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| Re: TA is obsessed with CBTC, and ''New'' tech for no reason. (578801) | |||
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Re: TA is obsessed with CBTC, and ''New'' tech for no reason. |
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Posted by Stephen Bauman on Sat Mar 1 09:16:28 2008, in response to Re: TA is obsessed with CBTC, and ''New'' tech for no reason., posted by Jeff H. on Sat Mar 1 00:08:11 2008. Any system which relies on fixed trips is what is called an "intermittent train control" system...Traditional cab signal systems with ASC are continuous train control, as is CBTC.That's a bit misleading because all are sampled data (discrete) systems. The position data is discrete, in that the data is the same regardless of where a train is within a block. The data can change only when the train crosses an insulated joint. Similarly, the instructions for a follower can change only when the follower is about to cross an insulated joint. So, where's the safety benefit for cab signals? The instructions will change the instant the follower crosses an insulated joint. This is usually where signals and trippers are placed for an all wayside system. Any benefit would be in allowing the follower to speed up more quickly, should the leader put more distance between them. Even here the benefit for a "continuous" cab signal is an illusion. T/O's are permitted to peek out the window and look at the aspect of the signal ahead of them. CBTC data is also discrete, even with a continuous block system. The sampling is in time, not distance. The control system knows the status of the leader and follower. However, these statuses were taken at different times. The control system must take into account the non-simultaneity of the data. One can equate the margins required for these time differences to those for a block system. Not surprisingly, CBTC is equivalent to a fixed block system with 100 to 150 foot block lengths. The average distance between signals on NYCT is 350 feet. The usefulness of such a small block length is questionable, considering how infrequently the distance between leader and follower needs to be within this range. Traditionally, timers were designed to "prove" that the train is under control and operating below a certain speed. It was assumed that, e.g. when approaching a bumping block, the motorman would not decide to commit suicide and wrap up the ccontroller the instant that the last stop arm goes down. The new signal design guidelines pretty much make that assumption, to the point that they are attempting to apply continuous train control principles to an intermittent system. Perhaps, the signal engineers should defer to the EE's who have contended with sampled data systems since the 1920's. The question of the suicidal T/O can be solved by using the Nyquist Sampling Theorem. Acceleration is analogous to maximum frequency. That determines the number of timers and their positions before the final tripper before the block. |
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