GB1592167A - Apparatus for monitoring signal lamps - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO APPARATUS FOR MONITORING SIGNAL LAMPS (71) 1, SECRETARY OF STATE FOR TRANSPORT, London, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to apparatus for monitoring signals lamps for failure thereof, more especially, but not exclusively, lamps in road traffic signal heads. Signal lamp monitoring has for a long time been used in railway and motorway signalling, as well as in other fields.

The difficulty with the techniques used when applied to traffic signals are that: 1) AC drive is used.

2) Each lamp has an individual transformer, which for reasons of prolonging the life of the lamps, has a high loss characteristic.

3) Up to 6 or more lamp circuits are normally connected to a single wire leaving the signal controller.

4) Normally no spare cable cores are available for separating out lamp drive circuits or for signalling purposes in a monitor circuit.

The lamp monitoring circuit required must be able to detect the failure of any one lamp on a junction, and should preferably also indicate a lamp failure when a lamp transformer fails, the disconnection of a lamp signal wire occurs, or in the event of the non-illumination of the lamp for any other reason. It need only indicate that a lamp has failed, no indication of which lamp or which type of aspect is necessary. The overriding characteristics however must be the reliability of the monitoring circuit and its cost.

In order to keep down the cost of the lamp monitoring circuits, all initial methods tried were based on having the total monitor unit installed in the controller case. Methods tried ranged from the measurement of current or power, to the transmission of current pulses during both the on and off periods of the lamps, or the transmission of high frequency waveforms during the lamp off period. The current that resulted from these pulses or high frequency waveforms was measured, and used to determine whether any lamp has failed.

These latter methods were designed to reduce the effects of losses in the transformers.

All of these methods suffered from the same major problem. Although they could be set to operate satisfactorily in the laboratory for a given set of lamps, the parameters were such that fairly careful set up was required, which could lead to drift problems. Of even greater importance however is that the lamps themselves have a wide tolerance on the current taken, so that even if drift considerations did not require frequent setting up operation, the replacement of the lamps would most likely necessitate resetting of the monitor. The alternative to this continual setting up would be a highly complex self-compensating circuit, which would be expensive and which would pose reliability problems.

Because of the problems associated with the methods employing only equipment installed in the controller case, attention has been turned to a method which employs some equipment installed in each traffic signal head, as well as some equipment in the controller case.

The requirements of such a method are that primarily the head based equipment must be extremely simple, reliable, and above all inexpensive, since even the simplest of junctions will have 6 heads, and most junctions have considerably more. Since it must be possible to add the monitoring equipment to traffic signals which are already installed on the street, no additional cable cores will be available for the transmission of data from the head to the controller case.

According to the invention, apparatus for monitoring signal lamps for failure thereof has current responsive means operable by the electric current through at least one of the signal lamps in at least one signal head; capacitors connectable across the power supply lines to said signal lamps; capacitor switch means operable by said current responsive means to connect the capacitors as aforesaid when less than a predetermined minimum lamp current flows through any one of said signal lamps; oscillator means for applying a voltage of at least a high audio frequency across the circuit points to which said capacitors are connectable; and current measuring means for measuring the resultant current so that a large capacitor current is measured by said current measuring means at said high frequency at some period during a cycle of signal aspects if less than said minimum lamp current can flow through any one of the signal lamps in said signal head, and substantially lower current is measured at said high frequency if said minimum lamp current is exceeded for oil lamps; said capacitors being chosen of such capacitance as to present relatively high impedance at the frequency of said power supply, and relatively low impedance at said high frequency.

The high frequency voltage is preferably applied across the capacitors through the supply lines to the signal lamps.

In one embodiment of the invention the said current responsive means is a current relay and the said capacitor switch means comprises contacts operated by said current relay.

Optionally the high frequency is applied to the capacitors through transformer means connected into a common power supply line, for example the neutral line, to the signal lamps.

Desirably the apparatus includes selector switch means whereby the oscillator and/or current measuring means can be switched to measure in turn any currents through the capacitors associated with each of a plurality of signal heads.

The invention will be further described, by way of example only, with reference to the drawings filed with the Provisional Specification, in which Figure I is a circuit diagram of a three aspect road traffic signal head.

Figure 2 illustrates modifications whereby current can be measured in the lamps in the signal head of Figure 1.

Figure 3 illustrates current measurement by means of solid state devices.

Figure 4 shows the connections in a signal head of capacitors and associated switching means.

Figure 5 is a schematic diagram of monitoring apparatus.

Figure 6 shows an arrangement of line selector switches.

Figure 7 is a circuit diagram of the monitoring circuits.

Figure 8 shows circuit arrangements for an additional lamp in a signal head.

Figure 9 illustrates phasing arrangements of a traffic light system with additional pedestrian signals.

Figure 10 illustrates phasing arrangements of a traffic light system with additional filter (green) arrow.

Figure 11 illustrates circuit modifications in a signal head required for an additional filter arrow.

Figure 12 shows the consequent modifications to the line selector switches as compared with Figure 6.

Figure 13 illustrates a way of feeding high frequency voltage to the lamp circuits.

There are several simple ways in which failure of a lamp can be detected at the head itself. All of the ways considered here rely on measuring the current taken by the lamp after the lamp transformer. (This has the advantage that by simply changing the current rating of the measuring device the method is immediately suitable for older types of traffic signal head, which use mains voltage bulbs without a transformer).

Figure 1 shows the circuit diagram of a straight forward three aspect traffic signal head of the modern type. Figure 2 shows the proposed circuit modification for current measuring to take place. Since one lamp should always be illuminated, the current sensor should detect current at all times, except during the very brief changeover period when the aspect is changing, or when a lamp has failed. Thus if the current sensor 10 (Figure 2) is observed during the red period, the green period, and the leaving amber period all three lamps will have been checked.

The parameters of the required sensor are quite stringent. It must not detect very small currents, up to say 0.25A, as this could represent the leakage current flowing across a wet lamp holder. The sensor must reliably detect the current which flows in a lamp under worst case conditions, and yet not be damaged by the current taken by 2 lamps under the highest tolerances. It must not cause a voltage drop in the circuit which exceeds 0.4V, or thereabouts, at the nominal lamp current, such that a noticeable drop in illumination level is evident.

At the low current end of the range the minimum dimming voltage is 120V t 5V ie 115 worst case, but with a - 20% supply tolerance this reduces to 92V. Assuming a signal head with a long cable run, and a junction with 12 heads, this further reduces to 90.5V. Using a nominal bulb rating of 11.7V this produces a bulb voltage of 4.41V, and causes a lamp current of 2.4A to flow. At the high current end of the range with a head close to the controller, and a + 15% supply tolerance the voltage to the head will be 276V, and the bulb voltage will be 13.45V, giving a lamp current of 4.2A. The device must not be damaged by a current of twice this value during the red-amber period, and a continuous application of this current, although it may damage the device must not cause an open circuit in the event of the controller sticking on the red-amber period.The nominal lamp current is 4.OA.

The simplest, and also possibly the cheapest and most reliable, method of current sensing is to use a current sensing relay as the current sensor 10. The coil of the relay is placed in the current sensor position, and the relay is energised by the lamp current. In the case of a relay the strict requirements of the current specification are very much eased by the fact that both devices normally require a surge current at switch-on. Since the lamp circuit provides a surge of up to 16.6A rms in the first half cycle, which reduces to the nominal value in 14 half cycles or 140ms, and at 120V nominal supply the initial surge is up to 8.6A rms decaying to the nominal value in approximately 26 half cycles or 260mS, the relay is energised in some 12mS during the surge period.In fact, if the supply voltage is further reduced down to a level of only 50V, the relay is energised during the surge period, and is released some 2 seconds later as the surge decays following the heating up of the lamp filament. At supply voltage of 80V, well below the 90.5V absolute minimum, the relay operates reliably at each switching, provided that the lamp has had a period of 2 seconds off, in which to cool down.

The alternative to a relay current sensor is a solid state device. The production cost of such a device is likely to be considerably higher than a relay because of the complexity of supplying the device from three different supply lines, and it will require three more connections to be made in each signal head. The overall life of such a device will not be greatly improved since a contact output will still be necessary, and a relay will have to be provided in the circuit, although it will be smaller and cheaper than the current sensing relay. An outline of the type of circuit needed is shown in Figure 3, but improvements would have to be made to prevent incorrect aspects being shown in the event of the failure of one of the supply diodes 12.However much improvement is made, the possibility must still exist of such a failure occurring, and must remain a fundamental problem of this type of circuit, unless three entirely separate circuits are used, or three independent transformers, with the consequent cost penalty.

Figure 4 shows the arrangement at the traffic signal head end, and Figure 5 shows a block diagram of the circuit at the controller and for the transmission of information using the signal head supply lines. When all the signal head lamps are functioning correctly, the current detection relay (CR) (10 in Figure 2) will be almost continually energised. If a lamp fails, then at the time when the lamp should be lit, no current will flow and the relay CR will be de-energised. The two normally closed contacts 14 in Figure 4 will then connect the two capacitors 16 between the Red and Green lines and the neutral as shown. The capacitors have values of the order of 0.1 F and are 600V dc working. When a line is energised they will-pass less than lOmA to the neutral (lamp current is approximately 280mA).

The controller circuit is connected to all the "dead" red or green signal lines at the same time, by the line selector switch 18 in Figure 5. Only those lines which are energised are disconnected from the system. The line selector switch circuit contains protection to prevent any line being energised inadvertently by another line, in the unlikely event of failure of the selector switch itself.

All the dead lines are connected to the output of an oscillator circuit 20 which is producing a sine wave oscillation at a frequency of 20KHz. The circuit is completed by an interrogation selection switch 22, and is connected back to the mains neutral via a current measuring resistor 24. Across this resistor are connected amplifier and filter circuits 26 and 28 and a level detection circuit 30 which operates a relay, 32.

When all the lamps are operating correctly the load on the 20KHz signal will consist solely of a number of lamp transformer circuits, one in each head. At 200KHz, the impedance of these lamp circuits is high, and the current as measured by the voltage across the resistor 24 is low. When a lamp has failed, a capacitor is now switched across the lines at the signal head and its impedance, although high at 50Hz is very low at 20KHz, and a comparatively large current flows. This produces a large change in the signal at the output of the filter 28 (around 15 times greater with an 8 head system) which is detected and used to produce an output. The filter is included partly to remove unwanted noise and frequencies appearing in the system, but mainly to remove the 50Hz components which appears on the current signal.Noise and transient suppression is mainly provided by delays on the output circuit of around 200mS, since fast response is neither necessary nor desirable. Even though all the lines selected on to the system are dead, each head will still contain one illuminated lamp, and the current for this lamp returns to the controller through the shared neutral. This produces a 50Hz voltage drop along the neutral wire which appears superimposed on the voltage measured across the current measuring resistor.

Because of the wide separation between the mains frequency (50Hz) and the measuring frequency (20KHz), the filtering required to remove the unwanted 50Hz signal is simple.

Even a straight forward resistor/capacitor filter, having a roll off of 20dB/decade, will provide 50dB of rejection, which is adequate for this purpose. The filter also includes a low pass network, to remove any unwanted high frequencies, to protect against the unlikely event of these arising due to pick-up on the cables or high frequency signals appearing on the neutral.

The main advantages of this method are that since the current measurement is performed in each head only a single lamp tolerance has to be accommodated, and this is well within the range of the current measuring device. This makes the head unit, which is the quantity item, much cheaper. Because the equipment measures current used by the lamp, and transmits the information back on the line of a different aspect, the system will detect failures due to lamp transformers or the loss of the line as well as the failure of the switching relay. It cannot however detect a failure if all lines to a head are disconnected, or if the neutral is disconnected, although the consequences of these two occurrences are more likely to be reported by the public, than a single lamp failure.

Figure 6 shows the line selector switches, and Figure 7 shows the monitoring and logic circuits use in the protytype equipment.

In the selector switches the red and green lines of each signal phase are presented to the opposite sides of a changeover contact GA to GD (Figure 6) of a relay associated with that signal phase. The relay 34A is driven indirectly from the green line so that the relay is energised when the phase is green. Indirect drive of the relay is necessary because of the range of voltage on the line due to the dimming arrangements. This arrangement ensures that the monitoring circuit is connected to the dead signal line (except for a few milliseconds at the start of each green). From this relay the now common line passes through a capacitor 36A.This capacitor serves two purposes, it provides dc isolation between the circuit and the lamp lines, although this function could be more simply achieved by having one common capacitor, but more important this capacitor while only having an impedance of 8 ohms at 20KHz has an impedance of 3.2K ohms at 50Hz, and is too high to allow a signal lamp to light. This capacitor acts as an additional safeguard to prevent a lamp being energised from a different line in the event of the other two major safeguards failing.

The circuit then passes through the MON relay 38A into the lamp monitoring unit. MON performs 3 functions. It prevents the mains from appearing in the monitoring circuit; it provides the correct periods at which monitoring should take place, to ensure that all lamps are monitored while not allowing the stage change points to appear as lamp failures (interrogation selection switch 22); and it provides isolation between the signal feed lines of the different phases. MON is driven by a simple timing circuit which causes the relay to close for a period of approximately 2 seconds, 0.5 secs after the commencement or termination of the green on each stage. An additional circuit is provided as a final safeguard against the incorrect connection of live to dead signal feed lines. A voltage sensitive circuit is connected to the monitoring circuit side of the MON contacts.If mains (or dimmed) voltage appears on the line the circuit operates immediately and causes MON to drop out before any incorrect lamps have had a chance to achieve full brightness in the event of the failure of both of the series capacitors. The circuit is retained preventing MON being energised again until the equipment has been examined. An indication of the operation of the circuit would be given to a remote observer.

Figure 7 shows the monitoring circuit. IC1 acts as an oscillator producing a sine wave oscillation at 20 KHz. This signal is passed to IC2 which is a 3W amplifier, which provides sufficient drive to power the signal feed lines. IC3 amplifies the voltage dropped across the current measuring resistor 24, and the signal is passed to IC4 which provides the filtering to remove the unwanted 50 Hz signals. This is a very simple filter, but is all that is necessary to achieve sufficient 50Hz rejection. The output of this amplifier is rectified and used to drive the output relay 32. A relay is used for the output since the whole of the circuit must be connected to the mains neutral, and the relay provides the necessary isolation. To complete the system a memory may be required after the relay stage to store the information, so that the operator does not have to observe the monitor output for a complete cycle.

Considering the operation in a little more detail. Assume initially Phase A becomes green, and all other phases are red. Relay GA (Figure 6) will be energised. Phase A monitor line will now be connected to the Phase A red line, all other monitor lines in the other Phases will be connected to the green line. Approximately 0.5 secs later the MON relay (contact 38A) will be energised. This short period allows the system to stabilise after the change of stage, and strictly need only be a few milli seconds. The MON relay is now energised for a 2 sec period, while the 20 KHz signal is fed onto all the signal lines. If any of the red lamps on Phases B to D or the green lamps on Phase A have failed the system will detect the presence of a failure capacitor and will give a failure output. After 2 secs MON will drop out.At the end of the stage GA relay will be de-energised and the monitoring line for Phase A will be connected to the green line as on all the other Phases. 0.5 secs later the MON relay will be energised. This time the lamps monitored will be the red lamps on Phases B to D for a second time and the amber lamps on Phase A. This process is then repeated for all the other phases. Table 1 shows the order in which the lamps are checked for a 4 phase controller.

TABLE 1 Time Period Phase A Phase B Phase C Phase D Red Amb Grn Red Amb Grn Red Amb Grn Red Amb Grn Phase A Start X X X X Phase A End X X X Phase B Start X X X X Phase B End X X X X Phase C Start X X X X Phase C End X X X X Phase D Start X X X X Phase D End X X X X From Table 1 it can be seen that on a 4 phase controller each red lamp will be checked 6 times per complete cycle, and the amber and green lamps will be checked once per cycle.

One final piece of circuit which would be included and which is not shown on the diagrams is a monitor test function. In the head based equipment the relay is operated in the fail safe mode, ie a failure of the relay will cause a failure indication to be given. It is not possible to detect the failure of a capacitor, but the use of a high quality component should minimise this possibility. Failures of the monitor circuit itself however are much more difficult to make fail safe, and the simpler and least expensive alternative will be to include a monitor test facility. This will consist of the ability to connect a capacitor across the monitoring circuit, to simulate the operation of the failure detection and output circuits.

The power supplies to the equipment have to be separate from other equipment supplies since, as explained earlier, it is necessary to connect the complete system to the supply neutral. To reduce the cost a small transformer, rectifier, and smoothing circuits are used which can supply the power requirements of both the green relays and the monitoring system. The unregulated supply is used to operate the relays, and a regulator on the monitor circuit board provides the 18V supply for the monitoring system.

The system description given so far has assumed straight forward 3 light heads only are fitted to the system. To cater for the needs of pedestrian signals and green arrows some modifications are necessary to the system.

Where pedestrian signals are provided, either where they are connected in parallel with the signals of the opposing stage, or as a separate phase, they will be provided with a sensor relay as for a main signal, and the failure capacitors can be connected in the same way.

Where they are connected in parallel with opposing stage signals the controller wiring will ensure that they are connected to the correct phase lines, where a separate pedestrian stage is provided it will have its own phase relay as with the other traffic stages. The only modification required therefore is to remove the amber sensing pulse. This is done by only monitoring such phases at the commencement of the green, and not after its termination.

The amber monitoring period is therefore omitted.

This would be achieved by having 2 MON relays, MON 1 and MON 2. With a system configured such that the equipment boards were designed for 4 phases, each relay would have 3 contacts (the standard configuration of the relays used in the prototypes). Where all 4 phases were normal traffic phases the 2 relays would be driven in parallel as a common relay. Where 1 or more phases had the pedestrian (or green arrow) facilities MON 2 would only operate on the commencement pulse.

With parallel connected pedestrian signals this will mean that, on the traffic stage concerned, it will not be possible to monitor the amber lamps with this arrangement. If this restriction is not acceptable two alternatives can be considered. Firstly the additional MON 2 relay is not used, and every pulse is monitored. However, in the pedestrian signal head not only is a current sensor fitted, but two voltage relays 40A, 40B are also fitted as in Figure 8. These relays ensure that the failure capacitors 42A, 42B are only connected at the correct time, and false amber failures are not indicated. This has the disadvantages that the method will only detect true lamp failure, the failure of the driving voltage for any reason will be undetected as the voltage relay will remain de-energised.Also since such a wide voltage range is employed for dimming, the voltage relays will probably only operate reliably while the lamps are bright, unless more complex measures are taken. The failure of a pedestrian lamp while they are dimmed could remain undetected until full brightness was resumed.

The second alternative can only be used where the pedestrian heads are mounted on a common pole with other signal heads. This method makes both the arrangements at the head and at the selection logic a little more complex. At the signal head the failure capacitors would not be wired directly on to the feed lines, but taken to the top of the signal pole where they would be wired on to a different phase. Figure 9 shows the signal phasing arrangements and the monitor periods on a 2 stage/phase controller which has pedestrian signals connected in parallel with phase A. The failure capacitors for the pedestrian head would be connected to the red and green lines of phase B. At the monitor circuit, phase A would need none of its pulses to be inhibited, but on phase 2 onstead of inhibiting both amber pulses, ie pulses 2 and 4 in Figure 9, only pulse number 2 would be inhibited.For phase B pulse No 2 only provides a second check on the red lamp while the phase A amber is being checked. Thus all lamps would be checked with the addition of a small amount of extra logic, and a slightly more difficult installation at the signal heads. The logic becomes a little more complx where more than 2 stages are required, and more than one pedestrian facility of this type is provided.

The modifications for green arrows are straight forward where 2 green arrows are fitted in place of a full green. One of the green arrows is wired into the circuit exactly as if it were a full green. A second current sensing relay is then provided solely for the second green arrow. The contact of this relay is then placed in parallel with the contact switching the failure capacitor on to the red line only. In this way its failure capacitor will only be monitored when the stage should be green.

The modifications for green arrows associated with left turn filters and right turn stages is again a little more complex. The simplest case is the left turn filter which comes on the same time as a green on an opposing stage. Figure 10 shows the sequence. A separate current sensing relay, 44 in Figure 11, is provided in the head for the green arrow, and the failure capacitor is connected to the full green in that head, which in Figure 10 would be phase B. If now pulse No 2 is inhibited in the same way as for the pedestrian heads, then all the lamps will be checked with no false indications, only the second check on the red of phase B being lost. If the left turn filter is demand dependent then it will be allocated a phase in the controller, and/or the monitoring system, and it will then be treated in a similar way as the right turn stage mentioned below.

Where a right turn stage green arrow is fitted the simplest method of detecting all lamp failures is to provide a second current sensing relay which is fitted for the green arrow. The failure capacitors for both the main head and the green arrow would be fitted between the red line to neutral and the green arrow line to neutral. The selector switch relay would then be driven from the green arrow phase and would switch the system between the main phase red and the green arrow phase green. This arrangement is shown in Figures 11 and 12.

Figure 11 shows the head arrangements, and Figure 12 shows the line selector logic which replaces the normal circuit (shown in Figure 6). The disadvantage of this circuit is that the main full green will only be tested when the right turn stage appears, which may not be as frequent as the main stage.

The equipment has so far been given thorough laboratory tests and is in the process of being evaluated while in service. All tests todate have been carried out on the relay type of current sensor.

A number of specially wound relays were obtained which meet all the requirements laid down, and considerably exceed some of them. The relay has been tested over a temperature range of -15 C to +550C.

The minimum supply voltage which will operate the device is 80V, well below the absolute minimum voltage of 90.5V. The device has also been tested with two lamps switched on at the maximum rated supply of 276V and was found to be satisfactory. One factor which has only been subjectively tested is the effect of the voltage drop across the relay on the level of illumination of the signal lamp. Observations by eye show that the loss in light output is only marginal, but if all other tests of the complete system prove to be entirely satisfactory, full photometric tests will be necessary.The voltage drop across the relay at nominal supply voltage is approximately 0.3V, so that if the photometric tests show that this drop is unacceptable the electronic current sensing circuits will need to be developed, since it is unlikely that the voltage drop could be reduced any further using a relay.

Initial tests were concerned with determining a suitable value of capacitor which is a measuring mode provides a low impedence compared to the impedence of the lamp transformer, and yet when the capacitor is connected across the mains the current which flows is small compared with the lamp current. Measurements of the full system showed that a considerable frequency drift could be tolerated on the oscillators. Tests have been simulated to investigate the effects of noise on the system and the effects of 50Hz current flowing in the neutral.

The system was set up with seven head equipments at the end of 50m of 0.75mm2 cable, which is considerably thinner than the cable normally used on traffic signal installations. In the tests which followed, initially a 50Hz current of 10A was made to flow in the neutral line. This is a current which would flow if some 45 heads were included on the junction, and far exceeds any normal situation. Although the signal appearing across the current measuring resistor contained more unwanted signal than wanted signal, the simple filter had no trouble in distinguishing between the two signals. Secondly the neutral current to an electric drill, producing large noise spikes, was passed along the common neutral. Again no problems were encounered.Lastly a highly inductive load of SA, consisting of a large 1.6KVA auto transformer, feeding a SA load was placed with its neutral current flowing through the common neutral line, and the supply was switched on and off at frequent intervals. No false or incorrect operations of the system could be induced.

The equipment has also been tested over a temperature range of - 15"C to +55"C and all tests simulating the operation of the circuit under normal circumstances have proved satisfactory.

Initially improvements can be made in the monitoring circuits. The existing circuit (shown in Figure 7) has a number of redundant components. In the design of the next circuit for further testing in a full trial, these redundant components will be removed, with consequent reductions in the cost of the equipment and improvement in its performance and reliability. As an example the 3W main amplifier used did not have sufificent data available to allow the oscillator to be made as an integral part of the amplifier, and a separate operational amplifier was included. This has introduced more distortion and noise into the system, and because of the excessively high gains which are now available some stability problems were also encountered. By using a main amplifier with more data and specification available an integral oscillator will be used, saving one IC chip and several components.The amplifier IC3 which amplifies the current signal developed across the measuring resistor is no longer necessary. Initial circuits contained a differential input, since the current measuring resistor was fitted at a different point in the circuit. This arrangement, although having certain advantages, had drift and stability problems and was abandoned. The gain in this amplifier can now be transferred to the main filter, and IC3 and its associated components removed.

All the above improvements are small details of the same basic design. It should however be possible to remove the majority of the switching logic if a transformer, 46 in Figure 13, could be introduced into the neutral line feeding all signal heads, through which the 20KHz measuring signal could be introduced onto the signal lines. The path would be completed at the controller end by a capacitor placed between each phase red and green lines to earth.

When all the lamps were operating correctly the circuit would have a relatively high impedence due to the lamp transformers at the head end. If a lamp failed the failure capacitor would complete a low impedance path and the current drawn from the monitoring circuit would increase. Advantages of the circuit would be the loss of the line selection switches, and removal of the exceedingly slight chance that the wrong lines could be corrected together to produce a wrong signal indication. Disadvantages are the cost of the capacitors and of the transformer which will need to pass all the neutral current, and which must be highly reliable since if this winding becomes open circuit the neutral would be disconnected and multi-aspect indications could be given to the traffic.

WHAT I CLAIM IS 1. Apparatus for monitoring signal lamps for failure thereof having current responsive means operable by the electric current through at least one of the signal lamps in at least one signal head; capacitors connectable to the power supply lines to said signal lamps; capacitor switch means operable by said current responsive means to connect the capacitors as aforesaid when less than a predetermined minimum lamp current flows through any one of said signal lamps; oscillator means for applying a voltage of at least a high audio frequency across the circuit points to which said capacitors are connected; and current measuring means for measuring the resultant current so large that a large capacitor current is measured by said current measuring means at said high frequency at some period during a cycle of signal aspects if less than said minimum lamp current can flow through any one of the signal lamps in said signal head, and substantially lower current is measured at said high frequency if said minimum lamp current is exceeded for all lamps; said capacitors being

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. Initial tests were concerned with determining a suitable value of capacitor which is a measuring mode provides a low impedence compared to the impedence of the lamp transformer, and yet when the capacitor is connected across the mains the current which flows is small compared with the lamp current. Measurements of the full system showed that a considerable frequency drift could be tolerated on the oscillators. Tests have been simulated to investigate the effects of noise on the system and the effects of 50Hz current flowing in the neutral. The system was set up with seven head equipments at the end of 50m of 0.75mm2 cable, which is considerably thinner than the cable normally used on traffic signal installations. In the tests which followed, initially a 50Hz current of 10A was made to flow in the neutral line. This is a current which would flow if some 45 heads were included on the junction, and far exceeds any normal situation. Although the signal appearing across the current measuring resistor contained more unwanted signal than wanted signal, the simple filter had no trouble in distinguishing between the two signals. Secondly the neutral current to an electric drill, producing large noise spikes, was passed along the common neutral. Again no problems were encounered.Lastly a highly inductive load of SA, consisting of a large 1.6KVA auto transformer, feeding a SA load was placed with its neutral current flowing through the common neutral line, and the supply was switched on and off at frequent intervals. No false or incorrect operations of the system could be induced. The equipment has also been tested over a temperature range of - 15"C to +55"C and all tests simulating the operation of the circuit under normal circumstances have proved satisfactory. Initially improvements can be made in the monitoring circuits. The existing circuit (shown in Figure 7) has a number of redundant components. In the design of the next circuit for further testing in a full trial, these redundant components will be removed, with consequent reductions in the cost of the equipment and improvement in its performance and reliability. As an example the 3W main amplifier used did not have sufificent data available to allow the oscillator to be made as an integral part of the amplifier, and a separate operational amplifier was included. This has introduced more distortion and noise into the system, and because of the excessively high gains which are now available some stability problems were also encountered. By using a main amplifier with more data and specification available an integral oscillator will be used, saving one IC chip and several components.The amplifier IC3 which amplifies the current signal developed across the measuring resistor is no longer necessary. Initial circuits contained a differential input, since the current measuring resistor was fitted at a different point in the circuit. This arrangement, although having certain advantages, had drift and stability problems and was abandoned. The gain in this amplifier can now be transferred to the main filter, and IC3 and its associated components removed. All the above improvements are small details of the same basic design. It should however be possible to remove the majority of the switching logic if a transformer, 46 in Figure 13, could be introduced into the neutral line feeding all signal heads, through which the 20KHz measuring signal could be introduced onto the signal lines. The path would be completed at the controller end by a capacitor placed between each phase red and green lines to earth. When all the lamps were operating correctly the circuit would have a relatively high impedence due to the lamp transformers at the head end. If a lamp failed the failure capacitor would complete a low impedance path and the current drawn from the monitoring circuit would increase. Advantages of the circuit would be the loss of the line selection switches, and removal of the exceedingly slight chance that the wrong lines could be corrected together to produce a wrong signal indication. Disadvantages are the cost of the capacitors and of the transformer which will need to pass all the neutral current, and which must be highly reliable since if this winding becomes open circuit the neutral would be disconnected and multi-aspect indications could be given to the traffic. WHAT I CLAIM IS

1. Apparatus for monitoring signal lamps for failure thereof having current responsive means operable by the electric current through at least one of the signal lamps in at least one signal head; capacitors connectable to the power supply lines to said signal lamps; capacitor switch means operable by said current responsive means to connect the capacitors as aforesaid when less than a predetermined minimum lamp current flows through any one of said signal lamps; oscillator means for applying a voltage of at least a high audio frequency across the circuit points to which said capacitors are connected; and current measuring means for measuring the resultant current so large that a large capacitor current is measured by said current measuring means at said high frequency at some period during a cycle of signal aspects if less than said minimum lamp current can flow through any one of the signal lamps in said signal head, and substantially lower current is measured at said high frequency if said minimum lamp current is exceeded for all lamps; said capacitors being

chosen of such capacitance as to present relatively high impedance at the frequency of said power supply, and relatively low impedance at said high frequency.

2. Apparatus according to claim 1 in which the high frequency voltage is applied across the capacitors through the supply lines to the signal lamps.

3. Apparatus according to claim 1 or claim 2 in which the current responsive means is a current relay and the capacitor switch means comprises contacts operated by said current relay.

4. Apparatus according to any one of the preceding claims in which the high frequency is applied to the capacitors through transformer means connected into a common power supply line to the signal lamps.

5. Apparatus according to claim 4 in which the common power lead is the neutral line.

6. Apparatus according to any one of the preceding claims including selector switch means whereby the oscillator means and/or current measuring means can be switched to measure in turn any currents through the capacitors associated with each of a plurality of signal heads.

7. Apparatus substantially as hereinbefore described with reference to any of Figures 2 to 13 inclusive of the drawings filed with the Provisional Specification.