In addition to the spill calculation/prediction, there are many more important functions for batch material transfers that should be built into control systems.
Where to build those functions in a batch control system?
If having some or all the functions described in the three boxes can be aligned among operational, process engineering and system control organizations, a decision must be made as to where to build these functions. A quick answer to this question might be in controllers where all the phase, equipment module and control module logic reside.
Most P&G batch control systems have had many of these functions built in to the controller level for more than 30 years. We realized the main advantage to this solution is that users can see the code for the functions, and create, develop and modify them without too much restriction. Disadvantages of this solution become also obvious:
- These functions occupy a lot of memory space in the controller(s).
- Users have to maintain the code.
- It is a platform-dependent solution since controllers are platform dependent.
Today, it makes sense to locate these functions in field devices, which can be scales or flowmeters where material transfers are measured. However, building these functions in field devices requires a strong commitment from associated vendors, which may not be easy to obtain. On the contrary, having these functions built into industrial controllers can be done at the user’s wish.
It seems that many of the disadvantages will disappear and more advantages will surface when some of these functions are built into field devices. With controls built into the field device, variations such as logic scan rate and command paths can be minimized. Since building and maintaining the code for these functions requires good technical skills, it may or may not be cost-effective to keep all skills “in house” for a long term. If, however, users do not want to know how the functions are coded and do not want to maintain the code for modification, troubleshooting and upgrades, having most of the functions built into field devices can be a good solution.
P&G recently has upgraded several manufacturing batch systems with some field devices containing most of the functions for material transfers. Experience with this technique has been very positive. Not only do functions in the field devices meet all our speed and accuracy requirements, but the size and complexity of the program in the controller have been significantly reduced.
Batch material transfer control seems an easy task to carry out. But the reality is that many functions are necessary if the system is to accomplish accurate batch material transfer control. Most functions for batch material transfer controls can be built in either industrial controllers or field devices. While building some of these functions into the controllers can be managed by the users, in field devices, it requires commitment and support from vendors. Either way has its advantages and disadvantages in engineering, maintenance, and supportability.
(Editor’s Note: This article is based on a paper, “Functions Ned to be Cosidered for Batch Material Transfer Controls,” given at the World Batch Forum, Chicago, IL (2004). For more information on the World Batch Forum, go to www.wbf.org.
- Mettler Toledo Inc., “Predictive Adaptive Control Applied to Material Transfers in Batch Processes,” presented at World Batch Forum 2002.
- Chappell, Dave, “Predictive Material Delivery to A Batch System,” presented at World Batch Forum 2002.
|About the Author|
Charlie Fu is technology leader in the Corporate Engineering Technology, Department of Power, Control & Information Technologies, Procter & Gamble Company, West Chester, OH.