Friday, October 14, 2011
Do a PRT vehicles need a way to for people to escape in an emergency? Many seem to think so, in that I am aware of a number of systems that have stated evacuation procedures. This is problem of elevated track, since obviously if the vehicle is on the ground one can just get out, so long as the doors can open. It is particularly difficult with suspended vehicles or systems that employ track that is too narrow to walk on. This is unfortunate, because the very real advantage of being minimal and out of the way becomes a disadvantage in this case.
All of this begs the question of what can stop a PRT system in the first place. The historically contemplated mode of failure is some sort of systemic computer problem. In a system with completely centralized control, a system outage would stop all traffic. Yet Google and others have demonstrated autonomously piloted automobiles. If all PRT vehicles can be sufficiently autonomous to find their way to a station, then that would seem to rule that problem out. Advances in battery technology have made it much easier to have ample on-board backup power to get to a station, so a systemic power failure wouldn’t seem to pose a problem either.
Then there is the in-vehicle failure. It should be noted that two such failures could trap all vehicles between the two and that a single such failure requires that all vehicles must be able to operate in reverse.
With a direct drive (hub motor) system, like I advocate, mechanical failure is exceedingly unlikely. After all, the only moving part is the wheel itself, so there is no drive train to break down. Each wheel turns on its own. In-vehicle control or communications failure? It would seem that there are a number of remedies for these possibilities as well, the most obvious being a redundant backup system. After all, the cost of computer boards these days is hardly worth mentioning. I suppose a last resort would be to pulse the motors very slowly (this will make them incrementally turn a few degrees with each pulse) without the computer systems. The steering guides would be set to exit at the next ramp, and the vehicle would emit a beacon to alert other vehicles. All of this could be triggered with simple relays or even manually. Furthermore, at least in the designs I am contemplating, the vehicles’ bogies, which are located inside the track, have bumper/coupling means. They can both push and pull other vehicles.
Then there is the possibility of a break in the track, say from an earthquake or large truck collision. This is a psychological barrier as much as an actual threat, in that the idea of flying off a broken track into free fall is a particularly frightening vision. With good brakes and the right software, it seems like thus too should be manageable, unless there are multiple breaks in the track, cutting off whole sections from a station. Such a case, it should be mentioned, would foil almost all evacuation plans, even if the vehicles were riding atop a wide causeway, unless it is one with very frequent exit stairs. I might mention here the break detection system employed by Disney for their rollercoasters: The pipes that comprise the track are filled with compressed gas. A reduction in pressure means that there might be a break. I would also add that with a hanging system, one of the advantages is that stations do not require lengthy ramps or elevated stations. This would favor stations being positioned with more frequency, reducing the number of potentially stranded passengers.
Then there is fire. With the motors being separated from the vehicle as they are in a suspended system,
even if there were a large amount of flammable materials in the motor, (which there aren’t) there
is still no way it could catch the cabin on fire. What about the cabin itself? This presents the one tricky
problem. How do you stop some idiot with boxes of papers and a lighter from starting his own fire? One obvious, but partial, remedy is to have a smoke detection system which automatically sends the vehicle to the next stop. I suppose that there is also the possibility of some release of noxious fumes from a power supply or other computer component overheating or burning out. The fact that computers are ever-shrinking and
requiring less and less power seems to indicate that this won’t be a problem. I suppose, also, that it possible that a passenger might spill a bottle of ammonia or puncture an aerosol can. The need for emergency outside air seems far-fetched, but is worth at least considering when weighing design options.
So it seems like a catastrophic earthquake, multiple separate vehicle failures, or a very foolish passenger are the main causes that would require evacuation, so long as the vehicles are at least semi-autonomous and have robust back-up power. That and simple human psychology. Perhaps there needs to be a way to evacuate passengers simply to make the system more saleable. After all, the fire and police departments might see this as just another potential drain on resources. And of course there is the law. Perhaps some well-meaning politician has put “public safety first” and created a legal hurdle. If there is such a statute, I am not aware of it, but of course this would vary between countries.
If there absolutely must be an evacuation means, for a hanging system I can see a few possibilities.
One is to have some sort of extra rail all along the track where emergency vehicles could travel, unimpeded by stalled vehicles, getting access to all. This is cumbersome to engineer well, but is at least worth contemplating.
Another solution is to have a means to lower the cabin or parts of it. This could be done with a very small winch, since it doesn’t need to raise the cabin fully loaded or be in any particular hurry. With gravity assisting, cables could be lowered with the most minimal of motors, or even by gravity alone. The tricky part is how far such a system could or should go before wind starts becoming a factor. Even with telescoping scissor-action stabilizers, diagonal cabling and every other means, there is still a problem if you go high enough. There is also the matter of limited choice as to what is below. Is the terrain level? Is it the middle of a highway? In the end Bubbles and Beams video, the vehicle leaves the system via an elevator of sorts, which is little more than a pole and some cables. The arrangement looks a bit flimsy, at least for going up and down on a regular basis. Going down in an emergency, however, is a whole different matter. Perhaps such poles could be placed periodically or some of the support poles themselves could be so equipped.
A variation on that theme is something I am currently working on. It would involve a fold-out platform or seat which the track support poles could be fitted with. This could be lowered via a cable running inside of a channel.
I can also envision such cable-inside-of-a-channel lowering means that can be mounted to the underside of the track, so they could swing down. Even rudimentary (very narrow) ladders could swing down in this way.
This all then brings up to more questions: How often along a track would escape equipment be appropriate? If money were absolutely no object, there is no end to the clever things that could be miraculously folded into the track. There is also the matter of the equality of escape means. If there were ladders integrated into the support poles where would that leave the elderly or disabled? Where do you draw the line between stairs and ladders and ramps?
If this all seems a bit extreme to you, join the club. I really think that having some control autonomy with onboard backup power is enough, but I may suffer less acrophobia/claustrophobia than most. Still it needs to be figured out. If there is a “safety” feature that can packed into the package, can you imagine any elected official NOT electing to include it? Or can you imagine anyone buying a system in which such matters haven’t been adequately addressed?
Finally, a note to my readers. Lately I have been designing more and blogging less. Originally I had hoped that with enough readers, I might get some help in the design work. It appears that isn’t going to be the case, so I will no longer chase readership with frequent posts. This blog was never about entertainment, after all. As designs progress, they become more difficult to explain. There is huge difference (in the amount of time involved) between an “artist’s conception” and something that can actually be built. I am a guy who builds things, so I am not content to just leave things at that early stage of development. This does not mean I won’t ever post opinion or general interest stuff. I will when something comes to me. Better to have quality than quantity, if readership numbers are not the object. Currently I am in the middle of a whole new bogie and track design, something which I have worked on almost daily. These things take lots of time!
Posted by Dan at 9:41 PM