Gents impulse clock

Alas, Business Case fails on cost. I found a few GPSDO modules for sale at about £100 each. Would be possible to make one cheaper except a design goal was a high-end Clock Analyser that anyone could replicate with cheap readily available modules. Plug and play rather than grown-up electronics.

Wondering now about using a Wireless technology such as Bluetooth to link a GPS unit in the clear to the analyser-part indoors next to the pendulum. Unfortunately, although the radio link is quick ( say 0.1µS delay over 30 metres), encoding and decoding Bluetooth messages isn't. And a wireless link is more cost and complexity.

Very tempted to steal Robert's collection of Rubidium Oscillators, drool...

John's drunkards walk comment has me thinking too. It reinforces my original notion that a pendulum analyser needs to be able to see both micro-variations in time keeping and subtle long term effects.

My test pendulum may be misleadingly good in the sense it's not a realistic mechanical clock, hence me not seeing a drunkards walk during testing. It's a steel bob on a carbon-fibre thread energised by an optically triggered weak electromagnet. I suppose because the mechanical and temperature effects on the pendulum are low, it keeps remarkably good time. Better than 20ppm when running well and I got one run of 8ppm. This with the bob dangling from a wobbly Meccano gantry.

So good was the test pendulums performance, I've toyed with the idea of a properly engineered rigid frame mounted inside a length of 4" plastic drainpipe from which most of the air has been removed. I think a simple arrangement like that would outperform all but the best mechanical clocks, essentially by dodging the friction and impulse problems that make building accurate mechanical clocks seriously challenging.

Not quite as attractive as a well-made brass clock. I doubt anyone would admire 18" of old sewage pipe perched on my mantelpiece...

Dave

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Point taken ... although I'm pretty sure that my [incomplete] collection of components includes all the clever bits, and has cost me much less than £100 so far.

... It might, however, be embarrassing to check how much, and how long it has taken to accumulate them

If it's any help; I have just rediscovered James Miller's simplified design: **LINK**

http://www.jrmiller.demon.co.uk/projects/ministd/frqstd0.htm

MichaelG.

Any decent OCXO will be accurate to 1 part per million in a year (<1x10^6, HP's 10811A is more than 10 times better than that) so that's 31 seconds in a year worst case. Over shorter time scales eg days it's typically a thousand times better so 0.001 seconds per day. How good do you need?

Robert G8RPI.

For a lowcost time interval counter with logging on a pc for checking clocks etc see

http://www.ko4bb.com/doku2015/doku.php?id=precision_timingictic

Robert G8RPI

Hmmm...that link doesn't work very well with an emoji in it Robert! Please could you try again?

.

I honestly don't know, Robert ... that's the problem

Not that I aspire to replicating one : The Shortt observatory clocks were thought to be accurate to about one second per year, but recent tests suggested that one second in twelve years might be nearer the truth !!

What I'm really interested in though is the shorter-term stability, and response to external stimuli, of a somewhat more practical pendulum than that.

MichaelG.

Michael and I are interested in the behaviour of individual pendulum swings where averaging hides useful detail. The easiest way (I think) is to compare the test pendulum with a much more accurate clock, and the more accurate the better.

With a pendulum clock, tens of microseconds are interesting, and picosecond resolution would be even more revealing. It's a bit like measuring length: to measure a thou accurately, you need an instrument that can get close to tenths.

Looking closely at the timing of a pendulum shows clock faults, for example I think I can detect the difference between a pendulum swinging true and one following an ellipse. On my test rig an elliptical swing has more jitter, I'm not sure why. I suspect it's due to a combination of the impulse being misplaced relative to the bob, torsion in the rod, and maybe bending at the pivot.

Dave

Thanks, Dave ... We're definitely on the same wavelength

Incidentally, my Jupiter GPS module was a few more £s than I remembered

The OCXO though was £16 from ebay [new old stock in 2007]

... probably totally obsolete by now !!

MichaelG.

As mentioned by Robert I can't see how the actual time, from GPS, matters when you can get a local clock that has high stability in the time period you are measuring, and if it is drifting you can measure over an extended period and compensate.

I think you would find it more instructive to be able to measure the actual physical position of the pendulum hundreds of times per swing to compare swings to see the difference made by the application of the impulse, especially when mechanically impulsed.

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But ... the 'actual time' from GPS is not being used. The principle is to make use of the timebase [thoughtfully provided to make GPS function] to fine-tune the local oscillator.

As with metrology ... The more accurate and stable the reference, the easier it is to obtain precise measurements.

MichaelG.

The PICTIC I linked to (corrupted by smiley) "http://www.ko4bb.com/doku2015/doku.php?id=precision_timingictic" Google KO4BB (O not zero) pictic to find it

is ideal for what you want. run it from a 10MHz OCXO and a optocoupler obscured every pendulum swing and it will send the time of every swing to a PC via the serial port.

Robert G8RPI.

Deleted

Edited By Robert Atkinson 2 on 16/06/2019 21:50:34

< I will try that again >

Edited By Michael Gilligan on 16/06/2019 21:59:01

< sorry, don't know what this forum is playing at >

Edited By Michael Gilligan on 16/06/2019 22:01:46

Thanks, Robert

Irritating little blighters, those smiley things !!

The link to PICTIC is on this page: **LINK**

http://www.ko4bb.com/doku2015/doku.php

... under 'Precision Timing'

MichaelG.

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I think that's where we are probably on different interpretations, Robert

... I anticipate needing to use several measuring points; being more interested in the perturbations within a swing than in its overall duration

MichaelG.

that's more of a challenge as to how you capture the data without affecting the pendulum. A high frame rate camera and software would be one option. A more interesting one would be line CCD or diode array sensor like those used in document scanners sensing the shadow of the pendulum rod. This would give better resolution, easily 1024 positions, and require less processing. The sensor and lens from a flat bed scanner would probably work as is. Disconnect the sensor from the mechanical drive (initially leave that is as is for home and end sensors) and just try the scanning software. The image should be a sine wave.

I just realised I made a mistake on the OCXO accuracy. 31.5 seconds per year is 1x10^-6 if it all went out in the same direction at the beginning of the year. A more typical 1x10^-9 average is 0.03 seconds in a year.

Robert G8RPI.

Thanks, Robert .... I think we're on the same page now.

Probably best to pause for thought and let 'Plasma' have his thread back.

MichaelG.