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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 |
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. |
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 |
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. |
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 |
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 |
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 |
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 |
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 |
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. |
A mechanical phase locked loop!
On 08/03/17 13:44, Brian Reay wrote:
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. Bulova Accutron. You can find them on US Ebay, not cheap, but even the example with the exposed internals. Pretty neat watches, but not sure how accurate they would be by now. Also like the early Junghans Mega msf clocks. Bought one of those around 1990. Still keeps spot on time and use it to rate the IBM clock and others... Chris |
A mechanical phase locked loop!
Jeff wrote on 8/3/2017 5:32 AM:
This is another false dichotomy. The aspect of the Shortt clock you are referring to is that it is *discrete* rather than continuous. Not correct the phases of the 2 pendulums are *never* in phase. Even when a kick is given, as of course if they were in phase there would be no need for a kick. You don't understand the meaning of "phase". If you said the two frequencies were never the same I would agree. The slave pendulum runs slower than the master with the intermittent impulse to adjust the phase. The relative phase varies with time as a sawtooth function and so at some point the phase *must* be aligned as the slave passes from being ahead to being behind. On the next adjustment the phase is adjusted or not. When properly adjusted the phase of the slave will only be "bumped" every other adjustment time. On the adjustment times when the slave phase is *not* adjusted the phase will be in alignment ideally. 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. When a electronic phase lock loop is locked there is no error as the 2 signals are perfectly in phase. There will only be a change in locked control voltage if the phase drifts. You need to go back to PLL 101 class. When the PLL is "locked" it simply means the error in phase is small enough that the loop can compensate by varying the VCO frequency. If you understand the math you will see that this means it will *always* hunt for the perfect alignment. If there is no integral term in the feedback loop, there will always be a phase error dependent on the dF/dV slope of the VCO. If there *is* an integral term in the feedback loop the loop will have small fluctuations as the frequency adjusts to correct the phase, but when the phase error reaches zero the frequency error will *not* be zero and the phase error will immediately become non-zero. 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. No, a phase locked loop has the same accuracy, or tolerance if you wish, as the reference. There is always jitter in the output of the PLL that is independent of the reference 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. No it will only vary in sympathy with the reference signal, or with signals that are not damped by the loop filter due to being faster than the loop filer can deal with. Please review your PLL materials. There is no such thing as a PLL that aligns perfectly with the reference. -- Rick C |
A mechanical phase locked loop!
Jeff wrote on 8/3/2017 1:17 PM:
You don't understand the meaning of "phase". If you said the two frequencies were never the same I would agree. Phase is fundamentally linked to frequency. The slave pendulum runs slower than the master with the intermittent impulse to adjust the phase. The relative phase varies with time as a sawtooth function and so at some point the phase *must* be aligned as the slave passes from being ahead to being behind. That is a ridiculous statement, if it were true you could say that any 2 random signals were 'in phase' just because at some point in time they both had the same phase angle. Not sure if you are referring to the Shortt clock or the PLL. But the statement applies equally to both. There is no magical stability in the PLL. It is a control loop and as such the thing being controlled will *never* remain in phase or at the same frequency as the reference. -- Rick C |
A mechanical phase locked loop!
On 08/03/17 17:27, rickman wrote:
Not sure if you are referring to the Shortt clock or the PLL. But the statement applies equally to both. There is no magical stability in the PLL. It is a control loop and as such the thing being controlled will *never* remain in phase or at the same frequency as the reference. I think the difference is that while a pll always has a phase offset the reference and vco are in phase lockstep once the loop has aquired lock. It's a closed loop system whereas the Shortt clock is an open loop system, only getting a kick back into sync from time to time. Like a hit and miss governor ?... Chris |
A mechanical phase locked loop!
Chris wrote on 8/3/2017 3:05 PM:
On 08/03/17 17:27, rickman wrote: Not sure if you are referring to the Shortt clock or the PLL. But the statement applies equally to both. There is no magical stability in the PLL. It is a control loop and as such the thing being controlled will *never* remain in phase or at the same frequency as the reference. I think the difference is that while a pll always has a phase offset the reference and vco are in phase lockstep once the loop has aquired lock. It's a closed loop system whereas the Shortt clock is an open loop system, only getting a kick back into sync from time to time. Like a hit and miss governor ?... I don't know what you guys are seeing. The two pendulums of the Shortt clock are in lock step. The fact that they are only compared every 30 seconds does not change the nature of the design. The phase comparison signal from a PLL is typically "grainy" in the same way and has to be filtered to become a control signal. The only reason you say they are in "lock step" is because the grain is very fine. The Shortt clock grain is very fine as well typically adjusting only every other 30 second period. I guess the difference is the Shortt clock is adjusting the instantaneous phase and the average frequency while a typical PLL adjusts the instantaneous frequency to try to keep the phase aligned. Both will see variations in phase over time. -- Rick C |
A mechanical phase locked loop!
On 08/03/17 21:31, rickman wrote:
I don't know what you guys are seeing. The two pendulums of the Shortt clock are in lock step. The fact that they are only compared every 30 seconds does not change the nature of the design. The phase comparison signal from a PLL is typically "grainy" in the same way and has to be filtered to become a control signal. The only reason you say they are in "lock step" is because the grain is very fine. The Shortt clock grain is very fine as well typically adjusting only every other 30 second period. I guess the difference is the Shortt clock is adjusting the instantaneous phase and the average frequency while a typical PLL adjusts the instantaneous frequency to try to keep the phase aligned. Both will see variations in phase over time. I would see the Shortt clock as a frequency locked loop, not the same thing as a pll. Different level of instantaneous precision. Semantics, semantics :-)... Chris |
A mechanical phase locked loop!
Chris wrote on 8/3/2017 6:33 PM:
On 08/03/17 21:31, rickman wrote: I don't know what you guys are seeing. The two pendulums of the Shortt clock are in lock step. The fact that they are only compared every 30 seconds does not change the nature of the design. The phase comparison signal from a PLL is typically "grainy" in the same way and has to be filtered to become a control signal. The only reason you say they are in "lock step" is because the grain is very fine. The Shortt clock grain is very fine as well typically adjusting only every other 30 second period. I guess the difference is the Shortt clock is adjusting the instantaneous phase and the average frequency while a typical PLL adjusts the instantaneous frequency to try to keep the phase aligned. Both will see variations in phase over time. I would see the Shortt clock as a frequency locked loop, not the same thing as a pll. Different level of instantaneous precision. Not sure why you say that. What is measured and adjusted is the phase. Either the slave is a bit ahead or a bit behind and it is either spurred on a bit or it is not. The frequency of the pendulum is not impacted other than at the moment of phase adjustment. -- Rick C |
A mechanical phase locked loop!
On 8/3/2017 3:05 PM, Chris wrote:
On 08/03/17 17:27, rickman wrote: Not sure if you are referring to the Shortt clock or the PLL. But the statement applies equally to both. There is no magical stability in the PLL. It is a control loop and as such the thing being controlled will *never* remain in phase or at the same frequency as the reference. I think the difference is that while a pll always has a phase offset the reference and vco are in phase lockstep once the loop has aquired lock. It's a closed loop system whereas the Shortt clock is an open loop system, only getting a kick back into sync from time to time. Like a hit and miss governor ?... Chris In this case I have to (surprise!) agree with Rickman. A phase locked loop is never in lockstep with the reference - there is always a bit of drift in the oscillator. It's no different than driving down a highway. You can aim your car straight down the road - but you need to continually make small adjustments to account for things like the road and the wind. The Shortt clock is not that much different, except that it purposely runs at a slightly lower frequency than the reference, and the frequency at which the comparison occurs is much lower. But the result is the same - a signal that is accurate due to compensation based on the instantaneous phase at specific times. -- ================== Remove the "x" from my email address Jerry Stuckle ================== |
A mechanical phase locked loop!
Jeff wrote on 8/4/2017 4:58 AM:
I don't know what you guys are seeing. The two pendulums of the Shortt clock are in lock step. The fact that they are only compared every 30 seconds does not change the nature of the design. What we are seeing is that even after the 30 second 'kick' the 2 pendulums are NOT in phase. They may well be 'a bit closer' in phase, but the kick just moves the difference a fixed small amount in one direction, which may be sufficient to bring the phases closer, or it may be too much and go through the in phase point. With the design there is no time where the 2 pendulums are *held* in phase. The design in fact relies on the fact that the phase of the 2 pendulums is constantly changing. As is true for any PLL. -- Rick C |
A mechanical phase locked loop!
In rec.radio.amateur.homebrew Jeff wrote:
I don't know what you guys are seeing. The two pendulums of the Shortt clock are in lock step. The fact that they are only compared every 30 seconds does not change the nature of the design. What we are seeing is that even after the 30 second 'kick' the 2 pendulums are NOT in phase. They may well be 'a bit closer' in phase, but the kick just moves the difference a fixed small amount in one direction, which may be sufficient to bring the phases closer, or it may be too much and go through the in phase point. With the design there is no time where the 2 pendulums are *held* in phase. The design in fact relies on the fact that the phase of the 2 pendulums is constantly changing. Jeff https://en.wikipedia.org/wiki/Shortt...sync hronizer "This feedback loop functioned as an electromechanical version of a phase-locked loop..." -- Jim Pennino |
A mechanical phase locked loop!
On 08/05/17 09:45, Jeff wrote:
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. 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 |
A mechanical phase locked loop!
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) |
A mechanical phase locked loop!
On 05/08/2017 14:57, Gareth's Downstairs Computer wrote:
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) I hope you have more success getting a copy of those books than getting the PA tuning instructions for a mass produced amateur TX. |
A mechanical phase locked loop!
Jeff wrote on 8/5/2017 5:45 AM:
What we are seeing is that even after the 30 second 'kick' the 2 pendulums are NOT in phase. They may well be 'a bit closer' in phase, but the kick just moves the difference a fixed small amount in one direction, which may be sufficient to bring the phases closer, or it may be too much and go through the in phase point. With the design there is no time where the 2 pendulums are *held* in phase. The design in fact relies on the fact that the phase of the 2 pendulums is constantly changing. As is true for any PLL. 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. You are making pointless distinctions. A phase locked loop is not defined by its mechanics but by the nature of its control. The Shortt clock maintains the relative *phase* of the two clocks by brief adjustments to the frequency via a spring. This is controlled by measuring the relative *phase* of the two clocks. It's that simple. You are just making things more complicated by talking about the details of how the adjustment works and the time function of the frequency. NO PLL can keep the two clocks perfectly in sync. Calling it open loop is just absurd. The loop is closed because it *measures* the phase of the clocks and adjusts the phase according to the measurement. It may be binary, but the adjustment is controlled by the measurement. -- Rick C |
A mechanical phase locked loop!
Chris wrote on 8/5/2017 9:34 AM:
On 08/05/17 09:45, Jeff wrote: 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. 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. You aren't making sense. The reference is never adjusted in a PLL. That's why it's the *reference*. 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. -- Rick C |
A mechanical phase locked loop!
On 05/08/17 14:34, Chris wrote:
On 08/05/17 09:45, Jeff wrote: 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. The amplitude is not, but the frequency is - why do you think the amplitude should be related to the difference in phase? 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. Ah, yes there is, see below. 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. Are you referring to the kick given to the master pendulum? That is not part of the PLL system. The kicks given to the master pendulum are specifically designed not to affect the phase of the master pendulum at all. If not, if you are referring to the kick given to the slave pendulum (these are quite different kicks) that is not how the clock works. The slave pendulum is kicked from time to time, ad kicked a little more often when the phases get too far apart - the difference in phases is the error signal mentioned above - and these kicks do affect the phase of the slave pendulum. 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. That is exactly what a PLL is - and it is almost (though not quite) what this clock does. It is certainly what the slave does. In a pll, there is continuous Not necessarily continuous - a bang-bang action is allowable, and does not prevent a system from being a PLL. feedback from the vco to the phase detector, closing the loop and keeping the phase offset constant, A PLL does not necessarily keep the phase offset constant, just within the interval =/- 2pi. The phase is continuously updated every cycle, Not necessarily continuously updated, or updated every cycle - as long as the offset is continuously within the range -2pi to 2pi, the phases are locked. whereas the Shortt clock can have significant accumulated error in the time between corrections... Yes - but that doesn't mean it is not a PLL, as long as the error is less than +/- 2pi. A phase-locked loop is a system which produces a (slave) vibration the integral of whose phase in comparison to the phase of another (master) vibration is continuously between -2pi and 2pi over long periods. A last requirement is that the phase-locked loop system should have no effect whatsoever on the master vibration. That's it. If it does that, the phases are locked - they may not be tightly locked, but the vibrations do not skip or add beats. More advanced PLLs might keep the difference between phases much smaller, as in this clock - but that is not a requirement of a PLL. There is no such thing as absolutely tightly locked, there is only unlocked or locked. Neither is continuous updating necessary, though the integral should be continuously in that interval. In this clock the hit-and-miss synchroniser action undoubtedly does act as a PLL. However it might be argued that the slave does subsequently have some (very small) input to the master, when it operates the gravity drive (whuzzat? I am not a clockmaker). That certainly has an effect on the amplitude of the master; although as the idea an intention and practical effect is that it has no effect whatsoever on the phase of the master, thus the slave clock action overall most definitely should be considered a PLL. -- Peter Fairbrother ps; the +/- 2pi bit is not really a requirement either, as long as the system can keep count of the missing/extra beats - but as most systems don't do that we shall just gracefully ignore that for now .. |
A mechanical phase locked loop!
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. -- Rick C |
A mechanical phase locked loop!
Peter Fairbrother wrote on 8/5/2017 11:01 AM:
On 05/08/17 14:34, Chris wrote: On 08/05/17 09:45, Jeff wrote: 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. The amplitude is not, but the frequency is - why do you think the amplitude should be related to the difference in phase? 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. Ah, yes there is, see below. 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. Are you referring to the kick given to the master pendulum? That is not part of the PLL system. The kicks given to the master pendulum are specifically designed not to affect the phase of the master pendulum at all. If not, if you are referring to the kick given to the slave pendulum (these are quite different kicks) that is not how the clock works. The slave pendulum is kicked from time to time, ad kicked a little more often when the phases get too far apart - the difference in phases is the error signal mentioned above - and these kicks do affect the phase of the slave pendulum. What they fail to see is that the amplitude of the kick *is* adjusted. It's just the adjustment is binary, on or off. But that is still *adjustment* and is in response to the measured phase. 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. That is exactly what a PLL is - and it is almost (though not quite) what this clock does. It is certainly what the slave does. In a pll, there is continuous Not necessarily continuous - a bang-bang action is allowable, and does not prevent a system from being a PLL. feedback from the vco to the phase detector, closing the loop and keeping the phase offset constant, A PLL does not necessarily keep the phase offset constant, just within the interval =/- 2pi. Not only that, but if you examine the equations for a PLL you will find it is *impossible* to maintain a constant phase offset with any variations in the reference or noise in the system. The phase is continuously updated every cycle, Not necessarily continuously updated, or updated every cycle - as long as the offset is continuously within the range -2pi to 2pi, the phases are locked. whereas the Shortt clock can have significant accumulated error in the time between corrections... Yes - but that doesn't mean it is not a PLL, as long as the error is less than +/- 2pi. A phase-locked loop is a system which produces a (slave) vibration the integral of whose phase in comparison to the phase of another (master) vibration is continuously between -2pi and 2pi over long periods. A last requirement is that the phase-locked loop system should have no effect whatsoever on the master vibration. That's it. If it does that, the phases are locked - they may not be tightly locked, but the vibrations do not skip or add beats. More advanced PLLs might keep the difference between phases much smaller, as in this clock - but that is not a requirement of a PLL. There is no such thing as absolutely tightly locked, there is only unlocked or locked. Neither is continuous updating necessary, though the integral should be continuously in that interval. In this clock the hit-and-miss synchroniser action undoubtedly does act as a PLL. However it might be argued that the slave does subsequently have some (very small) input to the master, when it operates the gravity drive (whuzzat? I am not a clockmaker). That certainly has an effect on the amplitude of the master; although as the idea an intention and practical effect is that it has no effect whatsoever on the phase of the master, thus the slave clock action overall most definitely should be considered a PLL. -- Peter Fairbrother ps; the +/- 2pi bit is not really a requirement either, as long as the system can keep count of the missing/extra beats - but as most systems don't do that we shall just gracefully ignore that for now .. In a typical PLL isn't the requirement to be within +/- pi rather than 2 pi? If you exceed a range of +/- pi from the intended alignment the feedback will start to push the controlled oscillator further out of alignment potentially aligning with another cycle of the master. -- Rick C |
A mechanical phase locked loop!
In rec.radio.amateur.homebrew Jeff wrote:
https://en.wikipedia.org/wiki/Shortt...sync hronizer "This feedback loop functioned as an electromechanical version of a phase-locked loop..." ..and of course everything on Wikki is correct!!! Jeff The usual cry of those who have not bothered to do any research on a subject and are shown a Wiki article that contradicts their position is that Wiki can be edited by anybody. Wiki is more correct than most of the babble on USENET and the Wiki article has 18 external references to back it up. Where is your annotated list of references? Here's another site that says the same thing: http://www.meccanotec.com/shortt.html "The slave is kept in synchrony with the master in a phase locked loop." -- Jim Pennino |
A mechanical phase locked loop!
On 05/08/17 16:19, rickman wrote:
Peter Fairbrother wrote on 8/5/2017 11:01 AM: On 05/08/17 14:34, Chris wrote: [.. The slave pendulum is kicked from time to time, and kicked a little more often when the phases get too far apart - the difference in phases is the error signal mentioned above - and these kicks do affect the phase of the slave pendulum. What they fail to see is that the amplitude of the kick *is* adjusted. It's just the adjustment is binary, on or off. But that is still *adjustment* and is in response to the measured phase. Yup. Compare with pwm (pulse width modulation) or ppm (pulse position modulation) - I forget what the actual modulation in the clock is called, but it is just another modulation, despite being binary and fixed in amplitude. A PLL does not necessarily keep the phase offset constant, just within the interval +/- 2pi. Not only that, but if you examine the equations for a PLL you will find it is *impossible* to maintain a constant phase offset with any variations in the reference or noise in the system. Indeed.. in some ultimate sense, perhaps that is the final purpose of a PLL. ps; the +/- 2pi bit is not really a requirement either, as long as the system can keep count of the missing/extra beats - but as most systems don't do that we shall just gracefully ignore that for now .. In a typical PLL isn't the requirement to be within +/- pi rather than 2 pi? If you exceed a range of +/- pi from the intended alignment the feedback will start to push the controlled oscillator further out of alignment potentially aligning with another cycle of the master. Yes, in a typical PLL - however I was considering a more theoretical one where eg the phase offset was known to be positive or negative. On reflection, is a system where the phases are several full cycles out-of-phase, but where the system over time adjusts the slave to (close to) the actual phase of the master, still a PLL? On further reflection, I think it must be - so perhaps a better definition might be that the integral of the phase difference remains close to zero over long periods time (while leaving how close and how long as an exercise for the reader) :) . -- Peter F |
A mechanical phase locked loop!
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 |
A mechanical phase locked loop!
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 |
A mechanical phase locked loop!
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 |
A mechanical phase locked loop!
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! |
A mechanical phase locked loop!
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 |
A mechanical phase locked loop!
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. |
A mechanical phase locked loop!
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 |
A mechanical phase locked loop!
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 |
A mechanical phase locked loop!
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 |
A mechanical phase locked loop!
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. |
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