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  1. #31

    Default Re: Bob temperature compensation (By: Tinker Dwight)

    Hi Tinker..
    The Pi software was designed for a Synchronome so expects a pulse every 30 secs. I added a /30 in hardware to make it work with mine..Every 30 seconds it records a time-stamp from a NTP time server on the net..
    I tried to word it so it didn't look like you needed to look up how a Hipp Toggle works ! But I don't think I succeeded.
    If you do Google "Hipp Toggle" it will be the first hit..Don't know why Google thinks it's the most relevant source of info out there....
    There is a full description with my simple circuit diagrams...
    Originally I timed it to 30 seconds but recently removed some resistance from in series with the impulse magnet (that was easy to get at without stopping the clock) and it went to 52..Sometime I'll try adjusting it to 60 seconds
    Roger

  2. #32
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    Default Re: Bob temperature compensation (By: rogerj)

    Quote Originally Posted by rogerj View Post
    Hi Tinker..
    The Pi software was designed for a Synchronome so expects a pulse every 30 secs. I added a /30 in hardware to make it work with mine..Every 30 seconds it records a time-stamp from a NTP time server on the net..
    I tried to word it so it didn't look like you needed to look up how a Hipp Toggle works ! But I don't think I succeeded.
    If you do Google "Hipp Toggle" it will be the first hit..Don't know why Google thinks it's the most relevant source of info out there....
    There is a full description with my simple circuit diagrams...
    Originally I timed it to 30 seconds but recently removed some resistance from in series with the impulse magnet (that was easy to get at without stopping the clock) and it went to 52..Sometime I'll try adjusting it to 60 seconds
    Roger
    What clock source are you using as a reference?
    It isn't a problem of software, it is a problem of reference used
    that I'm asking.
    Tinker Dwight

  3. #33

    Default Re: Bob temperature compensation (By: Tinker Dwight)

    Every 30 seconds the software is triggered to see what the offset error is between the pulse and the time off an NTP server. A value in milliseconds is recorded. The average error over every 5 min period goes to the graph..Does that make sense ? Roger

  4. #34
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    Default Re: Bob temperature compensation (By: rogerj)

    Quote Originally Posted by rogerj View Post
    Every 30 seconds the software is triggered to see what the offset error is between the pulse and the time off an NTP server. A value in milliseconds is recorded. The average error over every 5 min period goes to the graph..Does that make sense ? Roger

    I doubt the server is providing an accurate time standard to the level you need.
    The best you can use is either a temperature compensated quartz oscillator
    or a GPS that has a one second time pulse out.
    Your server shouldn't have any long term drift but I wouldn't rely on it for
    sub millisecond timing.
    Unless you want to see some other funny aliasing you might want to set
    the power pulses to an prime number, not near 30 or 60.
    Are you using a true mechanical switch or an optical interrupter
    for the Hipp Toggle?
    Tinker Dwight
    Last edited by Tinker Dwight; 04-19-2017 at 04:40 PM.

  5. #35

    Default Re: Bob temperature compensation

    Hi Tinker.
    In reverse order..yes I am using an optical interrupter as the Hipp detector.
    Next time I alter the impulse period I will try for 59....but I wish I understood the reason !
    On the 17th Mar I asked my friend to send me a graph of the un-averaged raw data over a short period..Here' a short sample of the going showing what we think are the small cyclic circular errors, albeit samples of a 52 sec period wave sampled every 30 secs.


    I'm not knowledgeable enough to debate the accuracy of an NTP server but I have discussed it with my friend who had thought of the possibility of short term jitter from the internet route and the WiFi to the Pi. I believe that's why he had decided on taken a 5 minute average of the previous 30 second samples. As for the long term accuracy we had never doubted it...I'd be interested in knowing why you question it..Certainly his Synchronome or my DIY clock are ever going to break any records..Roger
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  6. #36
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    Default Re: Bob temperature compensation (By: rogerj)

    Most computers only need to check the time once a minute.
    The build in time reverence is the over driven crystal used for the
    processor speed.
    Then they use an estimated error from a two way path over the
    net.
    They are usually within a second of the correct time but usually
    as much as 5-10 milliseconds fast or slow every minute.
    Some of the variation you see is likely from such differences.
    The daily drift rate is real but some of the ups and downs may just
    be the processor responding to its drift and variability.
    In the graph you're showing the short term are likely your pendulum
    but the general waviness may not be.
    Using an prime number of swings will shorten the time of sampling error
    caused by beating against the reference frequency. It ensures that you
    don't have something like a 1 hour beat in the sampling method and seeing
    it as some error in your pendulum. I might choose something like 53 swings.
    If you used something like 60 and the computer was correcting on 60 seconds
    the beat frequency might be hours that you see as long term drift in your
    pendulum. It might actually just be systematic errors in some part of the
    sampling noise.
    An interesting thing to do is to save all the samples and do an FFT on the data.
    Decimate the data at different amount and again run a FFT. This will allow
    you to see the different types of regular noise. These will usually show up
    as peaks at regular points. Your pulse rate of the pendulum should be clear
    but other errors can be hiding.
    Tinker Dwight

  7. #37

    Default Re: Bob temperature compensation (By: Tinker Dwight)

    Thanks for that insight..I will pass your info to my friend to see what he makes of it..
    As my clock outputs 1 pps and is divided by 30 in an external board, it would be possible to test the sytem by substituting 1 pps from a GPS receiver (which I already have) Roger

  8. #38

    Default Re: Bob temperature compensation (By: rogerj)

    Hi Tinker..Sorry for the delay in replying but I have been chatting with the writer of the program I've been using on the raspberry Pi computer. I ran a test for a week using a 1pps GPS pulse divided by 30 into the Pi instead of the clock. I'm not able to pass on all he told me because I don't follow all the technicalities but basically he doesn't disagree with anything you said.
    However we did decide that in spite of the points you raise (which he was already aware of ) the system is perfectly good for monitoring a pendulum clock running at atmospheric pressure and domestic temps.
    The graphical results we use are done with adapted open source software called MRTG.
    The graphical display resulting from the weeks trial of the GPS source was a perfectly flat line right on GMT. The resolution of the line does depend on the PC monitor resolution but we reckon it probably takes an error of 10 milliseconds to step up or down a level on my screen.
    The short term errors seemed to be within +/- 3 milliseconds and they generally occur quite slowly. The graph I've attached shows the previous 7 days of the actual clock rate which we think is a good enough indication of how it's going. The temp was not monitored and the scale of the graph is +/- 0.5 seconds.




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  9. #39
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    Default Re: Bob temperature compensation (By: rogerj)

    If these are absolute numbers, the clock is not doing well.
    One wonders what is the source of the variation. The numbers
    span about 3%.
    It would have been good to get a reading of the environment
    variables for this time span.
    Tinker Dwight

  10. #40

    Default Re: Bob temperature compensation (By: Tinker Dwight)

    ooch ! And I thought it was not going too badly. High pressure predominated until the middle of the period when the low arrived overnight. The lowest I saw was was 991 Mb but it may have been lower at some time..Today it's 996. Overall it's only 0.2 secs ahead of where it was a week ago (although it's still gaining) I suspect the rating is a tad out and may be inherently fast but I thought I'd wait another week before fiddling. Unfortunately I don't have a convenient way of recording ongoing temp and pressure. The monitoring continues...Roger

  11. #41
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    Default Re: Bob temperature compensation (By: rogerj)

    Is this a pressure measurement? I was thinking it was the pendulum.
    Tinker Dwight

  12. #42

    Default Re: Bob temperature compensation (By: Tinker Dwight)

    Yes Tinker..they are approximate pressure measurements added to the graph with Photoshop..Should have added the "Mb"..So, not so bad after all ? The middle line is GMT...Roger

  13. #43
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    Default Re: Bob temperature compensation (By: rogerj)

    Which direction is +0.5s and which is -0.5s.

    Phil

    PS: I assume it is s/day.

    With out ambient temperature I think yo are in the long grass.

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    Default Re: Bob temperature compensation (By: Phil Burman)

    A pendulum will lose time with higher atmospheric pressure due to flotation. If I read your graph correctly yours is apparently gaining at higher pressure. Bob friction in air and air mass movement both also have the possibility to alter the rate due to a change in air density (read pressure).

    There are several books that will give you insight into this somewhat complicated issue.

    Phil

  15. #45

    Default Re: Bob temperature compensation

    Phil..The graph has lost it references after cropping the full image. The centre horizontal line is GMT. The graph shows the error between the clock and GMT , plotted against time. Trending downward, the clock is losing and vice versa. The vertical height of the displayed section is 1 second, centred on GMT. The dotted verticals are midnight of successive days. The rightmost one is the start of May12th.
    So....at the end of the period shown the clock was ahead of GMT by about 0.25 sec.
    And during the period of high pressure the clock was losing and when the low arrived overnight 9th/10th it began to gain..following the law as expected.
    I hope this clears up the confusion I have inadvertently created. If I ever post another graph I won't make those mistakes again !
    There is a fair degree of temp compensation (the subject of the original post) and the actual temperatures prevailing, with the central heating off (broken), are around 20C with no great variation night or day at present.
    Last edited by rogerj; 05-13-2017 at 03:44 AM.

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