Sunday, April 25, 2010
I want to do a little update on my ongoing SMART project, which nowadays stands for “Standardized Modular Automated Rail Transit”. If that sounds like it encompasses a lot more than PRT, well, it does. If someone chooses to automate, standardize and modularize full-scale trains or monorails under this umbrella, more power to them. I am concentrating on specifications aimed at lighter, smaller systems.
For newcomers to this blog, here is yet one more rehash of my position and reasoning. If a start-up company tries to sell a PRT system to a municipality, it must convince that customer that it is selling both a system and a service that will be reliable for decades. Remember Worldcom? Enron? How about Lehman Brothers? PanAm? These were huge corporations with proven track records, but they, for one reason or another, went under. Who would trust a startup to change the cityscape with millions in infrastructure that only it understands, and could maintain for decades to come? The tack of starting out incrementally is equally troublesome, because a minimal PRT system loses all of its advantages. It is like the Internet with a half-dozen web sites trying to support itself with a half dozen customers. Going directly to any destination in a network isn’t very compelling if the network only has a couple of stops. A simple shuttle could do that.
I am not against PRT being supplied by a single firm. I just think it is a foolish notion that any city will accept a set of contracts that creates utter, unending dependency on the vendor, or leaves the city with infrastructure that nobody understands.
There are various ways to mitigate this problem, such as alliances and partnerships, and of course the contract language itself. One additional way, and this is what the SMART specification is all about, is to break the PRT system into manageable parts from the onset, so that those parts are easy to understand, improve and subcontract, both by the PRT vendor and, in the event of any failure to fulfill terms of a contract, by new partners. I do not think it is coincidental that the two currently awarded PRT contracts are both for vehicles that travel on a simple paved surface, in spite of efficiency and weather issues. If the deal goes south the buyers can always fall back on standard electric vehicles instead.
PRT is broadly comprised of track, stations, vehicles, and a control and communications infrastructure. In my view it would be extremely beneficial if all aspects of PRT construction and maintenance could be so divided as to be accomplished by local contractors with no previous PRT expertise. When all responsibilities and areas of expertise are co-mingled, it’s an organizational mess. This is not an easy task, however. Synchronizing high volume, high-speed vehicular merges is an example of a situation that calls for close association between control, communications, the physical vehicles and track. Some elements of centralized, integrated system architecture may, in the end, prove unavoidable. That doesn’t mean that we can’t chip away at the problem, however.
I have started with the track. One advantage that I have enjoyed in this project is that I have no deadline, no target price, no set corporate agenda. I can imagine anything and everything that might ever be required of PRT track, now or in the future, for any town or country in the world, and see if it can’t be fit in somehow. A track specification, in the broadest sense, need not exactly fit a specific vehicle or vehicle weight. It need not be designed for a specific target speed. It just needs to be versatile. Being a specification, it can rest on a foundation of broad generalities and be further constrained and defined as needed by adding additional version numbers.
The following is for hanging, gondola-style PRT in a basic box beam track. That does not mean that there is no value in doing the same for bottom mount track, or other schemes as well. I am only one guy, and I still have to try to scratch out a living whenever I’m not too busy!
Below is pictured a very short track section. (without any support structure) The red areas represent areas that take pressure from a bogie (PRT motor unit) traveling within. To see examples of how the bogie would fit, scroll down to previous posts. It should be pointed out that the areas that are gray on both sides could actually be eliminated. The red areas could be held in position by structural trusses, or by being connected to a building’s architectural structure, for example. There are, as yet, no provisions for electrical rails or rack and pinion means for steep slope travel. Such details are dependent on the specific bogie design, and therefore would be better classified under a bogie/track interface specification. (along with the running surfaces, shown in green, in the second illustration) One last thing to point out is the square tubing on top. I have made it height-adjustable, which is a must, especially when making an abrupt change of pitch, which changes the height of the bogie relative to the track.
The second picture describes the basic dimensions suggested by my research. I think it can be safely said that track built within these dimensional guidelines will always be useful and will prove very versatile. It is designed for weights up to those needed for larger group vehicles, as well as for highway speeds and beyond. For extremely high speeds fairly straight sections could probably be retrofitted to accommodate maglev technologies such as Inductrack II. It is compatible with any reasonable turning radius or pitch. It can go into buildings with average ceiling heights. A shorter “chopped off” version can be made for slower speed bogies used in material handling in factories or parcel handling or airport baggage. Such bogies would be capable of citywide travel without disrupting other traffic because of computerized routing and scheduling, probably in the middle of the night. I can even envision a very thin ceiling hugging version for indoor micro vehicles to deliver medicines, food and equipment around a hospital. Anyway, here are some “first draft” dimensions and brief comments on the reasoning that went behind them.
A - 230 mm. (9”) Considerations for this dimension are that the larger steering guide wheels contained therein will rotate more slowly, have more surface area and therefore last longer. While this is not in itself a big deal, especially since the wheels only engage when changing track, there is no particular reason to make the dimension smaller, other than to make dimensions D or E larger. See discussion of B,E.
B – 65 mm. (2.5”) This is one of the dimensions that was squeezed to get the overall height down. And it only leaves room for fairly thin, disc-like steering guide wheel. This means a small wearing surface, (made better by a 200 mm diameter) but this also has the advantage of being more aerodynamic.
C – 815 mm. (32”) This is as short an overall height as I am comfortable with. It allows a drive wheel diameter of 510 mm. (20”) This allows a 330mm. (13”) rim. This general height will enable a full system height of about 3 m. (10’)
D – 100 mm. (4”) This is dimension needs to accommodate some side-to-side movement by the “hold-down” wheel (about 50 mm) which engages it.
E - 600 mm. (24”) This dimension could be made more compact but there is no compelling reason to get it down. It adds stability by allowing a wider wheelbase, and there is also the matter of aerodynamics. If the bogey fits the track too tightly it will have to push air instead of slipping through it. This is one aspect that still needs looking into.
F – 50 mm. This should be designed to keep arms out, away from the electrical rails. I keep trying to figure out something fat that needs to go between “pod” and bogie, but it seems like a few tubes and wires are all there are. The tradeoff here is that to prevent the (bogie to swing-arm) connecting piece from bending, some thick (heavy) or stainless steel (expensive) or corrugated (complex) material would have to be used.
G – 65 mm. This is the same as B.
H- This is structural plate running crossways to the track. The size would vary depending on span and other factors. For example, if the track were attached to a ceiling this part would be of minimal size. This would not be part of any specification, but rather to be decided by structural engineers. There would probably be holes or channels in it for utilities.
I – I doubt this would be part of a main specification. There are many ways to design a bogie, and these concave surfaces are challenging to manufacture for curved track sections. They would probably be made as removable inserts. This has advantages of allowing sound (vibration) isolation and expansion joints can be produced with greater precision. There is also the matter of tire width vs. the radius of this piece. (The two would ideally be sized for each other although there is some flexibility here.)
J – 25 mm. Minimum. I would like to get that number up a bit higher.
K – 76 mm. This is a bit taller than B and G because some designs might use this space as a primary centering means, with wheels that are meant to be kept in constant contact, unlike the steering-guide wheels. Therefore larger diameter (half of E) and greater width would reduce the maintenance associated with wear on the wheel.
L – 6 x 65 mm. These are non-continuous, rubber mounted strips which taper out from the surface they are attached to at either end. They are placed leading into and out of junctions only.