Data processing: measuring – calibrating – or testing – Calibration or correction system – Timing
Reexamination Certificate
1999-03-23
2001-07-24
Shah, Kamini (Department: 2857)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Timing
C702S085000, C702S176000, C377S028000, C377S029000
Reexamination Certificate
active
06266625
ABSTRACT:
FIELD OF INVENTION
This invention relates to a method and system for calibrating high resolution measurements. In particular it relates to a method and apparatus of generating a high precision, high accuracy measurement from two measurements differing in precision and accuracy.
BACKGROUND OF INVENTION
The precision of a measurement is the smallest resolvable change. The accuracy of a measurement is how close the measurement is to the true value. The two are not the same.
An example of the problem to be solved is generating a high precision, high accuracy, time-of-day measurement, given two clocks. One is an accurate clock, but it only ticks at widely spaced intervals. This clock can sometimes be unpredictably resynchronized with a master clock, so causing step changes in the time. The other is a clock which ticks rapidly, but the time given is not directly related to the first clock, and that the actual rate it counts can drift from its nominal rate. It can also stop for unpredictable periods, for example if someone else uses it. The user wants a clock which gives the right time, but also ticks rapidly, allowing accurate measurements of short time intervals. The problem is to use both clocks to generate such information.
PRIOR SOLUTIONS
The simplest solution to acquire a real time measurement is to only use the high accuracy, low resolution clock (see FIG.
1
A). This give an accurate measurement but gives low precision.
A more precise solution is to take a reading from the high accuracy low resolution clock and high resolution counter at the same time at the start (see FIG.
1
B). Then read the counter and estimate the time by extrapolation from the time given by the clock at the first measurement. This extrapolation is referred to as a correction or calibration. This gives better precision and can be used to time short intervals. However this extrapolation will drift from the real time as the counter will drift and may also stop, giving persistent error thereafter. Furthermore any resynchronization of the clock to a master clock will not be detected or acted upon.
A third solution is shown in FIG.
1
C. At each time measurement a note of the high and low precision times is made. If the low precision time is the same as the last low precision time measurement, then the time is extrapolated from the previous time measurement using the difference in high resolution counters. If the low precision time differs from the last measurement, then return the low resolution time, and save both times ready for the next measurement. This makes for an accurate measurement but precision is lost over measurements greater than one low resolution clock tick. The precision is lost because if the low resolution clock has changed since the last measurement then the low resolution clock is used to provide the time without any interpolation from the high resolution counter.
If the time is accessed from several program threads at once, then synchronization is needed to protect updates to the last measurement of high and low resolution time. Synchronization using semaphores can cause deadlocks or poor performance of a computer system.
This solution allows short time intervals to be measured provided the accurate clock does not change between the measurements. It synchronizes every timing measurement, but loses all the previous information about the offset between the accurate clock and the precision counter if the clock has changed.
Semaphores are mutual exclusion controls which allow programs with multiple threads of execution to be written such that two threads cannot update the same variable at the same time by making the threads get exclusive ownership of the semaphore before they update the critical resource.
A fourth solution, based on solution
2
, is shown in FIG.
1
D. Synchronization of the clock and counter take place at externally decided intervals. This gives generally high precision and reasonable accuracy. However it is complex, leads to loss of precision when resynchronized because the resynchronizations may not occur at the same time interval after a clock tick. It is also difficult to decide how often to resynchronize. A semaphore may be required to protect the clock offset value.
SUMMARY OF INVENTION
According to one aspect of the invention there is provided a method of measurement calibration comprising:
obtaining a calibrated low resolution measurement;
obtaining a high resolution measurement;
calculating a range of possible corrections to the high resolution measurement to align it with the low resolution measurement;
adjusting the correction range to make it consistent with a previously stored correction range; and
calibrating the high resolution measurement with a value taken from the correction range.
This gives good accuracy and precision. The accuracy must be in the range allowable by the high accuracy clock. The counter can measure small time intervals, even after a long interval between time measurements when operating correctly. However disruption can cause the calibration of the counter to be lost.
For each time measurement, record the high resolution time, low resolution time, then the high resolution time. Calculate the upper and lower bounds of the correction factor to go from the first high resolution time to the real time, at the time the high resolution time was taken given that it must be in a range determined by the low resolution high accuracy time and the tick interval. Combine these limits with previously generated limits, so that both limits are true. If the limits are irreconcilable, then use the current limits. Use the first high resolution timestamp and the lower of the limits to generate the final time.
REFERENCES:
patent: 4165459 (1979-08-01), Curtice
patent: 4611926 (1986-09-01), Hayashi
patent: 4719375 (1988-01-01), Martin
patent: 4912734 (1990-03-01), Frauenglass
patent: 5333162 (1994-07-01), Condreva
Bui Bryan
Cameron Douglas W.
Dougherty Anne Vachon
International Business Machines - Corporation
Shah Kamini
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