MES or batch: What is the best answer?
Should an application use a manufacturing execution system or will batch software suffice? A small set of decision points can determine the most commonly used models for selection or integration of ISA 88 batch execution systems and ISA 95 manufacturing operations management (MOM) systems. Ask these questions to decide. The ISA 95 Technical Report TR95.02 – Integrating ISA 88 and ISA 95 workflows can help.
Many have manufacturing execution system (MES), a batch management system, or a combination of the two. Deciding what parts of the process are better served by an ISA 88 batch execution system (BES), and which parts are better served by an ISA 95 manufacturing operations management (MOM) system or MES can be a contentious debate. The methodology and decision tree defined here can be used to help determine the appropriate tool and implementation architecture based on the ratio of equipment-based actions to manual-based actions. The methodology is based on work used to develop the draft ISA 95 Technical Report TR95.02 – Integrating ISA 88 and ISA 95 workflows. It addresses the fact that the typical MOM/MES is focused on manual actions, while a BES is focused on equipment control. Two different systems in a complete automation environment can be used together by using the strength of each system.
New process control
Many companies have islands of automation in their facilities, with some systems using ISA 88 BES, some with paper processes for coordination of automated equipment, some using an ISA 95 MES/MOM system, and often every possible combination. When it comes time to expand a system or add new production lines, the same question always comes up: What system are we going to use to control the new production process?
- Will it be an MES system because many manual actions must be coordinated?
- Will it be a BES because a lot of automated equipment must be coordinated?
- Will it be a combination?
- What if it must integrate with existing automated equipment?
The usual answers are more likely to be based on local experience, or integrator experience, with the MES options or the BES options, rather than a careful evaluation of advantages and disadvantages. Selecting the best option requires some knowledge of both the MES and BES models.
The core model for many MESs and BESs is workflows. The workflows are what provide automated execution support for standard operating procedures (SOP), standard work instructions, and other production procedures. The workflows can be customizable or editable, or can be hard-coded (as is typical in weight and dispense procedures).
There are two basic models for workflows: an orchestrated model and a choreographed model. In the orchestrated model, a coordination workflow contains the logic to sequence lower level workflows. The lower level workflows don’t directly interact with each other, but only with the coordinating workflow.
Many business systems follow the orchestrated workflow model because it improves modularity and reusability of services. The orchestrated model also provides a single view into the coordinating workflows, making it easy to see the status of the work and determine the next steps.
While there is a performance penalty from the centralized coordination service, the impact is usually not significant at the relatively slow speed of business processes. The biggest problem with the orchestrated model is that it does not scale well to entire organizations. Orchestrated workflows work best when the job or task has a well-defined scope, such as in the execution of an ISA 88 recipe.
In the choreographed model there is no overall coordinating workflow. Any workflow may trigger the execution of another workflow, and wait for a response or continue asynchronously. The overall workflow process is defined by the collection of rules implemented in the individual workflows. Because there is no single view into the workflows, the choreographed model does not provide a simple way to view the status of the work and determine the next steps.
Choreographed workflows are used when there is a large scope of work, no single overall triggering condition, or no overall coordination is needed. The choreographed model can handle faster process times and is the model often used in manufacturing applications and in customer-facing Internet applications. Both of these applications must handle thousands of interactions per minute with minimal latency and delays.
MESs typically use the choreographed model. An MES will often handle manual tasks that are triggered by external events or other workflows. An MES workflow will often have significant logic to handle exceptions and errors.
BESs, following the ISA 88 models, use the orchestrated model. Each recipe is an orchestrating workflow, commanding the lower level actions (called equipment phases in the ISA 88 standard) in an orchestrated manner. The lower level procedures are phase logic, usually implemented in equipment modules. This model allows for a single view into the status of a batch and is designed for reusability of the lower level equipment phases in different recipes and across different equipment.
BES recipes are specialized workflows. They follow the orchestrated model, and their primary purpose is the control of equipment. The equipment control can be directly to automated equipment, or as commands to operators to control equipment. Because of the focus on equipment control, batch systems also implement mode and state control of the workflow. Modes allow operators to control the execution of the workflow, allowing it to run in automatic mode, single stepped using a semi-automated mode, or entirely manual with no automatic stepping through the workflow. State control allows for the workflow to be paused, resumed, stopped, and aborted. Not all MESs provide this ability to control their workflow execution through modes and states.
Workflow on top?
There is a typical pattern for operational workflows that is based on a simple decision. If the system will receive process orders that will require multiple separate jobs (batches or production runs), or there is work in process (WIP) that is managed at the operations level, then there is typically a workflow system that receives the process orders, supports breaking each process order into jobs, assigns jobs to work centers (process cells, production lines, etc.), and tracks WIP. In this case there is a workflow coordinating activities in work centers. The top level workflow can be configured, or can be dynamically interpreted depending on the tool chosen. In making a decision on the use of an MES or batch system at each work center, one can follow a decision tree to determine the appropriate model to be used. In any complex manufacturing facility, there may be different models for different work centers. For example, there may be a batch model for primary processing, a manual workflow for packaging, and a mixed model for incoming material and material preparation.
A decision tree
Selecting the best solution for a work center can be done using the decision tree.
The four basic decisions are:
1. What percentage of the basic actions that need to be controlled are equipment executed actions? This includes equipment actions that may have to be performed manually (opening valves, starting motors, setting up parameters to equipment, sequencing equipment steps).
- If there is a high percentage of equipment executed actions (for example, over 75%), then a BES is usually the best choice.
- If there is a very low percentage of equipment executed actions (for example, under 10%), then a workflow system is usually the best choice.
- Otherwise, the best choice requires an additional decision.
2. Is the primary coordination through equipment events or manual events?
- If the primary coordination is through equipment events, such as the completion of equipment phases, then Model #4 is usually the best choice.
- If the primary coordination is through manual events, then Model #5 is usually the best choice.
3. Is there existing automation code?
- If the automation code is to be written for the work center, then Model #1 is the best choice.
- Otherwise, the best choice requires the addition of proxy phases to encapsulate the equipment control.
4. Is there a PackML interface to the equipment?
- If there is a PackML equipment interface, then Model #2 is usually the choice.
- Otherwise, Model #3 is usually the choice.
Model #1 is the choice when there is a high percentage of equipment executed actions and no existing automation code for the automation equipment. In this model a recipe execution system coordinates the process. The recipe execution system would communicate to ISA 88 equipment phases, defined using the ISA 88 Equipment Module and Control Module patterns. Interactions with operators for manual activities could be directly through the recipe execution system using prompts and responses, or through the automation equipment using dedicated displays and lights and switches.
Model #2 with PackML interfaces
Model #2 is the choice when there is a high percentage of equipment executed actions, there is existing automation code for the automation equipment, and the equipment exposes a PackML (ISA 88 TR.02) interface. In this model, proxy phases are created to encapsulate the PackML interface. Proxy phases are either automation code or code running in the recipe execution system, that makes the PackML interface look like the recipe system’s phase logic interface (PLI). Because PackML provides a standard mode and state model for equipment control and a standard way to pass parameters and return results, a set of standard proxy phases can be used to interface between the recipe system and the automation equipment.
Model #2 with no standard interface model
In this model, because there is no standard interface to existing automation equipment, custom proxy phases must be created for each piece of automated equipment. The custom proxy phase would implement the ISA 88 phase mode and state model and convert the recipe commands into the equivalent equipment specific commands.
Model #3 workflow coordination
This model is used when there is very little interface to automated equipment and a high percentage of actions are not directly related to equipment control. In this situation an MES or MOM system with configurable workflows is usually the best choice. The workflow execution engine will provide direct interaction with operators and other users. Any required communication with automated equipment will handled on a case-by-case basis.
Model #4 recipe coordination
This model is used when the primary coordination of work within a work center is through equipment events. The events often take the form of completion of phases or operations within a coordinating recipe. Unlike Model #1, there are significant workflows associated with the manual actions that cannot easily be defined using the recipe’s orchestrated workflow model. In this situation choreographed workflows are used to coordinate the manual actions. Basically, an MES or MOM system provides a service to the recipe system for manual actions. Proxies are usually created to convert the recipe’s phase commands into commands to start small workflows in an MES or MOM system.
In this model the decision on how to implement the recipe-to-equipment interface is the same as those required for Models #1 and #2.
Model #5 workflow coordination
This model is used when the primary coordination of work within a work center is through manual events. Also, if there is a combination of equipment executed and non-equipment executed actions, then this model is often used. The overall workflow is controlled by an MES or MOM system, and batches are started from the workflow. The workflow usually continues at the completion of the recipe, where the MES will collect the batch record and add it to the overall work record.
In this model the decision on how to implement the recipe-to-equipment interface is the same as those required for Models #1 and #2.
Future, combined models
The decision tree is necessary because of the historical split between choreographed workflows that are primarily designed to handle manual interactions on MES/MOM systems, and the orchestrating workflows following the ISA 88 model in recipe execution systems.
In the future there may be combined systems that allow for both models of workflows, equipment control using the ISA 88 mode and state models, and sophisticated manual interaction capability. However, when putting these systems in place today, it is helpful to have a small set of decision points to determine the most commonly used models for selection or integration of BES and MES/MOM systems.
- Decision tree helps to choose between using a batch or manufacturing execution system.
- Future systems may be combined to allow for both models of workflows.
- Applicable standards include ISA 95 Technical Report TR95.02
- Integrating ISA 88 and ISA 95 workflows.
Does your application merit a manufacturing execution system (MES) system, a batch management system, or a combination of the two?
Search on Brandl atop www.controleng.com.
See the Manufacturing IT page.
Also see the following articles on batch control and MES, linked below.
See the BRL Consulting listing at the Global System Integration Database.