Module 1 - Industrial Instrumentation and Automation

H330 Automotive Engineering MODULE 1 INTRODUCTION TO PROCESS CONTROL: BLOCK DIAGRAM OF PROCESS CONTROL LOOP, DEFINITION OF ELEMENTS. SENSOR TIME RESPONSE: FIRST AND SECOND ORDER RESPONSES. REVIEW OF TRANSDUCERS: CHARACTERISTICS AND CHOICE OF TRANSDUCER- FACTORS INFLUENCING CHOICE OF TRANSDUCER Introduction to process control Process - A series of actions or operations to attain a desired output Classification of processes i) Natural process - A process existing in or produced by nature without any human intervention - Eg: volcanic activities, tidal activities ii) Self regulating process - Self-regulating processes are processes that are inherently self-regulating - Self-regulated processes have built-in feedback characteristics that cause the process to tend towards self-regulation - An example of a self-regulating process is a tank of water with an input of water entering the tank and an output of water leaving the tank. Let’s say the water level in the tank is constant at 10 inches. Water enters the tank at a rate of 20 gallons per minute and leaves the tank at a rate of 20 gallons per minute. 2 - A non self-regulating process is one where the process does not tend towards self-regulation. - These processes have no self-regulating feedback characteristics and will tend towards being unstable if not controlled externally. iii) Man made processes - It is a combination of electrical, mechanical, chemical or other mechanism together to get a desired output - Eg: Industrial processes Requirements of process - A process must satisfy several requirements imposed by its designers and the general technical, economic and social conditions in the presence of ever changing external influences (disturbances). - Safety of men and machine - Environmental regulations - Production specifications - Operational constraints and economics 3 Examples of Process - Unit process like an alkylation reactor - Unit operation like evaporator, distillation column or storage vessel. - Manufacturing Industries - Space heating/cooling - Water temperature control Process control - Process control is an automatic control of an output variable by sensing amplitude of output parameters from the process and comparing to the desired output set level and feeding an error signal back to control an input variable - The basic objective of process control is to regulate the value of some quantity - To regulate means to maintain the quantity at some desired value, regardless of external influences - The desired value is called the reference value or setpoint 4 • Let us have an example, a specific system will be used to introduce terms and concepts employed to describe process control • Consider a water tank system as shown in figure. • Liquid is flowing into a tank at some rate, Qin, and out of the tank at some rate, Qout • The liquid in the tank has some height or level, h • It is known that the output flow rate varies as the square root of the height, Qout = K . √h • So the higher the level, the faster the liquid flows out • If the output flow rate is not exactly equal to the input flow rate, the level will change • If Qout > Qin level lowers & if Qout < Qin, level rises • This process has a property called self-regulation. • This means that for some input flow rate, the liquid height will rise until it reaches a height for which the output flow rate matches the input flow rate. • Suppose we want to maintain the level at some particular value, H, as in Figure, regardless of the input flow rate. Then something more than self-regulation is needed. 5 Human-Aided Control • The figure shows a modification of the tank system to allow artificial regulation of the level by a human. • To regulate the level so that it maintains the value H, it will be necessary to employ a sensor to measure the level. This has been provided via a “sight tube,” S • The actual liquid level or height is called the controlled variable • In addition, a valve has been added so that the output flow rate can be changed by the human • The output flow rate is called the manipulated variable or controlling variable • Now the height can be regulated apart from the input flow rate using the following strategy: The human measures the height in the sight tube and compares the value to the set point If the measured value is larger, the human opens the valve a little to let the flow out increase, and thus the level lowers toward the set point 6 If the measured value is smaller than the set point, the human closes the valve a little to decrease the flow out and allow the level to rise toward the set point By a succession of incremental opening and closing of the valve, the human can bring the level to the set point value, H Automatic Control - In this 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 like the one in figure , which are designed to regulate the value of some variable to a set point, it is called process control. 7 Advantages of process control - Energy savings : energy wastage is reduced when your plant and machinery are efficiently operated - Improved safety : control systems automatically warn you of any abnormalities which minimizes the risk of accidents - Improved & Consistent product quality : variations in product quality are kept to a minimum and reduce your wastage - Lower manufacturing costs : detecting faults early means throughput, yield and quality are maintained - Improved environmental performance : systems can give you an early warning of a rise in emissions - Increase the productivity - Optimise the performance - Minimize the effect of external disturbance 8 Block diagram of process control loop - The block diagram of process control loop is shown in figure - Here, c = Controlled variable u = Process variable b = Measured variable r = Set point (reference) p = Output signal of controller - The control element operates on the process by changing the value of the controlling process variable u - Error, e = r-b PROCESS - A process can consists of a complex assembly of phenomena that relate to some manufacturing sequences. - Many variables may be involved in such a process and it may be desirable to control all these variables at the same time. 9 - - - MEASUREMENT Measurement refers to the conversion of the variable into some corresponding analog of the variable, such as a pneumatic pressure, an electrical voltage or current, or a digitally encoded signal. A sensor is a device that performs the initial measurement and energy conversion of a variable into analogous, digital, electrical, or pneumatic information ERROR DETECTOR The error detector compares measured value with reference value to find the error This error has both magnitude and polarity CONTROLLER The controller requires an input of both a measured indication of the controlled variable and a representation of the reference value of the variable, expressed in the same terms as the measured value CONTROL ELEMENT The final element in the process-control operation is the device that exerts a direct influence on the process; that is, it provides those required changes in the controlled variable to bring it to the set point. This element accepts an input from the controller, which is then transformed into some proportional operation performed on the process 10 Physical diagram of a process control loop and its block diagram representation 11 Objective of Control system - A practical statement of control system objective is best represented by three requirements: 1. The system should be stable. 2. The system should provide the best possible steady-state regulation. 3. The system should provide the best possible transient regulation. Steady state regulation • The objective of the best possible steady-state regulation simply means that the Steady State error should be a minimum. • Generally, when a control system is specified, there will be some allowable deviation, about the setpoint. • This means that variations of the variable within this band are expected and acceptable. • External influences that tend to cause drifts of the value beyond the allowable deviation are corrected by the control system. Transient regulation • Transient regulation specifies how the control system reacts to a sudden change of some process variable, which may affect the output. This is called transient response. 12 Sensor time response - Sensors do not change their output state immediately when an input parameter change occurs - Normally it will change to the new value over a period of time called the response time - The sensor time response can be defined as the time required for a sensor output to change from its previous state to a final settled value within a tolerance band of a correct new value. - The sensor time response is important for sensors because they are the primary element for providing knowledge of the controlled variable value. - There are two most common types of sensor time responses 1. First order response 2. Second order response - Consider a sensor that produces an output, b(t), as a function of the input, c(t) as shown in figure - The static transfer function determines the output when the input is not changing in time. - To specify the time response, the nature of the time variation in output, b(t), is given when the input exhibits a step change as shown. - Note that at t = 0, the input to the sensor is suddenly changed from an initial value, C i , to a final value, Cf. 13 - If the sensor were perfect, its output would be determined by the static transfer function to be bi before t = 0 and bf after t = 0. - However, all sensors exhibit some lag between the output and the input and some characteristic variation in time before settling on the final value. 1. First-Order Response - The simplest time response is shown in Figure as the output change in time following a step input. - This is called first order because, for all sensors of this type, the time response is determined by the solution of a first order differential equation. - A general equation can be written for this response independent of the sensor, the variable being measured, or the static transfer function. - The equation gives the sensor output as a function of time following the step input 14 b(t) = bi + (bf – bi) [1-e-t/τ] Where, bi = initial sensor output from static transfer function and initial input bf = final sensor output from static transfer function and final input τ = sensor time constant - The sensor output is in error during the transition time of the output from b i to bf. - The actual variable value was changed instantaneously to a new value at t = 0. 2. Second-Order Response - In some sensors, 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. - Such oscillation (and the decay of the oscillation itself ) is a function of the sensor. - This output transient generated by the transducer is an error and must be accounted for in any measurement involving a transducer with this behavior. - This is called a second-order response because, for this type of sensor, the time behavior is described by a second-order differential equation. - It is not possible to develop a universal solution, as it is for the first-order time response. EEE MITS of the response. 15 Instead, we simply describe the general nature • 16 - 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. Time Constant - The time constant, τ, is part of the specification of the sensor. - Its significance can be seen by writing Equation for first order response of a sensor as follows b(t) = bi + (bf – bi) [1-e-t/τ] - In this equation, the quantity on the left is the change in output as a function of time, whereas (bf - bi) is the total change that will occur. - Thus, the square-bracketed term in Equation is the fraction of total change as a function of time. - Suppose we wish to find the change that has occurred at a time numerically equal to τ. - Then we set t= τ in Equation and find b(τ) - bi = (bf – bi) [1-e-1] b(τ) - bi = 0.6321(bf – bi) - Thus, we see that one time constant represents the time at which the output value has changed by approximately 63% of the total change. 17 - The time constant is sometimes referred to as the 63% time, the response time, or the e-folding time. - For a step change, the output response has approximately reached its final value after five time constants, since from Equation we find b(5τ) - bi = 0.993(bf – bi) Q1. A sensor measures temperature linearly with a static transfer function of 33mV/ ◦C and has a 1.5-s time constant. Find the output 0.75 s after the input changes from 20 ◦ to 41◦ . Find the error in temperature this represents. Sol. We first find the initial and final values of the sensor output: bi = (33 mV/°C) at (20°C) bi = 660 mV bf = (33 mV/°C) at (41°C) bf = 1353 mV Now, b(t) = bi + (bf – bi) [1-e-t/τ] EEE MITS b(0.75) = 660 + (1353 – 660) [1-e-0.75/1.5] = 932.7mV 18 - This corresponds to a temperature of T = 932.7/(33mV/°C) = 28.3°C - Thus the indicated temperature differs from the actual temperature by -12.7°C because of the lag in sensor output. - After a time of about five times constants (≈7.5 s) the sensor output would be about 1.353 V, correctly indicating the actual temperature of 41°C. 19 Transducers - A transducer is an electronic device that converts energy from one form to another (Electrical energy – either voltage or current). - Common examples include microphones, loudspeakers, thermometers, position and pressure sensors, and antenna. - A sensor is a device that converts any form of energy into another. Classification of transducers - The transducers are classified as shown 20 1. Active and Passive Transducers Active Transducer - These transducers do not need any external source of power for their operation. - Therefore they are also called self-generating type transducer. - Eg. Photovoltaic transducer, Piezoelectric transducer, Electromagnetic transducer Passive Transducer - The transducers need an external source of power for their operation. - So they are not self-generating type transducers. - Examples: Resistive, Inductive, Capacitive transducers 2. Analog and Digital Transducers - The transducers can be categorized on the basis of the output which may be a continuous function of time or the output may be in discrete steps. Analog Transducers - This type of Transducers converts the input quantity into an analog signal output, which is a continuous function of time. 21 - A strain gauge, LVDT, Thermocouples, or thermistors are called analog transducers as they give an output that is a continuous function of time. Digital Transducers - Digital transducers generate an electrical output in the form of pulses which form a unique code. - Thereby a unique code is generated for each discrete value sensed. - Eg. Encoders 3. Primary and Secondary Transducers - Some transducers consist of a mechanical device along with an electrical device. - In such types of transducers, the mechanical device acts as a primary transducer and converts physical quantity into the mechanical signal (Eg. Mechanical movement) - The mechanical signal produced by the primary transducer converts into an electrical signal by the electrical device and it is called a secondary transducer. - Therefore, an electrical device acts as a secondary transducer. 22 - For example, in pressure measurement, as shown in the figure, Bourdon’s tube acts as a transducer which converts pressure into displacement and LVDT acts as a secondary transducer which converts this displacement into an equivalent quantity of electrical signal. primary 4. According to Transduction Principle - The transducers can be classified according to principle used in transduction. the Capacitive Transduction - In capacitive transduction, the quantity to be measured is converted into a change in capacitance. - A capacitor basically consists of two conductors (plates) separated by a dielectric insulator. - A change in the capacitor takes place either by changing the distance between two plates or by a change in the dielectric. Electromagnetic Transduction - In electromagnetic transduction, the quantity to be measured is converted into an electromotive force (voltage) induced in a conductor by a change in the magnetic flux, in the absence of excitation. 23 - By moving a magnet to alter the magnetic flux through a coil thus generating an electromotive force (EMF) according to Faraday’s law. Inductive Transduction - In inductive transduction, the quantity to be measured is converted into a change in the self-inductance of a single coil. - This is achieved by displacing the coil’s core, which is linked or attached to a mechanical sensing element. Piezoelectric Transduction - In piezoelectric transduction, the quantity to be measured is converted into a change in electrostatic charge, q or voltage, V generated by crystals when mechanically get stressed. Photovoltaic Transduction - In photovoltaic transduction, light energy is the quantity to be measured is converted in the voltage generated when a junction between dissimilar materials is illuminated. Photoconductive Transduction - In photoconductive transduction, the measurand is converted into a change in resistance (conductance) of semiconductor material by a change in the intensity of illumination incident on the material. 5. Transducer and Inverse Transducer - Transducers are those that convert-electrical quantity into electrical quantity whereas inverse transducer converts electrical quantity into non-electrical quantity. - Example of Transducer: A microphone is a transducer that converts the sound signal into an - Example of Inverse Transducer: A loudspeaker converts an electrical signal into a sound signal is an inverse transducer. Characteristics of Transducers - The performance characteristics of a Transducer are key in selecting the best suitable transducer for a particular design. - So, it is very important to know the characteristics of transducers for proper selection. - Performance characteristics of transducers can be further classified into two types: 1. 2. 3. Static Characteristics Dynamic Characteristics Overall, both static and dynamic characteristics of a Transducer determine its performance and indicate how effectively it can accept desired input signals and reject unwanted inputs 1. Static Characteristics - The static characteristics of transducers relate to its performance when the measured quantity is not a function of time i.e. it remains constant or varies very slowly with respect to time. - Following is a list of some of the important static characteristics of transducers. a) Sensitivity - The sensitivity of the electrical transducer is defined as the electrical output obtained per unit change in the physical parameter of the input quantity. - For example, for a transducer used for temperature measurement, sensitivity will be expressed in mV/◦C. - A high sensitivity is always desirable for a given transducer. b) Linearity - The output of the transducer should be linearly proportional to the input quantity under measurement. - It should have linear input - output characteristic. c) Resolution - Resolution of a transducer is defined as the smallest change in the measured quantity that can be detected by the transducer. - A high resolution is desired for a transducer 26 d) Precision (Accuracy) - It is defined as the closeness with which the reading approaches an accepted standard value or ideal value or true value, of the variable being measured. e) Span and Range - For a transducer, the operating range should be wide, so that it can be used over a wide range of measurement conditions. f) Threshold - Threshold is the amount of measurement change required before a measuring instrument reacts to a change in measurement output or produces a specified result. g) Drift - Drift is the variation of output for a given input caused due to change in the sensitivity of the instrument due to certain interfering inputs h) Stability & reliability - The output of the transducer should be highly stable and reliable so that there will be minimum error in measurement. - The output must remain unaffected by environmental conditions such as change in temperature, pressure, etc. 27 i) Responsiveness - It is the rapidity with which the transducer responds to changes in the measured quantity. - The speed of response of the transducer should be as high as practicable. j) Repeatability - The output of the transducer must be exactly the same, under same environmental conditions, when the same quantity is applied at the input repeatedly. k) Size - The transducer should have smallest possible size and shape with minimal weight and volume. - This will make the measurement system very compact. l) Dead Zone - Dead zone is the largest range of values of a measured variable, to which the instrument does not respond m) Hysteresis - Hysteresis is a phenomenon that illustrates the different output effects when loading and unloading. - Many times, for the increasing values of input an instrument, may indicate one set of output values. - For the decreasing values of the input, the same instrument may indicate its different set of output values. n) Ruggedness - The transducer should be mechanically rugged to withstand overloads. It should have overload protection. 28 2. Dynamic Characteristics - The dynamic characteristics of transducers relate to its performance when the measured quantity is a function of time i.e. it varies rapidly with respect to time. - The following are some dynamic characteristics that may be considered in selection of a transducer. a) Dynamic Error - The difference between the true value of the measured quantity to the value shown by the measuring instrument under varying conditions. - This type of error occurs when the input quantity is varying with time. - This type of error occurs where system contains energy storage element and due to this the output cannot follow input exactly. - The dynamic error can be made small by having a small time constant. b) Fidelity - It is defined as the degree to which a measuring instrument is capable of faithfully reproducing the changes in input, without any dynamic error. 29 c) Speed of Response - It is the rapidity with which the transducer responds to changes in the measured quantity. - The speed of response of the transducer should be as high as practicable. - It indicates how active and fast the system is. d) Bandwidth - The frequency range over which the transducer meets the minimum power requirement may be called the effective bandwidth e) Lag - Every system takes at least some time to respond, whatever time it may be to the changes in the measured variable. - For Example Lag occurs in temperature measurement by temperature sensors such as Thermocouple or RTD or dial thermometer due to scale formation on thermowell due to process liquid. 30 Factors influencing the choice of transducers for a particular application - Transducers are widely used in process control to sense various parameters - Before choosing a transducer for a particular application, we have to ensure that the transducer is suitable for our need. - The choice of a transducer used for measuring different quantities depends on many factors such as: 1. Operating Principle - The selection of transducers may be made based on its different operating principles like inductive, resistive, capacitive, piezoelectric, optoelectronic etc. 2. Operating Range - A transducer having a wide range of operation is an ideal choice because it does not break during an operation. 3. Sensitivity - It should have a high sensitivity for producing the correct, detectable output. 4. Cross Sensitivity - The measure of a transducer can vary across different planes due to sensitivity. - Hence, cross sensitivity is essential for accurate results. 31 5. Accuracy - Generally, transducers produce accurate values after calibration. - A small gap for variability from desired value is permissible and is necessary for industrial applications. 6. Environmental Compatibility - A transducer must be able to perform in any environment. - It must work well with shocks and high pressure. 7. Reliability and Stability - Transducers must have great stability for sustained operations. - They must also be reliable in order to avoid failures or errors. 8. Errors - The transducer should maintain the expected input-out relationship as described by its transfer function so as to avoid errors. 9. Transient and Frequency Response - The transducer should meet desired time domain specifications like peak overshoot, rise time, settling time and small dynamic error. 32 - It should ideally have a flat frequency response curve. In practice, however, there will be cutoff frequencies and higher cut off frequency should he high in order to have a wide bandwidth. 10. Loading Effects - The transducer should have a high input impedance and a low output impedance to avoid loading effects. 11. Insensitivity to Unwanted Signals - The transducer should be minimally sensitive to unwanted signals and highly sensitive to desired signals. 12. Usage and Ruggedness - The ruggedness both of mechanical and electrical intensities of transducer versus its size and weight must be considered while selecting a suitable transducer. 13. Electrical aspects - The Electrical aspects that need consideration while selecting a transducer include the length and type of cable required. - Attention also must be paid to signal to noise ratio in case the transducer is to be used in conjunction with amplifiers. 14. Static Characteristics. - Apart from low static error, the transducers should have a low nonlinearity, low hysteresis, high resolution and a high degree of repeatability. 33 Previous University Questions 1. Explain the factors that govern the output characteristics of a transducer. 2. Explain the factors influencing the choice of a transducer for an industrial instrumentation system. 3. 4. Draw and explain second order sensor time response. 5. Draw the step response of a first order sensor. Explain the effect of time constant on the nature of response of the sensor. Draw the block diagram representation of a process control system and explain the functions of each block. 34