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Old August 1st 17, 12:00 PM posted to uk.radio.amateur,uk.rec.models.engineering,rec.radio.amateur.homebrew
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Posts: 97
Default A mechanical phase locked loop!

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

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Old August 1st 17, 12:58 PM posted to uk.radio.amateur,uk.rec.models.engineering,rec.radio.amateur.homebrew
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Posts: 379
Default A mechanical phase locked loop!

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.


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Old August 1st 17, 01:49 PM posted to uk.radio.amateur,uk.rec.models.engineering,rec.radio.amateur.homebrew
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Posts: 980
Default A mechanical phase locked loop!

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.

--

Rick C
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Old August 1st 17, 06:19 PM posted to uk.radio.amateur,uk.rec.models.engineering,rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Aug 2013
Posts: 379
Default A mechanical phase locked loop!

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.




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Old August 2nd 17, 04:59 AM posted to uk.radio.amateur,uk.rec.models.engineering,rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Nov 2012
Posts: 980
Default A mechanical phase locked loop!

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.

--

Rick C


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Old August 2nd 17, 05:08 AM posted to uk.radio.amateur,uk.rec.models.engineering,rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Nov 2012
Posts: 980
Default A mechanical phase locked loop!

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.

--

Rick C
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Old August 2nd 17, 08:19 AM posted to uk.radio.amateur,uk.rec.models.engineering,rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Aug 2013
Posts: 379
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.


--

Suspect someone is claiming a benefit under false pretences? Incapacity
Benefit or Personal Independence Payment when they don't need it? They
are depriving those in real need!

https://www.gov.uk/report-benefit-fraud
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Old August 2nd 17, 03:04 PM posted to uk.radio.amateur,uk.rec.models.engineering,rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Nov 2012
Posts: 980
Default A mechanical phase locked loop!

Jeff wrote on 8/2/2017 5:09 AM:

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.


I think the confusion occurs because at no time, are the phases of the 2
clocks locked together, even at the point of the impulse. By the very nature
of the design the phase of the 2 pendulums (or should that be pendula to
please Gareth) shift in relation to each other.

In an electronic pll, even one using a bang-bang phase detector, the phases
of the 2 signals are locked together, within the constraints of the loop
filter.


This is another false dichotomy. The aspect of the Shortt clock you are
referring to is that it is *discrete* rather than continuous. So you can
clearly see the fact that the slave oscillator is not in perfect lock step
with the master (reference). The same is true in *all* PLL circuits. The
phase of the oscillator is adjusted by the error signal. There can be no
adjustments without error, so the oscillator will not be in perfect lockstep
with the reference. It will be within some tolerance... same as the Shortt
clock. A PLL can be discrete and the phase will move in patterns with small
offsets in frequency at all times. With a continuous phase comparison the
frequency will vary continuously but still will not be "locked" to the
reference with no error. In fact, PLLs are used to remove short term jitter
from clocks by the use of a slow filter on the control signal.

--

Rick C
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Old August 3rd 17, 01:43 PM posted to uk.radio.amateur,uk.rec.models.engineering,rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Aug 2017
Posts: 10
Default A mechanical phase locked loop!

On 08/02/17 07:19, Brian Reay wrote:
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.



I've had an interest in clocks as well. Working in computing, was
interested in the IBM master clocks, which have a Graham deadbeat
escapement and either an electrically wound spring, or weight
driven mechanism, + an Invar pendulum. Found a mid 1930's
example some time ago, which has been running now for about a
year. Stripped down completely and rebuilt. IBM claim around 15
seconds a month error, but after rating for a few weeks, it shows
an error of less than a second a month. There's noise on the
stability, drifting +/- half a second or so from day to day, but
was quite amazed at the accuracy of such an old clock...

Chris
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Old August 3rd 17, 02:44 PM posted to uk.radio.amateur,uk.rec.models.engineering,rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Aug 2013
Posts: 379
Default A mechanical phase locked loop!

On 03/08/17 13:43, Chris wrote:
On 08/02/17 07:19, Brian Reay wrote:
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.



I've had an interest in clocks as well. Working in computing, was
interested in the IBM master clocks, which have a Graham deadbeat
escapement and either an electrically wound spring, or weight
driven mechanism, + an Invar pendulum. Found a mid 1930's
example some time ago, which has been running now for about a
year. Stripped down completely and rebuilt. IBM claim around 15
seconds a month error, but after rating for a few weeks, it shows
an error of less than a second a month. There's noise on the
stability, drifting +/- half a second or so from day to day, but
was quite amazed at the accuracy of such an old clock...

Chris



I used to have a small, but nice, collection of pocket watches. I'd
collected them over the years, repaired them etc. Then some scum bag
thieved them. While I got a generous insurance payment, it wasn't the
same. I'd put 'sweat and blood' into them- they were in a poor state
when I got them but valuable when I'd restored them. I was tempted to
buy some more to restore but never got around to it- time was always
short. Now my dexterity isn't what it could be and I probably would
struggle with a pocket watch, let alone a wrist watch. One had a
cylinder escapement, not rare, but unusual and with a distinct 'tick'-
different to a normal escapement.

While I prefer mechanical watches, I favour Rolex (originally English,
BTW), I would quite like to get one of the 'tuning fork' watches,
ideally the version with the clear dial. Another classic.




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