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Sulaiman

Wed Mar 13 2019, 12:44PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Background:
I have played with R.C and L.C oscillators, XTO, TC-XTO, ovened-XTO and rubidium oscillators, but now I want to go back to basics and make a clock based on a pendulum.
(I already have a moderately accurate wind-up pendulum clock)

The 'clock' part is easy,
I bought a cheap quartz clock with hands via eBay for use as a pulse-counter / display.
When fed with alternating positive and negative pulses of c1.3v for 27ms (measured) at one second intervals, the display advances one second per pulse, i.e. displays the time.

The pendulum and its driving are my main concerns.
I would like to be able to see the effects of the moon's orbit and the tides changing my local gravity,
but that is the optimistic upper end of my expectations.

The Plan
1) Blog the development here - at my pace it could be a long time before finished - if ever.
2) The pendulum would be free-swinging in a reduced pressure environment for high 'Q', in a 1200mm tall x 160mm diameter glass tube.
3) the rod will initially be a borosilicate glass tube,
due to thermal expansion, if the rod were 1 degree Celcius from nominal then the clock would be innacurate by about 3.3 ppm which translates to c103 seconds/year error (per Kelvin) = unacceptable.
4) the bob mass will consist of NdFeB magnets and lead.
5) the pivot will be a knife-edge. I may also try the spring type.

All of the above is straightforward and just a matter of doing it.
Later I will make a temperature-compensated pendulum which needs some thought,
but most of my thoughts at the moment are regarding the sensing and driving of the pendulum.
There are a few designs for electronic/electromagnetic pendulum motors on the web but none suit my needs.

For now the power will be provided by
'6V 1W' solar panels and
'4.8V 800mA.h' Ni-Cd battery packs, (actually more like 560 mA.h)
with a 1N5339B 5.6V 5W zener across each battery pack.
So supply voltage is nominally 5.6 to 4.4 Vdc,
maximum drain of c30 mA continuous.

Ni-Cd is my choice because of the daily recharge/discharge cycling.
Using the zeners and solar panels they cannot get overcharged,
and Ni-Cd is the most tolerant battery chemistry if full discharges are anticipated.

As I have not started physical construction yet, all of the above are open to discussion and/or changes.

Now to the part that is occupying my thoughts ... the motor.

There are a few theoretical requirements;

A) the peak angular swing of the pendulum should be constant as pendulums are not truly iso-synchronous,
the period increases with increasing peak angular swing.

B) people smarter than me have worked out that the best driving pulse is ;
a consistent energy impulse,
delivered at the bottom of the swing (maximum kinetic energy),
to the centre of percussion (to reduce wear on the pivot)

C) I want to use only discrete electronic components - no ICs
(other than maybe a 3.3V series voltage regulator for a constant supply voltage)

Sulaiman

Wed Mar 13 2019, 01:36PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Has anyone here tried, or thought about, something similar ?

Blackcurrant

Fri Mar 15 2019, 01:57AM

Registered Member #2989 Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94

It's a very interesting experiment.
I did have a read of the wiki page a while back. I think some clocks are driven by changes in air pressure, ie bellows winding up a spring or something so I guess the energy needed to keep them going must be very small.
Send a beam of light through so the pendulum would cut it then detect.
How about using HV static charge to drive the pendulum a bit like the Oxford Electric Bell?
I wonder if you could hang the pendulum using magnets for lower friction than a pivot.
I was going to say zerodur glass for your arm part but that might be a bit difficult to find.
maybe ask a clock-maker for ideas or pitfalls etc

Sulaiman

Fri Mar 15 2019, 12:10PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The power required to keep a pendulum swinging is miniscule - measured in nano-Watts to micro-Watts

I am considering the use of magnets to significantly reduce the pressure on the rolling-rounded-knife-edge pivot,
but I've not yet got a configuration that does not also add a magnetic restoring force - independant of gravity.

I had not considered electrostatic drive because my power source is 5V +/-12%
and I'd have to generate consistent kV ms pulses,
I'm going to do a quick feasibility check now that you've mentioned it,
but I doubt that it will be a winner......

update:
I've ordered a 10mm dia x 1000mm long pultruded epoxy resin & carbon fibre rod - as the pendulum rod,
(possibly slightly over-engineered ... about 10 tons tensile strength :)
The mass of the bob should stretch the rod by less than 5 ppm.

I've also had a play at casting my stock of 1.4kg lead into a bob a few times,
and learned how difficult it is to drill into solid lead
and the pointlesness of screwing anything into it.
and wood shavings do help,
also, candle wax in the melting pot keeps the lead surface shiny..

My guess is that electrostatic drive will work well only in a vacuum,
the large areas required would cause a lot of wind resistance,
which loses energy every swing, giving a low 'Q' hence low accuracy,
and
operating in a vacuum comes much later in my 'plan', (plan...rotfl)
because safely maintaintng a vacuum in a 5mm wall 160mm i.d. x 1200mm tall glass tube, with a 3kg pendulum swinging inside, may require some thought :)

15Apr19... carbon fibre rod arrived ... 1000.0mm long and perfectly straight
(to the limits of my +/-0.1mm measurement accuracy, not that straightness or actual length matter in this case)
Too nice to cut or drill so I've ordered 12mm od x 10mm id roll-wrapped carbon fibre tube.(larger undefined temp.co.)
24Apr19 ... C.F. tube arrived, perfect fit for C.F. rod. (and my 10mm dia. NdFeB magnets for position sensing etc.)

Mattski

Mon Apr 29 2019, 08:25PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Sounds interesting. Have you considered an ovenized enclosure instead of, or in addition to, temperature compensation?

The magnets is the bob mass, is this to provide both a measurable impulse for the clock output, and will this also be used to provide the driving pulse to maintain the swing?

Sulaiman

Mon Apr 29 2019, 09:02PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I have briefly considered an ovenised enclosure,
the main consideration is power consumption,
as I want the entire clock to be solar/battery powered,
so I've decided to leave environmental control for a later option.
I'd like to study the temp.co. of the carbon fibre rod first.

The bulk of the Bob mass will be lead, another 1.8 kg arrived so I melted it together with the original 1.4kg to make a new single 100mm dia. 3.2 kg Bob.
Melted misc. quantities of old solder, a pewter plate and lead bits to form a second 100mm dia. mass of 624g.
I expect the final bob mass to be around 4kg

I've read that the losses of a good pivot are around 1% of air friction losses,
so a steel craft knife blade edge, pivoting on the sides of two more blades should be ok for version 1.

I had initially considered using my two largest N52 magnets (23 dia.x20 long),
located symetrically about the centre of percussion,
but I've decided that would be overkill ... for now.

The 10mm dia N52 magnet(s) will be used for position (and either velocity- or more likely, peak swing- ) detection,
and initially for drive also, based on this:

- but more gentle.

Project now on hold for probably months due to moving home.

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