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Process Control 2 (PSC261S)
Content
1. Review
2. Introduction
3. Explain the control system configurations
3.1 Single-Variable
3.2 Compound
3.3 Cascade, and
3.4 Multivariable control.
4. Standard measures of quality in a control system
5. Control valves and valve positioners
By Dr. Tshemese-Mvandaba
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Process Control 2 (PSC261S)
1. Review
 Overall control system Structure
&
 Sensor Selection

Plant floor communication in an industrial environment
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Process Control 2 (PSC261S)
2. Introduction
 Overview
 The purpose of a process
control system is to maintain
some process variable at the
setpoint.
 The configuration and
complexity of process
control system depend on
the characteristics of
process under control.
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Process Control 2 (PSC261S)
3. CONTROL SYSTEM CONFIGURATIONS
 A system is a collection of components which are coordinated together to perform a
function.
 System interact with their environment across a separating boundary,
 The interaction is defined in terms of variables:

System inputs

System outputs

Environmental disturbances
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Process Control 2 (PSC261S)
3. CONTROL SYSTEM CONFIGURATIONS
3.1 Single Variable
1. Independent Single Variable
2. Interactive Single Variable
3. Compound Variable
 Single Variable



The simple process control loop is based on a single variable loop.
The loop is designed to maintain control of a certain process variable.
This is done by manipulation of a controlling variable, regardless of the other process
parameters.
In control engineering, a single input and single output (SISO) system is a simple single variable
control system with one input and one output.
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Process Control 2 (PSC261S)
3. CONTROL SYSTEM CONFIGURATIONS
 Independent Single Variable
It is a variable that
stands alone and
isn't changed by the
other variables you
are trying to
measure.
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Process Control 2 (PSC261S)
3. CONTROL SYSTEM CONFIGURATIONS
 Interactive Single Variable
In this example, there is process tank. Water is fed
through an inlet tube. Water is taken out through
an outlet tube. There is a flow controller to
maintain constant flow through the inlet tube.
The water inside the tank is heated.
There is a temperature control system to maintain
the tank temperature.
Under nominal conditions, the flow into the tank is
held constant and the temperature is also held
constant. If the setpoint of the flow control system
changes, the flow control loop adopts a new flow
rate, which will appear as a load change to the
temperature control system.
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Process Control 2 (PSC261S)
3. CONTROL SYSTEM CONFIGURATIONS
 3.2 Compound Variable


In some cases, a single
process control loop is used
to provide control of the
relationship between two or
more variables.
This can be accomplished by
using measurements from
two sensors as input to the
process controller
A signal conditioning system must
scale the two measurements and add
them prior to input to the controller
for evaluation and action.
The analysis of such systems can
become quite complicated.
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Process Control 2 (PSC261S)
3. CONTROL SYSTEM CONFIGURATIONS
3.3 Cascade Control
Cascade control is a control algorithm in which the output of one control loop provides the target for another
loop, as shown in the diagram below. The ultimate goal of the cascaded loops is to control the end process.
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Process Control 2 (PSC261S)
3. CONTROL SYSTEM CONFIGURATIONS
3.3 Example Cascade Control
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Process Control 2 (PSC261S)
3. CONTROL SYSTEM CONFIGURATIONS
3.4 Multivariable Control Systems
 The use of the word multivariable in this section refers to those processes in which many
strongly interacting variables are involved.
 Such a multivariable system can have such a complex interaction pattern
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Process Control 2 (PSC261S)
3. CONTROL SYSTEM CONFIGURATIONS
3.4 Multivariable Control Systems
3.4.1 Analog Control
 When analog control loops are used in multivariable systems, a carefully prepared instructional set
must be provided for the adjustment of setpoints.
 Generally, such adjustments are carried out in small increments to avoid instabilities that may result
from large changes.
3.4.2 Supervisory and Direct Digital Control
 The computer is ideally suited to the type of control problem presented by the multivariable control
system

The computer can make any necessary adjustments of system operating points in an incremental
fashion, according to a predetermined sequence, while monitoring process parameters for interactive
effects.
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Process Control 2 (PSC261S)
3. CONTROL SYSTEM CONFIGURATIONS
3.4 Multivariable Control Systems
3.4.2 Supervisory and Direct Digital Control
 The problem in such a system is determining the algorithm that the computer must follow
to provide the control function of the setpoint change sequence.
 In some cases, control equations are used.
 Usually, in complex interactions, these relations are not logically known.
 In some cases, self adapting algorithms are used, causing the computer to sequence
through a set of operations and letting the result of one operation determine the next
operation.
 As an example, if the temperature is slightly raised and the pressure rises, then drop the
temperature, and so on.
 The computer can go through a numerous micro adjustments of setpoints looking for the
optimum adjustment path.
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Process Control 2 (PSC261S)
4. Standard measures of quality in a control system
In brief, given that a control system can provide a product that meets specifications.
 We ask questions such as how well does it perform this job, what variation in parameters
exists, what percentage of rejected product occurs, and so on.
 To answer these questions, we must first describe measures of quality in a control system and
then analyze how the loop characteristics affect these measures.
The definition of quality in control system is based on the following:

Loop Disturbance

Optimum Control

Stability

Minimum Deviation

Minimum Duration
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Process Control 2 (PSC261S)
4. Standard measures of quality in a control system
Loop Disturbance


The process control system is supposed to provide regulation so that disturbances in the system will
cause minimum deviation of the controlled variable from the setpoint value.
The quality of the control system is defined by the degree to which the deviations that result from
the disturbances are minimized.
There are three basic types of disturbances that can occur in a process control
system:
 Transient
 Setpoint changes
 Load changes
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Process Control 2 (PSC261S)
4. Standard measures of quality in a control system
Loop Disturbance: Transient
 A transient disturbance results from a brief change of some parameter in the system
that affects the controlled variable.
 It is not practical to use transient disturbances to define control quality because the
nature of a transient cannot be well defined.
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Process Control 2 (PSC261S)
5. Control valves and valve positioners
Control-Valve Types:



Quick Opening,
Linear, and
Equal Percentage
Quick Opening
Symbols for some final control elements.


This type of valve is used mostly for full ON/full OFF control applications.
The valve characteristic of the Figure shown above shows that a relatively small motion of the valve stem
results in maximum possible flow rate through the valve. Such a valve, for example, may allow 90% of
maximum flow rate with only a 30% travel of the stem.
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Process Control 2 (PSC261S)
5. Control valves and valve positioners
Linear


This type of valve, as shown in this Figure, it has a flow rate that varies
linearly with the stem position.
It represents the ideal situation where the valve alone determines the
pressure drop.
Equal Percentage

Equal Percentage A very important type of valve employed in flow control has a characteristic such that a
given percentage change in stem position produces an equivalent change in flow—that is, an equal
percentage.

Generally, this type of valve does not shut off the flow completely in its limit of stem travel.
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Process Control 2 (PSC261S)
5. Control valves and valve positioners

The different types of control valves are classified by a relationship between the valve
stem position and the flow rate through the valve.

This control-valve characteristic is assigned with the assumptions that the stem position
indicates the extent of the valve opening and that the pressure difference is determined
by the valve alone.

Correction factors allow one to account for pressure differences introduced by the whole
system
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Process Control 2 (PSC261S)
Conclusion
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Process Control 2 (PSC261S)
References:
1. Process Control Instrumentation Technology, by Curtis D. Johnson Eighth Edition 2014. Chapter 12.
Control-Loop Characteristics
2. William Bolton (2015), 2nd Edition Instrumentation and Control Systems
Next Lesson
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