Often, we see confusion concerning the role of the manufacturing execution system (MES) versus the role of the distributed control system (DCS) in facilitating a digitalized shop floor. Some attempt to leverage the MES as a DCS, while others try to incorporate MES functionality into the DCS. In reality, both systems are important pieces of a holistic digitalization architecture.

 

A distributed control system (DCS) is typically found in continuous and batch industries where real-time orchestration is required across various production units on the plant floor, and tight control of process values such as temperature, pressure, and flow throughout the facility is essential. A DCS may also introduce a degree of redundancy and fault tolerance to ensure that critical operations spanning longer time periods and multiple production units are never disrupted. Industries often leveraging a DCS include specialty chemical manufacturing, pharmaceuticals, beverage, power generation, and oil/gas production.

 

The DCS is an interconnected system of sensors, controllers, and associated computers distributed throughout the plant, facilitating data acquisition, process control, and feedback to the operator. A DCS can make coordinated adjustments to each of a plant's many interacting automated operations. A DCS also provides a front-end user interface alerting the operator of alarm conditions, allowing setpoint changes, and providing trending of process parameters such as temperature readings, flow readings, etc.

 

The purpose of the MES is to facilitate operations management on the shop floor. This includes enabling schedule execution, handling material transactions (consumption and production, genealogy, and reporting), and managing manual activities that must be done by operators, such as equipment changeovers, taking samples, weighing and kitting materials, performing manual equipment maintenance checks, etc. The MES replaces processes that were traditionally (and often haphazardly) tracked using paper processes. The MES also often provides insights concerning production, performance, and quality that can be leveraged by various stakeholders across the organization to make decisions concerning targeted improvements. While a DCS may need to react in a matter of seconds or even milliseconds to a process condition, the MES typically manages data with a granularity of minutes, hours, shifts, and days.

 

Given the different roles and responsibilities between the two systems, some manufacturers deploy both, but with no connectivity between the two. The DCS manages the equipment automation processes, and the MES manages the operational 'people' processes. While this is an improvement over a plant driven by isolated PLCs and paper, a far greater degree of improvement can be gained by connecting the MES and the DCS in a layered manner.

 

Some key examples of interaction points that can lead to more efficient execution include the following:

  • The MES typically possesses a view of the shop floor schedule, including products to be made, quantities, and associated bills of materials (BOMs). The MES can present a schedule view to the operator, allowing a process order to be activated and the resulting production batch to be staged to the DCS, which can associate the batch to a recipe and initiate execution.
  • The DCS typically possesses information concerning the amount of material transferred into a batch and the quantity of the batch produced but lacks the ability to associate transfer quantities to specific raw material lots and ERP process orders. The MES can receive raw transfer quantities over time from the DCS, associate these with the staged lots and process orders, and report consumption/production information to higher-level systems such as the ERP.
  • The MES can also perform broader genealogy/traceability reporting in the context of raw material/finished goods identifiers. For example, a report indicating all finished goods orders containing material from a specific raw material lot is more easily facilitated by the MES than a DCS, but the detailed raw transfer values are needed from the DCS to provide the proper quantity reporting.
  • The MES may be able to better calculate situational production parameters than the DCS that are reliant upon external data sources. For example, the MES may determine a blend percentage due to external data such as the CoA, quality sampling results, and finished goods tolerances that are not available to the DCS. The MES can then pass down these key parameters that are in turn leveraged by the DCS during production execution.
  • The MES can coordinate manual operations such as sampling but may need specific triggers from the DCS to know when to prompt a sampling operation to occur. For example, when a batch reaches the end of a certain phase, the DCS may send a signal to the MES, which the MES captures and leverages to trigger a specific sampling operation and oversees its execution, potentially interacting with lab instrumentation or an external LIMS. The result of the sampling operation may then be sent down to the DCS, permitting the batch to continue or a rework operation (such as an additional mix cycle) to occur.
  • The MES can allow contextualized, summarized information to be recorded based upon detailed raw data collected via sensors connected to the DCS. For example, the DCS may be connected to a temperature probe capturing the temperature of the batch over the course of its production. The MES may, in turn, leverage readings from this probe to capture the maximum or minimum temperature during the batch cycle and commit this to an electronic batch record.
  • While the DCS may possess specific batch procedural models for various units across the plant, the MES can facilitate the translation and abstraction of this detailed data into a standard batch reference model that allows for batch-to-batch comparison across the facility and even the enterprise, considering such aspects as batch duration, first-pass quality, key batch parameters such as concentration or yield, and other aspects.

 

The means of interfacing the MES to the DCS can vary greatly based upon the DCS vendor, but it is important in any scenario that standard, well-documented, consistent interface structures are leveraged across the systems to ease change management and maintenance concerns. Some DCS platforms, such as DeltaV, allow for the deployment of a batch historian layer on a SQL server that the MES can access to obtain data. Typical shop floor interface conduits using the OPC protocol are often the norm for data handshaking (trigger/response scenarios). Leveraging the concept of the universal namespace (UNS) can reduce the need for various interwoven direct connections between systems, further simplifying the data architecture and promoting data consistency.

 

There are some key 'red flags' that may trigger one to rethink the plant's overall systems architecture, including the following:

  • Relying upon the MES to facilitate real-time process alarming, such as alerting when a process parameter exceeds a certain limit, or a batch parameter such as mix time is exceeded.
  • Relying upon the MES to track batch production in real-time and stage setpoints across various isolated PLCs in real-time.
  • Relying upon the DCS to interact directly with the ERP layer to receive the schedule or report production/consumption.
  • Duplicating detailed batch recipe information across both the MES and DCS, making change management considerations more challenging when establishing new or editing existing formulations.
  • Requiring dual entry into both the MES and DCS, or requiring someone to read values off the DCS and manually record them on a paper log sheet.

 

All of these scenarios may warrant the establishment of both a proper DCS and MES, as well as integration between the two, to improve production efficiency, agility, and quality on the shop floor.