Precision pendulum techniques

[S KParticipant @sk20060]

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

[duncan webster 1Participant @duncanwebster1]

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 KyteParticipant @martinkyte99762]

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 HaineParticipant @johnhaine32865]

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 HaineParticipant @johnhaine32865]

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 KParticipant @sk20060]

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

[S KParticipant @sk20060]

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

[SillyOldDufferModerator @sillyoldduffer]

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 KyteParticipant @martinkyte99762]

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 GilliganParticipant @michaelgilligan61133]

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

.

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

[duncan webster 1Participant @duncanwebster1]

That's what I meant by rugby ball.

[John HaineParticipant @johnhaine32865]

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.

[SillyOldDufferModerator @sillyoldduffer]

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.

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:

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 GreenParticipant @masongreen60104]

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

[SillyOldDufferModerator @sillyoldduffer]

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 GreenParticipant @masongreen60104]

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

[John HaineParticipant @johnhaine32865]

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:

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

The CERN gentleman still has to to impulse his pendulum, and that is still electromechanical I think.

Regarding electromagnetic impulsing, the Fedchenko clock is an "existence proof" that it does work and indeed it was the most accurate pendulum clock ever made, clearly showing the tidal influence of the moon and sun in its rate. You can't make 'em better than that!

Thanks for the link to that GPSDO. I'm a little puzzled by it to be honest – we know that any frequency from the GPS device will have a small jitter because the internal 48MHz oscillator isn't itself disciplined to the satellite clocks – rather the circuit that generates the time pulse output introduces occasional corrections in 21ns increments. It's normal to use an external PLL to "smooth" this jitter but most designs have a rather complex digital system to give a very small loop bandwidth – this design given just uses an analogue circuit for filtering so I suspect it doesn't do a great job of jitter reduction.

[Mason GreenParticipant @masongreen60104]

Posted by John Haine on 04/03/2023 17:06:14:

The CERN gentleman still has to to impulse his pendulum, and that is still electromechanical I think.

Zero impulse. Swing time is measured with an optical sensor.

Will be interesting to see how this experiment plays out.

Some in this forum may consider dynamically adjusting the pendulum's center of mass to be "cheating;" while others may see opportunity.

All is fair in art, love, war.

[duncan webster 1Participant @duncanwebster1]

If it isn't impulsed why doesn't it stop? Perpetual motion machine? 0.1 seconds per day is fairly rubbish as precision clocks go, and disciplining it with an atomic clock is just cheating, you might as well use the atomic thing

Edited By duncan webster on 04/03/2023 18:55:59

[Mason GreenParticipant @masongreen60104]

Posted by duncan webster on 04/03/2023 18:55:23:

If it isn't impulsed why doesn't it stop?

Zero impulse, as in not altering the clock's original function.

One could argue that anything beyond 18th century technology is cheating, including the use of a vacuum, LOLZ.

[Martin KyteParticipant @martinkyte99762]

Posted by Mason Green on 04/03/I2023 19:10:06:

Posted by duncan webster on 04/03/2023 18:55:23:

If it isn't impulsed why doesn't it stop?

Zero impulse, as in not altering the clock's original function.

One could argue that anything beyond 18th century technology is cheating, including the use of a vacuum, LOLZ.

I think the cheating bit is using any kind of external clock to correct the going of the clock in question. As has been said monitoring is fine.
regards Martin

[Mason GreenParticipant @masongreen60104]

Posted by Martin Kyte on 04/03/2023 19:16:44:

I think the cheating bit is using any kind of external clock to correct the going of the clock in question. As has been said monitoring is fine.

Isn't that how the Shortt clock operated? Master/slave relationship. In the CERN case we are replacing the vacuum pendulum reference with an atomic clock reference, correct?

Please help me understand how they are fundamentally different in that respect.

[Martin KyteParticipant @martinkyte99762]

Well for a start the free pendulum of the Shortt combination is correcting the Synchronome which provides the impulse.

regards Martin

[duncan webster 1Participant @duncanwebster1]

The Schortt is a system, it has the Synchronome and the vacuum free pendulum. Without the Sync the free pendulum will stop, and without the free pendulum the Sync will not keep good time. In fact the Sync was set deliberately slightly off, can't remember which way. With MG's setup, the atomic clock does all the timekeeping on its own, the pendulum bit just acts as a slave, you could use a solenoid/ratchet system like a Gents or Sync slave. I could of course be misunderstanding it.

[Mason GreenParticipant @masongreen60104]

Thanks, Duncan. That makes sense from a systems perspective.

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