A Methodology for Batch Control Implementation—Real World Lessons
Dave Christie, Yokogawa Industrial Automation
Contents
Introduction
Understanding the Process
Batch Control Design
Recipe Procedures
Coordination of Application and Hardware Designs
State Transition Matrix Fundamental to System Behavior
Exception Handling Is Critical Element
Batch Reporting
Conclusion
Introduction (Back to Top)
I love to hike. Living in Georgia, I have plenty of fascinating outdoor areas for this hobby. But my trips don't always go so smoothly. The Okeefenokee has oceans of water, mud, vines, alligators, and bugs, and bugs, and bugs…. Without a good map and a compass, a person can have a pretty miserable time wandering around in the Okeefenokee.
The last time I went to the Appalachians, it rained. Then, on top of the mountain, the rain started to turn into sleet. I decided to take a shortcut to get back to the warm, dry jeep, and I eventually ended up down in a steep, slippery canyon full of tangled rhododendrons. As in the Okeefenokee, I needed a map and compass and a well thought out travel plan. Even then, the trip might not have been a dream, but with a little forethought and planning it would have been a lot less miserable.
The point is that we often have a vision of where we want to go, and how perfect and beautiful it will be when we get there, but we often bog down between the vision and the reality. Success requires two elements: a goal and a plan for reaching that goal. Without a goal, we do not know where we are going, why we are going there, or when we have arrived. Without a plan, we may never arrive and might even wish we had never started out in the first place.
This concept applies to batch control as well as to hiking.
As a practicing batch control engineer, I use and appreciate the importance of ISA S88.01 almost daily. The ISA S88.01 control models and terminology facilitate communication and understanding between vendor and customer, between vendor and vendor, and, most importantly, within the vendor/customer project team charged with implementing any specific batch project.
In hiking terms, ISA S88.01 is the equivalent of a newspaper's "Outdoor" section. It provides the goal. It allows the batch engineer to visualize what a completed batch control system will look like, what features it will have, and how everything will be perfect and beautiful in the end.
However, ISA S88.01 does not address project practices and methodology. It is not a map and compass, it provides a goal but does not dictate how we should get there. We are on our own to have a smooth journey or to wallow in the Okeefenokee through our misguided efforts. As a project journeys from concept to the plant floor, there are many pitfalls for those who do not plan ahead. On the other hand, there are many rewards for the engineering team that combines knowledge of ISA S88.01 with a proven design and implementation methodology.
Within my own company, the implementation concepts that have been learned from real world experience have been formalized in our ISO9000 procedures, peer review process, and Batch Engineering manual. Our ISO procedures dictate what activities should occur, in what order, and with what quality barriers. These internal practices are flexible to accommodate a wide scope of projects (such as validated or non-validated) but are specific enough to be our "map and compass" to keep us on the happy trail toward a successful project. I would like to share some of this experience.
Understanding the Process (Back to Top)
My first and foremost rule is, "Don't do anything until you thoroughly understand the process." This is easy to say, and even easier to overlook in the rush to get a project off the ground. The uses and interconnections of process vessels seem obvious at first. Then, upon further analysis, there always seem to be those non-standard recipes, which require unusual operations and make the life of a batch engineer so interesting.
Batch Control Design (Back to Top)
Rule number two requires good self-discipline (or a strong ISO auditor): "Don't implement until you have designed." How tempting it is to think, "I know where I'm going. I'll get ahead on the project," and then rush to start punching keys. Then, how confusing the design changes and software patches will be before the project gets finished. Successful batch control is very structured, consisting of ordered and interrelated pieces such as formula data, unit procedures, operations, and phases. These are difficult to implement and maintain if the underlying structure does not fit the process.
Recipe Procedures (Back to Top)
Regarding the recipe procedure itself, it is amazing how many different people will say, "Don't worry, we know exactly how to make that product. We've been doing it for years," then proceed to disagree among themselves exactly how the batch should be run. Automation of a batch process often provides an opportunity to re-think the "best" recipe procedure, and different internal groups may differ in perspective.
Coordination of Application and Hardware Designs (Back to Top)
In many organizations, different groups engineer the control hardware and the batch application. It can be a challenge to get these groups to talk to each other, and this can be a source of problems. In many cases, all that is required is a little up-front communications between the engineering disciplines.
State Transition Matrix Fundamental to System Behavior (Back to Top)
The State Transition Matrix describes the "rules" for switching between various recipe modes and statuses. The design team should not underestimate the importance of the State Transition Matrix. It is one of the most fundamental aspects of the batch control system; consequently, it may be one of the hardest things to change after the system is in operation. The State Transition Matrix is one of the first items, which should be designed, but is one of the easiest items to overlook in the rush to define units, flow chart unit procedures, and lay out recipe data.
Exception Handling Is Critical Element (Back to Top)
Exception handling is easily overlooked but is a critical element for achieving long-term control system success. Based upon my company's real-world project experience, exception handling constitutes 40–60% of the batch control design and implementation effort. It is easy to get caught up in the excitement of automating a recipe, but it is more tedious to go back and figure out how to handle all of the unpleasant complications that might occur.
Batch Reporting (Back to Top)
The batch report is mentioned last to emphasize that it should not be considered last in the batch design. A successful batch report is not an add-on and requires different considerations than the process report for a continuous plant. The structure of the batch reports should be considered early during the design. These reports should receive the same deliberation that is given to the handling of recipe formula data. Thus, there will not be any unpleasant surprises and hurried re-engineering when the report is formatted near the end of implementation.
Conclusion (Back to Top)
This article has focused on a batch control design methodology, which is equivalent to a map and compass, which are used to find one's way during a hiking expedition. So far, we have not laid a finger on the keyboard to begin control implementation, and that is exactly my point.
I leave you with three general rules of thumb:
- Do not design the controls until you fully understand the process.
- Do not implement the system until you have created a written design.
- Resist attempts by others to ignore Rules #1 and #2.
For more information: Dave Christie, Yokogawa Industrial Automation, 4 Dart Rd., Newnan, GA 30265-1040. Tel: 404-254-0400. Fax: 404-251-6427.
This article was adapted from a piece appearing in the Spring, 1999 Newsletter of the World Batch Forum, an engineering society devoted to the understanding of batch automation standards. World Batch Forum is located at 4032 E. Woodland Dr., Suite 100, Phoenix, AZ 85044. Tel: 602-759-9246.