WO2014023935A2

WO2014023935A2 - Method for calibration of timepieces

Info

Publication number: WO2014023935A2
Application number: PCT/GB2013/052003
Authority: WO
Inventors: Richard George Hoptroff
Original assignee: Richard George Hoptroff
Priority date: 2012-08-08
Filing date: 2013-07-26
Publication date: 2014-02-13
Also published as: WO2014023935A3; GB201214178D0; GB2501136B; GB2501136A

Abstract

This invention relates to the calibration of timepieces that keep time using oscillators such as quartz resonators. Rather than measuring the frequency of the oscillator directly, externally calculating an appropriate correction value and storing in non-volatile memory (106) in the timepiece, time corrections (109) made by the user during the course of use are compared to a stored value (104) of the time of last correction in order to determine the amount to adjust correction value stored in non-volatile memory (106).

Description

METHOD FOR CALIBRATION OF TIMEPIECES

This invention relates to the calibration of timepieces that keep time using oscillators such as quartz resonators. Rather than measuring the frequency of the oscillator directly, externally calculating an appropriate correction value and storing in non-volatile memory, time corrections made by the user during the course of use.

Electronic timepieces such as clocks and watches typically employ quartz crystal resonators as a frequency source from which the progress of time is calculated. Typically a crystal will resonate at 32768Hz which, being a power of two, is readily processed by a counter circuit produce a stable 1 Hz pulse, and thus count and indicate the passage of time. Due to variations in the physical and chemical characteristics of the crystal, and variations in the load capacitance of the resonator, the frequency will not be exactly 32768Hz, but will vary from circuit to circuit. Thus some kind of calibration is needed.

Calibration can be achieved using an accurate frequency counter to measure the actual frequency of the resonant circuit. Any inaccuracy in the circuit is then compensated for, possibly my modifying the capacitance of the circuit, or, as is the state of the art, by calculating a number of 1/32768-second counts to be added or subtracted, say, every minute, in order for the inaccuracy to be numerically corrected. This number is then stored in non-volatile memory in the counter circuit and duly added or subtracted as required.

The quartz resonator frequency will also vary with temperature. By careful measurement and design, the temperature variation is reasonably characterized accurately by a known square-law curve centred at a known temperature. Thus the state of the art timepiece, such as the Thermoline range from ETA, will additionally correct for variations in frequency due to temperature. Such corrections can be calculated directly from the measured temperature and do not require calibration due to variations from circuit to circuit. (The temperature sensor may require calibration, but that is another matter.)

A limitation of the state of the art is that it requires expensive calibration equipment that has an accurate time base, and an electrical, acoustic or magnetic sensor sufficiently well designed that it can determine the frequency of the oscillator, or a frequency derived from it by the counter circuit, without introducing any stray capacitance into the circuit. A skilled operator is also required.

A further limitation of the state of the art is that the calibration value must be stored in nonvolatile memory in the counter circuit. This again requires expensive equipment and a skilled operator.

A further limitation of the state of the art is that it does not compensate for changes in the resonant frequency with time due to factors such as crystal aging and physical shock. While a crystal may be accurate to within ±10ppm (parts per million) when it leaves the factory, it will drift by 5ppm every year and further drift by as much as 5ppm with each large physical shock. Thus any attempt to ship a timepiece as 'pre-calibrated' will not last the test of time - recalibration will need to be repeated over the lifetime of the timepiece in order to be effective. This is unlikely if expensive equipment and skilled operation is required.

This invention employs a different approach. Rather than measuring the frequency of the oscillator directly, over a short period of time and writing the determined calibration constant in non-volatile memory, the timepiece provides means for the user to correct the time indication on the timepiece. This correction modifies the time value stored in the counter circuit and updates the time indicated by the timepiece accordingly, but in addition it calculates the appropriate calibration constant from the extent of the correction required and the time elapsed since the last such correction. Thus a user who corrects the time indication on his timepiece is also calibrating it at the same time. Such operation can readily be repeated over the lifetime of the timepiece.

Accordingly, and with reference to figure 1 , the invention is the following method:

1. Resonator '101' generates oscillating signal which clocks counter '102', so it keeps track of time. Counter output is fed to display control circuitry '103' that drives display '108' to depict the time. 2. At regular intervals such as once per minute, counter '102' triggers add / subtract module '105' to add to the counter '102' the signed value stored in non-volatile memory '106' to correct for oscillator inaccuracies.

3. From time to time, user employs interface '109' to correct the time stored in counter '102' and shown on display '108'. When such a correction is made, manual correction module '107' updates the counter as required.

4. Memory '104' stores the time when counter '102' was last corrected by correction module '107', such that correction module '107' can determine the time elapsed since the last correction, or indeed if this is the first time it has been corrected since power-up. 5. When user employs user interface '109' to correct the time stored in counter '102', if this is not the first time the counter has been corrected, correction module '107' additionally calculates a new calibration value to be stored in non-volatile memory '106' from the existing value in non-volatile memory '106', the time elapsed since last correction, and the extent of the correction.

An embodiment of the invention might be a watch employing a microcontroller implementing all elements '102' - '107'. A 32768Hz quartz crystal serves as resonator '101'. Timepiece display '108' might be an analog stepper motor or an LCD display. User interface '109' might be a set of pushbuttons on the watch that allow the time to be corrected by means of a sequence of presses. Add/subtract module '105' is triggered once per minute, i.e. nonvolatile memory '106' stores the number of 1/32768 second pulses to add or subtract from the counter '102' every minute.

Figure '2' shows a flow chart of the operation of manual correction module '107'. Execution commences '201' when user corrects the time indication with user interface '108'. The number of counts to correct, C, is calculated '202' from the user input, and then this correction is applied '203' to the counter. (C may be positive or negative.) This instantaneously corrects for any error in the time indication and is rendered on display '108'.

If, '204', this is the first time a correction has been applied since power-up, it is assumed that the time is being correctly set for the first time. The new counter setting is stored in memory '208' and execution of manual correction module '107' stops '209' until the user corrects the time indication again. If, '204', this is not the first time a correction has been applied since power-up, the corrective factor and the elapsed time since the last correction can be used to deduce a new calibration value stored in non-volatile memory '104'. The number of minutes M since the last correction is calculated '205'. The quantity C/M is then calculated '206', this being the amount to adjust the calibration value to compensate for inaccuracies in the original calibration value and any subsequent aging or shock. This quantity is then added '207' to the calibration value stored in non-volatile memory '104'. The new counter setting is stored in memory '208' and execution of manual correction module '107' stops '209' until the user corrects the time indication again.

In alternate embodiments, user interface might be provided via an electrical or optical interface.

Claims

1. A method of calibrating a timepiece, the timepiece having a time display showing a first time value, a controller for updating the first time value based on a count of pulses, a calibration module for applying a calibration value to the count of pulses, and a user interface for receiving a time value input, the method comprising the steps of:

receiving a first new time value from the user interface; and

calculating the calibration value based on the number of pulses needed to correlate the first time value with the first new time value over a predetermined time period.

2. A method as claimed in Claim 1 , further comprising the step of receiving a second new time value from the user interface, and the predetermined time period is the elapsed time between receiving the first and second new time value.

3. A method as claimed in Claim 1 or Claim 2, further comprising the initial step of receiving a set-up time value from the user interface.

4. A method substantially as herein described with reference to and as shown in the accompanying drawings.

5. A timepiece having a time display showing a first time value, a controller for updating the first time value based on a count of pulses, a calibration module for applying a calibration value to the count of pulses, and a user interface for receiving a first new time value, wherein the calibration value is based on the number of pulses needed to correlate the first time value with the first new time value over a predetermined time period.

6. A timepiece as claimed in Claim 5, wherein the user interface is also for receiving a second new time value, and the predetermined time period is the elapsed time between receiving the first and second new time value.

7. A timepiece as claimed in Claim 5 or Claim 6, wherein the user interface is also for receiving a set-up time value.

8. A timepiece substantially as herein described with reference to and as shown in the accompanying drawings.