Member postings for Tony Jeffree

Hi Martin

At some point you have to use some kind of reference in order to determine what effect the parameters you are setting in your environmental chamber are having. I don't see the pedantic point you are making here.

Cheers,

Tony

I remember Dick Stephen doing some measurements on suspension springs Vs small ball races for pendulum suspension and concluding that there wasn't much to choose between them - the frictional losses in the ball race were comparable to the hysteresis loss in a spring. I don't know whether he published any results - he was certainly writing on & off for HJ and ME at the time.

Isn't the drive entirely controlling the pendulum frequency at that point?

It can certainly lead to Confucion...

I'll get my coat.

It's NBG if you don't know your TLA's 😂

I don't recall where those particular ones came from John, but Willys would be one of the places I would look, also Mouser and RS Components.

I don't recall where those particular ones came from John, but Willys would be one of the places I would look, also Mouser and RS Components.

True. As per Martin K's comment above, mine is brass sleeve with end caps and central steel tube, completely filled with lead, so not convinced the lead has anywhere significant to go to.

Is potting resin likely to be any more dimensionally stable over time than lead...?

There's no need to have a magnet on the bob - mine has a soft iron armature attached which the electromagnet attracts. I do use a very small magnet for the Hall effect sensors though - but as others have done, optical sensors could be used.

Interesting stuff Dave, thanks - there's plenty there for me to steal too!

Cheers,

Tony

JH - thanks for the links - very interesting articles.

Seems to me that there have historically been two rather different extremes in the approaches taken to building a precision pendulum clock - the Harrison approach, where he accepted that there were environmental and mechaninal effects that would affect the timekeeping and carefully balanced them in order to create an accurate clock, and the Shortt et al approach, where the pendulum was as far as possible isolated from mechanical and environmental effects and mostly left free to vibrate as it will.

My own rather amateurish messing around with a "free" pendulum that is electromagnetically impulsed under the control of a single board computer has led me to start thinking about a third possible approach which takes advantage of the processing power and electronic sensors that could be applied to the problem - measure the environmental factors that could affect the pendulum (temperature, pressure, humidity,...), feed those measurements into the SBC, and use its processing power to determine how much impulse to give to the pendulum in order to correct for the environmental effects. Even for the lowly BBC Micro Bit that I am using has add-on boards that will measure temperature, pressure, and humidity. Obviously the calculation is probably going to be non-trivial, and the details specific to a particular pendulum, but in principle...?

...or are you all way ahead of me...?

Edited By Tony Jeffree on 11/02/2023 12:13:30

Well worth having a read of "Harrison Decoded" to get the full picture - not a cheap book, but fascinating stuff. As you say, Harrison was, and mostly still is, massively underestimated.

If (as I am) you are making use of a single board computer to manage the sensing and impulsing associated with the pendulum, there are opportunities to measure the environmental conditions around the pendulum and alter its behaviour accordingly. For example, the BBC Micro Bit that I am using has an inbuilt temperature sensor - probably not a very accurate one, but it's a start. And I discovered yesterday that there are add-on boards available for it that can give it temperature, humidity, and pressure measurements. Seems to me like the basis for automated correction based on measurement. The only issue is figuring out the formula...

He used various corrections in such a way that they balanced each other out. Given the relative simplicity of an electronically impulsed pendulum, there are fewer factors to correct for, and it is potentially a simpler problem (than Harrison/Burgess/Clock B had to deal with) to bring them into balance.

I've been playing with the setup of this clock and have discovered that the BBC Micro Bit single board computer that I used is a Version 2 board - at the time I was working on it I was unaware that there were multiple versions of the board, and I bought 2 more which turn out to be Version 1 boards. To cut to the chase, during my messings around today I was programming a V1 board and plugging it into the breakout board, only to find that it wasn't working as it should, whereas the original V2 board continued to work just fine. I haven't yet figured out why there was a problem and I probably won't spend any time on it - my solution is going to be to sell off the V1 boards and get a couple more V2 - but just a heads up that the information in the article series relates to the V2 board.

I think I posted elsewhere that the code, which was printed in the articles, was a bit of an eye chart, and that the original JPEG files are in my album. If anyone wants the HEX file drop me a message and I will email the file.

I've just acquired a copy of "Harrison Decoded" which makes very interesting reading. Given that electronically impulsed clocks have rather fewer issues to deal with than there are with "Clock B" (no mechanical escapement, no issues with going trains and friction...etc), it ought to be entirely possible to achieve comparable performance to "Clock B" with an electronically impulsed free pendulum swinging in a "normal" environment (i.e., not temperature/pressure/humidity controlled). Has it been done?

The two sensors that determine the pulse length are set approx 13mm apart and the coils are offset approx 35mm from BDC.

I've just been experimenting with my clock and it is very easy to induce the dreaded "twang" by varying the timing of the impulse and the position of the coil. If you are getting a "twang" then essentially some of the impulse energy is being dissipated in unwanted vibration (and corresponding disturbance of the pendulum motion), so it is well worth figuring out why this is happening - I suspect it will actually prove to be a timing issue in your case. If the timing is wrong, then the magnet will initially accellerate the pendulum, but if you don't switch it off soon enough (i.e., before the swing reaches the point of max attraction) it then acts as a brake for the last part of the "impulse" and causes the pendulum to twang.

My impulse coil activates with the pendulum near enough at BDC, but I have offset the electromagnet sufficiently to the right of BDC so that the impulse is guaranteed to have finished before the armature attached to the pendulum reaches/passes it.

I can induce the "twang" very easily by shifting the coils left too far and/or by making the pulse length too long. With my current setup the pendulum swings for about a minute between impulses and the impulses are completely silent.

5V is actually plenty for these coils - the issue isn't voltage but amp-turns, which basically determines the magnetic strength. I suspect the Post Office coils used many more turns of much finer wire. Telephone exchanges were based on 50V DC electrics so it was probably easier to make coils to suit the voltage than to mess around with a lower supply voltage. I believe I am currently running mine on 12V but that is massive overkill.

The John Wilding 3/4 second Pendulum Clock which was the inspiration for my design (and in turn, for this one) used a 3V supply (2x "D" cells in series).

John - your use of core-less coils to drive the pendulum prompted me to unearth a couple of coils that I had to hand that were bought as components for a loudspeaker crossover. Thes kinds of coils can be readily purchased from the usual electronic supply suspects, are beautifully wound, and come in a wide variety of sizes. An easy solution that avoids the need for coil winding. The ones I unearthed are wound from 0.4mm copper wire, and have about a dozen layers of 25 turns.

Edited By Tony Jeffree on 29/01/2023 17:05:51