[time-nuts] Regulating a pendulum clock
namichie at gmail.com
Sun Aug 8 02:26:49 UTC 2010
the range of the adjustment is probably far wider than you would ever
as you will be applying it continuously instead of over a few hours
to correct the clock to the latest
So all you need is an amplifier run off + and - 15 Volts with enough
gain for the DAC output.
That should give you + or - 10Volts.
You may also need a reference diode, say 2.5 volts, to provide an
offset voltage to the input of the amplifier
if you want to set the zero of the control range and possibly offset
a single ended DAC output
to span either side of zero.
On 08/08/2010, at 12:10 PM, J. Forster wrote:
> Since it's inside a closed loop, the design is uncritical.
> One option is a high voltage Op-Amp with +/- 25 to 30 VDC supplies.
> would set the OA gain to about 10, so 2.5 V in would yield 25 V
> out. and
> sum in a negative offset voltage so that +2.5 from the DAC yields
> 0.0 V
> out. I'd use something like a 100 K FB resistor and a 10K from the
> assuming it's a voltage output DAC. A 1 M to the -25 V supply would
> provide the 2.5 V offset.
> Another option would be to use two series opamps with the first set
> up as
> above, and the second as a unity gain inverter with input connected
> to the
> output of the first. The coil would connect between the two OA
> outputs. As
> one output swings high, the other mirrors that and goes low (just
> as in an
> H bridge). Stability might be an issue, but this has the advantage
> of only
> needing a +/- 15 supplies.
>> Hi all,
>> I have a Seimens master clock with a Reiffler pendulum. A lovely
>> 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
>> 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
>> 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
>> 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
>> I'd appreciate some thoughts on circuits to do this. Software side is
>> not a problem.
>> Jim Palfreyman
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