rickman wrote on 8/5/2017 11:08 AM:
Gareth's Downstairs Computer wrote on 8/5/2017 9:57 AM:
On 05/08/2017 14:34, Chris 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.

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)

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/Elec...cks/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.

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.

--

Rick C

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


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

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



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...

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


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

Chris wrote on 8/5/2017 2:33 PM:
On 08/05/17 14:48, rickman wrote:


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

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

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.


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...

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


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.

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


Ok, the Shortt clock is probably as close as you can get to a classic
pll using mechanics :-)...

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.

--

Rick C

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

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.

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!

On 08/05/17 19:14, Gareth's Downstairs Computer wrote:
On 05/08/2017 20:06, rickman wrote:

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.

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!



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

On 05/08/17 20:14, Gareth's Downstairs Computer wrote:
On 05/08/2017 20:06, rickman wrote:

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.

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!



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.

Gareth's Downstairs Computer wrote on 8/5/2017 3:14 PM:
On 05/08/2017 20:06, rickman wrote:

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.

All pendulums have circular error where the frequency is determined by
the amplitude of swing,

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


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.

This has nothing to do with the circular error.


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!

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.

--

Rick C

Chris wrote on 8/5/2017 4:06 PM:
On 08/05/17 19:14, Gareth's Downstairs Computer wrote:
On 05/08/2017 20:06, rickman wrote:

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.

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!



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...

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

--

Rick C

Brian Reay wrote on 8/5/2017 5:10 PM:
On 05/08/17 20:14, Gareth's Downstairs Computer wrote:
On 05/08/2017 20:06, rickman wrote:

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.

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!



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.

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.


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.

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

--

Rick C

On 05/08/2017 22:24, rickman wrote:
Gareth's Downstairs Computer wrote on 8/5/2017 3:14 PM:
On 05/08/2017 20:06, rickman wrote:

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.

All pendulums have circular error where the frequency is determined by
the amplitude of swing,

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

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.

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.

This has nothing to do with the circular error.

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


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!

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.

.... which is virtually the range where sin( theta) = theta.