EP0217297B1 - Track circuit with or without insulated joints for railway systems - Google Patents

Abstract

The circuit is connected to an AC voltage source (5) and is equipped at the end with connection terminals (8, 9) for tapping off the AC voltage and for displaying the free or occupied state. In order to generate a dynamic signal at the end of the track circuit, a circuit arrangement is in the form of a series circuit of a low-impedance resistor RAP and a periodically actuable switch (10) for the periodic varying (modulation) of the amplitudes of the AC voltage having a frequency which is lower than this AC voltage and a device which compares the amplitude change of the AC voltage with a threshold value voltage and displays it is provided at one point of the track section. By means of the aforesaid configuration, track sections with lengths greater than 1GK can be monitored and also the respective state of the ballast can be displayed. <IMAGE>

Description

The invention relates to a track circuit with or without insulating joints for railway systems, in which the beginning of the track section to be monitored is connected to an AC voltage source via terminals and at the end of the track section of length l _GK the AC voltage still present is tapped and used to indicate the free or occupied Condition is used.

In the known track circuits of the aforementioned type, the free or occupied state is derived from the amplitude of the track voltage U _G measurable at the end of the track section. Such track circuits are known, for example, from the magazine "Electrical Railways", Volume 81, No. 7, July 1983, pages 216-226.

When a voltage U _a with the frequency f _{GK is} fed into the track acting as a line, the amplitude of the voltage decreases with increasing length l _{GK of} the track circuit as a result of the longitudinal and transverse damping given by the track and the bedding in the free state of the track section from, so that the track voltage U _{G is} still present at the end of the track section. Lead when busy the axes entering the track section to a short circuit so that there is practically no track voltage U _G at the end of the track.

The voltage that can be measured in the free state at the end of the track section depends on the condition of the bedding of the track section. With the worst bedding, the smallest track voltage U _GsF results in the free state of the track section, while the greatest track voltage U _GgF occurs at the end of the track circuit in the free state with the best bedding. If the axles entering the track section do not lead to a complete short circuit in the occupied state, the track voltage U _{G is} not zero at the end of the track section, but the relatively high voltage _value U _GgB occurs in the best bedding and the much smaller voltage _value in poor bedding U _GsB on.

In order to be able to determine from the voltage measurement at the end of the track circuit whether the track section is free or occupied, the most unfavorable conditions must be assumed, i.e. a sufficient difference between the lowest voltages occurring in the free or occupied state must be ascertainable. This results in a voltage-related working range A = U _{GsF -U GgB} with a reference voltage lying within this working range, which is also referred to below as threshold voltage voltage U _S. Since the measurable in poor bedding in the idle state voltage U _GsF and the measurable in the busy state in good ballast voltage U _GgB not only with increasing length of the track section are small, but the voltage U _GsF decreases faster than the voltage U _GgB is the working area A becomes smaller and smaller with increasing length of the track section or track circuit and finally reaches the value A = 0. This will make the theoretical limit length of the track section is determined, however no measurement is possible. In practice, there must always be a finite, ie evaluable, work area A, which is only given if U _{GsF is} sufficiently larger than U _GgB .

In order to increase the effective lengths of track circuits, it is known to reduce the damping coefficient α of the track acting as a line by continuously connecting capacitors to the track, for example at intervals of 100 m, and to work with the lowest possible frequencies in the case of insulation-free track circuits. Track circuits with capacitors connected in parallel between the tracks only work reliably if there are no interruptions in the leads and connections of the capacitors.

Another possibility for increasing the length of the track sections to be monitored is to cascade several shorter track circuits, but this represents a very complex solution in terms of circuitry.

The invention is based on the object of developing a track circuit of the type mentioned in such a way that track sections of substantially greater length can be monitored and the condition of the bedding can also be continuously displayed.

To solve the above task, the track circuit mentioned above is designed such that a circuit arrangement consisting of a series connection of a low-resistance resistor and a periodic one for generating a dynamic signal in the course of the track section between the tracks Actuable switch for periodic change (modulation) of the amplitudes of the alternating voltage with a frequency that is significantly lower than the alternating voltage and a change in the amplitude of this alternating voltage with the amplitude change that occurs when the switch is opened and closed at the end of the track section when the switch is opened and closed comparative and indicating device are provided.

The periodic change or modulation of the amplitudes of the alternating voltage that can be taken off at the end of the track section means that the absolute values of the alternating voltage that can be tapped off at the end of the track section experience different amplitude changes as a result of the modulation with the aid of the periodically actuatable switch, and that a comparison of these different changes in amplitude can be evaluated as a criterion for determining the free or occupied state of the track section. It is no longer important that the voltage U _{GsF that can be tapped} in the worst bedding and in the free state is greater than the voltage U _GgB that can be tapped in good bedding and the occupied state in order to have an evaluable working area A, but the new track circuit enables clear display of the occupied or free state of the track section depending on the absolute values of the two aforementioned voltages up to a length of the track section in which the amplitude changes of these voltages no longer differ significantly enough from one another.

The new track circuit can be operated with an alternating voltage of, for example, 100 Hz up to a few kHz. It is important that, depending on the frequency of the AC voltage, the periodic change in the amplitudes takes place by the periodic actuation of the switch at a frequency which is at least approximately 10 times smaller than the frequency of the AC voltage source.

The low-ohmic resistance must be of the order of magnitude of the axis resistance.

By closing the periodically actuatable switch, the occupied state of the track is approximately simulated or the occupied state at the end of the track section is simulated. In the free state of the track section to be monitored, the dynamic signal with the amplitude change ΔU _GF occurs when the switch is periodically actuated with the frequency f _sch , while in the occupied state, in which the track is practically short-circuited by the run-in axes, when the parallel is switched low-resistance, the amplitude change ΔU _{GB is} generated, which is vanishingly small, so that by comparing the amplitude change with a very low threshold voltage U _S, the free state for ΔU _{GF is} greater than U _S and the occupied state for ΔU _{GF is} less than U. _S can be easily distinguished even with a very long track circuit length and large fluctuations in the bedding resistance. The threshold voltage U _S is determined by the change in amplitude occurring at the best bedding in the occupied state when the switch is opened and closed at the end of the track section.

The condition of the bedding can be continuously monitored from the change in amplitude occurring in the free state, since the change in amplitude increases as the bedding improves and vice versa.

The circuit arrangement for periodically changing the amplitudes of the alternating voltage can be arranged at any point between the beginning and end of the track section. Appropriately, however, it is arranged according to claim 2 at the end of the track section. This simplifies the installation since no special connecting lines running along the track section are required.

In order to detect the change in amplitude of the track voltage U _G present at the end of the track section to be monitored and to transmit it to the display device, various circuit arrangements are possible, which are set out in claims 3 to 7.

While in an arrangement according to claim 3, the amplitude change U _G = f (t) occurring at the end of the track circuit as an envelope of an amplitude-modulated oscillation results directly from the demodulation of the track voltage U _G = f (t) as a variable that can be evaluated for the display, it can according to claims 4 to 6 can also be evaluated only after rectification or implementation in another frequency range.

According to claim 7, no constant threshold voltage U _{S is} provided for the evaluation of the change in amplitude as a reference variable, but the threshold voltage in this embodiment is a periodically linearly changing reference voltage with a frequency f _ab , which for example has a sawtooth shape changes so that with the threshold voltage U _S = f (t) the entire range of the track voltage U _{G is} continuously scanned periodically. The number n of changes between the free and busy display is a measure of the change in amplitude, which in turn represents the criterion for the free or busy display.
While maintaining the evaluation according to claims 3 to 7, a further variant of the track circuit according to claim 8 is possible.

An example of the invention is explained with reference to the drawing.

Fig. 1

einen Gleisstromkreis mit Isolierstößen und am Ende vorgesehener Schaltungsanordnung,

Fig. 2

den Verlauf der Gleisspannungen U_G in Abhängigkeit von der Länge des Gleisabschnittes l_GK im Frei- und Besetzt-Zustand für zwei verschiedene Gleisbettungen,

Fig. 3a

entsprechend Fig. 1 einen Gleisstromkreis bekannter Art nach Einlaufen einer Achse,

Fig. 3b

die Gleisspannung U_G am Ende des Gleisstromkreises nach Fig. 3a beim Durchlaufen einer Achse,

Fig. 3c

den zeitlichen Verlauf der Amplitudenänderung der Gleisspannung U_G eines freien Gleisstromkreises gemäß Fig. 1 bei guter und schlechter Bettung,

Fig. 3d

eine Darstellung gemäß Fig. 3c bei besetztem Gleisstromkreis,

Fig. 4a

eine erste Auswertschaltung für den Gleisstromkreis durch Demodulation der Gleisspannung U_G,

Fig. 4b

eine weitere Schaltung für die Auswertung der Gleisspannung U_G mit Hilfe einer mit der Schaltfrequenz f_sch arbeitenden Anordnung,

Fig. 4c

eine Schaltung zur Auswertung der Gleisspannung U_G mit Hilfe einer Gleichrichteranordnung,

Fig. 5

eine Schaltung zur Auswertung der Gleisspannung U_G mit Hilfe einer Anordnung zum Herausfiltern einer Grundschwingung aus dem Verlauf der modulierten Wechselspannung,

Fig. 6

eine schematische Darstellung einer Auswertung der Gleisspannung U_G in Verbindung mit einer linear ansteigenden Schwellwertspannung,

Fig. 7

die Schaltungsanordnung zur Auswertung der Gleisspannung U_G nach dem Schaltschema der Fig. 6 mit Hilfe eines Abtastgenerators.

Show it:

Fig. 1

a track circuit with insulation joints and circuit arrangement provided at the end,

Fig. 2

the course of the track voltages U _G as a function of the length of the track section 1 _GK in the free and occupied state for two different track beds,

Fig. 3a

1 a track circuit of a known type after running in an axis,

Fig. 3b

the track voltage U _G at the end of the track circuit according to FIG. 3a when passing through an axis,

Fig. 3c

the time course of the change in amplitude of the track voltage U _{G of} a free track circuit according to FIG. 1 with good and poor bedding,

Fig. 3d

3c with the track circuit occupied,

Fig. 4a

a first evaluation circuit for the track circuit by demodulating the track voltage U _G ,

Fig. 4b

another circuit for evaluating the track voltage U _G with the aid of an arrangement operating at the switching frequency f _sch ,

Fig. 4c

a circuit for evaluating the track voltage U _G using a rectifier arrangement,

Fig. 5

a circuit for evaluating the track voltage U _G with the aid of an arrangement for filtering out a fundamental vibration from the course of the modulated AC voltage,

Fig. 6

1 shows a schematic representation of an evaluation of the track voltage U _G in connection with a linearly increasing threshold voltage,

Fig. 7

the circuit arrangement for evaluating the track voltage U _G according to the circuit diagram of FIG. 6 with the aid of a scan generator.

The track current rice shown in FIG. 1 has a length l _GK . The rails 1 and 2 of this track circuit extend from the insulation joints 3 to the insulation joints 4. In the region of the insulation joints 3, an AC voltage source 5 is provided between the rails 1 and 2, which supplies an AC voltage with the amplitude U _A and the frequency f _GK feeds a resistor 6 into the track circuit, in which the rails 1 and 2 serve as current-carrying conductors.

In the circuit diagram, a resistor 7 is shown between the rails 1 and 2, which can have a different size depending on the bed conductance G _B.

The size of the damping coefficient α of the track and the wave resistance Z _{L of} the track depend on the above-mentioned bed conductance G _B , for which with good bedding the value G ' _Bg = 0.1 S / km and with poor bedding the Value G ' _Bs = 0.7 S / km can be considered.

In the example shown in FIG. 1, terminals 8 and 9 are provided at the end of the rails 1 and 2 for tapping off the track voltage U _{G of} the frequency f _GK still present at this end of the rails. In parallel to these terminals 8 and 9, a series connection of a low-resistance resistor R _{AP is provided} with a switch 10, which is actuated in the direction of the double arrow and opens and closes with a switching frequency f _sch . The low-resistance resistor R _AP has approximately the order of magnitude of the axis resistance of a train entering the track section. Currently an axis resistance in the order of magnitude of 0.5 to 1 ohm is expected.

When the switch 10 is closed, the occupied state of the track circuit is simulated. The connected resistor R _AP between the terminals 8 and 9 of the track circuit also results in a track voltage U _G , the amplitude of which approximately corresponds to the occupied state of the track circuit. If the track circuit is free, when the resistor R _AP is switched off, that is to say when the switch 10 is open, the amplitude of the track voltage U _G is greater by the amplitude change ΔU _GF than when the resistor R _{AP is switched on} , so that in the state shown in the right part of the Fig. 1 reproduced time sequence ΔU _GF = f (t) results.

Since the change in amplitude is also dependent on the track bedding, relatively good amplitude changes .DELTA.U _GF result in good bedding of the track and correspondingly lower values for .DELTA.U _GF in poor bedding, as can also be seen from FIG. 3c. 1 shows the circuit according to FIG. 1 with switch 10 open and closed and in the left part Part of the different amplitude changes for good bedding G ' _Bg and bad bedding G' _Bs in the free state of the track section.

If the track circuit is occupied, the amplitude of the track voltage U _GB is already very small, corresponding to the resistance of the run-in axes, so that when the resistor R _{AP is switched on there is} hardly any change in amplitude ΔU _{GB of} the track voltage. This also results from FIG. 3d, which in the right part shows a section of the circuit according to FIG. 1 with the axis running into the track circuit with the axis resistance R _A and the various positions of the switch 10, as well as the associated track voltages U _G in the left part of the figure reproduces depending on the time. Min recognizes that when the track is occupied, the change in amplitude .DELTA.U _GB due to the actuation of the switch is only very small, both with good bedding G ' _Bg and with bad bedding G' _Bs .

It can be seen from the different course of the track voltage U _G according to the bedding condition of the track that the design of the track circuit provided according to the invention enables evaluation by detecting the change in amplitude with the aid of the load resistance R _AP over substantially greater lengths l _GK than was previously the case .

The track voltage changes over the length l _{GK of} the track circuit, ie over the length of the track section assuming the adapted state in accordance with the function U _G = U _A .e ^-αl GK. Since the damping coefficient α depends on the bedding condition of the track, there are different curve profiles for the track voltage U _G. 2 are plotted the track voltages U _G at bedding conductance values of G '= 0.1 S / km and G' = 0.7 S / km, as they correspond to the limit values in practice, the course of U _G in the free state by the index BgF or BsF, and in the occupied state is characterized by the respective index BgB or BsB.

The working range of the previously known track circuits lies between the two curves shaded in FIG. 2. The corresponding working area A with the associated threshold voltage U _S is shown for the length l 1 as the mean value between the voltages U _GsF and U _GgB . The previous _{mode of} operation of the track circuit presupposes a higher voltage U _GsF compared to the other voltage U _{GgB in} order to provide a clear statement about the free or occupied condition of the track in every bedding condition in the range of the specified bedding _{guide values} when compared with the threshold voltage voltage U _S ensure. The working area A becomes smaller with increasing length l _{GK of} the track section and becomes zero at the crossing point of the hatched curves. The length l _{A = 0} assigned to the crossing point is to be regarded as a theoretical limit for the length of the track circuits, which can be monitored with the known circuits. In practice, however, this limit value cannot be achieved because a comparison of the voltages determining the working range with the threshold voltage voltage U _{S is} necessary, this comparison leading to significant overshoots or undershoots, which must result in values determined by the hatched curves.

If, on the other hand, the change in amplitude is carried out by switching on and off a resistor approximately corresponding to the axis resistance at the end of the track circuit, there are still clear evaluation results if the track circuit length I _GK goes beyond the intersection of the hatched curves in FIG. 2.

If one assumes that the mentioned resistance R _AP corresponds to the axis resistance at the end of the track circuit, then with a length l 1 of the track circuit with good bedding in the free state, amplitude changes ΔU _GgF and with poor bedding of ΔU _GsF , as in the case Length l 1 in Fig. 2 are entered. With the length l₂ of the track circuit, that is, in the area after the crossing point of the hatched curves, the amplitude changes have become smaller, but it can be seen that these are still clearly evaluable changes, as is also clear from Fig. 3c .

The right part of FIG. 2 shows the range of the limit length for the track circuit up to which the free or occupied state can be determined by evaluating the change in amplitude according to FIG. 1. In this part of FIG. 2, the course of the voltage with the switch 10 open and the dashed line with the switch 10 closed is shown in the solid line for the course of the track voltage with good bedding and in the occupied state. It can be seen that when switch 10 is closed and the bedding is good when the switch 10 is open and closed, an amplitude change of ΔU _GgB occurs, and that at this limit length of the track circuit with poor bedding and when the switch 10 is open and closed, an amplitude change ΔU _GsF occurs. In the border area, the two aforementioned changes in amplitude are approximately the same size, so that under unfavorable conditions the occupied or free state of the track section can no longer be determined. The value ΔU _GgB , which should be kept as small as possible by appropriate selection of the resistor R _AP , takes over the function of a threshold value that is independent of a working range A. The free state of the track section is present for all amplitude changes above this threshold value caused by the opening and closing of the switch 10.

From Fig. 2 it can also be seen that for a given length l _{GK of} the track circuit, for example at l _GK = l₂, the amplitude change ΔU _GF obtained in the free state with periodic switching on and off of the load resistor R _AP , which is between the limit values _{.DELTA.U GsF} , and _{.DELTA.U GgF} , in which FIG. 2 lies, provides a measure of the bedding condition of the track circuit due to their size and thus enables continuous monitoring of the bedding condition. The greater the amplitude of ΔU _GF , the better the bedding.

3a to 3d, in conjunction with the associated circuit arrangements, show the track voltage curves for a direct measurement of the track voltage at the end of the track circuit according to the prior art and when the track circuit is designed in accordance with the invention according to FIG. 1.

3a shows a track circuit similar to FIG. 1, but without the additional circuit arrangement consisting of the switch 10 and the low-resistance resistor R _AP . Instead, a track relay GR is shown between the terminals 8 and 9 of the track circuit as a symbol for an evaluation device. In the arrangement according to FIG. 3a there is a run-in axis with the axis resistance R _A at location X of the track circuit.

Fig. 3b shows the course of the track voltage when running through an axis, and that with good and bad bedding in the free state in each case outside the track circuit, the respective voltages U _GgF and U _{GsF are} above the threshold voltage U _S , while subsequently in Busy state, i.e. in the area of the track circuit with the axis of resistance R _A that has entered, the track voltages are smaller than the threshold voltage U _S , as shown by the voltage _curves U _GgB and U _GsB .

3c and 3d each show in their right parts in a reduced representation the end of the track circuit according to FIG. 1 in the two different switching positions of the switch 10 and in the left parts the course of the track voltages depending on the switching frequency f _{sch of} the switch 10. Fig. 3c shows the track circuit and the associated voltage curves and the amplitude changes in the free state depending on the time t, while Fig. 3d shows the corresponding circuit diagrams and curves of the track voltage with an axis running into the track circuit with the axis resistance R _A , i.e. in the busy state, shows.

As has already been described in connection with FIG. 2 with reference to FIGS. 3c and 3d, when the track section is free, the periodic switching on and off of the resistor R _{AP results in} large amplitude _{changes .DELTA.U GsF of} the track voltage, while the change in amplitude ΔU _{GgB is} only very small.

4a to 4c show various circuit arrangements for evaluating the change in amplitude ΔU _{G of} the track voltage.

Since the periodic actuation of the switch 10 converts the track voltage U _G into an amplitude-modulated oscillation, this oscillation ΔU _G = f (t), which occurs as an envelope curve, can be obtained by demodulation according to FIG. 4a.

4a shows the end of the track circuit with the switch 10 and the low-resistance resistor R _AP and symbolically indicates an actuating device 11 for opening and closing the switch 10 with the frequency f _sch . This switching frequency is significantly lower than the frequency f _{GK of} the track voltage U _G. A demodulator 12 is connected to the terminals 8 and 9 for tapping the track voltage U _G. The drawing of the demodulator 12 shows the voltage profile of the tapped track voltage U _G.

The demodulator 12 is followed by a low-frequency stage 13, which in turn is connected to a frequency multiplier 14 for multiplying the switching frequency f _sch and converts the switching frequency f _sch into the track circuit frequency f _{GK in} order to use a circuit arrangement and display device 15 to bring about an active display, an evaluation being made possible in the same way as was previously carried out without the amplitude modulation. However, the amplitude of the threshold voltage U _{S is} very much lower than in the known track circuits without the modulation of the track voltage, since according to the explanations for FIG. 2, the amplitude change ΔU _GsF for free display only shows the worst case in the occupied state with the best _Bedding occurring value ΔU _GgB must exceed.

According to FIG. 4b there is an immediate evaluation of the amplitude ΔU _{G of} the track voltage, which changes with the switching frequency f _sch . In this arrangement, the frequency multiplier 14 is not provided, but only the demulator 12 and the low-frequency stage 13 according to FIG. 4a are arranged in an attachment 16. In the circuit and display device 15 there is an active display corresponding to the switching frequency f _sch , as indicated in the display device 15 shown.

In the arrangement according to FIG. 4c, an attachment 16 according to FIG. 4b is again provided. In this case, the switching and display device 15 additionally has a rectifier 17 arranged upstream of the actual display device, so that the evaluation for the free and occupied state takes place with the direct voltage ΔU _G -.

FIG. 5 shows a further possibility of evaluating the modulated vibration in order to display the free and occupied state. In this arrangement, the amplitude-modulated oscillation, the envelope of which contains the amplitude change ΔU _{G to be} evaluated, is fed to a filter circuit 18. In this circuit, a sinusoidal fundamental _wave U _Gsch = f (t), which runs at the switching frequency f _{sch, is} filtered out. Since this filtered-out oscillation is proportional to the change in amplitude .DELTA.U _G , the following circuit and display device 15 can be used to display the switching frequency f _sch .

A completely different evaluation principle for recognizing the free or occupied state is shown in FIGS. 6 and 7.

The basic idea of this evaluation arrangement is to no longer work with a constant threshold voltage U _S as before, but to continuously increase the threshold voltage from 0 to a maximum value and to repeat this process periodically.

By changing the amplitude of the threshold voltage, the size of the ΔU _G range is scanned, as is shown in principle for the free and occupied state in FIG. 6. In the respective left part of the graphical representations of FIG. 6, the track voltage U _{G is shown} as a function of the time t, above for the free state and below for the occupied state during a switching operation by means of the switch 10.

The respective right-hand part of the graphic representations shows the threshold voltage voltages U _S as a function of time t.

The track voltage curves for good and bad bedding are shown in both figures.

It can be seen that, with the amplitude of the threshold voltage U _S increasing from zero, despite the switching on and off of the resistor R _AP, the free state is indicated by actuating the switch 10 until the threshold voltage U _{S reaches} a value at which the ΔU _G - Area begins. Within the entire ΔU _G range, as U _S continues to grow as a dynamic signal, there is a constant change between free and busy display in time with the switching frequency f _sch . For a given switching frequency f _sch , the number n of free / busy changes is directly proportional to the amplitude ΔU _G to be measured. If the threshold voltage U _S continues to increase beyond the maximum value U _{G of} the track voltage occurring in the free state, the dynamic signal disappears again and the occupied state is continuously displayed. Since a small amplitude _{change .DELTA.GgB is} possible even when the track circuit is occupied, a limit value n _min results when evaluating the number n of free / busy changes (see FIG. 6 below), so that the free -State for n large n _min , the occupied state for n smaller n _min . The number n of changes is at the same time a criterion for the bedding condition, since n increases as the bedding improves. The number n of changes, which represents the measured variable, can be measured either with a simple evaluation device with a single output or with an evaluation device which has an active display with two separate outputs for the free and busy display.

FIG. 7 shows a circuit diagram of an evaluation circuit for an active display according to the principle described in FIG. 6.

The device 11 operating at the switching frequency f _sch is designed as a clock generator in order to actuate the switch 10 periodically. The amplitude-modulated track voltage is fed to the evaluation switching device 19, which in turn is connected to a scanning generator 20 that generates a sawtooth-shaped threshold voltage U _S , which is compared with the respective amplitude of the amplitude-modulated track voltage U _G , and in connection with FIG. 6 described way leads to the change of the free / busy display, which changes with the frequency f _sch . In connection with a counting device 21 and correspondingly preset comparison and display devices 22 and 23, when there are a number of changes n less than n _min, the busy display takes place, while conversely with a larger number n of changes greater than n _min, the free state is shown is shown.

nur von der Amplitudenänderung U_G abhängig ist.The number n of free / occupied changes is also a measure of the bedding derivation of the track, since n with constant switching frequency f _sch and constant sampling frequency f _abt = $\frac{\text{1}}{\text{T}}$

is only dependent on the change in amplitude U _G.

In the arrangement according to claim 8, with the described evaluation circuits, the amplitude changes up to the respective limit of the evaluation can be detected from the beginning and end of the track section to its center, so that the track section can have a correspondingly greater length than in the example.

Claims (8)

1. A track circuit with or without insulated joints for railway installations, in which the beginning of the track section to be monitored is connected by means of connections to an a.c. voltage source (5) and at the end of the track section of the length l_GK the a.c. voltage which is still present is tapped and is used to display the clear or occupied condition, characterised in that in order to produce a dynamic signal in the course of the track section between the tracks (1,2) there is provided a circuit arrangement consisting of a series circuit of a low-ohmic resistor (R_AP) and of a periodically actuable switch (10) for periodically changing (modulating) the amplitudes of the a.c. voltage at a frequency (f_sch) which is substantially lower in comparison to the a.c. voltage, and there is provided an arrangement (15) comparing and indicating the amplitude change (ΔU_G) of this a.c. voltage with the amplitude change (ΔU_GB) which occurs with best bedding and in the occupied condition when opening and closing the switch at the end of the track section.
2. A track circuit according to claim 1, characterised in that the circuit arrangement, consisting of the series circuit of the low-ohmic resistor (R_AP) and the periodically actuable switch (10), is arranged at the end of the track section parallel to the connections (8,9) for the tapping of the a.c. voltage.
3. A track circuit according to claims 1 or 2, characterised in that in front of the indicator device (15) is arranged a demodulation circuit (12) for the modulated a.c. voltage tapped at the end of the track section.
4. A track circuit according to claims 1 to 3, characterised in that in front of the indicator device (15) for the a.c. voltage, which is tapped at the end of the track section and demodulated, is arranged a rectifier circuit (17).
5. A track circuit according to claims 1 to 3, characterised in that in front of the indicator device (15) for the demodulated a.c. voltage, tapped at the end of the track section is arranged a circuit (14) for converting into another frequency range.
6. A track circuit according to claims 1 to 3, characterised in that in front of the indicator device (15) for the a.c. voltage which is tapped at the end of the track section and demodulated, is arranged a circuit (18) for filtering out a first harmonic from the course of the demodulated a.c. voltage.
7. A track circuit according to any one of claims 1 or 2, characterised in that an evaluation circuit (19) operating as a comparison device is provided for the modulated a.c. voltage tapped at the end of the track section and is connected to a sampling generator (20) which produces a periodic and linearly changing comparison voltage (threshold value voltage), which has a substantially smaller frequency than the modulation frequency of the tapped a.c. voltage and a maximum value noticeably exceeding the amplitude change of the a.c. voltage and that the evaluation circuit causes a periodic change in the indication on comparison of the respective amplitudes of the a.c. voltage with the comparison voltage, if the comparison voltage is in the region of the amplitude change of the tapped a.c. voltage and causes a continuous clear or occupied indication with comparison voltages greater than or less than the amplitude of the a.c. voltage.
8. A track circuit according to any one of the preceding claims, characterised in that both at the end and at the beginning of the track section there is provided in each case a circuit arrangement, consisting of a series circuit of a low-ohmic resistor (R_AP) and a periodically actuable switch (10), and a device synchronously actuable with the switches, in connection with a comparison device for selecting the respective a.c. voltage with the lower amplitude change U_G and with a further device (15) comparing and indicating the lower amplitude change of the a.c. voltage with the amplitude change occurring when there is best bedding and in the occupied condition, when opening and closing the switch provided at the end of the track circuit.

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