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.
Sunday, April 18, 2010
Because Earth Day is coming up I decided to bring up a point that needs to be driven home again and again to a world that just doesn’t get it. I have resurrected (and added to) a drawing from my second post to help make it.
The point is this: The automobile/road system, as a primary transportation means, is so inefficient that scrapping it and replacing it with something better, like PRT, would simply transform the world.
This is a hard sell, because we have hundreds of years of societal conditioning telling us that we are doing the right thing. Paths became roads became highways. Wagons became cars. Everything seems to be advancing. We have become experts in roads designed for heavy freight and fast passenger vehicles designed for those overbuilt roads.
But it’s more than the roads being hugely overbuilt for the 1.2 passengers carried by the average car. It is the whole tradition of terrestrial travel. If we look at the process of moving people into and out of a city as an industrial machine the inefficiencies become abundantly clear. For example, can you imagine designing a factory where two assembly lines cross, so only one can work at a time, and you have to alternate production from one to the other? This is obviously a horrible, ridiculous design, one that would cut productivity in half. Yet we do the same to ourselves every day with stoplights. Don’t even get me started about coming to a complete stop at empty intersections with stop signs! A partial solution was found with the introduction of overpasses and the cloverleaf, an innovation that revolutionized road travel. But traditional roads are too costly to elevate except where absolutely necessary.
In the factory example, the design would be summarily rejected because of the effect it would have on the bottom line. But what about YOUR bottom-line? Somewhere along the way we seem to have forgotten that systemic societal efficiency brings prosperity. What is holding us down economically? Every stoplight. Every stop sign. Every traffic jam. Every accident. Every traffic cop, and every ticket. Every tow truck. Every parking lot. Every flat tire. Every oil change. Every insurance payment. Every car note. Every license renewal. Every pot hole, every drop of gasoline, and every minute spent pumping it. You and I are paying for this and much more. And for those of you that do not know, an automobile engine is, at best, 20% efficient in the first place. (when stuck in traffic it is 0% efficient) It’s all money and time thrown down the rabbit hole of an archaic system.
I know we will still need roads, especially for heavy trucks and interfacing with rural communities. But consider the tax revenue that would be gained by even returning 10% of a city’s streets back into commercial use, and how much taxpayer money would be saved by cutting back on the constant road widening. Or, from an Earth Day perspective, consider the “green-space” and bike trails you could get out of the deal, not to mention the 80% reduction in energy use per passenger-mile.
Each major advancement in transportation technology has historically ushered in a bright new economic cycle lasting decades. We could sure use that right about now…especially if the boom was also a way out of this (climate-change/dwindling reserves) pickle we’re in.
Sunday, April 11, 2010
Many readers might want to know what it is like to live the life of a famous PRT blogger/designer…
“How do you handle the glamour, the prestige?” Some might ask. I would like to assure you all that I put on my pants one leg at a time just like you. Away from the spotlights, amazingly, sometimes my PRT work can even be a bit, well, tedious. That’s right. Tedious. Why, in the course of writing the following piece I actually dozed off! P.S., Sorry, my non-American friends, for all the non-metric units but I want get this out… I’m getting.. (yawn) sleepy….
I have been looking into smaller wheel diameters because I really think full-sized motorcycle wheels are too much of good thing. True, they are capable of 120 mph speeds, and the larger wheel diameters improve the rpm/velocity ratio, leading to longer bearing and tire life. There is also the matter of motor availability. There are a fair number of stock motors in the 3 to 7 kilowatt range (hub and torque motors) that tend to be designed for RPM ranges that are bit slow. Making the wheels bigger makes the vehicle go faster for a given motor speed.
But the exercise of the last post demonstrates the downside, that the full system height is a tight squeeze between the floors of a standard building. The previous design was 10’ 9” tall and getting that down even a few inches would be worthwhile. Furthermore the same is true of freeway overpasses.
With this in mind I started looking for very big scooters or small motorcycles to use in a smaller version of the system shown in post 74. While 16” and 17” are the norm for rear motorcycle rims, a Honda CN250 “maxi-scooter” has 10” rims. More importantly it has a weight of 346 lbs with no riders and a top speed of 72 mph. Since most of the weight on a scooter is to the rear, even without a couple of passengers, it seems safe to say that tires capable of handling the needs of a 4-5 passenger, single bogie, highway speed-rated PRT vehicle are already on store shelves in sizes down to 10”. The smallest scooter wheel that I have found so far that is rated for 100 mph is the Burgman 400’s, at 13.”
There is more to this than using stock tires. There are the wheel bearings. Here I have to admit to being behind the times. Improving manufacturing techniques and material science continue to change the rules. It used to be that bearings for our application would need to be roller bearings, probably tapered, and that they would tend to overheat. I have just finished looking at a bunch of videos of motorcycle bearing replacement, (including the heavy, small wheeled but over 100 mph Burgman 650) and it looks like they are running on simple sealed ball bearings. Hmmm. Ceramic balls, super finished, super hard steel races… Onward and upward! The bottom line is this. The large wheel size that has driven in my track designs has always been a guideline more than a rule. It would now appear that off-the-shelf tires and ball bearings can be had that are designed for 2000 rpm, 450 lb. per wheel applications. Therefore there is little justification for a 36” high track, at least in town. What I am shooting for is a finished vehicle-plus-track height of 10’, without compromising speed or comfort. I would note, however, that existing products, like motorcycle bearings or tires, do not necessarily represent the last word in what can be done technologically. Just because there is currently no 9” 120 mph, 500 lb. tire doesn’t mean it would be difficult to make. I just would prefer not to base a track standard around it. My inclination at present is to go with a 10” rim, (16” outside diameter) even though there may not be any tires currently available which are rated for speeds over 72 mph. Sometime down the road…err … track, if PRT on this track standard ever takes off, someone will make such a tire. The same goes for motors. Nearly every manufacturer advertises that they will design and build to your specifications and needs. Still, a preliminary design needs to have some solid basis for its dimensions…
One other… another…. the.. zzzz zzzzzzzzzzz zzzzzz…..the zzzzzzz……
Posted by Dan at 10:14 PM
Sunday, April 4, 2010
I guess I have come up with a solution, of sorts, to the problem I posed in last week’s post. By introducing a rotating joint where the swing arm attaches to the cab, the cab can be pivoted sideways. This gives the clearance required for vertical travel. It also opens up some interesting station design options, such as the closely spaced front boarding shown in this video for Monic PRT.
The picture above shows the indoor station problem to scale. The rail here is fairly large, (nearly 36” tall) and is the high-speed design shown in post 74. The vehicle is 63” tall at the pivot point. This cannot be reduced much without making the seats too low or sacrificing headroom. (No, the rail is not part of the back wall; the viewing angle just makes it appear that way...)
The ceiling as shown is 10’ 9”. Luckily, most modern buildings have more distance than this between floors. That makes the bottom of the track at 7’9”, barely within reach of the average adult. Because being able to reach it at all is a bit troubling, work continues on trying to find the best way to shave a few inches off of that 36” dimension.
In the second illustration the cab has been turned sideways to the track in preparation for descent and in the last it is shown on a vertical track section. Note that the swing arm must be at least half of the width of the cab, but not so long as to push the overall height of the system higher than is necessary to keep track out of easy reach, lest the system be too tall to fit between the floors of most buildings.
One thing I would like to accomplish with these designs is to create a system architecture that enables a business model that is not as reliant on busy stations. I understand that previous designers have had to keep in mind that “the squeaky wheel gets the grease,” so any initial system is likely to be for a very busy area. Also, in the beginning, limited funding will mean that PRT will have to prove itself with a minimum of both track and stations. Still, PRT’s main strength lies in concept of point-to-point travel, and that means lots of stations. Trying to do otherwise is like having a taxi service that only goes to and from a few locations. We already have that; they’re called shuttles, and are most efficient when transporting larger groups.
Reducing the cost of the stations is a main factor favoring hanging vehicles over bottom-supported designs, which require extensive means to keep people away from the track, like elevators, gates, fences, etc. Cheaper stations will eventually pay off in higher ridership. Can you imagine, for instance, public buses trying to operate with two thirds of the bus stops removed? Who would want to walk that far, both before and after the ride, and presumably on the return trip as well? PRT is no different. It is unavoidable that initially PRT will have to start in an environment where shuttles would be competitive, but it is unwise to create a system architecture that is only economically viable in these situations. This is particularly true in many U.S. cities, where activities like shopping and entertainment are often done very far from the city center. By the way, the larger track size shown is fully compatible with larger group (GRT) vehicles, if it can be shown that they would be more effective in certain routes. The vertical or steep slope travel capability, however, is PRT only, and aimed primarily at situations where a small footprint is needed, yet the ridership isn’t sufficient to justify an elevator equipped station. Sharing space with a bus stop comes to mind.
Finally, I have to acknowledge the differences with European hanging designs, which have no swing-arm at all, although they have similar track heights, because they tend to be taller, walk-in style vehicles. A quick “walking view” tour in Google Maps of various European city centers reveals huge masses of pedestrians compared to the U.S… many, many times more. I can certainly see that a lower speed, inner-city centric approach could make a lot of sense there. While we struggle with “urban renewal” projects here in the U.S., it appears that they’ve “been there, done that,” in Europe, perhaps hundreds of years ago! (Some of the oldest U.S. cities seem to have more vibrant downtowns as well, and being constrained by water seems to help somewhat.) The whole structure of many American cities, especially in the fast growing “Sunbelt”, is about growth along freeways, giving the cities long tentacles of urbanization. The designs I have shown reflect this landscape.
Posted by Dan at 10:34 PM