01-08-2012, 02:42 PM
PROGRAMMABLE LOGIC CONTROLLERS AND LADDER LOGIC
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ABSTRACT
This report answers basic questions that may be posed by anyone in the Electrical
Engineering field, including electrical engineering technicians and electricians. The
reader does not need to have any background in controls or ladder logic; only an interest
in how PLCs work and a basic understanding of digital logic.
Introduction
Subject
Programmable logic controllers (PLCs) have become the most predominant
control elements for the discrete event control of a mechatronics system. Simplification
of engineering and precise control of manufacturing process can result in significant cost
savings. The most cost-effective way which can pay big dividends in the long run is
flexible automation; a planned approach towards integrated control systems. It requires a
conscious effort on the part of plant managers and engineers to identify areas where
automation can result in better deployment and/or utilization of human resources and
savings in man-hours or down time. Controls automation need not be high ended and
extremely sophisticated; it is the phased, step-by-step effort to automate, employing
control systems tailored to one’s specific requirements that achieves the most attractive
results. This is where programmable logic controls have been a breakthrough in the field
of automation and control techniques. This report looks at the role PLCs play in these
techniques.
Programmable Logic History
PLC History
PLCs were first introduced in the 1960’s. The primary reason for designing such a
device was eliminating the large cost involved in replacing the complicated relay based
machine control systems. Bedford Associates (Bedford, MA) proposed something called
a Modular Digital Controller (MODICON) to a major US car manufacturer. The
MODICON 084 brought the world's first PLC into commercial production.
When production requirements changed so did the control system. This becomes
very expensive when the change is frequent. Since relays are mechanical devices they
also have a limited lifetime because of the multitude of moving parts. This also required
strict adhesion to maintenance schedules. Troubleshooting was also quite tedious when so
many relays are involved. Now picture a machine control panel that included many,
possibly hundreds or thousands, of individual relays. The size could be mind boggling
not to mention the complicated initial wiring of so many individual devices. These relays
would be individually wired together in a manner that would yield the desired outcome.
The problems for maintenance and installation were horrendous.
Hard Wired Relay Comparison
At the outset of industrial revolution, especially during sixties and seventies,
relays were used to operate automated machines, and these were interconnected using
wires inside the control panel. In some cases a control panel covered an entire wall. To
discover an error in the system much time was needed, especially with more complex process control systems. On top of everything, a lifetime of relay contacts was limited, so
some relays had to be replaced. If replacement was required, machine had to be stopped
and production as well. Also, it could happen that there was not enough room for
necessary changes. A control panel was used only for one particular process, and it wasn’t
easy to adapt to the requirements of a new system. As far as maintenance, electricians had
to be very skillful in finding errors. In short, conventional control panels proved to be
very inflexible. Typical example of conventional control panel is given in the following
picture.
Programmable Logic Controller Components
Definition
A Programmable controller is a solid state user programmable control system with
functions to control logic, sequencing, timing, arithmetic data manipulation and counting
capabilities. It can be viewed as an industrial computer that has a central processor unit,
memory, input output interface and a programming device. The central processing unit
provides the intelligence of the controller. It accepts data, status information from
various sensing devices like limit switches, proximity switches, executes the user control
program stored in the memory and gives appropriate output commands to devices such as
solenoid valves, switches etc.
Input output interface is the communication link between field devices and the
controllers. Through these interfaces the processor can sense and measure physical
quantities regarding a machine or process, such as, proximity, position, motion, level,
temperature, pressure, etc. Based on status sensed, the CPU issues command to output
devices such as valves, motors, alarms, etc. The programmer unit provides the man
machine interface. It is used to enter the application program, which often uses a simple
user-friendly logic.
Conclusion
Summary
This report has discussed the role that programmable logic controllers have in the
efficient design and control of mechanical processes. Also discussed was the
understanding of ladder logic and the programming involved with it. Finally, the report
has discussed relay logic and the evolution that ladder logic made from it.