John Haine | 13/03/2021 13:56:00 |
5563 forum posts 322 photos | Humidity seems to have very little impact, it's air pressure and temperature that mainly affect the drag and hence amplitude. More pressure and lower temp = air denser. There is also a small viscosity affect, which gets higher with temperature. Some very successful clocks have been made with aneroid compensators which work very well. Or you can take the Harrisonian approach and get a balance between escapement and circular deviation so changes in amplitude with pressure compensate for buoyancy changes. The affect of changing impulse on period depends on where it is - if the centre of the impulse is after BDC a larger impulse slows the pendulum down; if before it speeds it up. Ideally the impulse is centred on BDC when it doesn't change timing. If you did use a pulsed light source to avoid ambient effects, it would be better to use a "synchronous rectifier" to detect the signal rather than a PLL. Just as a benchmark, Doug Bateman's electronically maintained clock uses an opto sensor to keep the amplitude stable to within a second of arc - described in Horological Journal Jan 1972. It is also fitted with an aneroid compensator and in a later article (October 75) he describes its performance with this which seems to reduce the error due to pressure variations by a factor of 10. |
John Haine | 13/03/2021 14:06:45 |
5563 forum posts 322 photos | Posted by Martin Kyte on 13/03/2021 13:44:04:
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 The condition that create more drag is greater density, but this makes the bob lighter through buoyancy, which increases the period. The effect of impulse on period may make it longer or shorter depending where it's applied in the cycle. For a perfectly central impulse it turns out that there is a nominal amplitude at which the buoyancy, drag and circular deviation effects cancel out and the pendulum rate is insensitive to barometric pressure. For a typical regulator pendulum this is about 2 degrees. However you have to let the amplitude change with pressure to actually get the compensation, so a system that regulates the amplitude can't be made to work in this way. There's a similar amplitude for other impulse positions, which may be at larger or smaller amplitude. In the case of Burgess' Clock B at Greenwich it's about 6.25 degrees. |
Martin Kyte | 13/03/2021 15:11:50 |
![]() 3445 forum posts 62 photos | Posted by John Haine on 13/03/2021 14:06:45:
Posted by Martin Kyte on 13/03/2021 13:44:04:
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 The condition that create more drag is greater density, but this makes the bob lighter through buoyancy, which increases the period. The effect of impulse on period may make it longer or shorter depending where it's applied in the cycle. For a perfectly central impulse it turns out that there is a nominal amplitude at which the buoyancy, drag and circular deviation effects cancel out and the pendulum rate is insensitive to barometric pressure. For a typical regulator pendulum this is about 2 degrees. However you have to let the amplitude change with pressure to actually get the compensation, so a system that regulates the amplitude can't be made to work in this way. There's a similar amplitude for other impulse positions, which may be at larger or smaller amplitude. In the case of Burgess' Clock B at Greenwich it's about 6.25 degrees. So my feeling is that for electronically impulsed clocks the objective is to create the best pendulum you can such as the above and use the electronics to generate the most stable impulse you can. Amplitude regulation causes too much interference with the pendulum. Essentially you want to provide enough energy to the oscillator to maintain it as near to it's natural frequency as possible in the most stable way you can. regards Martin |
duncan webster | 13/03/2021 15:16:23 |
5307 forum posts 83 photos | I hope John won't mind if I link to this which explains quite a bit Burgess clock. I've learned quite a lot from this thread, not least how little I knew before! If I shift my electromagnet so that it is offset from the centre and only impulses in one direction that could give impulse both before and after mid point. Probably not equally, but would it be worthwhile? |
Martin Kyte | 13/03/2021 16:00:32 |
![]() 3445 forum posts 62 photos | I hope you don't take my contributions as gospel without some conformation from elsewhere Duncan. I have some understanding but don't set up as an authority. Like you I have found this and the other similar thread emmensy entertaining and informative. regards Martin |
Tony Jeffree | 13/03/2021 16:09:08 |
![]() 569 forum posts 20 photos | Reducing the effective spring length to ~5mm seems to have controlled the rotational oscillation pretty well, so that will hopefully improve matters. I've been playing around with the height of the electromagnet (the gap between it and the armature on the pendulum) and it is easy enough to increase the time between impulses - up to just short of 3 minutes at one point. However, given some of the discussion above, it probably makes sense to increase the gap (reduce the force of the impulse) as that will result in a smaller variation in amplitude. Edited By Tony Jeffree on 13/03/2021 16:09:48 Edited By Tony Jeffree on 13/03/2021 16:09:58 |
John Haine | 13/03/2021 16:24:23 |
5563 forum posts 322 photos | Posted by duncan webster on 13/03/2021 15:16:23:
I hope John won't mind if I link to this which explains quite a bit Burgess clock. I've learned quite a lot from this thread, not least how little I knew before! If I shift my electromagnet so that it is offset from the centre and only impulses in one direction that could give impulse both before and after mid point. Probably not equally, but would it be worthwhile? Not at all Duncan, a good find. |
Tony Jeffree | 13/03/2021 23:03:37 |
![]() 569 forum posts 20 photos | Posted by John Haine on 12/03/2021 19:12:02:
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. Couldn't find clocksync on the Android app store, but I did find an app called "Atomic Clock & Watch Accuracy Tool (with NTP time)" which gives NTP time and in the unpaid version will allow you to record measurements for a single timepiece (more if you pay the sub). Pretty neat - you tap the screen when you want to take a measurement and enter the time from the clock & it records the NTP time at the moment you tapped. Nice graphs etc. and calculates a day rate from your data when you give it enough measurements. |
John Haine | 14/03/2021 06:46:56 |
5563 forum posts 322 photos | Tony, that's weird! I just checked and you're right, ClockSync is no longer available. However there is one called System Clock Monitor which does the same and actually seems better. Taking a photo of the clock face is an easy way to do the recording. |
Tony Jeffree | 16/03/2021 10:40:22 |
![]() 569 forum posts 20 photos | John, that seems to do the NTS thing, but nothing else - the one I mentioned earlier allows you to record measurements and will calculate day rates for you. There are a couple of other similar offerings that I am trying out: WatchCheck and Toolwatch.
|
Tony Jeffree | 17/03/2021 10:13:19 |
![]() 569 forum posts 20 photos | More by luck than judgement, I seem to have adjusted the rate to less than a second a day already just by adjusting the rating nut. |
Tony Jeffree | 18/03/2021 15:44:36 |
![]() 569 forum posts 20 photos | Posted by Neil Wyatt on 12/03/2021 11:12:43:
Look out for the next MEW Neil
That's a very pleasant surprise, Neil - I had almost given up on seeing that article appearing! Of course, in keeping with pandemic regulations, Skelly (as he is affectionately known) is now wearing a mask, but unfortunately, the ravages of salt water have now fused his bottom bracket, so he has been reduced to freewheeling. The unipivot is slowly boring its way through the top tube of the bike, so I fear it won't be long before he rests in pieces... |
Andrew Cattell | 14/11/2021 22:47:46 |
12 forum posts | I went down this path some 20 years ago with a constant amplitude pendulum controlled by optical interrupters. I used two slotted opto devices, one to detect the extremity of swing at one side and a second one to detect the bottom dead centre. The interrupter obscurer is a single steel pin, just 1mm diameter poking down from the centre of the bottom of the pendulum shaft. The impulse is delivered by a hollow electro magnet coil mounted to one side of the pendulum shaft attracting a steel pin fixed to the side of the pendulum shaft just above the bob. The whole lot was masterminded by a small PIC microcontroller. The thinking is that if the interuptor pin fails to break the light beam at the extremity the impulse magnet will be energised for the next time that the centre opto is obscurred by the pin thus giving an accurately centred "pull" to the pendulum. The whole assembly of pendulum suspension, optos and impulse coil is mounted in the corner of a room, the suspension bracket is a short length of 1inch X 1inch angle iron bolted diagonally across the corner using 8mm masonry bolts. After setting it running the pendulum was brought to time by adding a few clothes pegs to the shaft above the impulse pin. The swing of the pendulum was compared with a signal from the NPL MSF time reference in Cumbria using a two beam storage oscilloscope. The impulse was made very small so that it would do 8 or nine impulses to to restore the amplitude which was then maintained for a few swings without impulse. This seemed to be better than giving a big pulse and missing many to my mind. Once I got it all trimmed I set about recording its operation, taking readings of time shift compared to 1 second tick from the MSF, the barometric pressure and the room temperature every hour. I did this for 14 days! I wanted to take enough readings to assess the effects of air pressure and temperature with a view to making compensation devices to be added to the pendulum later. By taking readings on the whole minute the MSF one minute pulse (easily itdentifable) and can be compared to the 30 second impulse sent to the display clock mechanism drifts of up to 2 seconds fast or slow can be taken with confidence that the right pulses are being compared. My post was deemed to be too long so more follows in next posting! Rans6................................. |
Andrew Cattell | 14/11/2021 22:49:04 |
12 forum posts | So far, so good. After a couple of weeks I stopped taking the readings every hour (it plays havoc with your sleep quality) but continued to record the readings once per day when we are at home. I noticed after a while that the time keeping was beginning to drift off. While investigating the errors I found that the pendulum had gone out of beat, the centre opto was not at the lowest point in the pendulum swing and the extremity opto, being the same distance from the centre opto, was controlling the amplitude to a lower constant size. I reset the position of the optos to restore the beat. Some months later I found that the timekeeping was slipping off the other way and sure enough the pendulum was out of beat the other way and the amplitude of swing had increased. It turns out that the whole house moves on a cyclic basis with the seasons, probably due to it being built on very high clay content ground, the moisture content of the clay changes through the year as the rainfall varies. Unbelievable. Just when you think you have removed or allowed for all of the things that might affect your clock something else comes to surprise you. Rans6................................ |
Andrew Cattell | 14/11/2021 22:49:41 |
12 forum posts | I have yet to work around the issues of the house walls tilting with the seasons, to implement temperature and pressure compensation but the clock continues to run. I will change the software so that it monitors the time between consecutive passes of the centre opto breaks to allow detection of the out of beat-ness. I have enough spare pins on the PIC controller to allow it to drive a small stepper motor which will be used to "motor" a carriage with the two opto devices along a path in parallel with the direction of the tilt of the walls. This is not exactly in parallel with the swing direction of the pendulum so needs a bit of thought. When the house moves the pin on the pendulum gets dangerously close to one side of the slot in the extremity opto due to the movement hence the need to move the correction carriage on a non parallel track. The impulse coil has enough space around the pin that it will not drag due to the house tilt. This big air gap was a conscious design feature to minimise pneumatic drag as the pin enters and leaves the hole. The PIC controller is supplied from a wall wart psu and has a rechargeable battery backup which I built in at the beginning using some old NiCad cells found in my junk box. This battery has maintained the clock through a number of mains power outages, some lasting many hours. It also sends an impulse to an old factory repeater clock every 30 seconds so the time is on display, the display clock has its own battery which has also survived the power cuts so it has only needed attention twice a year when the clocks change. Rans6............................................. |
Andrew Cattell | 14/11/2021 22:50:10 |
12 forum posts | Other improvements I want to make. At the moment the optos LEDs are strobed by the PIC micro to reduce power consumption and increase service life. They are only strobing for a few milliseconds before during and after the time when there is predicted to be a pin passing through. The strobing causes there to be a few 10s of microseconds jitter in the detection time of breaking of the beam when seen by the PIC. This might have some effect on the accuracy of the extremity position on boarderline cases of detect/not detect. It also affects the accuracy of the BDC impulse. Both of these can be made better by using some hard wired logic gates between the opto and the PIC, putting the PIC in control of when the optos are on and when the impulse needs to be on but letting the logic gates actually drive the impulse. As others have noticed the slotted opts are affected by ambient light levels. I have done my best to shield them but as the pendulum assembly still has no case around it more can be done. What I am not sure about is whether the optos are showing any sign of sensitivity change due to aging. Thinking about how to detect/compensate for this. I also want to investigate if the performance of the opto devices changes enough with temperature to give a measureable change in the pendulum accuracy. After all of the work I did (and have yet to do) on my clock I came across a design that has taken the subject to a whole new level. See the work of Edward T. Hall and the Littlemore Clock, your favourite search engine will find it. Rans6............... |
Michael Gilligan | 15/11/2021 10:20:29 |
![]() 23121 forum posts 1360 photos | A great [group of 4] first posting, Andrew … Thanks MichaelG. |
Tony Jeffree | 15/11/2021 10:50:22 |
![]() 569 forum posts 20 photos | Great posts Andrew! Just a thought, but you could potentially fix the "house drift" problem by mounting the whole clock on gimbals. You would of course need to add sufficient viscous damping to the gimbal bearings and/or add mass to the structure so the pendulum oscillations themselves don't cause the whole contraption to swing, but it could be done. |
Tony Jeffree | 15/11/2021 10:53:33 |
![]() 569 forum posts 20 photos | Posted by Neil Wyatt on 12/03/2021 11:12:43:
Look out for the next MEW Neil
Talking of which Neil...do you have an ETA for my most recent scribblings...? |
John Haine | 15/11/2021 11:16:32 |
5563 forum posts 322 photos | Good work Andrew. Reading your last post reminded me that I believe that Hall was ultimately disappointed in the performance of his clock and attributed it to ground motion induced by a large nearby tree swaying in the wind. On the issue of optos being sensitive to stray light, I think I referred earlier to Doug Bateman's clock. This was designed long before slotted optos were available so he had to roll his own. The clock uses a single sensor driven by a vane on the rod just above the rod, with a central slot to drive the impulse. So the sensor is hidden from the light source, the vane providing "shading". Likewise the amplitude is sensed by the end of the vane. There's some discrete logic to decide what to do with the sensed signals. Doug reported that there is no discernible sensitivity to stray light, I think because of the shading the vane provides, so any ambient light just adds to the illumination from the LED. His clock has been running since the 1970s I think so no problem with diminishing sensitivity. Disciplining an oscillator to MSF or GPS is quite complicated. For the purpose of monitoring cycle-by-cycle variations it could be better to just use the TCXO signal "unvarnished" and if necessary calibrate it? When I was monitoring my "Arduinome" I used a "picPET" to time the opto edges using a 10MHz OCXO, logging the results wing-by-swing in an R-Pi. From this I periodically uploaded the data over Wi-Fi. I had two problems with this - one was that the Pi took too much current to be able to run it from the backup so a run would stop if there was a brown-out or power cut. Also the Pi would very occasionally lose Wi-Fi connectivity, which needed restarting to fix, again interrupting the run. This is a know Pi problem and though there is a fix even that didn't seem entirely reliable. In the end though the pendulum didn't have the short-term stability to make detailed monitoring worthwhile, and the clock keeps good enough time given it doesn't have a seconds hand (uses a modified Synchronome movement). |
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