Historians in general are considered a sub system or data source in the overall controls architecture. The following shows the S95 functional hierarchy model with the addition of the PI historian:
The MES system (Level 3) is the intermediate layer between the ERP system (Level 4) and the control system (Level 0, 1, 2). The MES layer provides a variety of different functions:
Resource Allocation & Status
Dispatching Production Units
Product Tracking and Genealogy
To perform these activities\tasks requires interfaces to connect to the different systems and data sources, data models to contextualize the information and a state machine workflow for execution. Workflows are either triggered based on time or events and state changes are captured in a SQL database.
In a PI-MES hybrid some functionality will reside in PI and some in the MES software. The functionality that should be performed in PI are:
Time Series Aggregates and other calculations
Process Models (Predictive/Forecast)
Equipment and batch data model
In a commercial MES solution 1) and 2) are typically provided as interface either through PI or as standard OPC interface. 3) and 4) are functionalities that are best performed in PI due to the wide spread availability of programming interfaces (API, SDK, OLEDB) and calculation engines (PI-ACE, PI-Analysis, CEP, …).
Equipment and batch data model exist both in PI (AF and EF) as well as in the MES. These objects require synchronization between the PI and MES system.
The next blog will explain how to set up the equipment and batch model in PI to be S95 and S88 compliant in order to be synchronized with the MES system.