Part 3 – Lot Tracking
Transcript
Welcome to part three in this series, What is an MES? In this part, we’ll take a look at lot tracking. You can think of lot tracking as the MES at runtime. Lot is the generic name for the thing that’s being manufactured.
If you’re building a Tesla, you might call it a vehicle. But because in the context of an MES we really don’t know what’s going to be built, we’ll just call it generically, a lot. Lot tracking combines a lot with a workflow.
Without a lot, a workflow is just a static set of instructions, a template, as it were, for what should happen in manufacturing. But when combined with a lot, the workflow’s instructions cause the lot to be gradually transformed into a finished product. The lot starts at the very first step of the workflow, where you get all the information needed to correctly process the lot at that step.
Once you’re finished processing the step, you track the lot out of the step, recording all the pertinent processing information, and into the next step. Each step has its own resource and data requirements. Some steps require equipment, while others won’t.
Some steps require chemicals, while others require parts, and still others, nothing at all. Ultimately, you come to a step where you evaluate the work that’s been done so far to determine if it’s been done correctly. These steps are typically placed periodically throughout the workflow in order to minimize the amount of work completed before a problem is discovered.
As you can imagine, it’d be an expensive mistake to discover a problem at the last step in the workflow that actually occurred at the very first step. At those decision points, an evaluation can be made based on all the data collected during lot processing up to that point. If the lot’s historical processing looks good, the lot is tracked out and proceeds on to the next step in the workflow.
In some cases, though, problems might be found. Sometimes the lot can be reworked and the problems corrected with very little additional work. In more severe cases, however, the damage is irreparable, and the lot has to be scrapped.
In any case, the MES records everything that happens so that reporting and analysis can occur later. At any workflow step that requires equipment for lot processing, lot tracking can be thought of as an intersection between lots and equipment. When the lot is tracked into a step, it queues up in front of equipment, ready to process. Once the lot is processed by the equipment, it’s tracked out and on to the next step in its workflow. When there are many lots queued up at equipment, the order in which they process must be prioritized. This is called lot scheduling.
Scheduling the order of lot processing can be very complex and influenced by a number of factors, including the relative priority of the lot, how far the lot is ahead or behind its expected schedule, whether the lot is a production or engineering lot, and even seemingly arbitrary factors such as the customer to which the lot will ship. When multiple pieces of manufacturing equipment are available to process lots at a step, the lots must be distributed among the available equipment to avoid idle tools and to minimize the time lots spend waiting before processing. This is called dispatching.
Rules for dispatching which lot to which tool can also be complex and influenced by a large number of factors. Lots can be thought of as following a horizontal flow through the factory as they progress along their workflow, touching every process area of the factory. By contrast, equipment can be thought of in a vertical perspective, confined only to a particular process, and often with little or no integration with other processes in the factory.
This notion of horizontal and vertical perspectives bears more discussion. The MES is all about what it takes to keep lots moving through manufacturing. You can think of the workflow as representing the physical flow of a lot through the factory.
Lots touch multiple process areas and therefore many different types of equipment as lots progress horizontally through the factory from the beginning to the end of the workflow. But a lot touches each equipment only once during its lifetime in the factory since each equipment contributes a different part in the lot’s transformation into a finished product. By contrast, a piece of equipment touches all lots.
It’s like a vertical cross-section of the factory. The customer who’s buying the finished product from the factory typically doesn’t care about the equipment used during processing. They care about the result.
Deliver a perfect product on time and they’re happy. Keep them updated along the way about their order’s progress and they’re happy. Because the MES is all about what it takes to keep lots moving through manufacturing and keeping customers happy, the primary focus isn’t on equipment. It’s on lots, because lots become the results that the paying customers care about. That’s not to say that the MES doesn’t care about equipment.
When we looked at MES process definitions in part two of this series, we certainly saw that the MES needs to know a lot about the equipment in order to ensure lots are processed correctly. However, from the MES’s perspective, the assumption is that given the proper processing parameters, the equipment will do its thing and process the lot correctly. But that may not always be the case.
So, equipment needs some special attention from outside the MES. Attention that requires a direct connection to each piece of equipment. We’d want to know if the equipment had some type of error or alarm during processing. And we’d want to monitor the equipment’s critical processing parameters to determine if any of them went out of control beyond the specified limits. Perhaps we could even use this information stream to predict when the equipment will need maintenance. That could be a lot more efficient than an arbitrary maintenance schedule.
To facilitate all this, we’d want some type of system that’s listening to the stream of data coming from the equipment, monitoring it, evaluating it in real time, making decisions based on the data stream and sending notifications when anything goes wrong. But this sounds more like an industrial internet of things or IoT application. And that’s exactly what it is.
To optimize equipment performance, we need a real-time focus on equipment and its data stream. The equipment’s perspective is about the equipment, not about creating a finished product. The equipment’s perspective is about ensuring its processing is as precise as possible, not about whether a lot waits too long in its queue. The equipment’s perspective is about ensuring it has the necessary resources and maintenance to consistently process flawlessly, not about ensuring the finished product is shipped to the customer on time. That’s the job of the MES. Because the MES assumes the equipment is going to operate flawlessly, it has far more modest needs from the equipment.
The MES needs to know which specific equipment processed a lot at any step along the lot’s workflow. And it needs to know when processing started and finished on the equipment. And it needs to know the recipe the equipment used, particularly if the recipe used was different than what was specified in the workflow instructions.
And if a person is required to start equipment processing, the MES needs to know who that person is that started the equipment. But none of that information requires connecting directly to the equipment. And it certainly doesn’t depend on real-time analysis of the equipment’s data stream.
Because the MES’s perspective is all about the lot and its horizontal flow through the factory, the requirements regarding the vertical perspective of optimizing equipment performance is out of scope for the MES. It’s not that optimizing equipment performance isn’t important for the factory. It certainly is, but it’s just not a function of the MES.
In part four, we’ll continue our examination of lot tracking and look further into lots and equipment. The key data they generate within the MES, and the metrics that can be derived from this data.
