Linking Recipes to Equipment
In this section you will learn
- More information about what physical model levels can run procedural logic
- How recipes link to equipment to run a process
So far, we’ve discussed equipment and recipes using the physical model and the procedural control model. Section 4 showed how recipes defined the batching process using procedures, and sections 5 and 6 was all about equipment, including modules.
But, somehow recipe procedures must be linked to equipment control. An operator can make an entire batch by controlling equipment one function at a time, but assumes responsibility for all kinds of details, such as coordinating equipment control and sequencing functions. If you want to make a product that has complex processing and you wish to avoid significant operator intervention, then you should really consider using a batch management system (or batch engine).
Formally put, procedural control directs equipment-oriented actions to take place in a given sequence in order to carry out a process-oriented task. Remember, it’s the recipe procedure that is used to describe the actions necessary to make equipment do its thing.
Figure 16 shows a matrix of the elements of procedural control that can be executed by the different levels of equipment. Since S88 was designed for implementing any level of automation, we’re not making any assumptions that the procedural control execution is being handled electronically. More than likely, you’ll use S88 software to run your process, but an operator can also execute a manual procedure.
Figure 16: Procedural Control Executed by Equipment
A recipe procedure is used to manage one or more units in making a product (or otherwise telling equipment how to sequence). Since a procedure spans units, only a process cell can execute it.
There are more types of control
Procedural control is just one of three types of control. If you’re interested in learning about the other two, basic control and coordination control, read Applying S88.
A unit has the most opportunity to execute procedural control, as it can run unit procedures, operations, and phases. An equipment module is confined to only executing a phase and a control module is not allowed to execute any procedural control.
This last point is what helps to clearly differentiate between equipment modules and control modules: An equipment module has procedural control associated with it (in the form of a phase), a control module does not. If you have a control module that is complicated enough that it must run some procedure (beyond basic state sequencing), more than likely you need to reconsider it as an equipment module.
Linking Recipes to Equipment Control
In order to make a batch, the instructions in a recipe must somehow be conveyed to the control system running the equipment. S88 suggests that a batch engine and an equipment control system should exist separately. However, procedures, unit procedures, operations, and phases could exist in both systems —although not simultaneously— depending upon how you run your operations. (But, be careful on putting instructions for executing a recipe and control code to run equipment in the same system. To learn more, see point 1 of the Technical Merits of S88.)
While S88 does not exactly dictate where the link between the control recipe procedure and equipment control occurs, it does require that a recipe procedure always exist in the batch management system and an equipment phase always exist in the equipment control system. Figure 17 shows this. Unit procedures and operations can either fall under the control recipe domain or the equipment control domain.
Figure 17: Recipe and Equipment Control Linking
Figure 18 shows what has emerged as the most common link, the phase level link. At the phase level in the control recipe, a reference is made to an equipment phase that is executed in a control system that performs a particular function. This reference is handled by sending parameters from the recipe phase to the equipment phase. As it’s executing or when it’s finished executing, the equipment phase then sends results back to the recipe phase that can be inserted into a database or included in a batch or run report.
Figure 18: Common Method of Linking
Linking at the phase level really means that the batch engine contains procedures, unit procedures, operations, and phases within the recipe and that the equipment control system contains only phase logic. Phase logic generally exists in process control equipment, such as a PLC or a DCS.
Figure 18 is very specific in that the recipe phase connects to a phase running on an equipment module. The equipment module phase (or equipment phase) then connects to control modules to command actuators or read sensors.
Remember that an equipment phase can be executed by a unit or equipment module. If executed by a unit, the phase is often called a unit phase. The unit phase can then link to equipment phases that are part of equipment modules. The equipment modules then command control modules, as needed. This is shown by Figure 19.
Figure 19: Unit Phase with Connections
Now we have learned about all three important elements that define a batch control system: recipes, equipment, and control activities.
We’re just about done. Go to the last section to finish up.