Member postings for John Haine

Michael was asking for more details of my claim of other mentions of a similar technique.

• Loseby's approach actually only applies the correcting force over a small part of the arc on one side.
• Bush/Jackson aim to exactly correct for the cubic terms at least for small deflections.
• The Bulle clock included a coil spring arranged in a very similar way to Bush's to keep period constant as battery voltage declined, in the early 20th century.
• David Robertson described the theory of the Bush type compensator in 1929 in one of his classic HJ articles.
• IIRC Peter Hastings described a scheme a bit like Loseby's in HJ a few years ago.

Also called bt30 or bt40

Chris Crew's comment about value being what someone is prepared to pay is very true. In my experience eBay attracts good prices for workshop equipment as it puts the advertisement in front of a lot of people and has a bidding system set up to encourage competitive bidding. Yes you have to pay them but the same is true of an auction house. Obviously you want to make it clear in the advert that "buyer collects" for the larger items at least.

This is Warco's responsibility, don't dilute it!

The other key reference on this is by Bush and Jackson "Correction of spherical error of a pendulum" in

Journal of the Franklin Institute

Volume 252, Issue 6, December 1951, Pages 463-467

alas only available if you pay or have institutional access. Vannevar Bush was an eminent US science maven in WW2 and after who played a major role in things like early computers etc. Also interested in clocks and there are a couple of articles in Scientific American describing their experiments dating from the early 1960s. The article above has a reference to Loseby but he only had the rather negative Grimthorpe reference and hadn't seen the Practical Mechanics stuff and the test report by Airy. The scheme used a tension spring pulling downward on a point on the pendulum, the other end being held vertically below the point of suspension. As the pendulum swings the spring is slightly stretched and the geometry can be designed to provide an additional restoring force proportional to the cube of the angle that can cancel the cubic term of the sinusoidal restoring force. In practice they used a cantilever leaf spring at right angles to the pendulum with a thin wire connecting it to the pendulum rod. Good results are reported, I haven't compared them with Airy's measurement yet.

As far as I can see Loseby's spring creates the cubic dependency through its shape and from my brief look at the description he doesn't say what this has to be mathematically. By contrast Bush only assumes that the spring is linear (a good approximation for small extensions) and the cubic term comes from the geometry.

Thanks Michael and Dave. Similar in principle to methods proposed by others except Loseby was first and certified by Airy no less! I will try to dig out the other references I've seen.

David,any chance you could post an image of the relevant page or a direct link please?

This kind of arrangement has been invented more than once to correct for circular deviation.

I think the coil shape is quite logical given the size of the bob - it just maximises the amount of ferrous metal exposed to the field.

Impulse timing - actually it doesn't matter as long as it's constant. If it isn't constant then it can become a source of error if either timing of amplitude of impulse changes. The grasshopper deliberately impulses late to provide some compensation for other variables.

I can't visualise your coil and magnet arrangement - any chance of a sketch?

Most escapement/crutch assemblies are more like 10% as the movement is much smaller than the pendulum. The 30% is roughly right for the Synchronome, I don't know how much science went into the number. It may be because pallet becomes a large enough target for the gravity arm roller and the travel on the pawl driving the count wheel has about the right travel.

**LINK**

This article from the latest HJ by Pepi Cima describes some experiments he did with a 'Nome where he impulsed the iron bob with a "frame" coil to one side. There is a photo showing the configuration well. The article is interesting in itself anyway as it shows experimentally why impulsing at BDC is preferred.

https://www.newscaletech.com/resources/technology/squiggle-micro-motor-technology/

**LINK**

One for Michael Gilligan, just the job for microscope stages?

Could we stick to the usual terminology, rod = shaft, bob = weight?

Where are you thinking of applying the impulse? The usual xNome configuration is about 30% down the rod, with a heavy bob, and both the original (3/8 invar rod IIRC) and my version (10mm CF tube) suffer from spurious oscillations on impulsing. How much of this is rod flex and how much sideways deflection at the point where the rod and suspension spring meet I'm not sure. With a lighter bob the centre of percussion should move up, so may be less of an issue.

I have no views on the supplier. But, if you are motorising both leadscrew and Xfeed then in my view you might as well go for full CNC.

On my S7n I have 2 standard NEMA23 steppers, the main leadscrew being driven with a 2:1 toothed belt reduction, and I think 2A drive current. I have never had an issue with missed steps except when I've managed to crash a tool (but that seldom). For a Myford I can't see that high torque or servos will be much benefit.

I'm looking forward to later episodes! I'm surprised that you don't think 1/4" invar would support a 14lb bob? 6mm CF tube is perfectly happy with my 10lb+ tungsten bob (though I have now gone for 10mm). 2lb sounds like it would seriously limit Q.

As you have used a plain ground rod for the transverse support, you could consider just using a flat-topped bracket rather than a "cradle". The Synchronome does this, though it also has clamps to hold the support steady once it has been located and settled. Dong Bateman's clock uses a rather larger diameter support rod and has a pair of ground steel bars bolted to a thick plate as "brackets, with no cradle or clamping.

We were looking at the Trinity clock recently and the Keeper showed that the mechanism tilts slightly when the tower warms in the sun - this puts the suspension axis slightly out of horizontal and the trunnion bearing which does have a cradle, has too much friction for the pendulum to hang exactly vertical. As a result the pendulum can start to swing in a slight ellipse. Doug's arrangement just allows it to adjust to hang vertical.

I find both MSF and DCF77 clocks don't like being close to computers, especially when trying to synchronise.

I'm sure there is scope for much better vlf antennas for clocks than the weedy ferrites usually used. Over on Timenuts there was a link to a WWVB 60kHz receiver with a clever antenna made of ribbon cable, with the conductors connected together in a staggered fashion to form a multi-turn loop. From memory this was a couple of metres diameter and hung in the maker's loft.

I haven't taken the time to work out how all the light gate works, but maybe the 100ns comes from a differential measurement. Each comparator may have a propagation delay of 1us but as he uses a device with 4 on the same die they may be quite well matched.

More interesting to me is the way he arranges the comparator threshold to track ambient light level, though he could equally well have just ac coupled the photodiodes to the comparator inputs. I'm planning such a scheme for the next iteration of my current clock if efforts to exclude ambient IR aren't good enough.

Thanks. Yes, I did that.

Having posted that of course it has sprung into life!

My latest "weather station" which has a radio controlled clock based on MSF no longer sets itself since replacing the batteries a couple of weeks ago. I believe it receives MSF from Anthony, and there was a scheduled maintenance outage earlier in the month. Are others having problems please?

When Harrison designed his gridiron pendulum he must have made or used expansion measurements for the two materials used. I'm not sure that's different in principle from applying a correction for accumulated temperature deviation as the clock runs? Likewise quartz movements sometimes use a digital correction for measured crystal frequency to eliminate a trimmer capacitor. Both my running clocks do something similar, correcting for the actual period error by occasionally swallowing seconds. I could make that adaptive and add terms for estimated temperature and pressure error. Potentially better to reduce the complexity of the pendulum assembly by eliminating extra joints adding temperature compensation and aneroid capsules etc.