Saturday, June 2, 2012
Well, as I write this, it is (for those of us who are US citizens) Memorial Day, or as we say here in rural New Hampshire, “Better-shoot-off-our-guns” Day. And so I am “sheltering in place,” in my cabin, trying my best to ignore the gunfire and musing over the state motto, “Live free or die!”.
In my last post I outlined a design that I am hoping ends the long quest for the perfect framework for a ultra-high performance PRT system, and I want to add a couple of additional comments. First of all, I would point out that my depictions illustrate both a track and bogie wheels, but not just for any bogie. It’s a high-speed one, and that is a very important point. You see, if we were just talking about the speeds normally associated with PRT, any one of a number of wheel designs would suffice. But I am looking for a design than can potentially well exceed highway speeds, and that explains my fixation with larger wheels. You see, I believe that the corridors created by highways are perfect for PRT, and that commercial development has tended to be along these areas anyway. In most US cities, almost all of one’s needs may be met within a block or two of a highway. I also don’t mean ONLY high speed. It must also be highly maneuverable to get everywhere you might want to go in a dense suburban environment.
Giving PRT longer-haul, faster capabilities has certain design ramifications, and those larger wheels are meant to address the wear multiplied by both higher speeds and greater distances. This does not mean that the larger wheeled system is the only one that can run on the track, but rather that the track profile is equally well suited for high speeds as low. The use of off-the-shelf steel makes the running surfaces extremely cheap and easy to fabricate, and can be housed in a variety of ways, such as trusses, a box-beam, or even open, such as in an roofed enclosure.
The profile is advantagious because it does not force the wheels to run on round pipe, something that concentrates wear on the center of the tires or solid wheels, yet the square tubing is easy to bend using a conventional 3 roll pipe bender, as shown below. (Needs V-grooved rolls)
Of course running PRT wheels on flat surfaces is nothing new, but established designs mandate that the beam that houses the running surfaces be of sufficient size to house those various guide wheels – generally oriented sideways. This limits those wheel sizes to less than one half of the interior of the truss or box beam. This has meant a trade-off between what you had to look at overhead, and the life of those wheels. Whereas that track girth can otherwise be justified for structural reasons if the spans are to be large, if it is convenient and cost effective to space track support poles more closely, then the track itself can be thinner, cheaper, and more attractive. Turning those flat running surfaces diagonally allows for either longer-lasting guide wheels, thinner track, or a little of each. Of course placing the main guide wheels outside of the track doesn’t hurt either!
That brings up the point by alert reader Rick, who noted that there has been an alarming departure from previous designs in that there is no failsafe for the possibility of a steering guide wheel failing to respond. This is true, but I stand by my design. A feature of many early designs, such as the one depicted in this patent drawing, is that they always had one or the other the of steering guide wheels engaged. That way if the something failed, the vehicle would still go one way or the other, and would never crash by trying to go both ways or somewhere between.
That is fine, and an excellent feature, but those wheels that stay engaged for safety sake must, in a high speed long distance system, do so for thousands of miles per week at high RPM. What if the system is to include an express shuttle that bypasses many areas that have no PRT service, so there are very long stretches with no off ramps? Would we want to be wearing out the steering guide wheels for that whole time?
I would note that there has been somewhat of a culture change underway in recent years in regard to the trust we put in our computer controlled equipment. When Anderson patented this mechanism, meant to “snap” the steering into full left or right positions, it was in a time when making the system centrally controlled was not a choice but rather mandated by the size and cost of even limited computational power. I am on record as advocating a more autonomous control architecture than many proposed systems, and autonomy certainly plays a part in any discussion on the safety of a system that requires an action at every junction. Google’s robocar, now licensed to drive in Nevada, could, on a twisty road, certainly veer into oncoming traffic by a similar failure to actively steer. Luckily the vehicle is not driving by Google Maps alone, but by an array of sensors and onboard computers all working in concert to create a vehicle that can act and react autonomously. Now, in the days of “cloud” computing (many seperate computers drawing from, and collaborating through, a central computer) and supercomputers made of dispersed computers sharing a common program, (such as SETI) the lines have been totally blurred. With today’s technology, steering gear deployment can be assured through a number of cooperative means, including autonomously from within the vehicle, from sensors within the track, from communication with a central computer and/or any combination therein.
Another, separate point that needs to made is that the track and bogie combination has been carefully designed to allow extremely tight turns, including changes of pitch, and the combination of this attribute with high speeds and switching is particularly challenging.
Anyway, to get back to a point I was making earlier, by dialing back the speed a bit, many other bogie designs are possible, and may be more practical than what I have shown for those speeds. My focus is to not PRECLUDE high speed (or other valuable attributes) in the track first, and then to design practical bogies for that track. I am really not sure what a downtown-use-only bogie for this track would look like at this point. I'll have to work on it between the bullets.