US2135530A - Track circuit for railroads - Google Patents

Description

Nov. s, 1938.

W. H. REICHARD TRACK CIRCUIT FOR RAILROADS 7 Filed May 26, 1934 IINVE TO w. A

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"44m fl ATTORNEY Patented Nov. 8, 1938 PATENT OFFICE TRACK CIRCUIT FOR RAILROADS" Wade H. Rei'chard, Rochester, N. Y., assignor to General Railway Signal Company, Rochester,

Application May 26,. 1934,. Serial No.. 727,691.

11' Claims.-

This invention relates in general. to track. circuits such asused inrailway signaling practice and has more particular referencetomeans. for.

automatically. discriminating between current. changes in :the, track circuit due. tothe. occupied condition' of. the track anddue. to. leakage. currents resulting frompoor ballast conditions.

In railway operation it is quite. essential that the. usual track relay connectedacross the track 1 sectionat. oneend of: a. signaling block be sufliciently energized when the block is unoccupied. to. pick upand holdup with certainty. It-is likewise essential that the. occupancy. of the track section will so effectively shunt. or condition. the relay that it will bepositivelyreleased. For ex-' ample, in the usual track circuitsif the normal holding currenttis. too-high, shunting may not be.- effective todrop the. relay and if the normal hold--- ing current is so lowthat shunting will be more positive it is. possible that at timestheholding current may. be insumcient.

Taking theabove intoiconsideration it will beseen that the usual variation -in ballast resistance encounteredin practice, .dueprimarily tochanges in weather conditions, may operate to interfere with the proper operation of the track relay unless some means is provided for discriminating between changes due to ballast resistance and track occupancy.

Asthe ballast resistance. increases, the leakage current between the rails. decreases so that the voltage across the terminals of the. track relay connected to the track circuit increases; Con-- versely, as therballast resistance decreases, ballast leakage increasesand the terminal voltageat the relay decreases.

The relay current will vary over a considerable range and cover a range at times which will prevent proper response of the relay under occupied v or unoccupied conditions of'the track section.

It is proposed in accordance with this invention to-provide means, associated. with. the track relay for discriminating or detecting a change due to track occupancy and. unoccupancy' and.

changes due to leakage currents caused by variation in ballast resistance between thelrails ofv the: track. In accordance. with: the present invention it is proposed to improve theshunting efiiciency' of'the trackcircuit and for this purpose. a; discriminating arrangement 'is included-in thecircuit of the track relay which discriminates be-' tween a slow change intrack. shunt due to track ballast and'a rapid change in. track shuntdueto train occupancy.

Further objects, purposes and. characteristic featuresof. theinvention will appear as the description progresses, reference. being made to the accompanying. drawing showing by way of example-.oneform. which the invention may assume.

The. ingle figure diagrammatically illustrates 5 one form of the present invention applied to a singlestretch of track.

Apparatus-Referring tothe drawing, a section of railway track including rails l is shown separated from; the adjacent track sections by 10 insulated jointsZ. Atrackbattery IB and a limiting resistance R. are shown connected to the trackrails atthe right-hand end of the illustrated track section.

As. typical-of-the control exercised by the track relay, asemaphore signal SG is illustrated at the left handend-of the. track section. A track relay T, havingtwocoils is illustrated, the control of which will be. explained in detail. A Pliotron or amplifier: tube R0 of any well known type is used, and as illustrated, is adaptedto control, by means of. the voltage generated in the secondary of transformer TR, the current for operating selecting relay. IS.

Associated withPliotron RC is theusual plate source of current which may be, a battery 2B andthe usual filament.- source: of current which may be abattery A; Bias. resistor BR is adjustably connected to thegrid. circuit of the amplifier in order toprovide a normal biasto the in-put of theetube-for. a purpose that will .be later. described. Slow actingselecting relays2S and 38 are provided and these relays in cooperation with relay IS-insertresistances IR, 2Rand BB. in the track circuit under certain conditions-in a manner and for a purpose that .will be explained in detail.

Normal conditions.-With the circuit in its normalicondition as illustrated in the figure, track relay T is normally picked up over a circuit extendingfromthe terminal of battery lB, resistance R, lower track. rail; lower winding and upper winding of relay T in series, primary winding of transformer TR,.front contacts ll, l2 and i3 of relays. IS, 28 and 3S respectively and upper track rail to the terminal ofbattery IB. The constants.of.the described energizing circuit for relay Tareof such'a value that suflicient current flowsunder the worst ballast conditions (maximumleakage betweenv track rails) to maintain the contacts oi'relay T' picked up. The best ballast conditions. (minimum leakage between track rails) allows more. current toflow over the above described circuit including relay T which of course willmaint'ain. its-contactsin their picked up. positions.

Any change in current flow over the above described circuit takes place through the primary winding of transformer TR but such a change due to a change in ballast conditions takes: place quite slowly, so that there is no appreciable effect on the secondary winding of transformer TR due to such a change.

Adjustable bias resistor BB is so adjusted that the grid of amplifier RC is normally sufficiently positive with respect to the filament that surficient current flows from the terminal of battery 2B, winding of relay IS, plate filament circuit of the amplifier and resistor BR to the terminal of battery 23, for maintaining relay IS in its picked up condition. Since slow changes in the track circuit current flow have no app eciable effect on the grid circuit of amplifier RC, this bias is maintained substantially constant so that relay IS is not affected by such changes.

Relay 2S is normally energized over a circuit extending from front contact I4 of relay IS and winding of relay 28, to Relay 3S is normally energized over a circuit extending from front contact I of relay 2S and winding of relay 38, to 'With relays IS, 2S and 3S picked up, resistances IR, 2R and 3R are short circuited at front contacts II, I2 and I3 so that they are not included in the circuit of the track relay.

Track occupancy.When the illustratedtrack section is occupied the shunt across thetrack rails, due to the wheelsand axles of the train, causes a sudden decrease in the current flowing through the windings of relay '1'. This sudden decrease inthe current flow through the circuit including the primary winding of transformer TR, which is in series with the track relay windings, sets up a potential across the secondary Winding of transformer TR in such a direction that the terminal connected to the grid of amplifier RC' becomes more negative with respect to the filament to such an extent that the current flow through the plate circuit of the amplifier is reduced to a value which causes relay IS to drop its contacts. Even though this potential is only momentary, relay IS has time to drop its contact-s and after the grid circuit returns to its normal state, with normal current flowing through the plate circuit of the vacuum tube, this current is insufiicient to pick up relay IS after it has dropped its contacts. This is due to the fact that the pick up current'for the relay is considerably higher than its holding current.

The dropping of front contact II removes; the shunt from resistor IR so that this resistor is inserted in the circuit. of relay T. This further reduces the currentcfiow through the windings, of

relay T so that'it has less holding power than when the track rails were shunted by the train. Relay T may or may not release under thiscondition. If it does not release within a predetermined time, relay 2S will drop its contacts because front contact I4 of relay lS deenergizes the winding of relay 2S. Relay 2S inserts resistance 2R in the circuitincluding relay T, by dropping its front contact I2. Relay '1' may or may not release under this condition. If it does not release after 'a predetermined interval of time, crelay SS is dropped because its energizing circuit is open at front contact I5 of relay 2S; .Relay3S then inserts resistance 3R in the track relay circuit by dropping its front contact I3.

. It will be understood that additional selecting relays may be provided if required, or if 1 ,condi tions are such that only one resistorIR need be track relay circuits.

lay Ta stick circuit is provided topreve'nt other selector relays being dropped, which circuit extends from back contact I 5 of relay T, front contact I! and winding of relay 2S, to The stick circuit is also efiective to stick relay 3S over a'circuit including its front contact I8 and its winding, to

It will thus be observed that relays IS, 28 and 3S function to sequentially insert resistance in the circuit of the track relay until it releases, after which those relays (if any) which were not released will be stuck up through a back contact of the track relay.i Since the stick circuits of the 2S and 3S relays are opened at their own front contacts, after the track relay is dropped I the selector relays are not againpicked up until the track becomes unoccupied as will be described.

Contact 2| of relay T and contact 3I of relay IS in series may be used to control the signal circuits of signal SG or for any other purpose required by the usual practice in connection with Front contact 22 of relay SS is not shown connected to any circuit but this contact is included to indicate that another selecting relay can be provided when desired and connected to front contact 22 of relay 3S in a similar manner to. which relay 3S is connected to front contact I5 of relay 2S. I

Track 'unoccupcmcu-When' the train leaves the track section, the quick removal of the shunt across the'track railscauses a sudden increase in current flow through the track circuit including the primary winding of transformer TR andthe windings of the track relay in series, This sudden increase in current flow sets up a potential across the secondary winding of the transformer in an opposite sense to thatpreviously explained so that the grid of amplifier RC 7 7 positive with respect tothe filament. This'positive potential on the grid'permits sufii'cient current to flow through the plate filament, circuit for picking up relay IS. Relay IS removes resistor IR from the track circuit and closes a circuit at its front contact I4 for picking up relay.

2S. Relay 2S removes resistor 2R from the track circuit and closes a circuit at its front contact I5 for picking up relay is. Relay 3Sremoves resistor 3R from the track circuit. It will be understood that in the event that'relay 3S or 28 were not dropped under the previous condition, then' when the track becomes unoccupied relay IS will pick up to remove resistor IR which under this conditionis the only one included. 7 With relays IS, 2S and 3S picked up, the circuits are in condition for maximum current flow becomes more' through the windings of relay T which causes this relay to be picked up; The circuits are now in normal condition.

It will be clear from the above description that this invention provides means whereby there is a' a positive discrimination between the track circuit conditions due to track occupancy and due to ballast leakage. s r

The above rather specific description of one form of the present invention has been given solely by way of illustration and is not intended 1. "In 'aitrack circuit, an insulated section or track, 'a source of current connected to said track, a track relay "connected to said track and energized in slowly varying degrees in accordance with the slowly changing current values due to variations in ballast resistance, the entrance of a train into said section of track producing a'rapid reduction in the current in the circuit of said relay, and means responsive only to said rapid reduction of current for sequentially-inserting resistance units in the circuit of said relay until the resistance of the circuit including said relay has increased to a value where said relay releases.

2. In a track circuit, in combination with an insulated stretch of track, a source of current connected across one end of said track, a track relay connected across the other end of said track, an energizing circuit for said track relay including the track rails, a thermionic device, a transformer having its primary winding connected in series with said track relay and its secondary winding connected to the in-put circuit of said thermionic device, a control relay connected in the out-put circuit of said thermionic device, the resistance voltage drop across a portion of said energizing circuit supplying the in-put circuit of said thermionic device with a normal bias which maintains said control relay picked up irrespective of changes in the ballast condition of said stretch of track, and the voltage induced in the secondary winding of said transformer due to the entrance of a train into said stretch of track being sufficient to overcome said bias to cause said control relay to release, and means responsive to the release of said control relay for effecting a control of said track relay.

3. In a track circuit, in combination with an insulated stretch of track, a source of current connected across one end of said track, a track relay connected across the other end of said track, an energizing circuit for said track relay including the track rails, a thermionic device, a transformer having its primary winding connected in series with said relay and its secondary winding connected to the in-put circuit of said thermionic device, a control relay connected in the out-put circuit of said thermionic device, the resistance voltage drop across a portion of said energizing circuit supplying the in-put circuit of said thermionic device with a normal bias which maintains said control relay picked up irrespective of changes in the ballast condition of said stretch of track, the voltage induced in the secondary winding of said transformer due to the entrance of 'a train into said stretch of track sufficiently changing said bias to cause said control relay to release, and means controlled by said control relay for gradually inserting resist ance in said track circuit until and only until said track relay is released.

4. In a track circuit, in combination with an insulated stretch of track, a source of current connected across one end of said track, a track relay connected across the other end of said track, an energizing circuit for said track relay including the track rails, a thermionic device, a transformer having its primary winding connected in series with said relay and its secondary winding connected to the in-put circuit of said thermionic device, a control relay connected to the out-put circuit of said thermionic device, the resistance voltage drop across a portion of amass-o said -energizing circuit supplying the 'in-put circuit-of said thermionic device with alnormal bias which maintains said control relay picked up irrespective of changes in =the ballast condition of said stretch of track, the voltage induced in the secondary winding of said-transformer due to the entrance of a train into said stretch of track sufliciently' changing said bias to cause said control relay *to release, means controlled by said 'control relay for gradually inserting resistance in said track circuit until and only until said track relayis released, and means controlled by the exit of the -tr'ain*'fromsaidstretch of track for sufliciently changing said bias to cause said control relay to pick up.

5. A track circuit comprising an insulated track section; a track relay; a resistance normally excluded from the circuit of said track relay; discriminating means including a thermionic relay; means responsive to the occupancy of said section for operating said discriminating means; means responsive to the operation of said discriminating means for including said resistance in the circuit of said track relay; and means controlled by said track relay for controlling the amount of resistance included in its circuit.

6. In a track circuit, an insulated section of track, a source of current and a track relay connected to said track, a plurality of resistance units, slow-acting means for slowly and sequentially inserting said resistance units in series with said track relay until it releases, means in cluding an impulse actuated device for starting the operation of said slow-acting means, and means controlled by the release of said track relay for stopping the operation of said slow-acting means.

7. In combination, a section of railway track, a source of current connected across the rails of said section, a relay receiving energy from the rails of said section and responsive to the steady value of current with which it is supplied, a transformer also receiving energy from the rails of said section, and means energized from said transformer and responsive to a change in the energization of the transformer occurring upon the entrance of a train into said section for decreasing the energization of said relay whereby said relay will release.

8. In combination, a section of railway track, a source of current connected across the rails of said section, a relay receiving energy from the rails of said section and responsive to the steady value of current with which it is supplied, a transformer having a winding included in the energizing circuit for said relay, and means energized from a winding of said transformer and responsive to a change in the energization of the transformer occurring upon the entrance of a train into said section for decreasing the energization of said relay whereby said relay will release.

9. In a track circuit for railroads, the combination with a section of railway track having a track relay and a source of uni-directional current connected thereto, an auxiliary relay separate from said track relay and effective when operated to decrease the current through said track relay and thereby cause release of its armature, and means including a transformer for causing operation of said auxiliary relay in response to the relatively quick change in the current in said track circuit caused by the entrance of a train into said section.

said section, a first relay receiving energy from the rails of said section and responsive to the steady value of current with which it is supplied, V I

a second relay, means for controlling said second relay in accordance with the-rate of change of current fiow'in said first relay when the value,

of current through said first relay is quickly decreased due to the entrance of a train into said section, and means controlled by said second relay for decreasing the energization of said 10 first relay. 7 s

WAD E BEI CI-IARDL

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