauge is an electrical transducer; it is used to measure mechanical surface tension. Strain gauge can detect and convert force or small mechanical displacement into electrical signals. On the application of force a metal conductor is stretched or compressed, its resistance changes owing to the fact both length and diameter of conductor change. Also, there is a change on the value of resistivity of the conductor when it is strained and this property of the metal is called piezoresistive effect. Therefore, resistance strain gauges are also known as piezoresistive gauges. The strain gauges are used for measurement of strain and associated stress in experimental stress analysis. Secondly, many other detectors and transducers, for example the load cell, torque meter, flow meter, accelerometer employ strain gauge as a secondary transducer.

15 STRAIN GAUGES THEORY OF RESISTANCE STRAIN GAUGES The change in the value of resistance by the application of force can be explained by the normal dimensional changes of elastic material. If a positive strain occurs, its longitudinal dimension (x-direction) will increase while there will be a reduction in the lateral dimension (y-direction). The reverse happens for a negative strain. Since the resistance of a conductor is directly proportional to its length and inversely proportional to its crosssectional area, the resistance changes. The resistivity of a conductor is also changed when strained. This property is known as piezoresistive effect. Let us consider a strain gauge made of circular wire. The wire has the dimensions: length L, area A, diameter D before being strained. The material of the wire has a resistivity ρ

16 STRAIN GAUGES

17 STRAIN GAUGES

18 STRAIN GAUGES

19 ELECTROMAGN ETIC FLOW METER

20 ELECTROMAGNETIC FLOW METER BASICS Unlike many other types of flow meters, they offer true non-invasive measurements. They are easy to install and use to the extent that existing pipes in a process can be turned into meters simply by adding external electrodes and suitable magnets. They can measure reverse flows and are insensitive to viscosity, density, and flow disturbances. Electromagnetic flow meters can rapidly respond to flow changes and they are linear devices for a wide range of measurements. In recent years, technological refinements have resulted in much more economical, accurate, and smaller instruments than the previous versions.

21 ELECTROMAGNETIC FLOW METER BASICS (CONT) As in the case of many electric devices, the underlying principle of the electromagnetic flow meter is Faraday s law of electromagnetic induction. The induced voltages in an electromagnetic flow meter are linearly proportional to the mean velocity of liquids or to the volumetric flow rates. As is the case in many applications, if the pipe walls are made from nonconducting elements, then the induced voltage is independent of the properties of the fluid.

22 ELECTROMAGNETIC FLOW METER

23 ELECTROMAGNETIC FLOW METER FARADAY S LAW OF INDUCTION

25 ELECTROMAGNETIC FLOW METER

26 ELECTROMAGNETIC FLOW METER NOISE SOURCES Stray voltage in the process liquid Capacitive coupling between signal and power circuits Capacitive coupling in connection leads Electromechanical emf induced in the electrodes and the process fluid Inductive coupling of the magnets within the flow meter

27 ELECTROMAGNETIC FLOW METER ADVANTAGES OF ELECTROMAGNETIC FLOW METER The electromagnetic flow meter can measure flow in pipes of any size provided a powerful magnetic field can be produced. The output (voltage) is linearly proportional to the input (flow). The major advantage of electromagnetic flow meter is that there is no obstacle to the flow path which may reduce the pressure. The output is not affected by changes in characteristics of liquid such as viscosity, pressure and temperature.

28 ELECTROMAGNETIC FLOW METER LIMITATIONS The operating cost is very high in an electromagnetic flow meter, particularly if heavy slurries (solid particle in water) are handled. The conductivity of the liquid being metered should not be less than 10 µω/m. It will be found that most aqueous solutions are adequately conductive while a majority of hydrocarbon solutions are not sufficiently conductive.

29 TEMPERATURE TRANSDUCTERS

30 TEMPERATURE TRANSDUCERS DEFINITION Application of heat or its withdrawal from a body produces various primary effects on this body such as: Change in its physical or chemical state such as phase transition Change in its physical dimensions Variations in its electrical properties Generation of an emf at the junction of two dissimilar metals Change in the intensity of the emitted radiation Any of these effects can be employed to measure the temperature of a body, though the first one is generally used for standardisation of temperature sensors rather than for direct measurement of temperature.

31 TEMPERATURE TRANSDUCERS - RESISTANCE THERMOMETER RESISTANCE THERMOMETER Resistance temperature detectors, or resistance thermometers, employ a sensitive element of extremely pure platinum, copper or nickel wire that provides a definite resistance value at each temperature within its range. The relationship between temperature and resistance of conductors in the temperature range near 0 C can be calculated from the equation where Rt = resistance of the conductor at temperature t ( C) Rref = resistance at the reference temperature, usually 0 C α = temperature coefficient of resistance Δt = difference between operating and reference temperature

33 TEMPERATURE TRANSDUCERS - RESISTANCE THERMOMETER

34 TEMPERATURE TRANSDUCERS - THERMOCOUPLE THERMOCOUPLE

35 TEMPERATURE TRANSDUCERS - THERMOCOUPLE ERRORS DURING THE MEASUREMENT Open Junction: There are many sources of an open junction. Usually, the error introduced by an open junction is of such an extreme magnitude that an open junction is easily spotted. By simply measuring the resistance of the thermocouple, the open junction is easily identified. Insulation Degradation: The thermocouple is often used at very high temperatures. In some cases, the insulation can break down and causes a significant leakage resistance which will cause an error in the measurement of the Seeback voltage. In addition, chemicals in the insulation can defuse into the thermocouple wire and cause decalibration. Thermal Conduction: The thermocouple wire will shunt heat energy away from the source to be measured. For small temperature to be measured, small diameter thermocouple wire could be used. However, the small diameter wire is more susceptible to the effects. If a reasonable compromise between the degrading effects of small thermocouple wire and the loss of thermal energy and the resultant temperature error cannot be found, thermocouple extension wire can be used. This allows the thermocouple to be made of small diameter wire, while the extension wire covers majority of the connecting distance.

36 TEMPERATURE TRANSDUCERS - THERMOCOUPLE ERRORS DURING THE MEASUREMENT Galvanic Action: Chemicals coming in contact with the thermocouple wire can cause a galvanic action. This resultant voltage can be as much as 100 times the Seebeck voltage, causing extreme errors. Decalibration: This error is a potentially serious fault, as it can cause slight error that may escape detection. Decalibration is due to altering the characteristics of the thermocouple wire, thus changing the Seeback voltage. This can be caused due to subjecting the wire to excessively high temperatures, diffusion of particles from the atmosphere into the wire, or by cold working the wire.

37 TEMPERATURE TRANSDUCERS - THERMISTOR THERMISTOR

38 TEMPERATURE TRANSDUCERS - THERMISTORS THERMISTORS

39 TEMPERATURE TRANSDUCERS - THERMISTORS

40 TEMPERATURE TRANSDUCERS - THERMISTORS THERMISTORS

41 PRESSURE MEASUREM ENT

42 PRESSURE MEASUREMENT DEFINITION The pressure, or force, measurement can be done by converting the applied pressure or force into a displacement by elastic elements which acts as a primary transducer. The displacement of the elastic element which is a function of the applied force may be measured by the transducer which acts as a secondary transducer. The output of the secondary transducer is a function of the displacement, which in turn is a function of pressure or force which is the measurand. Some mechanical methods are used to convert the applied pressure of force into displacement. These mechanical devices are called force summing devices.

43 PRESSURE MEASUREMENT FORCE SUMMING DEVICE The most commonly used summing devices are Flat or corrugated diaphragms Pivot torque Straight tube Single or double mass cantilever suspension Circular or twisted Bourdon tube Bellows

44 PRESSURE MEASUREMENT SECONDARY TRANSDUCERS The displacement produced by the action of the force summing devices is converted into a change of some electrical parameter. The various transducers used for this purpose are of the following types: Resistive Inductive Differential transformer Capacitive Photo-electric Piezo-electric Ionization Oscillation.

45 PRESSURE MEASUREMENT TYPES OF SUMMING DEVICES

46 PRESSURE MEASUREMENT - RESISTIVE TRANSDUCER RESISTIVE TRANSDUCER The electrical strain gauges attached to a diaphragm may be used for measurement of pressure. The diagram is shown in Figure The output of these strain gauges is a function of the applied strain, which in turn, is a function of the diaphragm deflection and the differential pressure. The deflection generally follows a linear variation with differential pressure P = P2 P1 (when the deflection is less than 1/3 of the diaphragm thickness). One of the disadvantages of this method is small physical area is required for mounting the strain gauges. Change in resistance of strain gauges on account of application of pressure is calibrated in terms of the differential pressure. Gauges of this type are made in sizes having a lower range of 100 kn/m2 to 3 MN/m2 to an upper range of 100 kn/m2 to 100 MN/m2

47 PRESSURE MEASUREMENT - RESISTIVE TRANSDUCER RESISTIVE TRANSDUCER

48 PRESSURE MEASUREMENT - INDUCTIVE TRANSDUCER INDUCTIVE TRANSDUCER This type of transducers has been successfully used as secondary transducers along with a diaphragm for measurement of pressure. Figure shows an arrangement which uses two coils; an upper and a lower coil which form the two arms of an ac bridge. The coils have equal number of turns. The other two arms of the bridge are formed by two equal resistances each of value R. The diaphragm is symmetrically placed with respect to the coils and so when P1 = P2, The reluctances of the path of magnetic flux for both the coils are equal and hence the inductances of the coils are equal.

49 PRESSURE MEASUREMENT - INDUCTIVE TRANSDUCER INDUCTIVE TRANSDUCER

50 PRESSURE MEASUREMENT - DIFFERENTIAL TRANSDUCER DIFFERENTIAL TRANSDUCER The linear variable differential transformers (LVDT) is used as a secondary transducer for measuring the pressure with bellows or bourdon tube acting as a primary transducers, i.e., as a force summing device. The pressure is converted into displacement which is sensed by the LVDT and transformed into a voltage. The two arrangements are shown in Figure and

51 PRESSURE MEASUREMENT - DIFFERENTIAL TRANSDUCER DIFFERENTIAL TRANSDUCER

52 PRESSURE MEASUREMENT - CAPACITIVE TRANSDUCER CAPACITIVE TRANSDUCER In this type of transducers, a linear characteristics can be achieved by using a differential arrangement for the capacitive displacement transducers. The arrangement using three plates is shown in Figure P1 and P2 are fixed plates and M is the movable plate to which the displacement to be measured is applied. Thus, two capacitors are there whose differential output is taken.

53 PRESSURE MEASUREMENT - CAPACITIVE TRANSDUCER CAPACITIVE TRANSDUCER

54 PRESSURE MEASUREMENT - CAPACITIVE TRANSDUCER CAPACITIVE TRANSDUCER

55 PRESSURE MEASUREMENT - CAPACITIVE TRANSDUCER CAPACITIVE TRANSDUCER

56 PRESSURE MEASUREMENT - PHOTOELECTRIC TRANSDUCER PHOTOELECTRIC TRANSDUCER

57 PRESSURE MEASUREMENT - PIEZO-ELECTRIC TRANSDUCER PIEZO-ELECTRIC TRANSDUCER When piezoelectric crystals are under the influence of some external force or pressure, they produce an emf. The force or displacement or pressure to be measured is applied to the crystal. The pressure is applied to the crystal through a force summing device. This causes a deformation which produces an emf which is a function of the deformation. This output emf may be measured to know the value of applied force and hence the pressure.

58 PRESSURE MEASUREMENT - PIEZO-ELECTRIC TRANSDUCER PIEZO-ELECTRIC TRANSDUCER

59 PRESSURE MEASUREMENT - IONISATION TRANSDUCERS IONISATION TRANSDUCER Ionisation is the process of removing an electron from an atom producing a free electron and a positively charged ion. Ionisation may be produced by the collision of a high speed electron from the atom. Figure shows the essential features of an ionisation-type gauge. Electrons are emitted from heated cathode using a filament and are accelerated towards the grid, which is positively charged. Some of the electrons are captured by the grid, producing grid current IG. Electrons having high kinetic energy pass through and cause ionization of gas atoms.

60 PRESSURE MEASUREMENT - IONISATION TRANSDUCERS IONISATION TRANSDUCER

61 PRESSURE MEASUREMENT - IONISATION TRANSDUCERS IONISATION TRANSDUCER

62 PRESSURE MEASUREMENT - OSCILLATION TRANSDUCERS OSCILLATION TRANSDUCERS These types of transducers use a force summing device to change the capacitance, C, or inductance, L, of an LC oscillation circuit. Figure shows the basic elements of LC transistor oscillator whose output frequency is affected by a change in the inductance of a coil. The change in inductance is caused by the force summing device acting upon an inductive device. The output of oscillator is a modulated output and can be demodulated and calibrated in terms of the pressure or force applied

63 PRESSURE MEASUREMENT - OSCILLATION TRANSDUCERS OSCILLATION TRANSDUCERS

64 IT S CONCLUSIO TIME!

65 CONCLUSIONS BASICS Sensor: A device that measures a physical quantity and converts it into signal which can be read by an observer or by an instrument. Transducer: Is a device that converts one type of energy or physical to another (generally electrical or mechanical) for various measurement purposes.

66 CONCLUSION TYPES OF TRANSDUCERS Linear Variable Differential Transformer (LVDT): uses displacement for measurement by excitation, resulting differential output Strain Gauges: detect and convert force or small mechanical displacement into electrical signals Electromagnetic Flow Meter: uses Faraday s Law of electromagnetic induction, and measure a flow using inductions of coils and electrodes

67 CONCLUSION TYPES OF TRANSDUCERS Temperature Transducers: Resistance Thermometers: employ a sensitive element of extremely pure platinum, copper or nickel wire that provides a definite resistance value at each temperature Thermocouple: consisting of two dissimilar metals joined together, is called a thermocouple and the voltage is called the Seebeck voltage Thermistors: semiconductors which behave as resistors with a high negative temperature coefficient of resistance

68 CONCLUSION TYPES OF TRANSDUCERS Pressure measurement: Resistive Transducers Inductive Transducers Capacitive Transducers Differential Transformers Photoelectric Transducers Piezoelectric Transducers Ionisation Transducers

69 REFERENCES VIDEO REFERENCES LVDT - Electrical Resistance Thermometer - Thermocouple - Electromagnetic Flow Meter - Photoelectric - Thermistor -

70 THAT S A WRAP! THANK YOU FOR LISTENING

Part 2. Sensor and Transducer Instrument Selection Criteria (3 Hour)

Part 2. Sensor and Transducer Instrument Selection Criteria (3 Hour) Part 2 Sensor and Transducer Instrument Selection Criteria (3 Hour) At the end of this chapter, you should be able to: Describe the definition of sensor and transducer Determine the specification of control