New developments in programmable logic controllers

Micro PLCs are now established as a key tool for the control and automation of virtually every industrial process.

The programmable logic controller (PLC) has been a mainstay of industrial control for many years.

Recognised as a key to automating processes and improving efficiency, manufacturers of PLCs constantly strive to improve their products by adding features and increasing program capacity.

The development of the PLC can be seen most clearly in the small and micro size devices.

Although sometimes still considered purely for relay, timer or counter replacement, today's typical micro PLC comprises powerful functionality and an extremely flexible range of hardware features.

Users are constantly demanding efficiency.

One of the key areas where manufacturers have helped achieve this is by increasing miniaturisation and connectivity of PLCs.

This has lead to reduced panel and enclosure size and minimised the time spent wiring to and from peripheral devices.

Matsushita has taken compactness one step further by producing the FP-e controller that integrates PLC, timer, counter, PID and axis control functions in to a single DIN48 size package.

One of the most important areas of advancement has been in communications.

As a standard, RS232C ports now allow communication at up to 115Kbaud with other serial devices like operator terminals and 2D code readers and there is a promise from manufacturers for even higher speeds in the future.

External communication modules are now available that are based on the IEC60870-5 standard that allow micro PLCs to communicate over Ethernet or TCP/IP networks.

These modules allow data within the PLC to be accessed or modified in information systems or via internet based web pages.

Energy and utility companies have seen the greatest benefit from this communications technology where remote control of pumps, valves and transformers is necessary.

In the absence of a land line or other direct connection the use of GSM modems has boomed over recent years so that control, monitoring and even fault analysis is possible at even the remotest site.

High speed transistor outputs and powerful microprocessors have allowed significant improvements in the motion control industry.

Acceleration and deceleration functions have been improved with the introduction of various s-type profiles in addition to the standard linear ramp profiles.

Other common functions such as "home", "return" and "jog" are now standard features of the PLC together with both linear and circular interpolation useful for laser cutting machines.

For more the demanding motion control applications addition expansion modules are now available.

The stand-alone processing power of these modules allows the realisation of simultaneous multi-axis motion control systems giving the designer a cost effective solution to the most demanding applications.

For some time analogue and temperature type expansion modules have been available but now it is common for micro PLCs to have direct thermocouple inputs built in to the controller itself.

These hardware features coupled with fast PID control algorithms, that are part of the PLC instruction set, allow for solutions to many varied temperature control applications.

The previously mentioned FP-e controller from Matsushita is an example of this and along with digital I/O has a two channel thermocouple type input.

The IEC6113-3 standard for PLC programming is fast becoming accepted as programmers realise the advantages the system has to offer.

Five programming languages that are supported: Instruction List (IL), Ladder Diagram (LD), Function Block Diagram (FBD), Sequential Function Chart (SFC) and Structured Text (ST).

Between them, these allow programmers trained in different disciplines to ease their way into the role of a PLC programmer.

With the ability of the vendor or programmer to create specific application orientated function blocks (FBs), the time saved is considerable as they eliminate the need to "reinvent the wheel" for each application that includes common code.

As an example, vendors are able to supply fully debugged FBs for applications involving PSTN and GSM modems, eliminating the need of the engineer to understand and debug these complicated applications.

Further examples of such FB libraries include Ethernet communications, motion control and temperature control.

Continual improvements in documentation, multi-language environments, improved structure and modularity mean that the programmer can feel safe in the knowledge that such a product will ensure their investment is safe for the long term.

In summary, the micro PLC is a comprehensive device that is capable of controlling more involved and complicated applications than they are often given credit for.

Originally designed as a replacement for local relay logic control, micro PLCs are now established as a key tool for the control and automation of virtually every industrial process.

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