Member postings for S K

Thinking about John's observation: I find it improbable, but not impossible, that the 3-5us time resolution both John and I have seen is tied to the wavelength of light. That is, reaching the fundamental limits of physics (e.g. uncertainty principles) without any special effort, vs. hitting other more mundane limits such as electronic noise and ordinary measurement issues, would have come far too easily to be likely. But I declare that it's John's responsibility to analyze this further! 😉

Also, my new pendulum has achieved perpetual motion. It's never stopped oscillating, albeit with a small but easily noticed amplitude. That's not very good news, actually, as it speaks to a pronounced negative impact of the environment on a non-enclosed pendulum.

Edited By S K on 09/09/2023 16:21:48

Oh no! The ATMega data sheet section 17.3 describes how pins T0 and T1 can be used as an external clock for the timer/counter registers clkt0 and clkt1. But it's not a direct input, it's actually sampled by the normal system clock! Apparently this is just part of the system-wide I/O synchronization. That being so, you can't have a genuinely independent external timer clock. Bummer!

I've only checked a few numbers, but it looks like the minimum output step and hence resolution is 400ns. Still, 400ns is 10x better than the 3-5us that the opto seems to deliver.

That would be too easy! I'd rather spend a few weeks tearing my hair out programming an FPGA! 😄

(But what's with the "100 ns resolution" claim when it appears to actually be 400ns?)

Edited By S K on 05/09/2023 17:33:20

Dave: Thanks for the comments. I've been wondering what "good enough" is or should be. And I still wonder about the philosophical question of what is marking time, the pendulum or a large assembly of electronic bits with a pendulum at the side? I thought I'd try for a mostly "pure" approach, with the electronics only doing rating and timing the impulsing. Other electronics, such as temperature monitoring, would be for examining performance, but not for correcting the pendulum (hence my attempt at mechanical compensation).

I envisioned the DS3231 as an inexpensive and easy to use medium-performance part prior to going nuts with GPS disciplined oscillators, etc. Its TCXO has about the same performance as any other TCXOs, and should yield about +/- 0.17 seconds a day. If I obtained close to that with my modest pendulum operating in open air I'd be pretty happy.

I've been researching the Arduino's hardware timer capabilities, but I want to avoid using the Arduino's own oscillator as the time basis. Not unless I can bash an external clock into it, anyway, and I have no idea how or if hacking it would work.

It seems the code you pointed to does use the internal oscillator. Using that requires continuously calibrating it against yet another time source, which is cumbersome. How are you monitoring / counting both the system clock and your external clock?

Documentation is a bit lacking, but I see that pin 5 (T1) can be used as an external clock input for Timer1 (16 bits). I'm not sure how fast it can go, but I'd like to hope for 16 MHz still. If that's possible, then an external 16 MHz OCXO, with a few parts per billion, could be used. That, I think, would simplify things by not having to calibrate the internal clock with an external clock, e.g with a 1Hz GPS reference.

Edited By S K on 05/09/2023 17:18:45

Tim: I like the idea of putting the compensator at the hinge, though it would probably complicate the hinge itself. But my main problem is that I have only found a reasonable source for Invar at 36" lengths. I did contact a few other dealers in the U.S. (purported, I think they really only drop-ship from somewhere else), but they both asked for about $500 for a longer length. So, if my total length is constrained, it doesn't matter much where the compensator is, and the bottom is easier. Maybe for next time! Thanks!

Progress on electronics:

I put together a prototype system consisting of a mini Arduino, a DS3231 real time clock, a MCP9808 temperature sensor, an SD card slot, and a vacuum florescent display unit. For now, the items are lightly tacked on using hot glue, and I used the "wire-wrap" technique for easy changes. Later I'll install everything more permanently.

• The popular DS3231 RTC is temperature compensated and is good for +/-2 ppm from 0 to 40C and +/-3.5 ppm from -40 to 85C.
• The MCP9808 temperature sensor has an accuracy of +/- 0.25C from -40 to +125C and a precision of 0.0625C.

For the moment, I'm just using Arduino's internal micros function for the period. It's understood that this is not good enough, and it's already evident that it's incorrect, being a fraction of a percent slow as tested. I'm still trying to decide how I'll do it "for real." The RTC, with 2ppm, is much better, and it has a 32kHz output that I might employ somehow. Otherwise, maybe I'll try hacking the Arduino's oscillator.

I chose the vacuum florescent display for its retro appeal. It's not as cool as Nixie tubes, but it's the next best thing. It turned out to be pretty convenient, as I discovered that the module I chose (Newhaven Display M0220MD-202MDAR1-3) is compatible with many of the similar LCD modules out there and with the Arduino libraries for them.

I have a faster Arduino-compatible microcontroller that runs at 120 MHz that I may swap in for the 16 MHz Arduino once things are running properly (it's a bit fussier to use), or else I have a few other considerably faster options on hand that aren't compatible with the Arduino IDE. One of those should improve timing resolution (but not necessarily accuracy). I am also considering getting an FPGA to do high speed timing. Anyway, deciding on how to do "proper" timing, and logging data on the SD card, are some of my next tasks.

Edited By S K on 04/09/2023 21:23:20

Get a tungsten bob. Seriously. Yours is not dense enough in comparison.

Edited By S K on 03/09/2023 21:04:04

You aren't alone. I recently read the old articles by Bateman on his clock. He reported similar drops in Q from such small add-ons. Really, every little nut and screw, etc., needs to be optimized for aerodynamics. That includes the surrounding apparatus that is near the pendulum, such as the opto, etc., that influences the air movement around the pendulum.

Until you are operating in a vacuum, you might as well just figure that, as far as Q is concerned, everything is about aerodynamics unless you are outright scornful of any other frictional sources such as the hinge.

Edited By S K on 02/09/2023 18:13:58

So the curve is an attempt to avoid adjustment or calibration. I suppose calibration - subtracting an offset, or perhaps an offset and higher-order figures, etc., from the data - would be excessively problematic?

My concern about round anvils is that they are pseudo-flat: neither with a sharp enough curve to accurately position the knives in one fixed place, nor flat enough to avoid skewness.

I also really don't like the "scratch" method, mentioned above, that seeks to keep the knives in position - that's begging for a lot of extra friction. But fine vertical scratches or marks for the proper position, and the use of a loupe, should allow positioning the knife edges with quite good accuracy.

My understanding is that the big downside of flat knife rests (whether knife on flat or flat on knife) is that environmental disturbances can cause the pendulum to "hop" minutely, and eventually walk right off the edge. I never saw any evidence of that myself (and I live in a geologically-active region), but my knife-edge pendulum was never intended to run for months or years, and indeed most of the time it was not even rocking. Nevertheless, the fear of that led me to switch to spring hinges for my current effort.

About alignment: Your effort is amazing, but I was surprised to see you have the square knife edge riding on an inside circular surface. All knife-edge setups I've seen have a knife or knives riding on one or two flat surface(s), or vice versa (as in my previous version, which was flats on knives).

Your setup makes theoretical sense, but I feel like your square shank could easily find itself miss-positioned, riding up higher than absolute bottom dead center on either or both sides (I think you have two pivots, right?), or skewed between the sides. Especially in the latter circumstance, I could imagine friction going up. You just changed from V's to flats on the other axis, so why round for your primary one?

Also, using two quite hard steel surfaces (your knife and your inner race) sounds good, but if they are close to each other in hardness, one will inevitably bite into the other. I used sapphire for the flat surfaces, and the steel knives just won't scratch it (dulling of the knives certainly can still happen, but I never got around to polishing their edges anyway).

On a different but related point, my limited experience leads me to feel that my new spring-hinge mounted pendulum is considerably more susceptible to wobble than my old knife-edge pendulum. In the latter case, the main source of wobble was flexibility of the rod (aside from obvious lifting off of the knife-anvil interface when ham-handling it), whereas the spring hinge can easily permit a certain amount of wobble by itself in addition to the rod flexing. I've felt like the spring hinge was a step backward.

Finally, something that I, at least, have been guilty of many times: presuming that this or that technique should and hence must yield superior results, followed by bafflement when it doesn't pan out. That's when the gods really start chuckling. 😉

Edited By S K on 02/09/2023 10:05:36

It depends on where and how you are measuring the amplitude, doesn't it? If you are using an angular sensor at the top, then it doesn't know anything about the length, and if the length of the rod grows, the pendulum will slow down.

If you are measuring the amplitude as the peak swing in X at the bottom, then longer length also means slower. To do this properly, the sensor for this should travel along with the pendulum, e.g. instrumented on a dummy pendulum. But most have the sensors mounted to the pendulum's support, which will typically shift in space independently of the pendulum itself as the temperature changes.

Even the placement of the sensors in X matters. John measured 1um position resolution of his opto. But almost any support fixing the opto's position will shift that much with a one degree change in temperature.

Edited By S K on 31/08/2023 19:21:05

I redid the temperature compensation based on the above data, and also to include an Invar isolator.

The Invar sleeve resides inside the bob from the bottom, supporting the bob at its 50% point. Its goal is to reduce temperature lag due to the large thermal mass of the bob by isolating the bob from the brass compensation. The Invar sleeve extends below the bob by about 0.15" to provide a small amount of additional isolation outside the bob. The main brass compensator sits below the Invar sleeve.

Will it work? It will still be off by some random amount. Hopefully it won't make things worse! If I measure a clear temperature dependence, I might alter the brass compensator to include a small (e.g. 3 piece) binary weighted stack for easier adjustment.

I also made a small pointer, positioned at the bottom of the rod, to trigger the opto-interrupter.

I'm almost ready to add electronics.

Edited By S K on 30/08/2023 21:07:42

In case there's any remaining confusion about how to calculate Q with this sort of run-down test, maybe you should state how you calculated it at each interval.

(It's not surprising that Q improves with lower amplitudes due to lower losses from air resistance per swing. Also, knife edges are cool and may be a bit superior to spring hinges, but air friction almost completely dominates Q at normal atmospheric pressure, as seen by the dramatic improvements in Q in a vacuum.)

Your work is an amazing effort!

Thank you all for your advice.

I went with Renaissance Wax. I first sanded the parts back to 400 grit, trying a little for a uniform grain direction, and I felt this gave a better finish in brass. The wax is easy to apply, and it seems to resist fingerprints to a degree. I'll be interested to see how well it holds up.

Here's an example after waxing. Thanks again.

Haha, the bob was too big for my tail-stock, even, so that's out. Work holding anything larger than a postage stamp is my biggest problem. I need to blow a bunch of money on some real equipment!

An interesting update: I found more information on the Invar rod. "Invar" is a trade-marked name, so this is "Dilaton 36," made in the U.S. It's "cold drawn, annealed." I recall that annealing was discussed as a step in preparing Invar for good stability, so that's fine.

The coefficient of thermal expansion is quoted as 2.3. That's larger than the canonical number, but at least I have it and I can adapt the pendulum to it. I've bought a short rod of the same material that I may use inside the bob, so that the main compensator can be completely outside it for lower temperature lag. If I feel ambitious enough, I also want to make a new hinge out of it, as the brass in my current hinge is already large enough to affect temperature stability.

Edited By S K on 28/08/2023 19:30:14

Yes, it's an aluminum tooling plate that I've repurposed as a temporary base. I want to be able to set it beside my computer while I'm installing and debugging the electronics, etc. Later, I'll wall mount it without the base.

I wish I could have chamfered it! Unfortunately, my puny 3" chuck can only hold the bob by tiny fingernails. There isn't enough grip. Or rather, I could do one end of it, but then I wouldn't be able to hold it at all when flipped around. The bob was the biggest and heaviest thing by far I've put in my toy lathe. It was on the ragged edge of its safe limit, and I spun it very slowly lest those tiny fingernails let go!

I realized that I needed a test stand, as it would be too difficult adding electronics and testing the pendulum while mounted to the wall it's intended for, so I printed a pair of brackets and mounted the whole pendulum to a base:

It's more than sturdy enough for functionality tests, but not rigid enough for highly detailed measurements. It doesn't look like it, but the bob easily clears the lower bracket:

I still have to add a flag, which I'll probably do in brass to match the rest of the pendulum. The electronics such as the opto sensor and the drive coil(s) will probably be mounted on 3D printed supports.

It's starting to look real! 🙂

All materials are unstable. It's a law of the universe. (People too, some more than others - hehe.)

Invar has been particularly well studied for this, given its use in advanced scientific instruments. But I don't believe the "sudden", "single day" instability claims or other anecdotal reports that are much more likely due to measurement errors. Creep over time under load I can believe, though, but the same is true for all metals. For example, the Young's modulus (not quite the value of interest, but it's something) of Invar is about twice that of aluminum, about the same as cast iron, but lower than typical stainless steel.

I'll take Invar over other metals with 10-20x the thermal instability, and over the quite unexplored instability of carbon fiber. (Indeed, "carbon fiber" is not one thing, there are hundreds of manufacturers using extremely-diverse manufacturing methods, weaving and extruding strategies, binding agents and finishing techniques.)

I'll also keep my "gorgeous" brass bob, with its ~20% higher density than steel or iron (but I'll remain jealous of John's tungsten bob!).

Invar has been successfully used in some of the most accurate clocks and other instruments ever made. But if I ever launch into making a superior pendulum, I'll hunt down something like a 2" diameter fused silica rod, and use that alone - no bob - in vacuum.

Edited By S K on 27/08/2023 17:09:03

Here's the 3" by 3" bob, with the short compensator and stainless support pin. I used wax on the parts. The magnet holder is up top:

It came out OK too. 🙂

Edited By S K on 26/08/2023 18:40:40