PROGRAMMABLE LOGIC CONTROLLER
A Programmable Logic Controller (PLC) is a general purpose digital
computer widely used in automation across all industries and are
very flexible in form factor, capacity and programming. A PLC
typically consists of a CPU and a number of I/O and communication
modules or ‘cards’ often mounted in an industrial rack, although
actual physical layouts vary enormously. They are programmed for
real-time operation with either simple Ladder Logic programming, or
more recently IEC61131-3 standardised tools. They are suitable for small to very large implementations with systems having thousands of I/O points easily possible. For local area distributed systems proprietary physical networks are common although Ethernet cabling and TCP/IP is rapidly replacing proprietary standards.
PLCs have many possible applications in the water industry including:
Pump Station Control
The programming capacity of the PLC is useful when complex requirements exist at a single pump station, like coordinating variable speed pumps or synchronising valves and pumps when starting and stopping a pump to prevent surge, hammer or back-flow.
Water Treatment Plants
PLCs are ideal to handle the large I/O and complex programming associated with the many processes within a treatment facility. For example, sequencing a filter back wash and air scour is easily within the PLCs capability.
Remote Telemetry Sites
PLCs are sometimes used instead of Remote Terminal Units (see RTUs) at remote sites like storage tanks and small pump stations, mainly due to the benefit of keeping a single supplier and programming tools across the entire enterprise. They do however tend to require additional hardware in terms of communications (e.g. serial communications card and a radio) which make them more expensive than an all-in-one RTU.
PLCs are rugged, industrialised, very reliable and compatible with standard programming languages like IEC61131-3. They are easily supported by a solid industry base of technicians and are very flexible in form factor, capacity and programming. To improve reliability, PLCs can be configured in “master-slave” pairs, where one unit is in control, and the other is in back-up mode, ready to take over if the primary unit fails. The slave PLC is fully synchronised to the master, called “hot backup mode,” so it can take over in microseconds, making the transition unnoticeable.
A PLC based system can be supplied and installed anywhere an electricity supply is available. They do not need special environments and can exist and operate for years unattended in hostile environments with extremes of temperature, electrical noise and vibration. However being electrical they should be protected from accidental water spray, and from corrosive gases (e.g. H2S) sometimes present in water treatment and distribution.