GB2164766A

GB2164766A - Improvements in clocks

Info

Publication number: GB2164766A
Application number: GB08423553A
Authority: GB
Inventors: Peter William Loose
Original assignee: EUROP ELECTRONIC SYST Ltd; European Electronic Systems Ltd
Current assignee: EUROP ELECTRONIC SYST Ltd; European Electronic Systems Ltd
Priority date: 1984-09-18
Filing date: 1984-09-18
Publication date: 1986-03-26
Also published as: GB8423553D0

Abstract

The invention relates to a method of synchronising a master clock with a slave clock in a building in which an encoded time signal is generated in a master clock 12 and is transmitted to the slave clock 14 through the power supply 16. In this way, it is possible to avoid the need to rewire a building to connect the master lock with the slave clocks. <IMAGE>

Description

SPECIFICATION Improvements in clocks The present invention relates to a method of synchronising clocks in a building.

In large office blocks, airports, and other buildings, it often desired to provide clocks at several different locations which display accurately synchronised time. If the clocks have their own movements then they tend to drift apart. Furthermore in the event of a time change, such as at the end or begining of British Summer Time, it is laborious to adjust all the clocks individually and, during the change over, the clocks may not all agree with one another for several days.

For this reason, in such buildings, it is preferred to provide a master clock, which includes its own time keeping mechanism and a plurality of slave clocks, which are essentially repeaters and do not have a time keeping mechanism. The slave clocks are connected to receive time informaton from the master clock, usually over a separate wire connection, and to display the time information on a suitable readout. The display is commonly a digital display but because the public finds such displays unacceptable it has also been proposed to employ an analogue display dial. Such a system is described in the copending Patent Appln. No. 8405552.

Difficulty is encountered in the installation of such a system in an existing building due to the need to provide a wired connection between the master clock and the slave clocks.

It is often not possible to route such wires without damaging the walls of the building necessitating redecoration and the cost of the materials and the labour involved in the installation is also substantial.

With a view to mitigating the above disadvantages, the present invention provides a method of synchronising a master clock with a slave clock in a building which comprises generating an encoded time signal in the master clock, feeding the encoded time signal into a conductor of a power supply circuit in the building which also supplies power to at least all the slave clocks, and detecting the time signal on the power conductor in the slave clock, the slave clock providing a time indication determined by the encoded signal and thus synchronised with the time indication provided by the master clock.

The invention therefore proposes the use of the power supply wiring to communicate time information between the master clock and a slave clock in a building.

In the case of an existing building, it is possible to use the mains wiring for the power supply and for the communication of the time information. However, in the case of a new building, it may be convenient to provide special power supply wiring for the clocks only, during the course of wiring the building, whereupon the time data would be carried by the power supply wiring for the clocks only and not by the mains supply which is connected to other equipment in the building. In this case, the time signal may be of relatively high power as the risk of interference with other equipment is no longer present.

In the case of a mains supply, the encoded signal is modulated onto a a carrier of low power in order not to interfere with other equipment.

Because the signal is subject to severe attenuation, its range will be limited. When the building is large it may be necessary to provide relays or boosters to allow the signal to reach reliably all the clocks in the system.

In the case where relays are used, each relay must remodulate the time data onto a carrier of a different frequency, in order to prevent positive feedback to the input of the relay. The clocks in the system then need to be tuned to different frequencies.

The invention offers the advantage of being capable of installation without requiring alterations to the building's wiring and as all slave clocks are connected to the same power supply no special care need be taken when siting a slave clock. This would not be the case if a radio, infra-red or acoustic link were employed. Such links also suffer from problems of either causing interference to other equipment or are themselves susceptible to interference.

The invention will now be described further, by way of example, with reference to the accompanying drawing which shows in block form an system for implementing the method of the invention.

In the drawing, there is shown a master clock comprising the blocks with the box 10, and a slave clock comprising the blocks within the box 12, the boxes 10 and 12 being shown in dotted lines. The drawing also shows schematically a time signal booster which though shown as a separate unit within its own box 14, also drawn in dotted lines, may be incorporated in a slave clock. The master clock 10 and each of the slave clocks 12 are connected to a common power supply represented by the wires 16.

The master clock comprises a power supply circuit 20 which may for example step down the power supply voltage, rectify and smooth it to provide a d.c. voltage of the appropriate magnitude for the remaining circuits. A clock circuit 22 in the master clock generates time information. This may be a quartz clock, an atomic clock, or most conveniently a radio clock. The latter is preferred for its accuracy and also because no special steps need be taken at the end and begining of Summer Time, as the radio time signal, as transmitted from MSF Rugby, will itself change automatically.

The master clock 10 may itself have a display 24 as shown, but this is not essential as all the visible displays may be provided by slave clocks 14. The time signal produced by the clock circuit 22 is fed to the display 24, if present, and also to an encoder 26. The encoder converts the time signal into one suitable for detection by the slave clocks.

The encoded time signal is next modulated in the modulator 28 onto a carrier of frequency different from the power supply frequency. The modulation may be amplitude frquency or phase modulation. The modulated carrier is then superimposed onto the power supply for transmisson to the slave clocks.

Where the power suuply is a three phase mains supply, then the time signal carrier is modulated onto all the phases of the mains supply.

It should be understood that the the power supply wiring need not be a mains circuit and, in particular in the case of a new building, power supply wiring especially dedicated for the clocks may be provided to carry both power and time data.

The slave clocks comprise power supply circuits 30 serving the same purpose as the power supply circuit 20 in the master clock 10. The mains supply is also fed to a demodulator 38 which demodulates the frequency of the carrier and extracts the time signal. The time signal is then decoded in the circuit 36 which performs a complementary function to the encoder 26 and passes a signal to the display circuit 34.

The display circuit is preferably an analogue clock the hands of which are driven by a motor capable of rotating in both directions. A position transducer may be connected to the minutes hand to detect each time the hand passes a predetermined point on the dial, the output of the transducer being processed to indicate the actual time indicated. The actual time display and the desired time display as represented by the decoded time signal are compared and the resultant error signal is used to control the motor, thereby closing a negative feedback control loop which acts to ensure that the displayed time agrees with the decoded time signal.

If the carrier signal is to travel a long distance between the master clock and the slave clock, a booster 14 may be required to counteract the attenuation of the signal and in a large installation several such boosters may be required at different levels in the building. As earlier mentioned, a booster may be incorporated in a slave clock.

Claims

1. A method of synchronising a master clock with a slave clock in a building which comprises generating an encoded time signal in the master clock, feeding the encoded time signal into a conductor of a power supply circuit in the building which also supplies power to at least all the slave clocks, and detecting the time signal on the power conductor in the slave clock, the slave clock providing a time indication determined by the encoded signal and thus synchronised with the time indication provided by the master clock.

2. A method as claimed in claim 1, wherein the power supply circuit is a mains supply and the encoded signal is modulated onto a a carrier of low power in order not to interfere with other equipment supplied from the same mains supply.

3. A method as claimed in claim 2, wherein the mains power supply is a three phase supply and the encoded signal is modulated onto each of the phases.

4. A method as claimed in claim 2 or 3, wherein relays are provided in the mains supply to allow the time signal to reach reliably all the clocks in the system, each relay being operative to change the carrier frequency of the encoded time signal.

5. A method of synchronising a master clock with a slave clock, substantially as herein described with reference to and as illustrated in the accompanying drawings.