[time-nuts] Regulating a pendulum clock

[Jim Palfreyman]

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