Arduino Pendulum Clock Design - Comments Welcome

This is all getting a bit esoteric. using a super accurate GPS clock to control the rate of a pendulum (which I have seen written up) seems like cheating to me, you might as well use the GPS clock to tell the time.

As long as SOD's pendulum is telling the Arduino when to trigger the magnet I don't see how the Arduino clock frequency comes in to it? You could achieve the same result with a board full of discrete logic chips, discharging a capacitor through the electromagnet ,no clock at all, unless you count a capacitor/magnet combination.

If still worried, you can get 3.3v Arduinos which work at half clock speed, if bot give same result then clock speed doesn't matter, or just add weights on top of the pendulum bob and see whether it changes the pendulum rate as predicted, if it does the pendulum is in control.

I still think that it doesn't matter how you impulse a pendulum as long as you do exactly the same every time, Temperature and buoyancy/drag effects need to be kept under control, I was going to put a heater in my clock, but failed miserably to make a decent fitting door!

I our house, no matter how accurate the clock, SWMBO will always be late, it's a matter of principle with her.

This is all getting a bit esoteric. using a super accurate GPS clock to control the rate of a pendulum (which I have seen written up) seems like cheating to me, you might as well use the GPS clock to tell the time.

Totally agree with that

As long as SOD's pendulum is telling the Arduino when to trigger the magnet I don't see how the Arduino clock frequency comes in to it?

The pendulum triggers the arduino timer which fires the impulse some fixed TIME later (Arduini timing). The impulse only occurs at the same POINT if the pendulum is travelling at the same speed.

regards Martin

Inching towards impulsing the bob. This is the relay driver circuit:

D13 is the pin connected to the built-in LED on the Arduino Board. Might see it flash!

Not had time to take in the other comments yet. Joe's soft magnet idea is interesting except I'm aiming to swing the pendulum by only 3° or so and the bob will never be far away. Still going to try it; the code is easy enough, but I think being on risks the magnet acting as a brake.

To self-start the clock I'm going to send powerful pulses at to match the pendulums period. When the bob is moving enough to break the beam, the code will switch to maintain mode in which much shorter and carefully timed pulses will be triggered optically.

Duncan's comments are reassuring. I'm worried I've accidentally built a phase-locked loop!

Breadboard the driver and write some code next.

Dave

I don't think we are trying to lock, but to avoid locking!

The end of the swing is the worst possible point to impulse. Ideally the impulse should be applied at the centre, or equally either side of it. Woodward shows that actually the impulse shape is largely irrelevant, if you get its "phase centre" in the middle of the swing - this is in effect the "centre of gravity" of the impulse. The shape doesn't matter because the pendulum's resonance filters out all the higher harmonics, you just want the fundamental to be applied as a force in phase with the pendulum to accelerate it at the centre of swing, where it has ideally no effect on timing. If there is a phase error, it causes a fractional change in time which is reduced by a factor of 2Q for small errors - so Q should be as high as possible. Q is determined by bob mass, period, and air losses.

I think there should be no worry about exciting spurious modes by a short spiky impulse applied directly to the bob provided it is in the plane of the swing. You don't want to be exciting a "back to front" mode as well. One thing I have noticed about the small diameter rod I have is that it's not circular. A non circular rod may induce spurious modes as it will have different bending moduli in different planes, and the modes will have different frequencies.

Clocks can be made with a small magnet on the bottom of the bob which passes over a coil at the centre of the swing - this triggers a transistor that applies an impulse (via the same coil) largely downwards at the centre, starting just before and ending just after the centre position. You could do this also using an opto to sense the centre. The opto could drive the coil directly rather than through a processor, which would avoid the processor delay affecting the timing. I believe (but haven't yet proved) that if you simply use a coil to attract the bob downwards for a short period starting just before BDC and ending just after, it will apply an impulse that increases amplitude.

The role of the processor will be to monitor and control amplitude, and correct the displayed time. On my Arduinome this is done essentially by moving the displayed time back by a second every "N" swings - the same approach is used in many quartz clock ICs.

I mentioned earlier that someone posting in the Synchronome1 group uses a coil off to the side, as described by Joseph, but rather a large flat coil, which is impulsed in the swing centre. Of course the magnetic efficiency is not very good (as it's probably 75 - 200 mm away from the bob). This configuration is dual purpose as it could also start the bob swinging.

Avoid an iron core in the impulse magnet, remanent flux can disturb the bob's motion.

One of the reasons I didn't use magnetic impulsing in my clock was to avoid interaction between the processor clock and the pendulum. The latter is impulsed by a gravity arm with a little roller which is placed on a "dead roll" on the pallet just before the centre of swing, where it has no effect. As the pendulum swings through the centre the roller "runs down" a little ramp which generates the impulse, then is caught on another dead roll until the processor lifts the arm up again. So the impulse timing is done by the pendulum itself. This is the same principle as in the original Synchronome, and the Shortt. All the 'Nome had to do was to lower the impulse roller onto the free pendulum's pallet just before BDC, so the impulse was taken by the pendulum exactly when it was wanted.

Not trying to do that at ALL! I Nowhere mentioned trying to lock the pendulum to GPS...I locked a reference oscillator to the pendulum...

And By Martin Kyte

The pendulum triggers the arduino timer which fires the impulse some fixed TIME later (Arduini timing). The impulse only occurs at the same POINT if the pendulum is travelling at the same speed.

Yes, of course, I get the picture, but against WHAT do you measure it???

My suggestion was not to lock to the GPS(DO), but to use the GPSDO as a reference to see how accurate and consistent the pendulum swing is. The second 'GPSDO', which IS locked to the pendulum, ie, converted to a PENDULUM LOCKED OSCILLATOR ( NOT the pendulum to it, if you read carefully..) is just so that you can easily compare two hi-frequency PHASE LOCKED clocks to measure accurately the delta and therefore the pendulum timing and jitter. The first clock is phase locked to GPS 1PPS, the second to PENDULUM 1PPx

If you are chasing the sort of accuracy implied in Dave's intent, just observing the pendulum opto- sense pulses on a 'scope is not going to cut it..

If your measurement tools are of lower accuracy or resolution than what is being measured, you ARE blowing smoke..

Joe

Ah, I hadn't twigged that SOD was proposing to impulse some time after the sensor detected the pendulum. I'll have to have another think.

Impulsing at the extreme of swing just makes life more difficult, and I haven't seen any advantage yet apart from stopping the Foucault effect.

I didn't suggest that Joe was referring to using GPS to correct the pendulum, but I have seen that written up somewhere. I think that changed the amplitude to fine tune the rate.

The advantage of impulsing at the end is that a single electromagnet can both start and maintain the clock. I've also a notion it will keep the pendulum on track, there being little to stop the bob swinging in an ellipse, perhaps a design fault. I'm thinking now of fitting a second electromagnet at the bottom of the swing to do the impulsing with the other dedicated to starting.

State of play when I downed tools for tea: the Arduino successfully starts the bob from rest after twelve 50% pulses.

Now I know how big the relay coil is I reckon a 3D printed holder for it (or them) and the IR transmitter/receiver is in order. Then the whole lot can be glued neatly as a module under the bob.

Dave

Dave, going back to your relay drive circuit, I don't know what pulse length you are aiming at but the configuration you have with the flywheel diode will lengthen the pulses because the current that was flowing through the transistor will flow through the diode when the it switches off, and decay exponentially depending on the inductance. This could be solved by adding a resistor in series with the diode, chosen so that the inductive kick plus the supply voltage remains within the max Vce rating of the 2n2222 (30V I think), and/or removing the iron core (advisable anyway to prevent remanent magnetism from perturbing the pendulum). Could also replace the 2n2222 with a MOSFET with a significantly higher max voltage.

Noted thanks. I've not noticed any residual magnetism in the relay core, but a Hall Effect device might show otherwise. I'm not too worried about the pulse length yet because the plan is to experiment: the pulse length will be slightly bigger than the minimum needed to keep the clock going, and it may not be applied to every swing. This morning's test showed more experiment to be necessary because last night the pendulum started reliably after twelve pulses, now it doesn't. I suspect it's because the magnet is temporarily mounted on a blob of blue tack and moved while I snored!

Main reasons for the 2N2222 is I got some and they're cheap. Max ratings up to about 40V and 800mA, well within the coil which is 5V at 70mA. My oscilloscope shows the diode managing the inductive spike; overshoot under a volt, negative rather less.

Does seem there are endless opportunities for minor improvements. At this stage I'm after the big things; can I make it work, then what causes the worst errors one at a time. For example, I took on board your suggestion that a tube is stronger than a tripod by adding a reinforcing ring to the design. Not proven yet that's good enough, though adding it noticeably stiffened the tripod - a good thing. Like as not there will be a Mark 2 design to address a shower of shortcomings.

I'm keen to get the clock starting and running reliably. Several suggestions and comments to check against the prototype once it goes. Out today unfortunately.

Ta,

Dave

Really interesting to follow this thread. I made the John Wilding "Simple Electric Clock" in 1987 and soon found it to be very unreliable. It works on the Hipp principle and the contacts were forever going out of adjustment and needing cleaning. Adding some transistors to help the switching didnt help so it was forgotten for years.

Eventually replaced the Hipp arrangement with two magnetic sensors (optical sensors soon failed with dust?)and 555 timers to control the pulse and it worked but soon found that the soft iron core of the magnet was magnetising the swinging armature and eventually becoming unreliable and stopping, abandoned again!

Fast forward a few more years and I made an energising coil without an iron core (fine wire 2K ohms) and replaced the swinging armature with a 6mm neodymium disc magnet so that there was no interaction when swinging and unenergised..On the 555 it worked but was complicated. Discovered Picaxe micros and got a version working after learning some programming. Eventually moved onto Arduino and the latest version measures the pulse width with a magnetic sensor and stops energising the coil when a predetermined pulse width is reached. When its being energised by the Arduino the armature with magnet is pushed away.

Been going a few months now and think its the best I've managed to get it running. The pundulum is on knife edges and quite a bit of friction in the train which I've reduced. but could be improved on.

Not in the precision league I'm afraid that Dave's trying for but I'm pleased with the progress!

Peter

Nice clock Peter! Hard to beat the character added by brass and moving parts; people like it. I like it!

Mine has no aesthetic qualities whatever. With the shield on it looks exactly like a foot of old drainpipe! The technical interest must be an acquired taste. My family think I'm daft! What's it for?

Thanks for confirming the iron core issue; it's on my 'fix' list. Also for the pulse width hint.

Cheers,

Dave

I was fortunate to be able to source some Swedish Iron for my Synchronome electromagnets.

PS You know when you are on th eright track when people think you are daft.

regards Martin

Really like Peter's clock.

For iron with no residual magnetism try old transformer laminations. Not easy to make it round, but no-one said cores have to be circular. Perhaps others could comment on the use of ferrite aerial cores, I have no idea.

Another approach to a central impulse self starting pendulum might be the Brillie style.

**LINK**

This could be duplicated with a neodinium magnet fastened to a curved brass member fixed to the end of the pendulum so the magnet swung through the coil

And in response to Martin's final sentence, most people think I'm barking mad!

One thing to remember is just how little energy is needed to keep the pendulum swinging. Mine gets impulsed once a minute by allowing a weight of ~10g to drop 2mm, or about 2e-4 joules. That's a dissipation in the entire system of 3.3 microwatts. Only a tiny force is needed to replenish the lost energy. With a neodymium magnet an air-cored coil will be fine and eliminate any concern about remanent flux. You can always increase the current, efficiency isn't really an issue.

Though it never got into a finished clock I experimented with a wound bobbin removed from a printer with its core removed (it worked some sort of interlock) and a 1x3mm neo magnet on the end of the rod and there was clearly far more force than needed even for moderate current.

My high friction clock is impulsed for 80ms but will run with a pulse width of 40ms. The magnetic sensor looks for a pulse length of 20ms before switching off and it can only manage a couple of swings before its re-energized .

If I switch off completely it takes 10 secs before it stops incrementing the count wheel and approx 60 secs before the pendulum is stationary. It takes a tiny push to get it going and around 30 secs to build up sufficiently before it is up to full swing and limiting.

Its quite nice to stop the clock and see it build up again with the led in series with the coil flashing then stop and bounce on/off when its limiting!

I have another more conventional experimental pendulum running on a similar impulse system but pre arduino which uses a pulse on time of around 5ms but its not driving anything.

Peter

Started reading Woodward's "My Own Right Time" which arrived yesterday. Excellent complement to "The Science of Clocks and Watches". Lot to take in though!

The tiny amount of energy needed to keep a pendulum swinging makes it less likely that the Arduino will dominate the pendulum I hope. Martyn has me twitching. As Woodward describes how to measure Q more simply than I knew, I'll have a go at that next. If my pendulum takes a lot of energy to keep swinging (low Q), then the risk the Arduino is accidentally dominating the pendulum is higher than if the pendulum is high Q and the Arduino only pulses it at low level.

I've corrected the start problem: someone who shall remain nameless messed with the Arduino code while checking pulse timings with his oscilloscope and forgot to put it back...

Dave

Measuring Q is a complete fiasco and the last attempt ended with me knocking the clock over. The method I'm using would take about 20 minutes on an ordinary pendulum clock; on this thing everything is against me. I keep disturbing the bob by bumping the table, and the cat is a menace. The tripod construction and tight space inside make it hard to measure amplitude because there's always something in the way. Also tricky to start the pendulum running straight by hand. Takes a long time to do the observations and I've yet to get a trustworthy result. Frustrating.

Plan B is to 3D print the magnet assembly and fit the IR beam to establish Q. Now where the h*ll did I leave the 3D printer's SD card?

Argh

Dave

Bit of progress.

I used Qcad to layout a plan of the clock base using the dimensions of the bob and magnets etc. I find it easier to do this sort of work in 2D.

Having established the fit in 2D, I moved to Fusion 360 and designed a box to hold two electromagnets with cheek pieces to hold the IR LED and detector.

The idea is to use one magnet at the side to start the pendulum swinging from rest, and the other underneath to impulse the bob. One end-wall is lowered so the 'start' magnet rests on it and tilts towards the bob. This late change bit back.

Exported the F360 design as STL and printed it via gcode with my Creality Ender 3D printer. Shown below with magnets glued in and the infra-red components plugged into the cheeks.

Below right under the terminal strip is an unmodified Arduino IR Proximity detector module. Normally the clear LED emits a beam that bounces back off obstacles and is detected to stop a robot hitting something. The module contains a comparator chip and sensitivity adjustment pot. Rather than design and build my own circuit, I modified a module simply by extending the LED and detector on wires.

Fans of duffer cock-ups will note the terminal strip swaps the connectioms over. Despite making notes and marking the LEDs to make sure they were reconnected the right way round, I soldered them back onto the module the wrong way. Ho hum.

Here it is connected and assembled for first fitting. Nothing is fixed down yet.

Although it looks reasonable, it's a very tight fit. I failed to allow space for the IR beam wiring. Worse, because the electromagnets are rectangular and asymmetric, the bob was pulled off centre. Rearranging the magnets fore and aft to correct that means the poles can't quite be positioned correctly under the bob. Might work but it's way off design.

I'll probably go back to a single electromagnet to test the pendulum while rethinking the dual coil idea.

Dave

Edited By SillyOldDuffer on 14/09/2020 20:26:54

I have had a test rig sitting on my bench for a while. It is a half-second pendulum suspended on a carbon fibre rod, and has a samarium-cobalt magnet attached under the pendulum. It originated with a series in ME by Dick Stephen. I built the rig because here were problems with the timing circuit and I thought I could do the job another way, using a microprocessor. There is a flat coil (no centre iron) which acts both as detector and as impulser (at least that's he theory). The original used two concentric coils, one for sense and the other for impulse, but I think more than one coil is unnecessary.

The pendulum will auto-start if timed pulses are applied to the coil.

The problem is that because the magnetic field is doughnut-shaped, the pendulum will tend to move in a slightly circular or elliptical path. Restraining the pendulum will mean frictional forces probably at the sides of the ball bearing at the top of the pendulum shaft. I am uncertain whether the fore-and-aft tendency is a result of the magnetic field, or simply created by minor random sideways forces - despite careful leveling of the rig, a rigid bench, and a concrete floor.

One of the problems with this kind of system, as Dick Stephen found, is that regular pulses tend to produce over-swinging, taking the pendulum far from any intended isochronous arc. Yes; we are trying to create a stable oscillator, but it must be resonant at the pendulum's natural frequency.

I don't think that a plan to apply regular pulses to the pendulum is necessarily a good way to ensure the pendulum swings at its own natural (resonant) frequency, and my plan is to use a statistical approach to try to identify what that frequency is, then try to maintain it. That means the adjustment of the frequency of swing will be by altering the effective length of the pendulum, which is what you would do on a mechanical clock, rather than attempting to drive the pendulum at a particular frequency to force it to time.

This is not a particularly urgent project. It has been maturing on the far end of the bench since 2007, and gets attention as the muse comes upon me.

It will be interesting to see your own results.

Marcus

Reading the post from Marcus made me realise where I got the inspiration to try an energising coil without a core, it was from reading the Dick Stephen article and was a breakthrough moment for me.

I also considered that the magnetic field would be a srange shape without a core to concentrate so I used iron filings on a sheet of paper above the coil and got a result with a dead position in the middle so I thought I had the centre of the bobbin was too large, however it worked fine on my simple clock pushing the magnet out of the way once started until the detected pulse width stopped over swinging..

I considered that the swing was going to be eliptical, my question is how can the circular error of the pendulum be measured, apologies if it's already been covered somewhere? Did wonder about a laser mounted on the swinging pendulum bob shining onto a card or something?

Peter