Basics of Process Automation
Automation is the use of
control systems and information technology to reduce human efforts by reducing
its intervention while controlling.
Process automation is specific to manufacturing or any such process wherein we are creating a product of required specifications with the help of automation. To control such a process first we need to sense the parameters to be controlled which is done through a sensor.
A SENSOR is a device used to convert a physical parameter such as room temperature, pressure, flow etc. into a signal that can be measured. It is not necessary that these signals are in the form of electrical signals. So to understand or manipulate the output of sensor, the signals of sensors should be in electrical form. This is achieved by TRANSDUCERS. The manipulation for control is done by a controller which can be a PLC or DCS. The controllers are placed at a distance from the field. For signals to reach there without any drop TRANSMITTERS are used.
Process automation is specific to manufacturing or any such process wherein we are creating a product of required specifications with the help of automation. To control such a process first we need to sense the parameters to be controlled which is done through a sensor.
A SENSOR is a device used to convert a physical parameter such as room temperature, pressure, flow etc. into a signal that can be measured. It is not necessary that these signals are in the form of electrical signals. So to understand or manipulate the output of sensor, the signals of sensors should be in electrical form. This is achieved by TRANSDUCERS. The manipulation for control is done by a controller which can be a PLC or DCS. The controllers are placed at a distance from the field. For signals to reach there without any drop TRANSMITTERS are used.
After processing the input
signal, the processor generates output depending on the logic which it has to
be performed. The output signal is then forwarded to an output device. If the
output device is a motor, then the signal from processor will give command to
the Motor Control Centre (MCC). If output device is a VFD or electrically
operated control valve then signal from processor can directly go to it. If
output device is a valve that too pneumatically operated then a device called
converter is used after controller to convert its electrical signal to
pneumatic signal which can then be given to the control valve.
A CONVERTER thus is a device which converts a standard signal to the signal which can be understood by the device to be interfaced to the converter. A CONTROL VALVE is an element which controls the flow of the substance to control process parameters such as flow, temperature, level, etc.
Thus there is a concept of Process Input/ Output and Controller Input/ Output. A sensor or transmitter acts as process output but acts as input to controller whereas a control valve acts as process input and output to controller. This basic understanding of the process and controller I/O is a base to start study of PLC hardware and its programming
Basic Parts of PLC :
1. Central
Processing Unit: This is the brain of
PLC which performs all the operation i.e. executes the logic or controls the
process. It has two parts:
Microprocessor: The computer centre that carries out mathematic and
logic operations.For performing these operations the processor needs to be
powerful which is decided by its clock speed and bit size.
Memory: The area of the CPU in which data and information is stored and retrieved. Holds the system software and user program. The part of the memory which holds the ladder logic or user program is called user memory. The part of the memory that holds predefined system variables is called data memory.
2. Addressing:Addressing in PLC, follows octal addressing. There are two types of addressing:
Internal Addressing: Internal addressing is the addressing made for
variables which are internally created in the memory for simulation purpose.
Also the tagging of timers, counters etc. i.e. variables of data memory come
into this type of addressing. With the change in the manufacturer of PLC,
change in the addressing takes place. The addressing of this type will be
explained while programming, in coming sections.
3. Input Module :
Basic work of Input Module.....
a) It senses the
presences or absence of an input signal at each of its input terminals. This
tells which of the switch, sensor or other signal is on or off in the process
being controlled.
b)
It converts the input
signal for high, or on, to a DC level usable by the module’s electronic
circuit. For a low, or off, input signal, no signal is converted, indicating
off.
c)
The input module
carries out electronic isolation by electronically isolating the input module
output from its input.
d)
Its electronic
circuit must produce an output, via output logic, to be sensed by the PLC CPU.
Basics
of Programming
In order to program the Programmable Logic Controller
there are several languages available. Each of the software available in the
market can support one of these languages or all depending on the manufacturer.
The languages available are as follows:
1. Ladder
Logic: Ladder Logic has been evolved out of relay control panels,
since PLCs adopted legacy concepts, which were applicable to such panels.Also to
facilitate maintenance and modification of the physically wired control logic,
the control panel was systematically organized so that each control formed a
rung much like a rung on a ladder.The development of PLCs retained the ladder
logic concept where control circuits are defined like rungs on a ladder where
each rung begins with one or more inputs and each rung usually ends with only
one output.This is thus the first programming language which was developed with
the advent of PLC in market. This language will be studied in detail over here
in coming practicals.
2. Functional
Block Diagram (FBD): This type of
programming evolved after the ladder logic programming came into market. This
is the easiest language in which we can program the PLC. This is an emerging
language in market nowadays. The concept used in this type of programming is
that a number of small instructions are combined to form a block which performs
a major function. E.g.: AND function Block, Timer Function Block etc. We just
need to configure these blocks in order that these blocks perform their
functions. This type of programming we will just be observing, we won’t be
learning this language in detail.
3. Sequential
Flow Charts (SFC):This language is
generally used in batch process control where a sequence of operation is
followed to obtain a product on a certain lot from a big amount of raw material.
This language is like flow chart preparation defining the sequence of
operation.This type of programming we will just be observing, we won’t be
learning this language in detail.
4. Instruction
Set Language (IS): This type of
language is similar to programming done in microcontroller assembly language
and C or C++ language. The programming of controller with this programming
requires an expert or a lot of practice. Thus this language is not popular in
industries. Thus we won’t be studying this language in coming sections.
5. Structural
Text Language (ST):This type of language
is similar to programming done in microcontroller assembly language and C or
C++ language. The programming of controller with this programming requires an
expert or a lot of practice. Thus this language is not popular in industries.
Thus we won’t be studying this language in coming sections.
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