On Fri, 20 Mar 2009, MIG wrote:

On Mar 20, 1:33*pm, wrote:
On 20 Mar, 13:15, MIG wrote:

On Mar 20, 11:48*am, David Cantrell wrote:

On Thu, Mar 19, 2009 at 06:38:09AM +0000, Neil Williams wrote:
A more clever system could use the GPS data to derive how far the
train travels on one revolution on that day and use it later when the
GPS was not available.

You're buggered if there's the wrong type of leaves on the line though,
cos then rotations has only an approximate correspondence with distance
travelled.

That would only apply to powered axles, but skids could affect any of
them.

Surely it would only apply to axles where the wheelsets have brakes.
If a train 'skids', some or all of the wheels could lock up, not just
the powered ones. On modern stock, all wheelsets have brakes.

Yes, it was a bit telegraphic wasn't it. I was inferring that the
main leaf-fall problems are to do with wheelspins, although that's not
actually in any of the words we wrote.

If there are any unpowered and unbraked axles ... not that I'm
commenting on the viability of the proposal in general.

Instead of a rev counter, how about an eddy current gizmo that measures
the passage of the rail? Same principle as a traditional speedometer:
passage of the conducting material (the rail) past the magnet (on the
gizmo) induces a force between them, which is measured by a force gauge.

Or sufficiently accurate inertial sensors. Given that trains follow highly
repetitive routes, with sufficiently clever software, they wouldn't even
need to be that accurate. It's certainly not a question of being able to
do fully general inertial dead reckoning, as on an aeroplane - you just
need to be able to tell when you've hit this curve or that station that's
listed in a database.

GPS just seems like a really bad idea for vehicles which regularly go
underground. Okay, it's fine for when they're in the open, but to build
the system round the assumption that a GPS fix will always be available is
pure madness.

The most sensible option by far would be beacons by the trackside at
stations, i suppose. These needn't be expensive - RFIDs rather than
eurobalises or something. Sensors on the train, and only open a door if
there's a beacon within a metre of it. Easy and cheap.

tom

--
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and infidelity). -- Dad Gone Mad, on rap