[time-nuts] Regulating a pendulum clock

[Jim Palfreyman]

In my post below I didn't understand how a constant electromagnet on
the side of the pendulum could adjust the pendulum rate.

Well I've worked it out. Despite common misconceptions pendulum swing
rate is not independent of amplitude. It is to the first order but not
when calculated properly.

The magnet when applied either reduces or increases the amplitude and
hence makes minor adjustments in timing.

Pretty neat for 40s technology.

Jim Palfreyman

On Sunday, August 8, 2010, Jim Palfreyman <jim77742 at gmail.com> wrote:
> Hi all,
>
> I have a Seimens master clock with a Reiffler pendulum. A lovely piece
> of work that used to provide time services in the 40s.
>
> Being a master clock it has contacts that open and close on each
> pendulum swing and so I can monitor it's accuracy quite easily using
> gps and my 5370B.
>
> I've adjusted it as best I can and the best I can get is about 50 ms
> over 24 hours. However that was a one off. Temp and air pressure cause
> variations of up to 300 ms and it changes direction too. Basically
> it's hard to keep accurate.
>
> It also has a coil mounted near the pendulum and a fixed magnet on the
> pendulum bar and this coil connects to a box down below with a meter
> and a knob. They are labelled in sec/day. The electronics in the box
> are not clear (being quite old) but by measuring the current in the
> coil it quite simply increases the current one way to slow the clock
> and the other way to speed it up. (I'll admit the physics of this
> doesn't make sense to me - but it works!)
>
> It's about 25v in the coil and goes up to 60mA max. Even at levels of
> 2mA has an effect.
>
> Using this control it's quite easy to manually bring the clock back to
> the right time if it's say half a second fast.
>
> What I want to do is control the current in the coil with a micro
> controller which I have attached to a rubidium oscillator. Getting the
> pps from the pendulum clock in and comparing to actual time is easy,
> but I need a way to control the current through the coil so it can
> dynamically adjust the clock.
>
> I need the current to go from say -10 to +10 mA (at 25v) and this
> needs to be controlled via a micro controller output (which goes from
> 0 to 5 with 2.5 being the 0mA point).
>
> I can either use the D/A in the controller (or PWM an output I suppose).
>
> I'd appreciate some thoughts on circuits to do this. Software side is
> not a problem.
>
> Jim Palfreyman
>