Wednesday, May 23, 2018

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PLC Examples

Programmable logic controllers are essentially nothing more than special-purpose, industrial computers. As such, they are built far more ruggedly than an ordinary personal computer (PC), and designed to run extremely reliable operating system software1. PLCs as a rule do not contain hard disk drives, cooling fans, or any other components with moving parts. This is an intentional design decision, intended to maximize the reliability of the hardware in harsh industrial environments where the PLC chassis may be subjected to temperature extremes, vibration, humidity, and airborne particulates (dust, fibers, and/or fumes).

Large PLC systems consist of a rack into which circuit “cards” are plugged. These cards include processors, input and output (I/O) points, communications ports, and other functions necessary to the operation of a complete PLC system. Such “modular” PLCs may be configured differently according to the specific needs of the application. Individual card failures are also easier to repair in a modular system, since only the failed card need be replaced, not all the cards or the whole card rack.

Small PLC systems consist of a monolithic “brick” containing all processor, I/O, and communication functions. These PLCs are typically far less expensive than their modular cousins, but are also more limited in I/O capability and must be replaced as a whole in the event of failure.

The following photographs show several examples of real PLC systems, some modular and some monolithic. These selections are not comprehensive by any means, as there are many more manufacturers and models of PLC than those I have photographed. They do, however, represent some of the more common brands and models in current (2009) industrial use.

The first photograph is of a Siemens (Texas Instruments) 505 series PLC, installed in a control panel of a municipal wastewater treatment plant. This is an example of a modular PLC, with individual processor, I/O, and communication cards plugged into a rack. Three racks appear in this photograph (two completely filled with cards, and the third only partially filled):

Siemens (Texas Instruments) 505 Series PLC installed in a municipal wastewater treatment plant control panel

The next photograph shows an Allen-Bradley (Rockwell) PLC-5 system, used to monitor and control the operation of a large natural gas compressor. Two racks appear in this first photograph, with different types of I/O cards plugged into each rack:

Allen Bradley (Rockwell) PLC-5 System used to monitor and control the operation of a large natural gas compressor

Both the Siemens (formerly Texas Instruments) 505 and Allen-Bradley (Rockwell) PLC-5 systems are considered “legacy” PLC systems by modern standards, the two systems in the previous photographs being about 20 years old each. It is not uncommon to find “obsolete” PLCs still in operation, though. Given their extremely rugged construction and reliable design, these control systems may continue to operate without significant trouble for decades.

A newer model of PLC manufactured by Allen-Bradley is the SLC 500 series (often verbally referred to as the “Slick 500” series), which is also modular in design like the older PLC-5 system, although the racks and modules of the SLC 500 design are quite compact by comparison. The SLC 500 rack shown in the next photograph has 7 “slots” for processor and I/O cards to plug in to:

Allen-Bradley SLC 500 PLC also referred to as

The first three slots of this SLC 500 rack acre occupied by the processor card, an analog input card, and a discrete input card, from left to right. The next two slots are empty (revealing the circuit board and connectors for accepting new cards). The last two slots hold discrete output and analog output cards, respectively.

A nine-slot SLC 500 system is shown in the next photograph, controlling a high-purity water treatment system for a biopharmaceuticals manufacturing facility:

Allen-Bradley SLC 500 controlling high-purity water treatment plant for biopharmaceuticals

A modern PLC manufactured by Siemens appears in this next photograph, an S7-300, which is a different design of modular PLC. Instead of individual cards plugging into a rack, this modular PLC design uses individual modules plugging into each other on their sides to form a wider unit:

Siemens S7-300 PLC, a different design modular PLC

A modern PLC manufactured by Allen-Bradley (Rockwell) is this ControlLogix 5000 system, shown in this photograph used to control a cereal manufacturing process. The modular design of the ControlLogix 5000 system follows the more traditional scheme of individual cards plugged into a rack of fixed size:

Allen-Bradley (Rockwell) ControlLogix 5000 System controlling a cereal manufacturing process

While the Siemens S7 and Rockwell ControlLogix PLC platforms represent large-scale, modular PLC systems, there exist much smaller PLCs available for a fraction of the cost. Perhaps the least expensive PLC on the market at this time of writing is the Koyo “Click” PLC series, the processor module (with eight discrete input and six discrete output channels built in) shown in my hand (sold for less than 80 US dollars in the year 2009, and with free programming software!):

Compact Koyo Click PLC Series

This is a semi-modular PLC design, with a minimum of input/output (I/O) channels built into the processor module, but having the capacity to accept multiple I/O modules plugged in to the side, much like the Siemens S7-300 PLC.

Other semi-modular PLCs expand using I/O cards that plug in to the base unit not unlike traditional rack-based PLC systems. The Koyo DirectLogic DL06 is a good example of this type of semi-modular PLC, the following photograph showing a model DL06 accepting a thermocouple input card in one of its four available card slots:

Koyo DirectLogic DL06 accepting a thermocouple input unit card

This photograph shows the PLC base unit with 20 discrete input channels and 16 discrete output channels, accepting an analog input card (this particular card is designed to input signals from thermocouples to measure up to four channels of temperature).

Some low-end PLCs are strictly monolithic, with no ability to accept additional I/O modules. This General Electric Series One PLC (used to monitor a small-scale hydroelectric power generating station) is an example of a purely monolithic design, having no “expansion” slots to accept I/O cards:

General Electric Series One PLC used to monitor a small-scale hydroelectric power generation station is an example of a purely monolithic design
 
 

1There are such things as soft PLCs, which consist of special-purpose software running on an ordinary PC with some common operating system. Soft PLCs enjoy the high speed and immense memory capacity of modern personal computers, but do not possess the same ruggedness either in hardware construction or in operating system design. Their applications should be limited to non-critical controls where neither main process production nor safety would be jeopardized by a control system failure.

 
Click here to go back reading the intro article, Programmable Logic Controllers

Click here to go to the next page, PLC Input / Output Capabilities

Go Back to Lessons in Instrumentation Table of Contents

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