Sunday, April 29, 2012
If you have followed this blog for any length of time, you have witnessed version after version of bogies and track. I started this blog with the naïve hope that I could recruit a bunch of mechanical engineers to create an open source PRT design. Such a consensual design would have had a credibility that was especially lacking back in those dark days before Heathrow and Masdar. Well, lacking the participation I had hoped for, progress has been pretty slow, although, in my own defense, I’ve set the bar pretty high, not just in terms of performance capabilities, but in terms of simplicity, cost and practicality. That’s a bit like trying to mix oil and water.
Anyway, folks, get out the Champaign and the party hats, because I am pretty sure the search is basically over, although I’m still studying tight-quarters switching, and a few other details. Let me reiterate, for new readers, what differentiates these designs. The challenge is to go faster, climb steeper, turn tighter, be less expensive, and therefore be more versatile and practical than other transportation alternatives. I seek a fully 3D solution instead of a system that requires raised stations or long ramps. The pictures here show only the bare essentials. Most structure, including the passenger compartment and the track’s support means have been left out for clarity.
Here are the highlights of the changes. First, I have replaced the round running guides (A) with square tubing tilted to make a diamond profile. This is almost as easy to bend as round pipe, but offers flat running surfaces that won’t wear grooves into the guide wheels. I have inverted the drive wheel flanges (B) to be convex instead of concave. This results in a single point of angular contact that will be very prone to wear, but for the fact is that these flanges only need to come into contact for specific, short term duties, most of them at very low speeds. For switching they are employed for only a few dozen revolutions. The steering guide wheels (C) and what were called “hold-down” wheels in previous iterations have been combined. Now, when ascending or descending very steep or even vertical runs of track, both of these wheels can be raised to serve the hold-down function. The new geometry makes their previous “anti-rock” function unneeded. The running guide wheels (D) and steering guide wheels (C) must now, though, all move independently, instead of in pairs. Here is how switching would generally work in four pictures.
1. The first of the four is the ordinary running configuration. The drive wheel flanges, although close, do not actually make contact, so all wear is on the larger softer, smoother, and quieter hard rubber wheel surfaces.
2. Several steps have taken place here. First, a steering guide wheel is raised. Next the pairs of diamond guides “taper” into a position where contact can be made. (They start spread out.) Meanwhile, the drive wheel guides make contact with the plastic wheel flanges. This will probably be accomplished by raising the guide wheel slightly on the side that isn’t being lowered. This will pull the bogie out of center, clamping the track between the flange and the guide wheel.
3. With the prior steps taken, the track sides can begin to diverge. Note that one side of the bogie can now be unsupported. This solves the situation often referred to as the “frog problem.” (The “frog” is the little piece of track that, in suspended designs, has no means of support.)4. The fourth pic shows the resumption of support under both sides of the bogie. Any “frog” would be cantilevered from this point, as the thin wall separating the two tracks is the only means of its support. At this point, the top guides can be discontinued and the drive wheel flanges can cease contact, to stop wear. From here the tracks can diverge into their respective directions. I have included a couple of additional screenshots, below. As for me, well, you’ll find me throwing out a couple of years’ worth of now obsolete designs. Oh, sweet victory!
Posted by Dan at 9:32 PM
Saturday, April 14, 2012
Recently I was reminded by one reader just how many posts I have written, and just how difficult it is to use this site as a resource. Blogs are just like that. The new builds on, but buries, the old. If you come in late, you just don’t know what is going on. Yes, I (ug!) need to update the table of contents and index… Anyway today I am going to go back to the very foundation of the designs found on all of those back pages, the primary design distinction that tends to define and distinguish one PRT system from another… the track.
The right track is everything. Think of it like a pipe for moving people. After all, our whole way of life is a study in the progress of learning to pipe goods and services with ever greater efficiency. Look at email vs. treemail! Gas pipelines! And, of course, there is the many mile journey of the water coming out of your garden hose and into your flowerbed. Back in Roman times, huge structures were erected to transport water because it had to be kept flowing gradually downhill. Once pressurized pipes and pumps were introduced, however, water could, for the first time, be efficiently moved anywhere in any direction. Our street system bears some resemblance to those ancient aqueducts, for reasons that contain elements of the same logic. Our vehicles are optimized for relatively level travel, abrupt turns require slow speeds, and of course, gravity is what keeps everything on track. In fact, it is a little appreciated fact that our city streets generally ARE aqueducts, insofar as they are carefully pitched to carry any rainwater away from homes and buildings, without ponding, to the nearest storm sewer. Conversely, highways, being designed expressly for speed, bear more resemblance to pipes or tubes. They are not referred to as traffic “arteries” for nothing! Unfortunately highways have severe limitations that mean that this analogy can only go so far. They are, frankly, huge. They rely on good weather and gravity and use acres of land for a simple turn. Pipes, on the other hand, can go anywhere – up, down, over and under. Their containment means higher speeds are possible and their compact form disguises the surprisingly high volume that a continuous flow can represent over time. There are places, in some countries, where the water truck still comes down the street so that people can come out and fill their containers. In rural areas, people still rely on fuel trucks stopping by. In the transit analogy this is like buses or light rail letting off passengers. It’s a cumbersome, stop and go world. Piping is so much better.
Futurists have been playing around with the people-pipe idea forever. Anyone out there old enough to remember the Jetsons? The facts on the ground are, however, that physical pipe for humans isn’t really the most practical way to go. It’s too large, and a main point of piping is to economically extend the reach of a resource into all required areas. So the challenge is, then, to devise a system that can move people as though they are being piped, yet be small and flexible enough to be easily extendable into a large network, the way other ”piped” resources are.
Before going any farther, one interesting point about moving people. Oddly, we have little knowledge of how fast this can actually be done. Those familiar with the designs in these pages know that the PRT vehicles herein are specifically designed to eliminate sideways and front-to-back G-forces. For example, a passenger’s beverage, set down, will not spill, much like a bucket of water tied to a rope can be swung in circles, even up-side-down, without spilling a drop. This is contrary to (and better than) our finest, most advanced vehicles, be they luxury cars or fighter jets. There are, however, unavoidable variations in ordinary, vertically oriented gravity and these can cause discomfort. I recall that when I was a child, elevators would give me that weird feeling in the gut. I’m not sure, but I suspect that, over time, the acceleration/deceleration control in elevators has been greatly improved - carefully calibrated to make the effect almost unnoticeable while getting you to your floor just as fast or faster. In any case, doing this in 3D is a brand new field. Nobody has actually tried moving people from here to there as fast as possible, without making them feel yanked around. I predict that the combination of simulated free-hanging and precisely calibrated acceleration and deceleration will prove to be surprisingly effective in “piping people” comfortably. PRT has historically been assumed to have vehicles all moving at the same speed. It was a matter of control. These days, and even more so in the future, PRT vehicles may be programmed to act more autonomously - to be opportunistic - to go as fast as passenger comfort will allow. After all, that is what would happen in fluid dynamics. The Bernoulli principle applied to transit!
There is a lot more to say on this subject, such as the possible dizzying effect of looking out the windows, but that is for a different post. Let’s get back to designing the “pipe”.
So what is the most minimalistic form factor that can hold a vehicle the way pipe walls hold in liquid? In theory, a simple pair of rails is all that it takes. Anyone who has seen a rollercoaster in action can attest to the fact that two rails can rapidly direct and redirect people through 3D space. (Remember, rollercoasters are purposely designed to throw you around, so I sort of hate to use them as an example) Anyway, though, it shows that two rails will work.
The notion of only two rails is a bit deceptive, however. The fact is that holding those two rails in proper orientation requires the lion’s share of the structure. So in actuality, it really doesn’t matter. Two, three, five rails or more…what matters is the overall weight vs. the span vs. factors like ease of construction and material cost. The big question for track designers becomes the tradeoff between many closely spaced supports vs. fewer, but with more massive and costly beams or trusses instead.
This brings up the question of what people will accept visually. Many current systems create a profound canopy effect, particularly where the track splits off to enter a station. If the system is to be two-way the problem is greater still. This effect weighs against the four-wheels-on-the-bottom paradigm seen on most vehicles. That said, the preferred profile for beams or trusses for making long horizontal spans is tall and narrow, not short and wide.
But there is one other factor. There is the tradeoff between more vibration and noise but long wheel life, on one hand, and a quieter, smoother ride with less wheel life on the other. An obvious example is wheels made of steel or hard plastic vs. rubber tires. This conflict arises from, and is proportional to, speed. At high enough speeds hard wheels are very loud and soft wheels wear out very quickly, as racing fans well know. The best remedy is to have larger wheels, which, of course, have more surface area. So this, too, gets weighed into the debate about PRT track profile for any fast, wheeled system.
A few decades ago, PRT pioneer Dr. J. Edward Anderson concluded that the best compromise for his single level, but raised, system was a truss about waist high and about two thirds of that wide. He would use the structure to enclose and support the rails. I have drawn the same conclusion, as far as general dimensions go. This really is sort of a “Goldilocks” compromise. It gives enough interior space for the larger wheels that high speeds with semisoft wheels require and it encloses any noise anyway. (Not to mention weather-proofing the running surfaces!) It allows spanning lengths sufficient to cross at least six lanes of traffic, (Two lanes, each way, plus a turning lane is a very common city intersection size.) It has close to the ideal proportions of width to height.
I have physically placed similarly sized objects, such as large trash cans at a distance to simulate how big the track would look from the ground, and I am satisfied that these dimensions will be found acceptable almost universally, especially if they can contain otherwise unsightly utility wires and improve street lighting in the process. Also, with a fully 3D system the track can be set as high as need be.
So that, for all you new or occasional readers out there, is a brief explanation of the thinking behind the track that you will see on these pages. What I propose is essentially a bogie within a slotted tube that pulls along a vehicle in such a way as to whisk people from anywhere to anywhere faster but more comfortably than any of us have so far experienced. In full 3D. It would, of course, have all of the system traffic management normally associated with PRT and enjoy the benefits of in-vehicle, rather than track-based switching. The bogies use wheels that turn themselves without any additional moving parts and so are exceedingly efficient, especially since they use track-supplied electricity. Oh, and one other thing… It’s fast. It’ll take you from here to there in no time…without snapping your head around!