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

[Jim Lux]

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.)