10,000 Foot View

A Control System Perspective

By Richard Bernett, CFPIM, C.P.M.

Software can be a real pain. But before your tear the rest of your hair out or call your software vendor in frustration you might want to think about some of the nasty little details that can cause the pain to begin with. The concepts below are not necessarily life changing. Yet many manufacturing specialists have ignored them and have needlessly suffered as a consequence.

First, let me be clear that software programmers and providers are NOT evil people. In fact, I am amazed and quite proud of how dedicated they are and how much progress has been made during the last three decades. Unfortunately, like any automated system, there are inherent limitations to what these systems can and can’t do. The purpose of this article is to help you understand that how you think about your operations and organize your work is as much of a factor for success of a software implementation as what code you choose and how you choose to configure it.

Special Note: Up until now, I may have given you the impression that ERP (Enterprise Resource Planning) is synonymous with MRP (Material Requirements Planning) and MRP II (Manufacturing Resource Planning); although I realize that they are distinct. I will address this in more detail later.

Here is some background…

I once worked in a process-based company where several process engineers worked to keep the complex process operational. Over lunch one day, the engineers were discussing the pros and cons of feed-forward and feed back control systems. Not being a process engineer, they were eager to rid me of my ignorance.

A feed-forward control system uses pre-defined inputs to manage the system. A good example of a feed-forward system would be your typical lawn sprinkler arrangement. You determine how much water you want, the time of day to start the watering, which part of the lawn goes first etc. All in all a very nice set up, unless of course, it rains. I’m sure many of us have seen sprinklers running in the rain and wondered why. That’s the pitfall of a feed forward system; it assumes that certain operating conditions are predictable.

A feed back system, on the other hand, takes information of the existing process (often real-time) and adjusts inputs accordingly. For example, in the sprinkler system example above, if we placed a moisture sensor to detect the current lawn moisture content, and programmed the sensor to allow water only below some set point, we would than have a feed-back control system. Feed-back systems are much more effective when the environment has several variables. The downside is that feedback systems tend to be more complex.

Recently, a client of mine was enthusiastically espousing the virtues of moving away from a traditional pull planning system (MRP) and towards a Kanban-based Pull system. He said that using actual customer demand, as opposed to the master production schedule, would be a blessing. But you may know the familiar story… “this will be GRRR-E-A-T once some of the kinks are worked out for getting it in place!” But the devil of these things is ALWAYS in the details.

Working with clients over the years I hear this quite a bit. Push is passé while Pull is ‘in’ because Pull is synched to true demand. Checking the APICS dictionary we get: PUSH -“production of items at times required by a given schedule planned in advance” and PULL – “the production of items only as demanded for use or to replace those taken for use”.

The great debate between so-called “Push” systems such as MRP and “Pull” systems such as Kanban is old news. Most practitioners by now would likely agree that demand pull is superior to push in that lead-time is reduced, work in progress (WIP) is reduced etc. However, the actual reason for the overall superiority, at least in terms of scheduling final assembly, is not what the common wisdom holds, it really has much less to do with ‘demand’ as it does with ‘feedback”.

So, what does this have to do with Push and Pull planning systems?

Despite the conventional wisdom that Pull and Push is differentiated by the nature of demand, the real differentiator is the nature of the system-type. A push system is a feed-forward control system where as a pull system is a feed-back control system.

For those whom have spent time and energy implementing a so-called closed loop MRP system, and constantly frustrated by the results, you can take solace in that there is a big difference in calling something ‘closed-loop and actually having a true feed-back system.

Recall that the first iteration of MRP in the late 60’s was followed very closely by ‘revised’ versions called “Closed-Loop MRP” in the early 70’s. It became apparent that without some feedback mechanism built into the system it was unworkable. By the 80’s the inclusion of several feedback modules (i.e. shop floor control and input/output control) was necessary to assure the planning side was kept valid. And from Closed MRP a new improved methodology manufacturing resource planning (MRP II) emerged to dominate U.S. discrete manufacturing in the 80’s and 90’s.

Unfortunately, the term ‘closed-loop’ was a bit of an exaggeration. Yes, the system technically has a feedback loop. It is self correcting over time, but that’s the catch – how much time?

In an effective control system, the feedback has to be time relevant. For example, in your car’s cruise control system, the feedback is virtually ‘real-time’. As the car senses a hill, it adjusts quickly. Imagine, if the delay between heading down hill and easing off the throttle was two minutes? Would you define this as being “in control”?

Yet, this is exactly what happens in MRP II, the feedback is simply not responsive to the real world changes that impact production.

Thus despite the implied ‘closed loop’ name tag, MRP II is in practice a feed forward system, with occasionally tweaks after the fact. Back to the sprinkler analogy, if we do hit a rainy period, we may adjust the sprinkler system accordingly – but usually, a dollar short and a day late.

Contrast that to a pull system. Here the system feedback is based on the material status itself – if an upstream resource has insufficient inventory, it automatically sends the signal to the downstream operation to stop. In this case the information and material are one in the same. Pull is a true closed loop system since the feedback mechanism is time relevant and the signal is very clear.

The real difference between push and pull is not whether we are building to ‘demand’ or to ‘forecast’. Rather the difference is because PUSH is inherently a feed-forward system. So, let’s be clear that the primary differentiator between Push and Pull is not what triggers the process, but what mechanism is used to control it. In MRP II, the ‘feedback’ mechanism is generally unreliable or not timely and thus is better described as a feed forward system. Pull systems (whether Kanban, or Drum Buffer Rope) are designed to adjust to immediate conditions.

Here are a few things to keep in mind:

• Purge and clean up any WIP that is physically on the floor and assure that the necessary MRP transactions reflect this reality.
• If you have a resource that is at or near its capacity, use this resource to implement traditional Input-Output control. The concept of Input-Output control is valid, but is too complex if used in more than a couple of key resources. Pick one or two strategic resources and use these to pace the inputs into the system.
• Do not release new orders into the system without validating that existing orders were actually completed and out of the system.
• Avoid the temptation of releasing any work that does not have all the needed material or resource time, since this order will invariably end up stalled in the process and add to confusion on the floor.

Knowing that MRP is not a true closed loop system can help guide improvement efforts. Here is what you can do to improve your existing MRP:

The real secret of Kanban as a scheduling tool is in its inherent ability to choke off release of new orders. This automatic ability of restricting WIP is the real Trump card of Pull. And while MRP is not as robust in doing this, with some awareness of the importance of input-output control at least you will be heading in the right direction.

Richard Bernett is an expert in operations within complex and precision manufacturing environments. He helps companies save time, money and aggravation by improving how they utilize their ERP or MRP systems.

Richard can be reached by email rbernett@3sixty solution.com or phone at 612-216-1629.