A Novel Free Pendulum Clock

As you mentioned the Hipp, you might be interested in this:

**LINK**

Being electronic it's inevitably called a (C)hipp toggle...

That's interesting, thanks...looks like a very similar sensing setup to mine, but all the logic hard wired rather than using a micro.

First, congratulations to Tony for sharing his interesting clock and showing what can be done with a micro:byte. I ought to try one, but too many toys too little time.

Martin's second point about establishing the angular precision of optical triggering is already on my 'Duffer Clock' To Do list. Tom de Baak mentioned the problem in a email and suggested collimating the beam with a slit and/or lenses to sharpen the trigger.

False triggering is a known problem on my clock: with the white PVC drainpipe cover off, the sensitivity of the IR sensor is effected by sunshine; and, with the cover on, the data log revealed occasional false triggering found to be due to internal reflections, and I hope fixed by blackening the inside of the drainpipe, bob, white plastic sensor holder, and the aluminium tripod. My pendulum is noisy when timed with microsecond precision and I need to investigate if the noise can be explained by poor beam precision adding randomness. Seem likely because my IR beam and sensor are both crude.

I hadn't realised there might be a relationship between circular and impulse error. Needs thinking about - it's on the list now.

Dave

Thanks Dave - yes, the Micro:bit is a great little device, and perfect for this kind of application.

I haven't attempted to characterise the precision (or otherwise) of the Hall-effect devices I am using - I suspect that they are likely to be inherently less precise than a well set up optical sensor, but they have their issues too as you have discovered. For the moment I am happy to have it up and running; I will see how it performs over the next few days.

Not so much a relationship as which generates the biggest variation. For a portion of the swing the pendulum is not swinging free but is being controlled by the impulsing system or escapement if you like. If the driven portion of the swing varies it will affect the period. For example in a electromagnetic impulsing system if the pendulum swing is not exactly central to the field it will get a sideways kick which disturbs it's path. It is unlikely that the swing will repeat its path each period and there may be all manner of periodic eccentricities or quasi stable states. How big these errors are will be a matter of testing. If they are more of a problem than variation due to circular error then that will determine how often you want to impulse for best stability.

That is probably as clear as mud for which I appologise but I hope you catch my thinking.

Keep up the good work

regards Martin

Tony, not sure how you are checking the clock but one way if you have an android device is to take a photo of the face. Android saves photos with the system timestamp as part of the filename. Also there's an app called clocksync that shows the error between ntp and system, on my phone it's usually less than a second.

All too clear I'm afraid! I think you're right. Some of my test runs with a stiff 'sprod' (the whole rod behaves like a spring) show a kind of sawtooth effect due to the impulse extending the period with a bump, and then speeding up as the amplitude decays.

Interesting stuff. I'm letting my imperfect clock do a long run while I'm bogged down redecorating and house-mending, but I'd like to get back to it and attempt a Mk2, plus something like Tony's clock for comparative purposes.

Currently typing with my screen tilted sideways at 30° to let red wine drain out, whilst two Arduinos dry out on a radiator. Knocked the glass over after the first sip. Aargh!

Dave

John's link is very interesting, but it's a lot easier with a processor! According to my clocky contact (an FBHI no less), carbon fiber has taken over from Invar, and is a lot is a lot easier to get hold of

My heretical thoughts are that it doesn't matter what you do to a pendulum, as long as you do the same every swing. Impulsing every swing and keeping the amplitude constant means that almost every swing will not be the true period of the pendulum, but they will be the same. I miss an impulse out when the amplitude grows, but over a sensible number of swings it all averages out.

I used slotted opto switches incorporating Schmidt triggers to get fast edges. I don't have a direct sunlight issue, if it was a problem then I'd try modulating the opto switch LED supply and sending the photo transistor output to a Phase Locked Loop chip LM567 PLL. No doubt someone who knows what he is talking about will be along in a bit to tell me why this won't work! The flag on my pendulum has appreciable length so it triggers the mid point opto a bit before the actual mid point, the magnet is then energised for a time generated by the processor so that it switches off at or before the mid point. If anyone were to object to this processor derived timing then the capacitor discharge system in John's link could be adopted.

While we wait for someone who knows what he's talking about, whilst modulating the beam certainly helps with sunlight doesn't it mean the trigger precision is limited by the modulation frequency and how long it takes the PLL to respond to change. Dunno about the PLL, but if modulated at 15kHz, detection would be not better than ±67µS, which is rather slower than a micro-controller can do with interrupts or input capture mode, .

Thinking about how long it takes to detect the pendulum, Tony's clock uses Hall Effect sensors. I've no idea how Hall Effect and IR sensors compare speed wise?

More deep water - it's a shame I can't swim!

Dave

Thanks John - that's very useful.

I have an HED on my "JH5" clock to sense the extreme of swing. It's an integrated device with a built-in schmidt trigger, and is described as "continuous time" which means that it doesn't use chopping to reduce drift. I don't have the data sheet to hand but my impression is that it's very fast - after all these things are used in tachometers and car gearboxes and stuff. My main concern is variability of the switching point with distance between the magnet and sensor - though I use a CF rod the pendulum support is mounted on 2 brass pillars with the sensor at the bottom end. As temp rises the pillars get longer, the sense magnet gets further above the sensor, and the pendulum has to swing further to activate it. So higher temp = larger amplitude = slower rate. On the other hand the sense magnet gets further from the coil so the impulse reduces and amplitude decreases. I haven't a clue how these interact!

So try out this train of thought.

If you have a pendulum with an escapement capable of delivering a set force for a set time and arrange things such that exactly the same amount of energy is given to the pendulum at the same point in it's swing then with varying atmospheric conditions the amplitude will change according to frictional interactions with the air. Suppose now the atmospherics change so that the amplitude is smaller and that the impulse is made correspondingly larger to restore the swing to the previous amplitude by, for example, extending the duration of the impulse. Although the pendulum has the same amplitude in both cases it does not follow it will have the same period. The arc through which it is driven by the escapement is longer in the second case. The period in the second case should be shorter than in the first case on the basis that the pendulum is driven for a longer portion of it's swing.

In the first case if the amplitude was less the period would also be less due to circular error. Whether the shorter period in the second case exactly matches I don't know but at least they are in the same direction. I suspect that the two case match in terms of energy budget but not neccessarily in terms of time.

regards Martin

So essentially I think I'm saying you cannot correct for circular error by amplitude regulation because all you do is move the circular error to the escapement error. I

regards Martin

What do you guys use as a suspension for your pendulums? I have used a conventional spring suspension and am noticing that I can get some rotational oscillation (rotating about the axis of the rod) - some of that was due to me using steel screws to mount the sensors on the backboard, and replacing those with brass has improved matters considerably, but the possibility is still there. Makes me wonder if I would be better off using knife edges or bearing races.

Knife edges are difficult to make well enough to take the loads of a bob without breaking, you need sapphire or something similar (carbide perhaps?). Ball races have been used but for low friction need to be very small and the limited angular excursion may lead to "brinelling" (see current thread on this subject). I used a single 16mm wide x 0.1mm CuBe strip on my first clock and rotation is an issue. On the second and also the current build I use a 2 spring version, it has 2 x 6mm wide strips spaced apart ~20mm sandwiched and soldered between brass strips.

You can just about see the details in this photo (in my JH5 album).

Duplicate post

Edited By John Haine on 13/03/2021 12:55:50

Point 1 I totally agree with.

Point 2 If you mean the amplitude of the impulse, yes. If you vary the impulse to regulate the amplitude of the pendulum you break your rule 1/point 1

Point 3 Missing an impulse again breaks rule 1.

regards Martin

I'm using a single 1/2" wide spring steel strip. I think part of the problem is that I have made it longer than necessary - just made up a pair of larger cheeks that will reduce its effective length.

Agree with your comments about the other alternatives. If the larger cheeks don't improve matters, I may try a similar setup to yours, but with two 1/2" springs.

Tony:

I made the same error at first, spring too long. My clocky man said no more than 10mm, so I reduced it. Some clever FEA man could no doubt tell us how short it could be without increasing the bending stress too much.

Martin:

I think you're right, but assuming higher humidity slows the pendulums down and more impulse speeds it up (both these assumptions might be wrong) they will tend to balance out. The only way to get rid of atmospheric effects is a vacuum enclosure as used on the best astronomical clocks. I'm not going there (yet), but it has occurred to me that just a hermetic enclosure might be beneficial, at least the atmospheric composition would not change, it's temperature would. The advantage is that you don't have in/out leakage, any interchange would be from small atmospheric pressure changes and diffusion.

SOD:

I need to have a think about possible delays to the impulse triggering. I'll be back

#2 son is here today, he has promised to show me how to post videos, bet you can't wait

Lower amplitude shortens the period. Higher amplitude increases the period.

So if atmospheric conditions create more drag the amplitude will decrease and the period will be shorter.

Correspondingly if the impulse is increased to correct for the lower amplitude that will also shorten the period (I think).

I don't think they cancel.

regards Martin