by Peter Raeth
When we talked about adaptive Proportional, Integral,
Derivative (PID) Controllers, we emphasized their adaptive nature. PIDs are not the only controller devices available.
We should also have a look at Programmable Logic Controllers (PLCs).
According to Hugh Jack,
Grand Valley State University, control of electrical machinery
was originally accomplished via mechanical relays implementing Boolean logic circuits. Relay logic produces a 1 or a 0 to
open or close the line feeding electricity to the machine. In operation, a mechanical-relay logic circuit takes its binary
input and exercises its Boolean circuits as appropriate. This is done automatically as electricity flows through the relay
coils to open or close the various contacts. Functionally, this is exactly what happens when electronic AND/OR gates are
exercised according to a Boolean equation. PLCs are a special-purpose computer whose programmed logic replaces the mechanical
relay logic, while the ultimate result may still be the opening or closing of a mechanical relay. The typical means of
programming PLCs, ladder logic, is derived from the approach used to design the on/off mechanical-relay logic circuits. The
PLC scans its inputs, exercises its program, and delivers its result. This process is repeated continuously with exactly the
same functional result as mechanical logic. To read more detail you really do want to
download his book. It
is a serious contribution to the field.
Basic PLC Tutorial
While it was not possible to determine the source, this is a good tutorial that will help you understand the fundamentals.
Something that stands out from this tutorial is that PLCs are capable of much more than could be achieved by mechanical-relay
logic circuits. The following topics are covered.
- History and Description
- Analog Inputs and Outputs
- Communications
- Various Comparators and Their Use
- Counters and Their Use
- Storing Data
- AND/OR Gates
- Numbering Systems
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- Scanning the Program
- Generate a Pulse/Edge Trigger
- Set and Reset of an Output
- Shift Registers
- Programming in STATEMENTS instead of LADDER LOGIC
- Creating and Using Time Delays
- Programming Examples
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Basics of PLCs
This briefing by Siemens starts with the task to be
performed on the assembly or inspection line, and continues through the application of PLCs to meet the control goals. You
may find yourself reviewing their other
briefings for additional background.
PLC Training Simulator
From The Learning Pit, this DOS-based animated training
tool comes complete with hands-on student
exercises. "PSIM is actually three distinct programs combined into a single package. First, PSIM contains a PLC Ladder-Logic
editor that allows users to create and edit PLC programs using Allen Bradley PLC-2 family instructions. Second, PSIM emulates
the scanning sequence of a PLC. When placed into the RUN mode, the user's program is scanned and the appropriate I/O are
updated just as would occur in an actual PLC. Third, PSIM contains a number of animated simulations that respond accurately to
the inputs and outputs of the emulated PLC. A conveyor-based filling line, Traffic intersection, and Batch mixing simulations
present life-like challenges for the student programmer."
Open Standards for Industrial Control Programming
"PLCopen is a vendor- and product-independent worldwide association supporting IEC 61131-3. Via this programming standard, we
provide greater value to users of industrial controllers. By implementing this standard in many program development environments
(known as Program Support Environments in the IEC 61131-3 terminology), users can move between different brands and types of
control with very little training, and can exchange applications with a minimum of effort. To reach this goal, the members of
PLCopen are committed to supply and/or use IEC 61131-3 compliant products. IEC 61131-3 is a worldwide standard. It harmonizes the
way people look to industrial control by standardizing the programming interface. This includes the definition of the language
Sequential Function Charts used to structure the internal organization of a program, and four interoperable languages: Instruction
List, Ladder Diagram, Function Block Diagram, and Structured Text. Via modularization and declaration of variables, each program is
additionally structured, increasing its re-usability, reducing errors, and increasing efficiency. In addition, IEC 61131-3
structures the way a control system is configured." The International
Electrotechnical Commission (IEC) is the international standards and conformity assessment body for all fields of electrotechnology.
Fuzzy PLCs
The Industrial Text & Video Company produces training
materials in industrial automation. Their site contains several public-access documents on PLCs. One in particular shows how to
implement fuzzy logic on PLCs. Thus, using a conventional control device, it is still possible to employ adaptive technology. However,
one gets the impression from reading the document that it would not be possible to implement fuzzy logic using ladder logic. However,
there is more than one language available for programming PLCs. The other documents available on their site are:
- PLC Primer
For those new to PLCs—Grasp the basics of PLCs
- Introduction to PLC Programming and System Implementation
Learn the techniques to implement a control program
- I/O Bus Networks Including DeviceNet
Slash wiring and installation costs using device networks
- PLC Start-Up and Maintenance
Learn six preventive-maintenance procedures to keep your PLC system running fault-free
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PLC vs. PC
If the PLC is a special-purpose computer, why not use a PC instead? One of
the biggest reasons that comes out is that PLCs are ultra-reliable, and PCs do not have that reputation. Here are links to
sites that discuss this issue.
PC Controls: Making Its Mark
Paul Morris, an Instructor/Engineer with Valentine Robotics,
writing for Robotics OnLine, offers a balanced view of the issue.
Pros for PC
- Start-up costs are typically less.
- The user interfaces are well known by the general public.
- Flexibility is increased via open standards.
- An array of programming languages can be employed.
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Pros for PLC
- Well-known interfaces invite unwanted users to "play" with the computer.
- The crash factor of a complex operating system is not desirable.
- The variety of available hardware creates many incompatibilities.
- The time and money to educate labor in the new skills is never invested.
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PLC vs. PC Lock Control Systems
The Construction Engineering Research Laboratory,
U.S. Army Corps of Engineers, has published a detailed briefing that greatly expands on the pros
and cons of PCs and PLCs in critical control applications. As in all choices, you will find that there is not a single best selection for all
circumstances. (It is best to download the briefing rather than view it online.)
When Controls Converge
Lawrence Gould, a Contributing Editor writing for
Automotive Design and Production, offers that "Many of the features that
differentiated programmable logic controllers (PLCs) from computer numerical controllers (CNCs), CNCs from motion controllers, and all of those
from industrial controllers based on personal computers (PCs), can now be found in all of these control systems."
Size and Connectivity Drive Hardware Strategies
Gary Mintchell, writing for Control Engineering, says that "New hardware
options are appearing with increasing frequency. But even more than PLC vs. PC, it's what kind of PLC or PC to use." He interviews a number of
industry experts to give us a large view of the options for implementing automated industrial controls. |
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Follow-Up
Max Dirnberger, a design engineer with Einstein Technology,
wrote to say that in addition to the links we reviewed in PID Controllers, he found
Motorola's app note AN1120/D
that describes doing the whole thing in an
HC05 micro.
While he felt that the code in this app note is flawed in a few areas, he found reading it to be a little more descriptive than the one for the
HC11 we mentioned earlier.
He actually ported his version of the HC05 code over to a TI
MSP430.
He found that exercise surprisingly easy. He created one PID channel to control a 40 A DC motor, and a second channel to drive a magnetic clutch
in controlled slip mode to achieve a continuously variable transmission. It came to a matter of roughly 1.5 HC05 instructions per one MSP430.
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