Monday, 11 July 2011

The ATC Bump

Many metro systems use Automatic Train Control (ATC), where the train is driven automatically and has built-in collision prevention called Automatic Train Protection (ATP). The ATP part is the safety system, while the automated driving control, called Automatic Train Operation (ATO), sits on top of it and drives the train much like a driver would.  What ATO won't do, unless it is specially programmed to, is fine tune the stopping to make it comfortable for the passengers.  This leads to a phenomenon which I call "the ATC bump".

As one of my students pointed out to me this afternoon, the ATC bump is what you feel when the ATC controlled train stops.  It stops hard with the brakes on and there is very little adjustment to reduce the jerk you feel at the stopping point.  If you like, you could say that the jerk is not limited.  Well, it should be.  "Jerk limit" is incorporated in most traction and braking systems, the technology is available and, although it requires some effort to get it to work well, it can enhance passengers' comfort.  It also helps to demonstrate to taxpayers' that their money has been well spent.

All the ATC operated lines in London (the Central, Jubilee and Victoria Lines) suffer from this problem.  In the case of the Victoria Line, it's rather annoying, since the old fleet (the 1967 Tube Stock) has recently been replaced and, not many years before its replacement, it was given a modification to incorporate a smooth stop.  The new fleet (the 2009 Tube Stock) that has replaced it has brought back the ATC bump.  What a shame.

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