[LEAPSECS] Of stepping motors and leap seconds
seaman at lpl.arizona.edu
Fri Feb 8 08:55:25 EST 2019
I suppose one could argue that even a manually disciplined clock (e.g.,
analog AA wall clock in the kitchen) is ultimately computer regulated
when flicking the hands while glancing at a cell phone.
The 10,000-year clock is disciplined by the Sun, though for some
functions this is relative to its equation of time cam machined to
deliver the result of computations performed by JPL:
On 2/8/19 6:29 AM, Tom Van Baak wrote:
> (steering the topic back to steps, smears, and leap seconds)
> Tony, Rob,
> It's not surprising if the Corpus Clock is disciplined against MSF. This trick is used more often than you think. John C Taylor presented at the Caltech "Time for Everyone" symposium in 2013:
> While there he revealed another of his inventions, the "intelligent pendulum", which you can see in the top third of this image from his web site:
> A more detailed set of photos and description of the prototype is found here:
> It's a battery operated device that functions both as a cosmetically appropriate pendulum bob (of sufficient mass) and an accurate quartz clock (of superior accuracy). The trick is to use an embedded accelerometer and microcontroller to detect and measure the periodic swinging motion and compute the mean period of the pendulum using the quartz oscillator as a time reference. Then, if the pendulum rate is found to be inaccurate, that is, if the pendulum is drifting in time, the device activates a geared-down stepper motor to climb up or down the pendulum rod some fraction of a mm in order to correct the rate. No different than NTP, really.
> For a standard seconds pendulum (length ~1 meter, period ~2 seconds) changing the location of the mass by 1 mm results in a 500 ppm change in rate, equivalent to ~40 seconds/day. To make a one second per day change one only needs to move the bob by 20 microns. Don't laugh. This is done. Precision pendulum clocks are a world where microns and microseconds matter a lot. And leap seconds are a pain for them too.
> There's a wonderful BBC video showing how Big Ben is adjusted for leap seconds:
> "Leap second: Slowing down Big Ben" 
> In this case they do not literally move the massive weight up or down, but instead, by adding tiny weights (usually penny coins) they change the effective center of gravity of the bob, which is all that matters. This is an unexpected application of the saying that "time is money".
>  If someone (UK only?) can capture this short video as mp4 I'd appreciate it.
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