[time-nuts] Regulating a pendulum clock

Sun Aug 8 10:46:38 UTC 2010

The attached single ended inverting driver is perhaps a better choice as 
it allows a dc coupled noniverting amplifier with gain and significant 
offset and drift to be substituted for the LT1010 buffer depicted if the 
frequency compensation is adjusted to suit.
The series RC across the coil damps the coil resonance and the 1nF caps 
approximate wiring and coil capacitance to ground.
A 10nF coil shunt capacitance and a series R of 400 ohms is included in 
the model.
In practice the compensation should be adjusted to suit the actual coil 
used.
A dc coupled discrete (or IC) audio power amplifier is one option for 
the noninverting amplifier.
The noninverting amplifier may also have higher supply rails should this 
be useful/necessary.

Bruce

Bruce Griffiths wrote:
> 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
>
> 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. You
>> 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 DAC,
>> 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.
>>
>> FWIW,
>>
>> -John
>>
>> =============
>>
>>
>>
>>
>>> 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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>>> and follow the instructions there.
>>>
>>>
>>
>>
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>
>
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