Critical Path Analysis

Problem

A potential client came to us in their search for simulation software. They wanted to understand how SIMUL8 could be applied to critical path analysis within a production machine that had many complex parallel steps. If they could easily understand the critical path elements, they could focus their attention on the best process steps for overall cycle time reduction. In this example, all process times are deterministic (fixed distributions with no variation).

Below is a simple example of a series of parallel processes. The values displayed (using SIMUL8's data graphics feature) contains the amount of time for an individual work item to complete the associated step and are included for information only.

Solution

To identify the work centers which are on the critical path, let's first refer back to the definition of a critical path.  If the change in process time of a work center results in a change in total cycle time (time in system), then the work center is indeed on the critical path. To demonstrate this, we added some control visual logic code to automatically run a series of scenarios, decreasing the operation time of each work center by a very small amount of time.  We then compared the resulting cycle time to that of the bench mark.  If the new cycle time was less than the bench mark, then that work center was indeed on the critical path. The simulation automatically repeated the experiment for each work center one at a time, recoding, in an information store spreadsheet, the result (1: on the critical path, or 0: off).

At the conclusion of all of the scenario runs, the simulation (using viusal logic Display+ commands) diplays red circle images overtop the workcenters, which are indeed on the critical path, and draws an arrow from one critical path work center to the next.  Below is an image of the resulting critical path analysis.

Summary

1. Run a base line simulation to establish the standard cycle time per the inputs.

2. Run a different simulation for each work center in the model whereby the process time for each workcenter is decreased by (0.01) time units. If the total cycle time decreases when compared to the standard, mark the work center as a critical path element.

3. Add graphical indications to the simulation identifying the critical path work centers.

Application

Critical path analysis has traditionally been associated with project management efforts. In this case, it is applied to process steps within a machine cycle. The potential client who requested this had been frustrated with simulation software in which a cirtical path solution had not been available. Although this example is very trivial, the same approach can be applied to machine cycles which have many serial and parallell, interacting process steps.