Sunday, February 5, 2012
My Pappi said, "Son, you're gonna drive me to drinkin if ya don't stop driving that HOT ROD LINCOLN!"
As long as people build and ride vehicles, there will be those of us that want to soup them up - to push the limits of acceleration, climbing power, or, more often, just plain speed. Personally, I am a student of structural geometry. From crystal lattices to geodesic domes, I have always been intrigued by the myriad ways shapes can placed and connected to swivel, slide, rotate, hold, reinforce, depress, hinge, stretch, bend….you get the idea. I just can’t look at a mechanism without dissecting every component piece to understand exactly why each surface and dimension is as it is. So when confronted with PRT, I really just can’t help myself – especially when that geometry limits practical functionality – even to the point that the whole concept struggles to find acceptance.
It is a fact that whatever form a new technology first takes becomes the de facto standard. Currently that PRT standard is the robocar model, and so cities considering PRT are looking at those requirements and limitations and making decisions accordingly. Road racing pushes the limits of automotive design and the military pushes the limits of aviation. If I don’t push the limits of PRT design, who will? And how will anyone ever know what PRT could really do for society without someone figuring out what the technology is really capable of?
In my quest to push back PRT’s limitations, I have made the following observation. PRT has been generally designed with the assumption that the track or guideway is continuous and equal throughout, except where it forks.
It is easiest to conceive these systems from a head-on view of the track’s running surfaces and corresponding wheels, as shown in this patent illustration. (Anderson 1985) The parts get arranged to enable the greatest capability possible. Job done. But wait! Those capabilities can be exceeded, and this is how.
The track’s running surfaces can have variations of all sorts for different purposes. Furthermore, the vehicle or bogie need not use the same wheels or guides for every purpose either. When you combine those two concepts, a lot of limitations fall away and new possibilities arise. You can turn that stuffy old PRT design into a real hot rod!
Let’s start with some main drive wheels and some guide wheels to keep them on track. This bogie is for a suspended system, so it is upside down from the bottom supporting design shown in the Anderson patent. This triangular geometry is designed allow for a minimum number of the larger, longer wearing wheels that are much preferable for high speeds. (The guide wheels in the Anderson design are limited in size to one half of the track width.) There is an unfortunate side effect however. Those angled guide wheel running surfaces are cumbersome to work with in curves, especially the serpentine routing that would enable system designers maximum flexibility in tight spaces. This is where it pays to remember that the track can transition. Here is a simple transition from double angle iron to pipe. There are many other ways to accomplish the same thing.
If the whole system used pipe, like on roller coasters, the wheels would wear in the center first, and more quickly need to be readjusted or replaced. By using the first configuration for straight runs and only using the pipe for curves, the wheel life can be maximized. After all, we hot rod guys know a thing or two about going through tires! I know that some of you are thinking that some miracle plastics out there can last a very long time anyway, and so this is all a wasteful complication. The problem is that noise and vibration are (roughly) inversely proportional to wheel hardness. Softer, quieter materials will always wear more and rob a bit of mechanical efficiency but that’s just a compromise that we have to live with. I suggest making the architecture capable of whisper-quiet operation first and then going with the hardest wheels that we can get away with.
Below is the concept adapted to ride on half of the track, which avoids the notorious “frog problem.” (unsupportable track area that results from “Y” interchanges) The additional bottom wheels (shown simply floating in space) in this position must be smaller and so would be the first to wear out but for the fact that they are not needed continuously. Then there are the upper wheels, which are designed for continuous contact, but are under minimal load, and so can be relatively skinny in profile. There is a pair of retractable upper guide wheels that, like the middle bottom ones, are only used for switching. A couple of notes: The top section shows a bogie that is not in an interchange. The bottom two show the running surfaces having split further apart, as would happen in a “Y.” The sprockets shown are for climbing, cog railroad style, not for driving the wheels, which drive themselves. The wheels are all independent, including the middle flanged wheels. As always, click image to enlarge.
The point to all of this is that there is a much higher inherent speed limit to such a design. Using wheel/hub motors there is room for over 300 hp worth of pure hot rod power, and these direct drive axial flux motors are what is used to win those solar car races, being over 97% efficient. Not only that, but since electric motors only draw as much power as their task requires, there is no compelling reason to keep them small and underpowered like their gasoline driven counterparts.
This picture shows an entirely different problem. Here the bogie is making a very tight turn, far too tight to take with ordinary rigid bogie designs. Here the turn is being accomplished “skid steer” style with the middle guide wheel keeping everything centered. This is would be extremely wearing, but for the slow velocity involved.
This picture illustrates the problems with tight radius vertical turns. The height of the bogie, relative to the internal structure of the track, changes completely, being too loose in one case and too tight in the other. Luckily this is not quite as difficult to work with as one might think. During assembly, a collapsible fixture is dimensioned to match the desired internal spacing. The precut and bent steel is clamped to this for welding. Oddly shaped and unwieldy pieces are routinely clamped for welding anyway. Like I said, there’s no reason why the track has to be uniformly dimensioned.
This little design exercise is obviously about more than speed. It is about having mobility capabilities that are closer to a personal helicopter than anything else. It is my belief that such capabilities are not that complicated or expensive. I also believe that integrating PRT infrastructure into an existing (mainly privately owned) cityscape is enough of a challenge to warrant exploring these solutions sooner, rather than later. So, if anyone calls, tell them I’m in the garage – workin’ on the hot rod!
Posted by Dan at 10:27 PM