Precision pendulum techniques

The GPSDO 10MHz disciplined oscillator output is gated with a signal from a counter that divides said 10MHz down to a 0.5Hz pulse. The pendulum pulse , a rising edge every 2sec , enables that counter array.

So, on the first pendulum rising edge, that counter counts down 2sec, which then enables the gate above. That gate feeds another counter array which also generates a 0.5hz train. That train feeds the TDC START. The pendulum 0.5Hz 'train' feeds the TDC STOP. at this point, the two 0.5Hz trains are 'almost' coincident and within TDC mode 2

Oh, I see, you have a GPS disciplined oscillator? Nice! Can you provide a link to it? Thank you.

No link I fear, as both of mine are home made...One using an osc from a defunct HP Cesium Beam reference, the other a new, very good OCXO from Golledge in the UK.

Can only give you photos and circuits...but we must do so on another topic or email, etc...

Here's one..

Very nice indeed! 🙂

I'm probably late to this party but had a sudden thought today. Though the measurements I did on the Sharp opto interrupter showed impressively low (sub micron) repeatability, I just realised that the emitter wavelength is 950nm, nearly 1 micron, so arguably whatever one does in the optical system will not I think improve the results.

Great observation! Neat!

What does this position repeatability say about time resolution? After all, slowly incrementing a flag across the sensor only differs in velocity compared to swinging it across. I haven't calculated it, but doesn't one micrometer of travel correspond to a few ms in time, and isn't that around what most of us have been seeing?

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

Too hot hot and tired to think clearly about wavelengths, but a period of 5uS is very much in the electromagnetic slow lane - 200kHz. 5nS is still fairly pedestrian - 200MHz. 5 picoseconds is more like it - 200GHz. Visible light is about 400THz

Really interesting question though - what limits the reliability of an IR beam break? Hmmm...

Dave

I think you should set your opto sensor up in the lathe or mill with the same electronics and test out its positional accuracy. I suspect it’s not going to be great.

regards Martin

I have done this and posted the results here somewhere.

I had a feeling someone had John.
regards Martin

If the position error of a sensor is 1um, and a pendulum is moving at say 1cm/s at the sensor, then there's an uncertainty of 1us in the sensor's timing - a reasonable number (but put in proper speed values).

So, if the wavelength of light dictates the position uncertainty, then it also dictates the time uncertainty? If so, we must break out the ultraviolet laser or better yet an electron beam immediately! Hey, you are putting your pendulum in a vacuum already, after all! 😄

This is my previous thread on the opto precision issue.

**LINK**

I wrote a longer article for HSN which I could share if of interest.

Please do.

Just emailed to you, Joe.

Well, if the S.D. was found to be 0.15um, then there should be plenty of room for improvements in timing resolution.

I still think that using a brighter light source (e.g. a laser) in conjunction with a fine slit should improve the timing as well as reducing the effect of stray light. I also bought a few of the optos that Joseph is using, but - warning - it seems I broke the first one I tried (they may be particularly sensitive to ESD, which the data sheet even prominently highlights in red letters). One day I'll get back to these investigations.

Also, I've mostly been placing the opto at the end of the swing rather than at BDC. This way, I conveniently get a measure of the period directly, as opposed to half of the period (and annoyingly-different measures for the left vs. right swings). I've believed that this is not optimal, though, since the pendulum is moving slowest at its apex vs. fastest at BDC. It's probably indeed not optimal, but by John's measurement, it may not be that bad after all.

Isolating a pendulum from the environment is difficult. This graph is from my clock's log just after the severe earthquake in Morocco at 2023-09-08 22:11Z. The epicentre is about 2400km from me. Might be a coincidence, but I think it caused my house to wobble.

Dave

Yes, that's quite likely due to that very serious and tragic earthquake.

If someone else here has similarly-good reference timing, you might even triangulate it.

If you place the opto at BDC and clock a D type flip-flop (SN7474 etc) with the Opto's output you get a single rising edge per period swing. You eliminate the right-left-right deltas and get a true period measurement at each rising edge.

Connect not-Q to the D input and the Opto output to the clock input - use Q or not-Q as required by your measurement system

Edited By Joseph Noci 1 on 10/09/2023 18:55:10

Yes, that would work. It's been a long, long time since I broke out the 74xx parts, though, and I tend not to look at them first for stuff now. (I did build my first computer that way, all hand-wired, with a staggering 1024 bytes of RAM.) Maybe for my final setup, though I'm toying with ideas involving multiple sensors.

I was once asked by a smart-ass: "If you built your first computer like that so long ago, how come you're not rich like Bill Gates?"

My response: "What makes you think I'm not!?"

...silence... 😄