The Model Railway
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Velocity MetersThe velocity meters on the model railway are the only possibility for a control software to get a feedback of the model railway's entire state. A velocity meter is built of two redundant reed-contacts. In order to trigger these reed-contacts, each train is equipped with two magnets. One is located at the front of the engine, the other one at the rear of the last car.
If at least one of the two contacts of a velocity meter is triggered by a passing train,
this event is reported to the electronical interface, where it is stored in a special
register until it is being read. This filtering of readings comes up with the following consequences:
The two reed-contacts of a velocity meters were originally designed to measure the speed of a passing train: One contact starts a counter in the electronic interface, the other one stops the counter. The faster a train passes the velocity meter, the smaller is the counter result. This result was stored in the register mentioned above. Since the actions of starting and stopping the counter are fixed to each contact, measuring the speed is limited to those trains running in a particular direction. If a train runs in the opposite direction, the counter is started but not stopped which results in an overflow. This overflow is detected and may be interpreted as train which has passed the velocity meter in the "wrong" direction. The measuring of a train's velocity was intended to be used for readjusting it, for example on descending lines. But it turns out to be impossible to get even a rough approach of what's really going on based on these readings.
Clemens Grelck generated on: Mon May 31 10:19:25 MET DST 1999 |