When the first PLC
offerings were brought before traditional manufacturing, the
engineering groups dependent on ice-cube relays, looked on with a
combination of curiosity and caution. Could this “black box” perform
reliably and offer more benefits? It’s safe to say the legacy PLC did
indeed out-perform its noisy predecessor.
In the past decade, a
similar transition has been occurring in the factory automation sector.
This time the shift is smaller, but the benefits are tremendous. The
current PC-based control strategy still depends on software based
control code installed on a processor and linked to inputs and outputs.
The big difference is the power of the processor and the surrounding
hardware and software architecture. The key to skillfully deploying
this strategy is to understand the specifics of the platform and
capitalizing on the inherent benefits of the shift, as well as the
significant list of platform specific advantages offered by the
manufacturer.
When referring to a
skillfully deployed PC-based control strategy, we are specifically
speaking about employing a strategy that provides the basic foundation
of the existing control strategy without compromising functionality or
precision. There should also be added benefits of flexibility, enhanced
programming features, increased connectivity, and additional time and
money saving aspects to create a fully integrated application
environment.
The basic foundations of
standard PLC technologies are rooted in over twenty years of control
fundamentals. These include complete real-time determinism for the
process execution, complete real-time determinism for the reading and
writing of input/output values, ensured robustness and reliability in
execution and continued operation, protection of the control code and
execution activities from outside influences and functional isolation
to ensure a dedicated control response. These are inherent features for
basic repeatability, reliability and operational safety. Without these
basic fundamentals, an integrated software replacement is a completely
unacceptable substitute for an existing control strategy.
However, if these
conditions can be met, then the user would immediately receive some
very basic, and often compelling, advantages. These basic advantages
would include significant processing speed improvement, significant
hardware advantages in availability; upgrade potential, cost, and
long-term improvements.
First, let us expand on the
foundations of a PC-based control strategy and how it can meet
fundamental control requirements. Once those fundamentals are met, we
can illustrate the immediate benefits of moving a control strategy to
the PC platform. Lastly, we will highlight the
expanding features and
benefits attainable with a highly integrated software strategy with a
PC running NT as a core component of the system.
The basic requirement of
existing and future control strategies is, and should continue to be,
complete determinism. Many times, the speed at which an application
runs is often confused with the actual deterministic nature of the
process. One can speak of generic software approaches using very small
scan rates. This is often confusing and irrelevant. The true
measurement of reliability and control accuracy is the determinism, or
repeatable and timely nature of the code execution. The benchmark of
the PLC, and of all PC-based control systems, is determinism.
Speed, like rate of fire
from a machine gun, is impressive, but accuracy is of ultimate
importance if the target is to be hit. The base factor must be
accuracy: hit the target each and every time. If you can add speed,
then this is beneficial, but adding speed in lieu of accuracy is the
same as closing your eyes and squeezing the trigger.
PC-based control systems,
such as our Visual Logic Controller, can provide this determinism by
using an RTOS (real-time operating system) that runs alongside the
Windows NT environment of the PC. This division of duty provides a host
of benefits including complete determinism for the control code,
efficient data integration, a unified hardware platform for control and
interface, and unwavering execution security.
Other cornerstones of PLC
technology include determinism in communication to I/O, reliable
execution and functional isolation to ensure a dedicated control
response. The PC system can address all these issues using a few simple
precautions. In order to guarantee deterministic communication, all I/O
drivers are part of the RTOS strategy. No API (NT-based) communications
are used, as NT introduces significant potential delays.
The PC system can also be
installed and configured to run without the aid of the hard disk or NT
operating system. In the event that either hardware or software systems
fail, the RTOS will continue to provide safe, reliable control. In
terms of functional isolation, the PC system can supply the most
complete integration of the control and NT operating system while
ensuring there is no functional overlap to interrupt the importance of
control tasks. The control code is executed first to ensure the
complete determinism of the process. After control code and I/O is
serviced, the processor is released for use by the NT operating systems
and associated tasks.
Realtime OS Priority
Additional precautions for functional isolation include using secured
RAM memory for run-time storage of the control code, the RTOS and
communications protocols. This feature ensures that NT task or programs
have no opportunity to dilute, erase or overwrite any control logic or
real-time values.
Now that the required
basics of a control model are fulfilled, we can move on to the obvious
and ingrained advantages of moving to PC-based control strategy. The
market supporting basic PC technology is roughly estimated to be about
220 billion dollars. This massive market can and does support a wide
range of development efforts focused on increasing the capabilities of
PC hardware, reducing cost, streamlining manufacturing and offering
more features to the general user. These improvements are directly
applicable to a skillfully deployed PC-based strategy.
Looking at the actual
hardware, speed is the first issue to be considered. Upon
investigation, one discovers that the standard PC on the market today
contains a Pentium processor running at or above 166 MHz. This basic
processor is capable of full 32-bit calculations and speed in excess of
9000 times faster than the legacy PLC of the American market. Here are
a few simple examples:
I
When we consider that a
166Mhz machine is most likely not available in a new PC purchase, and
the more likely option would be a 450,500 or even 650 MHz machine, the
obvious processing advantage is clear. This is not so say that
processing speed alone should be used as a gauge for comparing PLC vs.
PC-based technology, but rather, this inherent speed advantage combined
with the proper software strategy provides immediate benefits to the
PC-based control user.
The economies of scale that
surround the PC market also come into effect to offer the automation
user a wide range of choices, models, features, vendors and prices for
the hardware purchased. All these factors are in much greater abundance
than can be found in the specialized PLC market. The PC-based control
user can also enjoy the growing availability of industrial hardened
PCs, which offer the same physical dependability of PLCs. This includes
smaller din-rail mounted units that are specifically designed for
PC-based control applications.
With assured performance of
a RTOS, just like a PLC, significantly enhanced speed, and all the
benefits of purchasing equipment from a multi-billion dollar market,
it’s easy to see how PC-based control has captured the attention of a
growing number of manufacturers. Of course, the needs of these
manufacturers are widely varied and it’s important to look at various
cases in detail to realize the impressive list of additional benefits
of a reliable PC-based control system.
The need for data exchange
from the control application to other applications tops the list.
Whether you are sharing data with a user interface screen, a data
collection system, a factory-wide network or a high level scheduling or
management package, the basic concept is similar. Take data from the
production environment and share it, in various forms, with the rest of
the company. This data exchange can be seen in both directions as well.
From inventory control to production scheduling to maintenance to
management reports, the effectiveness of data management is another key
ingredient in a PC-based control strategy.
By providing a bridge
between the RTOS and the NT operating system, a PC-based control system
can allow for fast, effective sharing of data. Basic connections
include Windows standard DDE and OPC servers. There are also toolkits
for creating specialized DLL connection, which provide greater speed
and customization. And let’s not forget the ability to send packets of
information over the Ethernet. With a network driver, peer to peer and
peer to host communications can be easily configured for effective cell
and line management. By adding TCPIP protocols into the RTOS
communication pathway, data transfer can survive a Windows NT crash as
well.
A PC system can provide
integrated solutions for communications to all types of peripheral
devices and accessories. This would include high-speed serial devices,
specialized I/O cards, any device with dual-port RAM, any devices with
a definable I/O port, and almost every family of I/O and device-level
network. Rack-based I/O, high speed distributed I/O, motion modules,
drives, servos, CNC controllers, user interface screens and more are
included.
The reliability, speed,
availability, connectivity and cost effectiveness of a PC solution
would seem to satisfy the needs of the most discerning
critic, but these aspects
are only the beginning of this dynamic system architecture. The real
power lies in two simple words: productivity and profitability. With
increasing demands being placed on manufacturing, the engineers of
these systems are asked to squeeze more and more effectiveness out of
limited resources. The entire system represents a synergy of hardware
and software that has been carefully orchestrated by the designers.
Were they able to
streamline their development using modular code?
Did they implement code
that could be easily interpreted by engineers in the field?
Were the tools available to
integrate diagnostics that readily identify possible errors?
Were graphical recovery
systems available to maintenance engineers to reduce downtime?
Is the system inherently
ready for future upgrades, hardware swaps and re-design?
With multi-threaded project
architecture, now available in a PC system, designers can simplify a
system into smaller, more manageable design efforts. The resulting
programs are easier to develop, debug and maintain. The project as a
whole runs all threads seamlessly to create advanced functionality with
minimal design effort. Many documented cases of PC-based technology
have produced design times of 25, 50 and even 75% faster than
traditional methods.
With easy-to-read flow
charts, the project is understandable at a glance. In-line math
functions, integrated motion programming, I/O network configuration and
flexible tag assignment are helpful tools that increase ease-of use of
a PC system. Also, by separating the code from direct hardware
assignment and linear addressing, a PC project can work on any
assortment of IO platforms or device networks.
There are even automatic
tools (wizards) for error detection code you can instantly include
giving an unlimited number of fault detectors automatically linked to
error message displays.
There is an integrated user
interface in the system, as well as limitless connections to any
existing interface or SCADA. Data handling and manipulation are
simplified using arrays of value in three dimensions and there is a
limitless ability to re-use functional blocks of code as sub-programs.
A PC system can also offer full printing, cross-referencing and
find/replace features. There are also on-line programming capabilities
to ensure that the PC system can easily meet and exceed the needs of
the most advanced development teams.
Once the project is
complete and implemented in the field, on-line diagnostics and
maintenance features can provide long term savings on a PC application.
With a simple glance, operators can see each step in the flow of a
control scheme and view the real-time values. By tracing the code
execution, a PC system can also give engineers insight into the history
of execution.
Using a Diagnostic Manager
feature, when available as a PC system option, each error is
time-stamped and saved for later review. Using web technology and html
access, comprehensive repair and maintenance screens can be linked
directly to the control code. These are used to rapidly instruct
operators of machine malfunctions and the proper method for repair.
Just imagine having the machine tell you where the problem has
occurred. This in not just a pipe dream; in fact, there are documented
cases of PC-based control customers reducing machine downtime by up to
90%.
Maintaining machine uptime
translates into manufacturing productivity and profit. Also, the
modularity of the code lends itself well to system modification and
upgrade paths, as well as hardware changes with little or no
programming effort. The overall effect is a flexible system capable of
meeting the manufacturing needs for today and tomorrow: short-term
profit through faster design and start-up, increased productivity, easy
system upgrades, and long-term savings through increased uptime.
The concept of the PC-based
control system is based on the past three decades of control wisdom and
accepted practice. The actual use of this system is made possible with
commercially-available hardware and software tools. PC-based control
benefits are seen in the design, implementation, flexibility and
long-term use. The time of curiosity and caution are past and the
technology shift is underway. A new chapter of automation technology is
being written, read and lived by manufacturers all over the world.
