[time-nuts] Regulating a pendulum clock

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Aug 8 01:09:54 UTC 2010


A high voltage opamp (or a low voltage opamp with a discrete output 
stage with a voltage gain of at least 2) with -3V and + 30V supplies is 
perhaps the simplest method.
The opamp merely senses the current flowing in a current sensing 
resistor and regulates this voltage drop to equal the output of a DAC.

Alternatively it should be feasible to use a pair of opamps (plus output 
buffers) configured in a bridge arrangement to drive the coil from a 
single 30V supply.
If one end of the coil has to remain near ground then a unity gain 
difference amplifier (with a discrete buffer with voltage gain) could be 
employed to implement a current source.
A difference amplifier could also be employed together with an opamp 
(plus unity voltage gain discrete ouput stage) inverter to drive the 
coil from a single 30V supply.

Bruce

Jim Palfreyman 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
>
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