A railway signal system for detecting a train approaching a railway crossing or track section transmits in the track a periodically interrupted carrier wave signal the amplitude of which is attenuated by an approaching train effecting a variable shunt across the track. A receiver converts the received signal to a DC level with an impressed AC pulse for application through a DC blocking differentiating capacitor to an amplifier, which produces an AC output used to effect pick up of a signal relay unless an approaching train is detected. The system is self-checking and fail-safe using an astable multivibrator to control power supplied to the relay driver, and the system provides for increased sensitivity with increased train proximity. Additional circuits including a broken rail detector power monitor, island amplifier, loss of shunt detector, and disabling circuit for the latter provide added capability for the system.
An apparatus for detecting an approaching train within a track section utilizes a transmitter coupled to the rails at a feed point for applying a current to the track section. There is a receiver coupled to the track section for producing a signal representative of the impedance of the track section. There are devices for utilizing the received signal for detecting train motion, abnormally low impedance and abnormally high impedance of the track section. A normalizer circuit is connected to the receiver for increasing or decreasing the received signal gain within predetermined limits in accordance with variation in impedance of the track section.
This invention relates to a system and method of detecting a broken rail in a railway system. The track sensing circuitry of the present invention applies a voltage source at each end of a block of rails and senses the current flowing through the circuitry. The present invention will detect broken rails continuously in a block, even with a train present (except for a break directly beneath the train). Since the rail is continuously checked, the only restriction imposed on train spacing by this track circuit configuration is that only one train can be present in a block at a time.
A train approach detector uses frequencies in the audio spectrum to activate grade crossing protection devices in advance of and during the presence of railroad trains at grade crossings. The detector is completely solid state, and includes one transmitter and one receiver each connected to the tracks so that transmitted and received track signals, which are partially pulse modulated for optimum filtering and signal coupling and minimum third harmonic radiation, appear on opposite sides of the crossing. The section of track between the transmitter and receiver is designated as a crossing island and sections of rail outside of the island are known as approaches. In addition to its basic train movement detection function, the detector contains additional features that are designed to provide operation under a variety of application conditions, including automatic ballast compensation, loss of shunt, low signal/low gain condition, high signal/broken rail condition, delayed pick-up, delayed drop-out and reverse switch override, and bi-directional motion detection.
