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.

Tuesday, September 22, 2009

51> A New Kind of Motor

I want to say a bit more about motors. It has been my contention that linear motors are no longer the way to go for PRT, or at least won’t be for long. In a nutshell, when PRT was first developed, computing and networking were at their infancy and there was no choice but to control all vehicles from a single “mainframe” computer. Linear motors were ideal for that, since they were able to take commands from the track itself, and eliminated all of the moving parts but the wheels, and made vehicles immune to traction problems, even on ice. A “no-brainer”. The tradeoff was efficiency, both in use of energy and materials. (in the old days of motors driving chains, gears or belts there actually wasn’t any tradeoff at all) These, however, enormous computing power can be cheaply employed on every vehicle, and increasingly, those energy wasting belts, chains and transmissions are being replaced by motors catering to the specific torque and speed needs of the machine. In other words, direct-drive alternatives are the coming thing. In PRT, direct drive means that the only moving parts are the wheels and this doesn’t change. These days there are low RPM, high-torque motors that are essentially like a linear motor but in a lighter, more compact, more energy efficient package. Like a linear motor, a direct drive motor can have regenerative magnetic braking. The one trade-off is that such a vehicle relies on traction being maintained between the wheels and the track. In this aspect linear motors are still superior. Therefore the following solution is only for reasonably clean and dry environments. (Shrouded track) Emergency stops, (i.e. There is skidding) can, if need be, be accomplished by clamping the track.

So that is the background, and to regular followers of this blog, that is all old news, as is my love affair with wheel motors. (motors wherein the axle stays stationary and the motor itself revolves) So here is how all that background ties in.

I got to thinking about what specifically was so good about wheel motors and realized a couple of things. First and foremost is that it is a direct drive solution. Secondly, it is the form. Wheel motors get hi-torque and high positional resolution from having large diameters, with many magnetic poles. Generic motors generally get more torque from being made longer, a shape which is not well suited to PRT. I therefore (with the discontinuation of a number of wheel motors from PML) started looking into alternatives with search words like “flat motors,” “pancake motors” “high-torque” etc., and found a kind of motor which was new to me, the “frameless torque motor,” Illustrated below. Although it’s inner ring (rotor) turns instead of the outer, like a wheel motor, it can be housed in such a way that it will still provide true direct drive. One advantage to this motor is that the PRT designer can use readily available, standardized, replaceable automotive bearings.

I am a bit disappointed by the upper speed of the air-cooled models, however, as I was with PML’s wheel motors. A quick look at my drive unit of July 16th shows relatively large drive wheels, which, quite frankly, crowd the track, which is already taller than I would have preferred. (I would like the track and vehicle to be able fit in a building between typical floors, if possible, and, in my wildest dreams, two high under a highway overpass) Slow rotation necessitates large wheels to get speed. For example, to achieve a speed of 60 mph, (96k/h) a 14” (356mm) wheel must turn at 1440 rpm. This has been sort of a sweet spot for me, design wise, because I doubt that it would be beneficial to make the drive wheels much bigger or smaller, and I doubt any initial PRT iterations would go any faster.

The wheel diameter and motor speed information, together with presumed assumptions of adequate acceleration, form a sort of triad, which can be adjusted to size the motor and track. (In the June 16th post I showed how the track size could be decoupled from the truss size; The track itself need not be self-supporting over long spans) The track is, of course, the big kahuna of PRT design. Little pods may come and go, but the utility of the track design will be debated for generations if that track design becomes the de facto standard. It can be said, therefore, that it would be silly to base track dimensions on some arbitrary motor specifications. After all, motors can be custom wound. But, hey, wouldn’t it be nice to base track specifications on readily available parts?

To the minority of readers who are even remotely interested in form factors, torque or speed ratings of motors, I would refer you to

Finely a bit of personal news. As of tomorrow morning at 4:30 AM, I will be on route to New Hampshire, (USA) to close up my cabin for the winter, and take in the magnificent fall foliage. There is no internet, or even electricity to speak of, since I am a half mile from the nearest telephone pole. I can plug in at the town library, though, and hope to post at least once in the next few weeks. So “bear” with me, if I am slow to comment or return Emails. I’ll try and post a picture
- Dan The Blogger

Saturday, September 12, 2009

50> Dualmode and Modular Design

I must say, as measured by email, there is a lot of interest in dual-mode. For those readers not already totally familiar with PRT and it’s many branches and schools of thought, dual-mode is the idea of being able to take a PRT vehicle off of the track and take it other places that the track doesn’t go. Like home or city destinations not served by the system.

I have two problems with this approach. The first is, it seems to me, that a PRT vehicle would make a lousy car, and that a good car would make a lousy PRT vehicle. The second is, that as a business model, dual-mode makes a very difficult transition that much worse. Who wants to buy a vehicle that has no track to run on, and who wants to build track for vehicles that no one owns?

The bad news for dual-mode advocates is that I will not be releasing designs for them anytime soon. The good news is that I will not stand in the way of the future, and, I have to say, I think dual mode will probably happen, once there is sufficient track installed to make it attractive. What I can do for dual mode is to not stand in the way, and to support PRT designs that are dual-mode ready. This happens to be extremely easy to do, because I believe that a modular approach to PRT design would be better anyway.

The above pictures are of a “skateboard” concept chassis from GM, called “AUTOnomy.” Although it is a full 6” (152mm) thick, it is also designed for considerable range and performance. The basic concept is that this platform can be totally driven by joystick. (The pedals and steering wheel would only simulate actual mechanical connections.) Various body styles can be attached as needed.

From the point of view of an open-source PRT standard, the thing to do would be to establish some reserved attachment zones on the bottom of the cab, and have the top “hanger” hardware detachable as well. Ironically, this approach pretty much kills the bottom or top track debate, since the cab would now be capable of attaching to either. This idea is very reminiscent of the MAIT System.

Another thing that came to me while considering modular design was that some of my pet design features are application specific. For example, I have included the ability to travel vertically. The fact is that I seriously doubt that vertical travel would be of much use for PRT, but for industrial and parcel delivery applications this feature might be invaluable. The ability to self-bank around corners would relatively useless for well-packed freight. Clearly the hanger portion therefore should be modular. This is especially true because any suspension would be in this section, and, if ordinary autos are any indication, the perfect ride could take years to perfect, even with smooth track.

Pictured below are some SketchUp drawings of a swing-arm that is removable via sliding dovetail joints. I have not gotten around to the weatherproof power and data connections yet. Actually the pictures are comprised of practice art, as I am just learning SketchUp, (free from Google) so things are not really to scale, or completely thought through.

Saturday, September 5, 2009

49> In Search of Gridlock and Opportunity

I have, for the last couple of posts, been trying to figure out how various PRT designs would fare with limited track and stations. My quest deals with the recognition of the fact that no investors in their right minds would sponsor a full network as a trial. (I am not abandoning the possibility of private money just yet) Like it or not, we have to look for the most cash flow from the least network. This exploration has led me to recognize the fact that a few dispersed low capacity stations may not introduce passengers to the system fast enough. Paying passengers must come from somewhere. If they do not come from dozens of smaller stations, then they must come from a few larger ones.

I understand how PRT is supposed to work. It is not a hub-and-spoke, mega-station type of technology. Nobody has described PRT vehicle storage because it is not supposed to be required. But if there is any way some track can be put down, and some vehicles deployed, expansion could start from there. The “foot in the door” it seems, would need to be some situation where people are so inconvenienced that they would be happy to pay a premium to take a short ride.

So here is a building block. This is a bare-bones seven-berth station that can have a 60 second turnaround time. (In one minute all berths have been loaded and moved out, and a fresh set of vehicles has taken their place.)

This illustration shows a high-capacity station comprised of six of the seven-berth stations arranged in parallel, with three boarding scenarios. The arrows indicate foot traffic. The first (A) shows how the station could be adapted to accept a majority of passengers who are boarding. Green lights or programmable signs would direct passengers to berths. Red lights would indicate that people are disembarking from that area. The middle figure, (B) shows a balanced load between incoming an outgoing vehicles. The last figure (C) shows the station accepting large numbers of incoming passengers with few heading out. Green lights mark the three berths accepting passengers. Such a station could, with the 60-second turnaround time, process 42 cars per minute or 2,520 cars per hour. (Note; This uses the gondola design: bottom track designs would require a means to get passengers to the boarding areas without crossing tracks.)

The illustration above shows a “hub and spoke” system with the main station connected to six satellite stations. In this scenario, the time required to completely empty the main station is one minute, the same as the satellite stations. For the sake of simplicity, let us assume that the satellite stations are a half-mile from the main station and that the travel speed is 30 mph. (I originally designed this to take people from a stadium to satellite parking, so that’s why the distances are so close) The number of stored vehicles is 42, so that with the 42 in berths and the other 42 from the satellite stations, the stations would never run out of vehicles. (126 cars total)

These examples point out a few key concepts. First, it is a closed system. PRT advocates tend to see PRT as an open-ended network, with an endless supply of vehicles, which will always self-balance. (i.e. a BIG network) In this example, all stations would run out of cars in 60 seconds, were it not for the stored ones. It is only the close proximity of the satellite stations that allows replacements to get there in time. For every additional half-mile, another 42 cars are needed.

More can be learned from the example. With only 3 miles of track, it is extremely limited, yet it would still probably cost over 30 million dollars. Could it pay for itself? One thing worth considering is that the full capacity would only be utilized for, perhaps, 3 hours per day, 5 days a week. If the cost of money were, say, 10%, the daily interest alone would be $8,219. What price point and usage pattern could bring in, perhaps, $13,000. to put the project solidly into the black?

Again, I designed this for a stadium, where the time to just get the whole family into and out of the parking lot would justify some very high fares. (not to mention that 90% of the track would be on private land) After checking the schedule of a local stadium, however, I realized that these big events are so infrequent that the system would never be viable, even though it could generate upwards of $40,000. per event. I now put it forth as something to ponder only. If the main station was broken into two or three, and the system stretched a bit, it might cut driving time by 10 minutes each way for some particularly congested downtown areas. My logic was to think up a scenario where passengers would gladly pour into a super cheap station to hardly go anywhere for top dollar. Investors, after all, are not known for altruism. We can all imagine a fully-grown, distributed PRT network. Can we come up with a viable PRT seed? For, say, 30 million?