Single/multi-loop controllers

What’s new in single and multi-loop controllers? Ron Higginson

The term ‘controller’ spans a huge range of products. At one end of the spectrum there are powerful and complex process control systems in 1/2 DIN housings, and at the other there are tiny temperature controllers in housings so small that you need specially sized fingers to operate the buttons!

One factor is common to all of them however: advancing semiconductor technology and software together with automated manufacturing is making them ever more powerful while prices continue to fall.

Temperature controllers, typically in 1/4 – 1/32 DIN, are most usually specified for stand alone machine control applications – albeit, increasingly communicating with a PC that provides a powerful human machine interface and keeps records of the controlled parameters on its hard disk. Recent advances include improved control algorithms using fuzzy logic and neural networks. A few of these instruments are now available in multi-loop configurations, but without a PC or other intelligent HMI, the configuration using front panel controls is daunting to say the least.

Turning now to the instruments traditionally thought of as process controllers and usually produced in 1/2 DIN housings, these have undergone a complete revolution. Because of price competition from smaller DCS systems, the days of building even 20-25 loop control systems from single or dual loop process controllers are long gone.

Recently, these process controllers have been relegated to providing additional security in the form of an independent, fall-back, control device within a larger system (‘single’ loop integrity).

Multi-loop controllers, while more cost effective, were generally associated with proprietary DCS systems – until the latest generation of instruments that is.

The newest process controllers are really small DCS systems in a panel-mounting box, quite capable of performing complex machine control tasks unaided.

Communicating upwards by Modbus or even Ethernet, communicating downwards to remote I/O using one or more fieldbus protocols, programmed for both continuous and sequence control using familiar and efficient IEC1131-3 tools … only the housing distinguishes these units from a DCS.

Taking an example from Moore Products, the PAC 353 provides up to 25 process control loops and extensive sequence control configured by a highly intuitive integrated suite of function block and ladder logic programming tools.

Local I/O is supplemented by up to 128 additional I/O channels that can be located up to 2,000m distant, connected by a free topology LonWorks twisted pair link operating at 78kbaud that also supports peer to peer communications between intelligent remote devices.

Each loop has its own PID and A/M control function blocks, plus 99 blocks per loop of other functions, such as math etc. Instance-specific block naming is fully supported.

A typical industrial boiler burner management plus combustion control application can be configured using 60 rungs of Ladder Logic and PID loops and occupies around 40% of PAC 353’s memory.

Looking to the future, with open communications to simplify integration and the cost advantages of the simpler housings, units such as these will take an increasing share of the small DCS market.

Benefits will include simplified inventory and manufacturing for the vendors and reduced configuration and programming time for the systems integrators, especially as more and more pre-configured control strategies are supplied in the standard units. Users can only gain as costs tumble.

* The Author is with Moore Products