Member postings for Joseph Noci 1

Michael, with or without knowing it, you are EXACTLY correct!

Edited By Joseph Noci 1 on 03/10/2023 21:30:27

Unfortunately, the solid has a vast amount more resin than the thin walled tube, and its the resin that has the poor Tc..

From Michael:

The two lowest frequencies of a pendulum rod must surely [?] be its first bending mode and its first torsional mode … everything else, I posit, will have a higher frequency.

The magnitude and frequency of both of these will depend upon the construction details of Jo’s pendulum [and they should be simple for him to measure] … but I would expect both to be many octaves above the frequencies he is observing.

I believe I am with Michael on this one -

The rod is stiff and stretched, Its natural resonant frequency would be in the 10s of Hz - I can hear it when I Twang it...Standing waves would only result in higher frequency modes and I am measuring 25 second periods and up...It definitely is not the rod 'vibration' or flexing due to the drive coil.

But I believe I have a Eureka - I am not going to post the chapter, but chapter 15, page 106 onwards, titled 'Energy Coupling between Modes of Oscillation" in Matthys book, Accurate Clock Pendulums is I think definitive of my observation.

In essence , the three oscillation modes are left-right , front-back, and rotation about the rod axis. In his book, a test is done where the pivot is displaced 4deg from perpendicular to the front-back axis, which imparts a strong front-back drive during the left-right motion. That resulting front-back motion ( very small movement..) presented a 6minute period ( mine was 50sec..) where it reduces to 0 and then builds to a max, and repeats - this then couples into the amplitude of the left-right motion, which is exactly what I observe. The pendulum RATE remains largely unaffected , but its time keep is - it runs slower- also exactly what I have observed.

I may also suffer from a rotational rate as well - the front-back and left-right simultaneous motion describes an ellipse to some extent, and can induce a rotational mode as well - a ball or cylinder shaped bob will resist rotational motion to some extent, while my rugby ball bob has more inertial mass out on a limb to perhaps aid that mode's persistence.

It is difficult to detect these phenomena from timing data derived purely from the opto-sensor used at BDC by most folk - These modes are easily induced by the slightest of casual pivot implementations it seems - spring pivot with non-identical spring-pair behavior ( spring clamp or soldering stresses..), pivot centering in the plane of swing, etc....

The opto method does not give amplitude measurement, and although it can be derived from the data, it is not accurate enough I believe.

The clock rate is not affected by induced front-back motion, but as indicated, its time is - Matthys 4deg pivot bias test resulted in a pendulum running 0.68sec/day slower, but stable.

Edited By Joseph Noci 1 on 03/10/2023 21:26:39

Posted by duncan webster on 03/10/2023 18:33:42:

Duncan, yes, I did find a web copy of that during my search and did read, hence my comment regarding its applicability. There is just so much interesting stuff out there, I get waylaid and the hours just pass...

Not having a copy of Hooker's autobiography, I entered 'Hooker's autobiography' in the google search line.

I know I am plagued by a vibrating rod, but the first few search results were...interesting...

Anyway having got past that, I dug further to see how this might assist - I would assume it is not what is happening though. Since the Karmen vortex shedding frequency tends to be similar to the natural frequency or resonant frequency of a structure/wire/cylinder, I would expect to see a vibration frequency similar to that emitted by the rod after twanging it - not 25 to 50second,

I must say I am at a complete loss here. I can find no refence to this. Also I am not sure if this a potentially limiting effect, or literally just noise in the system.

I have been digging in internet to try find some info on this phenomena and come up dry. Searching 'pendulum rod' vibration brings up people who call the pendulum swing a vibration, and the rod vibrates at pendulum period...Even Alan Cromer's 'many oscillations of a rigid rod' does not help...

What is also interesting is that the modulation frequency as described has seemingly no relationship to the rod's 'Twang' ( there's that word again) frequency, which is in the many 10s of Hz region.

Edited By Joseph Noci 1 on 03/10/2023 10:47:20

My pendulum exhibits an interesting rod 'vibration' mode..

I suspect this is inherent in all pendulums but passes unobserved due to the way the period is measured by most folk.

Since my pendulum is amplitude controlled by measuring the PP swing sinus voltage, the quality of that measurement is key to generating a clean stimulation sinusoid. The PP measurement is done by means of a 2 op-amp Peak detector ( typical 3 diode/op amp peak detector). My PP swing logging shows noise, which I believe stems from the peak detector - the peak detector works by 'topping' up a storage capacitor to the peak value of the sinus, but the opamp that generates this top-up voltage operates open-loop til top-up time, and the top-up interval seems to vary, which means the peak voltage on the storage cap varies, which means my drive voltage to the pendulum coil varies... Not much, 8 to 14mv, which equates to 14 millidegrees swing variation max. I am still unsure how to equate 14 millidegrees variation to a ppm value in pendulum stability...

However..

In pursuing a cleaner PP measurement method, I notice a regular modulation on the logged PP swing sinus - at first I thought this was the Peak detector somehow feeding through - I replaced the peak detector output with an equivalent fixed voltage. The pendulum was now not amplitude controlled, but was driven, with no peak detector artifacts able to affect it. Made no change.

I then disconnected the sinus drive to the pendulum coil, and let is run down to 10% swing. Made no difference to the modulation phenomena - did a Q calc to verify the pendulum is still 'OK' - Q = 12900, so OK.

Then I reconnected every thing up, did a run and logged it -

This shows a modulation depth of around 10mv - 10milligrees at a 50 second period.

I then moved/fiddled with the knife pivot location point, ie, moved the knife forward about 0.2mm, so it was 'riding' up the inner ball race support surface. This meant the knife was twisted in its circular support pivot, and would add some odd motion to the swing.

I then logged data again - I did observe a new modulation on the Swing sinus, but at a 2sec period, ie, same as the pendulum period - I could also see the bob was swinging in an ellipse, rather than a straight line. However, the 50sec modulation period remained unchanged, so it was not induced by the knife or pivot or its friction, etc.

I then added a weight to the pendulum rod - an AA penlight cell fitted with masking tape.. - at 40% down from pivot, and logged data -

This showed the same modulation depth approx, but the period was now 35 seconds.

I then added 2 more weights ( AA cells) one at 20% down, one at 65% down.

The period changed to 25 seconds.

So - the rod somehow has a very long period vibration mode, or a High frequency vibration mode, which influences the pendulum period in a very cyclic manner. I suspect everyone using light weight 'stiff' rods has similar modes, but do not see them due the to measurement methods.

This shows the three logs, no weight, one weight 3 weights.

Expanded view of the bottom peaks of the sinus swing, shows the 3 varying modulation frequencies

No weight

1 Weight

The carbon fibre tube is a no-no - maybe a solid carbon fibre rod will improve matters, but that prevents fitting wires internally to my sensors in the rod and bob. And solid carbon rods temp coef is a lot poorer than a tube, so I would be better off using Invar...

And I am pretty sure the house is not vibrating at those frequencies...

The issue is evaluating how much of an effect this modulation has on the pendulum period - it is cyclic, 10 millidegrees, which can be validated in change of period, but the cyclic nature undoes what it causes on a regular basis. Not sure if I am chasing ghosts here.

Edited By Joseph Noci 1 on 03/10/2023 08:49:25

John, what phase error do you mean? Or which phase(s) are used to define this error term?

Apologies, thanks for correcting me - period is what I meant, and thanks for the explanation

Joe

A quick plot of Phase angle change and effect:

X axis is elapsed time in 100ms periods. Y axis is pendulum period delta from GPSDO 0.5Hz reference.

The left rising slope: Commanded phase angle = 86deg.

duration is 400seconds, with a pendulum period change of 5769us over said 400sec, giving a pendulum period increase of 0.005769sec in 400sec

The right falling slope:: Commanded phase angle = 93deg.

duration is 700seconds, with a pendulum period change of 5128us over said 700sec, giving a pendulum period decrease of 0.005128 sec in 700sec.

I believe this may just work...

I have re-designed almost all the electronics on the pendulum. The new ideas implement better op-amp voltage offset and temp drift control and I have implemented a linear voltage controlled amplifier for the pendulum sinusoidal drive voltage amplitude control- this was logarithmic in the previous design and was very sensitive. Also, the 90deg drive signal phase shifter is now controlled via a 24bit DAC, from the Nucleo processor. The phase shifter gives plus and minus 5deg shift around 90deg with a -3v to +3v DAC control voltage.

The amplitude control loop works well - it locks quickly and appears very stable

I will post circuits etc later , for anyone interested.

I am trying to determine the change in period wrt the change in phase at the moment, to see if I can derive a transfer function I can use in the final software control loop. This means setting the pendulum for a 'stable' drift rate and logging the period , and then changing the phase in 0.5deg steps while logging. From that I should be able to derive the change in period at phase change, and how that works through the system for some 10s of minutes after the change.

This is interesting, since if the pendulum period is increased, for example, the amplitude had to have changed as well, and the amplitude control loop will try to bring that back into closed loop control. So, maybe the phase change will change the period, for a short while, till the amplitude loop stabilises again. Then removing the phase change repeats the process in the opposite direction - this is how it simulates anyway, so I think the phase change method of control will be an application of phase change for a short period, seconds, etc, just enough to affect the change in period, and then revert back to 90deg...

Lots to learn down this road.

I have a 'theory' question please...

To mechanically adjust the pendulum period I have a nut under the bob that raises/lowers the bob, for coarse adjustment.

At the top of the pendulum rod I have two adjuster nuts, one small (in yellow), another larger (in red), just below the pivot. These give very fine rate adjustment.

As expected, if I lower the bob, the rate increases. However, if I screw the large adjuster nut DOWN (away from pivot, towards bob), the rate DECREASES, and UP INCREASES the rate. Can someone defend this mathematically??

Edited By Joseph Noci 1 on 30/09/2023 20:38:22

Graham, you are a true Craftsman - Absolutely stunning work.

Joe

you mean 1500 instead of 3500....

2kW 24/7 for heating only is only in the winter -June-Sept - so 4 months typically. Zero for heating for the other 8, while Geyser is heated from Solar PV all year.

(Apologies MichaelG)

Edited By Joseph Noci 1 on 28/09/2023 10:01:51

My house/workshop has been off-grid since Dec 2022 reliant on PV solar energy. Monthly usage averaged 1300KW over winter, with 2500KW generated. The excess is exported to the Electricity supplier at 60% consumption value.

I have hot water underfloor heating, which used to run from a solar heated water system, heating a 6000liter insulated water tank, buried in the ground. That heated water ( around 30degC in winter end of day) fed a 3phase 14KW boiler, and then into the pipes in the floors. The boiler was used during many low/no-sun days in winter.

In winter my floor heating energy used was around 50KW/24hour period - I did not run the boiler during day time, since electricity costs are almost double between 10H00 and 18H00. So the floors cooled during the day, and extra energy was needed at night to re-heat.

When I fitted the PV system ( 3phase, 28KW PV capacity) I fitted a Stiebel Eltron heat pump - a 3phase pump, with a variable speed compressor, to heat the floors.. The pump is capable of 12KW output, and is an Air extraction pump.

Since I heat the floor water to 35deg C max, and the floor water return temp is never below 20degC, the delta the heat pump sees is a max of 15deg, at start of heating, with the delta reducing as the floor heats up to 25deg setpoint.

All that gives me a pump COP of 10 (!) and I run day and night, so the floors don't cool in the day. That returns a daily heat output of around 33KW, with a consumption of around 3KW...

My monthly usage for floors in now around 100KW, instead of 1500KW - and THAT ticks all the boxes - energy cost, carbon footprint, comfort....

Lower COP's are to be expected if the water heating temp differential is higher, such as heating water for showers, etc ( 50degC plus..) - but this heat pump delivers a COP of 4 to 5 when doing that...

The heat pump did cost around 8K British Pounds...but is better than sliced bread.

Not really - think about it - the spring would need to lift it a few 100th's to clear cut metal, but not more than the new metal when stepped over - so what prevents the spring from lifting it to the extent it can? A fine adjust on the spring so it lifts just a smidge? And when friction on the clapper pivot changes ( oil, machine warms up, whatever), the spring lifts less - or more....And if it did lift just enough, it means there will be a little ridge at the start of cut , all along the edge of the workpiece, as the cutter is just a little higher than the set depth as the cut starts. The normal gravity drop starts the cut with the cutter fully down in place each time. The spring would mean the clapper never fully shuts ...or 'claps'..

If the tool is lifted, it won't make contact on the cut stroke either...Gravity is your friend..

Sorry about that - without reference to the actual parts it becomes anyone's guess...

See photos below of the part - if you need a dimensioned drawing, let me know, I will see what is possible..

Hi Jeff,

I don't have photos I fear - there are part breakdown docs on the web, google will find them, but most are in French or German, and the part drawings are not very clear it seems...

This photo shows where the pin - item 9 in parts list, impinges on the point ringed yellow. the pin is screwed in, rotating the eccentric which snugs the helix up against the ring gear to take up play and backlash. The red ring is the surface that is clamped in the table body, by cap screw item 16, preventing the eccentric from rotating. To relieve the helix from contact with the ring gear, loosen cap screw 16 and rotate the eccentric away from pin 9, and the helix rotates away and free from the ring gear. Pin 9 slide in the slot ringed yellow, 'up' towards the top end as the eccentric is rotated, Pin 9 keeps the eccentric from pulling out of the table.

For interest, some div. head calculators...

Div head spreadsheets1

Div head Spreadsheets2

Another Calculator

edit- smileys...

Edited By Joseph Noci 1 on 15/09/2023 08:02:37

Hello Jeff.

I will have a look tomorrow. I don't believe I took photos of those parts, but I may have a manual somewhere.

Joe