Tuesday, October 30, 2012

147> Pearls of Wisdom

Well, I said I was going to start from scratch, and that is what I am doing. Sorry for the delay in getting a post out. There were some minor family medical emergencies. And even in the best of times, a total redesign involves a lot of time chasing (and drawing) ideas that turn into dead ends. My apologies also go out to any new readers, since it has been some time since I have even shown an actual PRT vehicle. I have been preoccupied with the bogie from which such a vehicle would hang, to the exclusion of everything else. So if you have been scratching your head wondering what you were looking at, now you know. But fear not, dear readers, for I return bearing pearls of wisdom… Well, OK… Let’s just call it food for thought…

One of the most important concepts that I have learned since the inception of this blog (and the many designs and redesigns within) is that it is counterproductive to design a single track profile that is supposed to handle every type of situation. It may not be the case with every type of PRT track, but for track that is contained within elevated beams to be both sheltered and yet have a minimal profile, a single track style is, I believe, a profound mistake. A uniform track profile limits the curves to larger radii, both vertically and horizontally, and limits the angle of ascent and descent. It even limits speed. The result is a system that is less flexible and therefore less able to meet a city’s needs. PRT should not be restricted in terms of routes or performance – Let’s leave that role for light rail!

The instinctual response to performance limitations is to try to design the bogie to allow the most capability within a given track profile. It is quite counterintuitive to have the track change in midcourse as a first choice, but many of the track’s contact surfaces involved in switching, climbing or tight turns are simply not needed in straight sections of track. Why go to the expense of including them throughout?

A prime example is the “cog” method of handling steep slopes. This is my preferred method for climbing with PRT, and it is noteworthy that once so engaged, none of the regular track surfaces are needed. The track becomes nothing more than an enclosed pair of racks and some guides (orange) to keep the teeth engaged. Note that the counter-rotating guide wheels are positioned quite closely beneath the drive wheels. Whereas they would ordinarily hit the underside of the running surfaces when the track curves sharply up or down, the fact is that such abrupt changes in pitch can only happen at very low speeds and a rack could be used instead of the running surfaces in those instances anyway. Therefore the interference between those guide wheels and the running surfaces is remedied by simply removing that feature of the track, not redesigning the bogie.

In the process of trying to achieve higher speeds, I was confronted with the fact that the small surface area of the various guide wheels would tend to wear out rather quickly unless they were quite hard, which is a recipe for a noisy system. (Yes, I know “hard” does not necessarily mean durable, but I am referring here to ordinary, reasonably inexpensive wheel materials here.) The preferred small profile of the beam that would contain them (and restrict their size) seemed to indicate that the system could be either noisy and fast or quiet and slow, unless, of course, one wanted to replace the wheels every couple of thousand miles. But much of the problem wasn’t really a problem at all; the answer lies in disengaging the steering guide wheels when not in use – not by retracting them, but by simply having the track’s engaging surface end.

It is tempting, when trying to enable tight turns, to assume the bogie must be short. After all, our experience in the automotive world has taught us that smaller vehicles are more maneuverable… right? Actually that is a false analogy. With cars and trucks the road or parking spot width is generally limited by surrounding real estate. In a beam-style PRT system, the width of the beam is kept as small as possible primarily for aesthetic and cost reasons, neither of which have much to do with short little low-speed sections of track for particular, close quarters maneuvers. If the bogie doesn’t fit in the beam because of the tightness of a curve, an alternative is to simply widen the beam in those spots. This point, in particular, has taken a while to sink in with me. While there are some other performance reasons to keep the bogie short, the very limited space between the front and rear wheels in my designs has made it a challenge to cram in the steering and emergency brake components. Below is a snapshot of a slightly longer bogie that has enough space for an upper set of counter-rotating guide wheels. A single, pivoting upper steering guide wheel shares the axle. Even this slight lengthening of the bogie would greatly limit the turning radius were the track to have a minimal, uniform profile.

As a final example, there is the matter of air resistance within the track enclosure. Track that is meant for slower speeds is generally also the track placed where people would find it most intrusive, and so it should be as skinny as possible, and indeed can be, because aerodynamic drag at neighborhood speeds is a very minor issue. High speed track, say running along freeways, could be fatter, allowing the bogie to slip through the enclosed air more easily. A fatter track profile has far superior geometry for spanning long distances, and this will probably offset any apparent increase in material costs of such fat track. Again, one-size-fits-all is not the best solution.

It may seem complicated to have track with multiple configurations, but such track would be factory made anyway. Therefore there can be a fixed menu of standard track sections that would be brought in by truck and basically dropped into place. True, seldom used profiles might cost a bit more, but at least the system itself can be adapted to all cityscapes, station types, and storage/staging solutions with minimal engineering effort. Such a scheme also makes sense in view of improvements in design and technology over time. Making the track offer the widest possible range of bogie design choices bodes well for the evolution of better, more capable vehicles over time. Even though these bogies are designed to essentially last forever, that does not mean that they might not be resold on a secondary market or repurposed for freight, opening the way for next generation designs. This won’t be the case if the bogie is so spatially constrained that there is no room for design variation.

In the early days of this blog, I thought the first step in designing a superior PRT system was to design the perfect track/bogie combination by studying the problem from an end-view. Now I have come to see that the best way is to study (in 3D) how to overcome the various limits first and come up with track/bogie for those, and then design for straight runs from there. So for those of you designing your own systems at home, I hope this helps. If you need me, I’ll be at the drawing board.



Juho Laatu said...

There are thus multiple track types. Do you assume that there will be one single bogie type that can use all the different track segments? Or could there be different bogie types that can each run only on some of the track types? That could mean e.g. fast bogies that can not be run on some tightly curved segments of the track.

Dan said...

Hi Juho - Good question. At first glance my answer would be that there would be a single bogie type, and I am merely pointing out a preferred procedure for designing the most capable bogie/track combo. At second glance, however, two scenarios come to mind where that might not be the case. The first is bogies or helper “engines” meant for VERY fast speeds… say double that of a typical highway or faster. I can’t, at present, find any way to make such a beast compatible with the kind of multi-axis maneuverability that would be so helpful in an urban environment, where space is at a premium. (I think the diamond track “shines” in the .7 -1.4 x highway speeds, whereas the present design is better in the 0 – 1.1 range, which is where the lion’s share city transit would be)

The second instance is GRT, which I think is highly compatible, (even symbiotic) with PRT where the routing is poorly developed, (A situation that would always probably exist somewhere) particularly where one stop is far from the rest. I believe that GRT can run on the same track but with greater headways to compensate for the greater weight. Such vehicles could probably run on nearly all of a network if they use double bogies.

I have always envisioned some barebones stations, though, that could not handle bigger or superfast vehicles. Pure vertical climbing is an example of something that would be easy for my PRT designs but certainly could not be done with GRT. Obviously we don’t want to exclude vehicles to gain some incremental advantage. It is not, IMHO, unlike weight-rated bridges or alleys that can’t handle an eighteen wheeler. We can only hope for a matching bogie/track combo that can handle the vast majority of the traffic with as few exceptions as possible. That storm has to make you wonder about firefighting, evacuation and police use of the system though!

mani said...

Dan, as a regular visitor of your blogs and fascinated with PRT, I have been mulling the idea that pods can be both suspended and have a lower guide rail alternatively. The basic idea being that urban environments and lower speeds call for suspended bogie trains that can disengage the pod once engaged to a lower rail train once on higher speed guide-ways or freeways. I am sure you have come across this idea but it seems like it should be revised for various reasons.

1. The additional material, risks, etc. to create the "bridge" or cantilever for suspended guide-ways becomes unnecessary outside urban environments (drop offs and pickups scant, no vertical translation, etc...).

2. Higher load yields for GRT or medium load cargo for long distance transport.

3. Adaptability for Phase 2 PRT mass transit maintenance and maneuverability: pods can be detached and reattached by multiple mechanized trains, allowing quick track changes, easy maintenance of pods and trains not having to be constantly attached.

The pod only needs a human interface and attachment ports on the top and bottom, the trains are individually controlled and can run with or without the pods. Once a train has a pod, the simple instructions of destination/speed/weight/etc can be conveyed and determine the performance of the train.

I understand that this means redesigning a lower track that can handle high speed bifurcating junctions in and out, but maybe this way of imagining the entire system can help simplify and assign control speeds to each guide system.

To follow up with your current epiphany in designing a high performance bogie that accommodates the various specific movements you are envisioning, it may be time to start editing the system again. This trial and error process seems to be going somewhere.

I live in Mexico City and starting a research workshop with capable engineers where I would love to build some physical models of your continued research. This may soon prove useful when presenting these projects to the Mayor here. Keep up the great and inspiring work!

M. Gottfried

Dan said...

Thanks for stopping by, Mani! You seem to have a rather comprehensive transportation system in mind, probably thought out in more detail than I am envisioning… I have no particular argument with the idea of detachable passenger compartments per se; I even have posted designs for it. But I am not entirely sold on the value of the ground-based part of your proposition, at least at present, when such infrastructure is not yet in place. I assume you are proposing following limited access highways, as this is the only way to ensure non-stop travel safely at ground level. Such a scheme seems to require completed PRT infrastructure at both ends first, and so would seem to be way, way, down the road. It also competes with the seemingly more direct approach of platooning vehicles into an ad hoc train to achieve higher speeds with less aerodynamic drag. Clearly, though, if there is a super high speed train that can carry a bunch of PRT vehicles to some distant city, it would seem to beat all of them going individually. I like to keep all options open, though, so if there are specific design issues that should be addressed this early on for some reason, I would like to know about them.

As for the status of my design work…Yes, there are a lot of assumptions carried over from past work that need to be reevaluated. I am currently working on a design dimensioned around off-the-shelf parts, and I should be posting details soon.

BTW, I love your country! I spent the better part of two years back-packing around it when I was young. I still feel the pull of my old home base of Boca Andrea, Veracruz!

Andrew F said...

The problem with high speed trains carrying PRT vehicles in a group is handling rest stops, given that there are no facilities on board the individual PRT vehicles. Also, there is the requirement for an additional vehicle. Why not have passengers board a more conventional high speed train and use PRT on each end of their journey?

mani said...

Ok, I love this. You guys are right on, on many points.

Andrew, when we talk about success in a project in a short-term return, we must take the cultural and social viability as a stake in the matter I think. In Europe this makes sense, but in México i don't believe it would work. You take out the very market you are trying to get out of cars and into changeable transport. maybe i am wrong about this, please continue this discussion. Worsening chronic traffic is indeed a huge motivator. Even when the loss of productivity through traffic in the megalopolis is around +1,400,000,000 man-hours/yr + estra fuel, CO2, noise, etc. people still just sit it out. Still, people want their spaces, their things, where they can work or be productive, where they yearn to be hurled across the city at 80 km/h non-stop from point A to point B, a linear distance of 25 km in 18 min 45 sec.

I think people would have to own their pods apart from having a large part of autonomous taxi pods. Bogies are tracked and run by the association or the organization in charge. If you own a pod, you rent the bogies like your engine and energy consumption. If you don't own a pod, you take a Cabbie-Bot-Pod (CBP). All paid digital instantaneously. Indeed, you can take your pod to the train station and park it, maybe place it on a parking-4-wheel platform, or stacking tower for pods to then finally hop on a train.

I still find the versatility of another load bearing infrastructure on the pod a huge advantage in mobility within an urban environment. Considering how pod´s interfaces can vary, bringing in other companies/orgs to develop new lines and systems, renting their bogies and their technology, similarly to how the PC and Apple share the USB and firewire technólogies or rather the CD or the internet. All the way from pod receptions on private infrastructures to security and privacy issues, etc. One must ask how does each of us imagine it really happening?

Bad maintenance of your pod or damage done to a bogie is immediately investigated, fixed and charged by those responsible.

Does this sound too complicated?

Dan: you are wise. I like how you basically say: keep it simple stupid. I hear ya. Just start out with a damn taxi service, medium-tech with good surveillance and intelligent control systems, then go on from there! Another detachable bogie system and pod system can part from there once we learn from this.

What I believe you to be saying to me is correct. I completely agree. As an architect with a sociological background I am forced to look at reasons why PRT is unpopular. It is cold, robotic, foreign, sterile and reminds people of a roller coaster or fun-ride(?).

I guess my point is PRT needs to be talked about and sold as what it may be one day based on what the market would pay for. But in reality start with a funicular simple circuit or intersecting circuits just to see what happens, (as a crude example).
Or rather that by having ALL the control on a suspended bogie, you can pretty much do everything that is necessary, even own your own pod and bogie without detachment ability, or just make the feken pod a detachable tricycle with enough battery power to travel 5 km.

Seriously, people have to want it. It just seems to me like the technology comes after the idea is sold, not before. This blog seems to be going in that direction and it seems right. I like it, a lot.

The correct next step should be physical modelling and I think some kind of way to consider/assess that maybe PRT needs to be culture-specific. Cities with most potential may be cities like LA or Seattle, or rather like Bombay or Bangkok or like Cairo or México City, or just another Airport or resort and not much more.

Testing PRT in the urban/social environment will always be a huge gamble. Engineering certainty is still indeed a fundamental key, and it seems social-interface must drive this engineering.

Andrew F said...

I'm not opposed to detachable pods, but I think that is something for a more mature PRT deployment than a nascent one. Users aren't likely to invest in their own private pods for a small system with an uncertain future. Also, it adds to the engineering challenge of getting a system off the ground, as well as the public skepticism.

Dan said...

I think we all agree that it should be kept simple to start. One thing that you did not mention in your list, Mani, is that nobody wants it in their front yard, so to speak. I think the only thing that can help with that is a pencil-thin track, which really limits vehicle weight, once you are off of the trunk lines, at least. Because the track must span intersections and hold against vehicle momentum on curves, the track will necessarily have some girth, but every ounce of vehicle weight undermines that objective of dirt cheap and unobtrusive track.

Of course if PRT becomes widely used and is seen as beneficial, people will be a lot more tolerant of track size and cost, and feather-weight track may end up relegated to residential or other sensitive areas. But for now, I believe this is a major factor that must be kept “front and center.”