US2817012A - Inductive control system for railroads - Google Patents

Description

H. c. KENDALL 2,817,012

5 Sheets-Sheet l F ICJJ;

GAT E GENERATOR INVENTOR.

MASTER CHANNEL 16 SATURABLE PULSE fiTRETCHER- ANPLiFlER PULSE STRETCHER- PUL5E STRETCHER- AMPUHER PULSE TRANSFORMER CONTROLLER AMPLIFIER STRETCHER- AHPLlFiER CZP MWTOOTH 1 E ERNQR DFFERENIIAT AMPLlFlER ,25 CATHODE CATHODE FOLLOWER CATHODE INDUCTIVE CONTROL SYSTEM FOR RAIL-ROADS )TRMKWAY RECEIVING COIL l0 E l H.C.. KENDALL Hi5 ATTORNEY cameo CATHODE FOLLOWER 18 AHPLlTUDE AMPLlFlER 'GATED. AHPUHER" FoLLowER AMPuHER. FOLLOWER GATED AMPuFIER.

Filed Feb. 20. 1952 TRAIN CAR REE CONTROL C0111 OSCILLATOR 5WEEP 1:0 RESONANT UNiT MID DET.

BLOCKING FREQUENCY P-DETECTOR & OSQLLATOR (ACCEPT? H) Z- L-cREs0NAm UNIT AND on.

. mcEPTs f2) [:CRESONANT UNIT AND DET (ACCEPTS fa] Dec. 17, 1957 H. c. KENDALL INDUCTIVE CONTROL SYSTEM FOR RAILROADS Filed Feb. 20. 1952 5 Sheets-Sheet 2 IN VEN TOR.

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INDUC'I'IVE CONTROL SYSTEM FOR RAILROADS Filed Feb. 20, 1952 L 5 Sheets-Sheet 5 TYPICAL WAVEFORMS VOLTAGE v QAWTOOTH GENERATOR OUTPUT FREouENoI n I/ I I B FREQUENCY SWEEP FREQUENCY OSCILLATOR OUTPUT VOLTAGE AFFECTED BY TUNED CONTROL COIL I I I I v D kVOLTAGE k V k OUTPUT OF DIFFER- I 1 I I I ENTIATOR-AMPLIFIER 19 I OUTPUT OF L-C RESONANT UNIT AND DETECTOR FOR CHANNEL 1 F I m GATING VOLTAGE AT I I I I I I VOLTAGE PLATE OF TUBE 106 G OUTPUT 0F CATHODE FOLLOWER 25 FOR CHANNEL 1 Y OUTPUT 0F PULSE I I I TRETQHER AMPLIFIER V0 LTA RELAY DROPPED AWAY 1 F: 7" OPERATION OF RELAY c1 RELAY PIQKE'D UP I FIG-5A. FIG.5B. raw/v area/0 60/L6 r/xz'a Ha/r-uP'c'a/L- INVENTOR H. C. KENDALL H15 ATTORNEY CDCDCDI- L {23353 I I I ENVELOPE 0F SWEEP FREQ" UENCY OfiCILLATOR OUTPUT United States Hugh C. Kendall, Rochester, N. Y., assignor to General Railway Signal Company, Rochester, N. Y.

Application February 20, 1952, Serial No. 272,571

9 Claims. (Cl. 246-124) This invention relates to an improved system for transmitting controls inductively between moving trains and fixed Wayside locations.

This application is to be considered in the nature of an improvement over the control system disclosed in the patent to H. C. Kendall et al. No. 2,693,525, issued November 2, 1954; and no claim is intended to be made herein to any subject matter disclosed in such prior application.

The principles of this invention may be applied, as a particular embodiment thereof, to the transmission of controls from moving vehicles to selected wayside locations as, for example, when it is desired to determine the identity of trains as they pass a particular location. This train identification system may be used in manual block signal systems, for example, where the local operator at each signal location must receive information as to whether or not the block ahead is clear before he can allow trains to enter that block. By placing, at the exit end of such a block, the proper wayside apparatus, and on the rear car or caboose of each train the appropriate vehicle-carried apparatus, all according to the present invention as will be described, it becomes possible for the local operator to be informed as to when each train has fully left the block ahead.

In a train identity system of this kind, it is generally only required to be known that a train has passed a given location; information as to the particular type of train as, for example, its number designation or destination is generally not needed. However, in train describer systems where the route to be set up for a train is dependent upon the kind of train or its destination and also in train annunciator systems where train designations must be determined as well as the fact that the train has passed a certain location, additional means must be provided which controls the wayside apparatus so as to give distinctive outputs as trains of different classes or designations pass by.

The principles of this invention may also be applied, as another embodiment thereof, to a system for transmitting controls from the trackway to passing trains as, for example, in an intermittent train control system. In this disclosure, however, the features of the present invention will be shown and described more particularly in connection with a train identity system.

The train identity system of this invention comprises wayside apparatus which includes an electronic oscillator organized so as to have its output sweep rapidly over a selected frequency range. The oscillator output is applied to a receiving or pick-up coil which is mounted adjacent the trackway in such a manner that it can become inductively coupled to a tuned coil mounted upon the vehicle.

atent C F During the interval that the wayside pick-up coil and the tuned vehicle-carried coil are inductively coupled, the oscillator output sweeps over its entire range a plurality of times. The vehicle-carried control coil is tuned to a frequency Within the sweep frequency range of the oscil- 2,817,012 Patented Dec. 17, 1957 lator so that each time the oscillator frequency sweeps through the resonant frequency of the tuned inductor, an energy transfer takes place from the pick-up coil to the tuned control coil. The wayside apparatus is so organized as to detect the resultant loading effect upon the oscillator, thereby ascertaining that an inductor, tuned to a frequency within the range of frequencies swept over by the oscillator, has passed the wayside location.

Where additional information as to train designation is to be made known as each train passes the wayside location, all the trains with the same designation carry one or more resonated coils each of which is tuned to a particular preselected one of a plurality of resonance frequencies which are included within the range of frequencies swept over by the oscillator included in the wayside apparatus. The oscillator output is thus afiected each time that its frequency sweeps over the particular resonant frequency of the vehicle carried tuned coil with which it is inductively coupled. When the train identity system of this invention is to be used in such a system, the wayside equipment is organized to detect at what particular frequency of the oscillator each reaction occurs and so provides distinctive outputs for each of the differently tuned vehicle-carried inductors so as to properly distinguish between various types of trains as they pass the wayside coil location.

An object of this invention is to provide a train identity system having wayside equipment which is so organized as to be unresponsive to spurious effects which might tend to affect the oscillator in a manner similar to that produced by a resonated control coil.

A further object of this invention is to provide wayside apparatus included in a train identity system having an electron tube oscillator controlled to sweep over a selected frequency range in the same direction for successive sweeps, to thereby permit a finer tuning of certain tuned circuits which are included in the wayside apparatus with a resultant increase in ability to discriminate between the various tuned vehicle-carried inductors that may be used.

Further objects, purposes, and characteristic features of this invention will in part be obvious from the accompanying drawings and in part pointed out as the description of the invention progresses.

In discussing this invention in detail, reference will be made to the accompanying drawings in which like reference characters designate corresponding parts throughout the several views, and in which:

Fig. 1 is a block diagram diagrammatically illustrating the circuit organization of this invention;

Figs. 2A, 2B, and 2C, when placed in order, one above the other, comprise a circuit diagram showing in detail the various parts and circuits of this invention;

Fig. 3 illustrates graphically certain waveforms as an aid in describing the manner of operation of an embodiment of this invention,

Fig. 4 illustrates a modified form of master channel to be used in certain circumstances; and

Figs. 5A and 5B show how a plurality of coils may be provided to give a coded form of control.

To simplify the illustration and facilitate in the explanation, the various parts and circuits constituting the embodiment of the invention are shown diagrammatically and certain conventional illustrations have been employed. The drawings have been made to make it easy to understand the principles and manner of operation rather than to illustrate the specific construction and arrangement of parts that would be used in practice. The various relays and their contacts, for example, are shown in a conventional manner, and symbols are used to indicate connections to the terminals of batteries or other sources of current instead of showing all of the wiring connections to these terminals. The symbols and indicate connections to the opposite terminals of a source of suitably low voltage as is required for the operation of relays and the like. The symbol (B+) and the symbol for a ground connection indicate connections to the opposite terminals of a source of somewhat higher voltage such as is required for the operation of various electron tubes.

Without attempting at this time to discuss in detail the scope of the invention but merely to illustrate the general principles, Fig. 1 shows a receiving coil which is preferably mounted adjacent the trackway when the system is to be used for train identification purposes. The vehicle-carried control coil 11 which is resonated by an associated capacitor 12 assumed to be so mounted upon a vehicle that it will, during motion of the vehicle, become inductively coupled with the wayside coil.

The wayside equipment shown in Fig. 1 includes apparatus such as the blocking oscillator 13, cathode follower 14, sawtooth oscillator 15, and saturable transformer controller 16, all of which cooperate to cause the sweep frequency oscillator 17 to rapidly vary its output fre quency over a selected range. When the output frequency of this oscillator 17 sweeps over the resonant frequency of a control coil 11 that is inductively coupled to the receiving coil 10, there is a reaction on the oscillator 17 which is detected by the amplitude detector and amplifier 18 and so causes an output pulse to be supplied by the differentiator-amplifier 19 to the pulse stretcheramplifier 20 and to the gated amplifier 21 of each channel. The pulse stretcher-amplifier 20 of the master channel responds to each such pulse and, in turn,gove'rns the operation of relay MR. Thus, the moving of a vehicle carrying a resonated control coil 11 past the wayside coil 10 is effective to cause actuation of relay MR of the master channel, regardless of the particular resonant frequency of such control coil.

In addition to the master channel, there are a plurality of individual channels, one for each of the different control coil resonance frequencies that may be used in a given system. Each of these channels comprises resonated circuit elements included in an L-C resonant unit and detector 22, having the output of the sweep frequency oscillator 17 applied thereto as indicated by the connection over wire 23 from the oscillator 17 to each of these L-C resonant units and detectors 22. The L-C resonant unit and detector 22 responds to this input by causing a distinctive voltage to be produced each time that the output frequency of the oscillator sweeps over the frequency associated with that channel. Thus, it may be said that each of the individual channels includes sweep position-determining apparatus which, in effect, demarcates the time interval throughout which the output frequency of the sweep oscillator 17 is passing through the resonant frequency of a corresponding control coil by supplying a distinctive output voltage during such interval.

The gated amplifier 21 of each channel has applied to it over wire 24 the output pulses obtained from the differentiator-amplifier 19, these pulses being indicative of reactions produced on the sweep frequency oscillator 17 by resonated control coils. These output pulses supplied by the ditferentiator-amplifier 19 are all alike, regardless of the resonant frequency of the control coil which has caused them to occur. However, the output pulse produced by the differentiator-amplifier 19 in response to a control coil tuned to a particular one of the plurality of resonance frequencies can, of course, only occur at the time that the output frequency of the oscillator 17 is passing through this particular resonant frequency. The particular channel corresponding to this frequency always demarcates, as described, the interval during which the oscillator frequency is sweeping through the resonant frequency of the associated control coil, and the distinctive voltage that is produced is applied as a gating voltage to the gated amplifier 21 of that channel. The occurrence of a pulse from the'difierentiator-ampliiier 19 during the time existence of such gating voltage is an indication that a control coil 11 resonated to the frequency associated with that channel is then inductively coupled to the receiving coil 10. The gated amplifier 21 is, accordingly, so organized that it produces an output in the event that a pulse is applied to it over wire 24 at the same time that a distinctive gating voltage is applied to this particular gated amplifier.

The output pulses produced by the gated amplifier 21 of any channel are supplied to an associated cathode follower 25 which then supplies a corresponding output to a respective pulse stretcher-amplifier 26. This device, in turn, controls an electromagnetic relay C1, C2, or C3 associated with the channels l-3 inclusive. The actuation of a relay of one of the individual channels along with the actuation of the master relay MR controls associated circuit means to a distinctive condition, thereby providing an indication as to the type or designation of train that has passed the wayside location.

In the embodiment of the invention illustrated in block diagram form in Fig. 1 and also in detailed circuit form in Figs. 2A, 2B, and 2C, three individual channels have been shown in addition to the master channel so as to accommodate three different train identities. More or less channels may, of course, be used as desired.

With respect to the apparatus which is effective to control the'operation of the sweep frequency oscillator 17, Fig. 1 shows that the blocking oscillator 13 supplies its output pulses which occur at a preselected frequency to a cathode follower 14 which then applies these pulses to the sawtooth oscillator 15. It should be understood that various types of frequency sources could well be used in place of a blocking oscillator, and, if desired, the sawtooth oscillator 15, in a manner well-known in the art, may be so organized as to operate at its own natural frequency as determined by the values of its circuit components.

The output of the sawtooth oscillator 15 comprises a voltage waveform which varies in a somewhat linear fashion between a lower and a higher limit with the return to its origin occurring in a very abrupt and rapid manner as is approximately illustrated in Fig. 3 at line A. This voltage waveform is applied to a circuit organization termed a saturable transformer controller 16. This device acts to vary the current through the primary winding of a saturable transformer to thereby cause a variationin the inductance of its secondary winding. The saturable transformer controller 16 cooperates with the sweep frequency oscillator 17 by causing a variation in the inductance of the tuned circuits included in the sweep frequency oscillator, thereby causing the output frequency of this oscillator to vary rapidly over a selected frequency range as at line B of Fig. 3. Thus, the blocking oscillator 13 determines the sweep repetition rate at which the output voltage of the sawtooth oscillator 15 varies between its lower and higher limits and for each such voltage variation in the output of the sawtooth oscillator 15, the inductance of the secondary winding of the saturable transformer is caused to vary and thus vary the oscillator frequency.

The gate generator 27 is also controlled by the output of the sawtooth oscillator 15 and allows the differen'tiatoramplifier 19 to respond only for a limited time, as will presently be more fully described, so as to prevent its responding to transient voltage variations.

The trackway receiving coil 10 is actually included in the circuit organization of the sweep frequency oscillator 17 so that the coupling of a vehicle-carried control coil 11 to the wayside coil 10 causes a loading of the oscillator as the oscillator frequency sweeps through the resonant frequency of the tuned control coil. This loading efiect causes the envelope of the oscillator output to decrease and then to abruptly increase again when the coupling ceases as shown at line C of Fig. 3. The amplitude detector and amplifier 18 rectifies the output of the sweep frequency oscillator 17 and filters out the high frequency variations so that the output of this amplitude detector and amplifier includes substantially only the waveform of the envelope of the oscillator output as shown in Fig. 3 at line C. The ditferentiator-amplifier 19 responds to the sudden increase in amplitude of the input applied to it at the time the loading efiect ceases by supplying an output pulse (see line D, Fig. 3) over wire 24 to the various gated amplifiers 21, each associated with a respective channel and also to the pulse stretcher-amplifier 20 of the master channel.

Since the pulse stretcher-amplifier 20 of the master channel has applied to it the output from the differentiator-amplifier 19, it receives an input pulse each time that the frequency of oscillator 17 sweeps over the resonant frequency of a control coil coupled inductively to the receiving coil. During the time that the receiving coil and control coil are inductively coupled, the oscillator frequency sweeps through the resonant frequency of the vehicle-carried inductor a plurality of times so that a succession of output pulses is provided by the difierentiatoramplifier, one for each such sweep. The rate of occurrence of these outputs is determined by the rate of frequency sweeping and thus is determined, in effect, by the repetition rate of the blocking oscillator 13 output. The pulse-amplifier 20 responds to each pulse received from the difierentiator-amplifier 19 by producing an output waveform which is at a high level for approximately half of the time between successive outputs and at a reduced level for the other half of each interval as shown at line H of Fig. 3. Thus, an essentially rectangular waveform with successive half-cycles of approximately equal length is supplied to the associated full-wave rectifier 28, and the rectified current supplied by the output of this rectifier is utilized to energize the relay MR. Consequently, the picking up of the armature of relay MR is an indication that a control coil 11 has become inductively coupled to the receiving coil 10.

Relay MR associated with the master channel is provided with slow operating characteristics by reason of the capacitor 29 connected across its winding. As a result, a plurality of successive outputs from the differentiatoramplifier 19 must occur in order to allow the rectified output from the full-wave rectifier to persist for a long enough time to allow relay MR to pick up. In this way, random, spurious outputs which might appear in the output of the ditferentiator-amplifier 19 are not effective to cause actuation of relay MR.

The detailed circuit organization of one embodiment of this invention is shown in Figs. 2A-2C. The dotted line blocks in this drawing have been made to correspond in designations with the blocks of the diagram of Fig. 1.

As previously stated, various types of frequency generators may be used to determine the sweep repetition rate of the oscillator 17 even though the specific form shown in Fig. 2A comprises a blocking oscillator. This blocking oscillator 13 includes a tube 40 which has its plate connected through a winding of a pulse transformer T1 and resistor 41 in series to (3+). The control grid is connected through another Winding of the pulse transformer T1 and through resistor 42 and capacitor 43 in parallel to ground. A by-pass capacitor 44 is connected from the junction of resistor 41 and the plate winding of the pulse transformer T1 to ground, and another connection is made from ground, and through a third winding of the pulse transformer T1 to the control grid of tube 45 included in the cathode follower 14. The control grid of tube 45 is connected to ground through resistor 9 which thus effectively shunts this output winding of transformer T1 and thereby dampens transient voltages that tend to appear across this Winding. Thus, a conventional blocking oscillator is provided which supplies positive output pulses to the control grid of the cathode follower tube 45 at a rate which is primarily determined by the time constant of the parallel-connected resistor 42 and 6 capacitor 43. Since a blocking oscillator of this kind is well-known in the art, its operation will not be described in detail.

Each positive input pulse applied to the control grid of the cathode follower tube 45 causes a corresponding positive pulse to appear across the cathode load resistor 46 of this tube, and these positive pulses are applied through a capacitor 47 to the control grid of a triode tube 48 which is included in the sawtooth generator 15. The occurrence of a positive pulse on the grid of tube 48 causes a current to flow between the cathode and control grid of this tube so as to charge the capacitor 47. At the end of this positive pulse, the charged capacitor 47 begins to discharge but can now discharge only through the relatively high resistance of resistor 39 which connects the control grid to (13+). For the selected rate at which the positive input pulses occur, as determined by the blocking oscillator 13, the time constant for the discharge circuit of capacitor 47 is so selected that this capacitor can discharge only slowly between successive input pulses. Consequently, the control grid of tube 48 is maintained at a sufiiciently negative potential between successive input pulses to maintain this tube cut off. Upon each occurrence of a positive input pulse, however, the tube conducts momentarily to charge capacitor 47.

Each time that tube 48 becomes conductive in response to a positive input pulse, the capacitor 49 which is connected from its plate to ground and is normally charged to a relatively high voltage after the tube has been in a cutoff condition for some time, discharges through the plate-cathode circuit of conducting tube 48. This discharge takes place rapidly so that the plate voltage of tube 48 abruptly drops to a relatively low value.

A connection is provided from the plate of tube 48, through capacitor 50, to the control grid of tube 51 included in the gate generator 27. The control grid of tube 51 is normally at about ground potential since any increase in potential of this grid above ground causes a grid current to flow through the resistor 52 connecting this control grid to (B+) which tends to lower the grid voltage. This tube 51 is normally, therefore, in a conductive condition, and the capacitor 50 is normally charged to a relatively high voltage.

As the plate voltage of tube 48 is lowered as a result of its becoming conductive and allowing capacitor 49 to discharge, the capacitor 50 also tends to discharge, but since the discharge can take place only through the relatively high resistance of resistor 52, this capacitor 50 can discharge only slowly with the result that the grid voltage of tube 51 is driven negative with respect to its cathode and is cut ofi.

The cathode follower comprising tube 53 is associated with the tube 48 so as to provide means for increasing the linearity of the sweep output voltage obtained from tube 48. More specifically, by choosing the series-connected cathode load resistors 54 and 55 of this tube to be of a large value, the cathode voltage of tube 53 approximately equals the grid input voltage to this tube which is obtained from the plate of tube 48. Since this cathode voltage is applied through a capacitor 56 to the junction of resistors 57 and 58 which connect the plate of tube 48 to (B+), a substantially constant voltage difference appears across the resistor 57 so that the charging current for capacitor 49 is maintained at an essentially constant value. The voltage across the charging capacitor 49 thus increases at a substantially linear rate with respect to time.

When capacitor 49 begins to charge through resistors 57 and 58 after tube 48 has again become nonconductive, no charging current need at first be supplied for the capacitor 50 since the voltage existing at that instant between the plate of tube 48 and ground is less than the voltage then appearing across the capacitor 50 so that this capacitor 50 continues to discharge for a short time. The reason for this occurrence is that the slow discharge rate of the capacitor 50 prevents it from dischargingfully during the short interval of time that tube 48 conducts in response to thepositive output pulse obtained from the cathode follower 14. Therefore, at the ins'tanttube 48 becomes nonconductive once more, the capacitor 50 continues for a'short time to discharge still more. After capacitor 49 has charged for a short time, however, the voltage at the plate of tube-48 rises to a levelwhere it equals the then existing voltage across the capacitor 50. Any further increase of voltage at the plate of'tube'48 must now cause an increase in the charge on the capacitor 50. The effect of this occurrence is to cause the voltage at the plate of tiibe48 to rise more rapidly at first and then to 'increaseat' a somewhat slower rate during "the remainder of the voltage'rise when charging current must also ;be supplied" for the'capacitor 50 as'well as for the capacitor'49. As a result, 'the voltage that appears at the junction of resistors '54- and-55 in the cathode circuit 'of tube"53 has approximately the waveform shown in Fig. 3 atline A.

The saturable transformer controller 16 includes the pentode type tube 59 which has its cathode connected directly to ground and its control grid connected through a grid leak resistor 60 to ground. 'Theiriput voltage is applied through the coupling c'a'p'a'citor97 "to the control grid. The screen grid is connected to the plate, and the suppressor grid is connected to the cathode. The primary winding of the saturable transformer ST is included in the plate circuit of this tube in series with a resistor 61. The resistor 62 connected across the primary winding of the saturable transformer ST serves to dampen transient voltages which tend to appear in the primary circuit. It has been found that the Q of the secondary winding of transformer ST decreases considerably when there is no current in the primary winding and this would ordinarily tend to make the oscillator 17 momentarily inoperative at the end of each frequency sweep when the current through tube 59 drops to zero. Resistor 38 prevents this from happening by maintaining a certain minimum current through the primary'winding.

The outstanding characteristics of the saturable transformer ST that make possible its use in conjunction with a radio-frequency sweep oscillator organization are its very low secondary inductance and the very low degree of coupling between primary and secondary windings which minimize the shunting effect on the sweep frequency oscillator 17 By applying an input voltage to the control grid of this tube 59 having a waveform substantially as shown in Fig. 3 at line A, the variation in current through the primary winding of the saturable transformer is approximately linear with respect to time. The high inductance appearing in the plate circuit of tube 59 through the effect of the primary winding of transformer ST would ordinarily tend to prevent a rise of plate current at the start of each sweep; however, the more rapid increase of voltage occurring at the start of the upward voltage sweep tends to overcome this effect so that the plate current of tube 59 varies almost linearly. As will presently be more fully described, the secondary winding of this saturable transformer ST is included in the circuit organization of an electron tube sweep oscillator 17. The variation in inductance of the secondary winding serves to vary the frequency of the oscillator output.

It has been found that a substantially linear current variation in the primary winding of the saturable transformer ST tends to produce an inductance variation of 'the secondary winding effective to produce an almost linear thus also a more linear frequency variation with time can be achieved. It should be fully understood,'however, that although the circuit organization 'as' shown in Fig. 2 tends tolproduce an-equenc variation that is substantially linear with time, the principles of this invention do not 'requirethat the frequency sweeping be accomplished at a linear rate as it is only required that the frequency s'weepin'g'thr'ough aselected range be at a relativelyrapid and somewhatunirorm rate.

The voltage applied tothe control grid of tube 59 is shown at line A of Fig. as increasing from-a low value to some higher level and'th'en decreasing abruptly to its original value. This 's'tidden voltage flyback of voltage tends to produce aspariens output from the secondary of the saturable transformer ST and may be effective throughout the rest of the circuit organization 'to provide an output similar to that which occurs when a tuned control coil is brought into coupling range with the receiving coil. Aswill later be'apparent, anoutput from the wayside equipment caused by such-spur-iousyoltages can be prevented "from occurring by removing or sufficiently lowering 'the'pla'te voltage of tube 63 included in the diiTerentiato'r-amplifier 19. For this reason, the output voltage appearing at the plate of 'tube 48 included in the sawtooth oscillator 15 is supplied, as previously described, to the control grid "of the normally conductive tube 51 which is included in the gate generator 27. As a result, upon the occurrenceof the abrupt voltage drop at the plate of tube 48, tube '51 becomes momentarily cutoff as'previously described and the plate voltage of this tube'51 is "thus momentarily increased because of the reduced voltage drop'a'cross resistor 64. This voltage increase'is applied through capacitor 65 to the control grid of'tube'65 and causes this latter tube to become momentarily conductive. The resulting reduction in the plate"voltageo'f "this tube -causes a sufficient reduction in the plate voltage 'oftube 63 included in the differentiator-amplifi'er19tomake this tube inoperative for a time.

The positive plate 'pulse'of"tube 51'also causes the capacitor "*65 to quickly charge, and since the time constant for the discharge of this capacitor through the resistor 98 is chosentobe relatively long with respect to the interval between successive inputs, a negative cutoff voltage 'is'applied to 'th'e control grid of tube 66' between successive pulses. The high plate voltage that then results for tube 66 because "of 'the reduced voltage drop across plateresistor 99 causes "the normally high operating potential to be supplied to the plate of tube 63 for the remainder of each sweep cycle. Only at the beginning of each sweep cycle when "the abrupt voltage reversal occurs does the tube'66 become momentarily conductive and so disable tube 63 by reducing its plate voltage.

The sweep frequency oscillator 17 shown in Fig. 2A is relatedto the Colpitts type'of oscillator circuit, but other types of oscillator circuits may also be'used when desired. In this'oscillator, the grid-cathode circuit includes a parallel inductance-capacitance tank circuit which includes the secondary winding of the saturable transformer ST shown iu'the saturable transformer controller 16. Thissecondary winding is connected in parallel with the fixed inductor '67. The capacitance elements of this tuned circuit'comprise the capacitors 63 and 69 connected in series,'withthe cathode of the tube 70 connected to their junction. The cathode circuit of this tube also includesthe wayside receiving coil '10 so that this wayside coil is actually in parallel with the capacitor 69 and thus forms a .part of the tuncd circuit for the oscillator. The voltage appearing across this tuned circuit is coupled "through the coupling capacitor 71"to the control grid of the tube 70, and this control grid -is connected to ground through the grid leak resistor 72. The plate of the oscillator tube 70 is connected through the plate load resistor 73 to (13+), a'ndthis resistor is shunted for the range of output" frequencies of-the oscillator by the by-pass capacitor 74 which is -conne'cted from plate to ground. -The screen' grid of this tube 70 is connected directly to the plate, and the suppressor grid is connected to the cathode.

The output of the sweep frequency oscillator 17 is obtained from its cathode and applied directly to the control grid of tube 75 included in the cathode follower-amplitude detector 18 which also includes the tube 76. The cathode circuit of tube 75 includes the resistor 77 which is shunted by the capacitor 78. On the positive peaks of the input voltage to the control grid of tube 75, the cathode-plate current reaches its maximum value, and the maximum voltage then appears across the cathode resistor 77. The capacitor 78 tends to charge to a voltage level approximating this maximum voltage that appears across the cathode resistor. The time constant for the capacitor 78 and resistor 77 is so chosen that the capacitor 78 can discharge only slightly between successive positive halfcycles so that a relatively high level of direct voltage is maintained between the cathode of tube 75 and ground. This bias voltage is of a sufficient level to cut the tube off for the negative half cycles of the input voltage with the result that rectification of the input to this tube occurs.

The time constant for capacitor 78 and resistor 77 is properly chosen, however, to permit variations in the cathode voltage of tube 75 when changes occur in the level of the output voltage derived from the sweep frequency oscillator 17. In other words, this time constant is chosen to be sufficiently long so as to prevent a substantial discharge of capacitor 78 during the interval be tween successive cycles of the oscillator output, but is sufliciently short so as to permit discharging of this capacitor in the event, for example, that a decrease in output voltage of the oscillator occurs.

A resistor 79 and the capacitor 80 which are connected in series across the cathode load resistor 77 act as a low pass filter for the ouput of the amplitude tube 75. Thus, the capacitor 80 acts as a low impedance for the relatively high frequency output of the oscillator, but the slower variations in amplitude of the oscillator waveform are not shunted by this capacitor so that they are applied through a coupling capacitor 81 to the control grid of the amplifier tube 76. This tube is provided with a grid leak resistor 120 and plate load resistor 121 and operates as a conventional voltage amplifier.

The principal effect of the loading of the sweep frequency oscillator 17 that occurs when the receiving coil becomes inductively coupled with the tuned control coil 11 is a substantial reduction in the amplitude of the oscillator output. This drop in voltage may be compared to the decrease of output voltage of a generator that occurs when a heavy load is placed upon it. Thus, the presence of a resonated coil in the presence of the receiving coil which is included in the oscillator circuit causes a substantial transfer of energy to take place from the oscillator to the tuned coil with the result that a decrease of oscillator output voltage appearing at the cathode of tube 70 takes place.

Another effect of the loading action that has been observed is a tendency for the tuned control coil to momentarily lock the oscillator frequency to its own resonant frequency even though the oscillator frequency tends to continue its frequency sweeping action. In other words, the effects of the oscillator loading are so pronounced as the oscillator frequency passes through the resonant frequency of the control coil as to tend to retard for a very brief instant the normal variation in frequency of the oscillator that would otherwise occur.

As the inductance of the secondary winding of the saturable transformer ST is varied still further by the output of the sawtooth oscillator 15, the circuit constants for the sweep frequency oscillator 17 are eventually so altered that it can no longer oscillate at a different frequency than the frequency at which it would normally oscillate despite the effect of the resonated vehicle-carried coil. At such time, the oscillator output effectively becomes unlocked from the effects of the resonated coil, and at such time there is a rather abrupt jump in the level of the oscillator output; Observations show that this voltage variation is more abrupt than the decrease of oscillator output occurring as the resonated coil and wayside coil first become coupled together. The effect on the envelope of the oscillator output is approximately as illustrated in Fig. 3 at line C and this is the voltage variation that is applied to the control grid of tube 25' included in the amplitude detector and amplifier 18.

The dilferentiator-amplifier 19 is organized to detect this sudden shift in level of the oscillator output. Tube 76 included in the amplitude detector and amplifier 18 amplifies and inverts the waveform of the output of tube 75, and the output voltage of this tube 76 is applied to the control grid of tube 63 through capacitor 82 and resistor 83. The inversion of the waveform produced by tube 76 causes the abrupt increase of the oscillator output as the loading effect on the oscillator is removed to appear as a sharp decrease of voltage on the grid of tube 63.

The control grid of triode tube 63 is connected to (B+) through the current limiting series-connected resistors 83 and 84. The voltage at the control grid of this tube is, therefore, normally at approximately the same level as the cathode. Thus, the increase of grid voltage occuring when the oscillator first is loaded by the tuned vehicle-carried coil can only produce a further increase in the grid current of tube 63 so that the plate current of this tube remains substantially unchanged.

The time constant for the discharge of capacitor 82 is chosen to be relatively short so that the input circuit for tube 63 is effective to produce differentiation of the input waveform. Thus, when the plate voltage of tube 76 is abruptly decreased each time that the coupling effect on the oscillator is removed, a negative voltage pulse appears on the grid of tube 63 and causes this tube to be momentarily cut off. Since the junction of resistors 83 and 84 is maintained at a certain voltage level above that of the control grid, the pulse from the plate of tube 76 must lower the voltage at the junction of these resistors must be lowered by more than this amount before there is any decrease in the grid voltage of tube 76. This provision tends to prevent the occurrence of an output from tube 76 in response to spurious voltages of a lower level.

The positive trigger pulse that appears on the plate of tube 76 is applied through a resistor 85 to the control grid of a cathode follower tube 86. The cathode of this tube 86 is connected through resistor 122 to (B+). Consequently, even when the gate generator 27 causes a normal plate voltage to be applied to tube 63 so that a higher voltage results on the grid of tube 86, the positive cathode voltage of tube 86 cuts this tube off so that only a substantial positive pulse at the plate of tube 63 can make tube 86 conduct. A corresponding positive pulse then appears across the cathode resistor 87 for this tube 86 and is applied over wire 24 to the gated amplifier 21 provided for each of the various channels and also to the pulse stretcher-amplifier 20 of the master channel.

Where a single tuned coil is provided on each vehicle, the outputs obtained from the cathode follower tube 84 include only one pulse for each sweep of the oscillator frequency range. Since the rate of this frequency sweeping is determined by the blocking oscillator 13, the rate of occurrence of successive outputs from the cathode follower tube 86 is known.

The pulse stretcher-amplifier 20 of the master channel includes the triode tube 88 which has applied to its control grid the positive output pulses obtained from the cathode follower tube 86. Each positive pulse applied through capacitor 89 to the control grid causes the tube to become momentarily conductive and at the same time causes the capacitor 89 to become charged. At the end of each such positive pulse, the capacitor 89 is PIC;- vented from discharging rapidly because it can then discharge only through the high resistance provided by the resistor 90 connected from control grid to (B+).

The tube 88 is cut otf,therefore, following each positive pulse applied to its grid for a time which'is determined by the discharge time of the capacitor '89. The time constant for the discharge of this capacitor is properly chosen so as to hold the tube 88 cut off for an interval which is approximately equal to one-half the time between successive positive input pulses. The tube 88, therefore, is alternately cut off and then conductive for approximately equal time intervals as long as "successive input pulses are applied to 'it from the cathode follower tube 86.

The resulting pulses "at the plate of tube 88 are then applied through the capacitor 91 to the control grid of amplifier tube 93. The controlgrid of this tube -is connected through a grid leak resistor 94 to ground, and a cathode bias is provided 'for it by the -resistor95 which is shunted by the by-pass capacitor 96. The primary winding of a transformer T2 is included in the plate circuit, and the secondary winding of this transformer supplies an output to the full-wave rectifier 28 which has a relay MR connected across its output terminals. The amplified alternating input applied to tube 93 and appearing in its plate circuit is thus rectified so as to provide a direct current for the energization of relay MR.

This relay MR and the corresponding relays provided for the various channels are all provided with slow operating characteristics which may be provided by-a shunting capacitor as previously'described. Other means may also be supplied as desired to require a repetition of outputs from each channel before the relay for that channel is actuated. Thus, it might be required that a certain number of output pulses as determined by a counting means be supplied to each relay before the relay could be picked up.

Each of the various channels such as channel 1 shown in Fig. 2B includes an L-C resonant unit and detector 22 comprising a parallel tuned circuit resonated to a corresponding one of the plurality of control coil resonance frequencies. The frequency band for each of these tuned circuits is chosen so as to encompass one of the resonance frequencies of a vehicle-carried control coil. The tuned circuit for the resonant unit and detector 22 of channel 1 is included in the grid-cathode circuit of tube 100 and comprises the inductor 101'and capacitor 102 connected in parallel. The grid input voltage for the tube 100 is obtained from the grid circuit of the sweep frequency oscillator tube 70 over the wire 23. This input voltage is applied through a decoupling resistor 103 to the control grid of tube 100.

Each time that theoutput of thesweep frequency oscillator 17 passes over the frequencyfor which the inductor 101 and capacitor 102 exhibit resonance characteristics, the voltage on the grid of tube 100 increases to a maximum value as the frequency approaches the exact resonance frequency and'then decreases as the frequency sweep continues. The-plate-cathode current of this tube 100 varies-in'accordance with'this variation in grid voltage.

The circuit organization, including the cathode resistor 104 and the shunting capacitor 105 associated with tube 100 operates in a manner somewhat similar to that of the amplitude detector and amplifier 18 in that rectification of the input voltage to the tube 100 occurs. This output voltage does not include any abrupt variations as does the envelope of the sweep frequency oscillator output when this oscillator is inductively loaded by a tuned control coil. The reason for this is that the grid of the oscillator is sufficiently overdriven so that the loading that occurs does not greatly affect the alternating voltage on the grid although there is a substantial decrease of the alternating cathode voltage applied to tube 75. It is, therefore, possible to employ a capacitor 105 in the cathode circuit of tube 100 having suflicient capacitance to provide a filteringation for the oscillator frequency s'othatfihebutputvoltage obtained from the cathode of this tube represents approximately the envelope of the input grid voltage waveform. Additional filtering means such as is provided for the output of tube included in the amplitude detector and amplifier by resistor 79 and-capacitor 80 is not required.

The output voltage of tube is applied to the cathode of tube 106 included in the gated amplifier 21. T he'control grid of this tube is positively biased by having'its grid connected to the junction of resistors 107 and 108 which are included between (3+) and ground. Accordingly, when the amplitude of voltage obtained from the cathode of tube 100 rises above a predetermined level, the tube 106 becomes cut oif. Variation of the positive grid voltage for the tube 106 by changing the ratio of values of the voltage dividing resistors 107 and 108 aifects the amplitude of voltage that must be applied to its cathode in order to cut the tube off. Since the amplitude of the applied cathode voltage varies in accordance with the proximity of the sweep frequency voltage to the exact resonance frequency of the tuned circuit in the grid circuit of tube 100, the grid bias on tube 106 can be adjusted so as to cut off this tube for a selected range of frequencies centering about the frequency F1.

For example, if the resonant frequency to which a vehicle-carried coil may be tuned is 280 kc., then the inductor 101 and capacitor 102 are tuned to this frequency also. As the output of the swee frequency oscillator 17 approaches 280 kc., and then decreasing again as the frequency recedes from this value. The cathode output voltage of this tube varies in a similar manner. Since the selection of bias voltage for tube 106 controls the level of cathode voltage that must be applied to cut this tube off, this also determines the range of output frequencies of the sweep frequency oscillator 17 for which the tube is cut oif.

Since tube 106 is normally conducting because of its positive grid voltage, the voltage at the plate of this tube is normally at a low value, so that the voltage applied to the control grid of the cathode follower tube 110 is low. Since the cathode of tube 110 is positively biased by being connected through resistor 92 to (B+), the low grid voltage results in a grid-cathode voltage for this tube that is below cutoff.

Each time that the output of the sweep frequency oscillator 17 sweeps through the frequency F1, however, tube 106 becomes cut off as has been described. The voltage increase that then tends to appear between plate and cathode of this tube 106 causes tube 109 to conduct a substantial plate current because of the normally positive bias for this tube resulting from the connection of its control grid to the junction of resistors 118 and 119 connected between ground and (13+). The flow of plate current of tube 109 through the resistors 113 and 114 produces a substantial voltage drop so that a low voltage still appears on the wire 111 which is connected to the grid of cathode follower tube 110. However, if a vehicle-carried coil tuned to F1 is at'that time coupled to the wayside receiving coil 10, the loading of the swee frequency oscillator 17 that-occurs causes a positive output pulse to appear on the wire 24 in a manner that has already been described in detail. This positive pulse is applied through the capacitor 112 to the cathode of tube 109 and raises the cathode voltage appearing across resistor 122 to such an extent that the tube is momentarily cut 01f. The drop in current that then momentarily results through the resistors 113 and 114 causes a positive pulse to be applied to the control grid of tube 110. In other words, if at the time that the oscillator output is sweeping over the range of frequencies adjacent F1, an output is simultaneously received from the difierentiator-amplifier 19 resulting from aloading action upon the oscillator, this is an indication that the reaction which has been detected-is caused by a coil tuned to F1, and therefore a positive pulse is applied to "the cathode follower tube1 10 bf-channel 1.

During the interval that the control and receiving coils are inductively coupled together, a distinctive output pulse is obtained from the cathode follower 25 across the oathode resistor 115 for each sweep of the oscillator frequency through the frequency F1. Since the rate of frequency sweeping is dependent upon the relatively fixed output frequency of the blocking oscillator 13, the consecutive output pulses from the cathode follower 25 occur at a predetermined frequency. The pulse stretcher-amplifier 26 for channel 1 and for the other channels also is organized in a manner similar to that of the pulse stretcheramplifier 20 of the master channel previously described. Thus, in response to each output pulse from the cathode follower 25, the tube 116 is cut off for approximately onehalf of the interval between consecutive pulses and is then conductive for the rest of this interval so that a rectangularly shaped waveform having positive and negative portions of approximately equal duration is applied to the control grid of tube 117. The resulting variations in plate current are applied, through the action of the transformer T3, to a full-wave rectifier 30 and are effective to cause the energization of the relay C1. This relay C1 is also provided with slow action characteristics so that a plurality of outputs from the gated amplifiers 21 must occur before actuation of this relay occurs so as to ensure against its being operated through random spurious outputs.

Channels 2 and 3 of Figs. 2B and 20, respectively, represent additional channels which may be provided as desired. Each of these channels is organized in a manner similar to that of channel 1. Each receives a common input over wire 24 from the differentiator-amplifier 19. In other words, each channel receives an output pulse each time that the oscillator frequency sweeps over the resonant frequency of a control coil which is inductively coupled to the receiving coil, regardless of the resonant frequency of such coil. At the same time, each channel receives an input from the sweep frequency oscillator 17. The tuned circuit elements which are included in the L-C resonant unit and detector 22 provided for each channel provide a distinctive output only as the oscillator frequency sweeps over the range of frequencies adjacent the particular resonant frequency for that channel. Thus, the contemporaneous occurrence of an output from the L-C resonant unit and detector for a particular channel with the occurrence of a ositive pulse from the diiferentiatoramplifier 19 is an indication that there is then coupled with the wayside receiving coil, a vehicle-carried control coil which is resonant to the frequency associated with that channel.

The diiferent resonance frequencies of the various control coils must be properly separated so that any control coil will be effective only with respect to its associated channel and not with any of the other channels. The number of different control coil frequencies that may be used in a system is, therefore, afiected by the width of the frequency band that the sweep frequency oscillator 17 can be made to operate over. In one embodiment of the invention, the sweep frequency oscillator was organized to operate over a range of frequencies from 150 kc., to 300 kc. with four different control coil resonance frequencies included in this frequency range. Also, in this embodiment of the invention a sweep rate of 300 cycles per second was selected, thereby ensuring that the sweep frequency oscillator would sweep over its frequency range several times during the interval that control and receiving coils were inductively coupled together for even the highest train speeds contemplated. These specific values are mentioned here only for purposes of illustration; other values can equally well be used in practice.

As previously mentioned, the inductive control systemthe vehicle, while the resonated control coils are then positioned at selected locations along the trackway.

When the inductive control system of this invention is to be used for train description purposes, the number of different train descriptions involved may exceed the possi' ble number of different control coil resonance frequencies. In that event, it may 'be desirable to mount upon each vehicle a plurality of control coils, each resonant to a different one of selected resonance frequencies in either the manner shown in Fig. 5A or 5B. In this manner, the combination of different resonance frequencies determines the train description, and with a specified number of different resonance frequencies available, the number of different train descriptions is appreciably in-- creased by this method.

When more than one control coil is thus to be used at a single location, it may be desirable to modify the receiving equipment by adding the apparatus shown in Fig. 4 within the dotted line block titled pulse stretcher driver 13!). The reason for doing so under these circumstances is that a plurality of outputs is produced by the differentiator-amplifier 19 for each sweep of the oscillator frequency when more than one control coil is used at a location in the manner shown in Fig. 5B, one output being supplied for each control coil. It will readily be understood from the description of the pulse stretcheramplifier 20 previously given that its operation is dependent upon the reception of inputs from the differentiator-amplifier 19 at a rate of one for each frequency sweep, with this rate determined by the blocking oscillator 13. Consequently, if more than one input is applied to the pulse stretcher-amplifier for each frequency sweep a symmetrical, rectangular waveform is not produced.

The pulse stretcher driver shown in Fig. 4 has applied to it over wire 24, the output obtained across the cathode resistor 87 of tube 86 included in the differentiator-amplifier 19 which is shown in detail in Fig. 2A. The output of the pulse stretcher driver 130 is applied through capacitor 89 to the control grid of tube 93 included in the pulse stretcher-amplifier 20. The pulse stretcher driver 130 includes a one-shot multivibrator comprising the triode tubes 131 and 132, a differentiating amplifier including the tube 133, and

.cathode follower tube 134.

The circuit organization of a one-shot multivibrator is well-known in the art so that a detailed description of its manner of operation will not be presented. Briefly, however, tube 132 is in a normally conductive condition because its control grid is connected through resistor 135 to (B+). A relatively low voltage thus appears at the plate of this tube 132 because of the high voltage drop across plate resistor 136. Because of the presence of the biasing battery 137 in the grid circuit of tube 131 and also because of the low plate voltage of tube 132, the grid voltage for tube 131 is sufliciently low to cause this tube to be nonconductive. A high plate voltage therefore results for the tube 131 because there is then no voltage drop across the plate resistor 138. Capacitor 139 connected between the plate of tube 131 and the control grid of tube 132 is thus charged to a high potential.

As described, when more than one control coil is used at a single location, a plurality of outputs is obtained from the difrerentiator-amplifier 19 for each frequency sweep of the oscillator 17. Each of these outputs from the differentiator-amplifier 19 appears as a positive-going pulse on the wire 24 and is applied through the capacitor 140 and decoupling resistor 141 to the control grid of tube 131. The first-occurring of these positive pulses raises the grid-cathode voltage of tube 131 above the cutoif level so that this tube begins to conduct plate cur- 15 voltage of tube 132 then tends to overcome the negative voltage provided by battery 137 so that the grid voltage of tube 131 is held above cutoff.

At the same time, capacitor 139 begins to discharge through the conducting tube 131 with its rate of discharge determined primarily by its value of capacitance and the resistance of resistor 135. As capacitor 139 continues to discharge, the grid voltage of tube 132 rises until finally the grid-cathode voltage for this tube reaches cutoff. At that time, the tube 132 becomes conductive once more and the resulting low plate voltage of this tube then causes tube 131 to again become nonconducfive.

The time constant for the discharge of capacitor 139 is selected so as to keep tube 131 conductive for a time interval that is slightly less than the sweep repetition rate of the oscillator 17. Thus, the multivibrator can respond to the first-occurring output pulse from the differentiator-amplifier 19 but then cannot respond again until the next frequency sweep of the oscillator 17 results in an output produced by the diiferentiator-amplifier 19 in response to the same control coil that produced the response in the previous cycle.

As an example, if control coils resonant to the frequencies F1 and F2 are employed at a single location, a separate output will be produced by the diiferentiatoramplifier 19 for each of these control coils on each frequency sweep of the oscillator 17. If the first output from the diiferentiator-amplifier 19 results from inductive coupling of the receiving coil with the control coil tuned to frequency F2, then the multivi'brator will respond to such output by causing tube 131 to become conductive and tube 132 nonconductive. Before the complete cycle time of the sweep oscillator has elapsed, an output from the ditferentiator-amplifier 19 will be produced in response to the control coil tuned to frequency F1. At the time of occurrence of this output, however, tube 131 will still be conductive so that this pulse will have substantially no effect. The time constant for the discharge of capacitor 139 is so selected, however, that tube 131 will again become nonconductive just prior to the second occurrence of an output from the dilferentiatoramplifier 19 in response to the control coil tuned to frequency F2. As a result, tube 131 becomes conductive only once for each frequency sweep of the oscilla tor 17, regardless'of how many control coils are used at a location.

Each time that tube 131 becomes conductive, the normally charged capacitor 142 which connects the plate 'of tube 131 to the control grid of tube 133 is rapidly discharged. The time constant for the discharge of capacitor 142 through resistor 143 is chosen to produce differentiation of the plate voltage of tube 131 so that a sharp negative pulse appears on the control grid of tube 133 each time that tube 131 becomes conductive. Whenever tube 131 becomes nonconductive, the increase of voltage that ends to appear on the control grid of tube 133 merely causes grid current to flow so that the grid voltage does not rise appreciably.

Each negative pulse at the grid of tube 133 produces a corresponding negative pulse at theplate'of'this'tube because of the momentarily reduced plate current through plate resistor 134. The resultingpositive pulse appearing at the grid of cathode follower tube 144 produces a'similnr positive pulse across-the cathode resistor 145 so that a positive resistor is applied to the grid of tube 93 included in the pulse stretcher-amplifier 24).

By including the pulse stretcher-driver 130 in the receiving apparatus, it is-thus possible to supply to the pulse stretcher-amplifier 2t) an input which comprises only a single pulse for-each sweep of the sweep oscillator 17. -The pulse strctehenamplifier '20 can thus respond, as al- 1 Having described aninductive control system as one embodiment of this invention, it should be understood that this form is selected to facilitate in the disclosure of the invention rather than to limit the number of forms which it may assume. Also, various modifications, adaptations, and alterations may be applied to the specific form shown to meet the requirements of practice without in any manner departing from the spirit or scope of the present invention.

'WhatI claim is:

l. A system for transmitting controls at selected locations inductively between a moving vehicle and fixed wayside locations along the trackway through inductive cooporation between control coils at a control transmitting location and a receiving coil associated with receiving apparatus, each of said control coils being resonated by respectively associated capacitance to a selected frequency, said receiving coil passing through an inductive coupling relationship with each of said control coils during train movement, said receiving apparatus including an electron tube oscillator for energizing said receiving coil, tuning apparatus including a sawtooth generator for continuously-varying the output frequency of said oscillator over a range of frequencies including said selected resonant frequencies of said control coils, said tuning apparatus and said electron tube oscillator cooperating to vary the frequency of said oscillator at a repeti- 'tion rateto cause said oscillator frequency to sweep over said range of frequencies a-plurality-of times during the interval said control and receiving coils are inductively coupledat maximum train-speeds, circuitmeans governed by said oscillatorand effective to provide a distinctive output in responseto the loading effect produced on said oscillator when the output frequency of said oscillator sweeps over the resonant frequency of a control coil coupled inductively to said receiving coil, a controlled device, and means associated with said controlled device to permit actuation of said controlled device only in response to a plurality of successive occurrences of said distinctive outputs to prevent'spurious outputs.

2. A system for inductively transmitting controls between moving trains and the trackway at fixed locations comprising,controlcoils at a control transmitting location each being resonated to a selected frequency, a sweep frequency oscillator including a receiving coil at a control receiving location, said sweep oscillator energizing said receiving coil with an output frequency continuously sweeping between selected limits at a fixed repetition rate, said receiving coil becoming inductively coupled With each control coil at said control transmitting location during train movement, reaction detecting means governed by said oscillator and effective to provide a distinctive output in response to the loading of said sweep frequency oscillator by said control coil, a controlled device, circuit means responsive only to successive occurrences of said distinctive outputs for supplying a steady current to said controlled device, means associated with said controlled device and effective to cause actuation of said controlled device inresponse to said steady current only provided that said steady current persists for a selected time interval.

3. A system for transmitting controls at selected locations between a moving vehicle and the trackway through inductive cooperation between control coils at a control transmitting location and areceiving coil associated with receiving apparatus, each of said control coils being resonated to aselected frequency by respectively associated capacitance, an electron tube oscillator for energizing said receiving coil, electronic tuning apparatus for continuously varying the output frequency of said oscillator over a range of frequencies which includes said selected resonant frequency of said control coil, reaction detecting circuit means associated with said oscillator and effective to produce a distinctive output in response to the termination of the loading effect produced on said oscillator when the output frequency of said oscillator sweeps over the resonant frequency of a control coil inductively coupled to said receiving coil, said tuning means eifective to vary the frequency of said oscillator over said range in the same direction on successive frequency sweeps with an abrupt return between successive sweeps-to the starting frequency, resonant circuit means included in said receiving apparatus tuned to the resonant frequency of said control coil and having the output of said oscillator applied thereto, a controlled device, means responsive to the contempt raneous occurrence of said distinctive output and the increase of voltage occurring across said resonant circuit to actuate said controlled device, the unidirectional frequency sweep of said oscillator causing successive of said distinctive outputs to occur at substantially the same frequency of said oscillator, said resonant circuit means being tuned to resonate over a relatively narrow frequency band encompassing said same frequency of said oscillator, to thereby render ineffective spurious outputs from said reaction detecting circuit means occurring at other frequencies of said oscillator.

4. A system for transmitting controls at selected locations between a moving vehicle and the trackway through inductive cooperation between control coils at a control transmitting location and a receiving coil associated with receiving apparatus, said control coils at each location resonated by respectively associated capacitance to different selected frequencies, an electron tube oscillator for energizing said receiving coil, tuning means for continuously rapidly varying the output frequency of said oscillator over a selected frequency range including said selected resonant frequencies of said control coils, said tuning means being effective to vary said oscillator frequency at a repetition rate effective to cause said oscillator frequency to vary over said frequency range a plurality of times during the interval said control coils and said receiving coils are inductively coupled even at maximum train speeds, reaction detecting circuit means associated with said oscillator and effective to produce a distinctive output in response to each loading of said oscillator occurring as the output frequency of said oscillator sweeps over the resonant frequency of a control coil inductively coupled to said receiving coil, a master channel comprising a controlled device and circuit means for causing actuation of said controlled device only in response to a preselected plurality of occurrences of said distinctive outputs, an individual frequency channel associated with each of said control coil resonant frequencies, sweep position determining circuit means included in said individual channel effective to produce a distinctive gating voltage as the output frequency of said oscillator sweeps over said resonant frequency corresponding to said individual channel, gated circuit means included in said individual channel responsive to the contemporaneous occurrence of said distinctive gating voltage and said distinctive output from said reaction detecting circuit means and effective to produce an output voltage pulse, a controlled device included in said individual channel, circuit means governed by said voltage pulses and effective to cause actuation of said controlled device only in response to a preselected plurality of occurrences of said voltage pulses, circuit means responsive to the contemporaneous actuation of said controlled device associated with said master channel and said controlled device of said individual channel to designate a particular control.

5. A system for inductively transmitting controls between moving trains and the trackway at fixed locations comprising, control coils at a control transmitting location each being resonated to a selected frequency by respectively associated capacitance, a sweep frequency oscillator associated with receiving apparatus and including a receiving coil, said sweep oscillator energizing said receiving coil with an output frequency range continuously sweeping between selected limits at a fixed repetition rate, said receiving coil becoming inductively coupled with each control coil during train movement, reaction detecting circuit means effective to produce a distinctive output in response to the loading effect on said oscillator occurring when said oscillator frequency sweeps over said frequency of each control coil during the time said control coil is inductively coupled to said receiving coil, tuned circuit means resonated to said resonant frequency of said control coil, circuit means for applying the output of said sweep frequency oscillator to said tuned circuit means, two electron discharge tubes having a common plate load resistor, circuit means responsive to the envelope of oscillator voltage appearing across said tuned circuit means for controlling one of said tubes to a nonconductive condition, circuit means responsive to the occurrence of said distinctive output from said reaction detection means for controlling the other of said tubes to a nonconductive condition, a controlled device, circuit means responsive to the high voltage appearing at the plates of said tubes when said tubes are simultaneously controlled to nonconductive conditions to actuate said con trolled device.

6. A system for inductively transmitting controls between moving trains and the trackway at fixed locations comprising, control coils at a control transmitting location each resonated by respectively associated capacitance to a selected frequency, a sweep frequency oscillator associated with receiving apparatus, a receiving coil included in the circuit organization of said sweep frequency oscillator, said sweep frequency continuously varying between selected upper and lower frequency limits at a fixed repetition rate, said receiving coil becoming inductively coupled with each control coil during train movement, reaction detecting means associated with said oscillator and effective to provide a distinctive positive voltage pulse in response to each inductive loading of said oscillator by a resonated control coil, circuit means responsive to the repetitive output pulses from said reaction detecting means and comprising a grid controlled electron discharge tube, said grid being connected to a positive voltage source through a grid resistor, circuit means for applying said positive pulses to said grid through a coupling capacitor, said capacitor having a discharge time through said grid resistor effective to maintain said tube nonconductivc following the occurrence of each of said distinctive pulses for an interval equal substantially to one-half the sweep repetition rate of said oscillator, an electromagnetic relay, rectifying circuit means responsive to the plate current of said tube for energizing said relay, a capacitor shunting the winding of said relay, whereby a plurality of distinctive outputs must be produced by said reaction detector circuit means to cause the energization of said relay to persist for a long enough time interval to actuate said relay.

7. In a system for inductively transmitting controls at selected locations between a moving vehicle and the trackway through inductive cooperation between control coils at a control transmitting location and a receiving coil associated with receiving apparatus, each of said control coils being resonated to a selected frequency by an associated capacitance, said receiving equipment including an electron discharge tube, a saturable transformer having primary and secondary windings, a plate-cathode circuit for said tube including said primary winding, a gridcathode circuit for said tube, circuit means for applying to said grid-cathode circuits a voltage varying over a selected range at a predetermined repetition rate, said voltage initially varying rapidly for a short time interval and then varying more slowly in the same direction for the remainder of said interval, the level of said voltage at the end of said voltage variation returning abruptly to a fixed starting level, an electron tube oscillator, a tank circuit for said oscillator including said receiving coil and said secondary winding, whereby said voltage variation causes said oscillator frequency to vary substantially linearly over a selected frequency range including said resonance frequencies of said control coils.

8. A system for transmitting controls at selected 10- cations between moving vehicles and the trackway through the inductive cooperation between control coils at a control transmitting location and a receiving coil associated with receiving apparatus, a plurality of control coils at each control transmitting location, each of said control coils at a location being resonated by an associated capacitance to a different frequency, a sweep frequency oscillator for energizing said receiving coil, said oscillator output frequency continuously sweeping over a frequency range including said resonance frequencies of said control coils at a predetermined repetition rate, reaction detecting means effective to produce distinctive voltages in response to the loading effect upon said oscillator produced as said output frequency of said oscillator sweeps over said resonance frequencies of said control coils, a master channel for said receiving apparatus including first circuit means responsive to said distinctive voltages, said circuit means being rendered ineffective in response to an occurrence of one of said. distinctive outputs to respond to successive occurrences of said distinctive outputs for a time interval just less than thetirne required for said oscillator to sweep overits .entire frequency range, a controlled device, second circuit means governed by the output of said first circuit means for energizing said controlled device, means associated with said controlled device to permit said controlled device to be actuated only in response to a plurality of successive occurrencies of said distinctive outputs.

9. A system for transmitting controls at selected locations between a moving vehicle and the trackway through the inductive cooperation between control coils at a control transmitting location and a receiving coil associated with receiving apparatus, a plurality of control coils each being resonated to a different frequency by an associated capacitance at each of said control transmitting locations, a sweep frequency oscillator for energizing said receiving coil, said oscillator output frequency continuously sweeping over a frequency range including said resonance frequencies of said control coils at a predetermined repetition rate, reaction detecting means effective to produce distinctive voltages in response to the loading effect upon said oscillator produced as said output frequency sweeps over said resonance frequencies of each of said control coils, a master channel for said receiving apparatus including a one-shot multivibrator, circuit means for controlling said multivibrator from its normal condition in response to said distinctive voltages, said multivibrator controlled to return to its normal condition after a time interval just less than the time interval of a complete frequency sweep by said sweep frequency oscillator, circuit means governed by said multivibrator to give a distinctive output when said multivibrator is controlled from its normal condition, a controlled device, means responsive to said distinctive outputs for governing said controlled device, means associated with said controlled device to permit said controlled device to be actuated only in response to a plurality of successive occurrences of said distinctive outputs.

References Cited in the file of this patent UNITED STATES PATENTS 2,150,857 Edwards Mar. 14, 1939 2,291,715 Hepp Aug. 4, 1942 2,407,270 Harrison Sept. 10, 1946 2,407,644 Benioif Sept. 17, 1946 2,416,320 Jeanne Feb. 25, 1947 2,419,340 Easton Apr. 22, 1947 2,454,687 Baughman Nov. 23, 1948 2,488,815 Hailes Nov. 22, 1949 2,509,632 Field May 30, 1950 2,510,066 Busignies June 6, 1950 2,512,305 Clapp June 20, 1950 2,535,162 Rodgers Dec. 26, 1950 2,562,295 Chance July 31, 1951 2,596,167 Philpott May 13, 1952 2,597,517 Noble May 20, 1952 2,673,292 Treharne Mar. 23, 1954 2,693,525 Kendall et al Nov. 2, 1954

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