GB2157903A

GB2157903A - Lamp monitoring apparatus

Info

Publication number: GB2157903A
Application number: GB08410448A
Authority: GB
Inventors: John P Burgess; Eric W Burdis
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1984-04-24
Filing date: 1984-04-24
Publication date: 1985-10-30
Also published as: GB8410448D0; GB2157903B

Abstract

The apparatus monitors at least one group of lamps e.g. traffic lights SP controlled by a local controller CT, and is provided with measuring means for measuring on a regular sampling basis the value of electrical current supplied by the controller to each group of lamps. The measuring means includes current transformers T1 T2 T3 a multiplexer MX and an analog- digital convertor AD. A comparator (COMP) compares the measured values with the value stored in a memory (SRV) for that group. A fault detector (FD) generates a fault signal when successive measured values differ from the stored value. <IMAGE>

Description

SPECIFICATION Lamp monitoring apparatus The present invention relates to Lamp Monitoring apparatus for monitoring the state of signal lamps in a traffic light system.

Known monitoring techniques use a detection unit which is fitted into each traffic signal head and comprises a current transformer and relay. Such detection units are connected by cable to a monitoring unit at the traffic light controller. A drawback of this type of installa- tion is that work has to be carried out at each traffic light pole, and unless spare cable cores are available, extra cables have to be laid to each pole, which proves to be a costly exercise.

The invention overcomes this drawback by utilising apparatus which is installed at the traffic light controller and not at the poles, and therefore does not require extra cables to be laid.

However, the introduction of apparatus at the traffic light controller introduces problems which the invention seeks to overcome. The problems are as follows: In normal arrangements of lamp feeder cables, groups of lamps are connected to one cable core at the controller. Sixteen or more lamps may be connected to one core, and therefore, failure of one lamp will cause only a small change in total current.

Mains voltage fluctuations cause signficiant changes in current, which in many cases can be larger than the change due to a lamp failure.

Ageing lamps, cables and associated circuitry cause significant changes in current, as will changes in environmental conditions.

The current to a lamp will change considerably during the first moments of applied voltage before settling to a stable value.

An aim of the present invention is to provide lamp monitoring apparatus which is economic to install and which overcomes the above mentioned problems in an efficient manner.

According to the present invention there is provided lamp monitoring apparatus for at least one goups of lamps or lamp wherein the group or groups are controlled by a controller local to the group or groups, characterised in that the apparatus is provided with measuring means for measuring on a regular basis the value of electrical current supplied by the controller to each group of lamps, comparison means for comparing the measured values with the value stored in a memory for that group, and fault detection means for generating a fault signal when successive measured values differ from the stored value.

An embodiment of the invention will now be described with reference to the accompanying drawing, which shows a schematic diagram of the lamp monitoring apparatus connected to a controller.

Referring to the drawing, the controller CT is shown having by way of example two Red, Amber, Green lamp feeds RAG and a Green lamp feed G, connected to the appropriate signal posts SP at the traffic junction.

The current to each group of lamps is measured on a regular basis. In general, the lamps are grouped in phases, a phase being the sequence of conditions applied to one or more streams of traffic which, during the cycle of the controller CT, receive identical signal light indications. The current for each phase is measured using a respective toroidal transformer T1, T2, T3. The current is measured at a defined point of the alternating waveform, typically at the peak of the positive half cycle. The output signals generated by the transformers T1, T2, T3 are applied to an analog-to-digital converter AD by way of a multiplexer MX. The output from the converter AD is sampled by a sampler circuit SM which is arranged to sample each output from the transformers T1, T2, T3 three times per second.The output signals from the sampler SM are applied to a Comparator COMP where they are compared with reference values stored in a memory. A significant difference indicates a probable fault, for example, a drop in current could be due to an open-circuited lamp and an increase in current could be due to a short-circuited lamp. If the fault persists for several successive samples the discrepancy is indicated by a fault detector FD, the output condition of which is signalled to a central office CO by way of a transmission unit TU.

Current surges or other short duration transient disturbances are ignored because a fault is only indicated if it persists over successive samples.

The reference values are initialised when, for example the controller CT is commissioned. The lamp monitoring apparatus is provided with an initialise control, such as a push-button PB which activates initialisation circuitry INT, to instruct the memory SRV which may be a programmable memory, to accept and store the values of current being monitored for each group of lamps. For threeaspect traffic signals, as least one of the Red, Amber or Green lamps should be on at all times and one reference value would be sufficient for that phase. Alternatively one value could be stored for each state of the controller CT, that is, for Red, Red-Amber, Green, Amber, to allow for differences between lamps and their associated circuitry.

To allow for small changes in current due to ageing of components, the apparatus accepts a tolerance on each value before indicating a fault, furthermore, the reference values are allowed to change gradually to track small variations in current. A modifier MOD is used to change the stored values by the application of a moving average signal to the memory SRV.

To account for variations in voltage, the voltage is measured at the same time as the current to normalise the current to a common base. A voltage monitor VM applies a signal to a voltage compensator VC which also receives the output from the sampler SM and generates a current value signal which is applied to the comparator COMP for comparison purposes and to the initialisor circuit INT for use when the memory SRV is being loaded with the initial information at the commissioning stage. The voltage compensation circuit can conveniently be used to detect when the lights are dimmed at night and to enable the current reading to be normalised.

Alternatively a separate set of values could be stored in the memory SRV for night time use.

The apparatus may be provided with a link between the controller CT and the comparator COMP to convey the controller state to enable the apparatus to ascertain whether the lamps in a particular group require to be illuminated at that instant. Alternatively the lamps of that group can be monitored by the voltage monitors VM to detect when the lamps should be illuminated.

The invention described, is not limited to traffic lights, it may be used to monitor pedestrian crossing lights and street lights for example.

Claims

1. Lamp monitoring apparatus for at least one group of lamps or lamp wherein the group or groups are controlled by a controller local to the group or groups, characterised in that the apparatus is provided with measuring means for measuring on a regular basis the value of electrical current supplied by the controller to each group of lamps, comparison means for comparing the measured values with a value stored in a memory for that group, and fault detection means for generating a fault signal when successive measured values differ from the stored value.

2. Lamp monitoring apparatus as claimed in claim 1 wherein the values stored in the memory are gradually modified by a modifying circuit to compensate for changes in current due to ageing of the lamps, or associated circuitry.

3. Lamp monitoring apparatus as claimed in claim 1 or 2 wherein a voltage monitoring means and a compensation circuit are provided which monitor voltage fluctuations at the same time as the current is monitored and the compensation circuit normalizes the monitored value of current in accordance with the voltage fluctuations.

4. Lamp monitoring apparatus as claimed in claims 1, 2 or 3 wherein the memory is a programmable memory, and initialisation circuitry is provided which feeds the initial electrical current values prevailing at the monitored groups of lamps and causes those values to be stored in the memory for subsequent comparison with the measured values.

5. Lamp monitoring apparatus as claimed in any preceding claim wherein the controller is a traffic light controller and each group of lamps is one phase of a traffic light set.

6. Lamp monitoring apparatus substantially as herein before described with reference to the accompanying drawing.