Abstract: An island circuit detects the presence of a train on a crossing island, and, in response, de-energizes a normally energized relay to activate a warning. A random signature signal is generated and transmitted through the rails from a transmitter to a receiver connected to the rails on either side of the island. If a train is present within the island, the wheels and axles form an electrical shunt, preventing the receiver from detecting the transmitted signal. The circuit employs solid state micro-electronic components and ancillary components, and embedded software for controlling and monitoring the circuit functions. The random signature signal is characterized by a relatively high voltage level but a low duty cycle on the order of ten percent, and has three variables: (1) frequency; (2) duration time of the pulse, or burst, of the signal; and (3) the delay between successive bursts. Each of the three variables randomly assumes any one of eight different discrete values.

Abstract: A fail-safe voltage-limiting circuit for an audio frequency-controlled railway track circuit includes a pair of NPN transistors arranged in the manner of a darlington pair, wherein the base connector of the first transistor receives an amplitude-modulated signal from a filter and detector circuit connected to the rails. A first pair of resistors arranged in series between the positive and negative system battery lines, includes a first fail-safe resistors also connected to the collector of the first transistor. The ratio of the first and second resistors are selected to limit the positive excursion of the input signal, thus clipping excess noise at this limit. A second pair of resistors is coupled to the second transistor to limit the negative excursion of the amplitude-modulated input signal. The second pair of resistors includes a second fail-safe resistor connected to the emitter of the second transistor and a third resistor connected in series to the second fail-safe resistor at this junction as well.

Abstract: 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 impedance caused by the moving shunt. A constant current AC signal is applied to the track adjacent the crossing, and the resulting voltage changed in amplitude and phase is picked off by a receiver on the opposite side of the crossing. Voltage and current information derived from these signals is inputted to a microcomputer which calculates the track impedance and determines whether or not a warning device or devices at the crossing should be activated. Such determination is made on a repetitive basis in accordance with a repeating program loop and stored in memory.

Abstract: A railroad train detector and communication system relies on the track rails for communicating information among a plurality of locations. The equipment located at each of the locations eliminates relay based current detection, relay based current transmission, and relay based logic functions, and rather these functions are implemented with microprocessors. An extent of signal trackway is defined between a pair of head ends, intermediate the head ends at intermediate insulated joint locations, a microprocessor implemented transmitter/receiver pair is provided. One half of the transmitter/receiver pair manages communication flow for each direction of information flow at the joint. At each of the head locations, only a single transmitter/receiver is required. Associated with each microprocessor in a transmitter/receiver is auxiliary apparatus to couple the microprocessor's input/output ports to the track rails themselves.

Abstract: A receiver for sensing track currents includes a toroid coupled to the track rails via a single conductor. The toroid has an air gap in which a linear Hall sensor is inserted. The voltage of the Hall sensor is applied to signal processing circuitry including a low pass filter, amplifier and level detectors. In one embodiment of the invention comprising a track occupancy detector, second and third windings coupled to the toroid are selectively energized from a potential source to provide MMF's in the toroid of one and another polarity, and of a level less than the MMF provided by an occupied track section. The amplifier includes first and second channels for amplifying signals of opposite polarities. Each channel of the amplifier is coupled to a pair of level detectors, one of the level detectors in each pair is a vital level detector with the threshold set at about 50% of the threshold of the other level detector in the pair.

Abstract: The two inputs of a first flip-flop (FF) are alternately energized over a pair of opposite contacts of a code transmitter. Outputs of a second FF alternately enable a pair of logic gates to pass first or second output pulses of the first FF as clock pulses which drive a counter which produces an output pulse after each preselected count X. Each counter output pulse, equal in length to a half cycle of the code, triggers the second FF to its opposite state. A magnetic stick code repeater relay is driven between its two positions by energy supplied from a normally active driver circuit over another pair of opposite contacts of the code transmitter. The driver circuit is optically coupled to be turned off during each counter output pulse, thus inhibiting repeater relay operation during the corresponding half code cycle. This relay holds in its existing position, blanking a half cycle code period, and then is held by the again active driver circuit during the subsequent half cycle.

Abstract: A railroad highway crossing warning system including pickup coils for sensing current flowing in the track rails of an approach zone, a circuit for sensing the voltage across the track rails, band-pass filter circuits for filtering the voltage and current derived from the track rails, impedance calculator circuits for calculating the track impedance, phase detector circuits for detecting the phase angle between the voltage and current, a multiplier circuit for linearizing the track impedance by multiplying the track impedance by a function of the phase angle, a data sampler circuit for sampling the linearized track impedance at given periodic intervals, a motion detector and crossing predictor circuit for detecting the motion of a train and for predicting the time of arrival at the crossing by comparing the predicted time of arrival with an advanced warning time, and a logic gate circuit for activating the warning apparatus at the crossing when the predicted time of arrival is less than the advanced warning t

Abstract: A highway crossing warning system for monitoring the motion and predicting the time of arrival of an approaching train at the highway crossing and for detecting the presence of a broken rail in the approach zone by feeding dual frequency signals into the track rails and measuring the track impedances at the two frequencies and the phase angle of the lower of the two frequencies.