[time-nuts] Regulating a pendulum clock

[J. Forster]

OK. You know better.

BTW, op-amp noise is essentially irrelevant in this application, and the
C's across the FB resistors limit slew rates so there is no significant
dI/dt to cause voltage spikes.

-John

================


> Your naive stabilisation scheme wont work, try simulating it.
> 741's are somewhat noisier than necessary.
> Omitting the diodes with an inductive load almost inevitably leads to
transistor or opamp destruction.
>
> Bruce
>
> J. Forster wrote:
>> IMO, far too complicated.
>>
>> I'd use a series pair of u741s each with a complementary emitter follower.
>> 2 u741s, 2x 2N2102, 2x 2N4036, 5 resistors. Maybe 2x .01 caos to stabilize
>> the thing
>>             ---------------------------------------------
>>           |\|     |---|c
>> DAC --o--| \     |   |\  2N2102
>>        |  | / --o-o     |------CCCCC
>>        R  |/    | |   |/  2N4036
>>        |    |   | |---|c
>>        |    |--------------------------------------------
>>        |        |
>>        |--------o-------------------------to input of mirror image
>>
>> Best,
>>
>> -J
>>
>> =========
>>
>>
>>
>>
>>> The attached circuit schematic illustrates the Howland current source
plus inverting amplifier drive technique.
>>> It also illustrates a method of frequency compensation (series RC
connected across the coil).
>>> Of course one can either use discrete buffers or high current opamps.
However for improved accuracy using a difference amplifier with built
in
>>> pretrimmed resistors for the Howland current source may be preferable,
in which case a discrete buffer stage or equivalent may be required.
>>>
>>> Bruce
>>>
>>> J. Forster wrote:
>>>
>>>> There are cheap, split supply audio amp ICs that'd work, or you could
use
>>>> a u741 with a complementary-symmetry output buffer of discrete
transistors.
>>>>
>>>> Crossover distortion would be essentially irrelevant, keeping the parts
>>>> count very low.
>>>>
>>>> -John
>>>>
>>>> ============
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>> The 60mA load current would be problematic for most common opamps
without an output buffer stage.
>>>>> High voltage opamps are relatively rare.
>>>>>
>>>>> 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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>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
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>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>
>>
>>
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