EE404- Answerkey-MAY2019 PDF

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May 2019 EE404 question paper solution...

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APJ ABDUL KALAM TECHNOLOGICAL UNIVERSITY EIGHTH SEMESTER B.TECH DEGREE EXAMINATION, MAY 2019 Course Code: EE404 Course Name: INDUSTRIAL INSTRUMENTATION AND AUTOMATION Max. Marks: 100 Duration: 3 Hours ANSWER KEY PART A 1. Explain the factors that govern the output characteristics of a transducer. Sol: Operating Principles Sensitivity Operating Range Cross sensitivity Loading Effect Stability & Reliability Environment Compatibility Insensitive to Unwanted Signals 2. Explain the concept of Nano instrumentation. Sol: It is one technology in which the exceptional role of instrumentation systems is unique, leading to the successful realization of man's dreams. The instruments may be grouped as 1, stylus type 2, optical type and 3 scanning electron beam. The ultra precise positioning systems make use of the piezoelectric transducers and capacitive sensors that are known to possess near-ideal or infinite resolution apart from other worthy characteristics. The positioning 'mechanism consists of several stages, known as nano-mechanisms using the above devices along with flexures for guidance and positioning with sub-nanometric accuracy. They are designed to work either as single axis or x-y stages and also to provide even six degrees of freedom, in certain cases. The positioning system is a closed-loop control :scheme, wherein the precision attainable is totally decided by the capabilities of the sensor which is invariably the capacitive type. The instrument mainly consists of a sensor and a scanning system aided by a miniature interferometer for attaining high resolution

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3. List any five important features of instrumentation amplifiers. Sol: Very high input impedance, High CMRR, High and adjustable Voltage gain, high Slew rate, good sensitivity, low output impedance 4. What are the advantages and disadvantages of MEMS? Sol: Advantages Minimize energy and materials. Improved reproducibility. Improved sensitivity, accuracy and reliability Low cost and low power. Easier to alter the parts of a device as compared to its macro counterpart Disadvantages Farm establishment requires huge investments. Polysilicon is a brittle material. Design includes very much complex procedures. 5. Explain the selection criterion for control valves? Sol: Noise and vibration Flow direction Velocity of fluid Process temperature Speed of operation Leakages Cost Life span

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6. Define an industrial automation system and enlist its components. Sol: Industrial automation is the use of control systems, such as computers or robots, and information technologies for handling different processes and machineries in an industry to replace a human being Components 1) Sensing Elements

2) Actuating Elements

3) Final control element A final control element is defined as a mechanical device that physically changes a process in response to a change in the control system set-point. Final control elements relevant to actuators include valves, dampers, fluid couplings, gates, and burner tilts to name a few. Final control elements are an essential part of process control systems, allowing an operator to achieve a desired process variable output by manipulating a process variable set-point. 7. Compare programmable logic controller with personal computer. Sol PLC 1. Inputs are sequentially scanned and required outputs are updated 2. Lesser complexity 3. PLCs are rugged 4. Available with I/O modules for diff. voltage levels PC 1. Can be interrupted using interrupt systems 2. Greater complexity 3. Not rugged in construction 4. Can be interfaced with external equipment

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8. What are the key features of DCS? Sol: Better and more consistent control Reduced installation and wiring costs Higher operating efficiency Increased process safety Flexibility Better process management Extendibility Greater centralization of control functions PART B 9a) Explain the factors influencing the choice of a transducer for an industrial instrumentation system Sol: 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. 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

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9b) Draw and explain second order sensor time response Sol: Time behaviour is described by a second-order differential equation A step change in the input causes the output to oscillate for a short period of time before settling down to a value that corresponds to the new input Output transient generated by the transducer is an error and must be accounted for in any measurement involving a transducer with this behaviour

Transducer can be said to track the input when the input changes in a time that is greater than the period represented by the natural frequency. The damping constant defines the time one must wait after a disturbance at, for the transducer output to be a true indication of the transducer input. 10a) With the help of a diagram explain the working of an eddy current sensor. Sol: Eddy current displacement sensors measure distances, displacements, or positions of any electrically-conductive target. The principle enables non-contact and wear free measurements. The measurement objects may have either ferromagnetic or nonferromagnetic properties

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

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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 1/10 th the supply frequency 10b) Draw and explain the working of a capacitive differential pressure transducer. Sol: 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 dielectric for the two capacitors

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, meaning that its displacement is a predictable function of applied force. The applied force in this case can only be a function of differential pressure acting against the surface area of the diaphragm in accordance with the standard force-pressure-area equation F = PA. In this case, we have two forces caused by two fluid pressures working against each other, so our force-pressure-area equation may be rewritten to describe resultant force as a function of differential pressure (P1 − P2) and diaphragm area: F = (P1 − P2)A. Since diaphragm area is constant, and force is predictably related to diaphragm displacement, all we need now in order to infer differential pressure is to accurately measure displacement of the diaphragm. The diaphragm’s secondary function as one plate of two capacitors provides a convenient method for measuring displacement. Since capacitance between conductors is inversely

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proportional to the distance separating them, capacitance on the low-pressure side will increase while capacitance on the high-pressure side will decrease. A capacitance detector circuit connected to this cell uses a high-frequency AC excitation signal to measure the different in capacitance between the two halves, translating that into a DC signal which ultimately becomes the signal output by the instrument representing pressure 11a) Draw the block diagram representation of a process control system and explain the functions of each block. Sol:

Process: Complex assembly of phenomena that relate to some manufacturing sequence Many variables may be involved It may be desirable to control all variables at the same time Single variable process (one Variable) Multi variable process (Multi Variable) Measurements: Conversion of Variables into corresponding analog variable (Pneumatic pressure, an Electric Voltage/ Current or digitally encoded signal) Sensor performs Initial measurements and energy conversion of a variable Signal conditioning may be required to complete measurement function Error Detector: Difference between actual and set point level Has both magnitude and polarity Error determination is made before any control action can be taken by the controller Controller: To examine the error and determination of what action should be taken Performed by microprocessor based computers Evaluation consists of determining action required to drive the controlled variable to the set point Control Element: Device that exerts a direct influence on the process 7

It accepts input from the controller, which is then transformed into some proportional operation It is also called Final control element For intermediate operation between controller output and the final control element Actuator is required It converts small energy signal of the controller into larger energy action on the process

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11b) Explain the measurement of torque using strain gauges. Sol: Torque transducers consists of a circular shaft and with strain gauges applied at 45deg. Operating Principle Two strain gauges are mounted on a shaft at an angle 450 𝑡𝑜 𝑒𝑎𝑐ℎ 𝑜𝑡ℎ𝑒𝑟 T=

𝜋𝐺(𝑅4 − 𝑟4) 2𝐿

𝞱 Nm

G-- Modulus of rigidity R-Outer radius of shaft L- Length of shaft 𝞱-angular deflection of shaft r-- inner radius of shaft The strain gauges attached at 450 to the axis of the shaft 𝑻𝑹 ∈ 𝟎= ± 𝟒𝟓 𝝅𝑮(𝑹𝟒 − 𝒓𝟒) TORSIONAL STRESS ,stress produced when we apply the twisting moment to the end of a shaft about its axis is known as Torsional stress. Torsion is the twisting of an object due to an applied torque

In this arrangement two strain gauges are subjected to tensile stresses while the other two experience compressive stresses The gauges must be precisely at 45 o with the shaft axis. Gauge 1 and must be diametrically opposite as must 3 and 4 It is fully temperature compensated It provides automatic compensation for bending and axial loads It gives the maximum sensitivity for a given torque Slip rings are used for the connection of the bridge to its power source and display arrangement. Slip rings are conducting rings attached to the shaft, but insulated from it, with one of the slip rings connected to each of the bridge terminals

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Rubbing contact is made between the rings and stationary brushes, which are connected to the input and output equipment 12 a) Explain the importance of signal conditioning in industrial instrumentation systems. Sol The purpose is to amplify and convert this signal into an easy to read and compatible form for data-acquisition or machine-control. A signal conditioner helps to provide precise measurements, which are essential for accurate data acquisition and machine-control Functions: Signal Conversion The main function of a signal conditioner is to pick up the signal and convert it into a higher level of electrical signal. Linearization Certain signal conditioners can perform linearization when the signals produced by a sensor do not have a straight-line relationship with the physical measurement. Amplifying Process of increasing the signal for processing or digitization. There are two ways that signal amplification can be performed; by increasing the resolution of the input signal, or by increasing the signal-to-noise ratio. Filtering The signal frequency spectrum is filtered to only include the valid data and block any noise. Evaluation and Smart-functions To provide additional benefits for the user and the process, modern signal conditioners have extra functions for signal evaluation and measurement data preprocessing. This helps to monitor and evaluate warning and alarms directly via an electrical switching output rapidly. Interfaces Signal converters have to transmit the sensor signals via standard interfaces and protocols to the machine control. 12b) Explain the principle of operation of phase sensitive detector. Sol: A phase sensitive Detector is used to detection of measurement of the amplitude and phase of a periodic signal with respect to another reference signal of the same frequency PSD is used to process signals that are ‘buried’ in noise and present output voltages with high signal-to-noise ratio

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The reference signal is assumed to be a square wave driving the switch towards the contactor 1 during the positive half cycle and to contactor 2 during the negative half cycle The sinusoidal test signal is amplified to assume reasonable amplitude and then applied to the switching system The reference and test signals are of the same frequency and hence the output of the adder consists of full wave rectified signal, If the signal are in phase. The average value is read on a PMMC system and is maximum When the test and reference signals are in phase, the operation is similar to an dc to dc converter, but when there is a phase difference between them, the average output voltage and the meter reading become functions of the phase angle 𝞱 of the test voltage with respect to the reference voltage When 𝞱 =1800, reading reaches a negative maximum, and when 𝞱 =90 0,he reading is Zero The low pass filter removes the AC ripple free DC output voltage for indication 13a) With the help of a diagram explain the principle of MEMS accelerometer. Sol: Depends only on the capacitive value that occurs due to the change in distance between the plates.

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The movable plates and the fixed outer plates act as the capacitor plates. When acceleration is applied, the proof mass moves accordingly. This produces a capacitance between the movable and the fixed outer plates. When acceleration is applied, the distance between the two plates displace as x1 and x2, and they turn out to be a function of the capacitance produced. All sensors have multiple capacitor sets. All upper capacitors are wired parallel to produce an overall capacitance C1 and the lower ones produce an overall capacitance of C2.

If Vx is the output voltage of the proof mass, and V0 is the output voltage produced between the plates, then

When no acceleration is given (a=0), the output voltage will also be zero. When acceleration is given, such as (a>0), the value of value of vx changes in proportion to the value of v0. When a deceleration is given, such as (aQin, or rise, if Qout < Qin.

To provide automatic control, the system is modified so that machines, electronics, or computers replace the operations of the human. An instrument called a sensor is added that is able to measure the value of the level and convert it into a proportional signal, s. This signal is provided as input to a machine, electronic circuit, or computer called the controller. The controller performs the function of the human in evaluating the measurement and providing an output signal, u, to change the valve setting via an actuator connected to the valve by a mechanical linkage. When automatic control is applied to systems which are designed to regulate the value of some variable to a setpoint,

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16 a) Draw the PLC ladder diagrams to realize two input AND, OR and XOR gates Sol:

16 b) What are the main components of SCADA? Sol: The main components of SCADA are 1. Multiple terminal unit 2. Master station 3. HMI computers 4. Communication infrastructure 17 a) With the help of a neat diagram explain the working of butterfly valve. Sol: Butterfly valves are flow regulating, pivoted disc valves. The circular disc or plate, having the same radial dimensions as the pipe is pivoted exactly in the centre of the pipe, perpendicular to the direction of the flow. The disc if the butterfly valve is mounted perpenticular to the flow stream, The disc turns to open/block the flow stream when the actuator turns, Though the valve gives an unrestricted flow of fluid, it induces a pressure drop in the flow as the disc is always present within the flow. When the valve is closed the 15

disc turns so that the disc block the flow, When the valve is open, the disc turns quarter turn, so that to allow unrestricted flow. The valve may also open incrementally to regulate flow.

17 b) Give the significance of timers and counters in PLC. Sol: Significance of timers and counters in PLC 1. The complex industrial processes requires timing and counting functions 2. The timers are devices that count increments of time 3. The timer compares its current time with the present time. The output of the timer will be 1 or 0. 4. Counters are used for counting a specified number of contact operations

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