PROCESS CONTROL For
Instrumentation Engineering By
www.thegateacademy.com
Syllabus
Process Control
Syllabus for Process Control Feedback principles. Signal flow graphs. Transient Response, steady state-errors. Routh and Nyquist criteria. Bode plot, root loci. Time delay systems. Phase and gain margin. State space representation of systems. Mechanical, hydraulic and pneumatic system components. Synchro pair, servo and step motors. On-off, cascade, P, P-I, P-I-D, feed forward and derivative controller, Fuzzy controllers.
Analysis of GATE Papers (Process Control) Year
Percentage of marks
2013
4.00
2012
0.00
2011
0.00
2010
2.00
2009
5.00
2008
4.00
2007
4.00
2006
3.33
2005
4.66
2004
2.66
2003
6.00
Overall Percentage
3.24%
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Contents
Process Control
CONTENTS Chapters #1.
#2.
#3.
#4.
Page No.
Introduction, Building Block’s
1 – 25
1–2 3 – 12 12 – 16 16 – 19 19 – 20 21 – 22 23 23 – 25
Process Characteristics Terminology Dynamics of Simple System Interacting and Non-Interacting System Process Degree of Freedom Assigment Answer Keys Explanations
Modes of Controller Operation
26 – 69
26 – 27 27 27 - 30 30 – 58 58 – 61 62 – 64 65 65 – 69
Control Parameter Range Reverse and Direct Action of Controllers Modes of Control Actions Compostie controller modes Valves Assignment Answer Keys Explanations
Tuning of Controllers
70 – 83
70 – 72 73 73 – 74 74 – 76 77 – 79 80 80 – 83
Process Reaction Curve Method Quarter Amplitude Criteria Ziegler-Nicols Tuning Method Controller Tuning From Routh Array Method Assignment Answer Keys Explanations
Process Control System Configurations
84 – 92
84 85 86 – 87 87 – 88 89 – 90 91 91 – 92
Feed forward and Feed Back control Feed Forward Control Cascade Control Configuration Ratio Control Configuration Assignment Answer Keys Explanations
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Contents
#Module Test
Test Questions Answer Keys Explanations
#Reference Books
Process Control
93 – 117 93 – 105 106 106 – 117 118
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Chapter-1
Process Control
CHAPTER 1 Introduction, Building blocks of Process control Introduction Instrumentation may be defined as an art of using instruments to measure the physical and chemical properties and changes of matter. Since it is not possible to measure these changes directly, certain variable like temperature, humidity, level etc are measured which effects these changes of properties of matter. Thus instrumentation mainly deals with measurement and control of these variables The team “process” refers to physical or chemical change of matter or an ensemble of environment where a particular variable is measured and controlled. “Process Control” refers to regulation and manipulation of variables influencing the conduct of a process in such a way as to obtain a product of desired quality and quantity in an efficient manner.
Process Characteristics To understand the characteristics of process control system, consider a simple problem where the objective is to maintain the level of the liquid in the tank at a height ‘h’ (as shown in fig 1.1) irrespective of disturbance that tend to deviate it from desired level. (Inlet )
Level measuring instrument
h (Control Value )
Human operator
(Outlet)
Figure 1.1
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Chapter-1
Process Control
The tank has inlet flow-rate q and Outlet flow rate q which is controlled by a control value. To achieve the objective or to convert it into a control system, a human operator is placed near the control valve as shown. The operator has desired level in his mind also known as set point. He constantly monitors the level measuring instrument. If the level is above the desired level (set point) he performs an operation on control value by opening it till the level reaches the set point and fixes the control value at this position which makes outlet flow rate equal to inlet flow rate. Similarly, if level goes down the desired level, he performs an operation on control value by closing it. Thus, the control system shown in fig 1.1 is manual control system. This can be converted in to an “automatic control system” by: Replacing human operator with AUTOMATIC CONTROLLER as shown in fig 1.2.
Automatic Level Control System Set-Point (Inlet ) Automatic LT
Controller
h Final Control Element
LT – level transmitter
Figure 1.2
Block Diagram of A Control System Controller Block
Set Point
+
-
Automatic Controller
Final Control Element
Process
Output
Measurement
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Chapter-1
Process Control
Terminology 1. Process: As defined earlier, it is an environment where a particular variable is measured, monitored and controlled. 2. Process Variables: They are (a) Controlled variable (or) dynamic variable. (b) Manipulating variables (c) Load variables (d) flow variables (e)across variables 2(a). Controlled (or) dynamic variable: It is the variable which is measured, monitored and controlled. Ex : Level in the tank is an example of controlled variable. 2(b). Manipulated variable: It is the variable which is manipulated to make the controlled variable remain at set point value. Ex: In the above level control system, the output flow-rate is the manipulated variable. 2(c). Load variable: It refers to all the other variables that affect the controlled variable to deviate from set point value. Ex: In the above example, changes in inlet flow rate also change the level in the tank, so it constitutes to load variable. 2(d). Flow Variable: Any variable which is measured along the process loop. Ex:- Flow in a loop, current in a loop, heat transfer. 2(e). Across Variable: Any variable which is always measured as a differential value or with reference to some point. Ex:- Voltage (potential difference), Differential pressure, Temperature, etc. Process
Flow Variable
Across Variable
Relationship
Electrical N/w
I
V Voltage
v = i ∙ R R = electrical Resistance
Flow Process
Q Fluid flow rate
h Pressure head
h=q∙R
Current
R = Flow Resistance
Useful Conversations Pressure 14.7 psi = 1 atm 1 N/
= 1 Pa (=1 Pascal)
Pa = 1 bar THE GATE ACADEMY PVT.LTD. H.O.: #74, Keshava Krupa (third Floor), 30th Cross, 10th Main, Jayanagar 4th Block, Bangalore-11 : 080-65700750, info@thegateacademy.com © Copyright reserved. Web: www.thegateacademy.com Page 3
Chapter-1
Process Control
1 Psi = 0.07 bar 1 gallon = 3.78 liters
3.8 liters
Process Load The set of all load variables that cause a change or deviation of controlled variable from set point are known as process load.
Process Equation For the example in Fig1.2, process equation can be written, which describes the process mathematically. Here the liquid level ‘h’ is a function of changes in inlet flow rate and outlet flow rate. A simple process equation is ‘ ’ =
(
Measurement It is the determination of magnitude of a variable i.e. controlled variable.
Controller It is an automatic device having an output that varies to regulate the controlled variable. A controller performs operations such as comparison, evaluation and operation upon the next stage to meet the demands and control objectives.
Final Control Element The device that directly controls the value of manipulated variables in a process control loop is the final control element. Often the control element is control value.
Elements of Process Control The different elements with which a mathematical model may be formulated for a process are A. Resistance element. B. Capacitance element. C. Time constant element. D. Dead time element. The mathematical model thus obtained is known as Transfer function. A transfer function is defined as ratio of L T of output to L. T of input under the assumption that all initial conditions are set to zero. [L.T Laplace Transform] o p ( i.e T(s = ( | T Transfer function i p (
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Chapter-1
Process Control
(A) Resistance Type Element
Resistance may be defined as any obstruction or constriction which can be incorporated by external means also, (or) the ability to resist the transfer of mass or energy is called resistance. Example: Consider the operation of capillary flow system shown in fig 1.4. Two capillary tubes are inserted at the inlet and outlet sections of the pipeline where the fluid is flowing. The difference in the head or height (h) between the rise of fluid in both capillary tubes is known as “Resistance” effect.
Capillarly (1)
Capillarly (2) h
Fluid flow rate (q)
Pipeline
Figure 1.4 Resistance =
cross Variable Flow Variable
The flow-head equation for fig 1.4 may be written as: (head) h ∝ q ( flow-rate) ∴h=Rq
------------- (
Thus if q = input and h =output (1) represents a linear relation between input and output. Applying L.T to equation (1) H(s)=R Q (s)
Q(s)
R
H(s)
Electrical resistance, thermal resistance, in mechanical or rotational systems, the spring elements etc are all other resistance elements.
(B) Capacitance Element
Capacitance is the ability to store. Example is a tank with inlet as show in fig (1.5)
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Chapter-1
Process Control
h
q
Figure 1.5 The flow of fluid into the tank is the input and the level of the liquid in the tank (h) is the output. The change in the level of liquid in the tank ( ) is directly propositional to inlet flow rate dh i. e ∝q dt c =q ------ (2) C = capacitance Applying L.T to equation (2) CsH(s) = Q (s) H(s ∴ = (s Cs Q(S)
H(S)
Hence the transfer function representation of capacitance elements is Response of capacitance elements is obtained by integration of equation (2) ∫
d
= ∫ = ( )t.
(h)
( )
t
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