CS 452/652 Fall 2019 - Lecture 27

Train Control

Objectives

• overall: finish trips as fast a possible
• technical objectives
  • route finding
  • collision avoidance
  • sensor attribution
  • robustness

Sensor Attribution

• sensor hit is either
  • train - which one?
  • spurious
• keep track of expected sensor passing
  • train N, time T +/- error margin
  • error margin no necessarily symmetric
    • velocity estimate - symmetric
    • sensor reporting latency - asymmetric

Robustness

• error: unexpected sensor is triggered
• error detection critically depends on assumptions
• here: assume at most one failure, not two in a row
  • can observation be explained by a single turnout failure or a single sensor failure?
• spurious sensor hit?
  • testing: treat as critical error
  • demo: ignore and hope for the best
• enhanced error model?
  • one error → recover
  • two errors → abort
  • anything else → spurious/ignore?
• in general: application type?
  • online shopping (little at stake): drop transaction, recover by reset
  • train, car (lifes at stake): unexplained failure → stop!
  • unmanned mission to Mars (recovery difficulty): try to keep going at all cost

Route Finding

• offline: precompute (some) routing information at system startup
  • including knowledge about track deficiencies
• finish path computation online
  • e.g. Dijkstra previous hop tracing
  • e.g. find alternative routes
• on-demand: compute entire end-to-end path online
  • including knowledge about other planned routes
  • single-node Dijkstra feasible
• on-demand conflict resolution
  • avoid conflict in time: wait/slow
  • avoid conflict in space: reroute
  • planning horizon? direction of conflict?
  • chance of stand-off or deadlock
    → need train to go back to clear conflict?
    • planning horizon: trade off between safety and agility
• train driving / turnouts
  • switch all at once vs. on-demand

Collision Avoidance

• use reservation system (server)
• divide track into suitable segments
• full-path vs. on-demand reservation and release
• planning horizon ↔ velocity / stop distance
  and topology: when entering stretch of single-track
  • use same planning horizon for routing and driving
  • reservation can determine sensor attribution
• more than two trains: cascaded stand-off?