| 摘要 |
<p>A railroad train moving along an approach to a grade crossing (or other location of interest) is detected by apparatus that is responsive to the change in effective track inductance caused by the moving shunt. The tank circuit of an oscillator is connected to the track at the crossing and has an incrementally variable capacitor bank capable of compensating for the decrease in inductance that occurs as the train closes on the crossing, as well as slow inductance variations resulting from changing environmental conditions. The frequency of the oscillator output signal is continuously monitored, and correction pulses are produced when the signal frequency deviates from a preselected normal frequency. A digital control arrangement responds to each correction pulse and varies the compensating capacitance in increments to return the signal to the normal frequency. Motion detection circuitry also receives the correction pulses and activates the crossing warning system when the spacing between successive pulses corresponds to a predetermined repetition rate indicating that a train is approaching. Accordingly, under normal operating conditions, the oscillator frequency is held constant within a narrow range of deviation even when a train is approaching, but the rate of tank capacitance increase required to compensate for the continuous and rapid change in track inductance when a train is approaching is detected. The system also recognizes instantaneous changes such as would be caused by an artificial shunt or a broken rail, and distinguishes these from a moving train.</p> |
