Monday, September 28, 2009

52> 3D, 2D and Sky Hooks


I have been more and more confused as of late. It seems like there are so many competing visions of PRT, it’s almost impossible to define. A quick look at Wikipedia gives this seven point list compiled by ATRA in 1988.

1. Fully automated vehicles capable of operation without human drivers.
2. Vehicles captive to a reserved guideway.
3. Small vehicles available for exclusive use by an individual or a small group, typically 1 to 6 passengers, traveling together by choice and available 24 hours a day.
4. Small guideways that can be located aboveground, at groundlevel or underground.
5. Vehicles able to use all guideways and stations on a fully coupled PRT network.
6. Direct origin to destination service, without a necessity to transfer or stop at intervening stations.
7. Service available on demand rather than on fixed schedules.

There can be little argument about point one. Point 2, however, seems a bit restrictive. What if you want to detach the vehicle (cab) from the drive unit or bogey? “Why do that?” you ask. Perhaps the payload is cargo, not people, and you want the whole container. Another possibility is that of dual-mode, where the vehicle can drive away. An argument against dual-mode has been that if the vehicle was not well maintained, it could break down and clog up the system. If the drive unit, however, stayed in the track, a vehicle could be carried just like any other piece of cargo. Number 3 is, of course, subject to the previous argument. Cargo can help pay the bills especially in the middle of the night.
As per number 4. How small is small? To me the ULTra track seems like a full fledged road. I think the main thing is a commitment to 3D. (more on that later) Number 5 seems a bit arbitrary to me. My question is why? What is wrong with the possibility of private “spurs” that run into private property? It also seems possible that the track might come in various classes, such as a lighter, tighter turning version for industrial use. The passenger vehicles can obviously be “smart” enough to avoid going on such track. Number 6 and 7 seem fine, although again, one person’s attribute is another’s restriction.

I think it pays to consider that we humans have been largely earth-bound since the dawn of time, so that sometimes we don’t really recognize the nature of the problems it causes. Roads and cars are a 2D solution. Sure, we can build a bridge, or even a cloverleaf, but it’s still basically ground flattened to roll stuff on. Many of the definitions above are basically attempts at overcoming the shortcomings of being stuck on the ground. Isn’t PRT necessary primarily because there isn’t any more ground between point A and Point B? This seems especially true now that battery technology is enabling true electric vehicles. Why not just add six foot “electric only” lanes? Oh, yeah. No room.

Imagine a world with skyhooks, and you’ve imagined a world where all transit is point-to-point, on-demand, etc. The infinite PRT network. I like to think of the problem in terms of XYZ coordinates. Mary Poppins can apparently go anywhere within the atmosphere, but we must be content with the network of track and the capabilities of the vehicles they carry. Never-the-less, the ultimate objective of PRT, as I see it, is to enable a person or thing to be plucked from one place and dropped in another. Everything else is just a means to that end. Looked at in this light, it is clearly imperative that the track be as light and adaptable as possible, for this will ultimately determine how many XYZ coordinates the network serves and, therefore, its ultimate usefulness. In a world of so many competing PRT visions, let’s remember this metric for measuring their worth. We can call it the Mary Poppins test.

8 comments:

akauppi said...

I will be visiting a Helsinki traffic planners public meeting this week, and the "beam me up, Scotty" approach you envisioned (same as Mary Poppins', as you know) gives a good simple way of declaring the PRT approach.

Whether we are talking to the general public or to traffic professionals, describing PRT is somewhat of a difficult task. Mainly, because we don't really have an existing implementation, yet.

Think of describing mobile phones before they existed. "It's like the handset, only without the phone. And it works almost everywhere." Believable in the 80's? Maybe not. But the description would have been accurate.

We need a similar pre-focusing description. The "Physical Internet" you mentioned earlier is one, and the "Beam me up" is another.

What I want to emphasize to the audience in addition to the ease of transport is also the enjoyability of it. It's like traveling in your favorite rocking chair. One will be wanting for the trips to take longer. :)

cmfseattle said...

great post. so now we can begin a list of design criteria:
"1. should be capable of horizontal and/or vertical maneuvers."

Anonymous said...

Whether you agree with the list or not, that has been the definition of PRT.

It details what constitutes PRT & what differentiates PRT from other forms of transit.

Proposals like cargo carrying & dual-mode extend the capabilities of the system but are not necessary to define it.

Dan said...

Dan The Blogger has come into town.

Hi folks, sorry for my long absence. My hard drive is on its last leg and no longer works off of inverters or battery. I am trying to hold out until I get to civilization again.

Akauppi, nice to hear from you again. No doubt you have already given your presentation; I hope it went well. I want to say, for the record I got the phrase “physical internet” from the JPods web site. Credit where credit is due. It’s very educational and worth checking out.

Cmfseattle, I’m not sure if you are being polite, sarcastic, a bit of both or what. I know that you would like to see us push forward with the current leaders, (ULTra) and I don’t think I have suddenly changed your mind… I would like to say that the “Mary Poppins test” is really just one metric for comparing systems, and elevators, though not my favorite option, do get the job done.

Anonymous, I guess there is a fine line between defining and confining. Perhaps there should be a term that is more all-inclusive than PRT, because these ideas are not off-shoots of PRT, but rather come from a common route… Automated Transit Systems?
This may seem like splitting hairs, but I am trying to come up with a unified track design that can serve many purposes.

cmfseattle said...

I really do think it's a great post.

I remember reading the Skyloop docs, about the objections to elevated stations, and thinking that a "turntable lift" might solve the problem.

The arriving vehicle maneuvers into a berth, which could be a cylinder made of glass and steel and of vehicle-length diameter. It's lowered to street level, where the vehicle's and cylinder's doors open.

For departure, the supporting/suspending element can be rotated ~90 degrees, so the vehicle wouldn't have to go backwards.

ULTra could do something similar without the elevating mechanism. I think the Taxi2000/PRTI design could be modified with "steerable" front wheels, which reminds me:

Why not use wheel motors on the rear and linear motors on the front? (btw, the T2k vehicle's LIMs ride on their own "sub-bogie," which is how they maintain the ~3mm gap). Taxi 2000 Vehicle Components Weight Estimates (8/23/01)

Dan said...

Dan The Blogger checks in, occasionally.
Cmfseattle- where do you get this stuff? You’re like the Library of Congress of PRT.
I had considered the sub-bogie design myself. I have to confess that I do not know enough about the efficiency of inducted magnetism to absolutely make a call, but I would wager it’s not as good as tight tolerances to rare earth magnets. When I get to civilization and get a new hard drive, I’ll take a look at the weights of those puppies vs. those torque motors. I guess I just don’t anticipate any skidding, so I don’t really see much point in them. Oops. Library is closing..cya

Miko said...

A few responses. 1) I agree that definitions shouldn't become confinements. The important thing is to find ways to improve our environmental responsibility and improve quality of life. I worry sometimes that the PRT community gets a little too bogged down in what is and isn't PRT. For example, Morgantown is either proof of PRT's viability or proof of PRT's failure (because it isn't "true" PRT). The real lesson from Morgantown is that it works for them, regardless of what exactly it's called.

2) We should remove the 24-hour-a-day restriction from the PRT definition. A system that improves transit in an area but doesn't run 24/7 is still an improvement. For the forseeable future PRT will always need babysitting, and it might not make economic sense to have someone on call in the middle of the night.

3) I think in general vehicles should be ambidirectional, i.e. able to go in either direction. The pods at Heathrow are like that and that allows for herring-bone stations. While we're on that concept, has anybody considered a wishbone design for turning vehicles around? The idea is that the pod can go forward into a slot, then "backward" into another lane. This design could reduce the need for large turnaround circles. Here's a diagram of the idea.

an-148 said...

this wishbone design is a nice idea for: 1) public beam: some "cul-de-sac" locations in narrow and less frequented places, where it is physically impossible to build a turnaround at the end of the line and 2) as a private terminal on private ground in order to enter the public beam without backtrack.
Maybe some remarks concerning the pods in that eventuality: 4 seat "vis-à-vis" cars can keep their orientation, but the smalled 2-seated pods would need to have a supplementery desing to turn the cabin by 180° before departure.
Anyway, it's a very good solution for additional flexibility in matters of cargo.