ELECTRONICS MEASUREMENT III SEMESTER ELECTRONICS ENGINEERING
Definition of a transducer Transducer is any device that converts energy in one form to another energy. The majority either convert electrical energy to mechanical displacement or convert some non-electrical physical quantity, such as temperature, sound or light to an electrical signal.
Functions of transducer 3 1. To sense the presence, magnitude, change in, and frequency of some measurand. 2. To provide an electrical output that, when appropriately processed and applied to readout device, gives accurate quantitative data about the measurand Transducer Electrical output Measurand Excitation Measurand – refers to the quantity, property or condition which the transducer translates to an electrical signal.
Classification of Transducers The Transducers are broadly classified as: 1) Mechanical and Electrical transducers. 2) Active and Passive transducers. 3) Analog and Digital transducers. 4) Primary and Secondary transducer. 5) Transducers and Inverse transducer.
Classification of Transducers 5 Transducers On The Basis of principle Used Active/PassivePrimary/SecondaryAnalogue/Digital Capacitive Inductive Resistive Transducers/ Inverse Transducers Transducers may be classified according to their application, method of energy conversion, nature of the output signal, and so on.
Mechanical Transducers-The mechanical transducer are transducer that respond to change in the physical condition of the system and gives output in other form. Electrical Transducers-An electrical transducer is a device that converts the non electrical quantity to an electrical quantity that is proportional to the input quantity. Active Transducer-These transducer do not need any external source of power for their operation. Therefore they are also called as self generating type transducer. Passive Transducer-These transducer need external power supply for their operation.
Analog Transducers-The output of these transducer is in the analog form that means it is a continuous function of time. Digital Transducers-The output of these transducer is in the digital form that means it is the form of digital pulses discrete in time. Primary Transducers-The mechanical device converts the physical quantity to be measured into a mechanical signal. Such mechanical device are called as the primary transducer. Secondary Transducers-The electrical device then converts this mechanical signal into a corresponding electrical signal such electrical device are known as the secondary transducer. Transducer and Inverse Transducer-Transducer convert non electrical quantity to electrical quantity while inverse transducer convert electrical to a non electrical quantity.
Forms of Capacitance Transducers Rotary plate capacitor Rectilinear Capacitance Transducer Thin diaphragm
The capacitance of this unit proportional to the amount of the fixed plate that is covered, that shaded by moving plate. This type of transducer will give sign proportional to curvilinear displacement or angular velocity. Rotary plate capacitor:
Advantages: 1.Has excellent frequency response 2.Can measure both static and dynamic phenomena. Disadvantages: 1.Sensitivity to temperature variations 2.the possibility of erratic or distortion signals owing to long lead length Applications: 1.As frequency modulator in RF oscillator 2.In capacitance microphone 3.Use the capacitance transducer in an ac bridge circuit
Inductive Transducer Ampere’s Law: flow of electric current will create a magnetic field Faraday’s Law: a magnetic field passing through an electric circuit will create a voltage i v + - v1v1 v2v2 + - + - N1N1 N2N2 29
Inductive Sensor (c) When core is at Advantage of LVDT You have wide of range of measuring
Self-inductance Mutual inductance Differential transformer n = number of turns of coil G = geometric form factor = effective magnetic permeability of the medium Ampere’s Law: flow of electric current will create a magnetic field Faraday’s Law: a magnetic field passing through an electric circuit will create a voltage Inductive Transducer 36
Strain gauge When external forces are applied to a stationary object, stress and strain are the result.
Strain gauge Strain is defined as the amount of deformation per unit length of an object when a load is applied. Strain (ε) = ΔL/L
Strain gauge Fundamentally, all strain gages are designed to convert mechanical motion into an electronic signal. A change in capacitance, inductance, or resistance is proportional to the strain experienced by the sensor.
48 Strain Gauge The strain gauge is an example of a passive transducer that uses electric resistance variation in wires to sense the strain produced by a force on wires. It is a very versatile detector and transducer for measuring weight, pressure, mechanical force, or displacement. The construction of a bonded strain gauge (see figure) shows a fine wire element looped back and forth on a mounting plate, which is usually cemented to the member undergoing stress. A tensile stress tends to elongate the wire and thereby increase its length and decrease its cross-sectional area.
49 The combined effect is an increase in resistance: Where, ρ: the specific resistance of the conductor material in ohm meters L : length of conductor (meters) A : area of conductor (m 2 ) As consequence of strain, 2 physical qualities are particular interest: 1)The change in gauge resistance 2)The change in length The relationship between these two variables called gauge factor, K, is expressed mathematically as
50 Where K= the gauge factor R=the initial resistance in ohms (without strain) ∆R= the change in initial resistance in ohms L= the initial length in meters (without strain) ∆L=the change in initial length in meters ∆L/L same unit with G, therefore gauge factor
51 From Hooke theory, stress, S, is defined as internal force/area. Where S= the stress in kilograms per square meter F= the force in kilograms A= area in square meters Then the modulus of elasticity of material E or called Young’s modulus (Hooke’s Law) is written as: Where, E= Young modules in kg per square meter S= the stress in kilograms per square meter G= the strain (no units)
52 Metallic strain gauge – formed from thin resistance wire or etched from thin sheets of metal foil. Wire gauge (small) – to minimum leakage – for high T applications Semiconductor strain gauge – high output transducers as load cells Strain gauge is generally used as one arm of bridge Types of Strain Gauge
Strain gauge Strain may be compressive or tensile and is typically measured by strain gages. It was Lord Kelvin who first reported in 1856 that metallic conductors subjected to mechanical strain exhibit a change in their electrical resistance. This phenomenon was first put to practical use in the 1930s.
Strain gauge If a wire is held under tension, it gets slightly longer and its cross-sectional area is reduced. This changes its resistance (R) in proportion to the strain sensitivity (S) of the wire's resistance. When a strain is introduced, the strain sensitivity, which is also called the GAGE FACTOR (GF), is given by: GF = (ΔR/R)/(ΔL/L)
Strain gauge The ideal strain gage would change resistance only due to the deformations of the surface to which the sensor is attached. However, in real applications, temperature, material properties, the adhesive that bonds the gage to the surface, and the stability of the metal all affect the detected resistance.
Strain gauge Because most materials do not have the same properties in all directions, a knowledge of the axial strain alone is insufficient for a complete analysis. Poisson, bending, and torsion strains also need to be measured. Each requires a different strain gage arrangement.
Strain gauge The deformation of an object can be measured by mechanical, optical, acoustical, pneumatic, and electrical means. The earliest strain gages were mechanical devices that measured strain by measuring the change in length and comparing it to the original length of the object.
Strain gauge The most widely used characteristic that varies in proportion to strain is electrical resistance. Although capacitance and inductance-based strain gages have been constructed, these devices' sensitivity to vibration, their mounting requirements, and circuit complexity have limited their application. The photoelectric gauge uses a light beam, two fine gratings, and a photocell detector to generate an electrical current that is proportional to strain. The gage length of these devices can be as short as 1/16 inch, but they are costly and delicate.
Strain gauge The first bonded, metallic wire-type strain gage was developed in 1938. The metallic foil-type strain gage consists of a grid of wire filament (a resistor) of approximately 0.001 in. (0.025 mm) thickness, bonded directly to the strained surface by a thin layer of epoxy resin
Application of Strain gauge Strain gages are used to measure displacement, force, load, pressure, torque or weight. Modern strain-gage transducers usually employ a grid of four strain elements electrically connected to form a Wheatstone bridge measuring circuit. The strain-gauge sensor is one of the most widely used means of load, weight, and force detection. As the force is applied, the support column experiences elastic deformation and changes the electrical resistance of each strain gage. By the use of a Wheatstone bridge, the value of the load can be measured. Load cells are popular weighing elements for tanks and silos and have proven accurate in many other weighing applications.
Application of Strain gauge Strain gages may be bonded to cantilever springs to measure the force of bending. The strain gages mounted on the top of the beam experience tension, while the strain gages on the bottom experience compression. The transducers are wired in a Wheatstone circuit and are used to determine the amount of force applied to the beam.
Application of Strain gauge Strain-gauge elements also are used widely in the design of industrial pressure transmitters. Using a bellows type pressure sensor in which the reference pressure is sealed inside the bellows on the right, while the other bellows is exposed to the process pressure. When there is a difference between the two pressures, the strain detector elements bonded to the cantilever beam measure the resulting compressive or tensile forces.
Application of Strain gauge A diaphragm-type pressure transducer is created when four strain gages are attached to a diaphragm. When the process pressure is applied to the diaphragm, the two central gage elements are subjected to tension, while the two gages at the edges are subjected to compression. The corresponding changes in resistance are a measure of the process pressure. When all of the strain gages are subjected to the same temperature, such as in this design, errors due to operating temperature variations are reduced.
RTDs RTDs are made of materials whose resistance changes in accordance with temperature Metals such as platinum, nickel and copper are commonly used. They exhibit a positive temperature coefficient. A commercial ThermoWorks RTD probe
Thermistors Thermistors are made from semiconductor material. Generally, they have a negative temperature coefficient (NTC), that is NTC thermistors are most commonly used. Ro is the resistance at a reference point (in the limit, absolute 0).
80 Detectors of wire resistance temperature common employ platinum, nickel or resistance wire elements, whose resistance variation with temperature has high intrinsic accuracy. They are available in many configurations and size and as shielded or open units for both immersion and surface applications. The relationship between temperature and resistance of conductors can be calculated from the equation: where R = the resistance of the conductor at temperature t ( 0 C) R 0 = the resistance at the reference temperature, usually 20 0 C α = the temperature coefficient of resistance ΔT = the difference between the operating and the reference temperature Resistance Temperature Detector (RTD)
Application In sound track reader due to fast response time. In switching element. Photo cell or photo voltaic cell. Detector of modulated light in optical communication system.
Piezoelectric Materials Many polymers, ceramics, and molecules such as water are permanently polarized: some parts of the molecule are positively charged, while other parts of the molecule are negatively charged.
Piezoelectric Materials When an electric field is applied to these materials, these polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material.
Piezoelectric Materials Furthermore, a permanently-polarized material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. These materials are piezoelectric, and this phenomenon is known as the piezoelectric effect.
Piezoelectric Materials Conversely, an applied electric field can cause a piezoelectric material to change dimensions. This phenomenon is known as electrostriction, or the reverse piezoelectric effect. Piezoelectric Effect Reverse Piezoelectric Effect
Piezoelectric Materials Piezoelectric materials are used in acoustic transducers, which convert acoustic (sound) waves into electric fields, and electric fields into acoustic waves. Transducers are found in telephones, stereo music systems, and musical instruments such as guitars and drums.
Piezoelectric Materials Quartz, a piezoelectric material, is often found in clocks and watches. An oscillating electric field makes the quartz crystal resonate at its natural frequency. The vibrations of this frequency are counted and are used to keep the clock or watch on time.
VoVo To find V o, assume system acts like a capacitor (with infinite leak resistance): Capacitor: (typically pC/N, a material property) k for Quartz = 2.3 pC/N k for barium titanate = 140 pC/N For piezoelectric sensor of 1-cm 2 area and 1-mm thickness with an applied force due to a 10-g weight, the output voltage v is 0.23 mV for quartz crystal 14 mV for barium titanate crystal.
Measure physiological displacement and record heart sounds Certain materials generate a voltage when subjected to a mechanical strain, or undergo a change in physical dimensions under an applied voltage. Uses of Piezoelectric External (body surface) and internal (intracardiac) phonocardiography Detection of Korotkoff sounds in blood-pressure measurements Measurements of physiological accelerations Provide an estimate of energy expenditure by measuring acceleration due to human movement. Piezoelectric Sensors 98
Models of Piezoelectric Sensors Piezoelectric polymeric films, such as polyvinylidence fluoride (PVDF). Used for uneven surface and for microphone and loudspeakers.
View piezoelectric crystal as a charge generator: R s : sensor leakage resistance C s : sensor capacitance C c : cable capacitance C a : amplifier input capacitance R a : amplifier input resistance RaRa Transfer Function of Piezoelectric Sensors
Convert charge generator to current generator: Current RaRa K s = K/C, sensitivity, V/m = RC, time constant RaRa Transfer Function of Piezoelectric Sensors
Voltage-output response of a piezoelectric sensor to a step displacement x. Decay due to the finite internal resistance of the PZT The decay and undershoot can be minimized by increasing the time constant =RC. 102
Example A piezoelectric sensor has C = 500 pF. Sensor leakage resistanse is 10 GΩ. The amplifier input impedance is 5 MΩ. What is the low corner frequency? 103
C = 500 pF R leak = 10 G R a = 5 M What is f c,low ? Current Example 104 If input impedance is increased 100 times: (R a = 500 M ) Then the f c,low :
Hall Effect Sensors Used to provide a noncontact means to detect and measure a magnetic field Named based on their use of the Hall Effect, discovered by Edwin Hall in 1879 http://farm1.static.flickr.com/62/227729006_fab88c1668.jpg?v=0 Hall Effect Sensor Sensing a Shaft Speed
How they work Presence of magnetic field deflects electrons flowing through a conductive material As electrons move to one end of a conductive material, a potential is developed in the direction perpendicular to gross current flow This potential indicates the strength of the magnetic field http://upload.wikimedia.org/wikipedia/commons/a/ab/Hall_ effect_A.png Depiction of the Hall Effect
Applications IC Engine Electronic Ignition Systems ◦ Used to determine position of cam shaft Brushless DC Motor Control ◦ Sensors determine position of permanent magnet rotor Assembly Lines ◦ Shaft position and velocity sensors ◦ Contactless limit switches Current Sensing ICs ◦ Electrically isolated alternative to shunt resistors
Pressure in open tank A container filled with a liquid has a pressure (due to the weight of the liquid) at a point in the liquid of: P = F/A P = W/A P = ρgV/A P = ρghA/A P = ρgh P = pressure F = force A = Area W = weight of the liquid V = volume above the Area g = gravitation ρ = mass density h = distance from the surface A h
Pressure Sensing Pressure is sensed by mechanical elements such as plates, shells, and tubes that are designed and constructed to deflect when pressure is applied. This is the basic mechanism converting pressure to physical movement. Next, this movement must be transduced to obtain an electrical or other output. Finally, signal conditioning may be needed, depending on the type of sensor and the application. Figure illustrates the three functional blocks. Pressure Signal Conditioner Sensing Element Transduction element displacement electric V or I output
Sensing Elements The main types of sensing elements are Bourdon tubes, diaphragms, capsules, and bellows All except diaphragms provide a fairly large displacement that is useful in mechanical gauges and for electrical sensors that require a significant movement
114 Bourdon-tube (oval cross-section) detector transducer stage Increased pressure causes movement of tube in this direction Sector Sector and pinion are modifying stage Pressure source Pointer and dial are indicator stage
Potentiometric Pressure Sensors Potentiometric pressure sensors use a Bourdon tube, capsule, or bellows to drive a wiper arm on a resistive element. For reliable operation the wiper must bear on the element with some force, which leads to repeatability and hysteresis errors. These devices are very low cost, however, and are used in low- performance applications such as dashboard oil pressure gauges
Inductive Pressure Sensors Inductive Pressure Sensors Several configurations based on varying inductance or inductive coupling are used in pressure sensors. They all require AC excitation of the coil(s) and, if a DC output is desired, subsequent demodulation and filtering. The LVDT types have a fairly low frequency response due to the necessity of driving the moving core of the differential transformer The LVDT uses the moving core to vary the inductive coupling between the transformer primary and secondary.
Capacitive Pressure Sensors. Capacitive pressure sensors typically use a thin diaphragm as one plate of a capacitor. Applied pressure causes the diaphragm to deflect and the capacitance to change. This change may or may not be linear and is typically on the order of several picofarads out of a total capacitance of 50-100 pF. The change in capacitance may be used to control the frequency of an oscillator or to vary the coupling of an AC signal through a network. The electronics for signal conditioning should be located close to the sensing element to prevent errors due to stray capacitance.
Piezoelectric Pressure Sensors. Piezoelectric elements are bi-directional transducers capable of converting stress into an electric potential and vice versa. One important factor to remember is that this is a dynamic effect, providing an output only when the input is changing. This means that these sensors can be used only for varying pressures. The piezoelectric element has a high-impedance output and care must be taken to avoid loading the output by the interface electronics. Some piezoelectric pressure sensors include an internal amplifier to provide an easy electrical interface.
Piezoelectric Pressure Sensors. Piezoelectric sensors convert stress into an electric potential and vice versa. Sensors based on this technology are used to measure varying pressures.
Strain Gauge Pressure Sensors Strain gauge sensors originally used a metal diaphragm with strain gauges bonded to it. the signal due to deformation of the material is small, on the order of 0.1% of the base resistance Semiconductor strain gauges are widely used, both bonded and integrated into a silicon diaphragm, because the response to applied stress is an order of magnitude larger than for a metallic strain gauge.
Strain Gauge Pressure Sensors When the crystal lattice structure of silicon is deformed by applied stress, the resistance changes. This is called the piezo-resistive effect. Following are some of the types of strain gauges used in pressure sensors. Deposited strain gauge. Metallic strain gauges can be formed on a diaphragm by means of thin film deposition. This construction minimizes the effects of repeatability and hysteresis that bonded strain gauges exhibit. These sensors exhibit the relatively low output of metallic strain gauges.
Strain Gauge Pressure Sensors Bonded semiconductor strain gauge. A silicon bar may be bonded to a diaphragm to form a sensor with relatively high output. Making the diaphragm from a chemically inert material allows this sensor to interface with a wide variety of media
Manometer Manometer A mercury manometer is a simple pressure standard and may be used for gauge, differential, and absolute measurements with a suitable reference. It is useful mainly for lower pressure work because the height