Technical merits of S88
The framers of S88 will say that they formed the ISA committee to address batch control issues after companies spent years challenged with four basic problems:
- No universal model existed for batch control
- Users had a very difficult time communicating their batch processing requirements
- Engineers found it very hard to integrate solutions from different vendors
- Engineers and users had a difficult time configuring batch control solutions
These problems led to expensive batch control systems that often did not meet all the needs of the users and were difficult to maintain.
S88 isn’t just a standard for software, equipment, or procedures; it’s a way of thinking, a design philosophy. Understanding S88 will help you better design your processes and manufacture your products. Leveraging the knowledge and experience contained in the standard will enable you better identify customer needs, make recipe development easier, and help reduce the time needed to reach full production levels with a new system or for each new product. Following the concepts explained in S88, you could improve the reliability of your operations and reduce the automation lifecycle cost of your batch processes.
Unfortunately, simply stating all of this to your management may not give you the needed justification for your project. So, let’s review six technical merits of the standard and then in the later sections, we’ll translate those into a more business focus. If you read closely, you’ll see some business benefits already in these.
When the software that defines a product (recipe procedure) and the software to run equipment (phase logic) are in the same device (such as a PLC or DCS controller), the two different sets of code eventually become indistinguishable, and in some cases, inseparable. This makes recipes and equipment control difficult, if not impossible, to maintain. Every additional ingredient and process improvement can require far too many person-hours to modify the software.
Documenting and validating such a system is also extremely tough. If recipes are kept separate from equipment control, the manufacturing process is more flexible and can present a more appealing scenario: Automation engineers design control software based on the full capabilities and performance of the equipment, not based on requirements of the product. Similarly, the personnel who manage the recipe —scientists, process engineers, or lead operators— can easily create it and make changes to it directly.
You can benefit from the replication of recipes and equipment control code. If written properly, you can duplicate equipment functionality with minimal code changes, significantly reducing the time needed to implement subsequent projects. With S88, recipes are also more transportable between sets of equipment or between plants. Also keep in mind that a modular approach tends to reduce software complexity. This can lead to easier maintenance, troubleshooting, and validation.
A modular S88 design will allow you to validate (and revalidate) procedures and equipment separately.
Also, validating a recipe procedure is easier once the phases (and equipment modules and control modules) are validated. Since recipe procedure code is decoupled from equipment phase code, the need to revalidate a recipe procedure does not necessarily require that all phases be revalidated. Theoretically, you can validate new additions to a process or revalidate changes to a process faster. Moreover, the S88 modular design approach helps minimize the risk that a change to one part of the process will affect another. With well-written equipment phases, for example, once Phase A is validated, modifying other phases will not upset Phase A’s validated state.
Common terminology and models make communicating with customers and vendors easier and reduces integration effort. They help you, your customers, and your vendors communicate successfully, making it more likely that the system you install is the system your customers wanted. S88 helps better define the manufacturing process, since common terminology and models help us know what questions to ask. Working with more than one vendor can also be easier. If everyone follows S88, you have a better chance of successfully integrating products from different vendors.
S88 software packages track the state of the batch in a log that can be integrated with a database, data historian, or data warehouse. Many packages allow you to capture information like operator commands, the status of equipment phases, and failures encountered.
A robust batching operation that includes methods of holding and restarting is very important to the success of a manufacturing process. Recovering from abnormal events is one of the most difficult parts of batch control. In many non-S88 installations, automatic recovery is not implemented, and operators and/or engineers are needed to get equipment and the recipe back in sync. Since S88 suggests a standard set of states, it provides engineers with an opportunity to consider how abnormal events should be handled when designing the system.