Deleted
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ATO
Jun 29, 2021 22:09:50 GMT
Post by Deleted on Jun 29, 2021 22:09:50 GMT
I've always thought about this. When the train is approaching a 35kmh speed limit (crossover, etc.) The train begins to brake, then accelerate for about 1 or 2 seconds then returns to brake again. Anyone know why this is?
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ATO
Jun 29, 2021 23:22:27 GMT
Post by Deleted on Jun 29, 2021 23:22:27 GMT
I've always thought about this. When the train is approaching a 45km speed limit (crossover, etc.) The train begins to brake, then accelerate for about 1 or 2 seconds then returns to brake again. Anyone know why this is? Adding onto this - it appears places like Loughton have a speed limit. But the points going onto Roding Valley don't have any sort of speed limit? Why is this?
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ATO
Jun 30, 2021 7:29:02 GMT
Post by brigham on Jun 30, 2021 7:29:02 GMT
It's known as 'hunting', and is a phenomenon of feedback control systems in general. The governor finds the required speed by establishing the 'limits', then settling between them. In naval gunnery terms, it is the equivalent of straddling the target.
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class411
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ATO
Jun 30, 2021 8:43:36 GMT
Post by class411 on Jun 30, 2021 8:43:36 GMT
It's known as 'hunting', and is a phenomenon of feedback control systems in general. The governor finds the required speed by establishing the 'limits', then settling between them. In naval gunnery terms, it is the equivalent of straddling the target. Hunting should not apply in the middle of a period of acceleration or deceleration. It should only apply when a controller has reached its target state. A correctly functioning automatic train should never hunt to the degree that it would be noticeable by passengers. It would only do so if the increments in acceleration control were so large that the hysteresis needed to be set too wide.
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Deleted
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ATO
Jun 30, 2021 9:37:19 GMT
Post by Deleted on Jun 30, 2021 9:37:19 GMT
As seen here. The train begins to brake then accelerate, then returns to brake.
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class411
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ATO
Jun 30, 2021 10:06:53 GMT
Post by class411 on Jun 30, 2021 10:06:53 GMT
As seen here. The train begins to brake then accelerate, then returns to brake. That is not hunting.
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Deleted
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ATO
Jun 30, 2021 10:24:10 GMT
Post by Deleted on Jun 30, 2021 10:24:10 GMT
As seen here. The train begins to brake then accelerate, then returns to brake. That is not hunting. So why would it do it? It appears everywhere with a 35kmh speed limit. Why is it accelerating? Why not just brake without the acceleration?
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class411
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Post by class411 on Jun 30, 2021 10:51:35 GMT
So why would it do it? It appears everywhere with a 35MPH speed limit. Why is it accelerating? Why not just brake without the acceleration? It's either faulty, or it's because of some input from the signalling system. An automatic train control system can monitor the speed of the train over extremely short intervals of time - fractions of a second, certainly. During each interval the change in the speed of the train, and hence its distance from the target speed, will be very small and can probably be handled by making the smallest possible adjustment to the motor output. To design a system that allowed the train to slow down to such an extent that it needed to release the brakes and accelerate would imply an appalling level of incompetence and would certainly never be presented to a customer. When accelerating or decelerating the control system will be continuously calculating an optimal power or braking profile and, similarly to the steady state situation, only very small adjustments to power or braking effort should be needed. I have noticed manually driven trains do the brake - accelerate - brake profile, where the driver realises s/he's overcooked the braking, although it's far more likely to be brake - coast - brake. n.b. I'm obviously not a railway engineer, so the above is based on knowledge and experience of general control system theory. If cars and planes can have auto-throttle systems that maintain an extremely accurate speed, it would be bizarre in the extreme if trains had noticeably inferior systems.
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Deleted
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ATO
Jun 30, 2021 11:46:10 GMT
Post by Deleted on Jun 30, 2021 11:46:10 GMT
So why would it do it? It appears everywhere with a 35MPH speed limit. Why is it accelerating? Why not just brake without the acceleration? It's either faulty, or it's because of some input from the signalling system. An automatic train control system can monitor the speed of the train over extremely short intervals of time - fractions of a second, certainly. During each interval the change in the speed of the train, and hence its distance from the target speed, will be very small and can probably be handled by making the smallest possible adjustment to the motor output. To design a system that allowed the train to slow down to such an extent that it needed to release the brakes and accelerate would imply an appalling level of incompetence and would certainly never be presented to a customer. When accelerating or decelerating the control system will be continuously calculating an optimal power or braking profile and, similarly to the steady state situation, only very small adjustments to power or braking effort should be needed. I have noticed manually driven trains do the brake - accelerate - brake profile, where the driver realises s/he's overcooked the braking, although it's far more likely to be brake - coast - brake. n.b. I'm obviously not a railway engineer, so the above is based on knowledge and experience of general control system theory. If cars and planes can have auto-throttle systems that maintain an extremely accurate speed, it would be bizarre in the extreme if trains had noticeably inferior systems. Thanks for that. It's been going on for a long while now and I've just been wondering!
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class411
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ATO
Jun 30, 2021 11:56:47 GMT
Post by class411 on Jun 30, 2021 11:56:47 GMT
I suspect that if it occurs repeatedly, what is happening is something like this: (I'll use the term 'signal' for a whatever the signalling system uses as a target point where a colour light signal would have been placed.)
1) A signal ahead is not clear, so the train is braking. 2) The signal clears, so the train starts accelerating to line speed. 3) The next signal along is not clear, so at the appropriate point the train once again starts braking.
This could happen multiple time between stations.
I dare say an LU professional will give a more accurate description of what's going on at some point.
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Deleted
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ATO
Jun 30, 2021 13:09:39 GMT
Post by Deleted on Jun 30, 2021 13:09:39 GMT
I suspect that if it occurs repeatedly, what is happening is something like this: (I'll use the term 'signal' for a whatever the signalling system uses as a target point where a colour light signal would have been placed.) 1) A signal ahead is not clear, so the train is braking. 2) The signal clears, so the train starts accelerating to line speed. 3) The next signal along is not clear, so at the appropriate point the train once again starts braking. This could happen multiple time between stations. I dare say an LU professional will give a more accurate description of what's going on at some point. Not sure about the signals, places like Loughton, Liverpool Street and Holborn it happens in the exact same spot.
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ATO
Jun 30, 2021 15:03:53 GMT
Post by 100andthirty on Jun 30, 2021 15:03:53 GMT
Sometimes train accelerate up to the speed limit, then the speed limit changes to a lower value, and finally the speed limit rises again just before a station stop. For example, the journey from Bank to Liverpool St sees a typical train accelerating into Liverpool St station before it brakes to a stop. This is quite normal and is done to minimise headways.
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Tom
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Post by Tom on Jun 30, 2021 15:52:30 GMT
So why would it do it? It appears everywhere with a 35MPH speed limit. I'm very impressed that you can identify it's where there is a 35mph speed limit. Especially considering that speeds on the central line aren't measured in mph.
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ATO
Jun 30, 2021 16:03:17 GMT
Post by Deleted on Jun 30, 2021 16:03:17 GMT
So why would it do it? It appears everywhere with a 35MPH speed limit. I'm very impressed that you can identify it's where there is a 35mph speed limit. Especially considering that speeds on the central line aren't measured in mph. I have realised I got my units wrong! It's actually KMH. Sorry about that!
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Post by notverydeep on Jul 1, 2021 12:43:00 GMT
The point where the linked video starts appears to be as the train approaches then runs across the crossover between Bank and Liverpool Street EB. As the permenant speed restriction across the crossover ends, the higher permited speed allows the ATO to reaccelerate the train (as the train enters the platform) until it reaches the point where it has to brake to slow to stop at the stop mark. It is likely that many manual drivers would not reaccelerate at this point as the train is entering the platform. This is therefore a good demonstration of one of ATO's key benefits. This reacceleration will reduce the run time across a very busy section, helping to make many journeys a little bit quicker.
This speed profile is also likely to reduce the reoccupation time at Liverpool Street EB This is a busy platform and will have a typically high peak dwell time and could become a throughput / capacity constraint without a fast reoccupation time. Reoccupation time plus expected dwell plus recovery gives the available headway for a given location / time and the worst of these will be the 'constraint location' that will determine the frequency that can be planned...
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ATO
Jul 2, 2021 7:34:44 GMT
Post by brigham on Jul 2, 2021 7:34:44 GMT
The ability of the system to do the rail equivalent of 'accelerating towards red traffic lights' does NOT sound like a 'key benefit'. It sounds like crass stupidity.
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class411
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ATO
Jul 2, 2021 8:24:50 GMT
Post by class411 on Jul 2, 2021 8:24:50 GMT
The ability of the system to do the rail equivalent of 'accelerating towards red traffic lights' does NOT sound like a 'key benefit'. It sounds like crass stupidity. That was my first thought; a terrible waste of energy. But then I realised that there must be many sections of track that can be in contention, and on occasion it might be considered worthwhile to use extra energy to clear, say, a junction, to avoid delays to following services. One advantage of a computerised high level control system is that it could (I've no idea if it does), make a decision on an acceleration as described, on the basis of proximity of other traffic to the asset in question, and whether or not the subject train is on schedule.
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Tom
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Post by Tom on Jul 2, 2021 15:34:20 GMT
One advantage of a computerised high level control system is that it could (I've no idea if it does), make a decision on an acceleration as described, on the basis of proximity of other traffic to the asset in question, and whether or not the subject train is on schedule. The older ATO systems (Original Victoria, Central) couldn't do this - they simply drove the train at the highest speed possible based on the codes they received from the signalling system - though the original Victoria Line could coast on some sections as required. I'm not sure how refined the 'new' Victoria line is but I believe it to be similar. SelTrac does exactly what you describe; for each train the system looks at where things are around it, and how that particular train is doing against the timetable. If it needs to speed up a late running train, it will do so, where possible. If a train is on time or early it will hold a train or run it at a lower speed to keep the service to time.
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ATO
Jul 2, 2021 17:54:21 GMT
Post by MoreToJack on Jul 2, 2021 17:54:21 GMT
SelTrac does exactly what you describe; for each train the system looks at where things are around it, and how that particular train is doing against the timetable. If it needs to speed up a late running train, it will do so, where possible. If a train is on time or early it will hold a train or run it at a lower speed to keep the service to time. To an extent! It won't speed up an early/on time train if there's a late train behind it, for instance. One of the pitfalls of SelTrac if junctions are in First Come, First Served (FCFS) and the signallers are dealing with something else and thus unable to intervene.
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North End
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ATO
Jul 2, 2021 22:17:04 GMT
via mobile
Post by North End on Jul 2, 2021 22:17:04 GMT
One advantage of a computerised high level control system is that it could (I've no idea if it does), make a decision on an acceleration as described, on the basis of proximity of other traffic to the asset in question, and whether or not the subject train is on schedule. The older ATO systems (Original Victoria, Central) couldn't do this - they simply drove the train at the highest speed possible based on the codes they received from the signalling system - though the original Victoria Line could coast on some sections as required. I'm not sure how refined the 'new' Victoria line is but I believe it to be similar. SelTrac does exactly what you describe; for each train the system looks at where things are around it, and how that particular train is doing against the timetable. If it needs to speed up a late running train, it will do so, where possible. If a train is on time or early it will hold a train or run it at a lower speed to keep the service to time. The new Vic Line system originally worked by looking at how early the train was, and for each section calculating a coasting point, from where the train would coast to the next station. Since then I know there’s been at least one software change, and I don’t really understand how it works now - indeed the trains now seem to be driven in an outwardly odd way, but still seem to keep time so a plan must have come together somewhere! The TBTC as found on the Jubilee and Northern is quite primitive. Like the Vic it looks to see if a train is going to arrive early at the next station, however in this case it simply imposed a speed ceiling which it rigidly maintains. There is a “constant headways” feature which was used extensively during the first lockdown when a special service was being run, from experience with that it doesn’t seem to work very well in practice. Certainly during the lockdown service, with the odd slow driver added in it quickly turned into a very gapped service, with the signallers having to babysit it in a quite hands-on way, the opposite of what is supposed to happen!
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