What is a PLC?
Programmable
Logic Controllers (PLC) continue to evolve as new technologies are
added to their capabilities. The PLC started out as a replacement for
banks of relays. Gradually, various math and logic manipulation
functions were added. Today they are the brains of the vast majority of
automation, processes and special machines. PLCs now incorporate
smaller cases, faster CPUs, networking and various internet
technologies.
You can
think of PLC technology as a small industrialized computer that has
been highly specialized for reliability in the factory environment. At
its core, a PLC looks at digital and analog sensors and switches
(inputs), reads its control program, makes mathematical calculations
and as a result controls various hardware (outputs) such as valves,
lights, relays, servo motors, etc. in a time frame of milliseconds.
While PLCs
were very good at quickly controlling automation, they did not share
data easily. At best, PLCs would exchange information with operator
interfaces (HMI) and Supervisory Control and Data Acquisition (SCADA)
software packages on the factory floor. Any data exchange with the
Business Level of the company (information services, scheduling,
accounting and analysis systems) had to be collected, converted and
relayed through a SCADA package.
Typical of
most PLCs, the communication networks were unique to the brand and
limited in speed. With the acceptance of Ethernet, communication
network speeds have increased but are still sometimes using proprietary
protocols.
Trends: More Power, Wider Data Sharing
Overall,
PLCs are getting faster, smaller, cheaper and more powerful. As a
result, they are gaining capabilities that used to be the exclusive
domain of the Personal Computer (PC) and workstation arena. This
translates into critical data quickly and cheaply being shared directly
between the PLCs on the Factory Floor and the Business Level of the
company. These are not your father’s PLCs.
Some of
the features that a PLC can bring to your automation projects are Web
Servers, FTP Servers, sending E-mail and Internal Relational Databases.
The following is a brief overview of these features and some of their
uses.
Web Server
PLCs can host a Web site on the internet or your company intranet. So
what’s that going to do for you? How about give you access to Real Time
Data and Data Logging for starters. Do you need a backup Human Machine
Interface (HMI) for a machine(s) or work cell? How about as a tool for
your Maintenance group? Did you know that with some PLCs you can store
documentation with the web server that lets you view machine drawings,
schematics, maintenance and operator manuals and short video clips?
They are all just a mouse click away with your web browser.
Web
servers in PLCs are probably the most varied and widely used of the
newer technologies. PLC web server capabilities vary depending on the
manufacture and model from a single “canned” page to full blown sites
using XML and JAVA based technology.
JAVA web
servers can provide a high degree of versatility for interacting with a
PLC. Three of the JAVA classes of small programs that enhance web
server technology are; Applets, Modlets and Servlets. In general terms,
the “lets” let you view, manipulate and transfer data faster.
Applets
are small JAVA application programs that are sent from the web server
to your web browser the first time you open a web page the speed up the
data transfer for values.
Modlets
are JAVA modules that run independently of the PLC control program for
servicing non-process event driven tasks of data handling or updating
calculations separate of the PLC control program. Modlets are very
useful for parallel processing functions that interact with the PLC
database.
Servlets
run in the web server after they are requested by your web browser.
They are very useful for displaying live data and dynamically creating
data log files (CSV).
Send E-mail
A Send
E-mail functions automates and simplifies exporting critical and
production data out of a PLC. Production data and material usage
reports, status changes, alarms, internal PLC data and maintenance
requests can be issued from within a PLC control program. With a little
time and imagination you can send your alarm messages to the
Maintenance personnel who carry alphanumeric pagers or cell phones.
FTP Server
File
Transfer Protocol (FTP) is your tool for easily and quickly moving or
copying files in and out of a computer through an TCP/IP ethernet
connection. Now it is available in some PLCs. While on the surface it
does not sound like a big deal, this handy tool can be a major time
saver. Why walk out to the PLC to copy files when you can access it
though a network from your desk? How much time would you save by
dialing into the Ethernet network the PLC is on (or a stand alone
modem/router) if the PLC is in another city, state or country?
Internal Relational Database
One of the
most exciting and useful new features that just showed up in the
market from SoftPLC Corporation is the “Internal Relational Database”
embedded in a PLC. As an internal database it allows crucial data to be
accessed in one program scan (milliseconds) rather than having to wait
for it from an external source (another computer or PLC) sending the
data through a communications port.
This
feature opens the door for a whole host of cost savings. For example in
a sorting conveyor with a bar code reader, the bar code reader usually
connects to a PC. The PC looks up the bar code in the database that it
hosts and then sends the resulting information to the PLC. Only then
can the PLC use the data to control a diverter, gate or bin. There are
usually a minimum of 6 steps to get the information to the PLC.
-
Scan the bar code.
-
Send the data to the PC.
-
PC decodes the bar code.
-
PC looks up the resulting information from the database.
-
Move the data to the PC communications port.
-
Send the information to the PLC across a serial communication connection.
-
Move the data from the PLC communications buffer to the PLC program memory and use it (in some cases).
Using a PLC with an internal relational database reduces this to four steps:
-
Scan the bar code.
-
Send the data to the PLC.
-
PLC decodes the bar code.
-
PLC looks up the resulting information from the internal relational database and use it.
Using
a PLC with an internal relational database eliminates the weak link of
the communications from the PC to the PLC. A PC will usually perform
the database lookup faster than a PLC. However, moving the data from
the PC database across a serial network connection (usually limited to
19.2k baud) rate is much slower that a PLC that can retrieve usable
data in one scan of the control program.
With this
sorting conveyor example the first cost savings is reduced computer
hardware by eliminating the PC with the database and the database
software. A second savings is realized by eliminating the integration
time required to get the PC and the PLC communicating. Another cost
savings is from the lack of needing the Information Technology
department to continuously maintain, backup and upgrade the PC.
There are
many different applications that a PLC with an internal relational
database can control in a more cost effective way for both the
Integrator as well as the End User. Manufacturing machines and
processes that assemble product based on recipes or build multiple
products on the same machine are all good candidates for this
technology. Also, projects that require setting “environment variables”
for configuration of a machine or custom written instructions and
drivers as well as fast keyed look ups for control values would find
the power of a internal relational database especially useful.
Summary
The
technologies discussed are available in as many flavors and options as
there are PLC manufactures. This overview only scratches the surface of
the ongoing improvements in PLCs. It can’t begin to cover all the
features and uses available among these four technologies let alone the
important related subjects of networking, security and the continuing
shift to Open Architecture.
The
significance of the technologies is that control system complexity is
rapidly being simplified for greater company wide information exchange
so there no longer needs to be “islands of automation”
Source:-http://www.automation.com/library/articles-white-papers/programmable-control-plc-pac/the-plc-new-technology-greater-data-sharing
