GB2150372A

GB2150372A - Lamp failure detector

Info

Publication number: GB2150372A
Application number: GB08331551A
Authority: GB
Inventors: John Samuel Coulter; William Ritchie
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1983-11-25
Filing date: 1983-11-25
Publication date: 1985-06-26
Also published as: NL8403575A; DE3441824A1; NL190588C; NL190588B; GB8331551D0; GB2150372B; DE3441824C2

Abstract

A lamp failure detector, preferably for a traffic light installation, has a current transformer (1) for measuring the return current from all lamps. This current value is sampled at intervals by sampling means (14) and compared 10 with the value obtained at the same instant in the previous cycle of operations by comparison means (16). The output of the comparison means is applied to output means (18) which gives an alarm indication if a current change in excess of a predetermined value is indicated by the output of the comparator. <IMAGE>

Description

SPECIFICATION Lamp failure detector This invention relates to a lamp failure detector for light systems in which a plurality of lamps operate in cyclically-repeated combinations. Preferably the detector is for use with a traffic light installation for road traffic control.

Traffic light installations commonly require a fairly large number of lamps which are switched on and off in cyclically-repeated combinations.

Where individual functions are provided with their own autonomous controller it is common for lamps to be checked and/or replaced on a regular basis. This requires a considerable expenditure of lamps and manpower.

It is now common for a number of junctions, sometimes a very large number, to be centrally controlled by a computer installation, and in such system communications exist between the control units at each individual junction and a central control room. It is therefore now possible to provide warning to the control room when a lamp failure occurs, so that remedial action may be taken. In the past, however, lamp failure detectors for fitting to existing outstation control units have necessitated some alteration of the wiring at the installation.

It is an object of the invention to provide a simple lamp failure detector for light systems in which a plurality of lamps operate in cyclically-repeated combinations.

According to the present invention there is provided a lamp failure detector for a light system in which a plurality of lamps operated in cyclically-repeated combinations, which includes means for measuring the current taken by all lamps which are energised at any instant during a cycle of operation, sampling means operable to sample said current at intervals throughout the cycle, comparison means operable to compare the value of said current with that obtained at the same instant in a previous cycle of operation, and output means operable to provide a fault indication if the output of the comparator indicates that value of said current has changed by at least a predetermined value.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is block schematic diagram of an installation incorporating the invention; Figure 2 is a flow chart illustrating the opartion of the invention; and Figure 3 illustrates the current pattern in a typical installation.

Referring now to Figure 1, a typical traffic light installation is centred around the signal controller 10. This contains the circuitry necessary to switch individual signal lamps on and off as required. The signal controller may be self-contained, determining, its own sequence of operations and timing, or it may be operating under the control of a remote master controller. The signal controller is shown as having three sets of lamps connected to it and controlled separately, namely red, amber and green. A current transformer 11 is connected in the common return connection from all the lamps so as to monitor the total current taken by the installation at any instant. In its simplest form the current transformer 10 may comprise a ring-shaped core having the common return connection passing through its centre to form the primary winding and having a multiple-turn secondary winding.

Such a transformer requires minimum disturbance to an existing installation.

The output from the current transformer is applied to a rectifier and filtering circuit 12, and then through an analogue-to-digital converter 13 to a sampling circuit 14. The output of the sampling circuit is applied to a temporary store 15, the output of which is connected to a comparator 16. This comparator is also connected to a second store 17, the "previous value" store.

A check controller 18 controls and synchronises the operation of the sampling circuit 14, the comparator 16 and the two stores 15 and 17. The controller 18 also forms the output means and has an alarm output line 19, and possibly a control input 20 from an external control system.

Figure 2 illustrates the operation of part of the arrangement of Figure 1, namely the sampling means 14, stores 15 and 17, and the comparator 16. The operation of these units is synchronised with the operating sequence of the traffic lights so that the check cycle is performed at least once during each different step of the sequence. One check cycle will be described.

The first step of the cycle is to sample the digitised value of the current measured by the current transformer 11 of Figure 1. The sample is taken by the sampling circuit 14. A second sample is taken a short time later and the two are compared. This is done to ensure that no switching or other transients are present. If the two samples are different, then a further sample is taken. When two latest samples agree, then the value of one of these is stored in the temporary store 15.

The comparator 16 then compares the value of the sample in the temporary store 15 with a value in the "previous value" store 17, which contains the sample taken at the same point in the previous cycle of the sequence. If the present and previous samples are the same, to within say 3%, then the present sample is put into the store 17 in place of the previous sample. The circuit is then available for checking the next phase in the sequence.

If the present and previous samples differ by more than 3% then this is taken as indicating the failure of one or more lamps, and an alarm indication is given. The phase of the sequence and the colour of the lamp which is faulty may be determined as will be explained below.

Modern traffic light installations include arrangements for dimming the lamps at night by reducing the energising voltage. The reduction usually applied is greater than 30%, and is clearly applied to all lamps simultaneously, and hence in such installations the comparison performed by the check circuit is required to respond only to variations between 3% and 30%. It is also advisable to apply to the check circuit an indication of the supply voltage, so that variations in current due to dimming will be taken into account. The box 21 shown in broken outline in Figure 1 is for this purpose.

The detection of a change of between 3% and 30% in the sampled current is sufficient to indicate the failure of one or more lamps in a group of up to 30, all illuminated at the same time, and is therefore adequate even for very large installations.

Changes in current values of less than 3% will be due almost certainly to ageing of the lamps, and hence may be ignored.

Synchronisation of the sampling and checking has already been mentioned. If the installation is one which always operates on the same sequence than it is only necessary to provide synchronisation once per cycle. If, however, the sequence is variable then more frequent synchronisation is required, for example each time a phase or stage change occurs. This applies to remotely controlled or vehicle-operated installations.

Figure 3 illustrates the current flow during a single cycle of a simple installation. It will be seen that at time t1 the green lamps on one road (G1) will be energised at the same time as the red lamps on the other road (R2). At time t2 the sequence starts to change, with amber (Al) replacing the green (G1). Since the number of lamps illuminated remains the same the total current should not change. However, the current will increase after a short pause as amber on one road (Al) is combined with red and amber (R2 + A2) on the other. At time t3 the amber (Al) will change to red (R1) and the red and amber (R2 + A2) will change to green (G2). At time t4 the sequence will start to change back to the initial situation. The detection faults is shown by way of example by comparing the "present" and "previous" values of current at time t1 and t2 of Figure 3.If the values at both times are the same in two successive cycles then no faults exist in any lamps operative during this part of the sequence (i.e. G1, R2 or Al). If, however, the values for two successive cycles are the same at time tl but different at time t2 then the fault must be in the amber lamps Al. Equally, if the values for two successive cycles are different at time t1 but the same at time t2, then the fault must be in the green lamps G1. If the values for two successive cycles are different at each time, then the fault lies in the red lamps R2. Similar comparison of present and previous current values at times t3 and t4 enable faults affecting the second phase of the operating sequence be identified.

Whilst the indication of a fault does not identify the actual lamp which is faulty, it may provide sufficient information to enable a maintenance engineer to locate the fault easily without having to check every lamp through the entire sequence.

Clearly if the installation forms part of a large computer-controlled installation then the fault signal from the check controller will be passed to a central control desk. However if the installation operates under its own control it is possible either to provide an indication at the controller (such as an external lamp on a control pillar), or to arrange for automatic dialing over the telephone network to report the failure.

Whilst the above description is concerned with traffic light installations, it will be apparent that the invention may be applied to other light systems in which a plurality of lamps operate in cyclically-repeated combinations.

Claims

1. A lamp failure detector for a light system in which a plurality of lamps operate in cyclically-repeated combinations, which includes means for measuring the current taken by all lamps which are energised at any instant during a cycle of operation, sampling means operable to sample said current at intervals throughout the cycle, comparison means operable to compare the value of said current with that obtained at the same instant in a previous cycle of operation, and output means operable to provide a fault indication if the output of the comparator indicates that the value of the current has changed by at least a predetermined amount.

2. A detector as claimed in Claim 1 in which the measuring means includes a current transformer operable to measure the current flowing in the common return connection from all of the plurality of lamps.

3. A detector as claimed in Claim 2 in which the current transformer has a toroidal core having the common return connection passing through its centre.

4. A detector as claimed in any one of Claims 1 to 3 which includes an analogue-to-digital converter operable to digitise the measured current values.

5. A detector as claimed in any one of Claims 1 to 4 in which the sampling means is operable to sample the current repeatedly until at least two samples have substantially the same value.

6. A detector as claimed in Claim 5 in which a sample is applied to a temporary store prior to its application to the comparison means.

7. A detector as claimed in any one of the preceding claims which includes a store in which the latest sampled value of current are stored after comparison with values from a previous cycle.

8. A detector as claimed in any one of Claims 1 to 7 in which a sampled value of current is compared with that obtained at the same instant in the immediately preceding cycle of operation.

9. A detector as claimed in any one of the preceding claims in which current values are sampled after each change in the combination of lamps.

10. A detector as claimed in any one of Claims 1 to 9 in which the output means provides a fault indication to a control centre.

11. A detector as claimed in any one of the preceding claims in which the output means provides an indication of the identity of a faulty lamp.

12. A detector as claimed in any one of claims 1 to 11 which include means for measuring the supply voltage applied to the light system, the output means being operable to compensate for current changes caused by changes in the supply voltage.

13. A detector as claimed in any one of Claims 1 to 12 in which the plurality of lamps form a traffic light system for the control of vehicular traffic.

14. A lamp failure detector substantially as herein described with reference to the accompanying drawings.