May 2008

All in a batch's work

By Dennis Brandl

Work definitions can control the flow of material between process cells, provide a mechanism to integrate material handling and material movement, integrate laboratories, material movement, and maintenance work flows into an integrated view of production, and provide a method to map the business view (product definition) to the execution view (work definition).

The ISA88 standard provides a powerful model for effective operations and control at the process cell and master recipe level, but says nothing about the levels above the process cell. The ISA95 standard provides activity models and data exchange models, but does not formally tie these to the ISA88 model. How can a new model of work definitions in the ISA95 updated standard bridge the gap between the ISA95 and ISA88 models? You can use work definitions to define the routing of batches between process cells, production lines, storage, production units, and other work centers.

We have extensively applied ISA88 to the control of process cells; it provides an effective set of methods and patterns for recipe and batch management. However, ISA88 makes several implicit assumptions. The most important are:

  • Materials required for a batch are made available to the process cell prior to the batch starting.
  • Materials produced by the batch are removed from the process cell and sent on for further processing or storage.
  • Someone or some system has determined which batches to place on the batch list in the process cell.

In a fully functioning site, there is either a manual or an automated manufacturing execution system (MES) that handles the above functions. Unfortunately, a definition of the structure that describes the material routing between process cells, the sequencing of recipes required to make a complete product, and control and tracking of the movement of materials between process cells or work centers is not defined in either the ISA88 standards or the current versions of the ISA95 Part 1 and Part 2 standards.

The ISA88 Part 3 standard defines site recipes; these types of recipes define material dependencies between process stages and process operations but do not define specific equipment. Site recipes may be managed by product lifecycle management systems. The ISA88 Part 1 standard defines master recipes; these define the routing of the process through units in a process cell. Master recipes are managed by batch control systems. The ISA95 Part 1 and Part 2 standards define the product as seen by the business systems. The product definition defines the routing through product segments at defined inventory, scheduling, and accounting points. A product definition is often maintained in both ERP systems and MES.

An intermediate structure between the product definitions and master recipes has no formal definition in the current ISA95 standard. The missing structure defines the routing or work and materials between process cells (or work centers) and is normally maintained in an MES.

In ISA95 Part 1 and Part 2, the missing element is named product production rules. However, no structure is defined, and the IEC 62264-1 standard (International version of ISA 95 Part 1) says product production rules are the information used to instruct a manufacturing operation on how to produce a product. This may be called a general site or master recipe (IEC 61512-1 definitions), product data application protocol as defined in ISO 10303-1, standard operating procedure, standard operating conditions, routing, or assembly steps based on the production strategy used.

What work means

A work definition contains information at each work definition segment. 

1. Material specifications: What materials to produce and consume when the segment executes.

2. Equipment specifications: What equipment to use when the segment executes.

3. Personnel specifications: What personnel resources to use when the segment executes.

4. Work parameters: Any product specific parameters required when the segment executes.

5. Work instructions: Used for production outside the work definition associated with a work definition segment.

Work definitions are organized by segments. Each work definition segment defines the collection of resources required for a specific area or work center for the production of the specified product. Work definition segments may be nested, made up of sub-work definition segments. One such segment may define what is to be performed in an area, and then the sub-work definition segments define what is to be performed within each work center within the area.

A work definition segment may reference external work instructions and may define personnel resources, equipment resources, physical asset resources (a new element added in the planned update to the ISA 95 Part 1 and Part 2 standard), materials resources required for the work segment, and any parameters that may need to be specified when the segment is to be performed.

ABOUT THE AUTHOR

Dennis Brandl is president of BR&L consulting in Cary, N.C., and chair of the ISA88, Batch Control Systems standards committee.