[time-nuts] 24 hr clock movements...

Rex rexa at sonic.net
Mon Jan 20 21:33:43 EST 2014


Jim, I think you missed the main point I was trying to address.

It seems that many of the newer quartz movements do not move the second 
hand in one-second steps. They move it in some way that appears smooth 
to a human observer. (Even if there is no actual second hand, the same 
motive issues need to be looked at.) I assume the smooth motor is still 
some kind of stepper but being driven by pulses at a much higher rate 
than a one-second rate. If you receive one of these versions, you will 
have a more difficult job to drive it. You'd need to figure out what 
rate is driving it and generate that. The 1-second step versions would 
be easier for us to generically interface with.

The description on the Klockit page wasn't very clear about which type 
it is (also it was somewhat ambiguous about using a secondhand, at all, 
if desired -- there is a footnote that I couldn't quite decipher).


On 1/20/2014 4:04 PM, Jim Lux wrote:
> On 1/20/14 3:32 PM, Rex wrote:
>> That listing is a bit vague about if it has a second hand. For the kind
>> of pulse drive that has been discussed here, it seems you would want a
>> definite second capability and step vs. smooth second hand drive.
>>
>> I know nothing except a little web searching, but this one seems to have
>> the right features...
>> http://www.clockparts.com/clock-part/24-hour-high-torque-movement/
>>
>> but, although they mention a 24-hour dial available, the page for it on
>> the site has no content.
>>
>
> it is very much a matter of buying a few and trying them.
>
> If you don't install a second hand, then that solves the inertia of 
> the secondhand problem.
>
> The challenge is that because the "motor" for these things is 
> basically a step at a time, if the hand has too much inertia, then the 
> hand will either not move enough to get to the next tep (dying battery 
> syndrome we've all seen), or, it will move past (because the "braking 
> torque" isn't high enough.
>
> It's sort of the torsional resonance effect that afflicts stepper 
> motors in another form.  The magnetic impulse is basically driving a 
> spring (the magnetic field) with a mass on it.
>
> These things are always highly idiosyncratic. I would imagine that 
> fiddling with the duration and magnitude of the step pulses (or, for 
> that matter the "shape" of the pulse) could have a huge effect if one 
> wanted to optimize it. A couple decades ago we built a large (5-6 foot 
> diameter) stopwatch prop with a stepper motor, and we had to play with 
> the drive voltage, the capacitance and resistance in the step channels 
> to make it work right.  Today, you'd do microstepping, or use a clever 
> algorithm to customize the step waveform.  Generally you want a 
> voltage profile that's sort of a spike (to get the current flowing in 
> the winding) with a back porch, and then a reverse polarity at the end 
> (to stop the motion).
>
> (I note that this problem is not unique to AA powered clocks.  The 
> hands of the clock on the UC Berkeley Campanile are wood for a similar 
> reason.)
>
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