by Jim Anderson, on March 21, 2019
Meeting or missing deadlines is really the composite result of many individual decisions. And complex processes involve many, many decisions. Ideally, everyone involved would always know the optimal decision to make at any given moment, but the reality is that the most we can do is to provide them with information to guide their decision making.
In manufacturing, if an operator at a workstation has a choice of lots to run at a given time, it can be helpful for them to know which lot should take priority over others in order to meet customer deadlines or management objectives. We can determine priority by measuring how far ahead of schedule or behind schedule a given lot is when it arrives at a workstation. Critical ratio (CR) is a common measure we can use to determine “lateness” or “earliness” and is calculated by:
In addition to raw processing time, “remaining shop time” includes setup time, transit time and any other delay time. A CR value less than one means a lot is behind schedule, while a CR value greater than one means a lot is ahead of schedule. If at any given time an operator has a choice of lots to run on a particular piece of equipment, the lot with the smallest CR should have the highest priority.
Normally we calculate CR from a lot’s current position to the end of the manufacturing flow, but we can extend the concept to include waypoints in a process flow. Waypoint processing is a unique method of line management that establishes stages for “special” lots (for example, non-shippable lots, experimental lots, etc.) by calculating a lot’s critical ratio to defined waypoints in the processing line.
For example, calculating the CR to a waypoint could be useful to an engineer who may need the results of an experiment by a certain due date. In this case, an experimental lot may need to arrive at a certain point in the manufacturing flow to ensure that the experimental data is available on time.
In order for a dispatching system to respond to waypoints, the dispatcher must be able to determine the shop time at each step. With defined waypoints and shop time information, the dispatcher can prioritize lots prior to these waypoints so that late lots appear at the top of dispatch lists.
A few things to consider while designing and implementing a waypoint dispatching solution:
Written by Jim Anderson
Jim began working in the field of semiconductor manufacturing automation in 1996 and has held a variety of positions including MES modelling, equipment integration, software development and project management. Jim has been with SYSTEMA since 2015.
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