Thursday, July 26, 2012

144> Compromise

This is a post I have been struggling over for several weeks.  It touches on so many subjects that it is difficult to compact it into a single post.  It is basically a review of the wheel geometry issues that have taken up so much of my time and an overview of the design philosophy and limitations we face.

There are three “must have” criteria that are central to the design - inexpensive, unobtrusive and easy-to-produce track, extreme 3D maneuverability, and speed.  There are also two additional criteria that overlay the others.  These are mechanical simplicity and forward compatibility.

Each of the three initial priorities is easily attainable, but at the expense of the other two.  This leads to tough choices.  Maneuverability in 3D lends itself to a short, wheel-driven bogie.  Speed indicates large wheels, or maglev.  Minimal track cost means no maglev or LSM, and minimal track size means smaller guide wheels.  All in all, the best choice seems like some kind of wheel driven system, but there is no easy answer as to what the best guide wheel placement would be.  Making the track a bit bigger or smaller, or making the speed bit faster or slower completely changes the optimal geometry.  What is sad is that sometimes these kinds of things are never resolved.  Consider rear wheel vs. front wheel drive for automobiles. Apparently now rear wheel drive is making a comeback.  Clearly this kind of ambiguity does not engender a standards-based approach, something that was central to this whole project.  That does not mean that there isn’t a best way to design for a specific speed or track size.  It does mean, though, that a relatively exact track size, speed, and maneuverability must be decided upon before the best geometry can be determined. 

And for the second two design priorities … Well simplicity seems like it would go without saying, but actually it jumps right in with the initial three;  there are ways to partially remedy the conflicts between track size, speed and maneuverability that involve the addition of parts.  This is, of course, not unusual. No engineer wants to add parts unnecessarily, but functionality may demand it.  Trying to design an automatic transmission with five moving parts is obviously a fool’s errand.  Yet mechanical complexity is surely the enemy of any brand-new, yet-untested PRT system, so I have spent considerable time on such foolish pursuits!

Lastly, there is the matter of forward compatibility.  Since PRT track of this type represents new and presumably permanent infrastructure, there is a need to “get it right.”  The last time this happened, with the railroads, there was little incentive to get everything standardized around the very best engineering paradigm, but there was every incentive to get track put down ASAP so that money could be made transporting goods.  This led to many now-obsolete track specifications and, to this day, those early design choices still limit what rail can do. Too late now!

So where does all of this leave us?  There is a real maze of options to sort through.  All of the designs I have shown are basically workable, and in my mind there is a near tie between them, and all have drawbacks.  One thing that would be useful is to get a handle on exactly how small guide wheels can be and still perform well in continuous duty at specific, higher speeds. New urethane formulations, like Dupont’s Hylene PPDI have greatly extended the life expectancy of similar wheels used in rollercoasters, although I have heard of some costing well over a thousand dollars each.  Also such wheels almost universally run on round (pipe) track, and so quickly wear in the center before spreading out the load a bit.  How would they perform against a flat surface? How wide could the wheels be?  Is a very wide wheel of small diameter a decent substitute for a narrower, larger diameter one with an equal wear area?  How about asymmetrical guide wheel placement?  Could that allow for a reduced track size?

All fixed guide wheels must contact outwardly, from the center, and steering guide wheels must engage from the outside, towards the center of the track.  If the steering guide wheels double as running guide wheels, they can be large (allowing for very high speeds) the track can be small, and if both sides retract,  very tight maneuvers can be accomplished.  But this creates the problem that was pointed out regarding the “diamond track” design.  While large guide wheels outside of the track can essentially eliminate any speed limit restrictions, one side must disengage at every interchange. Is this a price worth paying for speed and smaller track? Probably, but that sure doesn’t mean I like it.

Perhaps it is possible to include designated zones on the bogey or track where future guide wheels could go.  This might enable simple, straight-forward designs for speeds that have traditionally been considered sufficient without foreclosing the option of higher speed routes in the future.  For the record, I see little problem with switching schemes such as Anderson’s nearly up to highway speeds, assuming a similar track size.   

I started this project with the belief that it was only the desire of PRT companies to quickly get to market that lowered the performance bar.  I was hoping that with persistence and an open mind, a truly simple design would emerge that could do it all.  I have learned that it will be a bit more complicated, and there will be performance limitations that will have a real impact on functionality.  These will be hard trade-offs, and the many judgment calls will mean that such a design will be ill-suited for any kind of standardization.  After all, any design that prioritizes the above criteria differently is apt to prompt significant changes throughout. Thus any standard will only emerge by the usual Darwinian means, with all of the uncertainty and waste that such experimentation entails.

For the moment I feel like starting from scratch yet again, despite the fact that it seems like I am spinning my wheels. (pardon the pun) Sometimes you get too close to a problem and can’t see the obvious. Also, it is time to “try on” at least a few, reasonable limitations to the design’s capabilities. Perhaps in the next post we can delve into the three criteria, and discuss what those limits should be and why.  ‘Till next time!


Juho Laatu said...

I'll present one extreme scenario that tries to demonstrate compatibility of small cheap tracks with some unorthodox steering wheel alternatives. Just to point out where the design limits might be.

The main track is simple and hosts just the main running wheels (no steering wheels). The main wheels can be steered slightly to stay in the middle of the track. Othrewise there are just some pads as a safety measure.

In addition to the main track, in switches there are additional side tracks that are used only for switching. The switches will thus become more complex, but most of the track stays very simple and cheap.

There are various steering wheel alternatives. Or if you want to be extreme, you could even steer with vertical wheels only (as in the switchless segments).

One more comment on forward compatibility and additional wheels. One could design the track with complete set of all possible wheels that may be needed, and then study the possibility of using simpler bogies (with less wheels) on the same track system. Also simpler tracks can be studied (half track, missing support for some wheels).

Andrew F said...

Dan, what level of speed are you thinking of as a limit? I see very little benefit in enabling speeds of more than 150 kph, or about 90-95 mph. You could cross a large city in under 30 minutes at that speed. Higher speeds than this would only be useful for intercity travel, and I think it would be an acceptible compromise to have to transfer to a different vehicle on a different guideway architecture (optimized for higher speeds).

Even 120 kph (75 mph or so) would be tolerable as a top operating speed. It would outclass cars for nearly all trips.

Dan said...

Dan the Blogger is getting sleepy...

One thing to keep in mind is that there is more than just centering, steering-wise, involved here, Juho. There is the possibility of sudden weight shifts in the cabin or gusts of wind. There is also the matter of some kind of rhythmic situation developing and going out of control, something that worries me about the pads you refer to. (Imagine the bogie bouncing from pad to pad) Also with the low center of gravity that results from the swing-arm, there is great leverage at work toward any kind of twisting of the bogie within the track, although I guess the vertical “hold-down wheels could keep that in check.

Switching to your point, Andrew, about what is fast enough, (although I don’t want to spoil the next post by writing it all here) I must say that you are hitting an important point. There is a speed at which individual vehicles become impractical. I am not really influenced much by the argument that a particular amount of time quick enough… What seems fast today is slow tomorrow – I am more influenced by the aerodynamic inefficiency of the form factor. The wind resistance starts to get pretty ridiculous at much above highway speeds and carrying around motors that is are that powerful seems like a foolish waste.

Perhaps the diamond track only enables speeds that are impractical for other reasons. Maybe we had better take Juho’s suggestion and design a track for all contingencies. Back in post 56 I outlined an idea for high-speed “engines” that would fit between and propel PRT vehicles, while joining them aerodynamically into a train. These engines would wait for and position themselves between incoming vehicles, propel them along high speed track sections, then exit, decouple, and wait for vehicles going the other direction. Maybe they are the only thing that needs something like the external steering guide wheels of the diamond track design.

Here’s a wild idea - (that I may or may not have recently posted… I’m tired…) – Have you ever seen a "ground-effect” vehicle?
I wonder if trapped air could be combined with guide wheels to create a situation where the guide wheels disengage or nearly disengage at high speeds... Standard roller coaster design universally matches diameter to load, and I suspect that evolved from experience.. In other words, reduce the load to the point that they don’t wear as much. This could be furthered with the idea of smaller, wider wheels I was pondering. One main drawback is the aerodynamic profile of little fat wheels – however, if they were embedded in an airfoil… who knows? Just throwing ideas out… Actually I guess the effect would be relatively slight. The bogie WILL need an air shroud of some type anyway, though…

OK that’s it!…Bed time!

Juho Laatu said...

Yes, my extreme scenario could be made more conventional by e.g. replacing the pads with small wheels that normally do not touch the track, and making the bogie longer and have shock absorbers, like regular cars. The long swing-arm will cause some twist that makes PRT different from old forms of transport. Tilted tracks could reduce the need of extra (continuously used) wheels a bit.

Speed is important, but there are limits. One practical limit is the ability of birds to escape. People are already used to cleaning the remains of insects from their windshield, but they might not like to do the same for birds. Underground tracks, tracks high up in the air and warnings to birds could help. And as Andrew F said, you could also change vehicle, or maybe your own gondola would automatically drive itself inside another bigger train like gondola (for fast and comfortable long distance travel), or it could connect automatically to a faster bogie.

Automated vehicles that can go anywhere make speed less critical. I'd like to step into my gondola on Friday, right after work, then read something, eat something, watch TV a bit, then sleep, and after that eat breakfast, get the morning newspaper, and then notice that I'm already in my holiday location. Those 10 hours that I spent in the gondola (maybe with a small break in some nice location) could have taken me already quite far (with low cost), even with moderate speed. I could also regularly travel to work while asleep, with very low average speed.

Andrew F said...

The vehicle you want for a 15 minute trip across town is different than the vehicle you would want for a several hour trip covering hundreds of miles. You would need a more comfortable vehicle, with better entertainment facilities. It would likely be more of a premium vehicle, not suited to heavy use (boarding 5 - 10 times per hour). I think that such as high speed, long-distance system is so far in the distance it may not be worth thinking too hard about. I have to say though, such a long-distance system seems hypothetical at best. The construction costs are not very competitive with high speed rail per unit of capacity.

Dan said...

Dan the Blogger's been in the deep woods...
Hi guys... I just wanted to mention that I'm probably going to be posting on speed next... so I don't have much to say here, but you might want to look at the (2nd?) illustration back in post 56 for how I would handle very high speeds. Also Andrew, I question your statement regarding costs of PRT track vs. high speed rail. PRT track without switches or stations running along a freeway would be ridiculously cheap. Railroad cheap? Depends on the terrain...