INTRODUCTION TO PROCESS
CONTROL
Prepared BySHIVAJI GOVIND THUBE
GP Mumbai
Introduction
Review of Process and Control Systems
 Control Systems
In general, all the elements necessary to accomplish the
control objective are described by the term control system.
 Process Control
The elements and methods of control system operation used
in industry to control industrial processes are referred as
process control or process automation or automatic control of
process.
Review of Process and Control Systems
 Process Control Principles
In process control, the basic objective is to regulate the value of
some quantity. To regulate means to maintain that quantity at
some desired value regardless of external influences. The desired
value is called the reference value or set-point.
 Self-regulation
Review of Process and Control Systems
 Process Control Principles
 Human-Aided Control
Review of Process and Control Systems
 Process Control Principles
 Automatic Control
Review of Process and Control Systems
 Servomechanism (Tracking control system)
The objective is to force some parameter to vary in a
specific manner. This mechanism forces the controlled
variable value to follow variation of the reference value.
Review of Process and Control Systems
 Process-Control Block Diagram
Review of Process and Control Systems
 Process-Control Block Diagram
Identification of Elements
 Process
Single variable processes or Multi variable processes
 Measurement
 Error Detector
 Controller
 Control Element
Review of Process and Control Systems
 Process-Control Loop
Review of Process and Control Systems
 Process-Control Loop
Review of Process and Control Systems
 Control System Evaluation
Control System Objective
In principle, the objective of a control system is to make the
error in Equation (1) exactly zero, but the control system
responds only to errors (i.e., when an error occurs, the control
system takes action to drive it to zero).
e(t) = r - c(t)
……………….(1)
A practical statements of control system objective
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.
Review of Process and Control Systems
 Control System Evaluation
 Stability
Review of Process and Control Systems
 Control System Evaluation
 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, +/- Δe , about the set-point.
Review of Process and Control Systems
 Control System Evaluation
 Transient Regulation
Transient regulation specifies how the control system responds
Upon set-point changes and disturbances changes.
If one of them suddenly changes value, the controlled variable
may be driven to change also, so the control system acts to
minimize the effect. This is called transient response.
Review of Process and Control Systems
 Control System Evaluation
 Transient Regulation
Transient regulation specifies how the control system responds
Upon set-point changes and disturbances changes.
If one of them suddenly changes value, the controlled variable
may be driven to change also, so the control system acts to
minimize the effect. This is called transient response.
Review of Process and Control Systems
 Evaluation Criteria
 The question of how well the control system is working is
thus answered by (1) ensuring stability, (2) evaluating steadystate response, and (3) evaluating the response to setpoint
changes and transient effects. There are many criteria for
gauging the response.
Damped Response
Cyclic Response
 ¼ Amplitude criteria (Decay Ratio)
 Time-domain criteria – time constant, settling time, %OS, etc.
 Frequency-domain criteria- GM , PM, etc.
Analog and Digital Processing
In the past, the functions of the controller in a control
system were performed by sophisticated electronic
circuits. Data were represented by the magnitude of
voltages and currents in such systems. This is referred
to as analog processing.
Most modern control systems now employ digital
computers to perform controller operations. In
computers, data are represented as binary numbers
consisting of a specific number of bits. This is referred to
as digital processing.
Analog and Digital Processing
Data Representation
•The representation of data refers to how the magnitude
of some physical variable is represented in the control
loop.
•For example, if a sensor outputs a voltage whose
magnitude varies with temperature, then the voltage
represents the temperature.
•Analog and digital systems represent data in very
different fashions.
Analog and Digital Processing
Data Representation
Analog Data
An analog representation of data means that there is a
smooth and continuous variation between a
representation of a variable value and the value itself.
Figure 11 shows an analog relationship between some
variable, c, and its representation, b.
Analog and Digital Processing
Data Representation
Digital Data: Digital data means that numbers are
represented in terms of binary digits, also called bits,
which take on values of one (1) or zero (0). When data
are represented digitally, some range of analog numbers
is encoded by a fixed number of binary digits. The
consequence is a loss of information because a fixed
number of binary digits has a limited resolution.
The representation cannot distinguish between 4.25
Vand 4.75 V because both would be represented by
(0100) .
Analog and Digital Processing
Data Representation
Special devices are employed to convert analog voltages into a
digital representation. These are called analog-to-digital
converters (ADCs). In a control system, the sensor often produces
an analog output such as a voltage. Then an ADC is used to
convert that voltage into a digital representation for input to the
computer.
Digital-to-analog converters (DACs) convert a digital signal into an
analog voltage. These devices are used to convert the control
output of the computer/controller into a form suitable for the final
control element.
Digital Control
True digital control involves the use of a computer in
modern applications, although in the past, digital logic
circuits were also used. There are two approaches to
using computers for control.
Supervisory Control
Direct Digital Control
Analog Control
True analog control exists when all variables in the system are
analog representations of another variable. Figure 14 shows a
process in which a heater is used to control temperature in an
oven.
Digital Control- Supervisory Control
Digital Control- Supervisory Control
Supervisory Control
When computers were first considered for applications in control systems, they did not
have a good reliability; they suffered frequent failures and breakdown. The necessity for
continuous operation of control systems precluded the use of computers to perform the
actual control operations.
Supervisory control emerged as an intermediate step wherein the computer was used to
monitor the operation of analog control loops and to determine appropriate setpoints. A
single computer could monitor many control loops and use appropriate software to
optimize the setpoints for the best overall plant operation.
If the computer failed, the analog loops kept the process running using the last setpoints
until the computer came back on-line.
Figure 15 shows how a supervisory computer would be connected to the analog heater
control system of Figure 14.
Notice how the ADC and DAC provide interface between the analog signals and the
computer.
Digital Control- Direct Digital Control
Direct Digital Control (DDC)
As computers have become more reliable and miniaturized, they
have taken over the controller function. Thus, the analog
processing loop is discarded. Figure 16 shows how, in a full
computer control system, the operations of the controller have
been replaced by software in the computer. The ADC and DAC
provide interface with the process measurement and control
action. The computer inputs a digital representation of the
temperature, , as an analog-to-digital conversion of the voltage, .
Error detection and controller action are determined by software.
The computer then provides output directly to the heater via digital
representation,
, which is converted to the excitation voltage, ,
by the DAC.
Digital Control- Direct Digital Control
Digital ControlNetworked Control Systems (NCS)
or
Distributed Control Systems
Digital ControlNetworked Control Systems (NCS)
When a plant uses DDC, it becomes possible to place the
computer-based controller directly at the site of the plant where
the control is needed. This is done by using smart sensors or by
placing the computer controller in hermetically sealed instrument
cases around the plant. In order to have coordinated control of the
whole plant, all these DDC units are placed on a local area
network (LAN). The LAN commonly provides communication as a
serial stream of digital data over a variety of carriers such as wires
and fiber optics. The LAN also connects to computers exercising
master control of plant operations, fiscal computers for accounting
and production control, and engineering computers for monitoring
and modifying plant operations as needed. In control systems,
these LANs are referred to as a field bus.
Digital ControlNetworked Control Systems (NCS)
When a plant uses DDC, it becomes possible to place the
computer-based controller directly at the site of the plant where
the control is needed. This is done by using smart sensors or by
placing the computer controller in hermetically sealed instrument
cases around the plant. In order to have coordinated control of the
whole plant, all these DDC units are placed on a local area
network (LAN). The LAN commonly provides communication as a
serial stream of digital data over a variety of carriers such as wires
and fiber optics. The LAN also connects to computers exercising
master control of plant operations, fiscal computers for accounting
and production control, and engineering computers for monitoring
and modifying plant operations as needed. In control systems,
these LANs are referred to as a field bus.
Digital Control- NCS
Figure 17 shows how the LAN or field bus connects the
computers in a plant together. Each of the process-control
computers operates one or more DDC loops like the one shown in
Figure 16. Bus users can monitor the operations of any of the
plant process-control loops, and those with authorization can
modify control characteristics such as setpoints and gains.
Special process-control bus standards have been developed for
how data and information are represented and transmitted in
these networks.The two most commonly implemented standards
are the Foundation Fieldbus and the Profibus (Process Field Bus).
The idea behind these standards is to have universal agreement
among process equipment manufacturers on how data are
represented on the bus line and how data are transmitted and
received. This is an extension of the “plug and play” concept used
for computer hardware
Review of Process and Control Systems

Process Industry- Oil Refinery
Process Industries
Pulp and paper, Chemical, Sugar, Petrochemical, Pharmaceutical and Power Industries.
Process
Process as used in the process industry,
refers to the methods of changing or
refining raw materials into end products.
The raw materials in a liquid,
gaseous, or slurry state during the
process, are transferred, separated,
mixed, heated or cooled, filtered, stored,
or handled in some other way to produce
quality end products .
Process comprise a single or series of
operations being performed over raw
materials to produce desired final
product.
Raw materials: reactants, auxiliary materials, energy;
End products: products, by-products, energy.
Types of Processes in Process Industries
Production processes in the process industries can be carried out in
several ways:
1. Continuous
2. Batch,
3. Semi-continuous (in which certain parts are done continuously and
some in batch form).
Continuous Processes
The continuous process
consists
of
raw
materials entering the
process and following a
number of operations
comes out as a new
product in continuous
manner.
The inputs and outputs
throughout the process
are continuous.
Continuous Processes
The continuous process is one in which inputs (raw materials, auxiliary
materials, energy, etc.), are fed into the system at a constant rate and at the
same time a constant extraction of outputs is done (products, by-products,
energy, etc.).
In the continuous process, all the stages are carried out simultaneously
(although possibly in different parts of the system), and so the overall time
required for the process is shortened.
Batch Processes
The batch process consists of
raw materials transformed into
a new product according to a
batch recipe and a sequence.
The raw materials typically are
fed into reactors or tanks
where the reactions occur to
produce a new product.
Batch Processes
The batch process in which a certain quantity of inputs (raw materials,
auxiliary materials, energy, etc.) are fed into the chemical reaction unit (of
the entire reaction) under conditions suitable for obtaining the desired
reaction (temperature, pressure, required time, etc.).
In the batch process, in the reactor and at any given period of time, various
actions take place in the wake of which a concentration of reactants and
products varies so long as the reaction progresses.
At the conclusion of the specific product is manufactured.
Batch Processes
In washing machine, into which a certain quantity of dirty washing is put.
The required inputs are water, electrical energy, washing powder, etc. A
“batch” of laundry goes through various stages that are programmed as
required: soaking, washing, various rinses, and extraction. All the actions
take place in one receptacle and at the conclusion of the process we obtain
wet laundry that is clean and ready for drying. From the washing machine
the wet laundry is transferred to the drier, in which a “batch” process of
separation/purification of the “end product” from water is also carried out,
until dry laundry is obtained.
Advantages and disadvantages of batch and continuous reaction processes.
Advantage of a batch process
• A variety of different products can be made using the plant.
Advantage of a continuous process
• More cost effective if large quantities of the chemical are
being made.
• Slow reactions can be carried out.
• Can use reactants in any state including solids.
Disadvantage of a batch process
• Risk of contamination if more than one than one product made
• No expensive ‘down time’ when plant is not being used.
• Automated process requires less labour.
Disadvantage of a continuous process
• High capital cost of setting up the plant
in reaction vessel
• Costs rise if plant not operated continuously.
• Expensive down time while reactor is being filled and emptied.
• Larger workforce required.
• Can be difficult to control highly exothermic reactions.
Process Control
What is Process Control ?
Process Control in process industries refers to the regulation of
all aspects of the process. Precise control of level, flow,
temperature, pressure, pH , & etc. is important in many
process industries.
Process control refers to the methods that are used to control
the process variables when manufacturing a product.
In order to produce a product with consistently high quality,
tight process control is necessary.
Process Control
What is Process Control ?
Process control is an engineering discipline that deals,
with architectures mechanisms and algorithms for maintaining the
output of a specific process within a desired range.
For instance, the temperature of a chemical reactor may be
controlled to maintain a consistent product output.
Process control is extensively used in industry and enables mass
production of consistent products from continuously operated
processes such as oil refining, paper manufacturing, chemicals,
power plants and many others. Process control enables automation,
by which a small staff of operating personnel can operate a complex
process from a central control room.
Process Control
Importance of Process Control
Process control is required in process industries for
following reasons:
• Increase product quality.
• Ensure safety to people, plant and environment.
• Increase product throughput.
• Decrease raw materials & energy consumption.
• Decrease environment pollution.
• Extend life of equipments.
• Decrease production labors.
• Maximize profit.
Process Control
Increased Product Quality
Process Control
Process Control
Manual Feedback Control System
Manual Feedback Control System
Automatic Feedback Control System
Automatic Feedback Control System
Process Control System’s Main Components
Most basic process control systems consist of a control loop as shown in Figure. This has three
main components which are:
• Sensor/Transducer/Transmitter: A measurement of the state or condition of a process
• A Controller calculating an action based on this measured value
against a desired value (set point) of process output.
• Final Control Element: An output signal resulting from the controller calculation which issued
to manipulate the process action through some form of final control element/ control
valve/actuator.
• The process itself reacting to this signal, and changing its state or condition.
Types of Process Variables in PCS
Controlled variables - are the process output(s) which quantify the
performance or quality of the final product, which are also called as
measured variables.
Controlled variables are called as measured variables as they are
measured for control purpose. In industry controlled variable are referred
as Process Variables (PVs).
Manipulated variables – are the manipulated process input(s) which
are adjusted dynamically to keep the controlled variables at their setpoint values.
Disturbance / Load variables – are non manipulated process input(s)
that can cause the controlled variables to deviate from their respective
set point values.
Set-point : Desired value of controlled variable.
Error is the difference between Set-point and Measured value of
controlled variable (for Negative F/B). Error is also called as deviation.
Error = Set-point -- Measured value of process output
(for Negative F/B)
PCS- Different Control Schemes
1. Feedback control scheme
2. Feed-forward control scheme
3. Cascade control scheme
4. Ratio control scheme
5. Selective control scheme
6. Split-range control scheme
7. Self-tuning adaptive control scheme
Advantages and disadvantages of batch and continuous reaction processes.
Advantage of a batch
process
• A variety
of different products can be made using the plant.
Advantage of a continuous
• More cost effective if large quantities of the chemical are
process
being made.
• Slow reactions can be carried out.
• Can use reactants in any state including solids.
Disadvantage of a batch
process
• Risk
of contamination if more than one than one product made
• No expensive ‘down time’ when plant is not being used.
• Automated process requires less labour.
Disadvantage of a continuous
• High capital cost of setting up the plant
process
in reaction vessel
• Costs rise if plant not operated continuously.
• Expensive down time while reactor is being filled and emptied.
• Larger workforce required.
• Can be difficult to control highly exothermic reactions.
Risk
More
High
No
A
Slow
Automated
Larger
variety
expensive
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workforce
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Manual Feed-forward Control System
PCS - “Feed-forward Scheme” Concept
Automatic Process Control System
“Feed-forward Scheme”
Feedback Vs Feed-forward Controls
Feedback Control: “Compensatory Control”
Distinguishing feature: measures the controlled variable .
Advantages:
• Simple, no detail knowledge of process is required.
• Corrective action is taken regardless of the source of the disturbance.
Disadvantages:
• Corrective action is taken only after disturbance upset the
process.
Feed-forward Control: “ Anticipatory Control”
Distinguishing feature: measures the disturbance variable(s).
Advantage:
• Correct for disturbance(s) before it (they) upset(s) the process.
Disadvantage:
• Must be able to measure disturbances.
• No corrective action for unmeasured disturbances.
• Must know the process model.
Process To Be Controlled – Home Heating System
Process Control for Room Heating System
Control Objective : To maintain the room temperature at
desired value.
Process Variables:
Controlled Variable: Room Temperature
Manipulated Variable: Fuel/Gas flow
Disturbance Variable : Opening/closing of windows and door.
Since an occurrence of Disturbance is not rapid and large and it
is unmeasured one, henceSelection of Control Scheme: Feedback Control
Required process control hardware : Temperature sensor,
Temperature Controller , Final control element-control valve.
Process Control for Room Heating System
Consolidation
1. Which of these industries the example of the process
industry?
Select all options that apply.
i. Pharmaceutical
ii. Satellite
iii. Oil and Gas
iv. Sugar
v. Power
2. What are the main reasons for manufacturers to control a
process?
Select all options that apply.
i. Increase product quality
ii. Ensure safety
iii. Reduce costs
iv. Increase efficiency
v. Increase productivity
3. The three tasks associated with any control loop are
measurement, comparison, and adjustment.
Is this statement true or false?
4. What type of control loop takes action in response to measured
deviation from set-point?
i. Feedback control loop
ii. Feed-forward control loop
5. What type of control loop anticipates and controls load
disturbances before they can impact the process variable?
i. Feed-back control loop
ii. Feed-forward control loop
Feedback Controllers
FEEDBACK
CONTROLLER
Feedback Controllers
The feedback controller plays an essential role in the feedback
control system. Of the four basic functions of a feedback control
system, (measurement, comparison, computation, and correction)
comparison and computation are solely achieved by the feedback
controller.
The transmitter performs the measurement, while a correction is
done by the final control element, but this is done according to the
controller's calculation. The goal of the feedback controller is thus
to reduce the error to zero in an appropriate fashion.
Feedback Controllers
The control mechanism in the feedback controller consists of two
sections:
• The comparator
•The controller – control algorithm
The purpose of the first is to compare the desired value, R(t) and
measured value, B(t) of process output (controlled variable) and
then compute the difference between them as error, e(t) = R(t)- B(t).
If there is no error, i.e. the controlled variable is at the set-point, then no action is
taken. If an error is detected, the second section of the controller decides “what to
do” based on the error. And then it operates to change the position of a final control
element in such a way as to minimize the error in the least possible time with the
minimum disturbance to the system.
Control Actions or Types of Feedback
Controllers
The control action is the manner in which the controller responds to the
error & makes correction in process input in order to minimize the error
between desired & measured values of process output.
The control actions are also referred by several other names such as control
modes, control algorithms , controller equations and/or control laws.
Basic Control Actions or Types of
Feedback Controllers
Discontinuous Control Actions: In these control actions, the controller output
does not change smoothly for error input.
Continuous Control Actions: These control actions calculate the controller
output which has been a smooth function of error or sum of past errors or rate of
change of error.
Control Actions or Types of Feedback Controllers
Two-Position Control Action (On-Off Controller)
Control Actions or Types of Feedback Controllers
Two-Position Control Action (On-Off Controller)
Most home thermostats are ON/OFF controllers. A typical example is the
thermostatically controlled domestic immersion heater. The thermostat will turn the
heater ON when temperature drops below the set value(error is positive) and OFF
when the temperature rises above set value (error is negative).
ON/OFF control is often called "bang-bang" control because the control output is
cycled between two extremes.
Control Actions or Types of Feedback Controllers
Two-Position Control Action (On-Off Controller)
Hunting Effect: Since controller output takes only two states either
ON or OFF based on error, its rapid cycling causes the fast operation of
final control elements such as control valves ,solenoid valves and relay
contacts, and may create damage to these final control elements .
Remedies: To keep the controller output from cycling rapidly ON
and OFF, most of the practical on-off controllers intentionally
incorporate “hysteresis or dead band ”. The hysteresis is the range of
error over which controller output does not change state. The
controller output remains in the current state until the error moves
out of the hysteresis.
Hysteresis prevents the output from “chattering ” or making fast,
and prevents damage to final control elements.
Control Actions or Types of Feedback Controllers
Two-Position Control Action (On-Off Controller)
Control Actions or Types of Feedback Controllers
Two-Position Control Action (On-Off Controller)
Advantages : i) Extremely simple and there is no parameter to adjust.
ii) Cheaper .
Disadvantages:
i)Limited use in process control due to continuous cycling of controlled
variable (Hunting/ chattering effect) --excessive wear on control valve or relay contacts.
ii) With on-off control , the controlled variable never reaches the set-point.
Synonyms :
"Two position" or “Two-step” or "bang-bang" controllers
Applications :
It is used in nearly every domestic appliance requiring automatic
control, e.g. refrigerators, irons, hot water services, home heating systems, etc.
Control Actions or Types of Feedback Controllers
Proportional (P) Control Action (P- controller)
In this control action, the controller output is directly proportional to the
error signal. A large error produces a large controller output and a small error
produces a small output.
Control Algorithm:
Mathematically, Proportional control is expressed as:
p(t) = Kp.e(t) + po
where,
p(t)=Controller Output (% or mA)
Kp= Proportional Gain (dimensionless, adjustable)
po= Controller output when error is zero or bias value (% or
mA)
e(t)= Error = R(t)-B(t) (% or mA).
Control Actions or Types of Feedback Controllers
Proportional (P) Control Action (P- controller)
Proportional Band, PB - This is defined as the range of error required to
move the controller output ( final control element) over its whole range,
and is expressed in percentage.
100
PB = ----------- %
Kp
Control Actions or Types of Feedback Controllers
Proportional (P) Control Action (P- controller)
Control Actions or Types of Feedback Controllers
Proportional (P) Control Action (P- controller)
P-Controller Response:
The response of P-control to various standard error inputs is shown in fig.
Control Actions or Types of Feedback Controllers
Proportional (P) Control Action (P- controller)
Offset :
This is a major drawback of P-control action.
Offset is a nonzero steady-state error produced in process output when a setpoint change or a large load change occurs in process control system.
P-controller has zero-error process output only for nominal (steady-state) load
conditions or particular set-point value. The small changes in load variables
cause transient error in process output. P-controller can maintain the
controlled variable at the desired value by adjusting controller output
proportional to transient error and reduces error to zero –”compensatory
control”.
When large error comes in process output after set-point changes or large
load changes, P-controller calculates new output and try to minimize error.
However as set-pt/load change is permanent , large error remains in process
output and controller output remains at newer sustained value because of
one- to- one relation between c. o/p and error. Hence the new controller
output always produces nonzero steady-state error called as an offset in
process output.
Control Actions or Types of Feedback Controllers
Proportional (P) Control Action (P- controller)
Control Actions or Types of Feedback Controllers
Proportional (P) Control Action (P- controller)
Offset Reduction Methods:
i)By increasing the P-gain(Kp) , offset can be minimized to some extent.
However, too higher P-gain causes oscillatory process response.
ii) Manually adjusting bias value of controller output ,offset can be
eliminated.
iii)Use Integral-action in conjunction with P-action to eliminate offset
completely.
Control Actions or Types of Feedback Controllers
Proportional (P) Control Action (P- controller)
Advantages :
i) It is relatively simple and easy to design and tune.
ii) It responds very rapidly and dynamically it is relatively stable .
iii)Hunting effect is prevented .
Disadvantages :
i) Offset results when a large load change or set-point change occurs.
ii) Oscillatory process response on increasing a proportional gain.
iii) Proportional Kick
Proportional Kick :
In P-action, if there is a sudden change in set-point, the controller output
changes abruptly which may undesirable.
Control Actions or Types of Feedback Controllers
Proportional (P) Control Action (P- controller)
Applications:
P-controller is used for the processes where small
magnitude load change occurs with small to moderate speed.
e.g. liquid-level and gas pressure processes, for such processes, it
is usually required to keep the controlled variable within a certain
range around the set-point and hence P-controller serves the
purpose.
Control Actions or Types of Feedback Controllers
Integral (I) Control Action (I- controller)
In this control action, the controller output depends on the integral of error
signal over time from when observation is started at t = 0.
Control Algorithm:
Mathematically, I- control action is expressed as:
where, KI = Integral gain (sec-1 or min-1),
pI(0)= Controller output at t=0 , (%)
Control Actions or Types of Feedback Controllers
Integral (I) Control Action (I- controller)
I-control algorithm can be expressed in terms of another adjustable
parameter, Integral time(I),
Control Algorithm:
where, I = Integral time (sec or min)
Control Actions or Types of Feedback Controllers
Integral (I) Control Action (I- controller)
Since the controller output, p(t) is function of
integral of error overtime , I-action will continue to
increase the controller output as long as error
exits. As a result the error due to any load change
will be finally reduced to zero.
Thus , I- action eliminates any undesirable offset.
This action is also known
as ‘Reset action’
because it applies correction until an error exits.
Control Actions or Types of Feedback Controllers
Integral (I) Control Action (I- controller)
Control Actions or Types of Feedback Controllers
Integral (I) Control Action (I- controller)
Control Actions or Types of Feedback Controllers
Integral (I) Control Action (I- controller)
I-controlled Process Response
Control Actions or Types of Feedback Controllers
Integral (I) Control Action (I- controller)
Advantage:
i)It eliminates any offset as I-action applies correction until
error exists
Disadvantages:
i) It tends to make process response more sluggish.
ii)It also produces oscillatory process response &
hence reduces the system stability.
iii) I-action increases the system type by 1.
Applications:
I-action can be used for the processes which have a
small process lags( fast process response) &
correspondingly small capacities.
e.g. Flow process with small process lag & single capacity.
Control Actions or Types of Feedback Controllers
Derivative (D) Control Action(D- controller)
The D-action predicts the future behavior of error & acts on that prediction in
advance by considering the rate of change of error.
Due to this advance action ,it is also called as Anticipatory control action.
Control Algorithm:
p (t )  K D
de
 po
dt
Where, KD = Derivative(Rate) gain (sec or min)
po = Controller output when error is zero or constant (%
p (t )   D
de
 po
dt
Where D = Derivative(Rate) time ,(sec or min)
Control Actions or Types of Feedback Controllers
Derivative (D) Control Action(D- controller)
Control Actions or Types of Feedback Controllers
Derivative (D) Control Action(D- controller)
Advantages:
i)As D-action anticipates the change of error, it has an ability to stabilize
the controlled process.
ii) It also improves the transient response of closed loop system, by
reducing overshoot and oscillation.
Disadvantages:
i)It is never used alone because when the error is constant, the controller
output is equal to its nominal(bias) value, po.
ii) If there is noise in the process measurement, noise will be amplified by
the derivative action.
Application:
Derivative action is never used alone to control processes,
it is always used in combination with the proportional or proportional plus
integral control action for controlling processes.
Control Actions or Types of Feedback Controllers
P-I Control Action (P-I Controller)
This is one of the important & most popular composite control actions. It has got
advantages of Proportional (P) & Integral (I) control actions.
Control Algorithm: Mathematically PI control action is written as:
t
p(t )  K P [e(t )  K I  e(t )dt ]  pI (0)
0
Where, KP = Proportional gain (dimensionless)
KI = Integral(Reset) gain (sec-1 or min-1)
pI(0) = Controller output at t=0 (%)
KP and KI are the two adjustable(tuning) parameters.
Control Actions or Types of Feedback Controllers
P-I Control Action (P-I Controller)
Control Algorithm: It can be expressed in terms of another adjustable parameters
as
t
100
1
p(t ) 
[e(t )   e(t )dt ]  pI (0)
PB
I 0
Where, PB = Proportional Band (%)
I = Integral (Reset) Time ( sec or min).
Here, PB and I are the two tuning parameters of PI controller.
Control Actions or Types of Feedback Controllers
P-I Control Action (P-I Controller)
PI Controller Response
Reset time (I): It is defined as the time required for I-action to repeat the
initial P- action amount in its output. Usually, reset time, I is expressed in
minutes sometimes it is expressed in minutes/repeat.
Control Actions or Types of Feedback Controllers
P-I Control Action (P-I Controller)
PI- Controlled Process Response
Control Actions or Types of Feedback Controllers
P-I Control Action (P-I Controller)
Advantages:
i) It completely eliminates any undesirable offset because I-action changes
the controller output as long as error exists in the process output.
ii) It provides a much faster process response than that of I-action alone.
Disadvantages:
i) If PI-controller is not properly tuned ,it makes the process response sluggish
or more oscillatory & thus reduces system stability.
ii) It increases the type of system.
iii) The integral action of PI-controller produces a saturation problem
called ‘Integral (Reset) windup’, when a large error persists for the
long time.
iv) More complicated to tune (PB, I) .
Applications:
PI-controllers are used for fast processes in which moderate to large change
occurs frequently. e.g. Liquid flow ,liquid pressure and/or vapor pressure
processes .
Control Actions or Types of Feedback Controllers
P-I-D Control Action (PID Controller)
It is generally believed that PID controllers are the most popular controllers used in
process industries. Because of their remarkable effectiveness and simplicity of
implementation, these controllers are widely used in industrial applications, and more
than 90% of existing control loops involve PID controllers .
It has three basic parts to give the control signal:
i) P-action-a part proportional to the present information of error(current error ).
ii) I-action-a part proportional to the past information of error (integral of error).
iii) D-action-a part proportional to the future state of error(rate of change of error).
Control Actions or Types of Feedback Controllers
P-I-D Control Action (PID Controller)
Control Algorithm:
Mathematically PID control action is written as
t
p(t )  K P [e(t )  K I  e(t )dt  K D
0
de
]  p I (0)
dt
Where,
KP= Proportional gain (dimensionless)
KI= Integral(Reset) gain (sec-1 or min-1)
KD= Derivative(Rate) gain (sec or min)
pI(0)= Controller output at t=0 (%)
KP, KD and KI are the tuning parameters of controller.
The control algorithm can be expressed in terms of another adjustable parameters as
t
100
1
de
p(t ) 
[e(t )   e(t )dt   D ]  p I (0)
PB
I 0
dt
Where,
PB =Proportional Band (%)
I = Integral (Reset) Time ( sec or min).
D = Derivative(Rate) Time (sec or min).
Here, PB, D and I are the three tuning parameters of PID controller.
The PID control algorithm is used for the control of almost all loops in the process industries.
Control Actions or Types of Feedback Controllers
P-I-D Control Action (PID Controller)
PID Controller Response
Control Actions or Types of Feedback Controllers
P-I-D Control Action (PID Controller)
PID- Controlled Process Response
Control Actions or Types of Feedback Controllers
P-I-D Control Action (PID Controller)
Advantages:
+ It provides the best control.
+ Better performance than PI-action.
+ I-action of PID-action eliminates the offset completely.
+ D-action of PID-action increases the stability of the system,
by reducing the overshoot, and improving the transient response.
Disadvantages:
-
Most complicated to tune (PB, I , D ) .
Cannot handle constraints on controlled variables
Derivative action may be affected by noise.
Integral Windup
Derivative Kick
Applications:
PID controllers are used for virtually any process conditions.
Usually PID controllers are used for sluggish,
multi-capacity processes to speed up the process response.
e.g. Temperature, composition and pH processes.
Control Actions or Types of Feedback Controllers
P-I-D Control Action (PID Controller)
Integral Windup or Reset Windup:
This is an inherent drawback present in PI- or PID controllers due to an Integral action.
PI-controller algorithm:
t
p(t )  K P [e(t )  K I  e(t )dt ]  p I (0)
0
PID-controller algorithm:
t
p(t )  K P [e(t )  K I  e(t )dt  K D
0
de
]  p I (0)
dt
Integral windup takes place when a PI- or PID-controller sees a large
sustained error. The sustained error typically occurs after a large set-point change
or as a consequence of large sustained load change. This error can also
come during startup or shutdown of batch processes , in cascade control and
when a final control element is driven by more than one controller, as in override
control schemes.
Control Actions or Types of Feedback Controllers
P-I-D Control Action (PID Controller)
When a large sustained error occurs , I-action terms becomes quite large & drives
the controller output to its maximum or minimum allowable value (0% or 100%) and
saturates controller output at last. In physical terms, the control valve driven by
controller becomes either fully open or fully closed before a control action is being
completed. As control valve can not be moved further, error remains nonzero for
long time.
And the integral action still continues to build up even after controller output
saturates. This further buildup of integral action while the controller is saturated is
called Integral Windup or Reset Windup (fig.4.28 ). After the load (disturbance) or
set-point returns to its normal level, the controller output remains saturated for a
period of time causing an upset in the process output.
Integral windup situation is not a deficiency of control algorithm, it
represents a shortcoming of process & control equipments (e.g. control valve).
Industrial Methods for Anti-Reset Windup :
i) Apply external reset feedback.
ii) Digitally turn-off integral calculation when the controller (or a control valve) saturates or a control loop is not
in use. ( Implement velocity-type digital PID algorithm )
iii) Clamp the controller output to be greater than 0% and less than 100%.(Use of batch
unit for batch processes)
Control Actions or Types of Feedback Controllers
P-I-D Control Action (PID Controller)
Derivative Kick :One disadvantage of the PID controllers is that a sudden change in set point and hence the error, e(t) will cause
the derivative term momentarily to become very large and thus provide a derivative kick to the final control element. This sudden
change is undesirable and can be avoided by basing the derivative action on the process measurement, B(t) rather than on the
error signal, e(t).
The modified PID-control algorithm is given by
t
p(t )  K P [e(t )  K I  e(t )dt  K D
0
d[ B(t )])
]  p I (0)
dt
Where, e(t)=R(t)-B(t)
Electronic Controllers
The control actions of feedback controllers can be implemented
by a direct application of standard op-amp circuits such as
inverter, integrator, differentiator, summing amplifier, & etc. Hence,
The resulting circuits are known as electronic controllers.
 Electronic Proportional (P) controller
 Electronic Proportional (P) controller
 Electronic Proportional (P) controller
Electronic P Controller
Control algorithm of electronic P-controller in voltage form:
V out K PVe  Vo
Where Vout = Controller output voltage (V)
Ve = Error voltage (V)
Vo = Controller output when error is zero
KP = Proportional gain (adjustable)
= R2/R1
R1 = Variable resistor
R2 = Fixed value resistor
Electronic PI Controller
Vout
R
 2
R1
Vout
t


1
V
dt
Ve 
  Vout (0)
e

R2 C 0


t


 K P Ve  K I  Ve dt   Vout (0)
0


Where, KP = Proportional gain (adjustable)
= R2/R1
KI = Integral gain (sec-1, adjustable)
= 1/R2C
Vout(0) = Initial controller output voltage
Electronic PID Controller
Circuit Diagram of Electronic PID-Controller
Vout
R
 2
R1
t

dVe 
1
V

V
dt

R
C
 e
  Vout (0)
e
D D

R
C
dt
I I 0


Vout
t

dVe 
 K P Ve  K I  Ve dt  K D
  Vout (0)
dt
0


KP = R2/R1 KI = 1/RICI KD = RDCD
Vout(0) = Initial output voltage (at t = 0)
Pneumatic Controllers
Pneumatic controllers are exclusively used in process industries
because of their explosion-proof feature, simplicity, ease of
maintenance & relatively less cost.
Now days, these controllers are preferred where safety is prime
factor, mainly in hazardous area.
A pneumatic controller is a device that uses a compressible gas or
air as a control medium to provide an output signal which is a
function of an input error signal.
The design of pneumatic controller is based on flapper nozzle
mechanism, which is same as the op-amp used in electronic
controller. To implement the control actions, pneumatic controllers
also use pilot relay, bellows, variable restrictions , in addition to
basic flapper-nozzle system.
Pneumatic PID Controller
Pneumatic PID Controller
The physical implementation of pneumatic PID-controller using four bellows,
flapper-nozzle system, pilot relay and two variable restrictions is shown in figure.
When the process measurement pressure(Pm) rises above the set-point
pressure(Psp), the flapper moves towards nozzle that increases nozzle back
pressure i.e the pilot relay output pressure (Pout) . The output pressure is fed to
the feedback bellows through a variable restriction (1) and also to reset bellows
via variable restriction(2). The output pressure increases rapidly until it bleeds
into the feedback bellows through variable restriction(1). When the increased
output pressure is leaks into feedback ,the flapper is moved away from nozzle to
balance a force exerted by the process measurement bellows. And as a result
output pressure decreases.
The reset bellows is still at the original output pressure because a
restriction prevents pressure changes from being transmitted immediately. As the
increased pressure on the output bleeds through restriction, the reset bellows
slowly moves the flapper towards nozzle, thereby causing a steady increase in
output pressure.
The variable restriction(1) permits the adjustment of derivative(rate)
time. The variable restriction(2) allows the adjustment of reset time.
Hydraulic Controllers
A hydraulic controller is a device that uses a incompressible fluid
as a control medium to provide an output signal which is a
function of an input error signal.
Elements of Hydraulic Controllers:
a hydraulic relay or amplifier
a signal sensing section
an error detector
Hydraulic PI Controller
Hydraulic PI Controller
Consider a hydraulic PI-controller shown in fig. , which is used to control the flow of a fluid passing through a
pipe line.
The sensor used for flow measurement is the orifice plate across which a differential pressure (P) is
produced such that flow rate is proportional to the square root of the differential pressure.
The high pressure (H.P.) & low pressure (L.P.) tapping are applied across a corrugated diaphragm
which together with a set-point spring mounted on opposite sides jet-pipe form an error detector assembly.
The error signal positions the jet pipe either towards d1, or d2 (distributor lines).
The PI-controller has two piston-cylinder arrangements.
1.Feedback cylinder with a needle valve ( or bypass integral valve).
2.Work cylinder which positions the final control element in the pipe-line.
Initially, let us assume that the flow of fluid has become more than the set point set on the set-point spring. This
pushes the diaphragm up & so the jet pipe moves up towards d1. This sends more fluid in lower chamber of the
feedback cylinder. Because of fluid pressure, the piston in the feedback cylinder moves up & the fluid in the
upper chamber is forced into the lower chamber of work cylinder. Hence, the piston in the work cylinder also
moves up closing the control valve(P-action).
Because of the upward movement of the piston in the feedback cylinder, a feedback is given to
the jet pipe through the pivoted feedback linkage . The jet pipe moves back to it’s central position. This results
in more fluid sent in the upper chamber of the work cylinder. This fluid opposes the easy upward movement of
the piston (P action).
Meanwhile the fluid in the lower chamber of feedback cylinder is sent into the upper chamber through the needle
valve .More fluid in upper chamber results in providing more fluid to lower chamber of work cylinder & hence, the
rate of movement of piston upward increases steadily (I-action).
Comparison
Electronic, Pneumatic and Hydraulic Controllers
Control
medium
Basic components
Pneumatic Controllers
Compressible gas or
Air.
Flapper-nozzle, pilot relay,
bellows, variable restrictions &
spring.
Hydraulic Controllers
Incompressible fluid.
Jet pipe(or spool)valve,
work cylinder, feedback cylinder,
diaphragm & spring.
Standard
transmission signal
(3-15) psig
(3-15) psig
Design
Start-up
period
Speed of
response
Transmission
distance
Maintenance
Accuracy
Initial cost
Operation in
hazardous area
Problem
Simple
Short
Moderate
Short
Output power
Life
Fast
Medium
Little
Low
Low
Safe
External leakage is permissible
to a certain extent but not the
internal leakage.
Medium
Moderate
Electronic Controllers
Current or Voltage
Signal.
Op-amp, resistors, capacitors, etc.
(4-20) mA dc
Complex
Long
Faster
Slow
Short
Long
Relatively low
Medium
Higher
Unsafe(safe with fire- resistant
fluids)
Internal leakage is permissible to
a certain extent but not the
external leakage.
Higher
Longer
Large
High
High
Unsafe (safe with proper
explosion proof housing)
Electrical noise
pick-up.
Less
Moderate
PID Controller Tuning
Tuning of PID controller is the process by which a control
engineers or technicians select the best values of controller
parameters [Proportional gain (Kp), Integral gain (KI) and
Derivative gain (KD)] to make the process being controlled to
respond as desired.
In short, PID controller tuning is “the process of correct
determination of Kp, KI and KD to achieve an optimal control”.
PID Controller Tuning
Tuning Criteria or Performance Indices:
To achieve an optimal performance of a PID-controlled process,
PID controllers must be tuned on the basis of one or more performance
Indices. The performance indices that are considered while tuning of
PID-controller are called as tuning criteria.
The tuning criteria:
1. Time/Frequency- domain tuning criteria
2. Integral Error criteria
Time/Frequency- domain tuning criteria:
Some examples are minimum overshoot, minimum settling time,
minimum rise time, zero offset at steady- state, an optimum (1/4) decay
ratio, positive gain and phase margins, and so on.
Of all, the most popular tends to be a quarter (1/4) decay ratio.
Conventional Methods for
PID Controller Tuning
1. Field tuning methods: Ziegler-Nichols’ tuning
methods:
a) Ultimate Gain Cycling method
b) Process Reaction Curve method
2. Tuning Relations: Cohen -Coon method
3. Direct Synthesis method
4. Internal Model Control(IMC) method
5. Frequency Response method
Conventional Methods for
PID Controller Tuning
Ziegler-Nichols’ PID Controller Tuning methods:
Ziegler-Nichols’ field tuning methods are the most popular ones and
mainly used for P, PI and PID controllers.
There are two methods for tuning PID controllers.
a) Ultimate Gain Cycling/ Closed Loop Response method
b) Process Reaction Curve/ Open Loop Response method
In each method, one experiment is performed on process to determine the
values of dynamic parameters of process. The process parameters are then
used to calculate the optimum values of controller parameters (Kp, I and D)
by referring tuning tables.
Ziegler and Nichols’ Ultimate Gain Cycling/
Closed Loop Response Method
While performing experiment on process, PID-controller is placed in ‘Auto
Mode’ with P-control action only (eliminating I- and D-actions). P-controller
gain is then increased until process response exhibits sustained
oscillations.
Ziegler and Nichols’ Ultimate Gain Cycling/
Closed Loop Response Method
Experiment is carried out through following steps:
1. Eliminate the integral and derivative action by setting D at its
minimum value and I at its maximum value.
2.
Set proportional action gain, Kp at a low value and put the
controller in automatic mode.
3. Increase the proportional gain Kp by small increments
until continuous cycling occurs. The term “continuous cycling” refers
to a sustained oscillation with constant amplitude as shown in Fig.
4. Note the proportional gain that results in a sustained oscillation as an
Ultimate Gain, Kpu and period of oscillation as the Ultimate Period, Pu.
5. Calculate the controller parameters (Kp, I and D) from tuning table
given by Ziegler and Nichols (based on quarter decay ratio criterion).
Ziegler and Nichols’ Ultimate Gain Cycling/
Closed Loop Response Method
Table- Ziegler Nichols’ Tuning Relations based on Ultimate Gain
Cycling Method:
Controller
Kp
(%)
I
(sec)
D (sec)
Type
P
0.5 Kpu
--
--
PI
0.45 Kpu
Pu /1.2
--
Pu /2
Pu /8
PID
0.6 Kpu
Ziegler and Nichols’ Ultimate Gain Cycling/
Closed Loop Response Method
Advantages:
1.It is less time consuming.
2.It requires no a priori information on process.
3.This method is applicable to all stable processes.
Disadvantages:
1.The tuning process is not applicable to processes that are open loop
unstable.
2.Some simple processes do not have an ultimate gain (e.g. first and
second order processes without time delays will not oscillate even with
very large controller gain).
Ziegler and Nichols’ Process Reaction Curve
/ Open Loop Response Method
A small step change in the controller output is introduced and measured
process response is recorded. This step response is also called as the
Process Reaction Curve which is characterized by two parameters: S, the
slope of the tangent through the inflection point, and , the time at which
the tangent intersects the time axis.
Ziegler and Nichols’ Process Reaction Curve
/ Open Loop Response Method
Experiment is carried out through following steps:
Steps:
1. With the controller in the manual mode, the controller output (p) is suddenly
changed by an amount p. The process output curve follows one of the
curves shown in fig.
2. In the Process Reaction Curve (PRC), a tangent at inflection point and
horizontal line are drawn to determine dead time (), time constant (), and
change in process output (T). Inflection point is the point at which PRC has
maximum slope.
3. Compute the average and normalized slopes (S, S*)of PRC using following:
Average slope, S = T/
Normalized slope, S* = S/ p.
4. Compute the controller settings from the following tuning table .
3
0
210.1.33
529
S *
Ziegler and Nichols’ Process Reaction Curve
/ Open Loop Response Method
Table- Ziegler Nichols’ Tuning Relations based on PRC Method:
Kp (%)
I
P
1/ (. S*)
--
--
PI
0.9/ (. S*)
3.33 
--
PID
1.2/ (. S*)
2.0 
0.5 
Controller
(sec)
D (sec)
Type
Ziegler and Nichols’ Process Reaction Curve
/ Open Loop Response Method
Advantages:
1.It does not require trial and error approach.
2.Applicable to self- regulating (stable) as well as non-self-regulating
(unstable) processes.
Disadvantages:
1.Ziegler-Nichols recommendations are very sensitive to the ratio / and
hence should be used for processes with
0.1 < / <1.
2.It may be difficult to determine the slope at the inflection point
accurately, especially if the measurement is noisy and a small recorder
chart is used
3.The method tends to de sensitive to controller calibration errors.
4.The method is not recommended for processes that have oscillatory
open-loop responses since the process model will be quite inaccurate.
Control Schemes
1. Feedback control scheme
2. Feed-forward control scheme
3. Cascade control scheme
4. Ratio control scheme
5. Selective control scheme
6. Split-range control scheme
7. Self-tuning adaptive control scheme
Feed-forward Control Scheme
The basic concept of feed-forward control is to measure important
disturbance variables when they enter the process, and take corrective
action in advance before they are going to upset the process output.
The feed-forward control begins to take corrective action as soon as
disturbance is detected.
It does not wait for the disturbance to propagate all the way through the
process .Due to this beforehand action, f/f control is also referred to as
anticipatory control.
Feed-forward Control Scheme
Automatic Process Control System
“Feed-forward Scheme”
Feed-forward Vs Feedback Controls
Feed-forward Control:
“ Anticipatory Control”
Distinguishing feature: measures disturbance variable.
Advantage:
• Corrects for disturbance before it upsets the process.
Disadvantage:
• Must be able to measure the disturbance.
• No corrective action for unmeasured disturbances.
• Must know the process model.
Feedback Control: “Compensatory Control”
Distinguishing feature: measures the controlled variable.
Advantages:
• Simple, no detail knowledge of process is required.
• Corrective action is taken regardless of the source of the disturbance.
Disadvantages:
• No corrective action occurs until after the disturbance has upset the
process.
Cascade Control Scheme
The second alternative to simple a feedback control is Cascade Control.
Cascade control is particularly useful when
i) there are two or more available measurements, one controlled
variable, and only one manipulated variable,
ii) the major disturbance is associated with the manipulated variable,
III) the final control element exhibits non-linear behavior (e.g. valve
hysteresis),
iv) and there are significant dynamics (for example, long time delays or
long time constants) between the manipulated variable and the
controlled variable.
Cascade Control Scheme
Cascade control is the combination of two feedback controllers, where
output signal from one controller acts as set-point for another controller .
Cascade control is built up by nesting the two feedback control loops
around two processes-I and II as shown in figure.
Cascade Control Scheme
There are two loops:
The inner loop is called the secondary or slave loop.
The outer loop is called the primary or master loop.
An inner and outer control loops are formed with separate feedback
controllers.
The outer loop controller is also known as the master or primary controller.
The input to this controller is the measured value of the variable to be
controlled. The set-point is supplied by the operator. The primary loop
controller is used to calculate the set-point for the secondary(inner) control
loop. This controller is usually designed with standard PID-control action.
The inner loop controller is known as the slave or secondary controller.
It measures a secondary variable whose value affects the controlled
variable. The set-point is supplied by the output from the outer loop.
Its output signal is used as to manipulate the process input.
This controller is generally designed with P- or PI-control action.
Cascade Control Scheme
The major benefit from using cascade control is that disturbances arising
within the secondary loop are corrected by the secondary controller
before they can affect the value of the primary controlled output. Cascade
control is especially effective if the inner loop is much faster than the outer
loop and if the main disturbances affect the inner loop first.
Cascade Control Scheme
Advantages:
i) Better control of the primary variable
ii) Primary variable less affected by disturbances
iii) Reject the disturbance in the slave loop
before it affects the main process variables.
Disadvantages:
i) Cost of measurement of secondary variable
ii) Additional complexity.
Ratio Control Schemes
Ratio control is a special type of feed-forward control.
The objective of a ratio control scheme is to keep the ratio
of two process variables at a specified value.
The two process variables are usually flow rates of a
manipulated stream(m) and a disturbance stream(d).Here,
the disturbance stream is also referred to as wild or load
stream.
Thus, the ratio (R) of two variables, m and d
R = m / d is controlled rather than controlling the
individual variables.
Ratio Control Implementation
Method-I
Ratio Control Implementation
Method-I
The flow rates of both the load and the manipulated streams are measured
and the ratio is calculated using a 'divider' element(FY-102). The output of the
divider is sent to the flow ratio controller, FFC (which is actually a standard PI
controller). The controller compares the actual ratio with that of the desired
ratio and adjusts the manipulated stream accordingly.
The main advantage of this method is that the actual ratio R is calculated.
A key disadvantage is that a divider element is included in the loop, and this
element causes the process gain vary in a nonlinear fashion.
1
 R 
Kp  
 
 m  d d
Because of this significant disadvantage, the Method II is preferred for
implementing ratio control.
Ratio Control Implementation
Method-II
Ratio Control Implementation
Method-II
Here the manipulated stream(m) is under standard feedback control. The flow
of the wild stream(d) is measured using flow transmitter(FT-101) and sent to a
'multiplier' (FY-102 ) which multiplies the signal by the desired ratio(Rd) yielding
the set-point for the flow controller(FC-102).The flow controller then adjusts the
flow rate of manipulated stream(m).
The main advantage of this method is that the process gain remains constant
because divider is not used.
Ratio Control Applications
Applications of Ratio Control:
1. Blending two or more flows to produce a mixture with specified composition
e.g water wastewater treatment plants.
2. Maintaining a stoichiometric ratio of reactants to a reactor e.g. A ratio control
scheme is to be used to maintain a stoichoimetric ratio of H2 and N2 as the
feed to an ammonia synthesis reactor.
3. Keeping a specified reflux ratio for a distillation column.
4. Maintaining the fuel-air ratio to a furnace at the optimum value.
Ratio Control Applications
The pH transmitter(AT-103) that measures the pH of effluent , is connected to pHcontroller (AIC-103). The output of that controller is the ratio of NaOH flow rate to
acid wastewater flow rate. To manually adjust the ratio, the operator may place AIC103 into manual mode and adjust the output. The multiplier (FY-102) performs the
multiplication of measured wild flow (acid waste water) and pH controller output.
Thus the output the multiplier becomes the set-point for flow controller(FC-102)
which then compares it with measured flow(NaOH) and adjusts the NaOH addition to
maintain the desired pH of effluent.
Self-tuning Adaptive Control Scheme
Self-tuning Adaptive Control Scheme