Member postings for S K

David,

I'm still debating that. I usually cut first and design later.

I like electromechanical impulsing as it is more romantic, but it's an objectively worse approach than electromagnetically.

OK, rod / bob it is.

I'm now contemplating getting a piece of cast iron for the bob, as it's about 1/3 the cost of brass. That could yield a bob in the 4-6 lbs range given the severe constraints of my puny Sherline lathe.

Where to impulse? The old "30% down the rod" rule of thumb seems left over from the use of an escapement + fork. My presumption has been that the center of percussion is the ideal point for the impulse. Even just considering the rod alone would place the CP 2/3 down from the pivot, and lower still including the bob.

Well, if cyanoacrylate isn't "extremely strong," when properly done it should at least be "more than strong enough!" 😉

The main problem with a string, even Kevlar, is that it stretches (chains have many issues). When a pendulum is at the far ends of its travel, the downward force of gravity remains downward as always, and so it does not pull in the direction along the string as much. Thus experiencing less force, the string becomes shorter, changing the length of the pendulum dynamically.

With a rod and weights of the sizes and masses we are using, I doubt that this should be a significant issue. So probably the impulsing is the greater problem? Really, I'm just working off Matthys' suggestions for shafts and weights.

About the bob: It's not that a 1/4" rod can't support more weight, it's that the rod is presumably too flexible for a disproportionally heavy weight. I was considering getting a larger hunk of brass anyway, though, of about 6 lbs. But that would cost $150, so using what I have on hand has its attraction.

I'll probably stick with a V-shaped cradle, maybe even with a spring retainer. The reason is that I live in earthquake country, and I'd rather not have it crash to the floor. I guess clamping on a flat surface after it settles would be fine too - I'll think about that.

Good quality fresh, thin cyanoacrylate (not the "gap filling" stuff), used in thin layers on tight-fitting, well-prepared surfaces, can be extremely strong. I absolutely could not break small test pieces apart without heavy tools. I suggest that it's as good as soldering in terms of strength, as well as being much easier. My only doubts are possible long-term failure issues, which I don't have information about.

The "well tempered" in the thread's title is because I'm aiming at a reasonable balance between accuracy and practicality. I'm not out to make this a world-beating clock. I'm also not planning on an enclosure (I don't have the skill or tools), so accuracy wouldn't be superior just on that point alone. And you will have noted that I used set-screws to fix the pendulum's shaft (I wanted the flexibility to change things). That can't be good either, but it's a tight fit and I could CA that in too if I pleased. Still, of course, I'd like it to keep good time.

Edited By S K on 21/07/2023 14:50:37

Dear Forum Members,

I've started work on an "Arduinome"-type hybrid mechanical / electronic pendulum clock. I'd like it to be simple, practical, and hopefully imbued with a touch of artistry. So far, I've completed the pendulum's hinge assembly.

The hinge itself consists of two 0.004" thick by 0.25" wide strips of beryllium copper. Cutting this using scissors did not work particularly well, as they leave a curled and uneven edge. Thankfully, John Haine (in this forum) made a creative suggestion, which I used: I clamped the cut strips between two 0.25" tool-steel blanks and used an old chisel, laid flat along the steel, to hammer-cut a sliver from the edges. After turning it around, I hammered off the other edge. This worked brilliantly, leaving very clean and straight cuts and a uniform width.

These strips are held by 0.25" thick two-piece "chops" for stiffness and stability. The upper chops were drilled and reamed for a 3" by 0.25" stainless steel rod. This will sit in a cradle that holds the pendulum.

The lower part consisted of two pieces: an "L" shaped main body, and a smaller piece that fits into the L. The "L" was used so that I could drill and ream the lower solid part to accept the main pendulum shaft. This was also drilled and tapped to accept two brass-tipped set-screws to fix the shaft in place.

Cyanoacrylate adhesive was used to join the chops. I'm a little uncertain if this will be sufficient in the long term, but I experimented with it, and it seems very strong. I suppose I could (and probably should) try to insert pins to further fix the strips in the chops. But for now, I'm more worried about ruining the hinge via mishandling!

The pendulum's shaft is a 0.25" by 36" rod of Invar, a low temperature-coefficient metal that is sometimes used in high-quality clock pendulums. It's quoted as having a 1.2 ppm TC. I may add further temperature compensation, or I may not, as this material should provide decent temperature stability on its own.

Next steps: I have a 1" by 3" brass disk which I will likely use as the pendulum's bob. This should weigh about 2 lbs, which is rather light for a bob, but the 0.25" shaft is too thin for the usual ~14 lb weight anyway. I also mocked up a cradle for the pendulum in 3D printed form, and will build it in brass, too.

Here's a picture of the hinge in its chops, the stainless support shaft and the Invar pendulum shaft:

It should be a fun and interesting project. 🙂

Michael: I have not looked at the details of that project beyond wondering which photodiode was used (I couldn't find it specified, actually) and which amp or comparator was used.

However, the speed of a comparator is related, in part, to the "overdrive" - the magnitude of the difference between the signal voltage and the threshold voltage. A typical comparator experiencing 100mV of overdrive will respond much faster than one with only 1mV. This dependency is sometimes called "time walk", and it can be a nuisance in some applications.

But more importantly, the project uses two sets of sensors, right? If so, it's actually the difference in time between the entry and exit sensors that is of interest, not necessarily the individual sensor's own speeds. This should be related to the bullet's speed and the distance between sensors, while folding in some of the individual sensor's performance too as noise in the measurement. It seems likely this is where the "100 ns" number comes from.

Edited By S K on 18/07/2023 14:23:01

Michael: All I did was add to the conversation by pointing out the raw potential of silicon photodiodes. Of order 30ns should be possible in visual wavelengths and, when using UV light, the response time could potentially be sub-nanosecond.

Until it gets down to 1ns or less, I'm not interested in that slow-ass project. 😉

Edit: Those randomly-inserted emojis. 🙄

Edited By S K on 17/07/2023 21:51:17

Sure. But with a ~1us propagation delay, those LM339's aren't particularly fast.

Interesting. But (way, way off topic) why would he trust it? It wouldn't be anything more than a mysterious and perhaps even demonic box to him. If you proclaimed "it's as accurate or even more so than observing the stars!," he might say "let's look, then" and find out that there are deviations all over the place. He might then congratulate you that your box at least comes close to matching the stars, but which would he ultimately trust more? As the old saying goes, "a man with two watches never knows what time it is," and aside from convenience likely the stars will be trusted more in the end.

Again, there is no such thing as "time." Only unified space-time, and so any attempt to fix or isolate "time" fails since it is always relative and therefore ephemeral. Our best efforts to define and measure it "accurately" are mostly for the sake of commerce, and at the human level it's really little more than a terrifying mutual hallucination that we grudgingly agree to believe in. 😉

To me, that relativity is what allows clock-making hobbyists to happily take pleasure in their pastime without needing justification.

Edited By S K on 17/07/2023 18:41:17

It seems like this implies that one need not use a microcontroller much faster than an Arduino, too.

But of course faster opto systems are certainly possible. One time constant of the charge collection of a typical bare silicon sensor in the red to near-infra-red would be about 30ns, so you could probably drop the opto system's measurement error by a factor of 100 if you tried.

Because the absorption depth is so much less in silicon, blue or ultraviolet would buy you an additional factor of 10. But by then you would need some fancy electronics to go with it, and my guess is that this would not remain the largest source of timing errors anyway.

Edited By S K on 17/07/2023 16:23:16

On the "philosophy" question, it may be beneficial to distinguish between pendulums and clocks, and further sub-divide the categories according to their features. For example:

• A pendulum. The only pure pendulum is a completely free one, with no deliberate external energy input applied except to start it (the initial lifting force). These are not very useful for clocks since they must eventually stop, exhibit non-isochronous behavior as their motion decays, etc.
• A pendulum with external energy input to maintain its motion. The external energy becomes "entangled" with the pendulum and alters its performance in both desired and undesired ways. A lot of the discussion here has been on how to apply external energy without disturbing the pendulums natural motion - something that can never be achieved perfectly.
• A mechanically compensated pendulum. Normally attempted by applying opposing actions (e.g. to temperature changes), and again a goal that can never be achieved perfectly.
• A pendulum clock: A pendulum with energy restoration plus a rating device (mechanical or electronic) to translate the pendulum's motion into standard time units. Most discussion here currently centers on electronic rating.
• An electronically-compensated pendulum clock: A pendulum clock with informational input from electronic environmental measurements (temperature, air pressure, local gravity, etc.) that is computationally processed to correct the translation between the pendulum's motion and standard time units.

I'm on the side of electronic compensation leaning into the fever-dream of the spectrum. It results less in a "pendulum clock" and more in "an instrumented computer system incorporating an anachronistic oscillator." But that can be fun too.

Edited By S K on 17/07/2023 14:17:28

It's fun to note that monitoring Jupiter's moons is not necessarily a good way of time-keeping. Why? Because of the finite speed of light, monitoring their positions, e.g. by eclipses, delivers times that vary based on the distance between Jupiter and Earth. Indeed, the error can be quite a few minutes worth as the planets move relative to each other.

In fact, the observation of varying times of eclipses inspired and delivered perhaps the first estimate of the speed of light, and quite a decent one at that!

That's a neatly-summarized insight!

My meandering thoughts concerning the practical philosophy of a new project of my own include:

• It should be a functional clock rather than a lab experiment.
• Mechanical design and execution choices should lean more old-school and favor simplicity and elegance, even at the cost of some accuracy. Few mortals expect a pendulum clock to be super accurate anyway, and bragging about it isn't going to impress anyone except - or especially - a very few clock gods.
• Electronic time-keeping, including the rating of an arbitrary-period pendulum, is OK.
• Decent-enough temperature accuracy can likely be obtained via use of an Invar shaft on its own (I have one on-hand), or perhaps one plus a brass compensator.
• Compensation for air pressure can remain beyond the scope of the project.
• Optical sensing of the swing is OK.
• I'd really rather do electro-mechanical (e.g. Synchronome-type) impulsing than electro-magnetic, despite - or perhaps because of - the added mechanical complexity.
• A commercial clock mechanism could be hacked to run the dial. Otherwise, the dial could be operated by a few small servos or steppers running each hand separately without gearing.
• Some artistic design, including of the dial's face, would be nice. I'm thinking a watercolor scene, perhaps of a sun surrounded by stars.

I'd hope that it would keep time well enough for long-term use without fussing over it. So I'm actually not adverse to periodic (e.g. once daily) corrections, such as off a high-precision real time clock module. Or, in a cunning if fatally-flawed plan to dodge the sin of directly correcting to a reference time, the use of a PID could merely chase the "real" time.

Plato would still roll his eyes. 😉

Edited By S K on 16/07/2023 00:06:25

Isn't it a measure of the energy loss per unit of oscillation?

Edit: Eh, there's probably a connection to noise in there somewhere, as for a pendulum the loss is via friction.

Edited By S K on 15/07/2023 18:30:28

I'd like to hear any commentary on the philosophy of efforts in pendulum projects.

I've paused my own considerations for a second pendulum project because I can't quite figure out what my goal should be. I've seen some aiming at the highest precision, e.g. "can I beat Harrison's Clock B?" That's a perfectly fine goal with many learning opportunities, but it's the how of that goal that seems less tied down, philosophically, other than "whatever it takes!"

For example there's a certain range of extremities that can be taken:

• Make a simple, "pure" pendulum, e.g. a pendulum that is not compensated, but uses the most stable materials one has available, e.g. Invar, carbon fiber, quartz, etc.
• Use traditional temperature compensation techniques.
• Add the reduction of buoyancy and air friction effects by placing it in a partial vacuum.
• Use optical measurements and electronic impulsing instead of a traditional escapement.
• Use other electronics to measure temperature and/or air pressure, and compensate for these computationally.
• Electronic compensation of errors, e.g. by periodic comparison to a known-accurate time source.
• Use an ultra-high-quality time source and modulate the pendulum's motion to match.
• And, these days, everything is automatically "better" if you throw in some AI somehow. 😉

In the realm of traditional clocks using escapements, and even in the electromechanical Synchronome-type clocks, it's the very maintenance of those traditions that seem to be an important, if unspoken, goal. There's a laudable beauty in those finely-made efforts in brass, etc.

But with the all-out addition of electronics, something feels lost. With all that, saying "it's a pendulum clock" is dandy, but now it's so far removed from the pendulum clocks of old that it's only true in a technical sense. One just winds up with a clock that, for all the surrounding gizmos, might as well use a quartz oscillator instead of a mechanical one.

Of course, all this is fair as it's a hobby and an opportunity to learn as you play. But what would Plato think? 🙂

To add another data point to the noise of the Sharp opto: I measured 5.5 us RMS noise using an old HP counter in muted daylight. That was with me stomping around the pendulum during the measurement, making material contributions to the noise. Relative to the fall time of the opto (about 20 ns, and I was only using this edge), I consider this to be poor.

It's possible that the mechanics of the pendulum contributes substantially, but given the two quite disparate pendulums and measurement methods, the above-mentioned 3.5-5-ish us RMS noise is likely irreducibly intrinsic to the opto itself.

Edited By S K on 15/07/2023 16:22:30

I am making plans to build an interferometer (e.g. the Mach-Zehnder type), but I think that's outside the scope of this forum. A Mk-II or compound microscope should also be coming, eventually. But for the immediate here and now, I'll probably return to pendulums, i.e. for a Arduinome-type clock.

As a coda to this thread, though: I had a couple of spare lenses and turned them into a pair of pretty awesome magnifying glasses. They are antireflective-coated two-element achromats. The larger one is 50mm diameter with a 200mm focal length, and the smaller one is 30mm diameter with a 150mm focal length. They are a little higher power than the nominal 250mm of a standard magnifying glass. They are super clear and sharp, and so much better than my old scratched up plastic one! I've found them to be very handy in my hobby room. 🙂

Edited By S K on 10/06/2023 18:42:39

Edited By S K on 10/06/2023 18:43:27

Diatoms are fascinating!

I'm trying to decide on a next project. I could make a Mk II version of this, or else maybe a compound microscope.

As with my previous gravity pendulum project, I have an interest in replicating classic scientific experiments. A few ideas might include be the Millikan oil drop experiment or Young's double-slit experiment.

There's an interesting variation of the double-slit experiment called a Mach-Zehnder interferometer. Instead of using two slits, photons are split across two paths by a half-mirror. It's important to not fall into the presumption that individual photons are traveling through one path or the other. In fact, they are traveling through both paths simultaneously in a "superposition" of the two paths, even though the two paths may be very far apart.

Other mirrors are then used to redirect the two paths back together, where they encounter another half-mirror. If light was considered to be particles, you should see half the light exiting the second half-mirror one way, and half the other. But if the photons are waves, interference between the single photon traveling in superposition down the two paths should occur. If set up right, a detector at one exit of the second half-mirror should show all photons exiting with probability 1 (due to constructive interference), while the other should show none (due to destructive interference). Indeed, that is what is seen.

But what if you destroyed the superposition of two paths by blocking one path (equivalent to blocking one slit in the double-slit experiment)? Then, there is only one path left, and hence no possibility of interference anymore. Both exits of the second half-mirror should now show photons exiting, with probability 1/2 for both. This experiment thus demonstrates the wave/particle duality in the same way as the double-slit experiment, but by using two discrete paths rather than a field of paths.

The next step is called the "quantum eraser" experiment. In this, you can apparently measure the path that a photon takes, thereby destroying its ability to interfere with itself, but then erase the measurement and allow it to interfere with itself again. This experiment is difficult for an amateur to do (properly, though a simpler version is more accessible) since it requires entanglement, not just spatial superposition. It's normally done via a special "down-conversion" crystal - one that produces two entangled photons of lower wavelength from one input photon. These crystals are available, but are quite expensive. Particularly sensitive and expensive avalanche photodetectors, etc., would also be needed, since you want to detect individual photons and not just "light."

The step after that is the "delayed choice quantum eraser" experiment. In this, you attempt to change the apparatus after a photon is already traveling through it (such as if you measure a state or block a path, etc.). The photon must then - seemingly, but not really - go back in time to change its state or path in response. This experiment is endlessly debated by youtubers and authors who are into woo-woo physics. Unfortunately, because light travels so fast, this experiment is extremely difficult to do.

Any other ideas?

Edited By S K on 07/06/2023 16:30:30

Build everything else and save the thinning for absolute last, then gingerly hang it. It should be good then, and possibly better - I don't know.

At least one world-beating clock used this technique, so it's worth a look.

Edited By S K on 06/06/2023 18:34:03

I don't view the transition to be steep where it matters - it's rather gentle, really. You can use a larger ball-end mill to make it more gradual if desired. You could round off the shoulders, too, but I don't see that as being a big source of problems.

But yes, you would want to polish out any machining marks in the troughs. I should have polished the edges of the strip before the milling, too, but it was just a test.

Even what I did so far was decent (light sanding with 400 grit using paper wrapped around a rod). But with hand-sanding it's almost impossible not to thin the edges of the hinge more than the center. Maybe a jig could help, but it still wouldn't be perfect.

That idea about thinning an already thin strip - have you tried it? I wouldn't have confidence that it would work out well for similar reasons.

The big take-away for me is that this metal, cold-worked 510 phosphor bronze in "spring temper," thinned to ~0.003", still does not have the right sort of spring to it, and is certainly too fragile, to be used in a practical clock pendulum.

Because I'm a romantic, I might still try to build it into a pendulum. But I'd want to compare it to an alternative, such as my knife-edge pivots, e.g. to see which results in the best Q.