The Model Railway
Components
Block Sections
Points
Signals
Relays
Velocity Meters

Velocity Meters

The 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.
Every renewed triggering of one of these two contacts that occurs within the next two seconds is being ignored. This proves to be necessary since it is very likely that both reed-contacts will open when the magnet of a passing train reaches a certain point right between the two contacts. While proceeding the magnet eventually leaves this point an the reed-contacts are being closed again. So by neglecting the contact for some time, it is intended to ignore "multi-triggerings" caused by one passing magnet. Perfoming some test runs revealed that two seconds seem to be a sufficiently large interval.
Besides form this, every reading from a contact on an unpowered block section is being suppressed by the interface.

This filtering of readings comes up with the following consequences:

• If a short train is travelling at high speed, it is possible that the engine and the last car cross a velocity meter within two seconds.
• If a train is moving very slow, it is possible that one of the mounted magnets needs more than two seconds to completely cross the velocity meter.
• If the engine of a train generates a reading for the velocity meter at the beginning of a block, an control program should, for safety-reasons, turn off the power on the preceding block. Through this, it is no longer possible to detect the last car passing the velocity meter at the end of the preceding block.

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.