CS 452/652 Winter 2022 - Lecture 27

Train Control (cont'd)

Though Experiment: Perfect Train Control?

• need exact problem specification, example:
  • given a topology, a set of trains, and a list of destinations for each train
    → determine optimal schedule
  • but: what is optimal?
    • time to last train at last destination?
    • average trip duration?
    • many other criteria possible
  • also: assume error-free operation
• with exact problem specification
  • formulate and solve optimization problem - computational complexity
  • centralized computation difficult for many real world control problems
  • or: design ad-hoc solution accommodate uncertainty (our approach)
• our goal is not overall perfection (which is not even properly specified), but:
  • build interesting multi-task, real-time application
  • model decentralized aspects of real-world system

Design Approach

• reservation server
  • continuous reservations (no leapfrogging)
  • consider directionality of reservation
  • reservation interface: train, segment → accepted
  • no time information
• travel server
  • travel plan with expected timing
  • entire plan, disregard conflicts
  • query interface: location, time → train(s)
• route server
  • determine path & feasibility (e.g. short move?)
  • compute only for slow/stopped trains? offload with lower priority
• sensor server
  • receive sensor reports via notifier
  • query travel server
  • send notification to train
• train task
  • source/dest → obtain route from route server
  • drive train
    • reserve track segments
      • reserve beyond error branches
      • reservation denied: slow/stop
      • reservation invariant: reservation distance > stop distance
    • release track segments
      • periodic or sensor-based updates
      • leave segment → release segment
      • pass turnout → release error path
      • stop → release rest of path
    • set train speed
    • switch reserved turnouts
    • handle sensor, timeout notifications
      • revise timeout?
• refinement: dynamic rerouting for conflict avoidance
  • routing - reservations - timing
  • conflict resolution:
    • wait until deadlock, then back off
    • move/avoid conflict zone by changing speeds
    • find alternative paths
  • computational overhead with more trains?
• failure model
  • sensor: spurious vs. silence
  • turnout: single-direction vs. derailment
  • can't fix everything...