Topics

Magnetic Impulsing


neil
 

Hi Bepi,
              Reporting progress to date. I have been running my clock now for the last 4 days. To reiterate - I have a cast iron bob on 3/8" invar rod. The rod is magnetically impulsed about 100mm above the bob and in one direction only - a pull to the left.
I have optos at BDC (bottom dead centre) and at the RHS extreme of the swing.
The total arc is 50mm at the bottom of the bob.
The electronics is controlled by a Atmega processor, and the code can be changed on the fly.
 I have a Mumford microset 3 connected to the BDC opto, and it is reading beat time to .000004 secs/beat.
The clock is bolted to a concrete wall
I have a GPS ref coming.
The graph below is the last 3 hours after I started the clock today, and is seconds/beat   v  time.
At present i have software in place to do either:
  • Electronic hipp toggle with variable impulse (in ms) with the implulse starting at BDC
  • Electronic hipp toggle with variable impulse (in ms) with the implulse symmetrical about BDC (the graph is with 150ms impulse symmetrical about BDC)
  • Impulse every 15,20,30..60 secs at any impulse length 
I can change the impulse algorithm without opening the door or stopping the clock.
Later today or tomorrow, I'll do your simple experiment of stopping the impulsing and let the clock coast to a halt.


Neil 
On 8/02/2020 05:22 am, James Meaton via Groups.Io wrote:

Hello all.
I am fascinated by all this high science and would not decry it for one moment.
During the time I was employed at the Westbury factory I remember how these now items of much interest were often sighted in the the dingy and damp atmosphere of the clients cellar.  Some did bother to just the pendulum swing for regulation by the threaded part of the Invar rod and other who were even more keen used a set of micro-weights that we could supply to set on the top face of the pendulum weight.  Other’s just received occasional adjustment by the NRA lever.
 Jim Meaton, aged 72 and counting.  (Still ironically enough dreaming of my time there as the toolmaker and later as one of the draughtsmen 🙄).
James Meaton


On 7 Feb 2020, at 13:30, Bepi <pepicima@...> wrote:

In "Circular error and my pseudo-synchronome" you can see data showing an unexpected relationship between period and amplitude over a large swing range. I was expecting the circular error to show up in all its might.

No experiment can be simpler than this, the recordings refer to winding down free-pendulum data, with the case door open and closed, I'll show those data again later on in this post. Q values of 5,000 and 8,000 respectively.

At the same time we were noticing a large instantaneous frequency shift at opening and closing of the case door, with a time dependence indicating a lack of symmetry of the air resistance, in the "tangent rule" logic. See also the "Effect of open case vs. closed case" topic.

In both experiments I couldn't think of anything else other than aerodynamic causes to explain what I was seeing.

Noticing that my synchronome bob is unexpectedly low density, 6.9 g/cm3, I thought it would have been interesting to test a more aerodynamic, denser, equal weight, bob made of lead (11.3 g/cm3).

Chatting on skype to an excellent mechanic, friend of mine and also a retiree, I asked him how difficult would have been to make a lead bob, in a soft iron skin for magnetic impulsing, and a few days later I got in the mail a very heavy box.

<Pasted Graphic 1.jpg>

We designed it like a cylindrical lens, 59 x 86 x 203 mm, to make it more streamlined and to keep it another 12 mm away from the back wall. My intuition tells me that those 8-10 mm from the back wall of the conventional bob might be responsible for high speed, large turbulence, flows, which means more undesirable effects.

Repeating the free pendulum experiment mentioned earlier I got a clearly different behavior.

 

About the period vs amplitude dependence, with the old cylindrical bob and the door closed I was measuring this:

<Pasted Graphic 3.jpg>

with the new lead bob (the first ramp is with the door closed, the second with the door open):

<Pasted Graphic 7.jpg>

 

Quite a different picture, now the period decreases with a decreasing angle (green bottom trace, left scale), linearly instead of parabolically, but now it decreases instead of increasing. A drop of - 270 μs instead of the predicted - 400 μs (bottom red trace, right scale), an indication that there is something else lurking in the muck. The spring? Case asymmetries? Hidden degrees of freedom? There is not much else to think about.

Energy wise another big difference, Qs roughly double.

Old Q , door closed:

<Pasted Graphic 9.jpg>

Lead bob Q, door closed, hr 0 to 3, door open, hr 3.5 to 7:

<Pasted Graphic 8.jpg>

 

Data are from the same series as above where one can get the angle evolution with time. Typical old Q = 5,000 at 2.5 deg, with the new bob at the same angle Q= 13,000.

Similarly the instantaneous frequency shift when one opens and closes the case door is much reduced with the lead bob, from 80 μs to 20 μs. More experimentation is needed to understand if at case closed, the normal condition for the clock, the "tangent error" associated with this effect is already geometrically optimized or not.

 
 

Lead bob:

<Pasted Graphic 11.jpg>

 

Traditional bob:

<Pasted Graphic 12.jpg>

 

To summarize: the new streamlined lead bob is a lot more efficient than the old cylindrical INVAR (?) one. Not surprisingly it shows also in the power needed to run the clock in steady state at the same angle, it more than halves. With the new bob we lost our cosy flat spot in the period vs angle dependence but we gained a more conventional "circular error" behavior. Still a bad agreement with the simple picture, almost a factor of two smaller than predicted. Of the roughly factor of four reduction in the anomalous period jump at case door opening, more than a factor of two is coming from the more than doubled Q and the remaining is presumably due to a reduced interaction of the new pendulum with the case walls.

A denser bob for the same weight is certainly reducing also the buoyancy error.



--
Bepi



Bepi
 

Are you sure 50 mm is the total arc and not half that? Is NZ metric?

Which kind of GPS are you adopting? I just purchased an adafruit module to interface with an arduino. I intend to start to log temp and pressure independently from the clock full time for months to check on their spectral properties for my "intercontinental interferometry" proposal. Incidentally I haven't managed to find better than daily atmospheric pressure, long term, records for italy, I need at least 10 times better.

Arduino works on a 16MHz clock so it can log time intervals with the resolution of its clock inverse period, the 4 μs limit is for the micros function, the easiest to program option. If one wants more there are lot faster, more expensive, arduinos. I would love to get a TCXO arduino, it's certainly possible, one of those situations where it's difficult to make a choice, too many ways to do it and I am next to completely inexperienced about electronics.

This is todays record for my nome, the smaller period fluctuations between 12 and 16 hrs, mid graph, are the night readings, the bottom trace tells you that too, the ramp corresponds to night hours when the heating is off. The SD of the 5 period cycle goes from 2 to 1 μs depending on hour of the day. In daytime the clock senses the traffic in the canal out of my window, only 10-20 m away. One more snag of living in Venice, higher SDs.

With the new lead bob the temp sensitivity has increased by more or less 50%.

It looks to me that your period measurements are integrated with a longer time constant than mine. Incidentally with the higher Q bob a (24h limited like the one shown) full spectrum SD has roughly halved, compared to the cast iron one, everything else kept constant.


Did you compare your different impulsing strategies regularity wise? not easy, it critically depends from what one wants from his clock.

Are you recording the pendulum max speed now? I was imagining you could do that easily triggering from the two rising fronts of the adjacent photo-interrupts, a near to ideal separation for this measurement. From that time interval you can get the swing amplitude right away and the pendulm Q in real time, a critical piece of info, most "intrinsic" clock errors depend on it (impulsing error for ex.).

--
Bepi


Bob Holmstrom
 

Pepi, 

With respect to you increased temperature sensitivity.....

How are you supporting your new bob?  As has been pointed out, lead does not have a low expansion coefficient.  In order for the bob to not contribute to the temperature error it needs to be supported slightly below its vertical centerline. That is because the mass moment of inertia of the part of the bob above the vertical centerline is lower than the bottom half.  The effect is usually small, but your measurement sensitivity may be sufficient to see it.  A detailed calculation of the exact support point is not difficult if you have all the dimensions and expansion coefficients of all of the pendulum parts.

Bob 

On Feb 9, 2020, at 5:39 AM, Bepi <pepicima@...> wrote:



Are you sure 50 mm is the total arc and not half that? Is NZ metric?

Which kind of GPS are you adopting? I just purchased an adafruit module to interface with an arduino. I intend to start to log temp and pressure independently from the clock full time for months to check on their spectral properties for my "intercontinental interferometry" proposal. Incidentally I haven't managed to find better than daily atmospheric pressure, long term, records for italy, I need at least 10 times better.

Arduino works on a 16MHz clock so it can log time intervals with the resolution of its clock inverse period, the 4 μs limit is for the micros function, the easiest to program option. If one wants more there are lot faster, more expensive, arduinos. I would love to get a TCXO arduino, it's certainly possible, one of those situations where it's difficult to make a choice, too many ways to do it and I am next to completely inexperienced about electronics.

This is todays record for my nome, the smaller period fluctuations between 12 and 16 hrs, mid graph, are the night readings, the bottom trace tells you that too, the ramp corresponds to night hours when the heating is off. The SD of the 5 period cycle goes from 2 to 1 μs depending on hour of the day. In daytime the clock senses the traffic in the canal out of my window, only 10-20 m away. One more snag of living in Venice, higher SDs.

With the new lead bob the temp sensitivity has increased by more or less 50%.

It looks to me that your period measurements are integrated with a longer time constant than mine. Incidentally with the higher Q bob a (24h limited like the one shown) full spectrum SD has roughly halved, compared to the cast iron one, everything else kept constant.
<Pasted Graphic.jpg>


Did you compare your different impulsing strategies regularity wise? not easy, it critically depends from what one wants from his clock.

Are you recording the pendulum max speed now? I was imagining you could do that easily triggering from the two rising fronts of the adjacent photo-interrupts, a near to ideal separation for this measurement. From that time interval you can get the swing amplitude right away and the pendulm Q in real time, a critical piece of info, most "intrinsic" clock errors depend on it (impulsing error for ex.).

--
Bepi


Bepi
 

Harvey and Bob, you are quite right temperature can be an important player for actual long term time measurements, i am trying to keep this parameter out of the picture for now while performing "short time scale" experiments at constant temperature. With a better look at the lead bob data I think that I have overestimated the period sensitivity, I believe now that I can't really tell the difference between the two bobs, ΔP/P/ΔT =.47 (lead) vs .42 μs/s/ºC (cast iron) as far as the temperature sensitivity is concerned.

The temperature measurements are inside the case and the predicted bob heating time constant confirms thermal equilibrium. The lead in the new bob is encased in a steel box, not obvious that it should expand as a rigid body. I also have a clamp at the top of the bob to prevent accidental rotations since it's not cylindrically simmetric any more, which acts also as a vertical stop.

As far as the more than double expansion coefficient of lead with respect to gray cast iron is concerned I did evaluate on paper its effect obtaining ΔP/P/ΔT = -.87 μs/s/ºC and 0 for the cast iron, taking into account the expansion of the INVAR rod (1.21 10-6 ºC-1). I noticed that the original gray cast iron bob has just the right coefficient of expansion (1.08 10-5 ºC-1) and height to compensate for an invar rod (1.21 10-6 ºC-1) when the two are connected at their bottom, which is the reason for the 0 prediction quoted above. The same can be achieved with a lead bob, in this case by shortening it from 225 to 83 mm, it would also improve its aerodynamics. Does anybody know if this design feature is typical of all synchronomes. If cast iron was adopted for this reason sacrificing Q I am not sure it was a good idea but it all depends on the clock use, they were not manufactured for constantly thermostatically heated apartments.

In the 2 ºC experiments cited above the angle was tracking well the temperature, with opposite sign, with an amplitude whose corresponding circular error was 4 μs. Wrong sign and wrong amplitude to describe the period observations. I have no explanation for all of this. Other clocks data might help to understand.

 
--
Bepi