Module 4 - Industrial Instrumentation and Automation

H330 Automotive Engineering MODULE 4 APPLICATIONS OF TRANSDUCERS DISPLACEMENT MEASUREMENT: RESISTANCE POTENTIOMETER, CAPACITIVE & INDUCTIVE CAPACITIVE DIFFERENTIAL PRESSURE MEASUREMENT, TORSIONAL, SHEARING STRESS AND ROTATING SHAFT TORQUE MEASUREMENT USING STRAIN GAUGE. FLOW MEASUREMENT: HOTWIRE ANEMOMETER, CONSTANT RESISTANCE CONSTANT CURRENT TYPE EDDY CURRENT SENSORS, VARIABLE RELUCTANCE TACHOMETERS PHASE MEASUREMENT: ANALOG AND DIGITAL PHASE DETECTORS NANO INSTRUMENTATION DISPLACEMENT MEASUREMENT 1. Resistance potentiometer - A resistance potentiometer is a resistive transducer - A transducer whose resistance varies with a change in the physical parameter to be measured is known as resistive transducer - The change in resistance is measured using AC/DC measuring devices - The resistive transducers are used for measuring physical quantities like temperature, displacement, vibration etc. - The resistive transducer can work as the primary as well as secondary transducer. - For temperature and displacement measurement, it can act as a primary transducer - For pressure measurement, it will act as a secondary transducer. The primary transducer changes the physical quantity into a mechanical signal & secondary transducer transforms it into an electrical signal. Construction - Figure shows a simple mechanical picture of a potentiometric displacement sensor - There is a resistance wire wound around a form, making a wire wound resistor with fixed resistance R between its endpoints 1 & 2. 2 - A wiper assembly is connected in such a way that motion of an arm causes the wiper to slide across the wire wound turns of the fixed resistor - An electrical connection is made to this wiper - Therefore as the arm moves back & forth, the resistance between the wiper connections, 3 and either fixed resistor connection will change in proportion to the motion Working - A potentiometric sensor is simply a 3 terminal variable resistor - The resolution of the sensor is limited to the distance between individual turns of wire - During the sensing operation, a voltage Vs is applied across the resistive element. A voltage divider circuit is formed when slider comes in contact with the wire - The output voltage V0 is measured - The output voltage is proportional to the displacement of the slider over the wire 3 - Then the output parameter displacement is calibrated against the output voltage V 0 V 0 = I RA But, I = VS/(RA+RB) Therefore, V0 = VSRA/(RA+RB) - We know, resistance R=ρL/A, where ρ is the resistivity of the material, L is the length of the resistor & A is the area of cross section Then, V0 = VSLA/(LA+LB) - There are 2 types of potentiometers, linear and rotary potentiometer Advantages of resistive transducer - Both AC or DC voltage or current is appropriate for the measurement of variable resistance - It gives fast response - It is available in various sizes and ranges Disadvantages - Since the wiper is sliding over the resistance wire, there is a possibility of friction and wear. Hence the number of operating cycle is limited - Limited bandwidth 4 - Inertial loading 2. Capacitive displacement measurement - Capacitive transducers are used for measuring displacement, pressure and other physical quantities - It is a passive transducer which require external power for operation - It works on the principle of variable capacitances - The capacitive transducer consist of 2 parallel plates. These plates are separated by the dielectric medium which is either air, material, gas or liquid. - In a normal capacitor, the plates are fixed, but in capacitive transducer, the plates are movable. Principle of operation - The capacitance between the plates of a capacitor is given by C = εA/d where, A = overlapping area of plates in m2 d = distance between plates in m ε = permittivity of the medium - From the above equation, it is clear that, the capacitance can be varied by varying a) the overlapping area of plates b) distance between the plates c) dielectric medium EEE MITS 5 - Capacitive transducer is mainly used for measurement of linear displacement - In displacement, force and pressure measurement, with variation in the physical quantity, the distance between/overlapping area of the plates is varied. - In measurement of liquid/gas level, the dielectric constant is varied to get a change in capacitance value. Capacitive transducer using the change in the area of the plates - This type of transducer can be used for sensing both linear and rotary motion - Here one of the plate of the capacitor is kept stationary and the other plate is kept movable. - The object whose displacement is to be measured is attached to the movable plate. - The arrangements are shown in figure 6 - When the object moves, the plate also moves resulting in a change in overlapping area of plates. - As a result, the capacitance value changes, which can be measured with the help of an AC bridge circuit Transducer using the change in distance between the plates - Capacitance is inversely proportional the distance between the plates. - The one plate of the transducer is kept fixed and other movable - The displacement to be measured is linked to the movable plates - The arrangements are shown in figure - When the object moves, the plate also moves resulting in a change in distance between plates. - As a result, the capacitance value changes, which can be measured with the help of an AC bridge circuit 7 Advantages of capacitive transducer - It is very sensitive - It gives good frequency response - It has high input impedance hence they have a small loading effect - It require less power for operation Disadvantages - The metallic parts require insulation - The frame of capacitor need to be earthed for reducing the effect of stray magnetic field - Sometimes it shown a non linear behaviour - The cable connecting the transducer causes an error 8 3. Inductive displacement measurement - Inductive transducers work on the principle of inductance change due to any appreciable change in the quantity to be measured - For example, LVDT, a kind of inductive transducers, measures displacement in terms of voltage difference between its two secondary voltages Principle of operation - The inductive transducers use one of the following principles for its working. a) Change of self inductance b) Change of mutual inductance c) Production of eddy current a) By change of self inductance of inductive transducer - The self inductance of a coil is given by, L = N2/S where, N = number of turns, S = reluctance of magnetic circuit, S = l/(Aμ) where, l = length of magnetic circuit, A = cross sectional area of magnetic circuit, μ = permeability of material of the core 9 - i.e, we can vary self inductance by varying number of turns or by varying geometrical shape of magnetic circuit (l & A) or by varying the permeability of magnetic circuit - For measuring the displacement using an inductive transducers, it should change any of the above parameter for causing a change in self inductance. b) Change of mutual inductance - Here transducers work on change of mutual inductance principle - Mutual inductance between these two coils is given by, M = K √(L1L2) where, K = coefficient of coupling, L1 & L2 are self inductance of 2 coils - Thus mutual inductance can be changed by varying self inductance or by varying coefficient of coupling, K. - The methods of changing self inductance we already discussed. EEE MITS 10 - Now coefficient of coupling depends on the distance and orientation between two coils. - Thus for the measurement of displacement we can fix one coil and make other movable which moves with the object whose displacement is to be measured. - With the change in distance in displacement coefficient of coupling changes and it causes the change in mutual inductance. - This change in mutual inductance can be calibrated with the displacement and measurement can be done. c) Production of eddy current - When a conducting plate is placed near a coil carrying alternating current, a circulating current is induced in the plate called “EDDY CURRENT”. This principle is used in such type of inductive transducers. - When a coil is placed near to coil carrying alternating current, a circulating current is induced in it which in turn produces its own flux which try to reduce the flux of the coil carrying the current and hence inductance of the coil changes. - Nearer the plate is to the coil, higher will be eddy current and higher is the reduction in inductance and vice versa. - Thus inductance of coil varied with the variation of distance between coil and plate. - Thus the movement of the plate can be calibrated in terms of inductance change to 11 measure the quantity like displacement. Advantages of inductive transducer - The responsivity of this transducer is high - Load effects will be reduced. - Strong against ecological quantities Disadvantages of inductive transducer - The operating range will be reduced due to side effects. - The working temperature should be under the Curie temperature. - Sensitive to the magnetic field Capacitive differential pressure measurement - The capacitive pressure measurement involves sensing the change in capacitance that results from the movement of a diaphragm due to pressure variation - The sensing element is a taut metal diaphragm located equidistant between two stationary metal surfaces, comprising three plates for a complementary pair of capacitors. - An electrically insulating fill fluid (usually a liquid silicone compound) transfers motion from the isolating diaphragms to the sensing diaphragm, and also doubles as an effective 12 dielectric for the two capacitors. - The arrangement is shown in figure. - Here, one stationary plate and sensing diaphragm act as one capacitor and second stationary plate and the sensing diaphragm act as second capacitor - Any difference of pressure across the cell causes the diaphragm to flex in the direction of least pressure. - The sensing diaphragm is a precision-manufactured spring element. - The capacitance of a capacitor is given by C = (εA/d), where A is the overlapping area of two plates of capacitor and d is the distance between plates - If a pressure difference occur as shown in figure, capacitance on the low-pressure side will increase while capacitance on the highpressure side will decrease since capacitance between conductors is inversely proportional to the distance separating them. - The variation in capacitance can be measured by using any AC bridge arrangement. 13 Torsional and Shearing Stress Measurement of Axis - When an object is twisted, shearing stress τ occurs. - At the same time, tensile stress and compressive stress, which are equivalent to the shearing stress, occur in 2 directions inclined by 45° from the axial line. - In measurement of axial twist under simple shearing stress condition, a strain gauge does not directly measure the shearing stress. - Instead, a strain gauge detects tensile or compressive strain resulting from tensile or compressive stress simultaneously generated with the shearing stress. - These stress conditions on a surface of axis are illustrated below. 14 - The shearing stress γ is defined as illustrated below, and the magnitude is calculated through the following equation: γ = τ/G where, G = shearing modulus, τ = shearing stress - When the axis is twisted, point A moves to point B, thereby initiating torsional angle θ. θ = 2lγ/d Stress Measurement with Quarter-bridge System - Bond the strain gage on the twisted axis in the direction inclined by 45° from the axial line. - The relation between strain ε and stress σ are expressed with the following equation to calculate tensile or compressive stress σ = (ε0 E)/(1+γ) where, ε0 = indicated strain E = Youngs modulus γ = Poisson's ratio - Stress σ and shearing stress τ are equal in magnitude, and thus τ = σ 15 Rotating shaft torque measurement using strain gauge - A strain gauge converts torque into an electrical signal. - The sensor is bonded to a rotating shaft that deforms when a torque is applied. - Generally, four strain gauges are attached to the rotating shaft in the form of a Wheatstone bridge circuit. - They are placed precisely at 45 degrees with the shaft axis with gauge 1 and 3 being diametrically opposite, as must gauge 2 and 4. - When torque is applied to the shaft, the shaft gets twisted to the direction of rotation, thereby producing shear strain. - This causes elongation in gauges 1 and 3 and compression in gauges 2 and 4. - These changes in the strain gauges lead to an increase in the circuit resistance due to tensile strain generated by one pair of gauges and a decrease in the circuit resistance due to the compression strain generated by the other pair. - This results in an unbalanced bridge, which produces an electrical output corresponding to the applied torque. - To transmit power or signal between the rotating strain gauge and the stationary signal receiver, slip rings are used. - Measuring torque on a rotating shaft using strain gauges is impactful only when the gauge is operated in a low-to-moderate rotating speed. - Higher rotating speeds will severely degrade their performance. - Also, in heavy-duty applications that deal with dust, dirt, extremes of temperatures, and vibration, the reliability and accuracy of torque output may get compromised. - Excessive vibrations can cause wear due to the limited lives of brushes and the rings which eventually lead to premature failure. - Similarly, high temperatures can have an impact on the electrical connections of slip rings and bushes and promote corrosion. 17 Flow measurement Hot wire anemometer - The hot wire anemometer is a device used for measuring the velocity and direction o flow of the fluid - This can be done by measuring the heat loss of the wire which is placed in the fluid stream - The wire is heated by electrical current - The hot wire when placed in the stream of the fluid, the heat is transferred from wire to fluid and hence the temperature of wire reduces - The resistance of wire measures the flow rate of the fluid - The hot wire anemometer is used as a research tool in fluid mechanics - It has 2 main parts, conducting wire and Wheatstone's bridge - The conducting wire is housed inside the ceramic body. - The wires are taken out from the ceramic body and connected to Wheatstone's bridge. - The Wheatstone's bridge measures the variation of resistance 18 Constant current method - In the constant current method, the anemometer is placed in the stream of the fluid whose flow rate needs to be measured - The current of constant magnitude is passed through the wire - The Wheatstone's bridge is also kept on the constant voltage - When the wire is kept in the stream of liquid, the heat is transferred from the wire to the fluid - The heat is directly proportional to the resistance of the wire - If heat reduces, that means the resistance of wire also reduces - Due to this, the galvanometer which was initially at zero position deflects and this deflection of the galvanometer becomes a measure of flow rate of the liquid Constant resistance method - In this arrangement, the wire is heated by the electric current - The hot wire when placed in fluid stream, the heat transfers from wire to the fluid EEE MITS 19 - Thus the temperature of the wire changes which also changes its resistance - The principle in this method is to maintain the temperature and resistance of the sensing wire at a constant level - Therefore, the current through the sensing wire is increased to bring the sensing wire to have its initial resistance and temperature - The electrical current required in bringing back the resistance and hence the temperature of the wire to its initial condition becomes a measure of flow rate of the fluid when calibrated. Measurement of flow rate of a fluid using hot wire anemometer - In hot wire anemometer, the heat transferred electrically to the wire which is placed in the fluid stream - The Wheatstone's bridge is used for measuring resistance of wire and hence its temperature - The temperature of the wire remains constant for measuring the heating current. Thus the bridge remains balanced 20 - The standard resistor connected in series with the heating wire - The current across the wire is determined by knowing the voltage drop across the resistor and the value of voltage drop is determined by the potentiometer Advantages of Hot wire anemometer - Cost is relatively cheap - Frequency response is high - Size: small measurement volume - Good spatial and temporal resolution - Turbulent flows can easily be measured with it - Multi component measurement: Simultaneous temperature measurement available with multi-sensor probe - Accuracy is as good as a Laser Doppler anemometer - Signal-to-noise ratio is low 21 Disadvantages - Liquid flows can contaminate the probe more easily - Probe breakage - Noise in the signal (Radio frequency signal noise, Circuitry noise) - Heat transfer problems Eddy current sensors - Eddy current sensors are non contact devices capable of high resolution measurement of the position and /or change of position of any conductive target - Eddy current sensors are also called inductive sensors, but generally eddy current sensors refers to precision displacement instruments and inductive sensors refers to inexpensive proximity switches 22 - Eddy currents are currents induced in a conductor due to the magnetic field produced by the active coil. - The conductor is placed in a changing magnetic field and the current is produced according to the change of magnetic field with time. - The amount of eddy current produced will be more if the field strength is greater. - When there is high field strength, the conductivity of the metal conductor increases, causing faster reversals of the field and hence more flow of eddy currents. - The circuit arrangement of an eddy current sensor is shown in figure 23 Working - The active coil is kept closer to the conducting material and both of them are placed inside a probe. - The compensating coil is kept further away from the conducting material. - The high frequency source acts as the bridge circuit and feeds the coil across the two capacitors. - The amount of eddy current produced becomes more as the distance between the conducting material and the active coil becomes less. - This causes a change in the impedance of the active coil and thus unbalances the bridge circuit. - The bridge circuit produces an output proportional to the amount of closeness between the conducting material and the active coil. - The output of the bridge circuit is given to a low pass filter (LPF) and then its dc output is calculated. - The high frequency allows a thin target to be used and also with this, the frequency response becomes good up to a target frequency 1/10th the supply frequency. - It should be noted that the diameter of the conducting material should be larger or at least same as that of a probe. - If not, the output is prone to reduce linearity. If shafts are used as conducting materials, they EEE MITS 24 should have a bigger diameter so that their curved surfaces effectively behaves as flat surfaces. Advantages of eddy current sensors Measurement of distance can be carried out even in rough or mixed environments. Cost-effective. The device is insensitive to material in the gap between the probe and the conductor. The device is less expensive and has higher frequency response than a capacitive transducer. Disadvantages • The result will be precise only if the gap between the probe and the conductor is small. • The device cannot be used for finding the position of non-conductive materials. Another way is to connect a thick conductor onto the non-conductive material. • There always occurs a non-linear relationship between the distance and impedance of the active coil of the device. This problem can be overcome only by calibrating the device at fixed intervals. • The device is highly temperature sensitive 25 Variable Reluctance Tachometer - The variable reluctance tachometer consists of a metallic toothed rotor mounted on the shaft whose speed is to be measured and a magnetic pickup is placed near the toothed rotor. - Magnetic pickup consists of a housing containing a small permanent magnet with a coil wound around it - When the rotor rotates, the reluctance of the gap between the pickup and the toothed rotor changes giving rise to an induced e.m.f. in the pickup coil. This output is in the form of pulses. - The frequency of the pulses of the induced voltage will depend upon the number of teeth and the speed of rotation. - Since the number of teeth is known, the speed of can be determined by measuring the of pulses with an electronic counter. rotation frequency - If number of teeth is ‘T’ and rotational speed is ‘n’ rps and number of pulses per second is ‘P’, then n= P/T rps = (P*60)/T rpm. - A typical rotor has 60 teeth. - Thus if a counter counts 60 pulses, it will directly display the speed in rpm. 26 This is the most common type of tachometer used today Advantages - It has Simple and rugged construction - Maintenance free - Easy to calibrate - Information can be easily transmitted. Phase Detectors - A phase detector or phase comparator is a frequency mixer, analog multiplier or logic circuit that generates a voltage signal which represents the difference in phase between two signal inputs. - It is an essential element of the phase-locked loop (PLL). - Detecting phase difference is very important in many applications, such as motor control, radar and telecommunication systems, servo mechanisms, and demodulators. - A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. 27 Analog Phase meter - The difference between 2 sinusoidal waves of same frequency is indicated by a simple analog DC voltmeter as shown in figure - The scheme shows that the 2 waves are sent through 2 channels, each consisting of i) an amplifier ii) a zero crossing detector and iii) a differentiator. The outputs from the 2 differentiators are used to trigger a bistable circuit - With a phase difference between 2 waves, the bistable is switched alternately by pulses received from 2 differentiators and the bistable output produces a series of pulses of width dependent on the difference of the phases of the signals in the 2 channels. - A voltmeter connected to the bistable output with an appropriate filter would indicate the phase difference when properly calibrated - The figure shows the waveforms and pulse outputs at appropriate points of the block diagram 28 Digital phase meter - A kind of digital phase meter also uses zero crossing phase detectors - The outputs from such detectors are processed by a digital processor to which the digital display unit may be attached - A frequency counter along with a standard frequency source can simultaneously give the value of the frequency as well. - A simplified scheme of such a phase frequency meter is shown in figure 29 NANO INSTRUMENTATION - Integration of mechanical components and electronic devices is known as mechatronics - Nano 10^(-9) 1nm - It consisting components of size 1𝞵m to 1nm - Fabricated by the new technology are termed as microelectromechanical systems(MEMS) - Several sophisticated instrumentation systems are necessary for checking dimensions of components during manufacture and later calibration and product quality checking. - Nanotechnology relies heavily on the effective measurements of the surface properties of components, during and after production, by means of highly sophisticated instruments. - Investigation of the surface properties demand different techniques as they depend on the material, its state, the on going reactions if any on the surface and the presence of ions and molecules chemical or biological in character. - The measurements are intended to establish the topographic features, shapes, edge sharpness, detection of defects, movement of atoms and structural characterization. - Selection and application of instruments require thorough understandings of the processes. - Nanotechnology is treated as the ultimate and is termed as enabling technology implying that all the faces of science and technology lend to their application in an interdepended fashion 30 Nano stylus instruments - A stylus is a pen shaped instrument with a round rubber piece that moves effortlessly across touchscreen devices. - It is an instrument used to navigate on a phone or tablet. - A stylus is used by placing it on the touchscreen surface. - Some styluses include a pen incorporating 2 writing instruments into 1 tool. - Conventional stylus methods involving contact with the surface under investigation using a sharp diamond stylus are seen to result in slight surface damage. - But it has become possible to produce stylus with tips of small as that of an atom, for use of nano stylus instruments and the techniques such as ion beam milling has enabled such tip production. - The instrument primarily consists of a sensor and scanning system, aided by a miniature interferometer for attaining the desired high resolution. - However, newer designs, employing components of zero thermal expansion, have come up for application , with better performance in all respects. EEE MIT S size as 31 Previous University Questions 1. Explain the concept of Nano instrumentation? 2. With the help of a diagram explain the working of an eddy current sensor? 3. Draw and explain the working of a capacitive differential pressure transducer. 4. Explain the measurement of torque using strain gauges. 5. Explain the principle-of operation of a variable reluctance tachometer. 6. Explain the measurement of flow using a hot wire anemometer? 32