Member postings for S K

Yes, the knife edge is essential, but for a different reason: A round pivot would result in the hinge of the pendulum rocking back and forth too, disturbing its performance. Of course, even sharp knife edges have a non-zero radius, and changing knives could result in changes in performance as well.

Edited By S K on 02/02/2023 13:24:41

Hopper: Knife-edge pivots for pendulums have been quite rare. Only the finest master time-keeping clocks have used them, and even then not often. They are just too sensitive and problematic for general use. But I'm not really trying to make a clock, I'm just making a "precision pendulum" in a fashion simple enough to implement using my meager tools. I've mostly completed the pendulum itself, and I'll start a separate thread once I get just a little further with the mount.

But to your point: Some have used an orthogonal pivot with higher damping to mount the main pivot, such that the pendulum adjusts the orthogonal axis on its own due to gravity. Before I start cutting absurdly-expensive brass (wow, what a shock!), I'll give that a bit more thought, but it's more complex to make.

Pete: Nothing wrong with a fine-pitch screw. It's all the rest, to achieve a rigid setup, that I've inquired about.

Thank you. Yes, I could use a micrometer head instead of the ball-tipped fine set screw. And in combination with a spring-loaded bolt, the net result should be easier to use than messing with two counter-acting screws.

At the pivot end, I think a single rod sitting in book-matched V's should do, no? I suppose a spring-loaded bolt at that end would help, too.

Yes, I did understand: Using two slightly different same-handed threads would amount to a vernier-type adjustment mechanism.

Now I'm also thinking of a combination: A fine set-screw (e.g. the 100 tpi one) to set the distance, and an opposite-handed differential screw to lock the two surfaces together (the base and the pivoting part).

As John Haine will surmise, this is to level a pendulum that is hanging on knife-edges.

Thinking through some math: My machinist's level (6" Starrett 98) has a purported resolution of 0.005" per foot. Over a 6" baseline, that would target 0.0025" as an accuracy limit at the adjustment thread. At 100 tpi, that would be a quarter-turn, which seems practical. If the adjustment error is half that resolution, then the error from vertical at the bottom of a 24" pendulum would be 0.005" (or up to twice that - not sure how Starrett is quoting the accuracy). It's hard to say if that's really good enough, but it doesn't sound horrible, and in any event it's all I would get from the Starrett.

Edited By S K on 01/02/2023 21:09:48

Edited By S K on 01/02/2023 21:10:18

Oh! I saw those but didn't understand the point - thank you for the explanation.

Ok, I'm seeing 1/4-20 to 1/4-28, but this is a coarser difference, and these are claimed to be intended to mate differing threads rather than the fine adjustment purpose. Anyway, I'll explore the options.

Thanks again.

Edited By S K on 01/02/2023 20:15:27

Ah, yes!

The ones I can find are only 20 tpi, though, so a quarter-turn would be 0.0125". In metric, the finest is M5x0.8, so 1/4 turn would be ~0.08". I have to dust off my trig, but it just might do if the baseline is long enough.

Edit: There are carbide insert screws that are finer-pitched. I see one that is 1/4-28, with adequate thread lengths, or 5/16-32 with marginal thread lengths. This could work, though the range of adjustment would be rather tight.

Thanks!

Edited By S K on 01/02/2023 20:02:07

Thanks for your response Bernard, but it seems I left something out:

The leveling needs to be done relative to gravity, not merely to some other surface. So it would need to be leveled each time it's moved. That's why an easy to use screw-type adjustment is desired. I've corrected this in my original post.

Thanks again.

Edited By S K on 01/02/2023 19:20:51

Edited By S K on 01/02/2023 19:22:29

Dear All,

I wish to be able to level a plate or bar relative to gravity via a very fine adjustment with near-micrometer sensitivity. Only one axis (vertical, i.e. on one side of a bar or plate relative to another) is particularly important, and only a small range of adjustment would be necessary (a few degrees).

The leveling adjustment needs to be done to say a thousandth of an inch. A micrometer head or a very fine-pitch screw could be used. For example, I can obtain stainless-steel ball-pointed 80 or 100 threads per inch set-screws and associated brass threaded inserts that look appropriate for the task.

My question is how to arrange things to maintain rigidity. Lateral as well as vertical rigidity is needed. I can't get away with simply setting and adjusting the screw(s) on some surface as one might do with a tripod. I've thought about adding hold-down force via spring tension or pressure, but I'm not sure that would be good enough.

The easiest (and crudest) way I can think of is using a fine-pitched threaded rod or screw (seems 32-48 tpi is the maximum) with two nuts to capture the floating element that needs leveling. The other end can can be similarly captured, but is just slightly loosened during adjustment, as it mainly acts as a hinge. This is how my bubble-level is adjusted, so perhaps it's OK? But it still feels unsatisfactory compared to just turning a micrometer head or equivalent.

Is there a better way of doing this?

Thank you.

Edited By S K on 01/02/2023 19:04:17

Edited By S K on 01/02/2023 19:20:15

Thanks for pointing out that book. The introductory comment about quartz clocks should be accompanied by a sober-sounding narrator intoning "little did they know ..."

Unfortunately, it's written in that overly wordy and flowery fashion that was common in earlier times, which makes it a slog for modern readers.

So that figure 82. What are the points being made? Seems like:

(1) That the amount of energy needed purely for keeping the pendulum in motion is the same whether spread over 30 impulses or applied in one larger impulse? Seems logical.

(2) That more energy is wasted if 30 small impulses are applied vs. 1 larger one? Also seems logical in a mechanical system, but likely irrelevant in magnetically-impulsed systems.

(3) That more energy is wasted if the impulse is applied at the bottom vs. near the top of the pendulum? I have more trouble accepting this, but I could imagine that additional friction could be encountered at the bottom because a larger throw of the impulsing mechanism could be necessitated due to the larger amplitude near the bottom. But again, this supposed additional wasted energy is likely irrelevant in magnetically-impulsed systems.

As for the ideal position for impulsing, I'll stick with the center of percussion, which would be near the bottom in a pendulum with a heavy bob.

One other point made is that the mechanical impulse was designed to be applied with nearly zero force at its onset, growing to some maximum before release. I think this could be pertinent to magnetically applied forces as well, and I had imagined using an RC network or similar to avoid an abrupt application of force.

Edited By S K on 01/02/2023 16:05:22

Thank you for the detailed explanation. The second coil was almost hidden in the head-on photo, so thanks for mentioning that too.

I think a horizontal arrangement should be OK in principle even when activated at vertical, except that the amount of energy needed would be higher. One pendulum I saw (from memory) included symmetrical rod "horns" extending towards the coil, and partially encompassed by the coreless coil, so it was operating essentially as a solenoid.

Do you have any comments about the use of the Hall effect sensors? I looked up the sensor you are using, and it looks there's a non-trivial temperature effect on the "operate" and "release" of the sensor about room temperature. I have never used them, but I've been presuming that optical would be superior.

That chain timing controller is interesting. But I have to imagine it causes additional friction, as well as other disturbance due to the free hanging region of the chain applying slight side-ways resistance on the shaft as it rocks. Also, I think it inevitably has to lengthen and shorten slightly as the tray rocks back and forth too, meaning the amount of weight on the tray changes slightly too. In the case of this clock, these effects are evidently lower than the temperature and barometric pressure effects, though.

I've imagined a battery-operated servo of some sort mounted to the shaft, or perhaps better-yet hidden inside a bob's case, to adjust the weight without such effects.

Very nice!

I'm working on something similar. A question: I note that you have positioned the coil drive pointing vertically. I've seen that elsewhere at least once, too. I wonder why that is? Wouldn't it be more efficient pointing horizontally? Also, does it have an iron core or is it open?

Thanks.

It's amusing to think about all the corrections that can be made to a pendulum based on sensors and math, some of which has been discussed and implemented in this thread: temperature and humidity at the least.

It's even likely possible to "discipline" a pendulum such that its motion precisely matches a GPS pulse-per-second signal, for example by adjusting the amplitude via a PID controller to take advantage of its slightly non-isochronous behavior. That reduces it to an "executive toy," but it's a fun idea.

Mine will not be a second's pendulum, so I figured I'd just rate it mathematically by comparing it to a precise RTC over a reasonably-long time period, e.g. a week or so. Ticks would be in floating-point rather than integers, or else if one felt the need for integer seconds, they could be generated by interpolation.

Of course, it's then tempting to correct it daily, or hourly, or even continuously. Is that any different from someone with a fancy new mechanical watch who obsessively monitors and corrects it every day?

In an age of atomic clocks, we are all just making "executive toys." 😀

Edited By S K on 26/01/2023 20:13:03

Also: If you have an oscilloscope, I'd put it on long or infinite persistence and see if any of the waveforms that should be uniform aren't.

I wish I could find good quality drawings or photos of the Riefler pivoting spring mechanism. I guess I could look up his old patent.

Some used a ratchet-like system to time the gravity-driven impulse, e.g. every 30 seconds exactly by counting swings, and others used the Hyp toggle, whose timing is variable based on the amplitude of the swing.

All those that I've seen used a gravity "escapement" (used loosely), with the lever reset electromagnetically. I haven't seen a direct magnetic impulse (no gravity escapement) on any of the original historic models. I've wondered why, or maybe I have just missed them?

It's surprising how well those old systems worked despite the mechanical impulses. One would hope that a modern "frictionless" interpretation would do better. Of course, it's all just for fun and satisfaction these days.

(I love being pedantic too!)

Edited By S K on 26/01/2023 17:49:17

After I posted I wondered some more about the difference between impulses on every beat vs. every n beats.

The synchronome-type clocks only impulse based on the retardation of the swing below a trigger level. But it's via a messy mechanical detection and impulse scheme, and I could see them wanting to minimize the frequency of doing that. Your essentially frictionless optical and electromagnetic version shouldn't suffer much from that.

And of course ordinary clocks do impulse on every beat. I'd expect clock makers would have devised n-beat impulse schemes long ago if that was really superior.

Anyway, so now I'm doubtful that a 1 beat impulse is better or worse than an n-beat one. But what I did think is that a scheme which skips the impulse-beats and somehow counts time only on the non-impulse beats could be superior. That would naively be hard to measure time from, but in hybrid mechanical / computational system, all kinds of magic can happen.

If I were you, I'd stop thinking of your measurements as "amplitude" and just call it what it is: "period." Yes, I know it's related, but you aren't actually computing the amplitude (e.g. in degrees) from period, are you? You are just measuring and plotting the period.

I still think there's some beat-type situation going on. I can totally see ~22 x ~3ns errors adding up to 64 ns and then starting over somewhere in your system.

A few things come to mind:

First, I believe you are only using a single position sensor (e.g. an opto-interrupter), right? In that case, how are you measuring the "relative amplitude?" Calculated from slight changes in period?

I believe you are also applying an impulse on every swing. Are you absolutely sure that the impulse width is exactly the same every time, and not off by +/- your clock period? If not, your variation in amplitude may be due to a beat-like phenomena. Maybe that 64ns or whatever your timing resolution is can be found in your data? For example, if the interrupt signal (or something similar) cannot be detected or acted on to better than 64 ns resolution, then the width of the pulses can vary slowly between x and x+64 (or x-64) ns and then flip back to x?

Stepping back, I think you should consider switching to a two-sensor arrangement (e.g. using those Sharp sensors): one for the bottom dead center (where the impulse should be applied) and one for an extremity of the swing.

I'm not sure it's a good idea to be applying an impulse every swing. That requires a finely-balanced impulse that can maintain a certain swing amplitude just based on the strength of the impulses alone. That's difficult to maintain, and I suspect that's leading to some of your trouble as in the "relative amplitude" graph. Instead, it should be applied only when the swing fails to reach the extremity-sensor. The impulse should then be just strong enough to cause the swing to exceed the extremity sensor for some number of swings, like say 30 on average. That way, the strength of the impulse is not critically important, it just has to be "strong enough" (though everything will eventually be found as flaws in the data if you look close enough!). You might choose to note or exclude the impulsed swing from your data, as it probably causes a non-ideal swing.

Edited By S K on 26/01/2023 11:09:10

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

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!

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!"