Today we are going to talk
about the basics of a PLC, the workhorse of industrial automation. Before we get into today’s
video, if you love our videos, be sure to click the like button below, and make sure to click subscribe and the bell to
receive notifications of new RealPars videos. This way you never miss another one! The programmable logic controller, or
PLC, is a special purpose computer. It has no display, no keyboard,
no printer, no hard drive, and hides in the control panel out on the
factory floor, but it is still a computer. Initially, the PLC was a
replacement for panels of relays, devices that turn on and off. Making a machine run using relays as logic
was time-consuming and challenging, and making a change in the logic
or the operation of the machine was almost as complicated
as starting over. Relays also fail far more frequently
than computer components, so that relay logic required more downtime to
keep it running than newer PLC-based controls. On top of all of that, relays
use a lot of electricity, make lots of heat and soot,
and take up lots of space. An installed PLC is deceptively
simple in appearance. You have a CPU module and input/output
devices, referred to as I/O. Sometimes the I/O is part of the CPU,
and sometimes it is in other modules. The CPU communicates with the I/O, so in most systems they share a backplane
that physically holds them in place and electronically connects them. In other equipment, the I/O modules
can be miles away from the CPU and connected with data cables, so that the PLC is not limited to a
single cabinet or even the same building. Since the PLC is a computer,
there is no need to limit it to only digital inputs and outputs. Over the years, the manufacturers added analog
and other numerical inputs and outputs. To make these numerical devices useful, they included calculation capability
in the programming as well, so that we can now calculate, for instance, statistical process control (SPC) values and put
PID controllers directly into the PLC program. Since this is called a programmable logic
controller, we need to program it. Most PLCs are programmed using an application
running on a standard desktop or laptop PC. They communicate with the PLC using Ethernet
or a proprietary communication bus, depending on the manufacturer. Most manufacturers seem to have settled
on Ethernet and/or USB in recent years. Unfortunately, the PLC manufacturers have failed
to agree on how we program the control logic. Although most of the manufacturers
claim some form of “Ladder Logic,” the specifics of that language are
different for each manufacturer, including different capabilities,
different ways of doing the same thing, and even differences in the order in which the
CPU processes the various pieces of logic. One attempt to bring some order and
inter-operability to this chaos is the IEC 61131 standard from the American
National Standards Institute (ANSI). The excerpt from the ANSI web page shown
here gives a flavor of what is there. The IEC 61131 standard defines
several “programming languages” with different strengths and weaknesses. Local programming standards
(your boss or the customer) will usually specify what language you use. That wraps up this short discussion
of the structure of PLCs. I hope you enjoyed this little
introduction to the underlying architecture of many
industrial control systems. Keep watching as we continue our exploration
of the basics of industrial controls. Make sure that you head
over to realpars.com. To find even more training material
for all of your PLC Programing needs. We offer many videos to assist you
in learning PLC Programing and landing that job in a high-paying, highly thought after field of
automation and controls engineering. Go to realpars.com and subscribe to our
highly effective training series now!
