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Precision pendulum techniques

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S K01/03/2023 22:21:58
288 forum posts
42 photos

That document is interesting.

After wading through (sometimes) politely-expressed but numerous long-standing grievances (what is it with old-time clock experts and their in-fighting? Or even modern ones - hehe.), we find that a pendulum in vacuum is some sort of over-balanced perpetual motion machine that can't be used to tell time! Air friction is a life-saver!

I guess geniuses are often "special."

 

Edited By S K on 01/03/2023 22:32:39

duncan webster02/03/2023 15:26:52
5307 forum posts
83 photos

My interpretation is that Harrison found really ingenious ways of managing temperature and air density variations. More modern astronomical clocks use low temperature coefficient materials, and get away from density variations by running in vacuum. Add the ability to very accurately control amplitude and Harrison techniques are no longer appropriate.

Those like me who only want to know what time it is to a reasonable accuracy can stick to low coefficient pendulum rod and controlled amplitude. And no I'm not going to define reasonable.

Michael Gilligan02/03/2023 15:37:12
avatar
23121 forum posts
1360 photos
Posted by duncan webster on 02/03/2023 15:26:52:

[…]

And no I'm not going to define reasonable.

.

You don’t need to, Duncan …”The man on the Clapham Omnibus” defines reasonable angel

MichaelG.

S K02/03/2023 17:18:41
288 forum posts
42 photos

I wonder what master-level clocks would have evolved to today if pendulums were still the most precise time-keeping technology?

I'd presume all the last developments prior to quartz oscillators (controlled vacuum, low COE materials, high-Q, small amplitudes, decoupled pendulums, etc.) but with electromagnetic drive and electronic measurement and compensation techniques much as folks here are playing with.

I'm still wondering how a synchronome-style mechanical gravity-impulsed drive compares to electronic drive. Certainly the best of the historical mechanical ones are still amongst the best of all. But I have to imagine that electronic drive can be made better due to the lack of friction if nothing else.

My next project will be a clock of sorts (probably still just a pendulum, with everything else electronic), but as I would not be aiming for ultimate accuracy, I've been toying with the idea of making a mechanical drive for it as a steam-punk flourish. Therefore, I've been wondering what a modern interpretation of that would look like. For example, electronic triggering is an obvious improvement over a mechanical counting wheel. And perhaps a servo might reset the drive weight rather than an electromagnet? Any other ideas?

S K02/03/2023 17:22:08
288 forum posts
42 photos

Oh, and does anyone have an answer to my pendulum bob orientation question?

duncan webster02/03/2023 17:27:24
5307 forum posts
83 photos
Posted by S K on 02/03/2023 17:22:08:

Oh, and does anyone have an answer to my pendulum bob orientation question?

Apart from looking distinctly odd I can't see any down side, and as you say it can't be facing the wrong way. Best shape is I believe a rugby ball, followed by a sphere.

Martin Kyte02/03/2023 17:36:27
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3445 forum posts
62 photos
Posted by S K on 02/03/2023 17:22:08:

Oh, and does anyone have an answer to my pendulum bob orientation question?

The only thing I can come up with is the potential to create lift as it moves through the air (or down force of course). Probably trivial though. Apart from that it doesn’t look as pretty.

As for where free pendulum clock would have got to, we are all waiting on Dave.

regards Martin

John Haine02/03/2023 18:44:52
5563 forum posts
322 photos

S K, I think that's a very interesting point about having a horizontal lenticular Bob. The only snag might be clearance to the case? On adjustment weight, its shown in many places that a weight anywhere on the rod will decrease the period, the amount being a maximum at the middle of the rod. If it was at the bottom it would,d be at the centre of gravity so just makes the Bob a bit heavier, if at the top it has zero MoI so can't change the period at either.

John Haine02/03/2023 18:49:25
5563 forum posts
322 photos
Posted by S K on 02/03/2023 17:18:41:

I wonder what master-level clocks would have evolved to today if pendulums were still the most precise time-keeping technology?

....

.......For example, electronic triggering is an obvious improvement over a mechanical counting wheel. And perhaps a servo might reset the drive weight rather than an electromagnet? Any other ideas?

**LINK**

S K02/03/2023 19:15:09
288 forum posts
42 photos

Do you have any comments about the mechanical vs. your newer electromagnetic impulsing?

S K02/03/2023 19:33:10
288 forum posts
42 photos
Posted by John Haine on 02/03/2023 18:44:52:

On adjustment weight, its shown in many places that a weight anywhere on the rod will decrease the period, the amount being a maximum at the middle of the rod. If it was at the bottom it would,d be at the centre of gravity so just makes the Bob a bit heavier, if at the top it has zero MoI so can't change the period at either.

I don't think that's strictly true if the pendulum's shaft has a non-trivial mass compared to the bob, right? For example, if the pendulum was just a solid bar?

That was close to my situation, especially on the "light" side of the pendulum, in which the shaft and the small "bob" (the small pivot) were somewhat equal in mass. In any event, I wound up adding mass above the pivot when hung in the normal direction, which slows it down.

(But yeah, I spoke too loosely about it without explaining that the COO was actually above the bob, which is not typical for very heavy bobs.)

Edited By S K on 02/03/2023 19:37:45

SillyOldDuffer02/03/2023 19:46:15
10668 forum posts
2415 photos
Posted by John Haine on 02/03/2023 18:44:52:

S K, I think that's a very interesting point about having a horizontal lenticular Bob. The only snag might be clearance to the case? ...

The main objection I can think of is a flat disc is more likely to collect dirt. It would alter the period by moving the bob's centre of gravity, and drop Q by making it harder for the bob to pass through the air.

Both effects would be small, but as I've learned from painful experience, small things matter in precision clocks!

Cylinders aren't ideal. I'm using a cylindrical bob of about 3:1 shape which the experiments reported in 3rd Edn Rawlings found to be the worst possible shape. I used a cylinder because they're easy to make! As tall thin cylinders are bad, I guess SK's short fat cylinder might score by being least worst of the clan!

A 2:1 parabolic spindle did best in Rawlings, presumably mounted vertically, so the aerodynamic shape cut through the air.

Be interesting to compare the Q of a pendulum with a flat disc mounted vertically and then horizontally. Swinging the bob through the smoke from a Joss Stick might show if one alignment is more turbulent than the other.

Dave

Martin Kyte02/03/2023 20:17:45
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3445 forum posts
62 photos

The Trinity clock has its aneroids and weights attached above the pivot point I believe. Lower pressure causing the bellows to expand raising the weights and slowing the pendulum to compensate.

regards Martin

Michael Gilligan02/03/2023 20:30:42
avatar
23121 forum posts
1360 photos

Posted by SillyOldDuffer on 02/03/2023 19:46:15:

[…]

A 2:1 parabolic spindle did best in Rawlings, presumably mounted vertically, so the aerodynamic shape cut through the air.

.

Au Contraire if I remember Matthys correctly

… I will have a look for my previous posting.

MichaelG.

.

Edit: __ https://www.model-engineer.co.uk/forums/postings.asp?th=184124&p=14

.

202aec1c-6c44-43b5-aec9-041acffa3c71.jpeg

Edited By Michael Gilligan on 02/03/2023 20:38:59

duncan webster02/03/2023 22:31:04
5307 forum posts
83 photos

That's what I meant by rugby ball.

John Haine04/03/2023 12:18:03
5563 forum posts
322 photos
Posted by S K on 02/03/2023 19:15:09:

Do you have any comments about the mechanical vs. your newer electromagnetic impulsing?

Synchronome-style impulsing causes a lot of spurious rod oscillations, which in my version are (a) worse than the classic design because of the very light rod, but (b) probably not as bad as they could be as the impulse is much more gentle. Comparing my ADEV with that of a conventional 'Nome they look pretty similar. The jury is out on my new impulsing method, there seems to be a lot of "microsecond level" noise in the measurements and I'm uncertain what is causing it, whether it's some spurious rod mode or possibly some play in the suspension. Currently trying to diagnose.

SillyOldDuffer04/03/2023 13:53:30
10668 forum posts
2415 photos
Posted by John Haine on 04/03/2023 12:18:03:
Posted by S K on 02/03/2023 19:15:09:

Do you have any comments about the mechanical vs. your newer electromagnetic impulsing?

... The jury is out on my new impulsing method, there seems to be a lot of "microsecond level" noise in the measurements and I'm uncertain what is causing it, whether it's some spurious rod mode or possibly some play in the suspension. Currently trying to diagnose.

Me too. I lack a comparison because I've not built a mechanically impulsed clock, let alone a good one. However, I have a notion that an electromagnet can be tuned to apply a softer accelerating impulse than any mechanical method. My electromagnet is side-on and about 15mm from the bob when it fires. As can be seen from the graph, the resulting field is low at that distance, and it's strength rises gently into a knee at about 5mm after which the field strength rises rapidly as distance to the pole drops. The idea is that the bob accelerates gently up the slope, and never jolted because the electromagnet is switched off before the bob gets too close.

The field strength can be controlled by altering pulse width, which is how I'm doing it at the moment, and/or by controlling the shape of the pulse with a resistor/capacitor. A key advantage, as I see it, is the ability to control the impulse across a wide range of values and timings, so that it can be optimised without having to rebuild a precision mechanical system. Might even be possible to optimise the magnetic impulse on the fly.

magneticfield.jpg

Seems to work but it remains unproven! Cause for concern includes the microsecond level noise noted by John. I hope my noise is explained by the mechanical shortcomings of my suspension, but I may also have spurious rod modes, that is the pendulum rod being twanged even by tiny impulses.

For what it's worth, next graph is a Fast Fourier Transform of my clock's last log file. FFT extracts frequency information from a data stream, revealing what the frequencies are, and how strong:

fftlast.jpg

Ideally, the FFT analysis of my pendulum, which has period of about 0.83s, should have only one frequency in the output, 1.2Hz. Oh dear, it doesn't: FFT identifies a significant amount of energy at 0.6Hz, and 9 spikes 0.159Hz apart. These indicate other sources of oscillation, such as the rod twanging, the suspension twisting or rocking, or the frame shaking.

Dave

Mason Green04/03/2023 13:58:02
19 forum posts
1 photos
Posted by John Haine on 04/03/2023 12:18:03:
Synchronome-style impulsing causes a lot of spurious rod oscillations

Rather than impulsing the pendulum, one could take a "radical" approach and alter the pendulum center of gravity like this gentleman at CERN:

Elektročas HH3 - the most accurate pendulum clock on the planet

Unfortunately, regular civilians are unable to PLL a 10Mhz atomic clock signal on premise, although one may fetch a nice clock signal via GPS:

10Mhz frequency reference for mortals.

More than one way to skin a cat.

--MG

SillyOldDuffer04/03/2023 15:42:11
10668 forum posts
2415 photos
Posted by Mason Green on 04/03/2023 13:58:02:
Posted by John Haine on 04/03/2023 12:18:03:
Synchronome-style impulsing causes a lot of spurious rod oscillations

Rather than impulsing the pendulum, one could take a "radical" approach and alter the pendulum center of gravity like this gentleman at CERN:

...

10Mhz frequency reference for mortals.

More than one way to skin a cat.

--MG

Trouble with internet forums is new members may not realise stuff has already been discussed, perhaps as recently as last month! John Haine and I are both into OCXO, and I just calibrated mine with GPS.

A couple of months ago the forum discussed whether or not we approved of disciplining a pendulum with a more accurate clock. It's not difficult to do, and can even happen by accident! We decided it was cheating. We're happy with comparing our pendula to more accurate clocks to see how well or badly they're doing, but we want our pendulums to keep time on their own.

My goal is a pendulum clock that runs as well as a Shortt-Synchronome using straightforward modern components. Boy, is it difficult! After over a year's effort I'm still orders of magnitude away. Hours of fun though!

Dave

Mason Green04/03/2023 16:12:08
19 forum posts
1 photos

Posted by SillyOldDuffer on 04/03/2023 15:42:11:

My goal is a pendulum clock that runs as well as a Shortt-Synchronome using straightforward modern components. Boy, is it difficult! After over a year's effort I'm still orders of magnitude away. Hours of fun though!

I'm not convinced an electromagnetic impulse is the way to get there. Maybe you can change my mind.

Advantages of dynamically adjusting the center of mass:

1) Simplicity and accuracy

2) If there's a loss of power, the clock will still function

With WWIII on the horizon, advantage #2 isn't such a crazy consideration.

--MG

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