Technical Notes

What on earth is an SP88 or an S88?

Starting with the industrial revolution, regulatory control has interested both process and control engineers. That kind of basic control has been developed over the years and has largely defined the control engineering profession. It has become very sophisticated, robust and reliable. Although it can be very complex in execution, it serves a very simple purpose: to set things-equipment or process parameters-to a state and keep them there. That was pretty much all we thought we needed for continuous processes. Unfortunately, some companies insisted on making things in batch processes and for that, there was no really good standard way to think about or implement control. We usually resorted to some combination of relays, PLCs, proprietary "batch" systems or other devices like drum sequencers, but none of these approaches was an overall or general solution to the problem.

About 15 years ago the ISA SP88 committee took on the task of writing the S88 standard to define Batch Control-a way to standardize what we call the bits and pieces of batch control and how the various pieces should fit together. Through a combination of a lot of experience, fortunate timing and spectacular luck, it turned out to be a hit. It has served, for the last 10 years as the basis for almost all of the newer batch oriented control systems on the market. The new approach to batch control has been accepted broadly and has had a real impact on batch control market. The worldwide sale of batch control systems is now about 4.5 billion dollars a year.

What did the S88 standard add to our knowledge of control? It defined a hierarchy of modular control that lends itself to reusable control components. It promotes flexibility by allowing a recipe to determine the product related characteristics of a process while equipment control functions are imbedded in the reusable modules. It is all about structure of a process and its control. It defined and gave names to the modules we have always had in basic control that set things to a state and hold them there. On top of that it defines a layer of procedural control that allows sequences of control actions to step a process through a recipe-defined procedure. Finally it defined coordination control that keeps all the other pieces and parts sorted out. Because of the structure it provided a way for good data to be collected in understandable context from a process that, by its nature, is constantly changing state.

S88 was adopted as an ANSII standard about ten years ago and became an international standard called IEC61512-01 about four years later. Since that first part of the work, two more parts have been issued and approved and a fourth part is underway. However the seminal work was part one and all of the implementation-oriented work that has followed is based on it. One of the more interesting realizations that have evolved from the work is that S88 principles are just as applicable to continuous processes as they are to batch. It is also interesting that S88 principles are even being applied to packaging machinery.