US3307168A

US3307168A - Signalling system

Info

Publication number: US3307168A
Application number: US238407A
Authority: US
Inventors: John F Zaleski
Original assignee: General Precision Inc
Current assignee: General Precision Inc
Priority date: 1962-11-19
Filing date: 1962-11-19
Publication date: 1967-02-28
Anticipated expiration: 1984-02-28
Also published as: DE1230693B; DE1230693C2

Description

Feb. 28, 1967 ,J zALE$K| 3,397,168

SIGNALLING SYSTEM Filed Nov. 19, 1962 2 Sheets-Sheet a v Ja/// F X44616? lNVENTO ATTORNEY United States Iatcnt (Mike 3,307,168 SIGNALLING SYSTEM John F. Zaleski, Pleasantviile, N.Y., assignor to General Precision, Inc., Binghamton, N.Y., a corporation of Delaware Filed Nov. 19, 1962, Ser. No. 238,407 16 Claims. (Cl. 340-258) This invention relates to signalling systems, and more particularly to an apparatus for signalling between one or more movable devices and one or more fixed locations.

Generally, in such fields as transportation and material handling, by way of example, it is often desirable to provide at a central station, information concerning the identity, location, and other characteristics of a plurality of movable devices. These devices, including such diverse items as railroad cars, buses, as well as bags or containers on a conveyor line, normally are routed to anyone of a number of locations throughout the transportation and/ or handling system.

Specifically, with respect to railway transportation systems, a train of railroad cars arriving at a switching terminal are first individually identified and then uncoupled and sorted in a predetermined manner, with selected cars being directed to various tracks and storage locations. Further, when an outgoing train is to be assembled, it is often necessary that a search be made among a large number of railroad cars in order to locate a particular car. As is well known in the prior art, various data processing systems have been deviced which are readily adaptable to sort, classify, and store data associated with any desired number of cars, yet the deviation of suitable input data relating to the characteristics of the individual cars has proven to be relatively expensive, so that full use of the available data processing machinery has not been extensive.

In order to derive the necessary input data, various prior art apparatuses have been designed, several of which are next briefly described. Photoelectric sensing means have been proposed for reading binary stripes on railroad cars, wherein the binary data is encoded in either dark or light stripes. Additionally, microwave power has been beamed at passive responder units which include fractional wave length apertures to provide a predetermined and unique reflected signal pattern. Further, radioactive sensing means have been proposed to detect radioactive elements carried in otherwise passive responders. However, most of the systems of the prior art have, as yet, not enjoyed extensive commercial success. This results from the fact that in many of the systems, the accuracy and reliability of the derived input data is critically affected by the distance between the interrogating element and the responder element, which, of course, is movable over relatively wide limits. Also, several are drastically affected by the presence of such foreign bodies as dirt, water, ice, etc. Notwithstanding the fact that some systems of the prior art have yielded acceptable performance in small scale installations, they are inherently incapable of use in a system wherein an extremely large number of vehicles or other movable objects are to be identified.

Yet another system of the prior art which overcomes many of the disadvantages of the typical systems briefly described above is shown in Patent No. 3,125,753 assigned to the assignee of the present invention. As there disclosed, the system utilizes completely passive responder elements which are inductively energized as they intercept an interrogation station, and each responder element provides a uniquely coded response signal, when interrogated by the interrogation station, which response signal is thereafter decoded by the interrogation station to provide the desired input data relating to the char- 3,307,168 Patented Feb. 28, 1967 acteristics of the moving vehicle upon which the responder is mounted. Apparatus of this general type exhibits several advantages over systems of the prior .art, including an improved signal-to-noise ratio which is uneffected by environmental conditions, as well as readily adaptability to large scale installations wherein many thousands of different objects must be individually identified.

According to the present invention, however, there is provided an improved signalling system, which also includes passive responder units, wherein each responder unit provides the desired input data at a markedly reduced cost, since filters or other complex components are 'not required for each binary bit, all without affecting either the system reliability or the system accuracy. Briefly described, the apparatus of the invention includes one or more interrogation stations and one or more responder elements. The interrogation station first provides a constant magnetic field which is effective to energize a responder element. Each responder element, upon being energized by this magnetic field, thereupon provides a number of signals at predetermined radio frequencies, selected in accordance with the input data to be derived therefrom, which :are thereafter accepted by the interrogating station. Next, the interrogating station decodes the signals received from each energized passive responder element in a novel manner, which includes an electronic counter operating with a fixed time base, and provides the input data relating to the characteristic of the movable vehicle upon which the responder elements :are mounted in a form determined by the overall data handling system.

It is an object of the invention, therefore, to provide an improved signalling system.

Another object of the invention is to provide an improved apparatus for signalling between one or more movable devices and one or more fixed locations.

A further object of the invention is to provide a signalling system including improved passive responder elements for deriving input data relating to the identities of or characteristics of movable vehicles. I

Still another object of the invention is to provide a more economical signalling system for transmitting predetermined information between fixed and movable locations.

Another object of the invention is to provide an improved signalling system for transmitting information between fixed and movable locations wherein the order of information is insensitive to the direction of relative motion between the locations.

Yet another object of the invention is to provide a signalling system including improved means for interrogating one or more passive responder elements.

A still further object of the invention is to provide an improved signalling system for identifying and classifying the movable vehicles of a transportation system.

A related object of the invention is to provide an improved signalling system for identifying and classifying the movable items on a conveyor line in a material handling system.

The above and other objects, features, and advantages of the invention will be apparent from the following detailed description of a preferred embodiment taken together with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a preferred embodiment of the invention.

FIG. 2 is a further schematic of a portion of the apparatus of FIG. 1.

FIG. 3A illustrates the magnetic field provided by a bar magnet.

FIG. 3B is an enlarged view of portions of FIG. 3A.

FIG. 4 indicates the location of a responder element and an interrogation station as installed in one embodiment of the invention.

FIG. 5 illustrates the output signals provided by a responder element of the invention.

Referring now to the drawings, FIG. 1 illustrates a preferred embodiment of the invention, which includes a single responder element and an interrogation station 12, it being understood that a plurality of such responder elements and one or more interrogation stations are employed in any large scale system. In general, the responder unit is mounted upon the movable device to he identified and includes a pair of passive oscillator units, each operable at a predetermined frequency. When the responder unit passes over or near an interrogation station, each of the passive oscillators is sequentially automatically operated in a novel manner, which is independent of the relative direction of motion between a responder element and an interrogation station, as more particularly hereinafter described.

As shown in FIG. 1, responder element 10 includes a pair of pickup inductors 13 and 14, each of which is coupled to a simple yet reliable crystal-controlled transistor oscillator. During the time interval that the total flux linking inductors 13 and 14 is changing, a voltage is induced in each inductor in the conventional manner, and this voltage is sufficient to provide the necessary power for the oscillator associated therewith. It should be noted, however, and this is an important feature of the invention, that the oscillators are oppositely connected to their respective inductors, and for this reason, only one oscillator is operable at any instant of time. By way of example, for the polarity indicated in FIG. 1, which may result from an increase in the number of flux lines linking inductors 13 and 14, only transistor 15 is supplied with the proper potential polarity to be operable and transistor 16 is effectively cut-01f. Conversely, when the number of flux lines linking inductors 13 and 14 is decreasing, the polarity of voltage induced in inductors 13 and 14 is reversed from that indicated in FIG. 1 to thereby render transister 16 operable and render transistor 15 inoperable. This feature, that one and only one oscillator is operable during a particular time interval, will be better understood as the descriptionproceeds.

With the polarity indicated in FIG. 1, a current flows through the collector-emitter circuit of transistor 15 and a resonant feedback tank circuit which includes a coil 17 and a variable capacitor 18. In responder element 10, coil 17 additionally functions as a loop antenna to radiate the generated signal at a frequency determined by a crystal 19, and the loop is distributed over as wide an area as is possible in order to obtain maximum radiation. Additionally, a capacitor 20 contributes significantly to oscillator stability and greatly increased efiiciency at the lower crystal frequencies. However, as will be understood by those skilled in the art, it must be maintained at a minimum value in order to avoid excessive crystal ringing. The distributed capacitance of inductor 13 is effective to by-pass the RF. signal around the inductor impedance. The oscillator associated with transistor 16 is identical to that described above, and, since the oscillators per se form no part of the invention, various other types and circuits may be substituted therefor, provided only that they supply the necessary stable predetermined output frequencies. Finally, a wide choice is available with respect to the inductor design other than that as illustrated in FIG. 1, including, but not limited to, a bifilarly wound inductor, or even a single inductor with the pair of oscillators coupled thereto by means of a pair of oppositely-poled diodes.

Turning now to interrogation station 12 of FIG. 1, it is seen first that a pair of permanent magnets 36 and 38 are associated therewith. Alternatively, an electromagnet could also be employed. Either of these magnets individually provides the necessary energizing power for each of the oscillators of a moving responder unit, as will be better understood in conjunction with the hereinafter detailed description of the operation of the system. Forming .a part of interrogation station 12 is a typical receiver unit which is broadly tuned to accept any frequency generated by any of the plurality of responder elements. This receiver is coupled to loop antenna 39 and is represented by amplifier block 40. Block 40 provides the necessary gain to amplify the minimum signal from a responder unit to a level sulficient for use with the output circuitry employed. Additionally, coupled to block 40 are a gate 42 and a level detector 55. Gate 42 isolates the output of block 40 from further decoding circuitry until the level of the output exceeds a predetermined level at which time detector 44 is effective to open gate 42, and to maintain the gate open until the output signal fails to exceed the predetermined level.

Although many and varied types of output circuits may be employed in order to decode the frequency information received by the interrogation station, in the preferred embodiment here being described, the decoder comprises an Events-Per-Unit-Iime Meter (or commonly an E-put meter), indicated as block 44 in FIG. 1, which is coupled to the output of gate 42 by a line 46 and to detector 44 by a line 48.

A typical E-put meter comprises an electronic pulse counter effective to count all input pulses occurring during a predetermined time period, the length of which is controlled by an accurate and highly precise time base. A simplified E-put meter, corresponding to block 44 of FIG. 1, is shown in somewhat more detail in FIG. 2. A crystal-controlled clock pulse oscillator 50 provides a continuous and precisely timed train of pulses. Since the accuracy of the frequency measurement is directly determined by the pulse timing, this timing accuracy is generally held to 1 part per million or less. In order to attain this precise timing, however, it is usually necessary to operate at crystal frequencies in the order of 10 me., a frequency which does not provide for sufficient counting time. For this reason, frequency division is accomplished in FIG. 2 by a binary counter 52 and a gate circuit 54. Gate circuit 54 is connected to the stages of binary counter 52 in order to determine the presence of a predetermined count therein, at which time an output signal is provided along a line 56. Upon the next occurrence of the predetermined count in counter 52, another output signal is provided along line 56. The signals along line 56 are coupled to flip-flop 58, a first output of which is applied to the reset and readout inputs of a decimal counter 60 and the second output of which is fed to an input of a differentiator 62.

The output of diiferentiator 62, which consists of a sequence of alternate positive and negative accurately timed pulses, is coupled to one input of a gate 64, the operation of which is controlled by level detector 44 (see FIG. 1) along line 48. The output of gate 64, comprising a series of pulses, timed by flip-flop 58 when a signal above a predetermined level is being received by interrogation station 12, is then fed to a further flipflop 66 which controls the operation of yet another gate 68. In this manner, signals appearing along line 46 are coupled to counter 60 only during the predetermined time periods. The readout data from counter 60 is coupled to a printer or output data converter, the latter being effective to link the counter to a strip chart recorder, a card punch, a paper, a punch, or the like, in accordance with the overall system requirements.

In order to illustrate the operation of the system described above, it will be employed in an automatic railroad car identification system for purposes of explanation only, it being understood that it is also adaptable to systems in other fields.

As employed in an automatic railroad car identification system, a responder element is secured at a common loca tion of each car, with the frequencies of the crystals therein selected in accordance with the input data to be derived from the individual car, as more particularly hereinafter explained. An interrogation station is then installed at a desired location with the magnets associated therewith positioned to ensure that a magnetic field, of at least a predetermined magnitude, is present in the area to be traversed by the responder elements. Alternatively, in selected applications, the interrogation station may be located in the movable car, with the responder elements installed in the railroad roadbed. It should be noted, and this is an important consideration, that to obtain input data insensitive to the direction of motion of a responder element, the magnetic field provided by the interrogation station must be parallel to the directions of motion of the railroad cars; i.e., parallel to the tracks, and the major axes of inductors 13 and 14 of the responder element (see FIG. 1) must also be parallel to the tracks. Now, when a railroad car, including a responder element, traverses an interrogation station, the responder inductors link the flux of the magnetic field and the change in flux resulting from the moving inductors, and the stationary magnetic field induces a voltage in the inductors of first one polarity, for example that shown in FIG. 1, thereby energizing transistor 15, to provide a frequency f and then of the other polarity, thereby energizing transistor 16 to provide a frequency f as hereinabove explained. This sequence of induced polarities which provides first a frequency A and then a frequency f is independent of the direction of motion of the responder element past the interrogation station, that is, the polarity sequence is the same whether the railroad car is travelling from east to west or west to east. Although this feature may not be readily apparent, it should become obvious from FIGS. 3A and 313.

FIG. 3A illustrates the conventional flux lines which indicate the magnetic field produced by a standard bar magnet. FIG. 3B is an enlargement of the dashed portions of FIG. 3A. Consider now a pickup loop moving from the left of FIG. 3B towards the right. This loop first cuts fiux lines 70 thereby inducing a voltage of, by way of example, positive polarity. Next, the loop intercepts fiux lines 72, whose direction is opposite to that of flux lines 70, and therefore a negative voltage is induced. Now, returning the loop from the right towards the left of FIG. 3B, the loop first cuts flux lines 72, but since its .motion is in a sense opposite to that previously described, a positive voltage is induced, and when the loop next cuts fiux lines 70 a negative voltage is obtained. Thus it can be seen that there is an effective reversal of flux direction with reversal of loop motion thereby providing the same polarity sequence independent of the motion direction, and it is this important feature that allows the economical responder elements and the interrogation thereof.

Although, as described immediately above, the sequence of frequencies generated by the responder element, that is, f -f is independent of the direction of motion between the responder element and the interrogation station, the frequency sequence is not independent of the orientation of the responder element with respect to the magnetic field, and it is for this reason that a pair of separated magnets are employed in the apparatus of the invention. Referring now to FIG. 4, there is illustrated a typical installation of the apparatus as applied in a railroad car identification system. As shown, loop antenna 39 is positioned along the center line between the tracks, while magnets 36 and 38 are offset therefrom. In the general case, wherein the inner flanges of the tracks are separated by a distance of five feet, the center of each magnet is aligned eighteen inches inwardly from the adjacent flange, resulting in a two foot separation between the magnets to thereby prevent the magnetic fields generated by the magnets from interacting one to another. A responder element 10 is secured to the lower surface of a railroad car and offset one foot from the center line thereof as shown. In specific applications, wherein the movable device or railroad car, is movable only back and forth between a pair of fixed locations only a single magnet is required. In the general case, however, wherein the probability of the movable device being rotated exists, it is necessary that the double magnet installation illustrated in FIG. 4 be employed. Note should also be made of the fact that the poles of the magnets are oppositely positioned as indicated in FIG. 4 in order to counteract the 180 rotation of inductors 13 and 14. Alternatively, a single magnet could be employed in a general installation provided that f never exceeds a predeterminer frequency and that f never be selected to fall below the predetermined frequency. Such restrictions, of course, materially reduce the maximum number of individually identifiable responder elements.

Continuing now with the description of the system operation, by determining the frequencies of the combination of response signals from a given responder element when it passes an interrogation station, the identity or other selected characteristic of the railroad car is established. Because of the many different combinations of system variables possible, such as oscillator frequencies, counting time, number of responder elements required in the system, maximum and minimum vehicle speeds, etc., only a typical system will be described in detail.

An exemplary system uses response frequencies ranging from 2.5 me. to 7.5 mc. spaced at uniform 10 kc. intervals, providing 500 different response frequencies. Since two crystal-controlled oscillators are used in each responder element, approximately 250,000 different responders may be provided, each of which produces a unique set of two response signals. By way of example, employing fre quencies of 4,020 kc. and 6,170 kc. in a responder, and a time base of 1 millisecond in the E-put meter, the resulting output identification would be 4020 and 6170, identifying, for example, Car Number 402617.

It might appear at first that by reducing the channel spacing to 1 kc. an eight digit code could be realized, but the 10 kc. spacing is preferred in order to accept the errors introduced by oscillator drift and time base tolerance.

Note should be made to the fact that the time base should not be less than three times the time of the shortest response signal to ensure obtaining a positive count, that is, to ensure that gate 68 (see FIG. 2) is open dur ing a counting interval, and, in general, the time base is made longer than this.

Although a wide choice is available for each of the parameters of the system, the below-listed specific values are typical in a large scale installation. Remembering now that the components associated with transistor 16 of the responder element 10 shown in FIG. 1 are identical with those associated with transistor 15, the following table lists the values of a responder element oscillator:

Transistor 15 Type 2N338.

Capacitor 20 15 mmfd.

Capacitor 18 500 mmfd. max.

Resistor 21 50,000 ohms.

Inductor 13 35,000 turns, No. 37 heavy Form- Coil 18 8 rogation station in the range of l to 60 miles per hour, the peak power level induced into coils l3 and 14 is at least 4 milliwatts and ensures a signal-to-noise ratio of about 30 db. Further, the operation of the responder element is unaffected by ferrous or non-ferrous structures in the vicinity thereof provided the structures are separated from the responder element by at least 3 inches and do not themselves generate a magnetic field.

It should be noted that, during system operation, the oscillator has a varying impulse of voltage applied to it. Since the impedance of the transistor is a function of this varying voltage, the crystal controlled voltage should also be expected to vary. However, in the circuit shown and described, this variation is only 0.02% or less of the selected frequency. Further, over a temperature range of 50 C. to +85 C. the maximum oscillator frequency deviation is about 0.002%. This is a cumulative deviation due to crystal and transistor changes. Amplitude change over this same temperature range is less than 1 /2 decibels. A typical response provided by a responder element is illustrated in FIG. 5.

In summary then, the preferred embodiment of the invention as above described provides an automatic vehicle identification system relating to moving vehicles, wherein the vehicle is provided with a coded responding identification unit which is not dependent upon any conventional built-in power supply subject to statistical failure or require replacement or servicing of the supply elements. The responder element comprises a pair of radio frequency oscillators powered by internally connected inductors which are energized as the vehicle carrying the responder element passes over a permanent bar magnet located in the roadbed. The nature of the connections of the oscillators to the inductors is such that, regardless of the direction of travel of the vehicle along the roadbed the radio frequency signal combinations are always emitted in an h-f sequence.

What has been described as an improved signalling system which includes a passive responder element and, effectively, a passive means to interrogate the element, together with novel circuitry to decode the responder response.

While only the fundamental novel features of the invention as applied to a preferred embodiment have been shown and described, it should be understood that various changes and modifications can be made without departing from the spirit of the invention, and it is the intention, therefore, only to be limited by the scope of the following claims.

What is claimed is:

1. A signalling system comprising,

at least one responder element;

at least one interrogation station;

said responder element including a pair of inductors, a pair of normally passive oscillators, and means coupling each of said oscillators to one of said inductors;

said interrogation station including means to provide a time-invariant magnetic field and means responsive to response signals from said pair of normally passive oscillators of said responder element to determine the frequencies of said oscillators;

means for providing relative motion between said responder element and said interrogation station whereby said magnetic field induces an operating signal in said pair of inductors of said responder element,

said signal being effective to sequentially energize said oscillators.

2. A signalling system comprising,

(a) a responder element including first and second normally passive oscillators, said first oscillator being operable in response to a signal of one polarity and said second oscillator being operable in response to a signal of the other polarity, each of said oscillators being compi ed to a pickup inductor;

(b) an interrogation station including means for providing a time-invariant magnetic field; and

(0) means for moving said responder element through a portion of said magnetic field in a predetermined direction to cause said inductors to sequentially provide induced signals of each of said polarities to selectively energize said first and second oscillators.

3. The system of claim 2 wherein said interrogation station includes measuring means to measure the frequencies of said first and second oscillators, said measuring means including means for counting each of said oscillator signals for predetermined times.

4. A signalling system comprising,

(a) an interrogation station including means for receiving and identifying radio signals within a predetermined frequency range and means for providing a time-invariant magnetic field;

(b) a responder element including a pair of normally passive crystal oscillators, each of said oscillators including a resonant circuit consisting of a coil and a capacitor wherein said coil additionally operates as a radiation antenna, and an inductor coupled to each of said oscillators, each of said inductors being oppositely coupled to its associated oscillator; and

(c) means for providing relative motion between said interrogation station and said responder element in a direction normal to each of a pair of spaced-apart mutually-parallel oppositely-extending components of said magnetic field, whereby a voltage is induced in said inductors, said voltage having a pair of successive excursions operable to sequentially energize said inductors, said voltage operable to sequentially energize said oscillators to provide first and second radio signals within said predetermined frequency range.

5. A signalling system comprising,

(a) a movable station including means selectively operable to transmit information signals at a plurality of discrete frequencies lying within a predetermined frequency range;

(b) a fixed station including means for receiving information signals within said frequency range; and

(0) means for operating said selectively operable means of said movable station including means positioned adjacent said fixed station for providing a predetermined time-invariant magnetic field in the path of said movable station.

6. A signalling system according to claim 5 in which said means for transmitting said information signals comprises a plurality of successively operated oscillators.

7. A signalling system according to claim 5 in which said time-invariant magnetic field in the path of said movable station includes a plurality of components of mutually-opposite sense whereby inductor means traversing said components successively will provide successive operating signals of opposite sense, and in which said means selectively operable to transmit said information signals comprises a plurality of oscillators connected to be selectively operated by said successive operating signals.

8. A signalling system according to claim 5 in which said means for receiving said information signals includes amplifier means, level detector means for providing a first control signal, timing means for providing a second control signal, a pulse counter, and gate means responsive to said control signals for applying signals from said' amplifier means to said pulse counter.

9. A signalling system according to claim 8 in which said timing means comprises a further oscillator, a counter connected to be advanced by signals from said further oscillator, and gating means connected to be operated by said couter to provide said second control signal.

10. A signalling system, comprising, in combination:

an interrogator device and a passive responder device,

said devices being relatively movable with respect to each other along a line in first and second opposite directions, said interrogator device including means for providing a unidirectional magnetic field having a component extending in a third direction normal to said line and means for receiving and decoding a response signal,

said responder device including an inductor operable to generate current upon traversal of said responder device through said magnetic field, and oscillator means connected to be powered by said current to generate said response signal.

11. A system according to claim in which said means for receiving and decoding said response signal comprises a pulse counter and means for applying said response signal to said pulse counter for a predetermined length of time.

12. A system according to claim 10 in which said unidirectional magnetic field includes a first portion having a component extending in said third direction and a second portion having a component extending in a fourth direction opposite to said third direction, said first and second portions being spaced apart from each other along said line, and in which said oscillator means comprises a first oscillator connected to provide response signal at a first selected frequency upon generation of said current with a first polarity and a second oscillator connected to provide response signal at a second selected frequency upon generation of said current with an opposite polarity.

13. A railroad car identification system comprising;

an interrogation station including means stationed at a railroad trackway for providing a time-invariant magnetic field and means to measure the frequency of received response signals within a predetermined frequency range;

a plurality of railroad cars to be identified, each of said cars being movable along said trackway and each including a responder element,

each of said responder elements including normally passive oscillator means for transmitting to said interrogation station first and second response signals when said responder elements enter into and leave from said magnetic field,

the frequencies of said first and second signals as determined by said interrogation station providing identification indicia for each of said plurality of railroad cars.

14. The system of claim 13 wherein said means for providing said magnetic field consists of a single bar magnet.

15. The system of claim 13 wherein said means for providing said magnetic field consists of a pair of spaced apart bar magnets.

16. A railroad car identification system comprising;

an interrogation station including means for providing a magnetic field having first and second spaced apart mutually-parallel components extending in first and second mutually-opposite directions, means for receiving signals within a selected band of frequencies, and means for determining and indicating the frequency of any signal received within said band; a plurality of railroad cars to be identified, each of said cars including a responder element secured thereon and positioned to intercept said magnetic field when said cars pass said interrogation station thereby to produce first and second successive operating signals of mutually-opposite sense, the sequence in which said operating signals are produced being independent of the direction of travel of said car with respect to said station; each of said responder elements including normally passive oscillator means for transmitting to said interrogation station first and second response signals upon generation of said sequence of operating signals when said movable responder elements traverse said first and second magnetic field components,

frequencies of said first and second response signals as determined by said interrogation station providing identification indicia for each of said plurality of railroad cars.

References Cited by the Examiner UNITED STATES PATENTS 6/1962 Van Allen 324-43 9/1965 Prucha 246

Claims

5. A SIGNALLING SYSTEM COMPRISING (A) A MOVABLE STATION INCLUDING MEANS SELECTIVELY OPERABLE TO TRANSMIT INFORMATION SIGNALS AT A PLURALITY OF DISCRETE FREQUENCIES LYING WITHIN A PREDETERMINED FREQUENCY RANGE; (B) A FIXED STATION INCLUDING MEANS FOR RECEIVING INFORMATION SIGNALS WITHIN SAID FREQUENCY RANGE; AND (C) MEANS FOR OPERATING SAID SELECTIVELY OPERABLE MEANS OF SAID MOVABLE STATION INCLUDING MEANS POSITIONED ADJACENT SAID FIXED STATION FOR PROVIDING A PREDETERMINED TIME-INVARIANT MAGNETIC FIELD IN THE PATH OF SAID MOVABLE STATION.