Sunday, October 30, 2011
As many of you know, I have been advocating the standardization of key PRT technologies in order to allow PRT to be developed, produced and deployed by a consortium, rather than a single company. This, in turn, requires that this development be started on a basis of the most promising design approaches. I have concentrated my efforts on a suspended system, rather than the bottom supported approach, even though I suspect that the majority of readers prefer the latter. One key to why I think suspended systems represent the best way forward is referred to in my latest iteration of the acronym “SMART,” which is what I call this effort. (“Standardized Multi-axis Automated Rail Transport”)
As I have pointed out numerous times before, all “ground” transportation suffers from the same problem: Vehicles or people going in different directions will run into each other unless they stop and wait their turn. Going over or under solves this problem, but that solution is too expensive to deploy universally with current modes of transportation. That indispensable artery of modern commerce, the freeway, clearly shows how effective high-speed non-stop transportation can be. This is simply the result of what happens when a transportation system is modified to be multi-axis instead of existing on one plane - the ground. Unfortunately making multi-level (multi-axis) routing for large vehicles such as trucks and trains takes huge amounts of money and space. When it comes to multi-axis transport, smaller is better. Luckily, we mortals are pretty small.
A multi-axis automated rail transportation system is essentially a new infrastructure designed to do what the freeway can’t. Go to any street, to any bus stop, to any building. It would be designed to be faster and safer than driving, more energy efficient than the most advanced electric car, and expandable for a fraction of the cost of roads. Being natively multi-axis, a suspended system can be employed in areas where long ramps are undesirable (that’s basically everywhere) and the system can be elevated higher than would be practical for supported systems. This can minimize visual impact. While it is true that a supported vehicle can be made to self-bank and keep its cabin level on slopes, it is much more cumbersome to engineer. Vehicles with wheels on the bottom are just ill-suited for extremely steep travel, while hanging vehicles have no such problem. Traditional PRT designs require raised, elevator equipped stations because otherwise the entrance and exit ramps into the station would block driveways, be subject to climbing, and be visually intrusive. A native 3D system has no such restrictions. A suspended vehicle can either taxi in like an airplane or come down like a helicopter. This means that stations can be put nearly anywhere, and they can be very minimal and inexpensive. They do not require high traffic volume to pay for themselves, so they may be placed with high frequency, like bus stops rather than actual stations. This will increase ridership. The question is this: If we are going to build a whole new infrastructure, do we want it to be raised, single-level, multilevel with ramps, or natively multi-axis? A consideration of the various routing situations likely to be encountered in a widely deployed system leads me to believe that it would be better to have true multi-axis capabilities from the start. Anyway, here is the latest iteration of the SMART PRT vehicle concept. Hmm… How about “SMARTPOD?”….Sorta has a ring to it…
Unlike previous versions, the steering guide wheels have been moved outside and under the track. This shaves off about five inches from the track height, bringing it down to about 30 inches/76 cm. (less if it the track is hung from a ceiling.) What is shown here is a high-speed vehicle, (highway speeds and higher) designed for many tens of thousands of miles between tire changes. (hence the large wheels)
The wheel flanges are designed to outlast the tires in two ways. They turn independently of the wheels, so if they contact the track at a different diameter than the tires there will be no conflict. Secondly, they are only deployed during actual turns. Otherwise the bogey is centered by leaving both left and right steering guide wheels in the upright position. The upper “hold-down” wheels replace the upper steering guide wheels of previous designs, prohibiting any rotation of the bogey within the track. This design is extremely maneuverable with a turning radius of a mere 8 ft., including vertical turns. (The spacing between various track surfaces must vary, however.) The pictured design is missing most of the components of the bogie at this stage of development. The sprockets pictured are for vertical climbing, although I plan to adjust the sprocket size somewhat.
The track has been designed to be extremely easy to fabricate into sections that are straight or curved. There would be no problem finding shops willing to bid this work, even in small towns if the pipe bending is outsourced. Removal of a left or right truss section will not mean that vehicles cannot pass, although there is a small temporary rail that needs to be placed as insurance against any freak events that would make the whole vehicle sway with great force. I am still working the best way to attach sheathing, although the reader will note that there is a slight arc to the outer edges of the truss. This is to make light-weight metal or plastic sheathing more rigid.
Alert readers will notice an air scoop. At this point I am leaning toward liquid cooled motors. This greatly increases the performance-to-weight characteristics of the motors, and hub motors are ideally suited for this, as the copper coils that need cooling are stationary and accessible radially from where the wheel attaches to the frame. A simple little electric pump that is remote from the motor itself is all that is needed. No moving parts are added to the motor. The scoop is for a radiator/heat exchanger.
Finally, I want to emphasize that this vehicle has capabilities that go well beyond what is likely to be deployed early on. Nonetheless, I am designing with the future in mind so aspects that are practical today but foreclose later improvement can be avoided. Water cooling, high speeds and vertical climbing are features that might be expensive complications to first deployments. However I see little point in building an infrastructure project whose inherent design limitations will become apparent as soon as it is deemed a success. This is, in part, why I favor a full multi-axis approach. Future cities are only going to get more crowded and time is only going to get more precious.
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