Improved Experimental Pendulum

I'm aiming for an amplitude of up to 5°, which will take the bob to within about 1mm in the new configuration. I can adjust the impulse so the bob doesn't quite hit the electromagnet.

Dave

I did, and the result is much softer. I see this method as a backup like having a crank handle available for a car. Normally better to have a working starter motor, but If I can't get it working, this is how I'll start the pendulum, thanks!

The impulse is software controlled and can be applied at any point in the swing. In practice, I impulse as soon as the beam is broken, when the bob is just past BDC.

Dave

Yes, that's possible, and it's partly why I increased the weight of the bob and stiffened the rod. But the affect of the impulse isn't as sudden as might be thought. The inductive coil smooths the jolt as it charges up, and the strength of the resulting magnetic field decreases rapidly with distance. However, running the clock will reveal whether the twanging is fixed, or not...

Dave

"I did, and the result is much softer. I see this method as a backup like having a crank handle available for a car. Normally better to have a working starter motor, but If I can't get it working, this is how I'll start the pendulum, thanks!

The impulse is software controlled and can be applied at any point in the swing. In practice, I impulse as soon as the beam is broken, when the bob is just past BDC.

Dave"

I haven't been following this topic closely, but I think the position of the electromagnet might be flawed.

I assumed it was located directly beneath the pendulum and acted to add impulse by pulling in line with gravity.

My understanding is that the impulse should only occur on the acceleration side of the swing before BDC, rather than after BDC, as you appear to have it.

Martin.

Dave, I played with one of those relays, stripped as you have, here on the bench now. Applying 12 to 15v pulsed gives a large increase in the magnetic field and the relay is just warm to the touch with a 100ms pulse every second after running for 4 minutes.

SInce you can't get the E_magnet closer to the bob for starting, if you have a 12/15v supply around the electronics, use a relay to select between +15v for starting and then switch back to +5v when running. That should work OK...

Edited By Joseph Noci 1 on 23/04/2023 20:30:11

Dave, it would be useful to try to separate clock noise from electrical noise. In your earlier clock, did you notice much  noise difference when running on battery power (as in power cut mode) ?

dave8

Edited By david bennett 8 on 23/04/2023 21:15:00

Now that's a good idea! I've made a holder to physically hold the electromagnet closer but been busy fixing mum#s plumbing to try it, maybe this evening/ If it doesn't work, I can up the volts instead.

Ta,

Dave

I ran it for about an hour on a battery and got much the same standard deviation. I have three main suspects:

1. Even if it was well-made, and it wasn't, the suspension required the clock to be level, which was difficult to do. So the pendulum was likely swinging on a twisted spring - not good, because it means the bob can;'t fly in a consistent straight line. The new version lets the pendulum naturally hang vertical, is better made, and the clock now has proper levelling screws.
2. The pendulum rod was flexible, not a good combination with a twisted suspension spring. I've upgraded to a stiffer one.
3. The beam break sensor is basic and my design made no attempt to create a sharp beam due to shortage of space. The sender and receiver are now mounted in long slotted tubes, which I hope the resulting constrained beam will make bob detection more consistent.

Electrical noise could contribute to false triggering, but I've not found any evidence yet. Lack of evidence may be because it was masked by problems 1,2 and 3 above. It's possible fixing them will reveal electrical noise is an issue at a lower level. The new circuitry already has some extra decoupling "just in case", but I still have to get the clock running before I can measure the pendulum again. Fingers crossed, the period will be much more consistent

Dave

It's a deliberate part of the experiment. In a mechanical clock where the escapement is usually at the top, it's definitely best to impulse when the bob is travelling at maximum speed because it minimises mechanical disturbance.

The same logic applies to magnetically impulsed pendula, so builders often often locate the coil directly under BDC. However, because a magnetic field doesn't deliver mechanical thump, I conjectured that the impulse can be applied at any point in the swing provided energy is added to the bob without disturbing it. The jury is still out - it's easier to impulse close to BDC, but I have run the clock successfully by impulsing close to end of swing. So much else wrong, with the earlier build though, that results are inconclusive.

Side mounting the electromagnet means it can also be used to start the pendulum swinging from a dead stop. This is useful when the pendulum can't be touched because it's in a vacuum! Self-starting and remote control also allows the clock to be physically located for best advantage - somewhere low vibration in the house where I won't trip over it! If the next version runs very well in a vacuum I might dig a safe hole for it in the back garden! Same hole might make a suitable grave if it runs badly...

Dave

Basic oscillator theory says that the fractional change in period due to a phase difference of "phi" radians between BDC and the phase centre of the impulse is tan(phi)/2Q. Generally known as the "tangent rule" but actually one of "Airy's formulae". You can clearly see this in an occasionally impulsed clock such as a Synchronome if you look at the period of each cycle, it can be seen to have a significant error on each swing where the impulse happens (e.g. every 15 swings).

Edited By John Haine on 24/04/2023 20:06:36

And I thought all I had to do was keep the amplitude constant....

Dave.

It seems wrong to me because the magnet will cause the pendulum to accelerate within the deceleration phase of its arc.

However, if the bob was changed to be a permanent magnet and the electromagnet was polarised so as to repel the already (just) departing pendulum, then I can see how it might work, providing the on/off period is correct.

Martin.

Dave (and John Haine), perhaps that phase change error would be seen by your clock as an amplitude error and be already compensated for?

dave8

.

At the risk of getting overly philosophical about it, Dave

… If you could do that, then the statement would be true

My understanding is that your system is working by a process of active adjustment, and therefore the amplitude cannot be kept constant.

MichaelG.

The system is experimental, and I've tried both. The pendulum is simply that - apart from the suspension, there is no mechanical connection to it. It's position is detected by breaking an IR beam connected to a microcontroller, so what happens next is decided by a computer program. As I wrote the program, it can do almost anything, and since starting the project I've tried various timing combinations:

• Impulsing at top of swing.
• Impulsing at bottom of swing,
• Impulsing after every 'n' beats
• Impulsing when the amplitude falls below a trigger level
• Impulsing on every beat

I've also tried various impulse lengths - anything from 8uS up, so the impulse can be set from almost imperceptible to completely OTT.

By firing the impulse on every beat, it's possible to adjust the impulse such that amplitude doesn't vary much. Most constant by over-impulsing, but this disturbs period. Manually reducing impulse to slightly more than needed to keep the bob swinging produced a fairly constant period and amplitude. As both exhibited noisy random jumps the oscillator was a bit unstable. Not awful, but clearly in need of improvement.

My diagnosis was of mechanical issues in the design and build of the pendulum, notably a whippy rod, a badly made poorly designed spring suspension, and no sensible way of levelling the frame. Also, a strong possibility that the IR beam was too broad. The latest build addresses these issues, but I made a mistake with with the electromagnet such that the pendulum can't be started swinging by the microcontroller. There's a chance I'll fix that today, fingers crossed.

Once the pendulum and software are running, I shall revisit the earlier experiments. Indicative rather than conclusive because results were flawed by mechanical and software development problems. Now I'm more confident of the build and the software, it will be interesting to see how well the improved pendulum performs. I'm expecting better, which is why this version of the design has tackled vacuum containment seriously. The earlier version never performed well enough to be worth pumping out. Fingers crossed, this one will be!

All this is quite confusing, because a forum thread isn't a good way of documenting a skittish development programme with deliberate experimental features. The experimental aspect means I've changed tack several times when others pointed out flaws and improvements. Also confusing!

I'm doing a Zoom presentation to the SMEE Digital Group on the 13th May, which I hope will clarify the 'big picture'. Well worth joining SMEE just to see what Joe Noci thinks of my electronics and computing; hearing John Haine's critique of the theory; and enjoying Duncan Webster's no doubt pungent remarks on my approach to statistics! And there will be several other well-qualified engineers present. Should be fun.

Dave

The pendulum decelerates for two reasons - it's climbing against gravity, and losing energy. If the attractive force just balances the friction force (or rather the energy given equals the energy lost), the pendulum won't know the difference except that if the "centre of gravity" of the impulse is not at BDC the timing will be affected. If the magnet was repelling it's exactly the same principle, except that you don't want permanent magnets anywhere near a steel bob or iron base as the magnetic force will affect the effective gravity.

I'd love to see Dave's presentation but alas it clashes with the AHS AGM where they have some talks on the Big Ben restoration.

Edited By John Haine on 25/04/2023 11:09:16

Comments inline above...

Joe

Hi John.

I understand that the impulse is required to replace only the lost energy and should be as 'unobtrusive' to the natural movement of the pendulum as possible. This is where I think Dave's current setup may be lacking.

I also understand that a steel/iron structure will interact with permanent magnets and so will cause problems with timekeeping. My example was just to illustrate that I think a repelling force might be more appropriate for what Dave would like to achieve.

We know that a perfectly free pendulum will tend to be a better timekeeper, because of the lack of interference by impulse mechanisms. By impulsing in the deceleration phase of the pendulum's movement, I think he is causing more disruption to its natural movement than is optimum. Once past BDC, a free pendulum will gradually decelerate until it stops and reverses. Unfortunately, Dave's setup changes the process and causes the pendulum to accelerate when it would normally be slowing down, by the fact that the closer the bob gets to the magnet, the stronger it will be attracted to it.

In contrast, if the electromagnet were to be fitted directly under the bob, it would be easier to impulse on either every swing, alternate swings, or even sporadic swings. The pendulum would only see the impulse as a slight increase in gravitational force, rather than an occasional pull to one side.

Martin.

It will probably be recorded so SMEE members can watch later. Impulse should be equally disposed either side of centre. This is what you get with synchronome/pulsinetic. The only way I can see of doing this non mechanically is using an aluminium vane and a linear motor. I'm not going to attempt it.

I'm hoping there will be a recording.

Several types of very successful escapements impulse after BDC when the bob is decelerating. Perhaps the best example is the grasshopper, which is a recoil escapement where the impulse torque increases from just before BDC when the "legs" interchange right up until the peak excursion on the "other side". A good example is on Clock B which doesn't suggest that this is a disadvantage.