John Haine | 30/01/2021 11:49:17 |
5563 forum posts 322 photos | Just a quick update on timekeeping. 8 days ago based on the extension of the run reported above I changed the second modulus in the code from 92 to 134 as I finally worked out the correct algorithm to calculate it. Up to this morning the estimated rate of the displayed time was ~0.02 s/day. Here's a new plot. The wiggles look much bigger as the errors are less. The recording spreadsheet also calculates the "ideal" correction number based on the rate and it's giving 134.6 as of this morning. |
duncan webster | 30/01/2021 14:33:10 |
5307 forum posts 83 photos | I posted some pictures of my PIC controlled clock at steel pendulum As it's now got cold in my workshop it started to gain due to contraction of the steel pendulum, so I've finally got round to substituting carbon fibre. I shortened the suspension spring at the same time, as it was deemed too long. I was a bit worried that the joint at the bottom which is a 4mm dia spike 20mm long up the inside of the CF tube might not be strong enough, but according to the interweb shear strength of Araldite is 15 MPa so it should hold 384 kg. My bob is ~2 kg, mild steel inside stainless tube to make it look pretty Positioning the bob by measurement got it within 1 minute a day, now to regulate it, time it over a week next
Edited By duncan webster on 30/01/2021 14:33:29 |
John Haine | 31/01/2021 10:51:48 |
5563 forum posts 322 photos | Duncan, is the rod actually a CF tube? On my "Arduinome" the rod is a 10mm CF tube with 8mm bore. I have a lot of 8mm aluminium rod which is a sliding fit in the bore. So I cut two slugs of this, probably 20mm - ish long, and araldited then in the ends. At the pendulum end the slug is tapped M4, originally for a rating nut but now there's just a nut that a step in the bore of the bob at its centre sits on. At the top the rod is cross drilled through after the slug was glued, with an M3 bolt holding it into the lower suspension chop. No sign of the araldite bond giving up yet (but only 18 months). That bob weighs 7 Kg. Rod length was originally found by calculation (using models for the MoI of all the components) and that gives a period near enough for easy digital compensation except for one M8 nut on top of the bob. On my second clock there is just an M3 screw through the lower chop and rod, no glued-in slug. The bob is held by a grub screw on the centre line clamping the rod. Again length set by calculation, and all regulation digital. Have you thought of doing the regulation in the PIC?
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duncan webster | 31/01/2021 13:33:35 |
5307 forum posts 83 photos | Yes it's 6mm OD 4mm ID CF tube. The top fitting was tapped 1/4 *40 for the steel rod, so I drilled it out 6mm leaving a scratch to key the glue, the bottom end has a 4mm dia spike on a bit of 1/4 steel threaded 1/4*40 (actually the end of the old rod) for the rating nut. The spike has small vee shaped grooves in it, and I poked an M4 tap up the tube, which roughed it up nicely. I then bored the bob at the top and made a split collet to fit round the CF tube so there is less chance of the bob wobbling on the nut. It's only split because i forgot to fit it to the CF before gluing the ends on I'm not messing with the PIC, I did think of replacing it with an Arduino, but it works, so leave it alone. If I did replace it I'd alter the opto switches to be in line vertically and have a second flag offset to one side of the rod to detect end of travel. Would look a lot neater and allow me to change the arc of travel. Aesthetics means you want use a lot more swing than you actually need, and less is more in the case of pendulums, although I'll repeat my mantra, it doesn't matter what you do to a pendulum as long as you always do the same. Having adjusted it once it's now within a second in 24 hours, I'll leave it for a week and have another calculate. Now for the next project, does anyone have any drawings to make a Lavet type stepper motor. These were used in a lot of silent slaves and in all the quartz clocks I've ever seen. Be nice to make one rather than buying a stepper motor. Lavet The slave I'm using now is Gents and needs 24v, be nice to run off <12, make the power supply simpler |
John Haine | 31/01/2021 13:59:27 |
5563 forum posts 322 photos | Ah, Lavet motors! I can't point you to drawings but you can get diametrical magnets that could potentially be used, though they might be embarrassingly strong for the job! I'd be interested too, having used a quartz movement by connecting to its stepper motor. I've also been looking into how these continuous sweep quartz movements work. As far as I've been able to find any useful information they use standard Lavet motors but drive them with trick waveforms to get continuous motion. I'd like to find out more because the dial on this clock (above results) is rather noisy. Any info anyone has would be very welcome. |
Michael Gilligan | 31/01/2021 15:01:34 |
![]() 23121 forum posts 1360 photos | John & Duncan You might be interested in Bowell's patent : **LINK** https://worldwide.espacenet.com/patent/search?q=pn%3DUS882186A MichaelG. |
duncan webster | 03/05/2021 15:11:12 |
5307 forum posts 83 photos | My carbon fibre pendulum clock has now lost 1/2 minute in 36 days. Not yet in the precision league, but it will do for me. Losing is more convenient than gaining as I have a fast advance button. |
John Haine | 03/05/2021 16:04:55 |
5563 forum posts 322 photos | 0.833 s/day - that's good. I have had "another..." clock keeping 0.22 s/day at one stage but have recently "upgraded" it with a dedicated PSU rather than a USB from PC or wallwart. It seems quite sensitive to supply voltage so currently losing ~3 s per day but consistently so should be able to trim that out by adjusting the correction coefficients. Upgrade also added the ability to set hands + and - for convenience. Has been running for 8 days so soon time I think to adjust the trim. |
Kitwn | 07/06/2021 05:16:46 |
2 forum posts | This is my first post on this forum, having stumbled across this thread whilst looking for other people's ideas on driving a pendulum with the aid of an Arduino. I thought followers of the thread may be interested in how it works. I have already made a prototype clock which can be seen at the link below. This one is wood but the same ideas can be applied to any design. The pendulum is driven by a solenoid and the movement is driven by a ratchet on the pendulum. This one does not use an Arduino but discrete electronics, however I want to modify it to be a simpler circuit board. The point of my design is that the period of the pendulum is locked to an external one pulse per second (1PPS) waveform derived either from a crystal oscillator or, for perfect accuracy, a GPS receiver. The amplitude of the pendulum swing is larger than for a normal clock and because the period of any pendulum rises with amplitude, regulation of the clock is possible if you can control the amplitude. In my design the timing of the pendulum passing through it's centre position is compared to the 1PPS pulse time. If the delay between them is too large the solenoid stops pulsing, the pendulum amplitude and it's period fall so the clock speeds up until the two pulses, pendulum and 1PPS, are within the prescribed window. Solenoid pulses are large enough to drive the pendulum to an amplitude above that required for exact timing so simply cancelling some pulses is all that is needed to keep the pendulum in step with the 1PPS. Obviously the pendulum must be adjusted to give the correct timing at around the middle of the acceptable range of amplitude. There is also a mechanism which measures the amplitude and prevents it going outside set limits. I will describe this in more detail if anyone is interested. https://vimeo.com/343781598 |
Joseph Noci 1 | 06/09/2023 15:03:27 |
1323 forum posts 1431 photos | Perhaps the Smart Folk here can assist.. If the period of the pendulum depends only on the length, what is the physics behind correcting the pendulum timing by pulsing more or less often, or regularly with more or less energy? Does the pulse not just add to the current velocity , 'speeding' the pendulum up? If that is all that happens, then according to conservation of energy, the pendulum should just go higher since kinetic E has increased. If it goes higher, it will accelerate more on the downstroke, due to the increased potential E - so the pendulum swings faster now, but also higher, so the period remains the same. What have we actually changed here, other than total mechanical energy? Since some objectives appear to be to alter the timing electronically, or alter the period electronically, so as to be able to compensate for environmental changes - How does retarding or advancing the pulse change the pendulum period in order to 'correct' its time if only length does this? Edited By Joseph Noci 1 on 06/09/2023 15:07:01 |
John Haine | 06/09/2023 15:29:30 |
5563 forum posts 322 photos | Ah, Joe, deeper waters here... A pendulum's period depends on its amplitude, the fractional rate being given by (angle^2)/16 where angle is in radians. For small angles this is nearly negligible but gets more significant as the amplitude gets larger. Remember that 1s per day is nigh on 1 in 100,000. A pendulum designed for exactly 1 second period would lose 1 second per day at an amplitude of only 0.7 degrees. This is why we want to control the amplitude. Traditional "regulators" also use rather small swings because that also reduces the sensitivity because of the square law. This is called "circular deviation" or sometimes "circular error". (CD) If you impulse the pendulum exactly at the centre of its swing that has no effect on its period. If you apply the impulse early (or advance the phase of your sine wave drive) it makes the pendulum period slightly less; if late it makes it longer. This is "escapement deviation". (ED) Its magnitude depends on the phase shift and pendulum Q and for small errors the fractional change is (phase error)/2Q. (Strictly tan(phase error)/2Q.) What one generally does is control the amplitude to keep it constant so the CD is constant and can be "adjusted out". Likewise one tries to keep the impulse phase constant so ED can be adjusted out. Many clocks have non-central impulses and when the impulse strength changes (for example because the spring unwinds), the amplitude reduces so it runs faster and the phase angle changes. If the impulse lags and gets later it slows the pendulum down and if you're lucky the two will cancel out. The ultimate is Clock B which plays these effects off against Q changes due to varying barometric pressure to compensate for that too. There have been designs that deliberately change the amplitude to adjust the clock, event to regulate it to an external reference. A problem with this is that the amplitude, and hence period, responds only slowly to impulse changes so the loop is very slow. What a few people (well certainly me and Dave I think) are proposing is to have the pendulum running as stably as possible with controlled amplitude (and in has case eventually in a vacuum), accumulate the phase of the pendulum then digitally correct this "post hoc" based on environmental observations before displaying the "time".
Edited By John Haine on 06/09/2023 15:44:59 Edited By John Haine on 06/09/2023 16:21:52 |
duncan webster | 06/09/2023 15:32:03 |
5307 forum posts 83 photos | I'm sure cleverer people than me will be along, but here's my take on it. Period depends strongly on length, and to a smaller degree on amplitude. If you keep the amplitude constant, then you only (only?) need worry about length. Clocks have been made where the amplitude is deliberately varied to keep the balance wheel in the case of which I'm aware in synch with a processor driven by a xtal. This to my mind is cheating! Pulsing the pendulum also changes it's period, theory says the minimum disturbance is by pulsing right at the centre. Mine pulses as the pendulum approaches centre, Fedchenko type pulse just after, by repulsion Edited By duncan webster on 06/09/2023 15:34:21 |
Joseph Noci 1 | 06/09/2023 16:13:19 |
1323 forum posts 1431 photos | Posted by John Haine on 06/09/2023 15:29:30:
Ah, Joe, deeper waters here... If you impulse the pendulum exactly at the centre of its swing that has no effect on its period. If you apply the impulse early (or advance the phase of your sine wave drive) it makes the pendulum period slightly less; if late it makes it longer. This is "escapement deviation". (ED) Its magnitude depends on the phase shift and pendulum Q and for small errors the fractional change is phase/2Q. ....................... There have been designs that deliberately change the amplitude to adjust the clock, event to regulate it to an external reference. A problem with this is that the amplitude, and hence period, responds only slowly to impulse changes so the loop is very slow. ............................. What a few people (well certainly me and Dave I think) are proposing is to have the pendulum running as stably as possible with controlled amplitude (and in has case eventually in a vacuum), accumulate the phase of the pendulum then digitally correct this "post hoc" based on environmental observations before displaying the "time". Thanks John: I wrestle with the distinction between a few of your statements.. If you impulse the pendulum exactly at the centre of its swing that has no effect on its period I presume this holds only if the impulse amplitude is a constant - will higher impulse amplitude decrease the period? Velocity is max at this point and a higher impulse will add to that? Or is this as I first stated - that all we are then doing is increasing the swing, not the period? Impulsing Early - Is that before BDC, and late , after BDC? Why would a late impulse increase the period? Are we not increasing the velocity just after the point the bob begins slowing down? There have been designs that deliberately change the amplitude to adjust the clock, event to regulate it to an external reference. A problem with this is that the amplitude, and hence period, responds only slowly to impulse changes so the loop is very slow. Intuitively I grasp why the response is slow in this mode - Reducing the pulse energy input does not in itself slow the pendulum - it has to run itself down - but pulsing with higher amplitude - would that not increase the pendulum amplitude in the same way and at the same rate as a phase shifted pulse ( the 'early pulse' ) What a few people (well certainly me and Dave I think) are proposing is to have the pendulum running as stably as possible with controlled amplitude (and in has case eventually in a vacuum), accumulate the phase of the pendulum then digitally correct this "post hoc" based on environmental observations before displaying the "time". If i understand this correctly, then this is what I am trying to do as well.....Use my amplitude control loop to try fix the amplitude as hard as possible, use the adjusters to get close to 2sec at an environmental 'norm' (room temp, midway Baro reading, etc) and then let it run - the shift from that timing reference is then measured and used to correct computer time ( not pendulum timing) against the environmental changes. What worries me though is that from what you say, it would seem I am would be wasting my time trying to perform timing adjustments on the pendulum, such as your early/late impulses would do, by changing only the amplitude of my sine wave drive, and not its phase.... edit - smileys... Edited By Joseph Noci 1 on 06/09/2023 16:15:17 |
SillyOldDuffer | 06/09/2023 16:45:48 |
10668 forum posts 2415 photos | Now the clever people have answered, here's my take. The curve followed by a pendulum bob isn't isochronous, so its period varies with amplitude. The bob's curve is nearly isochronous at small amplitudes, so most pendulum clocks operate with amplitude less than 5°. This reduces amplitude error considerably. I can use the effect to decide when more energy is needed. Measuring period with high resolution allows small changes of amplitude to be detected, and used to govern impulses. The goal is to keep amplitude constant whilst minimising any disturbance of the bob. (Over impulsing is causes many problems!) I've tried two strategies:
My first clock ran best with impulse every beat. The second runs better with n about 3. My hypothesis is that:
If I'm right there is no instantly obvious physical law because how best to achieve constant amplitude with minimum disturbance depends on how the pendulum is constructed and what it is running in. A pendulum suspended in air inside a tight box is very different to the same pendulum swinging in a hard vacuum. I guess a well-made pendulum in a vacuum will perform better with 'n' beats per impulse, whilst a competently made pendulum swinging in air might do better with 1 impulse per beat. I have a notion of applying graduated impulses. When more energy is needed, do a succession of weak impulses rather than one big one. Not done it yet - I'm already up to my armpits in pendulum problems! Dave
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Joseph Noci 1 | 06/09/2023 16:56:24 |
1323 forum posts 1431 photos | Posted by SillyOldDuffer on 06/09/2023 16:45:48:
Now the clever people have answered, here's my take. The curve followed by a pendulum bob isn't isochronous, so its period varies with amplitude. The bob's curve is nearly isochronous at small amplitudes, so most pendulum clocks operate with amplitude less than 5°. This reduces amplitude error considerably. I can use the effect to decide when more energy is needed. Measuring period with high resolution allows small changes of amplitude to be detected, and used to govern impulses. The goal is to keep amplitude constant whilst minimising any disturbance of the bob. But you are trying to keep amplitude constant by varying the period which you said will not vary if length is constant, and if the swing angle is small... Your method would seem to reduce to spreading impulse energy over time to get the required amount of impetus, ie, 1 pulse every few swings..and not where the impulse is applied. This does not seem to be in line with John's concept of changing phase of the pulse, ie, its position relative to the bob zero. Will you concept therefore also suffer from the slow response time issue? In fact you confirm that - I can use the effect to decide when more energy is needed and not where in time energy is needed...
Edited By Joseph Noci 1 on 06/09/2023 16:58:47 |
John Haine | 06/09/2023 19:07:59 |
5563 forum posts 322 photos | Posted by Joseph Noci 1 on 06/09/2023 16:13:19:
Posted by John Haine on 06/09/2023 15:29:30:
Ah, Joe, deeper waters here...
.............................
Thanks John: I wrestle with the distinction between a few of your statements.. If you impulse the pendulum exactly at the centre of its swing that has no effect on its period I presume this holds only if the impulse amplitude is a constant - will higher impulse amplitude decrease the period? Velocity is max at this point and a higher impulse will add to that? Or is this as I first stated - that all we are then doing is increasing the swing, not the period? Sorry I wasn't clear. Impulsing at the centre does not change the pendulum's phase instantaneously. If the impulse is just strong enough to make up for the lost energy then there is no net change to period. Impulsing Early - Is that before BDC, and late , after BDC? Correct. Why would a late impulse increase the period? Are we not increasing the velocity just after the point the bob begins slowing down? Again, if we are impulsing each period and just replacing the lost energy, the late impulse causes a phase jump that lengthens the period. Similarly an impulse that is early shortens the period. There have been designs that deliberately change the amplitude to adjust the clock, event to regulate it to an external reference. A problem with this is that the amplitude, and hence period, responds only slowly to impulse changes so the loop is very slow. Intuitively I grasp why the response is slow in this mode - Reducing the pulse energy input does not in itself slow the pendulum - it has to run itself down - but pulsing with higher amplitude - would that not increase the pendulum amplitude in the same way and at the same rate as a phase shifted pulse ( the 'early pulse' ) No, the same time constant determined by Q applies, and anyway the impulse will be applied at BDC. What a few people (well certainly me and Dave I think) are proposing is to have the pendulum running as stably as possible with controlled amplitude (and in has case eventually in a vacuum), accumulate the phase of the pendulum then digitally correct this "post hoc" based on environmental observations before displaying the "time". If i understand this correctly, then this is what I am trying to do as well.....Use my amplitude control loop to try fix the amplitude as hard as possible, use the adjusters to get close to 2sec at an environmental 'norm' (room temp, midway Baro reading, etc) and then let it run - the shift from that timing reference is then measured and used to correct computer time ( not pendulum timing) against the environmental changes. What worries me though is that from what you say, it would seem I am would be wasting my time trying to perform timing adjustments on the pendulum, such as your early/late impulses would do, by changing only the amplitude of my sine wave drive, and not its phase.... You could use amplitude to adjust its mean rate. But my approach is to have the pendulum deliberately running fast and not to adjust it to a particular period, and deal with the "rating" in code. edit - smileys... Edited By Joseph Noci 1 on 06/09/2023 16:15:17
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Joseph Noci 1 | 06/09/2023 20:52:47 |
1323 forum posts 1431 photos | Posted by John Haine on 06/09/2023 19:07:59:
Posted by Joseph Noci 1 on 06/09/2023 16:13:19:
Posted by John Haine on 06/09/2023 15:29:30:
You could use amplitude to adjust its mean rate. But my approach is to have the pendulum deliberately running fast and not to adjust it to a particular period, and deal with the "rating" in code.
A little clearer..Thanks. Why running fast, as opposed to slower, or nearly correct? |
John Haine | 06/09/2023 20:57:47 |
5563 forum posts 322 photos | So I only need to skip pendulum pulses, just seemed easier. |
duncan webster | 06/09/2023 21:02:40 |
5307 forum posts 83 photos | I've linked this in another post, but it explains large amplitude penduli, and does the sums for you My clock is set to run deliberately slow, as I have a button which advances the 30 sec slave every second to set it right. But I'm not a pendulista, as long as it keeps decent time I'm happy |
SillyOldDuffer | 06/09/2023 22:18:09 |
10668 forum posts 2415 photos | Posted by Joseph Noci 1 on 06/09/2023 16:56:24:
Posted by SillyOldDuffer on 06/09/2023 16:45:48:
Now the clever people have answered, here's my take. The curve followed by a pendulum bob isn't isochronous, so its period varies with amplitude. The bob's curve is nearly isochronous at small amplitudes, so most pendulum clocks operate with amplitude less than 5°. This reduces amplitude error considerably. I can use the effect to decide when more energy is needed. Measuring period with high resolution allows small changes of amplitude to be detected, and used to govern impulses. The goal is to keep amplitude constant whilst minimising any disturbance of the bob. But you are trying to keep amplitude constant by varying the period which you said will not vary if length is constant, and if the swing angle is small... Your method would seem to reduce to spreading impulse energy over time to get the required amount of impetus, ie, 1 pulse every few swings..and not where the impulse is applied. This does not seem to be in line with John's concept of changing phase of the pulse, ie, its position relative to the bob zero. Will you concept therefore also suffer from the slow response time issue? In fact you confirm that - I can use the effect to decide when more energy is needed and not where in time energy is needed... My electromagnet is mounted side on so I can pulse the bob at any point after the beam is broken. I had the idea that pulsing the electromagnet as the bob approached top of swing (max amplitude) would lift the bob a little higher by it flying into an area of apparently reduced gravity, thus reducing the jolt. As the strength of a magnetic field weakens rapidly with distance, I thought the grab would accelerate the bob softly. However, George Airey did the maths about 200 years ago and showed least disturbance occurs when the bob is travelling at it's fastest, ie when it passes bottom dead centre. So I measure amplitude, and if it is below an experimentally determined value, I pulse the magnet on the next beam break, with the beam set to trigger at BDC*. Pulse when is easy, I don't know about a timing a sinusoid to energise a bob.. Dave Due to a build error, the electromagnet fires after BDC at the moment. Hopefully explains why my pendulum is noisy...
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