AU2006236020B2

AU2006236020B2 - Timekeeping apparatus

Info

Publication number: AU2006236020B2
Application number: AU2006236020A
Authority: AU
Inventors: John Stuart Calvert
Original assignee: Secure Int Holdings Pte Ltd
Current assignee: SECURE INTERNATIONAL HOLDINGS Pte Ltd
Priority date: 2005-11-16
Filing date: 2006-11-15
Publication date: 2011-07-28
Anticipated expiration: 2026-11-15
Also published as: AU2006236020A1; GB2432432B; GB0523363D0; GB2432432A

Abstract

A timekeeping apparatus, suitable for use with a utility meter, comprising a first clock which is preferably a crystal based real time clock, and a means for determining a 5 correction based on an oscillating signal, which is preferably a frequency regulated mains supply. The correction is applied to the time of the first clock.

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant: Polymeters Response International Limited Invention Title: TIMEKEEPING APPARATUS The following statement is a full description of this invention, including the best method of performing it known to

US:

1A TIMEKEEPING APPARATUS The present invention relates to a timekeeping apparatus, in particular for use in a utility meter, and to a method for correcting the time in a clock. 5 In a typical utility meter, the internal time kept by the meter and used in processing functions is kept on the meter's real time clock which is usually based on a crystal oscillator. Where the crystal is used to keep the time, the short term accuracy is good, but the time can drift over longer periods, resulting in poor long term accuracy. The drift will increase over time in an unbounded manner and needs to be reset in order 10 to maintain the accurate time. Another way of monitoring the time in a utility meter is to use a mains clock which is the preferable method in the United Kingdom for example. This type of method operates by counting the number of mains frequency cycles assuming that each cycle has a time period equal to the exact time period of the nominal supply frequency. is This is possible in such cases where the electrical grid frequency is controlled by the generating companies to produce a fixed number of cycles over an extended time period (usually a day); such count of cycles being equal to that would be produced by a perfectly stable nominal frequency system in the extended time period. A drawback of this system is that whereas this type of system may have good and bounded long term 20 accuracy, the short term accuracy will be poor. Moreover, in the absence of mains power, the clock will also be switched off and have to be maintained by other non volatile means during this period. Some current systems base their time primarily on a mains clock and revert to the crystal oscillator clock when power fails. However, this type of combination results 25 in an inaccurate time in the short term due to perturbations of the grid frequency. It is therefore desirable to provide a meter with improved accuracy in both the short term and the long term. The present invention provides a time keeping apparatus comprising a first clock, and means for determining a correction based on an oscillating signal and 30 applying the correction to the time of the first clock, the apparatus further comprising a means for providing a suppression of a short term component in the applied correction. In an embodiment, the oscillating signal is a frequency regulated alternating voltage source, such as a mains supply. 2727558_1 (GHMatter) P62875.AU 2 Another aspect of the invention provides a utility meter incorporating the above described timekeeping apparatus. One of the clocks may be a battery backed crystal controlled clock with high short term accuracy and unbounded long term accuracy, and the second clock may have poorer short term accuracy than the first clock, but bounded s long term accuracy. Particularly, in the case of a utility meter, the second clock may be derived from a mains frequency counter. In order that the present invention be more readily understood, embodiments thereof will be described with reference to the accompanying drawings, in which: Fig. I shows a timing graph comparing the proportioned and integral feedback 10 signals applied to a real time clock signal A using a perfect mains frequency signal; Fig. 2 shows a timing graph of a real time clock correction signal when the proportional and integral feedback signals are combined and applied to the real time clock signal A; Fig. 3 shows a timing graph for an imperfect main frequency signal wherein the 15 correction signal uses proportional and integral feedback. In a preferred embodiment of the invention, the timekeeping apparatus comprises a crystal based real time clock which keeps the internal time of a utility meter. Such a clock is powered by a battery and considered to have good short term accuracy but unbounded long term accuracy. The error will increase over a long period 20 of time because of the steady monotonic drift (e.g. 0.5 second per day). A second clock in the form of a mains frequency clock is also provided and in contrast to the crystal based clock, it has a large short term error, but the error is often bounded. For example, the clock error may be +2 seconds at 8 AM in the morning, -7 seconds at 5 PM and again +1 second at 6 AM the next day. Such a clock will have very 25 little error when its time is averaged over a long time period. In order to correct the drift in the crystal based internal clock, the controlled variable is made to follow the mains clock, the command variable, by means of a 2255315_1 (GHMatters) 3 servo control loop. This is shown in Figs. 1 and 2. The servo control loop applies a correction to the controlled variable A, in this case the meter's internal crystal based time, in accordance with an error, which is the difference between the controlled variable (the internal crystal time) and the command variable (mains time). 5 As shown in Fig. 1, if the correction applied by the servo control loop is proportional to the error, the controlled variable will be corrected more as the error gets larger. The amount of error that is needed to correct any given rate of drift of the time is proportional to the rate of drift. This means that if the drift is non-zero, the controlled variable will not equal the command variable, as shown by corrected signal 10 Al. If the correction applied by the servo control loop is proportional to the integral of the error, any persistent error is corrected by an increasing rate of correction. Clearly, this type of behaviour is needed to eliminate the permanent error that is otherwise needed to correct a persistent drift. By a suitable choice of the gains that are applied to the proportional and integral errors in the servo control feedback, the. controlled 15 variable can be made to follow the tracked variable with a damped oscillatory error A3. The period of the oscillations is determined by the integral gain (a higher integral gain gives a shorter period), and the damping rate is controlled by the proportional gain (a higher proportional gain gives more damping). Once the following error has been reduced to zero, the rate of drift is 20 determined entirely by the integral error A2. The integral error can therefore be initialised according to a measured frequency error, to reduce the time needed for the servo feedback to converge on the correct value, thereby effectively anticipating the drift in the meter's internal time. The integral error can also be used to calculate the amount of drift there has been while the power is off, so that compensation for this 25 can be applied when the power Is restored. Another embodiment is shown in Fig. 3 where the combined proportional and integral feedback is applied by an imperfect mains frequency signal. In the UK, for example, the mains supply frequency averages out to about 50Hz over a period of 8 days. This is taken into account by adapting the correction signal A4 to have a first 30 half-period of 8 days, thus allowing time for the frequency to average out and give a more accurate correction. Oscillations can be reduced by introducing a -damping factor of for example 50%.

4 Accordingly, the preferred way to obtain a stable clock is to use a proportional-integral servo loop with appropriately chosen gain and rate so that any errors are quickly settled to zero. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

Claims

2. The apparatus of claim I wherein the first clock exhibits short term accuracy of a first quality but unbounded long term drift and the oscillating signal exhibits short io term accuracy poorer quality than that of the first clock but bounded long term accuracy.
3. The apparatus of claim I or claim 2 wherein the first clock is a crystal based real time clock and the oscillating signal is a frequency regulated alternating voltage source. 15
4. The apparatus of claims 1, 2 or 3 wherein the oscillating signal is derived from a frequency regulated mains supply.
5. The apparatus of any one of the preceding claims wherein the suppression of the 20 short term component is achieved by means of a proportional feedback of the error.
6. The apparatus of any one of claims I to 4 wherein the suppression of the short term component is achieved by means of a proportional and integral feedback of the error. 25
7. A utility meter comprising the timekeeping apparatus according to any one of the preceding claims.
8. A time keeping apparatus according to any one of claim 1 to 6, and substantially 30 as herein described with reference to the accompanying drawings.
9. A utility meter according to claim 7, and substantially as herein described with reference to the accompanying drawings. 2727558_1 (GHMatters) P62875.AU