Abstract:
An audio frequency (AF) track circuit whereby a railway track tuned loop transmitter operating in a dual resonant mode, for handling, two signal frequencies, can be coupled to the rails. Each frequency is conducted through a tuning unit having series and parallel branches tuned to each frequency. The track circuit using the tuned loop transmitter eliminates DC return current imbalance effects while maintaining suitably high track circuit shunting sensitivity.

Description:
DESCRIPTION 
     The invention relates to railway signalling systems, and particularly to track circuits having plural frequency coupling circuits whereby different audio frequencies can be coupled to a track section, as for cab signalling and train detection purposes. 
     In track circuits, the presence or absence of a train along a designated track section under yard control is detected by means of an electrical signal transmitted onto the rails and sensed by a remote receiver. A train entering the section places a short across the rails, prevents transmission of the signal to the receiver, and causes the yard controls to operate. Simultaneously, during the presence of the train on the track a second controlling signal operating at an alternate frequency is coupled to the cab of the train via the tracks. Several circuits are needed, each for allowing transmission of a signal of different frequency in each track section. A number of track coupling techniques which have been proposed are mentioned in U.S. Pat. No. 4,373,691; issued Feb. 15, 1983. In U.S. Pat. No. 3,897,921 there is shown interlocking track circuits with audio frequency train detection and cab signalling capability. An advanced and secure method of train detection involving multiple loops and operating at different frequencies is shown in U.S. Pat. No. 4,053,128, issued Oct. 11, 1977 to K. Frielinghaus. The loops and impedance bonds described in U.S. Pat. No. 3,897,921 are untuned and therefore do not allow shunting senstivities (the highest value of impedance of the shunt across the rails provided by the wheel axle which will be detected) of greater than 0.1 ohms. On the other hand, impedance bond type audio frequency track circuits can achieve shunting sensitivities of 0.25 ohms or greater, but they cannot operate in areas where the DC imbalance currents exceed more than a few 100 amperes. In special track work areas, such as interlockings, the DC imbalance currents may exceed 1000 amperes. 
     It is the object of this invention to provide improved track circuits having multi-frequency signalling capability and at lower cost than with such multiple frequency loop systems as have heretofore been available. 
     It is another object of the invention to provide improved AF track circuits which can transmit both train detection, and cab signal frequencies through a track loop which can achieve high shunting sensitivity and which is capacitively coupled to the rails so as to obtain immunity to DC currents. 
     It is a further object of the invention to provide improved track circuits which obtain improved track circuit shunting sensitivity and also obtain immunity to high levels of DC imbalance currents. 
     These objects are accomplished in accordance with the invention by providing a combination frequency coupling for introducing multiple signals of different frequencies and enabling the flow of signal currents of different frequency along a defined section of railway track. A defined section includes the tuned loop transmitter with its shorting bar, the rail sections and an impedance bond or tuned loop used to couple the signal to the receiver. The signals can be from a track transmitter and a cab signal transmitter operating at different frequencies. The coupling provided by the invention is more economical and readily implementable than prior multi-frequency couplings, while providing adequate equivalent source impedance to allow for suitable track circuit shunting sensitivity. 
     Briefly, the invention provides loop coupling means whereby a coupling loop can be tuned to two different frequencies for the transmission of alternating current (preferably audio frequency) signals for train detection and transmission to a cab receiver for train control. The coupling loop is placed between the rails and adjacent to a shorting bar. This shorting bar is a heavy conductor connected between the rails to provide a return path for the electrical propulsion system and to provide increased coupling for the transmitter loop. 
     The loop has a split inductance arrangement and is connected to other inductive and capacitance circuit elements which are connected to be resonant at two different frequencies. The coupling loop uses no magnetic materials, thus making the coupler impervious to the DC traction return currents. This is particularly advantageous for special trackwork areas such as sections employing restraining tracks or interlockings which can cause large DC imbalance currents. The tuning elements may be economical, commercially available capacitive and inductive components. The coupling loop is constructed of standard multi-conductor cable and is placed in close proximity to the shorting bar. The bar can be tapped and used for current equalization. The bonds which can complete the track circuit section are standard tuned impedance bonds, and to prevent the flow of DC imbalance currents through them, a capacitor is placed in series with the bond primary and the rail connection. This capacitor presents a very low AC impedance while blocking all DC current flow in the bond primary. The signal receive function could also be accomplished with a tuned track receiver loop and a shorting bar, instead of a standard tuned impedance bond. 
    
    
     The foregoing and other objects, features and advantages of the invention will become more apparent from a reading of the following description in connection with the accompanying drawings in which: 
     FIG. 1 is a block diagram schematically showing a track circuit interlocking similar to that shown in U.S. Pat. No. 3,897,921 having the invention embodied therein; 
     FIG. 2 is a circuit diagram of a typical one of the tuning units shown in FIG. 1 showing the equivalent circuit of the inductively coupled loop, rails and shorting bar; and 
     FIG. 3 is a block diagram of another track circuit having loop coupling means in accordance with the invention. 
    
    
     Referring to FIG. 1 there is shown a track circuit having an interlocking in sections defined between shorting bars (SB) and central bonds (CB). The track circuits are similar to those shown in U.S. Pat. No. 3,897,921 and reference numerals like those used in that patent in FIGS. 2 and 3 thereof are used in FIG. 1 hereof. 
     In accordance with this invention, the loops 40, 43, 44, and 47 are tuned via the tuning unit signal sources TUSS 100-103. The center bonds 48 and 49 are connected across the rails by capacitors C5. A track circuit is provided which is immune to DC imbalance currents, can transmit both train detection and cab signal frequencies in a single loop, and can achieve shunting sensitivities comparable to normal impedance bond type audio frequency track circuits. The performance of the interlocking track circuits of U.S. Pat. No. 3,897,921 is therefore enhanced. 
     As shown in FIGS. 1 and 2, the tuning units 100-103 containing elements T1, L1, C1, C2 and C3 is connected to the coupling loops 40, 43, 44, and 47 having inductance LT. These coupling loops have multiple turns (e.g., 90 turns) a tap is made to the loop (e.g. between turn 30 &amp; 90). This circuitry has two frequency resonant points. The series circuit formed by capacitor C1 and the loop inductance LT has a resonant point at frequency f(1), where f(1)=1/2π√LT C1. At f(1) this series conduction path represents a low impedance to the transmitted signal. The parallel circuit formed by L1 and capacitor C2 is also chosen to be resonant at f(1), where f(1)=1/2π√L1 C2. 
     Since at resonance this parallel circuit has a high impedance, this conduction path to the transmitted signal may be represented as an open circuit. Therefore at f(1) the transmitted signal is coupled to the rails via the capacitor C1 and inductance LT series tuned circuit. The equivalent source impedance of the tuned loop reflects an impedance of about 0.25 ohms into the rails, which provides a shorting sensitivity of approximately 0.25 ohms. 
     At a second frequency, f(2), the circuit elements C1, LT, L1 and C2 present an inductive reactance as seen in series with C2 across the secondary of the transformer T, and can be reduced to some equivalent inductance LEQ. he LRs and LSB are the equivalent inductances of the shorting bars B and rails. the capacitor C3 is then chosen to form a second resonant circuit with LEQ where f(2) =1/2π√LEQ C3. Thus, a second frequency f(2)) can be coupled to the rails using the same coupling loop. f(2) is greater than f(1). 
     Each duel frequency tuned loop has the same circuit. However, different f(1) and/or f(2) frequencies may be created by selection of the component values L1, C1, C2 and C3. Typically, different frequencies for train detection will be used in adjacent sections while the frequency used for cab signalling will be the same in all sections. The bond 48 and 49 shown intermediate the rail joints 17 and 18 and 19 and 20 in FIG. 1 is connected in series with a capacitor C5. This bond serves to couple the signal in the rails to the receiver. The capacitor C5 is included to block the flow of imbalance DC return currents through the primary of the bond. Such return currents can arise in interlocking track sections which may include other rails, as from sidings 21 and 22. 
     FIG. 3 shows a track without insulating joints (such as the joints 17--FIG. 1). A shorting bar 32, the rails and dual frequency bonds 33 and 34 define track circuit sections 35 and 36. These sections separately carry train detection signals of frequency f(1) and f(3). They also carry vehicle or cab signals of frequency f(2) when they are transmitted. Receivers 39 and 91 coupled to the bond 33 detect the f(1) signals and f(7) signal which are transmitted in the westerly section adjacent to section 35. Receivers 92 and 93, coupled to the easterly bond 34, detect the f(3) signals and f(5) signals in a section easterly of the section 36. Tuning unit signal sources (TUSS) 37 and 38 having the transmitters couple the signals of different frequencies f(1) or f(3) and of the frequency f(2) via loops 94 and 95 on opposite sides of the shorting bar 32. Because of the dual resonances in the tuned loop couplings at f(1), f(3) and f(2), in the TUSS 37 and 38 both f(1) or f(3) and f(2) frequencies may be transmitted in each section. The receivers in each section are sensitive to only one frequency (f(1) or f(3) for example) while the vehicle is sensitive to the common frequency f(2) for cab signalling. 
     The coupling bonds 48 and 49 (FIG. 1) and the dual frequency bonds 33 and 34 (FIG. 3) may be bonds which are commercially available from the General Railway Signal Co. of Rochester, New York under the name &#34;Wee-Z Bond&#34;. Such bonds are described in U.S. Pat. No. 4,074,879. As an alternative receive function means, two single tuned loops inductively coupled to a shorting bar, could be used in place of coupling bonds 48 and 49 (FIG. 1) and dual frequency bands 33 and 34 (FIG. 3). Such alternate receive function means should be understood as included within the term &#34;dual frequency bond&#34;. 
     While a perferred embodiment of the loop coupling circuit and presently preferred embodiments of track circuit systems which embody the invention have been described, it will be appreciated that variations and modifications thereof within the scope of the invention will undoubtedly suggest themselves to those skilled in the railway signalling art. Accordingly, the foregoing description should be taken as illustrative and not in a limiting sense.