[time-nuts] Regulating a pendulum clock
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sun Aug 8 04:28:10 UTC 2010
J. Forster wrote:
> 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.
>
>
Noise is never irrelevant.
You havent shown that its insignificant either.
In the real world such dv/dt assumptions with inductive loads lead to
fried parts.
For example if the circuit oscillates at high frequency because the
compensation isnt correct/effective or the feedback wire becomes
detached or the power supply goes down suddently due to a crowbar event
then high dv/dt at the opamp/buffer output is possible.
> -John
>
> ================
>
>
Bruce
>
>> 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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