Arduino Pendulum Clock Design - Comments Welcome

Peter, what do you mean by circular error? It has a specific meaning in horology, being the increase in period with amplitude. Do you mean this, or the ellipticity?

Regarding elliptical swinging, this can happen if the impulse is not exactly coaxial with the bob, but also if the axis of suspension is not quite orthogonal to the suspension. For example, often the suspension has a back-to-front pivot to allow the pendulum to hang vertical, but if this has some stiction it may settle slightly out of vertical. Then when the bob swings in the "wanted" direction there will be a small force in the back/front plane so the pendulum will swing in that as well though with low amplitude. Usually the length in that plane is slightly longer so the period greater too, so the axis of ellipticity can precess - such behaviour has been observed. The period of the wanted resonance will also slightly wander at the difference frequency.

I'm not sure I understand the logic of a pancake coil. A simple solenoidal coil seems intuitively to be better as it focuses the impulse and the sensing too if dual purpose. Also is probably easier to adjust to get it on-axis.

There is a beautifully simple scheme for using an Arduino to drive one of these magnetic pendulums here. The code has 6 lines in its main loop().

.

Great link, John ... Thank You !

This reduces the drive ‘mechanism’ to what it should be [a motor]

... leaving experimenters free to concentrate on the timing performance that can be extracted from a pendulum.

MichaelG.

Very instructive to compare and contrast how others approach these problems!

Ages ago I made a single-coil sense/impulse pendulum driven on every swing by a couple of transistors. A Meccano novelty with a Super-magnet on the bob rather than a clock. Problem as Marcus says, impulsing caused the bob to gain energy - good fun watching it swing about 60°, but useless for time-keeping! Learnt it's hard to time and control impulse strength with electronics, because the circuit gets ever more complex as it's improved. Microcontrollers make the same job easy.

Microcontrollers have a built in clock and manage multiple inputs and outputs with almost unlimited logical flexibility: many possibilities open up without needing a soldering iron.

I dislike sense coils because they're slightly intrusive; generating electricity absorbs energy. While tiny the disturbance can be avoided with a beam breaking sensor. So my version doesn't have a magnet on the bob (plain mild-steel, maybe cast-iron later), and swings are detected with an infrared beam. The microcontroller can be programmed to impulse the pendulum whenever I choose and by altering the length of the pulse I can control the amount of energy applied to the bob. Also possible to program it to not pulse on every swing, and to measure the pendulums amplitude so it only impulses when needed.

Agree it's vital not to force the pendulum. Martyn made the point earlier and it's still worrying me. Proving my clock depends on it's pendulum to keep time rather than being locked to the microcontroller's crystal oscillator is top of my 'to do' list once the clock runs.

Another feature of my design is it doesn't matter much what the pendulum period actually is; there's no need to set it to a particular value. Instead, the period can be accurately measured - whatever it is - and the ticks converted to human display time by calculation in the microcontroller. In effect the clock's 'gear train' can be any ratio I want, and the rate.

Doughnutting is another worry, or at least making sure the pendulum swings straight. My lightweight construction is asking for trouble; the bob only weighs 30g. I hope firing a short impulse at just the right time will minimise disturbance and the bob will always move towards the centre of the magnetic field. The field is generated with a relay coil, which is more sharply focussed than a pancake. What we're on to here is an example of noise, which I'm reading about in Woodward's "My Own Right Time". at the moment. Noise due to friction, temperature and pressure changes, and tidal effects on gravity etc. sets the ultimate limit on pendulum accuracy, so getting rid of avoidable problems is good. John Haine recommended Woodward earlier in the thread, and it's a good 'un!

I may be doing something original, or not. As there's nothing new under the sun in that pendulum clocks have been intensively explored in the past, I'm a tad concerned I'm working on an idea not mentioned in the books because it's naive and doesn't work. Or just maybe development of pendulum clocks stopped slightly short of the art of the possible when they were surpassed by crystal oscillators. Keeping me amused however it ends up. If nothing else fans of traditional methods may get to enjoy me failing to produce an accurate clock despite modern 3D CAD, 3D printing, and advanced electronics!

Dave

Agree it's vital not to force the pendulum. Martin made the point earlier and it's still worrying me. Proving my clock depends on it's pendulum to keep time rather than being locked to the microcontroller's crystal oscillator is top of my 'to do' list once the clock runs.

John Harrison stated that "The Pendulum must have dominion over the escapement" which I find very usefull to return to when the risk of getting lost in the complexity looms large.

My other question is does it matter if the pendulum moves in an ellipse provided the oscillations in each axis are independent.

regards Martin

The problem of excessive amplitude mentioned by Marcus is what lead me to have an end of travel sensor and miss out the next pulse until it was back within bounds. Dick Stephen used a copper pate/magnet damper at the end of travel to take energy out, this didn't appeal to me at all. I seem to recall that this idea was used by Brille as well.

The Minimag chap used to sell a kit for impulsing a pendulum as described in John Haine's link. Quick look on his website doesn't reveal it..

I wish this issue hadn't raised its head as I've promised myself not to start anything else until I've actually finished some of the half complete jobs, but the temptation is getting too great. Get thee behind me Satan!!!!!!

The problem of excessive amplitude mentioned by Marcus is what lead me to have an end of travel sensor and miss out the next pulse until it was back within bounds. Dick Stephen used a copper pate/magnet damper at the end of travel to take energy out, this didn't appeal to me at all. I seem to recall that this idea was used by Brille as well.

The Minimag chap used to sell a kit for impulsing a pendulum as described in John Haine's link. Quick look on his website doesn't reveal it..

I wish this issue hadn't raised its head as I've promised myself not to start anything else until I've actually finished some of the half complete jobs, but the temptation is getting too great. Get thee behind me Satan!!!!!!

Yes, because as soon as the pendulum moves out of its intended plane there is a coupling to the other mode, so if the modes are at different frequencies they will beat and the here will be a cyclic variation in the wanted period. Bob Matthys observed this, and at his request I built a simulation which also shows the effect.

That's one concern I have about Dave's approach, that a thin CF "rod" used as a spring/rod combination will have a rather complex motion as the rod may not be circular - certainly the thin CF "wire" I have is decidedly out of round.  So the period in different planes could be different and they will inevitably couple.

Edited By John Haine on 15/09/2020 11:04:05

Bold assertion ... Feel free to contradict me:

It doesn’t matter a jot what path the pendulum describes, or the extent of its displacement ... provided that it is consistent ... The real problem is finding ways to achieve that consistent behaviour.

That was the logic behind my first post, on p1 of this thread.

MichaelG.

I can see possibilities of energy being transferred from one axis to the other via the escapement/impulsing system which was not my actual question which was for a pendulum alone. Perhapse I wasn't clear The periods must be identical as it's the same pendulum and provided the suspension is from a point I cannot see a mechanism for coupling or resonance.

regrds Martin

Edited By Martin Kyte on 15/09/2020 12:14:09

As an aside if anyone fancies swinging with a transistor circuit, this is what I used. Not sure were it came from.

Couldn't get an NE555 based circuit to work, at the time didn't have a scope. The transistors 'just worked'.

Dave

For a pendulum suspended from a point with a massless, completely floppy and weightless string that's true. But a pendulum which has for example a conventional suspension, if for example the back-front axis is not orthogonal to the LR axis, in both planes, there will be coupling. Dave's pendulum has nearly a point suspension as it uses a CF rod which has some bending compliance, so its period is not fixed just by the bob but partly by the rod's springiness. If this is different for bending in different planes because the rod isn't exaclly round, this will make the period different for oscillations in different planes as well as introducing coupling.

If you consider a pendulum oscillating in a circular "ellipse" as an example, this is a "conical" pendulum and behaves rather differently from a conventional one. For example, its period decreases with increasing amplitude rather than increasing.

.

A quick question, if I may, John

... The iPad was playing ‘Nanny’ and not letting me view the downloaded code

I have worked-around that by opening it ‘as text’ in the Goodreader App

Could you please check that the following is complete and correct ?

__________

#define THRESHOLD 10
#define PUSH_TIME 30
#define A_PIN A0

void setup() {
pinMode(A_PIN,INPUT);
analogReference(INTERNAL);
}

void loop() {
if(analogRead(A_PIN)>THRESHOLD) {
pinMode(A_PIN,OUTPUT);
digitalWrite(A_PIN,HIGH);
delay(PUSH_TIME);
pinMode(A_PIN,INPUT);
}
delay(1);
}

__________
.

much appreciated

MichaelG.

John,

Lots of good debate today! I meant a swinging error caused by the doughnut shaped magnetic field pushing my magnet on the bottom of the pendulum, perhaps thats ellipticity? I'm assuming it will affect the time keeping as I am switching the drive off for some swings to prevent excessive travel. I'm assuming the pendulum will then take a slightly different line as there is nothing to deflect it on these swings.

The sensor which monitors the lenght of the pulse also fires the drive coil, previously I had a end of travel sensor like Duncan to prevent excessive travel.

Thanks also for the Arduino driver, ingenious and looks very simple with a nice description but I dont think it has provision to prevent excessive amplitude.

Michael, copied your code and it complies so should be ok.

Peter

Yes, that's exactly the code Michael.

I'm planning to use something like that code but adding a Hall sensor for amplitude.

Thanks, John and Peter 

MichaelG.

Edited By Michael Gilligan on 15/09/2020 20:54:35

Frustration galore.

Something strange going on with my code, swings should take about 800mS, but every so often I get an odd reading. Checking with an oscilloscope shows pulses are being detected correctly (except see below!), it's as if millis() and micros() are seizing up. Bit odd, I've written similar code many times before without any bother. May have been overconfident and slapdash!

Ignoring software issues, I connected the magnet and found the transistor didn't work. Turned out to be connected back to front because a P2N2222A has a different lead-out to the PN2222A. I looked up the wrong one and connected cbe instead of emitter-base-collector. Still no joy, until I noticed the coil was connected to ground instead of +5v. Fixing that I got the magnet diodes the wrong way round. Air turned blue.

Now the magnet is pulsing, but as I've made no attempt to get the impulse timing right it can't possibly work. To my surprise the clock started while I was thinking about how to fix it. Yer wot?

Celebrations were premature. Looking at the Arduino report showed measured pendulum period to be all over the place (last column) in milliseconds:

All lies, but the oscilloscope confirmed bad behaviour:

Yellow trace is pendulum swings detected by the beam breaker. Blue trace is the impulse. Yellow shows 'tick tick pause tick tick' - wrong. Blue shows the impulse mostly fires correctly when the beam is broken but sometimes on the falling edge too, which is wrong. Also the blue pulses should all be the same width and they aren't. Hmmm.

Turns out the clock started due to bright sunshine on the bob. The extra shadows and reflections accidentally produced extra impulses at just the right time to swing the pendulum. Able to prove it by drawing and undrawing a curtain. Fairly sure the 'tick tick pause tick tick' effect is due to my genuine but incorrectly timed impulses braking the bob rather than letting the sun do all the work! Clock doesn't run now the sun has moved round.

Ho hum,

Dave

Not sure I dare try and do timings on my pendulum Dave!

Seeing your pics I couldnt resist looking at what mine looks like.

This one has the output from the sensor which is triggering the Arduino on theblue top line with the yellow energising coil on the bottom. The input pulse is quite long as the pendulum is jus swinging a small amout and gaining speed. The output pulse length is the yellow line and the energising coil is driven directly by the arduino.

The swing has increased and the pulse length has reduced to 21ms, the energising pulse is also reducing with less power being given to the pendulum.

Finally its got up to speed and the energising pulse is being switched off as the input goes under 21ms, the pendulum slows slightly input length increases and its energising again. Tried fiddling with the energising strength but with my high friction clock train it didnt make much difference.

Peter

Good stuff Peter, you're well ahead of me at the moment. I like the missing impulse technology!

Can I encourage you to cursor your pendulum pulses? Measuring their consistency, or lack of, might be food for thought. My early Clock Analyser tests showed that my lash-up pendulum's period was subject to small variations that mostly cancelled out over time. Be interesting if your clock has the same characteristic.

I'm making slow progress with software, constant distractions, but I'm also attempting something a little advanced. It's a fire and forget single pulse function that doesn't stop other processing by waiting for delay(). Pseudo code:

if ( pendulumDetected ) sendImpulse( xMilliSecondsLong, afterWaitingyMilliSeconds );

afterWait allows impulses to be applied to the pendulum anywhere during it's swing, so the mechanical position of the electromagnet and beam break detector aren't critical. xLong allows the strength the impulse to be controlled from zero up, hopefully allowing a later version of the code to maintain the clock with minimum energy.

Bit like a car engine management unit using knock detection to dynamically optimise when the spark is applied.

Another age thing I'm sure. Used to write software in noisy open plan offices and was able to concentrate whatever the racket. Now almost anything puts me off. Need to unhook the phone, lock the cat out and put 'Do Not Disturb' on the door. And now it's lunchtime...

Dave