Yet another Arduino clock thread!

[S KParticipant @sk20060]

Posted by duncan webster on 24/01/2023 22:09:51:

Nope! I suggest you Google centre of percussion

OK, got it! Thanks.

I believe this means that the optimum place to apply an impulse is at the center of percussion. Neat!

[duncan webster 1Participant @duncanwebster1]

Ah you replied whilst I was editing. I agree with you, but it's not easy to achieve. As the rod has mass the COP does not coincide with the CG of the bob. I seem to remember a pendulum which had 2 bobs, one above the other, with a means of applying the impulse in the gap, but it's all getting a bit complicated.

[S KParticipant @sk20060]

If the pendulum is simple enough, it can probably be calculated to within a few percent. My pendulum, so far, is designed for easy and accurate measurement and calculation. For example, the bob is a simple brass disk, but oriented "sideways" compared to a normal circular bob, so the shaft pierces the center of the disk rather than its edge. Not aerodynamic, but simple to model.

Also, it can probably be found experimentally to a few percent as well. Hang it from its pivot by a line to the ceiling and start tappy-tap-tapping. A video camera with an engineer's rule as a background can be used to detect motion at the pivot.

It may also mostly be a case of "don't be too wrong!"

[duncan webster 1Participant @duncanwebster1]

I suspect this is of very little importance as long as it's somewhere near. As someone above said, Fedchenko was impulse right at the bottom of the rod, and that kept pretty good time.

[John HaineParticipant @johnhaine32865]

On the question of suspension spring free length, a longer spring is more susceptible to torsional resonance around the rod centre line at frequencies comparable to the wanted resonance. My first clock has a 16mm wide spring with ~10mm free length and is quite susceptible to this if the escapement wasn't carefully set up. That's why I went to the more complex structure with two springs quite widely separated, which is much stiffer torsionally without needing a small free length. Short springs have higher bending stress, will lose more energy and be more susceptible to fracture.

I believe that whilst the CF matrix does absorb water slightly and this may cause it to shrink/expand, the fibres themselves don't. In a pendulum the CF layup should be longitudinal and with the bob weight the load should be largely taken by the fibres so changes in the matrix should have less effect. There is a small affect on density because of the weight of water, but I calculated that the impact of this on timekeeping is minimal.

**LINK** 4th or 5th post down in the thread.

Edited By John Haine on 25/01/2023 10:51:00

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Haine on 25/01/2023 10:50:31:

…

I believe that whilst the CF matrix does absorb water slightly and this may cause it to shrink/expand, the fibres themselves don't. In a pendulum the CF layup should be longitudinal and with the bob weight the load should be largely taken by the fibres so changes in the matrix should have less effect. There is a small affect on density because of the weight of water, but I calculated that the impact of this on timekeeping is minimal….

I agree. My unsuspended carbon fibre rod used as a spring was strongly effected by humidity, but the effect of humidity on period disappeared after adding a conventional spring suspension. I believe the matrix, which is a polymer resin – a plastic – was responsible for the rod's springiness. However, the fibres dominate pendulum length, and aren't effected by humidity.

In other words, although carbon fibre rod makes a poor spring vulnerable to wet air, it's still a good pendulum rod material – length not effected much by either temperature or humidity.

Be interesting to compare the stability of carbon-fibre versus Invar rods. Carbon fibre rods will move a little due to humidity effecting the matrix whilst Invar moves a little because the alloy is unstable – noticeably so in a high-precision clock.

Dave

[duncan webster 1Participant @duncanwebster1]

Fused quartz rod has a similar low expansion coeff, and should be less susceptible to humidity, but it's a bit fragile.

[S KParticipant @sk20060]

Invar is somewhat hard to find and rather expensive, but at least you know what you are getting. Same with fused quartz (probably the finest pendulum material), though good luck finding sizes appropriate for a second's pendulum!

Carbon fiber tubes and rods, though, are made by so many different outfits in so many different ways, that "carbon fiber" isn't one thing at all, it's a roll of the dice.

Speaking of hard to find materials, I need to ask the OP again: Where was the thin BeCu strips acquired? I'd like to hunt some down. Thanks!

[John HaineParticipant @johnhaine32865]

Posted by S K on 25/01/2023 14:58:01:

Invar is somewhat hard to find and rather expensive, but at least you know what you are getting. Same with fused quartz (probably the finest pendulum material), though good luck finding sizes appropriate for a second's pendulum!

Carbon fiber tubes and rods, though, are made by so many different outfits in so many different ways, that "carbon fiber" isn't one thing at all, it's a roll of the dice.

Speaking of hard to find materials, I need to ask the OP again: Where was the thin BeCu strips acquired? I'd like to hunt some down. Thanks!

Look at your PMs.

[duncan webster 1Participant @duncanwebster1]

Posted by S K on 25/01/2023 14:58:01:

Invar is somewhat hard to find and rather expensive, but at least you know what you are getting. Same with fused quartz (probably the finest pendulum material), though good luck finding sizes appropriate for a second's pendulum……..

Quick Google shows Robson scientific sell 6mm dia * 1m at not too stupid a price <£30.

[SillyOldDufferModerator @sillyoldduffer]

Posted by S K on 25/01/2023 14:58:01:

Invar is somewhat hard to find and rather expensive, but at least you know what you are getting.

…

Ah but do you know which Invar you're getting? Several Invars available, and, depending on the alloy, coefficient of expansion varies from about 0.55ppm/°C to 12ppm/°C.

In comparison Borosilicate is about 3ppm/°C, and Carbon Fibre in the range 0.4 to 5.6ppm/°C

A perfectionist might argue it's necessary for a serious clock-maker to measure all of them and compensate appropriately.

I believe the ultra low expansion glass used to make big telescope mirrors has the lowest known coefficient of expansion. Unfortunately I've no idea where to buy it or if it's strong & tough enough to make a pendulum rod.

Best thing about chasing perfection in pendulum clocks is it's a never ending quest!

Dave

[duncan webster 1Participant @duncanwebster1]

Several sources quote fused quartz at 0.5 ppm/C. It still doesn't tempt me, I'd break it

[S KParticipant @sk20060]

Posted by SillyOldDuffer on 25/01/2023 19:10:02:

Ah but do you know which Invar you're getting? Several Invars available, and, depending on the alloy, coefficient of expansion varies from about 0.55ppm/°C to 12ppm/°C.

Invented well over 100 years ago (and winning a Nobel Prize!), "Invar", or "Invar 36" or "FeNi36" or "64FeNi" is 64% iron and 36% nickle. Fe-Ni has a single pronounced dip in the coefficient of thermal expansion right at 36% nickle, reaching about 1.2 ppm/C.

Moving away from that percentage quickly makes the CTE worse, at least if it's expected to perform over a wide temperature range. But if you restrict the potential operating temperature range, you can still do better a little off those percentages. Other similar materials are designed to match the CTE of ceramics or glasses rather than just being the lowest.

I bought mine (Invar 36) from McMaster Carr in the U.S., where it's currently about $45 U.S. for 1/4 inch by 3 feet or $92 for 3/8 by 3 feet, etc. Unfortunately, they don't stock longer than 36", so a second's pendulum was out.

It's a little gummy, and the threads I cut turned out a bit tragic.

Edited By S K on 26/01/2023 00:32:17

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by S K on 26/01/2023 00:24:58:

Posted by SillyOldDuffer on 25/01/2023 19:10:02:

Ah but do you know which Invar you're getting? Several Invars available, and, depending on the alloy, coefficient of expansion varies from about 0.55ppm/°C to 12ppm/°C.

Invented well over 100 years ago (and winning a Nobel Prize!), "Invar", or "Invar 36" or "FeNi36" or "64FeNi" is 64% iron and 36% nickle. Fe-Ni has a single pronounced dip in the coefficient of thermal expansion right at 36% nickle, reaching about 1.2 ppm/C.

Moving away from that percentage quickly makes the CTE worse, at least if it's expected to perform over a wide temperature range. But if you restrict the potential operating temperature range, you can still do better a little off those percentages. Other similar materials are designed to match the CTE of ceramics or glasses rather than just being the lowest.

I bought mine (Invar 36) from McMaster Carr in the U.S., where it's currently about $45 U.S. for 1/4 inch by 3 feet or $92 for 3/8 by 3 feet, etc. Unfortunately, they don't stock longer than 36", so a second's pendulum was out.

It's a little gummy, and the threads I cut turned out a bit tragic.

Edited By S K on 26/01/2023 00:32:17

Anecdotally, Invar does seem to vary considerably from sample to sample. I came across a report somewhere back in the mists of time of a sample that had a negative expansion coefficient.

And yes, threading an Invar rod can be a less than satisfying activity!

[SillyOldDufferModerator @sillyoldduffer]

Posted by S K on 26/01/2023 00:24:58:

Posted by SillyOldDuffer on 25/01/2023 19:10:02:

Ah but do you know which Invar you're getting? Several Invars available, and, depending on the alloy, coefficient of expansion varies from about 0.55ppm/°C to 12ppm/°C.

…

I bought mine (Invar 36) from McMaster Carr in the U.S., where it's currently about $45 U.S. for 1/4 inch by 3 feet or $92 for 3/8 by 3 feet, etc. Unfortunately, they don't stock longer than 36", so a second's pendulum was out.

It's a little gummy, and the threads I cut turned out a bit tragic.

…

Buying it from a known source is going the extra mile. Expensive, but reliably predictable. Contrasts with quite a lot of hobby work, which is done with whatever materials come to hand – scrap, ebay, etc. In this way of working, metals tend to be classified very broadly as 'Cast Iron', 'Brass', 'Steel', 'Bronze', or 'Aluminium' etc without worrying that these are alloy families each with a wide range of properties. Sometimes it matters – you can't harden mild-steel, and pure Aluminium is so soft and sticky that it's difficult to machine.

Preaching to the converted I'm sure, but every so often construction articles appear in the mags calling for specialist materials almost designed to frustrate enthusiasts. I exaggerate, but maybe the project calls for a 5lb block of depleted Uranium and a 3" x 12" x 9' Mahogany plank! Then rubbing salt in the wound, material sources are covered unhelpfully in the text: 'a friend of my father's worked at Aldermaston in 1952', and 'I found the Mahogany in a lay-by off the B1147 in 1977'

Thanks for sharing the info that Invar36 is gummy. I'll remember that if I go that way myself. It's not unlikely, because I've often thought how useful it would be to compare my highly experimental pendulum with a well-made best-practice pendulum like John's. For example, how noisy is my super-cheap beam-break circuit compared with John's pro-user Sharp device, and how much does it matter for what I'm doing?

Dave

[duncan webster 1Participant @duncanwebster1]

I've got a memory rolling around that says Invar is very dependant on heat treatment, and that it has been known to spontaneously change its properties after time. Could be complete rubbish of course 

Edited By duncan webster on 26/01/2023 11:24:38

[John HaineParticipant @johnhaine32865]

Posted by duncan webster on 26/01/2023 11:24:05:

I've got a memory rolling around that says Invar is very dependant on heat treatment, and that it has been known to spontaneously change its properties after time. Could be complete rubbish of course

Edited By duncan webster on 26/01/2023 11:24:38

This is correct, see:

https://academic.oup.com/book/9250/chapter-abstract/155957399?redirectedFrom=fulltext

**LINK**

Paper by Robert Matthys

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by John Haine on 26/01/2023 13:36:34:

Posted by duncan webster on 26/01/2023 11:24:05:

I've got a memory rolling around that says Invar is very dependant on heat treatment, and that it has been known to spontaneously change its properties after time. Could be complete rubbish of course

Edited By duncan webster on 26/01/2023 11:24:38

This is correct, see:

https://academic.oup.com/book/9250/chapter-abstract/155957399?redirectedFrom=fulltext

**LINK**

Paper by Robert Matthys

Very interesting. The "free machining" variant would be most welcome!

[John HaineParticipant @johnhaine32865]

A quick update after 5 days running. I think it's time the clock was moved to a better location now it doesn't need much fiddling. Being in the workshop and with the shop vac living right next door I've notice that it's prone to being jolted and sometimes as a result its amplitude reduces and it can get into a mode where it "forgets" which way it's supposed to be impulsing – then it impulses the pendulum the wrong way and brakes it instead! As long as there are occasional swings that exceed the amplitude set point it is reminded of which way is which so them I have to give it a nudge to get it above the line again. So it can go back on the wall of the cold studio where it should have been, I moved it into the warm! It will then have a more rigid mounting so I hope the Q will be larger. I also want to do some mods to the electronics to improve the power supply arrangements.

Meanwhile I've been working further on the dial, adding a mounting for the chapter ring, mounting the printed chapter ring I bought on a rigid and strong backing, adding some attachment points for the dial to the case when I eventually get round to that.

[duncan webster 1Participant @duncanwebster1]

If you had a second opto right alongside the centre one it might be possible to have a sort of quadrature set up so it knows which way it's going. Probably not worth the effort

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by duncan webster on 27/01/2023 13:56:43:

If you had a second opto right alongside the centre one it might be possible to have a sort of quadrature set up so it knows which way it's going. Probably not worth the effort

I used a second sensor in my clock – partly for that purpose and partly as a means of determining the duration of the impulse.

[John HaineParticipant @johnhaine32865]

My first clock ("Arduinome&quot used a single sensor which is quite offset from the centre so tghe processor can work out which side the pendulum is – the actual impulse position is completely determined by the mechanics so a central sensor position isn't needed. And I used blocking "delay" statements liberally in my code in a way which meant that I never actually had to work out where the pendulum was.

[John HaineParticipant @johnhaine32865]

Back on the wall. Actually much quicker than I expected. It does seem to be happier with a more solid mount, one can see from this.

Top trace is the pendulum sensor, which is also the drive pulses if enabled.  Bottom trace the amplitude sensor, each blue pulse is where it just catches the amplitude peak.  When that happens the following two drive pulses are disabled.  In this trace you can see that the 4th and 5th pulse are disabled, also the 6th and 7th, then there are two where there will be drive pulses (then it's actually the previous trace…).  So more or less the control system is doing hit/miss alternately.  This is doing significantly better than when screwed to the bench as I hoped.  Now to rebuild my time measuring system…

Also I can now walk round the workshop without having to carefully avoid the thing.

 

Edited By John Haine on 27/01/2023 17:16:15

Edited By John Haine on 27/01/2023 17:18:21

[Tony JeffreeParticipant @tonyjeffree56510]

John – your use of core-less coils to drive the pendulum prompted me to unearth a couple of coils that I had to hand that were bought as components for a loudspeaker crossover. Thes kinds of coils can be readily purchased from the usual electronic supply suspects, are beautifully wound, and come in a wide variety of sizes. An easy solution that avoids the need for coil winding. The ones I unearthed are wound from 0.4mm copper wire, and have about a dozen layers of 25 turns.

Edited By Tony Jeffree on 29/01/2023 17:05:51

[John HaineParticipant @johnhaine32865]

Posted by Tony Jeffree on 29/01/2023 16:57:43:

John – your use of core-less coils to drive the pendulum prompted me to unearth a couple of coils that I had to hand that were bought as components for a loudspeaker crossover. Thes kinds of coils can be readily purchased from the usual electronic supply suspects, are beautifully wound, and come in a wide variety of sizes. An easy solution that avoids the need for coil winding. The ones I unearthed are wound from 0.4mm copper wire, and have about a dozen layers of 25 turns.

Edited By Tony Jeffree on 29/01/2023 17:05:51

Would be good to know where they came from Tony*. But HH coils are quite easy to make and you have the advantage then of a predictable flux density so the current needed can be calculated. But if I ever make another one I might well buy two of those coils…

* https://willys-hifi.com/collections/audio-inductors-air-core ?

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