Wednesday, November 30, 2011
I know I said that my next post would be about standards, but I think this topic should come first since it weighs into that discussion. There has been something in the back of my mind, in every design that I have posted on this blog. It is the recognition that progress often happens in baby steps, yet baby steps often don’t go in a straight line or a logical direction.
I am talking about forward compatibility. The vexing thing about designing PRT (or anything else) to be forward compatible is that it requires a design that allows evolution toward an end that must first be defined itself. In the case of PRT, that eventual product would ideally be very fast, silent, comfortable, able to be deployed with minimal cost, and be adaptable for every contingency a city could throw at it. These adaptions would include a variety of station types for different locations and passenger volumes, being practical within buildings, being able to be elevated to a level deemed acceptable by the effected parties, etc. The vehicles themselves can be assumed to be, in this future world, mass produced to point of enjoying economies of scale.
I am sure that many readers have dismissed my designs as being too ambitious. The vehicles, in particular, have very advanced electro-mechanical systems which are designed to remedy situations which could largely be avoided in the first place with little negative impact. Well now you know the reason. By designing the vehicles as though Toyota had been building them for decades, one can better consider the best design for the track and stations that may still be around when that day comes.
In the meantime, however, PRT will be subject to the restrictions that our current economic, political and technological realities place on it. The problem is that designs for today and designs for tomorrow are strikingly different. The main culprit is the tire wear and noise/vibration associated with speed. It is true that smooth running surfaces and track clamping emergency braking capability enable harder rubber, solid tires which don’t need to flatten out on pavement to achieve the high traction requirements associated with gripping slippery roads. Still, highly wear resistant plastics, such as are seen in rollercoaster wheels, are a recipe for a very noisy system at high speeds, especially on pipe, which is notoriously good at amplifying sound. (This is the basis for many musical instruments)
I should point out that the design I show in the Oct. 30 post enables both steering guide wheels to be raised for high speeds, eliminating contact (and therefore noise and wear) by the plastic flanges. Also, these flanges are to rotate independently of the drive wheels, so that they may make contact with the track (pipe) anywhere on their surface and create their own rotational speed based on the diameter established by that point, rather than the smaller diameter of the tire. This reduces wear on what is a tiny contact point.
The main point remains, however, that high speed systems should have larger wheels (OK, not Maglev) or risk lots of wear and/or lots of noise. A system that requires wheels or tires to be changed every few thousand miles would be a disaster. But longer wearing, larger tires means bigger track, something that is nearly as bad, in that it raises the cost and visual impact of a system which will, in the beginning, be under intense scrutiny from critics. Also initial systems will probably be slower anyway, because such trial systems will have to first prove themselves for short-distance downtown use.
Could there be a two tier system? Would it be crazy to start with a system for downtown that would preclude high speed vehicles? I know it sounds like a terrible idea, but it would probably shave 20% off of the track costs, and vehicles would be discounted considerably more. And let’s face it. The other PRT systems out there aren’t exactly fast or flexible either.
In the illustration above, the system on the left, which is obviously simplistic and incomplete, would only need to raise and lower the small pairs of wheels to steer. I do not believe that the middle “hold-down” wheels (illustrated in previous posts) would be required, so that’s really all there is. There are inexpensive “off the shelf” hub motors available that would fit in the drive wheels, and the flange and hubs could be cast as one, (in urethane) so that solid rubber tires would slip on. Such a bogie would be extremely cheap to produce. The complex (expensive) articulation capabilities of the swing-arm and gondola could also be dialed back in such a “starter” system.
The obvious problem comes from the fact that the fast vehicles wouldn’t fit in the smaller track, although the slow vehicles could run in the high speed track. So what is a city to do? Well, there are a couple of things to note here. First, the high speed track would be equally usable for GRT. (Group Rapid Transit.) A track going out to an airport, for example, might well be a good stand-alone investment used in this way. Passengers coming from the airport to downtown would need to change vehicles to use the downtown PRT, but the upside is that they didn’t have to make the long trip at 30 mph. Faster, express PRT could share the track at some point, and slow vehicles could use the track at certain times of day. Because PRT is a smart technology, if high-speed track is running through a grid of low speed track, the slow vehicles could still, in theory, get on and off without disrupting the high speed service. (Assuming sparse high speed traffic) If the track is modular, standardized and interchangeable, the slow track could be removed (during an upgrade) and be reused elsewhere. In the airport example, for instance, slow track taken from downtown could be used to build a network around that airport. In such a case changing vehicles would be a minor inconvenience for relatively few passengers. It is also noteworthy that, in a downtown environment with mixed track, fast vehicles can’t get up to speed anyway, because of sharp turns. Therefore slow vehicles sharing the (fast) track would be no problem. In such a case the system could simply send a slow vehicle if a trip would involve a stretch of slow track.
I have come to the conclusion, reluctantly, that there are theoretically reasonable migration paths from slower, inexpensive PRT to faster systems capable of tackling longer distance commuter traffic. The examples above are just a sample of the possibilities. They also show that it takes some creativity to undo what many of us would say is a very shortsighted decision. (To put down track that can’t take fast vehicles) But at least it’s better than having no forward compatibility at all! It is unfortunate that such a complicated situation should ever exist in the first place, but I have my doubts that we can ever get to PRT 2.0 without first dabbling in PRT 1.0.
Sunday, November 13, 2011
Suspended PRT has the advantage (over bottom supported systems) of being well suited for very steep slopes. As I have pointed out in previous posts, I believe this attribute is essential to achieving flexibility in routing and station placement because it enables a completely three-dimensional transportation solution. Such flexibility could be important for keeping costs down and increasing ridership. I do not think that starting with the supposition that each station must handle a minimum of many dozens of passengers per hour gives the kind of flexibility that cities need. Such pedestrian-rich station locations may not be close enough to each other to make a walkable grid of coverage and getting to such strategically placed stations might well involve more car travel than the PRT trip is likely to save. This makes dirt-cheap stations very important. Bare-bones ground level boarding, say, at bus stops, is an option that is out of the question with most supported systems because the ramps would be in the way.
The picture above shows a minimal station in a park. While it clearly is a low throughput design, it also would not require a lot of passengers to pay off. Small neighborhood parks are very common in the US, often created by the real estate developers to help attract home buyers to a subdivision. Better still, they are usually located at or near a subdivision’s entrance. Such small neighborhood stations could exist along a routes that are important but aren’t part of a grid, say from an airport to a downtown area, or between two metropolitan areas that have grown together. Once residents realize the benefit of such an access point and station’s capacity becomes a problem the station could be upgraded, this time based on real-world ridership numbers. The foot-in-the-door approach to eco-friendly commuting!
This (more multi-axis) approach should also allow options that will help in the “not-in-my-front-yard!” confrontations that any new transportation infrastructure is sure to create. For example, it enables the track to be run at a much higher altitude than systems that rely on elevators or gradual ramps. Instead of a “take it or leave it” approach, there is the option of running the track above the tree tops if necessary.
None of this, however, is possible without a straight-forward and inexpensive way to engineer this capability into the system.
Here is such a method. I have illustrated it with only the relevant wheels and engageable surfaces of the track shown. The last post shows the track and vehicle in more detail. (To any newbies to the site out there, I advocate direct-drive “hub motors” which rotate around electrically fed axles, so actually the whole drive system is shown!) This design is fashioned after the rack and pinion system used by cog railways. The trick is to make it engageable and disengageable without complicated mechanisms that would drive up costs and reduce reliability. It uses two stretched lengths of modified roller chain as a rack, and a sprocket that is of much reduced diameter (compared to the wheels) to eliminate the need for a low gear for slow but powerful climbing. This method adds next to nothing to the costs or complexity of the system. Such custom links as I use here (and, indeed, whole specialty chains) are widely available because they are commonly used for material handling tasks in industrial production lines. Here is how it engages, step-by-step.
The forward wheels move off of the flat running surface, so they are supported by the flanges alone, which rotate separately and freely from the motorized, rubber-clad drive wheels. For a moment the front wheels can then spin freely as the bogie is being pushed forward by the rear wheels alone. The motor controller then increases the rotational speed of the front wheels to compensate for the smaller diameter sprocket. (The front and rear wheels must now be synchronized at two different speeds.) As the sprocket engages the chain there is near certainty that the teeth will not be in the exact position to mesh exactly. To adjust for this, there is a compressible rubber cushion (shown in green) which allows the chain to “stretch.” Additionally, it is possible to design some limited rotational play into the sprocket hub. Since we are talking about roller chain here, any occasional slipping will not “grind the gears” causing excessive wear as would be the case in a typical rack and pinion. Keep in mind that this transition is not expected to be taken at anything approaching normal operating speed. Once the chains mesh with the front sprockets, engagement is kept captive by an upper bar which presses on a freely rotatable ring (blue) mounted on the wheel’s axle. This pushes the sprocket teeth into the chain and holds them from slipping out. Although this may seem redundant because the middle, hold-down wheels do the same thing, (push down) I added this component because I was worried that the compression of the rubber tire would allow the sprocket teeth to slip, and I prefer that rubber because of noise and vibration dampening at high speeds. Anyway, at this point none of the track’s normal wheel support surfaces are necessary for the front wheels to pull or stay centered. With the front wheels engaged, the rear wheels will follow a similar sequence but without any question about meshing, since the distance between wheels can be precisely set to match the chain. Of course the whole process can happen in reverse as well, to go from chain back to ordinary track .
Some readers might be concerned that the chain and sprocket are not strong enough to be used in this fashion, especially since normally at least a half a dozen teeth of each sprocket engage a chain. Each sprocket tooth, however, will have a test strength of at least 3500 lbs. The test strength of chains of the size pictured is at least 12,500 lbs. each.
Footnotes… The design as shown is based on four drive wheels, with the hold-down wheels being free-turning. This is a somewhat arbitrary decision. (The hold-down wheels make the system “half-track capable,” meaning the vehicle can travel on the either its left or right wheels alone without possibly twisting inside the track from lack of support – an essential attribute for switching tracks without the tracks themselves having moveable parts.) These hold-down wheels could just as well share propulsion and braking duties, and so could also be configured with additional sprockets and chains themselves.
Finally, A personal opinion: I think PRT designers should take a page from the late Steve Jobs. A lot of what he did was to take existing ideas and products and redefine them by engineering them so they were a joy to the senses. He had zero tolerance for the slightly funky design choices that help rush products into the marketplace, only to limit their acceptance in the long run. When I look at the current PRT offerings out there I see such design compromises by the boatload. We, at least in the developed world, will never fully embrace any system that is clunky, bumpy, noisy or slow! We must recognize that the bar has been set very high by a hundred years of automobile engineering and try our best not to disappoint. But then again even a Ferrari can’t rescue you from the congested city streets by going straight up!