Member postings for S K

By "narrow," I meant the net width of the spring material, not the spacing between springs in a two-spring configuration. So, using two of the thinnest, shortest and narrowest springs (spaced apart appropriately) that can hold the weight and not permanently deform under abuse, etc., are likely to be best.

It's clear that a lot of clock-making black magic is in trying to balance one unavoidable adverse effect against another opposing adverse effect. But I'm not patient enough to fuss with all that. I just want to do a respectable job based on good materials and straight-forward techniques, maybe with a touch of artistic flair, and hopefully get decent results.

Well, yes, I'm sure all materials will exhibit some elastic hysteresis, but it would be very hard to measure any in a nice thin strip of well-tempered spring steel or beryllium copper, etc., and when flexed only slightly.

My query was whether the "spring" in a spring is actually beneficial at all to a pendulum, or whether it's a detriment that's only used because it's functional and convenient in other ways. I think the latter.

Anyway, the adverse effects of a spring are likely minimal compared to air friction (like maybe 1% of air friction), and the ancillary benefits (support and some anti-twist) are useful. Alternatives such as silk threads or knives have their own problems that are typically worse. But you still want a spring that presents as little interference with the motion of the pendulum as possible, including by being thin, narrow (including when split in two) and short.

Some springs with designed-in hysteresis have been used in things like old-school keyboards to provide a tactile sensation by buckling under load. But a spring exhibiting any noticeable hysteresis (i.e., one that doesn't obey Hooke's law) would be very poor for a pendulum application.

My gut and my limited experience tells me that knife edges are about as good as it gets - and superior to springs - for pendulums, at least when their finicky nature isn't an obstacle. That's rare, though. For example, I live in earthquake country, and I'd never propose using knife edges as a long-term solution, as sooner or later it would be jostled out of position.

I fully expect my spring-mounted pendulum to have lower Q than my knife-edge version, all else being equal.

Edited By S K on 31/03/2023 22:26:11

A spring can do that, but the "storing and returning energy" part is supposed to be done by exchanging potential and kinetic energy through the force of gravity.

It seems to me that this aspect of a spring is at best a distraction and at worst an influence that corrupts the natural behavior of a pendulum. So, shouldn't this particular quality of a spring be minimized, e.g. via a spring that is as thin, narrow and short as practical given it's other jobs (to support the pendulum, minimize twisting, etc.)?

Edit: Now I'm thinking of starting over. 🤔

Edited By S K on 31/03/2023 17:37:45

I think it would have to be stabilized somehow, like clamped between sacrificial metal.

I think I'll just use it as-is. At least it should be robust, as I had wished when I initially laid it out. I'll make a better-thought-out Version 2 if I get that far.

It does lead me to wonder "why a spring?" in the first place. I had thought the springiness was irrelevant, and that the goal was just to act as a pivot. But this strip is springy enough to have an influence on the pendulum vs. my knife-edges. Is there more to the spring being a spring than providing a pivot?

How might I do that? It's just a thin strip hanging in space from the chops. I can't think of a clean way to cut a slot in it at this point.

Buoy is frequently pronounced more like "boy" here, but tinged with "buoy" (our English is so sloppy), and buoyancy is most frequently pronounced sounding like "boy" as well, maybe with a slight tinge of "buoy" too.

I made some progress towards a spring-hinged pendulum (for yet-yet-yet another arduino-based clock), but now I have some doubts.

The spring is from 0.004" (0.1mm) beryllium copper. I wanted to make it simple and robust, so it's a single piece spring that is 13/16" (~20mm) wide.

Now, looking at it and comparing notes with other pendulum designs, I think it's too wide. This may be especially true since I intend on a much lighter weight of 2 lbs, compared to the 14 lb or so bobs that seem common. It's already mounted on one side to chops, so it would be hard to trim it any smaller at this point, neatly anyway.

What do folks here think?

I was in error - not sure what I was thinking.

There is at least one configuration that does have zero influence on the stand: a type of pendulum with an infinite period. But that's not very practical for a clock.

Edited By S K on 25/03/2023 17:05:36

Something I've been thinking about:

An "invertible" pendulum that has the same period hung either way (see my "precision gravity pendulum" project) has its centers of oscillation right at its pivots. This configuration should therefore result in a minimum of motion of the support structure and hence less energy loss, etc.

Achieving that balance is fairly easy to within say 0.1%, as long as you are willing to tune the pendulum's basic parameters. In the end, it wouldn't have to actually be invertible, it would just have to be balanced as such.

It may be worth considering, especially for attempts that would benefit from higher Q.

Edited By S K on 25/03/2023 00:38:49

Seems to me you know how to calculate most of the correction and are just worrying about getting the system delay offset right. You need to find a way to measure that accurately, including the jitter, preferably in hardware. If it's solid, then no problem. Otherwise, it will limit the accuracy. Perhaps running a PID for a while would help dial it in, but you can't leave it running or it will subvert the reason for the pendulum.

Edited By S K on 24/03/2023 00:38:22

I think part of my confusion is due to the lack of context; the "why" part. Why is it important to you to synchronize the two clocks? Just don't want to be off by up to a second? Do you expect to hold accuracy of <1s for very long?

I'm a bit confused about what you wish to do, including what you mean when you write "clock" or "pendulum clock."

Is your pendulum an accurate seconds one? Or does the "clock" convert its beats to seconds via math? Do you mean a clock circuit driven by the pendulum, meaning that the pendulum is the thing that actually needs to be synchronized? If so, obviously the pendulum will start out of phase and it may take considerable effort over time to coax it into synchronization, if that's even possible.

A possible approach for initial synchronization would be to capture the bob with an electromagnet or some other sort of electro-mechanical trigger, and release it electronically on a beat. One would want to move the electromagnet, etc., out of the way after the release. (I thought about trying that for starting a genuinely-free pendulum's swing at a fixed amplitude for repeatable tests.)

Or do you mean you just wish to synchronize the "clock" (sans pendulum) to reset simultaneously with the external time source without a significant delay, but thereafter be incremented by an out-of-phase (and possibly not even seconds) pendulum? If so, you may need a faster circuit (e.g. an FPGA) or else yes, a repetitive correction in the fashion of a PLL sounds needed.

The circuit looks intended to drive very high currents through the LED, maybe 100A or more.

But R1 is 100 Ohms and so that limits the maximum DC current to 0.05 A. Instead, the circuit relies on C2's stored energy to create a short, rapidly-decaying burst of current through the LED and T1.

My guess is that the D1 diode is just a representation of a parasitic reverse-polarity diode inherent in the LED, yes?

T1 has 6,500 pf of input capacitance, which is why U1 is inserted. The latter can drive T1's input capacitance in under 50 ns. T1 has an 80 ns rise / fall time. I wouldn't recommend using a standard CMOS input instead of U1.

Except for sheer convenience, I'd avoid USB power supplies, including battery-based ones, as they are all "switching" supplies (for cost and efficiency reasons). These are very noisy.

You want a nice, quiet linear supply with good regulation, like a decent laboratory bench supply, but those are somewhere around the $300-500+ U.S. mark.

A suitable battery followed by a common "LM"-type regulator would be quiet too, though long-term regulation would be an issue as the battery runs down, temperature changes, etc.

It's not impossible that a mechanical drive (a falling weight) could be more consistent over the long term than all but very well regulated electronic drives.

Some form of low-pass filtering could help with those sharp edges, which can't be that great for the pendulum's performance.

I'm far from an analog guru, but I suspect John would be able to provide some guidance, perhaps even a step towards that proposed raised-cosine profile.

Unless you are completely satisfied with whatever residual horizontal movement is in the hinge area, why not add some cross-bracing in the direction of the swing?

As I'm not intending to try for a "world beating" pendulum-based system, and because I'm motivated to learn more about machining, I've been musing about developing some sort of "improved" mechanical impulsing technique.

I happen to have a small servo that purportedly can be set to apply no more than a preset maximum torque. That's not what I want, though: I'd like to apply a varying torque, e.g. in a raised-cosine. If the baud rate is high enough, perhaps a continuously-varying torque could be approximated just using the servo, though.

I've also seen cam-based systems that can apply varying torque, but they are not adjustable without cutting a new cam. In combination with a programmable-torque servo driving the cam, perhaps they could be made adjustable, but it feels like a fixed cam would inevitably have to be redesigned several times and perhaps never be quite right without some insane math to help figure out the profile.

The nice thing about a servo is that they are so easy to control. But another problem may by how to reduce the torque, as a servo may be too strong compared to the tiny amount of force needed. I guess a 1 out of n swing approach could mitigate that.

I was also thinking about a lever made of thin spring steel that would soften the torque application by flexing (I saw something like this in an ancient clock patent). Perhaps an appropriately-designed spring like this could, in conjunction with a fixed-torque servo and a feedback loop (strain gauge, capacitive sensor...) could work.

A voice-coil is another possibility, but it would require a more complicated drive circuit than a servo would. And at that point, you might as well make an electromagnetic drive anyway.

(Also, because I like quirky, amusing things that might make someone else smile too, I was even thinking of making a little hand or finger that applied the impulse to the pendulum - hehe.)

So Matthys explored a continuous drive approach using clipped sine wave (apparently first tried in 1960). The notion is to continuously restore the lost energy. So at the extremes of the swing, the restoring force should be zero, and at BDC, when the speed and hence air resistance is at its maximum, the restoring force should be at its peak.

I don't understand the original motivation for clipping, though - it seems plainly contrary to what is desired, as Matthys eventually points out. Also, while continuous drive seems to make sense at some level, it's long been known that the ideal time to impulse (at least for a brief one) is at BDC. Continuous drive would also have to be quite precise about its phase relationship, or near the extremities it may start pushing when it should still be pulling and vice versa.

Matthys eventually comes to the conclusion that continuous drive is not ideal, and that a brief impulse at BDC is likely superior. He also expresses concern about the stability of the drive, which goes back a long way in the earliest attempts at electromagnetic drive. He did mention that a continuous drive could at least be more gentle over its profile than a shorter one at BDC.

My thought is that a raised-cosine drive or similar profile concentrated at BDC is likely to be superior to a "digital" one, as that "twanging" - the shock of a sudden sharp impulse that some here have observed - would likely be lessened.

Ah, good. Thanks for that.