elec3320- lecture notes 1

5/27/2018 ELEC3320- Lecture Notes 1

1/36

Introduction to Process Control

Process control is a branch of controlengineering.

It deals with the operation of plants inindustries such as petrochemicals, food,steel, glass, paper and energy.

Control Systems

The diagram below shows a general controlsystem. This diagram can be applied to all

control situations:


2/36

For example, the measurement devices willrange from yourself deciding if the process

is operating as it should be, to a pulse flow

sensor and transmitter arrangementautomatically measuring rate of flow offluid.

The controller may be simple logic levelswitching equipment or a universalmicroprocessor-based controller with manyinternal control algorithms.

The actuator converts the small energysignal from the controller to a large energyaction of the process. For example in a

liquid flow/level control situation theactuator may consist of a current topressure converter together with a

pneumatic valve.


3/36

Process Control and Servo Systems

The one basic difference between a processcontrol system and a servo system is thatgenerally the emphasis in process controlis on the performance of the loop as a

Regulator, i.e. disturbance rejection.

In servo systems the emphasis is on howwell the control system can follow changesin the reference or desired input signal.

In a typical process control system thereference value will not change frequently.For instance, the required temperature of a

particular product may be constant fordays.


4/36

Process Control Concepts

The term Set Pointis used to representthe reference input to the process controlsystem. The reference represents a desiredconstant operating point. The set point can

be imposed by turning a knob, typing in avalue, or it can be transmitted to acontroller from a computer or anothercontroller.

The term Measured Valuerepresents theoutput of the measurement system(transmitter, sensor or transducer). Themeasurement system produces a signalwhich is a function of the actual value of

the physical process variable being

controlled. The signal may be electrical,pneumatic or mechanical.

The set point is compared to the measuredvalue to produce the deviation, which is

simply the difference between the two. Thecontroller then uses this deviation to make

the set point and the measured value asclose as possible.

Safety considerations are alwaysparamount in process control. Therefore,


5/36

safeguarding and monitoring systems mustbe included to deal with cases of

equipment failure. All possible failures

must be considered, so that the system isprevented from failing or, if failure cannotalways be avoided, then that it will fail

safely.

Instrument Classifications

There are generally two kinds ofinstrument used in process control. Firstly,instruments for monitoring processvariables such as temperature or pressure

and that give an audio or visual indicationof the magnitude of the physical quantitymeasured. An example is a liquid-in-glass

thermometer.

Secondly, instruments referred to astransmittersin process control

engineering are those incorporated in anautomatic control system. They are neededto provide (transmit) information about the

plant status to the controller, and hencetheir output must be in a suitable form(electrical, hydraulic, pneumatic) to beaccepted by the controller.


6/36

The primary component of both types ofmeasurement instrument is a transducer,

or a sensor that converts the measured

physical quantity from one form to another.

Active and Passive

Another classification is between activeand passiveinstruments. If the

instrument output is entirely produced bythe quantity being measured, theinstrument is termed passive.

A pressure gauge is a passive instrumentsince the pressure of the fluid is translatedinto movement of a pointer against a scalewithout any external power source.

A liquid level indicator is an activeinstrument, since the change in liquid level

moves a potentiometer arm, in which casethe output signal is a portion of the


7/36

external voltage applied across the twoends of the potentiometer.

Null vs. Deflection

A further distinction is nullversusdeflectioninstruments.

In a deflection type of instrument the valueof the quantity being measured isdisplayed in proportion to the displacementof a pointer.

A dead-weight pressure gauge is anexample of a null-type instrument. The

pressure being measured displaces apiston, and weights are placed on top ofthis piston until it reaches its zero position

again. The value of the weights needed toreach this position represents the pressuremeasurement.


8/36

Null-type are more accurate than deflectiontype instruments because of the calibration

accuracy in the former. However,

deflection type instruments are generallymore user-friendly.

Analogue and Digital

A final comparison is between analogueand digitalinstruments.

An analogueinstrument gives an outputsignal that is a continuous function of theinput signal being measured. An example

of an analogue instrument is the deflectiontype pressure gauge.

However, a digital instrument gives anoutput that varies in discrete steps. Theadvantage of using a digital instrument is

that it can be directly connected to acomputer so that digital control of theprocess can be carried out.


9/36

If an analogue instrument is used in adigital control system, an analogue-to-

digital converter is needed to convert the

analogue output signal from the instrumentto a digital form, for processing by thecomputer.

Instrument Characteristics

Accuracyis a measure of the deviation ofa reading from the true value, and isusually quoted as a percentage of the full-scale reading of the instrument.

Precisiondescribes the extent to which aninstrument is free from random errors,

these are errors due to electrical noise,environmental changes etc.

A large number of readings of the samequantity taken by a high precisioninstrument should differ very little. Highprecision does not imply anything about

measurement accuracy. A high precisioninstrument may have low accuracy. Lowaccuracy measurements from a highprecision instrument are normally caused


10/36

by an offset in the instrument and can becorrected with calibration.

Dead spaceis the range of input valuesfor which there is no change in the output.Dead space is also sometimes calledDeadband.

The Hysteresis Curveis shown below:

It consists of two curves identical in shape. Theupward and downward arrows describe theway in which the output reading varies as the

input quantity to the instrument increases and

decreases respectively.

The measurement resolutionis theminimum change in the input measured


11/36

quantity that will produce an observablechange in the instrument output.

Analog Data Representation

4-20 mA current and 3-15 psi pressure aretwo commonly used analog signals forinformation transmission in a processcontrol plant.

Sensor in the above figure should becalibrated so that 4-20 mA currentrepresent the full measuring range of theprocess variable.


12/36

Similarly the 3-15 psi signal should becalibrated to represent the valve fully

closed to fully open orientation.

Current and pressure signals can betransmitted over long distances withoutloss of signal independent of line resistance.


13/36

Process Control Principles


14/36


15/36

Discrete State Control

In process control not only it is important to regulatecertain variables at predefined values but also certain

events need start and stop at certain times.

Discrete State Control is about controlling thesequence of events in a process control environment.


16/36

ON/OFF Control

Final control element has only two states, it either ONor OFF, OPEN or CLOSE.

The above system exhibits hysteresis and a deadband.


17/36

Analog Control

All variables in an analog control system are analogrepresentations of another variable.

Analog control systems allow continuous variation ofsome parameter.


18/36

Digital Control

There are two types of Digital Control:1.Supervisory Control2.Direct Digital Control

Supervisory Control

Loops are analog Computer monitors the variables and updates set point


19/36

Direct Digital Control

Smart Sensor

The measuring sensor, ADC, computer controller,DAC producing 4 to 20 mA current output are all built

into one unit called a smart sensor.

It also provides serial interface for another computerto update the set point.

The most current technology allows smart sensors tobe connected to a LAN. These LANs are referred toas field bus.


20/36

Networked Control Systems

Several DDC units are connected to the plant andregulate various process variables.

DDC units are also connected to a LAN and able tocommunicate to various other computers in the

network.

Bus users can monitor the operation of the plantprocess control loops and those with authorization can

modify set-points and gains.


21/36

Distributed Control System (DCS)

DCS is an example of a networked control system. There are various data highways, i.e. Slave bus,Controlway and the central communication network. Raw data from the process plant is placed on the slave

bus and passed to Multifunction Processors (MFP).


22/36

MFPs can alter the set points and also send control

signals to the final control elements such as actuators.

MFPs will also place any important data on thecontrolway and will pass this data to the CentralCommunication network via the Network Processing

Units (NPU).

Authorized users can login to the CentralCommunication Network from anywhere in the world

to monitor and change process parameters.

Block Diagram Representation ofControl System


23/36


24/36

Control System Evaluation

A control system objective is best represented by thefollowing requirements:

1.The system should be stable2.The system should provide best steady state

regulation

3.The system should provide best possible transientregulation


25/36


26/36

1.Minimum Area Criteria2.Quarter Amplitude Criteria


27/36

Process-Control Drawings (ISAstandards)

A standard set of definitions are used to represent aplant and its associated control systems.

This standard was developed and approved by acollaboration between American National Standards

Institute (ANSI) and the Instrumentation Systems

Automation (ISA) society.


28/36


29/36


30/36


31/36


32/36


33/36


34/36


35/36


36/36

Draw the following figure in the standard PI&D symbols.

elec3320- lecture notes 1

Documents

process control concepts

control systems

process controlsystem

process controlis

elec3320 lecture notes

servo systems

processcontrol system

themeasurement system