Precision pendulum techniques

[John HaineParticipant @johnhaine32865]

Posted by duncan webster on 10/03/2023 00:34:39:

To add a finale to SK's lifting suggestion, putting energy in does make the bob lift higher than it would have done (at the extreme of swing), but a given amount of energy (ie that lost since the last impulse) will lift a heavy pendulum less than a light one. It will therefore increase the amplitude of a light bob more than a heavy one, and so reduce the period more. If you're impulsing every swing I don't suppose it matters.

Or increase the period if after bdc.

[John HaineParticipant @johnhaine32865]

Posted by S K on 10/03/2023 01:30:52:

Hi John,

Would you have a recommended magnet-wire gauge for these applications?

Thanks.

I use 0.15mm because I have some!

[duncan webster 1Participant @duncanwebster1]

Posted by John Haine on 10/03/2023 09:27:19:

Posted by duncan webster on 10/03/2023 00:34:39:

To add a finale to SK's lifting suggestion, putting energy in does make the bob lift higher than it would have done (at the extreme of swing), but a given amount of energy (ie that lost since the last impulse) will lift a heavy pendulum less than a light one. It will therefore increase the amplitude of a light bob more than a heavy one, and so reduce the period more. If you're impulsing every swing I don't suppose it matters.

Or increase the period if after bdc.

I didn't mean the change due to impulse, if the amplitude is increased by whatever means the period changes, so lighter pendulum iis more affected.

[SillyOldDufferModerator @sillyoldduffer]

Changing the subject slightly, I've been looking at a Keyes Laser module with a view to using it in a beam break sensor. The red beam is much sharper than an IR LED, and it's detected OK by an IR photo-transistor, but it's extremely bright, likely to cause trouble inside the pendulum case due to multiple reflections.

Whilst it was powered up, I grabbed the opportunity to see if my Mk1 Pendulum tripod is vibrating by bouncing the laser beam off a mirror.

The laser module was held in a chemical clamp-stand:

And the beam arranged to bounce off a mirror laid on top of the tripod:

So that the spot hits the ceiling about 6 metres away:

Then the pendulum is set swinging about 15° and the spot examined. It could just be seen to move in time with the pendulum, about a millimetre. The test shows the tripod assembly, total weight 571g and on rubber feet, is moved slightly by a bob weighing 40g.

Not much, but my tripod moves…

Dave

 

 

 

 

Edited By SillyOldDuffer on 10/03/2023 14:26:38

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 10/03/2023 14:24:45:

[…]

Then the pendulum is set swinging about 15° and the spot examined. It could just be seen to move in time with the pendulum, about a millimetre. The test shows the tripod assembly, total weight 571g and on rubber feet, is moved slightly by a bob weighing 40g.

Not much, but my tripod moves…

Dave

.

Disappointing — Yes

Surprising — No

MichaelG.

[david bennett 8Participant @davidbennett8]

Dave, would reducing the length of the pendulum and the whole structure add to it's stability ?

dave8

[SillyOldDufferModerator @sillyoldduffer]

Posted by david bennett 8 on 10/03/2023 17:32:37:

Dave, would reducing the length of the pendulum and the whole structure add to it's stability ?

dave8

It would, but the pendulum would speed up, a 50mm rod vibrating at about 0.45s

Interesting idea which I hadn't thought of at all – a short squat clock, about 80mm high beating at about 0.5s. Perhaps one of the experts will know, but I can't think of an obvious reason for insisting on a long rod.

In a mechanical clock, I guess there's practical advantage in a long slow pendulum, such as making it easier to set the period to exactly 1s. But my clock doesn't care what length the pendulum is, and could beat much faster.

I went for a pendulum rod of about 170mm because it was about the right size to fit into a 67mm pvc drainpipe without it being too tall. I went for a ratio of height to base diameter between 3 and 4 to 1, not very scientific!

Now I'm wondering how a short fast pendulum would perform as a precision time-keeper. Unless someone explains why it's a bad idea, I shall have to try it.

Dave

[S KParticipant @sk20060]

Every pendulum support will move (Newton had something to say about it). The only question is by how much. I presume you have set it up to amplify the motion? Have you calculated the amount of motion right at the pendulum, and for a more realistic amplitude?

I've wondered about how pendulum length affects performance. Matthys claimed that shorter pendulums have higher Q, but without explanation, and that's about all I've seen on the subject.

A longer pendulum is slower, and can move over a shallower angle while still covering enough distance at its bottom to fit in your measurement apparatus. My guess is that it should encounter lower losses at the hinge and in air due to these factors, though a longer shaft would induce some additional losses too. I'm not sure it matters much if you are going to have a partial vacuum and a shallow swing on a low loss pivot, though.

Edited By S K on 10/03/2023 20:18:50

[duncan webster 1Participant @duncanwebster1]

Posted by S K on 10/03/2023 19:59:27:

Every pendulum support will move (Newton had something to say about it)…….

Pedant alert, it was Hooke actually…. Ut tension sic vis as they say in scientific circles

[Martin KyteParticipant @martinkyte99762]

Regarding shorter pendulums especially in a free pendulum timekeeper.

Just intuitively I would say that the shorter the pendulum the bigger percentage of the swing is taken up with impulse interaction. I would say that the percentage of time the pendulum is truly detached the better.

In addition to keep circular error low the angular arc must be reasonably small which results in smaller movements of the bob and less stored energy so more susceptible to disturbances.

Maybe someone has the maths for all this. I tend to go by feel more often than not.

Mind you Fermi was reported to be able to ‘think like a neutron’ so maybe there is hope for me yet.

regards Martin

[david bennett 8Participant @davidbennett8]

The pendulum swings the clock. The pendulum support is base mounted in this case. To maximise stability, the base could be impractically wide, or the pendulum shortened. Of course this introduces it's own problems. Surely the experimental nature of this endeavour is to find out more.

dave8

[Clive SteerParticipant @clivesteer55943]

There is a reaction at the pendulum pivot point which is used in dual pendulum clocks to provide coupling between the pendulums that bring them into mutual resonance. It would be interesting to know if a dual pendulum clock has more stable amplitude and reacts less to noise and vibration.

CS

[John HaineParticipant @johnhaine32865]

Dual pendulums synchronise in anti phase and should be much less sensitive to lateral vibration. A couple of people have made dual pendulum clocks where only one is impulse and the other swings in antiphase in sympathy. This works provided the coupling is quite strong, which favours a less rigid support. One of the clocks actually had the pendulums suspended from a platform which was itself suspended on short ribbons.

[S KParticipant @sk20060]

Posted by duncan webster on 10/03/2023 21:24:39:

Posted by S K on 10/03/2023 19:59:27:

Every pendulum support will move (Newton had something to say about it)…….

Pedant alert, it was Hooke actually…. Ut tension sic vis as they say in scientific circles

Off topic, but you are saying that Hooke's law = Newton's third? Or that Hooke's is the one that applies here, not Newton's?

[duncan webster 1Participant @duncanwebster1]

I'm saying that if you put a load on a structure it will deflect, no matter how small the load or how rigid the structure. If I walked across the Forth bridge it would deflect. Not a lot, in fact I doubt even the best metrologist could measure it, but deflect it would.

Newton says that as the structure pulls on the pendulum, the pendulum pulls on the structure. Not quite the same.

[S KParticipant @sk20060]

I think electromagnetic impulsing is the way to go for higher control and precision, but I've wondered how modern techniques could apply to traditional approaches. So:

In a mechanical impulsing design such as in the Synchronome, a weighted roller applies force to an anvil on the shaft. The profile of the force is determined by several factors, such as how close the tolerance is at the beginning of the force (e.g., does it strike the anvil with an initial shock or very gently) and the profile cut into the anvil, etc.

But what if you applied the force with a servo through a load-cell or capacitive measurement or some other force-monitoring system. With electronics to monitor the force, a profile such as a pseudo-Gaussian force curve could be applied. This would avoid sudden shocks to the pendulum that could cause vibration.

For electromagnetic impulsing, I've thus far only seen a more-or-less digital approach: The coils are simply turned on and off via a digital signal, with little other control except timing and sometimes magnitude. But a proper electronic filter could soften the blow in a more controlled fashion. Or, even better, a DAC and associated circuitry could implement an arbitrary programmable force curve.

Either way, it's not clear to me that anything much different than a pseudo-Gaussian is worthwhile, but being able to control it at a fine level sounds interesting and possibly beneficial.

Does this make sense?

[S KParticipant @sk20060]

Posted by duncan webster on 11/03/2023 18:55:35:

Newton says that as the structure pulls on the pendulum, the pendulum pulls on the structure. Not quite the same.

Right, but that's exactly why the frame rocks as the pendulum does, as SOD observed (Newton). Anyway, enough pedantry for me.

[Michael GilliganParticipant @michaelgilligan61133]

Before we wish goodnight to pedantry … I would just mention that in scientific/engineering usage rigid is an absolute, not a variable. … i.e. It’s a condition that we presume, for the convenience of not needing to include stiffness in our calculations

Rather like Euclid’s point, line, etc. … rigid is an hypothetical concept … in this case of something which is infinitely stiff.

Our perfect hypothetical pendulum is suspended at a point, has a rigid rod, and a bob which inhabits another point.

… Reality is different, and the harder we look, the more different it turns-out to be.

MichaelG.

[S KParticipant @sk20060]

Would "rigid" apply to the character of someone who insists that the word "rigid" must be banned, and those using it excoriated, except if we are talking about an imaginary world of abstract ideas? 😛

[Martin KyteParticipant @martinkyte99762]

On a more practical pedantry Hooke had a lot more involvement with pendulum clocks than did old Isaac so if anyone directly commented on pendulum supports it was likely to have been Robert.
regards Martin

[Martin KyteParticipant @martinkyte99762]

Posted by S K on 11/03/2023 20:26:17:

Would "rigid" apply to the character of someone who insists that the word "rigid" must be banned, and those using it excoriated, except if we are talking about an imaginary world of abstract ideas? 😛

In fact rigid and flexible materials are taught as a subject as part of some degree courses. So it seems perfectly acceptable to consider practical rigidity as well as theoretical rigidity.

😊

[Michael GilliganParticipant @michaelgilligan61133]

Posted by S K on 11/03/2023 20:26:17:

Would "rigid" apply to the character of someone who insists that the word "rigid" must be banned, and those using it excoriated, except if we are talking about an imaginary world of abstract ideas? 😛

I wouldn’t know … I was just trying to explain a point [no pun intended] not to dictate anything.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Martin Kyte on 11/03/2023 20:39:52:

[…]

In fact rigid and flexible materials are taught as a subject as part of some degree courses. So it seems perfectly acceptable to consider practical rigidity as well as theoretical rigidity.

😊

.

Then I give up …

MichaelG.

.

Edit: __ valuable resource for those with a more flexible vocabulary:

https://www.twinkl.ca/search?q=rigid+materials&c=244&r=parent

Edited By Michael Gilligan on 11/03/2023 20:56:20

[duncan webster 1Participant @duncanwebster1]

Posted by S K on 11/03/2023 19:14:38:

Posted by duncan webster on 11/03/2023 18:55:35:

Newton says that as the structure pulls on the pendulum, the pendulum pulls on the structure. Not quite the same.

Right, but that's exactly why the frame rocks as the pendulum does, as SOD observed (Newton). Anyway, enough pedantry for me.

Yes but Newton says it pulls, Hooke says it moves

[S KParticipant @sk20060]

If I may make a point ("you can't!" says a voice from the peanut gallery), I'd like to draw a line ("again: can't!" ) between pure math ("Math? Ha! Any so-called consistent formal system can't prove it's own consistency!!!" ) and reality ("Hahaha, you and everything you think of as real is all just a holographic projection on the surface of a black hole – and even that is just a computer simulation!" ), … wait, I forgot my point ("GROAN!" ). Oh, never-mind! ("Mind? As in 'free-will?' Lol, you haven't heard of super-determinism, have you…" ) 🙃

 

Edited By S K on 11/03/2023 21:17:19

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