A Novel Free Pendulum Clock

Thanks for the suggestion Tony J,

There might be some merit in mounting the whole clock on gimbals even though it goes against the grain, robust and solid "feels" like the right approach to gaining stability. Having an assembly balanced by gravity and then impulsing one part of it against the other seems to be asking for another set of motion variables to interfere with the system.

I have thought about mounting the opto sensors all on a static pendulum pivoted on the same axis as the moving pendulum and damped to stop it swinging in sympathy. What slowed me down at that point was dealing with the tension in the wires causing the static pendulum to hang off centre. I then came up with a complicated arrangement which put the sensors on a fixed bracket but used mirrors on the static pendulum to get the light beams all across the path of the obscuring pin. Beam divergence reared its ugly head and then laser diodes looked promising...............

Option paralysis and life have prevented any further work..

Rans6.....................

I was chatting with Jim Arnfield, onFriday … and he mentioned his latest [and apparently successful] Opto configuration on his double pendulum clock: He is putting each sensor at the end of a long thin tube, blackened internally.

Seems like a very good idea to me

MichaelG.

Great idea.

I just had another watch of the video in the OP of this thread. The first thing that struck me is the size of the pendulum swing. My understanding is that a large swing is asking for trouble with circular error which may or may not be significant with a tightly controlled amplitude. It does, however increase the amount of energy the system is losing to air resistance. In my clock the pendulum is 60cm long (3/4 second) and the swing is controlled to plus or minus the closest distance that 2 slotted opto devices can be mounted, approx 6mm between BDC and peak. This gives an angle of swing of less than 1 degree each way from the centre.

Rans6...................

Ah well, that's a perennial debate! Circular "error" is only an issue if the amplitude changes - and if it does then it may change the escapement error too. Circular deviation can be used to advantage to nullify the effects of barometric pressure as noted above. In "Clock B" the amplitude is over 6 degrees! In Doug Bateman's clock amplitude varies by only ~1 second of arc with an amplitude of 1.3 degrees.

In practice the energy lost by the pendulum to air resistance is probably a fraction of that lost to running the opto LEDs and the processor. There is a school of thought that says a high energy throughput makes a clock more accurate though without much scientific justification. I do believe though that a high amplitude is beneficial as it maximises the "signal to noise ratio".

I make no claims about how well or badly my clock performs - my intent was to build a pendulum that operated with no direct mechanical contact and the minimum of indirect mechanical interference, and to that end, I have succeeded. As it is currently set up, the clock runs for ~100 seconds between impulses; that interval, and also the choice of arc of swing, can be adjusted to tase by re-positioning the Hall effect sensors, changing the distance between the electromagnet and the pendulum bob, and changing the electromagnet current. With a sifficient supply of round tuits I'm sure all of the potential permutations could be investigated, along with their effect on the clock performance.

However, as John has observed, as the arc of swing is closely controlled, circular error shouldn't itself be a major problem.

18 years ago, i read Accurate Clock Pendulums , published 2004 after listening to a R4 book review, of course.

Shortt are out of my price range, Enthused I made an oak 2 part mould for a bi-linear bob cast in type metal also from Keighley 11Kg bob, split the mould, not enough weight and got it cast at metcalfes in Keighley. Bought quartz pendulums 2 , ground a recess in each end to hold a split washer. everything made from ironless aluminium bronze. 3/4 inch steel pendulum head platform with dual single piece suspension springs

I cracked the first pendulum attaching the bob , the spare was broken in a move.

2021 got a JD2 tube bender for another project and ended up building a frame for the top pivot , Ive got a steel pendulum rod as a place keeper, .

Q 100K ~ 300K , thats what I measured i16 ? years ago with the top assembly bolted to a cellar wall

Just seen this thread , thanks for posting. I will read it all tomorrow. .

I'll post some pics , looks like we have a common appreciation of something that is (almost)? undefinable. Time

do you have a siesmometer.

So I put my pendulum together with a electromagnet, teensy 4.1 . a reed relay with a magnet on the bob to time the fet coil power from 12V . It went so well the pendulum smashed into the glass reed relay and smashed it,

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?

Harrison's Clock B was recently tested and found to be accurate to 1 second over a 100 day period. That's 0.1 ppm!

The COSC standard for quartz "chronometers" is +/- 25.55 seconds per year, which is much worse. (Whether "COSC" certification is meaningful here is debatable, but it's a point of reference.)

Excellent and somewhat-pricey temperature-compensated oscillators are also about 0.1 ppm.

So Harrison's design (actually made by Burgess) beat quartz movements by a mile and achieved parity with modern oscillators with electronic compensation. Sounds like it would be hard to match, even with the advantages of an optically monitored and electromagnetically driven free pendulum, but it's a fun target.

So who will be the first!? (Not me, my work-in-progress will be too small and light to compete.)

Edited By S K on 04/02/2023 15:37:34

It's a good target to aspire to and I'd be very pleased to get performance half that good.

Strictly speaking, the performance wasn't achieved by Harrison's Clock B, because he never made one! Rather it was delivered by a clock built on Harrison's principles. The build was started by Mr Burgess who completed Clock A in 1987, but didn't finish Clock B. 30 years later the parts were bought in 2009 by a collector who had the clock finished by Charles Frodsham & Co. I share their pain - my clock project is also taking years, with no end in sight!

The Burgess/Frodsham clock uses technology not available to Harrison, like Polyaryletheretherketone pallets. Whether Harrison, who was a genius, could have got the same performance in his lifetime is questionable, but I'm sure it would have been brilliant.

Does anyone have any details on Clock B, such as how it compensates for changing air-pressure? Although the effect is small my crude pendulum responds to it, and it's nowhere near 0.1ppm

Dave

The key aspect of Clock B is that the pendulum automatically compensates for barometric pressure variation by balancing the variation in rate caused by buoyancy against the opposite change in rate caused by amplitude change resulting from varying air drag. It is of course temperature compensated by using an invar pendulum rod and by other means. It's a complex system that is very hard to "design" as not all aspects of it can be modelled but have to be set up on test.

Any pendulum that is allowed to vary its amplitude according to air drag has a nominal amplitude where it has zero first-order sensitivity to air density variations. So it would be possible to apply the same method as Clock B does, though with much less complication.

Dave, I sent you a PM.

From my understanding of Harrison's techniques was that he recognised that atmospheric conditions were connected. That there was a correlation between density, air pressure, temperature etc. His answer was to use circular error to compensate for the changes. On the RAS clock he fitted cheeks either side of the pendulum suspension so as amplitude increased so the effective length of the pendulum was lessened. Rather than have this as perfect amplitude compensation only, he over compensated to allow for the variation in pressure essentially allowing one effect to work against another. The brocklesby Park clock pendulum has evedence of a vane on the back to increase drag which has been suggested that Harrison was aware of these effects at a very early stage and did do some experiments. Compensation on the RAS could really be only done by patient running, measurement and adjustment untill things were brought in balance.

regards Martin

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 gather that the RAS Regulator reproduction under test at Upton Hall (one of several built), based on the same design principles as Clock B but much more representative of Harrison's methods and materials, is performing very well though I don't think details have been published yet.

From John Haine's PM, I learned the way Clock B balances out environmental effects is remarkably clever. I used to think Harrison was a genius, now I think I underestimated him!

I agree an electronically impulsed pendulum ought to do well compared with a mechanical clock, but I'm not finding it easy! Fascinating subject though, full of subtle challenges and interest.

Dave

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...