[time-nuts] Regulating a pendulum clock
J. Forster
jfor at quik.com
Sun Aug 8 14:52:46 UTC 2010
You are picking very unimportant nits.
If there were a small noise spike from the opamp, it'd goose the pendulum
a tiny amount. That would be corrected on the next swing.
You are turning a trip to the corner store into an Apollo Moon Mission.
BTW, since the =drive does not to be bipolar, one of the NPN and PNP
transistors can be deleted. They never turn on. So you are left with two
opamsa, =each with a simple emitter follower.
-John
==============
> 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
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> time-nuts mailing list -- time-nuts at febo.com
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>>>>>>>>> and follow the instructions there.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> time-nuts mailing list -- time-nuts at febo.com
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>>>>>>>> and follow the instructions there.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>
>>
>>
>>
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>
>
>
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