|
What to do (if anything) with our
limited knowledge of instability and chaos? A
definitive study to
detect true chaos in a scheduling system is
limited by system complexity and the need for
sophisticated mathematics. Moreover, sorting
out true chaos from external randomness is near-impossible.
What we can do is understand the fundamental causes and effects
and then minimize these causes.
This is
what Toyota did in their early lean efforts even though their
knowledge was only vague and intuitive.
Root Causes of Instability and Chaos
Time Delays--the most ubiquitous cause of instability in
inventory systems. They are especially destructive in the
feedback or information loops. Batching is the most common form
of time delay. In the forward loop we often batch products which
sit for hours, days, weeks or months.
In MRP/ERP systems, time delays are especially common.
We often batch transactions prior to entry in an MRP/ERP system.
The system also assumes fixed delivery times. In fact, such times are highly variable
and people inflate them to
cover worst-case scenarios.
Amplification--occurs primarily from the human elements. Buyers
order more than is necessary, "just in case…" Production
supervisors overbuild because the machine is already setup.
Complexity--Complex systems are more prone to chaos than simple
systems.
Non-Linearity--Linear systems have a response that is
proportional to the input. Re-order point systems, for example,
are very non-linear. Multiple withdrawals occur in a reorder point system and
nothing happens. Then, when the reorder point is reached, the
system triggers a large order even though
the final withdrawal may have been small. |
Minimizing Chaos and Instability
To minimize chaos and instability, we must deal with root causes.
The table summarizes these root
causes and common preventive strategies. Note that Kanban,
by its nature, addresses most of the root causes.
|
Cause |
Prevention Tactics |
|
Reduce Time Delays |
|
|
Minimize Amplification |
-
No Arbitrary Increases On Orders
-
Eliminate Overproduction
-
Order/Build Frequently In Small Batches
-
Restructure Feedback Loops
-
Disseminate Information Widely
-
Correct Variations Immediately
|
|
Complexity |
-
Cellular Manufacturing
-
Simplify The Process
-
Flatten BOMS
-
Employ TQM
-
Simplify Product Mix
|
|
Minimize Non-Linearity |
-
Implement Kanban
-
Close Min-Max Points
|
Stabilization Series
Stabilizing Production Scheduling
Feedback System Basics
Instability In Feedback Systems
Complexity
Chaos In Scheduling
Using The Knowledge |
