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01 October 2003

Is continuous control just batch?

If we take a new look at process manufacturing control, it appears that, with proper control equipment, making stuff is really just making stuff.

Not too many years ago we knew with certainty there were three types of manufacturing: continuous, batch, and discrete. It was a comfortable world. The fundamental differences in the types of manufacturing were simply accepted, especially when we had to control them.

Continuous processes required panel boards or distributed control systems. Discrete production lines called for relays or programmable logic controllers (PLCs). Batch was a little like both but more different than alike. Batch was kind of like continuous, because it dealt with bulk quantities of material, but it was also kind of like discrete, because it progressed stepwise. We usually ended up with a "batch system" or PLCs or some sort of sequential stepping equipment such as a drum programmer. Even with our uncertainty about the best way to control a batch process, it definitely seemed different from both continuous and discrete. There was a nagging problem, though. Many processes had elements of batch and continuous processing. In those cases we had to either separate the process into two parts or ignore the problem.

In spite of our certainty about the three kinds of manufacturing, we had to take notice when the batch control standard and modern control systems for batch came on the scene. If nothing else, we had a new and common approach to the automation of batch processes. When we began to understand batch manufacturing and batch control (thank you ISA and S88), what we thought we knew became less certain. It turns out that batch control principles can apply directly to continuous process manufacturing. Following that line of thinking leads to the conclusion that from a control standpoint, there might really be only two kinds of manufacturing: discrete and process. One kind of manufacturing makes things and the other makes stuff. Although both convert raw or intermediate materials into something else and many of the control approaches for making stuff can work for making things, controlling the two still seems comfortably different.

PROPER EQUIPMENT

If we take a new look at process manufacturing control, it appears that, with proper control equipment, making stuff is really just making stuff. Batch and continuous coalesce into just process manufacturing. Although this requires a new way of thinking about processes, especially those we call "continuous," it is also comforting because the processes that are a mixture of batch and continuous now fit nicely under the same roof.

Until S88 taught us to think in terms of three different kinds of control (basic control, procedural control, and coordination control), it was easy to see differences that now just fade away if we have proper tools. Control for continuous processes was (and is) all about setting things to the right state and keeping them there (regulatory or basic control). Regulatory control in today's world is sophisticated technology. Unfortunately, it is easy to think of it only in terms of continuous processes. However, the need for good regulatory control doesn't go away when we approach a batch process. In batch manufacturing it is still necessary to set things (equipment and process variables) to a state and keep them there—they just don't necessarily stay in that state very long. Once state-oriented control exists, the operator or a control system can then step the process through a planned sequence of states (a procedure—procedural control). Because many things can happen independently, the operator or the control equipment must also keep multiple procedures sorted out (coordination control). The requirement for procedural and coordination control is what made batch look so different.

DIFFERENCES

A control system that can control batch-wise processing has (or should have) all of the regulatory control needed to handle traditional continuous processing. The only difference is that the system can also control procedures and contains the necessary coordination control to keep it all working together.

If we can control all kinds of process manufacturing with one kind of control system, that seems nice, but should we? Is there any reason to want procedural and coordination control in a control system for a "continuous" process? Well, yes—even a continuous process needs to start up and shut down every once in a while. Until they reach steady state conditions, all processes and their subparts (units) sequence through a series of states. That often occurs manually, but it is still a product specific procedure. If we want to automate start-up and shutdown sequences, we need control that looks a whole lot like batch (procedure and coordination). In addition to start-up and shutdown, many continuous processes go through "on the fly" product changes. In that case, we need to manipulate the individual parts (units) of the process through the proper procedures to change from one product to another with a minimum of waste. Again, this obviously requires procedural control and coordination control. One of the most telling benefits, though, may well be the ability to use a single integrated control system for a process that contains batch and continuous processing equipment.

The problem is that none of this is easy (or even possible) unless we have good tools. Our control systems need to let us use procedural and coordination control as well as regulatory control to make our processes behave properly. While today's control systems offer one or more of these features, not all of them provide all capabilities. With the emerging view of process manufacturing as a single kind of manufacturing, fully functional systems that can handle all of the challenges of process manufacturing may well become the new benchmark for process automation. IT

Lynn W. Craig is president of Manufacturing Automation Association, Inc., a consulting firm specializing in manufacturing methods and technologies for the batch processing industries.