Understanding Workload Control
Releasing orders in the Order Pool is typically based on order priority, order due date & availability of components.
Yet, it is also important not to release too many orders at the same time, as you may build up an excessive amount of WIP on the shopfloor. On the other hand, releasing too few orders may lead to workstations running idle. Hence, one needs an approach to release the right amount of orders and balance throughput and WIP. This is what Workload Control is all about.
Workload Control is a concept developed by Land, Thürer & Stevenson in 2009 (Youtube intro). Here is how it works.
You define a workload norm, a target amount of work for each team, e.g. 2 days of work for each team. This workload norm is configurable and can be gradually decreased over time.
For each team, the current workload is defined as the cumulative normalized workload of all orders that have been released and yet have to be processed by that specific team. In other words, it is not only based on the orders currently in buffer at that workstation, but on all released orders that are still on their way to that workstation. Also, when a team finishes the work for an order, the workload of that order is subtracted from the workload of that team. Normalized means that a weighting factor is applied, the further the team is in the sequence of operations, the lower the weighting factor.
When considering the list of orders to release in the Order Pool, Workload Control checks whether releasing an order will increase the workload of one or more teams beyond the workload norm. The basic idea is to release those orders that keep the workload for each team under or around the workload norm.
Workload Control versus POLCA
Historically, Workload Control (WLC) and POLCA were treated as competing paradigms: WLC focuses on centralized order release (controlling when a job enters the shop), while POLCA focuses on decentralized production authorization (controlling how a job flows between cells using capacity signals).
Academic studies indicate that Workload Control (WLC) outperforms POLCA, but the best performance is achieved by combining both. More specifically, the combination of both further reduces lead times in production. In practice and for simplicity of implementation, most companies deploy either Workload Control or POLCA.
In summary:
|
Feature |
Workload Control (WLC) |
POLCA |
|
Primary Focus |
Order Release: Prevents the shop from becoming overloaded at the entry point. |
Flow Control: Ensures downstream capacity is available before a job moves. |
|
Information |
Centralized: Uses a pre-shop pool to manage all incoming orders. |
Decentralized: Uses cards between pairs of cells to signal capacity. |
|
Strengths |
Excellent at reducing lead times and improving due-date adherence. |
Excellent at managing shifting bottlenecks and preventing local congestion. |