# A mechanical phase locked loop!

Rubbish, the function of a phase locked loop is to keep the phase of the 2 signals the same, within the constraints of the loop filter.
The clock *never* achieves this, it is open loop and applies a 'kick' to one pendulum the amplitude of which is NOT related to the difference in phase of the 2 pendulums.
A fixed kick is given without any knowledge that it will be of the correct amplitude to achieve an in phase or near in phase condition. There is NO feedback of an error signal that relates to the phase difference between the 2 pendulums.
The only time phase comes into the picture is the timing of when the 'kick' is given, so as not to disrupt the normal swing of the pendulum, and whether or not to give a kick at all.
It is and ingenious system, but not a phase locked loop.
I guess it could be closer to a PLL if the kick had its amplitude varied by the phase difference between the 2 pendulums, but you still have the problem that if you were in the state where no kick was required there is no way of slowing the second pendulum without waiting for it to drift back, so it is still open loop.
Jeff
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On 08/05/17 09:45, Jeff wrote:

Exactly. The control is single path, master to slave, with no feedback to the reference, making it an open loop design. The master has no knowledge of the state of the slave at any time.
In a pll, there is continuous feedback from the vco to the phase detector, closing the loop and keeping the phase offset constant, The phase is continuously updated every cycle, whereas the Shortt clock can have significant accumulated error in the time between corrections...
Chris
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On 05/08/2017 14:34, Chris wrote:

Untrue.
The matter starts off when the slave signals to the master and drops the gravity link in the master, then, when the master pendulum is in a position to accept the impulse from that dropped gravity link, it signals back to the slave
But ... I'm still trying to google for the exact mechanisms because most URLs only hint at what is happening. (I'm also awaiting delivery of a couple of hope-jones' books about electric clocks)
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On 05/08/2017 14:57, Gareth's Downstairs Computer wrote:

I hope you have more success getting a copy of those books than getting the PA tuning instructions for a mass produced amateur TX.
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Gareth's Downstairs Computer wrote on 8/5/2017 9:57 AM:

What you are describing is how the phase measurement of the master is made. The gravity lever is simply a remontoire providing a consistent push to overcome the force of friction. It is designed to be invariant of small changes in timing of its release. You can see that in the animation linked below. The gravity arm is released at the point when the wheel is directly under the end of the gravity lever. A small change in timing changes the force only a tiny amount. This is critical to maintaining the swing of the free pendulum without affecting its period.
http://www.chronometrophilia.ch/Electric-clocks/Shortt.htm
The animation happens in real time so it is hard to see the details of what is going on. The gravity lever and accompanying control is the magic of the clock. The rest is pretty straight forward. You need Flash to view this page. There is a button to see the wires.
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Rick C

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rickman wrote on 8/5/2017 11:08 AM:

One other part of the Shortt clock that requires careful thought is the relay and spring that perform the phase detection and correction. The slave pendulum has a leaf spring parallel to the rod and the control relay has a pick which is activated under control of the master gravity lever. The pick can intercept the leaf spring or not, depending on the timing. There is an issue with this which is impossible to eliminate, only minimize and that is metastability. A decision is being made and it can not be done with infinite resolution. So the pick and leaf spring must be designed to minimize the problem, likely done by making the spring thin as possible and making the edge on the pick as sharp as possible.
We see the same problem in electronics when trying to make decisions on the state of an input that is changing.
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Rick C

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Chris wrote on 8/5/2017 9:34 AM:

You aren't making sense. The reference is never adjusted in a PLL. That's why it's the *reference*.

There is no requirement in a PLL for continuous action or even frequent action.
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Rick C

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On 08/05/17 14:48, rickman wrote:

Just where did I say that ?. Having worked with pll's since the 4046 and earlier, I should know the difference.

That's probably why the Shortt clock is described as a hit and miss system and correction is unipolar, whereas a classic pll continually updates the vco every cycle, not multiples thereof.
Ok, the Shortt clock is probably as close as you can get to a classic pll using mechanics :-)...
Chris
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Chris wrote on 8/5/2017 2:33 PM:

You snipped the part I was replying to but you talked about the master knowing the status of the slave which would only be useful if you were adjusting the master.

"Classic"??? There is no such definition of a PLL to "continuously" update anything.

Yes, because it *is* a PLL. In fact the problem most people have with it is that it doesn't adjust the phase by adjusting the frequency of the slave. It adjusts the *phase* so clearly it *is* a phase locked loop.
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Rick C

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On 05/08/2017 20:06, rickman wrote:

All pendulums have circular error where the frequency is determined by the amplitude of swing, so for the half cycle where the phase is adjusted by abridging the swing by the hit of the hit and miss stabiliser, the frequency of the slave is, indeed, changed.
The standard formula given for the cycle time of pendulums ..
2 * PI * root( L / G)
... is only valid for those small angles where sin( theta ) = theta, and such angles are so infinitesimal that no visible movement of a pendulum would be seen!
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On 08/05/17 19:14, Gareth's Downstairs Computer wrote:

This just won't go away, will it :-). Here we are, arguing over the semantics of phase locked loops, but the term pll didn't come into wide use until the 1960's, decades after the Shortt clock. I'll continue to think of it as a hit and miss governor, as it was originally described...
Chris
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Chris wrote on 8/5/2017 4:06 PM:

And that is what it is, not at all unlike a PLL using a bang-bang phase detector.
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Rick C

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On 05/08/17 20:14, Gareth's Downstairs Computer wrote:

You seem to be confusing two different things
The error you refer to is due to the pendulum not actually taking a direct line between the ends of its travel, the error is small for small amplitudes. There was a famous experiment by a Frenchman in, I think Paris, he hung a huge pendulum and let it trace its path in sand, rather than it going 'to and fro' it actually went in arcs as it went to and fro.
The effect is minimised by reducing the amplitude.
As you correctly say, the frequency of a pendulum is given by the formula you state. If you 'give it a nudge' you may shorted one swing but the overall frequency is still determined by the formula.
The 'nudge' will change the phase of the swing, not the frequency- ie it will shorten one cycle.
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Brian Reay wrote on 8/5/2017 5:10 PM:

I believe you are thinking of the Foucault pendulum. This had nothing to do with elliptical paths of pendulums. This was a pendulum free to swing along any axis. As the earth rotates the pendulum continues to swing in its original path and the earth turns beneath it. Of course the pendulum appears to rotate the plane of swing.

Yes, that is right. The change in frequency (phase change rate) is only momentary.
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Rick C

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Gareth's Downstairs Computer wrote on 8/5/2017 3:14 PM:

All *uncorrected* pendulums have circular error. The Fedchenko clock has a mounting spring for the pendulum that corrects for circular error.

This has nothing to do with the circular error.

This equation is an approximation which ignores the higher terms of the power series of the full equation. It is only truly valid for no swing at all.
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Rick C

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On 05/08/2017 22:24, rickman wrote:

Hadn't heard of that one. At the BHI lecture there was mention of another correction of circular error by a colied spring attached somewhere at the bottom, but I wasn't paying full attention at that point.
There were also other means such as cycloidal cheeks around the suspension spring.

It has everything to do with the circular error and the variation in frequency that comes with varying amplitude of the swing.

... which is virtually the range where sin( theta) = theta.
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Gareth's Downstairs Computer wrote on 8/5/2017 5:57 PM:

You seem to be completely misunderstanding the operation of the Shortt clock. The slave pendulum has no need for correction of circular error. It is a good pendulum, but not a great one. It doesn't need to be great, it is corrected every 30 seconds by the electromechanical escapement of the master pendulum. It only has to be good enough to provide an appropriately timed release of the gravity lever.
So the small circular error has no bearing on the slave pendulum.

Exactly. This *is* the range where sin(theta) = theta. Anywhere other than zero it is an approximation.
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Rick C

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On 05/08/2017 23:25, rickman wrote:

I'm sorry, but you totally misunderstood what I was saying, which was that because all pendulums exhibit circular error, when the hit occurs in the hit and miss synchroniser and foreshortens the swing, then, for that half-cycle, and only that half cycle, the frequency is changed, as it must be.
Just as in the electronic PLL, instantaneous changes of phase have instantaneous changes of frequency, no matter how short lived, associated with them.
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Gareth's Downstairs Computer wrote on 8/6/2017 5:26 AM:

What you say about frequency vs. phase is true and how the Shortt clock adjusts phase, but it has nothing to do with circular error of the pendulum. The correction of the phase is from the added spring resistance shortening the time as well as the travel of the pendulum. The fact that the swing is shorter and the second order circular error will create a tiny error in the timing is pretty much irrelevant. The real change is from the added spring constant changing the first order effect in the pendulum equation. The coefficient of the gravitational constant is effectively changed by the spring.
Is that more clear?
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Rick C

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On 06/08/2017 17:18, rickman wrote:

You continue to misunderstand. Any pendulum swinging with circular error speeds up for shorter amplitude; speeding up means increased frequency. Therefore, for the half cycle inwhich there is a hit, a shorter amplitude and hence instantaneous higher frequency exists.