Heat is on for analogue vs digital temperature control

Bob Postlethwaite suggests that the need for higher accuracy and repeatability has renewed the focus on analogue process control systems

Very tightly controlled batch production processes, such as those found typically in the food, pet-food and pharmaceutical industries, now mean that manufacturers in these and similar areas are looking hard at the accuracy, and repeatability of their control sensors and systems. What was originally a matter of maintaining product quality and ensuring that output was consistently acceptable to the market over a long period of time, has now become more a matter of meeting international regulations and fulfiling demanding requirements.

In fact, as manufacturers now strive to compete, their focus has changed to consider close tolerance manufacture. For example, in the food processing industries previously acceptable temperature tolerances of +/-2 or 3 degrees C have now given way to typical values of +/-0.5 degrees C. Where statistical process control (SPC) is employed as a cost cutting tool some manufacturers expect even tighter control of their processes. Fortunately, the developers and manufacturers of process control systems have generally been able to evolve their products and systems to meet these more demanding requirements.

APPEALING CONCEPT

A popular view in these quality focused industries until about five years ago was that the answer to providing more accurate process control was essentially one of using more digital control techniques. This was certainly an appealing concept, since the availability of low cost PLCs, interfacing where required to centralised PC-based control solutions, and possibly using the inherent integrity of digital bus communications, appeared to be both practical and economic. Such systems however, have now been proved generally unable to economically meet the growing requirement for higher accuracy.

Wherever questions of accurate and repeatable control arise, the most cost effective answer is to use local PID controllers. This is particularly true in a range of food and drink production processes like sterilisation, melting, baking, direct and pressure cooking, wrapping, sealing, and continuous processes. Analogue systems may utilise discrete stand-alone instruments or be interconnected as part of a distributed control solution.

The principal reason for this development is that discrete instruments incorporate highly developed – and often highly proprietary – software algorithms and can be programmed to do a particular job. With electrical heating loads for example, power feedback can be used to stabilise the controlled temperature against supply voltage changes, leading to greater heating accuracy. Dedicated cooling algorithms can likewise ensure optimum control of fan, water and CO2 cooling systems.

A BALANCING ACT

Whilst the evolution of process control systems has seen a number of developments, particularly in bus communications, what we are now seeing is the establishment of a balance between analogue and digital systems. The strengths of digital – including distributed data processing and reliable in-plant communications – are happily being married to the local accuracy and repeatability afforded by more traditional PID techniques. The result is that process control operators are enjoying what Voltaire called `the best of all possible worlds’.

Bob Postlethwaite is with Eurotherm Controls