Wednesday, November 26, 2014
At this time of year, seems like everybody’s sharing culinary secrets. So here's my recipe for perfect PRT. First you take individual desirable attributes, like speed, energy efficiency, and so forth, and, one by one, design for that attribute. The idea is to take each one as far as you can before it is clearly too much of a good thing. Take that design, set it aside for future use. Then you repeat the same process for the other, often competing attributes you have identified. Once all of that has been done, you have premixed the ingredients for truly great PRT, and the fun begins. Like adding spices to a soup by experimenting with small quantities, you add a little of this and a dash of that, backing off when you add too much, until the proportions are just right.
Recently I have been mulling over where I left off with SMART PRT, and have realized that there is still a lot of work left undone. If attributes like small turning radius, high-speed switching or low maintenance are the object, then I am done. But at what expense? I designed a system that is "track-compatible" with GRT, (larger group vehicles, such as scheduled shuttles) equal a car in speed and comfort, can even climb straight up… but what about all of the single passengers only going a couple of miles in the inner city? Have I excluded this important demographic? After all, to many who live and work in the inner city, this is what PRT is really about. Many do not even see an important role for suburban systems, as I do. Not everyone lives in a freeway-dependent mega-city. Perhaps I omitted an important ingredient from the dish.
This reflection has also been prompted by a couple of factors. After my last post, about programmable motors, I discovered that since last time I looked into it, a huge amount of progress has been made in this field, and not just to make servo motors run programmed routines. They are using specially shaped waveforms for optimal speed or torque performance as well, in a process called Frequency Oriented Control. (FOC) Unfortunately wheel motors with matched FOC controllers (with position control) have not yet arrived, and I hate promoting designs that rely on parts that are not available at practical prices. I love direct-drive wheel motors, but the advantages of FOC, when it comes to integrating motors into a larger system software architecture, are huge. Also, the appropriately sized wheel motors that are available are for two-wheeled scooters and e-bikes, and they have through-axles that are meant to be supported from both sides of the wheel. My designs do not reflect this, although these motors could never achieve higher speeds anyway.
I also have been troubled by my abandonment of a totally enclosed bogie. In a quest for high speed and smooth switching, I designed-in some very large diameter centering and steering guide wheels, leaving guide surfaces exposed on the bottom of the track and those guide wheels semi-exposed to the elements. That large diameter would be consistent with a softer, quieter ride via softer, quieter wheel materials, which, of course, wear more quickly - thus they are bigger for more wear surface. That diameter could be maintained, but with harder materials, for heavy vehicles like GRT. At least that was my rationale. Yet I have never been a fan of what this does to the track, not just for the reasons just mentioned, but also in regards to compatibility with vehicles that don't need such big guide wheels since they are for slower, urban use. Furthermore, the high speeds or heavy vehicles are likely to make the most noise where it doesn’t matter much, such as along freeways. And not fully enclosing the guide wheels inside the track makes them louder anyway.
I think, in retrospect, I designed for the exception rather than the rule, and it is time to see how a lighter, smaller system can be modified toward the larger and more powerful, instead of starting with the heavier system and trying to design lighter vehicles for it. At the very least, those reverse rotating centering wheels (golden-yellow, below) can to be reduced in diameter. 15" is just way too big to require (due to track proportions) of lighter PRT vehicles. The load on these wheels may be continuous, but it is not that great, except perhaps for GRT, which could even use steel wheels if wear is a problem.
This illustration, (not to scale) shows the current track design. The bogie is highly abbreviated to more clearly show the guide wheel positions. The configuration shown is for straight travel, as even the raised steering guide wheel (left, light brown) is not quite touching. This is because the steering guide rails are discontinuous, (and therefore not shown) and only taper into contract in advance of a switch-point. It can be seen that the track (blue) is not wide enough to wrap the guide wheels in this design.
Also at issue are those multipurpose "hold-down" wheels. (maroon) These are a holdover from early designs that had no other options to deal with the tremendous twisting forces (red arrows) that could be exerted by a swinging passenger cabin. These also doubled as upper steering guide wheels, securely locking the bogie to one side or the other as the track splits into separate directions. As one such wheel would be in continuous rotation for long periods, these were also designed to be of a softer material, like rubber, and of large diameter. The problem is that this solution, which engages the top of track, means that a smaller, lighter bogie, which might optimally run in a smaller track, would have to be inconveniently tall to reach these “ceiling mounted” guideways. The key to solving this dilemma has been partially solved by current position of the large counter-rotating running guide wheels. (golden yellow) They are now positioned so as to directly absorb the brunt of any sideways forces, making continuously engaged upper guide wheels unnecessary. Any upper guide wheels would be for switching and extraordinary events, such as a sudden gale force wind gust or emergency stop. The position of those large counter-rotating guide wheels does, however, directly add to the height of the track, and competes for vertical space with the drive wheels. (Purple) These may have to be reduced from typical automotive size, even for heavy or fast systems. By the way, if you are wondering about those things sticking out of the axles, they’re for climbing... Check out post 131 for details.
So anyway, I have concluded that my feast is not quite ready. I need to add two more ingredients... Bogies designed for drop-in motors that are available now, and a lighter, lower performance vehicle equally at home in the heavier track. What I am aiming for is a three tier system for vehicles - a “papa bear” (GRT) a “mama bear” (high speed, longer range PRT) and a “baby bear” (light-duty PRT for inner city and campuses.) Right now I see “mama bear” track as universal, being sharable with either the larger or the smaller, but not both at once. For example, track running along freeways wouldn’t allow any “baby bear” pods, and “papa bear” GRT vehicles would be only for connecting large stations that are far apart, while the mid-size, “mama bear” vehicles would be able to go anywhere. (Actually they are more like a compact car, very tight for 4 average adults.) That size, after all, can comply with US disability laws, whereas anything catering to individuals or pairs could not.
And speaking of the US,.. and catering,.. Tomorrow is Thanksgiving, a US holiday about giving thanks while eating ourselves into a coma. So happy Thanksgiving, everyone. If you need me, I’ll be in the kitchen. I’m preparing a feast!