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Arduino Pendulum Clock Design - Comments Welcome

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John Haine24/10/2020 20:33:56
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Yes, which in my case is a 10 MHz OCXO, and (I think) for Dave it is the Arduino clock. But the AV is a variance - it is a measure of how the samples of period depart from some sort of mean. A simple variance would just take the long term average and the sum of squares of the deviation from that average, but for some types of variation the variance gets larger and larger without limit as the number of readings increases. Once you have some sort of timebase to measure the period you don't need another one to compare with.

Joseph Noci 124/10/2020 21:32:36
778 forum posts
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I think we vehemently agree...The 'another one' you talk of would be a THIRD - thats NOT at all what I suggested.....The Timebase is the second ( the reference, same as your 10MHz OCXO.)..My point was that you are not measuring the clock variance against ITSELF, but measuring its variance referred to the 'accurate' timebase, the reference...

SillyOldDuffer24/10/2020 22:11:17
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Joe & John's Q&A has clarified my thinking. What I need to do is upgrade my Arduino code to compensate for the deficiencies of its resonator. I can continually re-measure how many Arduino ticks are in a GPS second and use the info to adjust the pendulum measurement. Doing so will compensate for temperature and resonator drift, at least within the 4uS resolution of the Arduino timer plus however long it takes to service the interrupt.

A Nucleo would squeeze a little more accuracy than a Nano. 3x faster and they have a 20ppm crystal oscillator too.

Dave

Joseph Noci 125/10/2020 05:52:08
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Dave, I get a bit lost with the size of the numbers we are dealing with here - I am chasing down in the 10 minus 12/13 arena, and this is way overkill for what you are measuring - I don't yet have a handle on what the real effect will be of, say, a 1ppm osc versus your, what, maybe 30ppm one? Also, what is the effect of the 4us jitter, and the interrupt jitter..I suspect that the oscillator stability effects will be drowned in the noise of the rest..Trying to model this a little and see...Lament the day I discovered TimeNuts...

BTW, 1ppm TCXO's are easily available from Digikey and Mouser for under $3.00 - easy to fit.

Joe

John Haine25/10/2020 06:30:35
3422 forum posts
184 photos
Posted by SillyOldDuffer on 24/10/2020 17:31:23:
Posted by John Haine on 24/10/2020 14:02:26:

...............

Interpreting the graphs is another walk in the dark, but I read your Arduinome chart thus:

  • The line generally falls from left to right, which is good.
  • The wave like wiggles in the first 100 seconds could be settling errors, maybe due to the pendulum vibrating after start-up, or the clock being bumped whilst the measurement was set up.
  • Y axis values are small, which I think indicates good stability.

My clock's chart:

  • The tick shaped line is bad - rising to the right indicates a consistent rate error. Sharp falls are characterised by Rowland as observation error. In my case, pretty certain the observations are OK, and the real cause is pendulum bounce due to restarting the clock from scratch for each test.
  • Y-axis values are big, which I think indicates poor stability.

Could be wrong!

Meanwhile, I broke the pendulum rod again. I frayed the rod and made a brass clamp to hold the fibres OK but got clumsy reassembling the clock. Showstopper - looks like I've run out of 0.5mm rod and will have to order more.

Dave

Just to clarify, my plot is of a set of measurements taken when the clock is in equilibrium, not from the start of the run. I don't know where the wiggles come from - there is one key periodicity in the clock which is the impulsing, which during that run was probably happening every 40 swings or 80 seconds. (Currently I impulse every 30 swings.) Another option in TimeLab is "Modified Allan Variance" - if I choose that the wiggles go away.

capture_1.jpg

What it means I still have to figure out!

You can buy this stuff which might be an alternative to burning the resin out of the composite?

John Haine25/10/2020 06:46:46
3422 forum posts
184 photos

Today's job is to move the Arduinome back an hour - currently manual, really I ought to add a couple of commands to deal with DST/UTC shifting. Would need to generate 400 pulses to step the dial backwards or forwards, +/- an adjustment to allow for the time taken.

Joseph Noci 125/10/2020 09:30:57
778 forum posts
984 photos
Posted by John Haine on 25/10/2020 06:30:35:

Just to clarify, my plot is of a set of measurements taken when the clock is in equilibrium, not from the start of the run. I don't know where the wiggles come from - there is one key periodicity in the clock which is the impulsing, which during that run was probably happening every 40 swings or 80 seconds. (Currently I impulse every 30 swings.) Another option in TimeLab is "Modified Allan Variance" - if I choose that the wiggles go away.

capture_1.jpg

John, do you have Adev's for longer periods? - 10 hours plus? Interested to see that - if so maybe we should PM so as not to hijack this thread any further..

My GPSDO Adev for approx 12-1/2 hours @ 1 sec samples:

jono_gpsdo_allaldev.jpg

John Haine25/10/2020 10:06:33
3422 forum posts
184 photos

Not yet, I haven't properly understood how to use TimeLab so far. I have been posting some results here - may be aa better place as it isn't precious about file formats?

SillyOldDuffer26/10/2020 17:16:54
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A surprising result from yesterday's endeavours.

First, my attempts to fray and hold the 0.5mm rod in a clamp failed miserably. Superglue wicks into the fibres, making them stiff and easily snapped. I'm completely out of 0.5mm rod now.

Needing to test a software upgrade, I fitted a 0.3mm diameter rod. (No 0.3mm drills available but I was able to shim the thinner rod with some 0.5mm O/D, 0.3mm I/D brass tube bought for another purpose.) Also took the opportunity to fit a slightly bigger bob, foolishly forgetting to weigh or measure it before putting the shield back.

Dropping the rod from 0.5 to 0.3mm made a big improvement. Q about 3000 and it takes much less power to impulse the bob. Currently getting about 20 swings per impulse. Zooming in on an undisturbed section of the run (I was working outside, so no vibration indoors) shows the Period holding steady while the amplitude sawtooths between impulses. The period can just be seen to jump when the impulse is applied.

bobplusandpr3.jpg

I guess the 0.5mm rod was acting as an inefficient spring and absorbing a lot of energy. Presumably the 0.3mm rod being about 1/3rd the weight and much less stiff makes it more efficient. Still surprised a small change made such a big difference.

Dave

Sam Stones26/10/2020 18:22:56
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780 forum posts
308 photos

Roughly 1/8 as stiff?

i.e. D^4

However, I can't be sure about resin composites.

Sam

John Haine26/10/2020 20:02:46
3422 forum posts
184 photos

Is it 0.3mm CF or steel?

duncan webster26/10/2020 20:23:15
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2839 forum posts
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It will undoubtedly be less stiff, and Sam's calculation is right, but if the springiness were 'ideal' I don't think it would make any difference to Q as it wouldn't absorb any energy. However even steel has (mechanical) hysteresis, that's why springs get hot if rapidly cycled. Stress/strain curves are done slowly to allow time for things to recover. I have no evidence for this but I suspect harder steel is better than soft. I can't find any information on CF/resin composites, but it would be interesting to try a steel rod of the same stiffness. There is some info about mechanical hysteresis at Royal Society

The best of the best clocks use knife edge suspension, Is this why?

My theory of 'do what you like, but do it every time' says SOD should now turn the wick down on the impulser so that it impulses nearly every time. No doubt the clever software could change the impulse so that it missed one in 100.

In passing my clock is now stopped, I stopped it as it had gained a couple of minutes (been cold) intending to start it when it got correct, but got distracted and forgot, so I'm waiting until it's correct again.

John Haine26/10/2020 22:24:34
3422 forum posts
184 photos
Posted by duncan webster on 26/10/2020 20:23:15:

....

The best of the best clocks use knife edge suspension, Is this why?

...

Do they? Not Harrison, or Clock B, or the Shortt Synchronome, or the Riefler.

duncan webster26/10/2020 23:56:12
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2839 forum posts
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Well I thought they did, shows how wrong one can be

According to this Riefler has both knife edges and a spring suspension

Edited By duncan webster on 27/10/2020 00:13:43

John Haine27/10/2020 06:52:59
3422 forum posts
184 photos

Ah, you're right, the upper spring chop pivots on a knife edge to do the impulsing.

Martin Kyte27/10/2020 07:58:12
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2113 forum posts
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John Harrisons RAS regulator also had the pendulum suspended on a knife edge.

regards Martn

Michael Gilligan27/10/2020 08:38:43
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16616 forum posts
723 photos
Posted by John Haine on 27/10/2020 06:52:59:

Ah, you're right, the upper spring chop pivots on a knife edge to do the impulsing.

.

Nicely illustrated in this Meccano version surprise **LINK**

http://www.meccanotec.com/riefler%20borrill.htm

... and detailed in his patent: **LINK**

https://worldwide.espacenet.com/patent/search/family/002577589/publication/US508760A?q=sigmund%20riefler

MichaelG.

John Haine27/10/2020 10:04:58
3422 forum posts
184 photos
Posted by Martin Kyte on 27/10/2020 07:58:12:

John Harrisons RAS regulator also had the pendulum suspended on a knife edge.

regards Martn

Actually no. Key to that clock, as also to Clock B, is the use of circular chops round which the suspension spring wraps to give partial compensation of circular deviation. This is obvious from the published descriptions of the RAS clock (The Lost Science of John "Longitude" Harrison), Clock B and the series on the RAS Regulator replicas in HJ at the moment. They do use knife-edge suspension, hard brass on glass blocks, for the crutch but that carries much less load than the main suspension, and arguably friction doesn't matter since the crutch is driven by the train.

duncan webster27/10/2020 12:37:00
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2839 forum posts
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I think this is where I got the idea that knife edge was good Knife Edge. If you go to Q3 there is a good description. However reading it again, he had a problem with the knife edge slowly walking sideways.

Mine's going again!

Martin Kyte27/10/2020 13:18:12
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2113 forum posts
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Posted by John Haine on 27/10/2020 10:04:58:
Posted by Martin Kyte on 27/10/2020 07:58:12:

John Harrisons RAS regulator also had the pendulum suspended on a knife edge.

regards Martn

Actually no. Key to that clock, as also to Clock B, is the use of circular chops round which the suspension spring wraps to give partial compensation of circular deviation. This is obvious from the published descriptions of the RAS clock (The Lost Science of John "Longitude" Harrison), Clock B and the series on the RAS Regulator replicas in HJ at the moment. They do use knife-edge suspension, hard brass on glass blocks, for the crutch but that carries much less load than the main suspension, and arguably friction doesn't matter since the crutch is driven by the train.

Obviously my faulty memory. I did recall reading about the inclusion of a knife edge running on agate in the description of the RAS and just assumed it was the pendulum suspension. Just looked it up agin and seeyou are correct in it's use on the crutch.

regards Martin

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