# Supporting the Management of Rolling Stock Maintenance with an Ontology-Based Virtual Depot
## 6 Development of the Process Model  
Whereas the first two steps described in Sections 6 and 7 deal with the static and structural aspects of the fleet maintenance system, in this next step, we map out the possible data flows through the key maintenance processes. To perform the capture, we use a standard method-- the Structured Analysis and Design Technique (SADT) actigram (known as IDEF0 [32]), and use as input both general information about maintenance depots and specific information from our collaborating company. An SADT activity diagram has a set of inputs from the left side of the box, a set of outputs from the right side of the box, a set of controls from the top side of the box, and a set of mechanisms (tools and techniques) from the bottom side of the box. SADT activity diagrams are used to capture the preventive maintenance scheduling process covering the communication between the involved processes/subprocesses. Figure 7 shows a process description of an example preventative maintenance scheduling system that we developed in SADT. In this example, routine PM Exams and wheel turning (i.e., use of a lathe to remove the top surface of the wheels to remove small cracks and return the wheel cross-section to the design profile) are scheduled depending on the cumulative unit's mileage since the last maintenance intervention whilst heavy maintenance and component overhaul are scheduled based on the cumulative component's mileage since the last maintenance intervention (see Figure 7, A1. A1.2, and A1.3 processes input). Scheduling of these PM interventions can be viewed as a three-stage process: (1) long-term, (2) short-term, and (3) daily schedule (see Figure 7). The maintenance activities: PM Exams, wheel turning, overhaul and heavy maintenance scheduling processes are represented with activities A1, A1.2, and A1.3, respectively. The PM exams scheduling activity (A1) takes the units' mileage since the last maintenance intervention as an input (taken from dedicated software) and produces the upper-bound limits (due date) as an input for short-term scheduling. Hence, each of the three activities is performed based on the unit's mileage (except the overhaul activity as mentioned above) and all of them are constrained by the depot capacity such as the maximum number of units accepted per day and fleet availability. We assume that all these three long-term schedules are generated using a dedicated maintenance management tool. A0 is a process updating the units' (components') mileage. Note that the use of the term \"schedule\" here is approximate long term, as the output of the process (A1) is more accurately described as a sorted list of due dates for activities. The short-term scheduling (A2) is a week-based framing of the long-term schedule where fragments of the long-term schedules are used as inputs, and the upper-bound limits (due dates) for each maintenance activity are used as constraints in addition to the fleet availability, depot capacity, and staffing rosters. Short-term scheduling (A2) tends to be manually performed in depots and relies on the planning team's experience and skills. The daily scheduling (A3) is one day of the short-term schedule where any new tasks (unplanned) might be dealt with at this level based on the outputs of the re-scheduling activity (A4), which is mainly controlled by the supervisor's decisions. The re-scheduling activity (A4) output can also impact the long-term and short-term schedules. The daily update activity (A5) deals with the daily reports where the supervisor's decisions about the reported failure reports are considered to update the maintenance decisions; the daily reports about the unfinished/finished maintenance tasks are also used to update the status of the units/components and their mileage. In the long-term schedule, only mileage and maintenance intervention intervals are available while a clear view and forecast of the depot resource availability are available at a short-term scale.