Many people would consider Manufacturing Execution Systems (MES) and Advanced Planning and Scheduling (APS) to be completely unrelated, to the point where you might well have completely different project teams investigating and installing each type of system.
Driving forces
Typically the APS project would be driven by the planners, who want to be able to carry out "what ifs" on their plan to improve adherence to company wide KPIs (Key Performance Indicators), whilst the MES project could be driven by Facilities Management who are striving for a better planned maintenance regime, or by production/IT who are looking for more accurate data to update the existing ERP system. Yet the systems are, in fact, two sides of the same coin, and when both are implemented together you can "close the loop" in your production control.
Closing the loop is a control system term, used to describe the necessity of providing feedback from a process if you are to control it accurately. To illustrate the problem, imagine that you are operating a simple light switch but you don't know which way is on and you can't see the light you are controlling. Can you tell whether the light is on or off? No, because the feedback you would normally get from looking at the bulb is not available, so what you have is an "open loop" control system which can only make assumptions about how the process (light) is responding.
Crossing your fingers
If we now move to a manufacturing environment, the light switch will be replaced by our APS. The APS will take in our manufacturing orders and schedule them against our resource capacity constraints, material availability and delivery promises, etc., to produce our production schedule. If we simply issue the schedule to the shop floor with no feedback in place we also have an open loop control system, because we are assuming that the production process will follow the schedule. Pursuing open loop control is like returning to the old days where we simply threw the orders over the "glass wall" into manufacturing and then just crossed our fingers and hoped that the products would turn up on time.
Of course in the real world things can and will go wrong and we need the shop floor feedback into our APS to allow the planner to decide what to do to correct the problems. We now come to the issue of the rate or timeliness of the feedback and the required rate is determined by the characteristics of our process.
Aiming towards MTO
If we are making to stock with large batches that take several days to produce, then feedback once a day would be adequate. Unfortunately few manufacturers today can afford the luxury of the classical EBQ (Economic Batch Quantity)-based make to stock environment and the pressure is on to become leaner and more agile so we can aim towards Make to Order (MTO).
Once we have created our lean, agile and flexible process (a significant problem in its own right), the issue we face is getting timely feedback. We are now rapidly making many small batches in flexible sequences so traditional end of shift paper-based feedback mechanisms will no longer cope. For example, an order could pass through many production stages (operations) between updates and could be well off the planned schedule before the planner is aware of the problem.
Fast feedback
What we need is a system that can quickly feedback the shop floor status to our APS, and this is where MES comes into play. A good MES system will use bar codes, RFID, machine cycle counters, etc. to monitor our production process in close to real time, and then provide timely feedback to our APS so our planner can also monitor the process and take corrective action where necessary.
Some MES systems can also disseminate the Work to Lists (WTL) produced by our APS in electronic form thereby removing the need for paper WTLs, which can be a barrier to re-scheduling more frequently. With our APS and MES now working hand in hand we are in a position to control our flexible, lean and agile production process, where ever possible making to order, and where we can't, making the smallest possible batch of each product.
