peterbalch
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- Feb 18, 2023
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What is the best way to synchronise a pendulum clock with a quartz oscillator? It seems like a simple question but I've searched the web and can find no convincing answer.
I have 150 year-old "black forest shield clock" and I want it to be as convenient as a cheap quartz clock. I realise that's not a direction that many horologists would go in but I don't want something I have to fiddle with. I want to change the battery once a year and I want it to keep time to a few minutes a year - like a clock I can buy in a car-boot sale for £0.50. I've already built an auto-winder (An Auto-winder for a Weight-driven Clock).
The pendulum should have a period of exactly 3168 ticks per hour (i.e. 6336 beats per hour). The pendulum has a wire link as its pivot. Its adjustment is by turning the whole bob (a disk) round by +/- one turn - i.e. 0.5mm or 0.15%. So, on its own, the clock has never been more accurate than a couple of minutes a day.
I want to build a quartz oscillator that runs at 3168 ticks per hour (to a few ppm). I can do the electronics, that's no problem. I'm assuming there will be a magnet (neodymium?) attached to the pendulum and an air-core electromagnet that's switched on occasionally. That leaves a lot of questions unanswered.
Does the circuit blindly send pulses to the electromagnet or is there a sensor to measure the current pendulum period and somehow to adjust the pulse? Huygens-style sync depends on blindly applying the impulses. Instead one could try to adjust the average period without attempting to sync.
Where is the magnet? I'd like it up just under the clock so it's inconspicuous. Does the north pole face the wall or does the north pole face to the side?
Let's assume the north pole faces the wall and the electromagnet is centrally placed on the wall. Is the electromagnet energised to attract or to repel the magnet? Is it pulsed once per cycle or once per beat? How long is the pulse? How powerful? Should the impulse from the magnet be as big as the impulse from the escapement? Bigger? Smaller? How would I measure it?
Should the electromagnet be on continuously to attract or repel the magnet? That would be equivalent to increasing or decreasing the force of gravity to change the period. There would then be some way of measuring the pendulum to know whether to advance or retard it.
Or let's assume the north pole faces sideways and the electromagnet is to the side just beyond the furthest swing of the pendulum. Should the electromagnet attract or repel the magnet? Or perhaps do both: attract then repel? Should it sense the current period and, if it needs to be shorter, wait until the pendulum is approaching then turn on the electromagnet to speed it up. Does speeding it up shorten the period? Doesn't it just increases the amplitude without changing the period?
You would think that the pulses from the electromagnet would simply entrain the pendulum but will they?
What is the "capture period" or "lock-in frequency"? Most people who sync a pendulum build a precision pendulum then assume the quartz driver will add those final few ppm accuracy. So the two oscillators' resonant frequencies differ by less than 0.01%. I have such a rubbish pendulum I need a capture period of, say, 0.3%. Is that even possible?
To get a big capture period you need a bigger impulse from the electromagnet. The pendulum becomes a driven oscillator. That's a classical example of a chaotic system. With a small frequency difference and a small impulse, you get lock. With a bigger difference you lose the lock and when you try to compensate by increasing the impulse you get chaotic behaviour.
What about the circular error? Can that be used to control the period? Altering the swing by +/-20% changes the circular error by a few ppm - nowhere near the 0.3% control I need.
How have other people done it?
I have 150 year-old "black forest shield clock" and I want it to be as convenient as a cheap quartz clock. I realise that's not a direction that many horologists would go in but I don't want something I have to fiddle with. I want to change the battery once a year and I want it to keep time to a few minutes a year - like a clock I can buy in a car-boot sale for £0.50. I've already built an auto-winder (An Auto-winder for a Weight-driven Clock).
The pendulum should have a period of exactly 3168 ticks per hour (i.e. 6336 beats per hour). The pendulum has a wire link as its pivot. Its adjustment is by turning the whole bob (a disk) round by +/- one turn - i.e. 0.5mm or 0.15%. So, on its own, the clock has never been more accurate than a couple of minutes a day.
I want to build a quartz oscillator that runs at 3168 ticks per hour (to a few ppm). I can do the electronics, that's no problem. I'm assuming there will be a magnet (neodymium?) attached to the pendulum and an air-core electromagnet that's switched on occasionally. That leaves a lot of questions unanswered.
Does the circuit blindly send pulses to the electromagnet or is there a sensor to measure the current pendulum period and somehow to adjust the pulse? Huygens-style sync depends on blindly applying the impulses. Instead one could try to adjust the average period without attempting to sync.
Where is the magnet? I'd like it up just under the clock so it's inconspicuous. Does the north pole face the wall or does the north pole face to the side?
Let's assume the north pole faces the wall and the electromagnet is centrally placed on the wall. Is the electromagnet energised to attract or to repel the magnet? Is it pulsed once per cycle or once per beat? How long is the pulse? How powerful? Should the impulse from the magnet be as big as the impulse from the escapement? Bigger? Smaller? How would I measure it?
Should the electromagnet be on continuously to attract or repel the magnet? That would be equivalent to increasing or decreasing the force of gravity to change the period. There would then be some way of measuring the pendulum to know whether to advance or retard it.
Or let's assume the north pole faces sideways and the electromagnet is to the side just beyond the furthest swing of the pendulum. Should the electromagnet attract or repel the magnet? Or perhaps do both: attract then repel? Should it sense the current period and, if it needs to be shorter, wait until the pendulum is approaching then turn on the electromagnet to speed it up. Does speeding it up shorten the period? Doesn't it just increases the amplitude without changing the period?
You would think that the pulses from the electromagnet would simply entrain the pendulum but will they?
What is the "capture period" or "lock-in frequency"? Most people who sync a pendulum build a precision pendulum then assume the quartz driver will add those final few ppm accuracy. So the two oscillators' resonant frequencies differ by less than 0.01%. I have such a rubbish pendulum I need a capture period of, say, 0.3%. Is that even possible?
To get a big capture period you need a bigger impulse from the electromagnet. The pendulum becomes a driven oscillator. That's a classical example of a chaotic system. With a small frequency difference and a small impulse, you get lock. With a bigger difference you lose the lock and when you try to compensate by increasing the impulse you get chaotic behaviour.
What about the circular error? Can that be used to control the period? Altering the swing by +/-20% changes the circular error by a few ppm - nowhere near the 0.3% control I need.
How have other people done it?