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This information is detected by a sensor, such as an infrared device placed between the tracks, a reed switch or a device which senses current draw in an isolated section of track. Feedback relays an electrical signal from the sensor hardware back to the digital central unit.
When the signal is at 'clear' or green ("off"), the electromagnet is energised. As the train passes, the permanent magnet sets the system. A short time later, as the train moves forward, the electromagnet resets the system. Once so reset, a bell is sounded (a chime on newer stock) and the indicator is set to all black if it is not already so.
Digital Command Control (DCC) is a standard for a system for the digital operation of model railways that permits locomotives on the same electrical section of track to be independently controlled. The DCC protocol is defined by the Digital Command Control Working group of the US National Model Railroad Association (NMRA).
An axle counter sensor comprises two independent sensors (so the device can detect the direction and speed of a train by the order and time in which the sensors are passed). As the train passes a similar axle counter sensor at the end of the section, the system compares count at the end of the section with that recorded at the beginning.
The design of the saturable inductor current sensor is similar to that of a closed-loop Hall-effect current sensor; the only difference is that this method uses the saturable inductor instead of the Hall-effect sensor in the air gap. Saturable inductor current sensor is based on the detection of an inductance change. The saturable inductor is ...
Like the manual block systems outlined above, automatic systems divide the route into fixed blocks. At the end of each block, a set of signals is installed, along with a track-side sensor. When a train passes the sensor, the signals are triggered to display the "block occupied" aspect on the signals at either end of that block.
Wheel sensors along the tracks feel for flat spots on the train's wheels. Any flat wheel that becomes too dangerous to travel on (a big flat spot on the train wheel) will be counted as a defect. Typically, these systems utilize accelerometers, strain gauges, fiber optic methods, or the very latest wheel impact phase detector (WIPD).
When no train is present, the relay is energised by the current flowing from the power source through the rails. When a train is present, its axles short the rails together. The current to the track relay coil drops, and it is de-energised. Circuits through the relay contacts therefore report whether or not the track is occupied.