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Old August 2nd 17, 08:19 AM posted to uk.radio.amateur,uk.rec.models.engineering,rec.radio.amateur.homebrew
Brian Reay[_5_] Brian Reay[_5_] is offline
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Default A mechanical phase locked loop!

On 02/08/2017 05:08, rickman wrote:
rickman wrote on 8/1/2017 11:59 PM:
Brian Reay wrote on 8/1/2017 1:19 PM:
On 01/08/17 13:49, rickman wrote:
Brian Reay wrote on 8/1/2017 7:58 AM:
On 01/08/17 12:00, Gareth's Downstairs Computer wrote:
Continuing my googling following last night's
BHI lecture, and following up on the Shortt
and Hope-jones clocks, here is a mechanical
phase locked loop, and in Meccano! ...

http://www.meccanotec.com/shortt.html

While the article refers to a 'phase lock loop', it isn't really..
There
doesn't seem to be any measurement of error in the slave which is
then use
use used to 'pull it' to reduce the error- which is how a true
phase lock
loop works.

The system seems to operate more as follows, the slave is designed
to run
'very nearly right'. It receives precise pulses from the master
which it
will naturally sync to. The same will happen if you have two
oscillators on
nearly the same frequency if you 'feed' the output of one to the tuned
circuit of the other. (Including harmonics.) This is used, for
example, by
some amateurs to lock radio oscillators to GPS locked references.

Still, it is an clever system and of interest.

The Shortt clock *does* make a measurement of the phase. It checks
to see
if the phase is fast or slow. In one case it invokes a spring that
tweeks
the phase of the slave. In the other case it does not invoke the
spring
allowing the clock to continue running unadjusted. The default
behavior
of the slave clock is to run a bit slow and the adjustments speed it up
(or the other way round, I can't recall exactly).

The measurement may be binary and the adjustment is the same, but that
does not make it anything other than a phase locked loop.


Hmm, I half see your point but I'm not entirely convinced.

I'm just not convinced that the description truly 'maps' to that of a
true
PLL.

I don't doubt that it works nor do I suggest it isn't a very clever
bit of
design. I'm just not sure about the terms used.


Ok, but I don't see what you can be confused about. I believe in
electronics this phase detector is referred to as "bang-bang" where it
outputs a 1 or a 0. So on every measurement the VCO frequency control
signal receives an impulse of one polarity or the other.

The only difference between that and the Shortt clock is the Short clock
only has one polarity of impulse and is adjusted to run a bit off so the
required intermittent impulses will keep it in phase with the master.

If you are interested in mechanical clocks (the Shortt clock uses
electricity to isolate the master and slave even though the master is
purely
mechanical) you can read about the Fedchenko AChF-3 time piece. It came
well after the Shortt clock and not long before quartz and atomic clocks,
but was amazingly accurate without any fancy footwork with master slave
complexity.

Fedchenko used a compound spring for want of a better name. I've read
that
it corrects for the parabolic distortion introduced in the timing of a
circular pendulum swing. This is a second order effect in that the
coefficient in the term is rather small. But in these clocks it makes a
difference. The way most clocks correct for it is to keep the
amplitude of
the pendulum swing as constant as possible minimizing the second order
deviation. The Fedchenko clock uses a pendulum spring with two distinct
lengths. This causes a different rate of spring over the range of angle.
Some descriptions seem to say it actually causes the pendulum to swing
in a
parabolic arc. Either way it corrects for the second order term in
the time
equation of the pendulum making it less sensitive to variations in the
amplitude of oscillation.


Thought I'd mention John Harrison's 'Clock B' too. It was designed 250
years ago, but never built that I am aware of until recently. It has
proved to be nearly as accurate as the Shortt and Fedchenko clocks even
though it was a much, much earlier design. I don't know any details of
why it is so good other than that Harrison took into account every
source of error and included a compensating factor to balance it out. I
haven't see any further detail. Pretty impressive. Clearly the man was
a genius.



Oh yes, I recall the B clock- I have an interest in clocks (actually
more watches) - and read up on Harrison's history, partly due to his
work on clocks / watches directly but also as much of my engineering
work was navigation related.

I recall reading of the building of the modern version of the B clock -
it must have been in the 70s or early 80s.

As you say, Harrison was a genius- albeit an largely unrecognised /
unappreciated one in his own time- at least by the Gov. of the day. I've
seen the examples of his work in the National Maritime Museum- the
quality is unbelievable, especially when you consider the technology of
the time.


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