Programmable Logic Controllers (PLCs)

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Programmable Logic Controllers (PLCs)
This course deals with PLCs, Programmable Logic
Controllers. PLCs are the industrial
computer/controllers that are in the heart of
most industrial controls. Examples would be 1.
Machine tools 2. Packaging equipment 3. Robot
workcell controller 4. Traffic light controller
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What is a PLC
PLCs are computers that have been optimized for
industrial applications. They have been
industrialy hardened. Which means that they
are more tolerant of hostile environments such as
higher temperatures and vibration. Most PLCs
have a modular structure that allows different
modules to be easily added. Modules such as
Input and Output modules.
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PC Control
The difference between PLCs and PCs is becoming
smaller. A PC (Personal Computer) such as an IBM
compatible computer is usually not used for
real-time control. Windows95, Windows NT are not
considered real-time operating systems
Real-time control - a control system that reads
inputs and turns outputs off or on and solves the
program logic on a fixed schedule. Also called
deterministic because every x mSec the cycle
repeats
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PLC Control
PLCs read inputs and control outputs on a fixed
time basis. Often in the mSec range. A typical
scan is around 10 mSec. So the PLC is
controlling some process and making adjustments
100 times a second. PLCs are usually programmed
in ladder logic. This is an extension of relay
ladder logic with some minor differences.
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PC Control
When the PC accesses the disk drive, communicates
with a modem, reads the CD-ROM drive, etc, other
tasks must wait. This is not a long time,
usually in the mSec range, but is not acceptable
in machine and process control under most
circumstances. The other type of PC control
deals with microcontrollers designed into custom
control solutions. The circuit board,
programming and interface electronics are all
programmed at design time.
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How do PLCs and PC differ
PLCs tend to be more modular, general purpose
controllers, easily programmed, configured and
fast. PCs are best used to solve math (spread
sheets), provide a graphical user interface
(GUI), and store vast amounts of data
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Specialized PCs
Since PLCs operate in the mSec range and
sometimes we need to control things in the uSec
range, custom control boards are designed. These
use a dedicated microprocessor or usually
a microcontroller. Popular microcontrollers
are BASIC Stamp http//www.parallaxinc.com/ Inte
l 8048, 8051, 8096, etc Motorola 6805 6811, etc

BASIC stamp
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Dedicated microcontrollers
Dedicated microcontrollers with custom interfaces
are used in either very specialized applications
or in high volume situations. Examples of
dedicated controllers A packaging line has a
glue dispensor that needs to put 16 glue dots on
a box every 100 uSec and visually inspect to make
sure that no dots were missed. If a glue dot is
missed the box must be rejected down the line.
See http//developer.intel.com/design/mcs96/desi
gnex/2351.htm for additional examples from Intel
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Advantages of dedicated controllers

• Fast
• Can often do more than a PLC
• Often the best choice for complex programs
• Less expensive in high unit volume
• Often are smaller in physical size

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Dedicated controller examples
Example The pump at a gas station needs to
control which gas is selected, how payment is
made, displaying price, dispenced amount, etc.
Since many gas pumps are made, the cost of the
custom design, testing and fabrication makes
this a cost effective solution.
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Disadvantages of dedicated controllers
-Usually custom designs - expensive in low
volume -Difficult and/or expensive to modify or
make software changes. -Parts availability may
be an issue. Sometimes, only one or two boards
are made.
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Advantages of PLCs

• Flexible I/O configuration
• Large number of knowledgeable programmers
• Easy modifications
• Parts availability

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Programming overview
Computers only work with ones and zeros. This is
called machine code. The next level up is called
assembly language, which is a representation of
groups of 1s and 0s in very simple statements.
Higher level languages are usually more
effiecient to program in. Examples include BASIC,
C, Fortran, Java and Ladder logic. In the higher
level languages, commands and statements that are
easier for people to understand and work with
are broken down or compiled into 1s and 0s for
the CPU
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Ladder logic programming
Since PLCs started out replacing relay panels and
there were many people who could hardwire
program these panels. The first PLCs allowed the
programmers to draw similar ladder logic diagrams
on the CRT to describe the control process.
These representations were compiled down to 1s
and 0s for the CPU inside the PLC. Some modern
PLCs program in an English like language.
Others allow you to program in C. There are more
ladder logic programmers than any other kind!