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Client: How do I model unscheduled or flexible downtime of my resource?

This article generally applies to iGrafx Process and iGrafx Process for Six Sigma Client tools where simulation is possible. The setup, however, can be defined by iGrafx FlowCharter.

You may model downtime of a resource, whether it is equipment or some sort of human resource, using some common methods. For example, unplanned downtime of a human is a 'sick day,' where as unplanned downtime of equipment might be a breakdown or equipment failure.

There are at least 4 ways to model unscheduled or flexible downtime in iGrafx, listed in order of increasing capability to model real-life situations, and in the time needed to implement:

  1. If reasonable, don't go into this much detail; The simplest way to deal with unscheduled downtime is to not model it at all. If it's not an important constraint on the process (i.e. scheduled or unscheduled maintenance is not really preventing the work, other factors), then don't model it.
  2. Go to less detail ("squint" a little): If the net effect is that you have X people or machines available at any given time due to careful scheduling, such that there are no breaks at the same time, then simply schedule those X people or machines for the full time. For example, if you have 12 people during the day shift, but you effectively only have 10 people available due to breaks, work outside the process, etc. then simply indicate you have 10 people that are 100% available during the appropriate times. If this is not realistic enough, then use one of the methods below.
  3. Use the "Availability %" feature in iGrafx in the Define Resources dialog box (from the Model menu, choose Resources) to randomly interrupt the resource for some percentage of their scheduled time per day. Please be aware that while this may be fine for most models, it may not be totally 'realistic', and you may need to use the option below to model exactly how interruptions occur.
  4. Place an activity (shape) on the diagram that requires the resource to be used (unavailable for other work) for the correct amount of time, and hook up a Generator to the activity that flexibly drives in the number of interruptions to your resource's availability. Please note that this is a flexible method that will allow equipment breakdown, scheduled maintenance, vacations, sick days, meetings, bathroom breaks, etc. This method, and several other useful modeling methods, may be found in the tips and tricks files on the website. Please visit for more information. In particular, take a look at the Scheduled and Unscheduled downtime examples.

Please note that if you have scheduled downtime, or something that effectively looks like scheduled downtime, then use the scheduling mechanism. For example, if a forklift must be serviced every 200 hours of use, and you know the forklift is used a certain amount of the time for any scheduled work day (which you can get from the simulation statistics through utilization%), then you can calculate roughly when 200 hours of use will occur, and simply mark a period of downtime as "Out Of Service" in the schedule. Look into composite schedules if you don't want to build one schedule for this.

Please also note that if the downtime is essentially scheduled, but depends upon behavior of the model (e.g. our forklift that must be maintained every 200 hours of use), then it's possible to use method 4 above, as long as the hours of use are also tracked. To track the hours of use, a Scenario attribute can track the total hours of use for the resource (assuming you only have 1; it gets harder w/multiple resources), and use that Scenario attribute to trigger a generator (e.g. an attribute change generator) when it hits a certain point (and reset your Scenario attribute to zero). This would, however, require recording in each activity that uses the resource (forklift) how long that activity used the resource for. This can be obtained by using the ElapsedTime function and transaction attributes before & after the Task page duration. For more details on modeling cycles based downtime, please see the information below and attached model.

We will now discuss equipment downtime in more detail. In modeling processes that use equipment resources, downtime is an important consideration that drives process capacity. Equipment downtime typically takes on three forms:

  1. Planned (Scheduled)
  2. Unplanned (Unscheduled)
  3. Cycles based

Planned or scheduled maintenance is typically for Preventative Maintenance (PM) and is based on a scheduled interval. For example, the drive gear box in a conveyor line gets an oil change every 8 weeks. Cycles-based maintenance represents cleaning or maintenance that is based on the number of cycles that a piece of equipment is used for. For example, a stamping machine may require cleaning after 200 cycles or a LASER may require calibration after 30 cycles.

Unplanned or unscheduled maintenance exists because of equipment failures or breakdowns. Typically breakdowns are modeled based on Mean Time Between Failures (MTBF) and Mean Time To Repair (MTTR) data. Ensure you select the proper random distributions to represent MTBF and MTTR.

All three types of downtime will employ one fundamental concept: Creating a request to acquire the equipment, acquiring the equipment and holding it for a specified time, and then releasing the equipment. In each case a specification called “Out of Service” is used for purpose of reporting resource statistics in a meaningful way. There are modeling subtleties between the three types of downtimes to represent them accurately.

The attached iGrafx model, "Acquire - Equip Downtime (Origins File Format).igx", demonstrates these downtime concepts.

Acquire - Equip Downtime (Origins File Format).igx

The Main Process diagram uses the "LASER' equipment resource in the manufacturing of a part. You may be familiar with a simpler version of this file if you have attended an iGrafx simulation training class. Also see the Downtime Process diagram in this file to see how downtime is modeled for the LASER equipment.

The activities in the Downtime Process could easily have been included in the Main Process diagram to keep it all on one diagram. However, it is a good practice to have a separate process map so that all of the maintenance processes are together in one location rather than spread around a diagram in a larger process than this example. This is a process-centric approach and the process of producing a good is a different process than that of maintaining or repairing equipment that produces the goods.

The attached iGrafx file includes text describing the specifics of the modeling constructs to represent downtime right where it is pertinent. Pay special attention to the shape notes for modeling details. Since there are multiple diagrams, remember to use your Explorer Bar to navigate between the two Process diagrams and the Report.

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