Sunday, April 25, 2010

83> Get SMART

I want to do a little update on my ongoing SMART project, which nowadays stands for “Standardized Modular Automated Rail Transit”. If that sounds like it encompasses a lot more than PRT, well, it does. If someone chooses to automate, standardize and modularize full-scale trains or monorails under this umbrella, more power to them. I am concentrating on specifications aimed at lighter, smaller systems.

For newcomers to this blog, here is yet one more rehash of my position and reasoning. If a start-up company tries to sell a PRT system to a municipality, it must convince that customer that it is selling both a system and a service that will be reliable for decades. Remember Worldcom? Enron? How about Lehman Brothers? PanAm? These were huge corporations with proven track records, but they, for one reason or another, went under. Who would trust a startup to change the cityscape with millions in infrastructure that only it understands, and could maintain for decades to come? The tack of starting out incrementally is equally troublesome, because a minimal PRT system loses all of its advantages. It is like the Internet with a half-dozen web sites trying to support itself with a half dozen customers. Going directly to any destination in a network isn’t very compelling if the network only has a couple of stops. A simple shuttle could do that.

I am not against PRT being supplied by a single firm. I just think it is a foolish notion that any city will accept a set of contracts that creates utter, unending dependency on the vendor, or leaves the city with infrastructure that nobody understands.

There are various ways to mitigate this problem, such as alliances and partnerships, and of course the contract language itself. One additional way, and this is what the SMART specification is all about, is to break the PRT system into manageable parts from the onset, so that those parts are easy to understand, improve and subcontract, both by the PRT vendor and, in the event of any failure to fulfill terms of a contract, by new partners. I do not think it is coincidental that the two currently awarded PRT contracts are both for vehicles that travel on a simple paved surface, in spite of efficiency and weather issues. If the deal goes south the buyers can always fall back on standard electric vehicles instead.

PRT is broadly comprised of track, stations, vehicles, and a control and communications infrastructure. In my view it would be extremely beneficial if all aspects of PRT construction and maintenance could be so divided as to be accomplished by local contractors with no previous PRT expertise. When all responsibilities and areas of expertise are co-mingled, it’s an organizational mess. This is not an easy task, however. Synchronizing high volume, high-speed vehicular merges is an example of a situation that calls for close association between control, communications, the physical vehicles and track. Some elements of centralized, integrated system architecture may, in the end, prove unavoidable. That doesn’t mean that we can’t chip away at the problem, however.

I have started with the track. One advantage that I have enjoyed in this project is that I have no deadline, no target price, no set corporate agenda. I can imagine anything and everything that might ever be required of PRT track, now or in the future, for any town or country in the world, and see if it can’t be fit in somehow. A track specification, in the broadest sense, need not exactly fit a specific vehicle or vehicle weight. It need not be designed for a specific target speed. It just needs to be versatile. Being a specification, it can rest on a foundation of broad generalities and be further constrained and defined as needed by adding additional version numbers.

The following is for hanging, gondola-style PRT in a basic box beam track. That does not mean that there is no value in doing the same for bottom mount track, or other schemes as well. I am only one guy, and I still have to try to scratch out a living whenever I’m not too busy!

Below is pictured a very short track section. (without any support structure) The red areas represent areas that take pressure from a bogie (PRT motor unit) traveling within. To see examples of how the bogie would fit, scroll down to previous posts. It should be pointed out that the areas that are gray on both sides could actually be eliminated. The red areas could be held in position by structural trusses, or by being connected to a building’s architectural structure, for example. There are, as yet, no provisions for electrical rails or rack and pinion means for steep slope travel. Such details are dependent on the specific bogie design, and therefore would be better classified under a bogie/track interface specification. (along with the running surfaces, shown in green, in the second illustration) One last thing to point out is the square tubing on top. I have made it height-adjustable, which is a must, especially when making an abrupt change of pitch, which changes the height of the bogie relative to the track. 

The second picture describes the basic dimensions suggested by my research. I think it can be safely said that track built within these dimensional guidelines will always be useful and will prove very versatile. It is designed for weights up to those needed for larger group vehicles, as well as for highway speeds and beyond. For extremely high speeds fairly straight sections could probably be retrofitted to accommodate maglev technologies such as Inductrack II. It is compatible with any reasonable turning radius or pitch. It can go into buildings with average ceiling heights. A shorter “chopped off” version can be made for slower speed bogies used in material handling in factories or parcel handling or airport baggage. Such bogies would be capable of citywide travel without disrupting other traffic because of computerized routing and scheduling, probably in the middle of the night. I can even envision a very thin ceiling hugging version for indoor micro vehicles to deliver medicines, food and equipment around a hospital. Anyway, here are some “first draft” dimensions and brief comments on the reasoning that went behind them. 

A - 230 mm. (9”) Considerations for this dimension are that the larger steering guide wheels contained therein will rotate more slowly, have more surface area and therefore last longer. While this is not in itself a big deal, especially since the wheels only engage when changing track, there is no particular reason to make the dimension smaller, other than to make dimensions D or E larger. See discussion of B,E.
B – 65 mm. (2.5”) This is one of the dimensions that was squeezed to get the overall height down. And it only leaves room for fairly thin, disc-like steering guide wheel. This means a small wearing surface, (made better by a 200 mm diameter) but this also has the advantage of being more aerodynamic.
C – 815 mm. (32”) This is as short an overall height as I am comfortable with. It allows a drive wheel diameter of 510 mm. (20”) This allows a 330mm. (13”) rim. This general height will enable a full system height of about 3 m. (10’)
D – 100 mm. (4”) This is dimension needs to accommodate some side-to-side movement by the “hold-down” wheel (about 50 mm) which engages it.
E - 600 mm. (24”) This dimension could be made more compact but there is no compelling reason to get it down. It adds stability by allowing a wider wheelbase, and there is also the matter of aerodynamics. If the bogey fits the track too tightly it will have to push air instead of slipping through it. This is one aspect that still needs looking into.  
F – 50 mm. This should be designed to keep arms out, away from the electrical rails. I keep trying to figure out something fat that needs to go between “pod” and bogie, but it seems like a few tubes and wires are all there are. The tradeoff here is that to prevent the (bogie to swing-arm) connecting piece from bending, some thick (heavy) or stainless steel (expensive) or corrugated (complex) material would have to be used.
G – 65 mm. This is the same as B.
H- This is structural plate running crossways to the track. The size would vary depending on span and other factors. For example, if the track were attached to a ceiling this part would be of minimal size. This would not be part of any specification, but rather to be decided by structural engineers. There would probably be holes or channels in it for utilities.
I – I doubt this would be part of a main specification. There are many ways to design a bogie, and these concave surfaces are challenging to manufacture for curved track sections. They would probably be made as removable inserts. This has advantages of allowing sound (vibration) isolation and expansion joints can be produced with greater precision. There is also the matter of tire width vs. the radius of this piece. (The two would ideally be sized for each other although there is some flexibility here.)
J – 25 mm. Minimum. I would like to get that number up a bit higher.
K – 76 mm. This is a bit taller than B and G because some designs might use this space as a primary centering means, with wheels that are meant to be kept in constant contact, unlike the steering-guide wheels. Therefore larger diameter (half of E) and greater width would reduce the maintenance associated with wear on the wheel.
L – 6 x 65 mm. These are non-continuous, rubber mounted strips which taper out from the surface they are attached to at either end. They are placed leading into and out of junctions only.  


Juho said...

I guess this is the latest specification of the track. Here are some quick questions and comments that I collected when reading the text. (It seems I have to break the message in smaller pieces to fit below the 4096 character limit.)

You mention that the gray areas of the first picture and the blue areas of the second picture are not really part of the specs. I think the specs should consist of the red areas that take the pressure and the free space that the bogie and gondola are allowed to use. The allowed empty space can be guessed quite well from the pictures, but to be exact, it should be specified. I mean that in the specification picture you could draw only the empty space and the pressure areas that lie next to it.

One could describe separately the profile of a straight track, and the profile of a turning/switching track. The turning/switching track is roughly one half of the first picture since no assumptions can be made on the proximity of the pressure areas of the other half. The pictures are ok as they are now and the text (or picture) can also tell the role of each part while switching. This is just one approach to make visualization easy.

Of course also simplified industrial tracks, high speed, low speed, heavy weight, light weight etc. variations have to be agreed in order to understand all possible interoperability related requirements (of different bogie and track types).

This specification is a very natural approach to a beam like track with suspended vehicles. I considered also some alternative approaches. For example, what if the hold-down wheel(s) would be under the main track leaning towards the "I" section from below. Did you assume that the thickness of "I" section is fixed or could a light weight track have a leaner "I"?

A general question on the hold-down wheel. How much is it for traction in uphill climbing, for braking, to control tilting in curves, to eliminate jumps in bumps, ... ? Does it have continuous contact and pressure?

What are the running surfaces for? Noise elimination, traction, to contact the turn wheels in the switches?

What is the round corner of the running surface in the "I" section for?

Have you considered further noise reduction, weather protection etc. by adding one more light weight cover below the track (near "E" in the second picture)?

Juho said...

(second part of the message)

The specs have two turn wheel supports ("B", "G") and one steering-guide wheel support ("K"). Both turn wheels are above the steering-guide wheel. In some extreme situations in a switch (turn at high speed with side wind and no support from the other side of the switch) most of the horizontal pressure could be on "G", and "B" could lose contact to the track (maybe also "K") when the bogie tilts. I guess the hold-down wheel is expected to keep the main supporting wheels in contact with "I" and thereby eliminate tilting of the bogie. The hold-down wheel is however quite high up and could move a bit sideways under horizontal pressure. Is this ok, or will this not happen with the planned speeds and with the current hold-down wheel design, or should one try to avoid also this kind of behaviour?

If one wants to avoid this kind of tilting problems, then one approach would be to move "K" higher, between the two turn wheels (or add "K2"). That would mean somewhere above the "I" section. This kind of triangular position of "K2", "B" and "G" wheel positions would in principle be more stable in some extreme situations. (There is a small vertical section of "I" too but there was no mention of a wheel leaning towards that section.)

In high speed turns in a straight track segment (=not a switch) similar tilting of the bogie might occur, but the "K" section of the other side gives some additional support against tilting.

Having two steering-guide wheels ("K", "K2") would in principle be a more stable solution than having only one steering-guide wheel. I guess the turn wheels are not intended to give support against tilting of the bogie (they could be in the wrong position anyway).

These tilting problems might be relevant only to some high speed tracks.

One alternative arrangement of the turn and steering-guide wheels could be to have only one turn wheel position and two steering-guide wheel positions ("G", "K", "K2"). Have you intentionally kept the number of wheels with continuous contact (steering-guide) wheels small? (There are arguments in both directions concerning the number of turn and steering-guide wheels.)

Note that if one uses the "G", "K", "K2" scenario above and moves the hold-down wheel down (below "I"), then there would be no need for the square tube.

The lower position of the hold-down wheel(s) would be somewhat safer against tilting. It could also allow the hold-down wheels to have lighter contact or be narrowly separated from the track. One could use them also to transmit power to the track in uphills / downhills and when breaking.

You mentioned that the steering-guide wheel at "K" could be of size "half of E". It could be also larger if needed.

That's all for now.

Juho said...

I must explain one more thing. In the case that the hold-down wheel would be below "I" it would have to be lifted/dropped away in switches just like the turn wheels of the other side. Or alternatively one would have to position the hold-down wheel pressure area below "G" or "K".

ItsEric said...

Don't forget periodic inspections would be needed. A special bogie with a platorm could carry equipment & cameras to do this and either relay it to an attached pod or to the maintenance facility.

ItsEric said...

How would you assemble the long it? One way would be to build & weld them together in say 100 ft/30 meter long pieces in a warehouse where you have jigs and computer controlled equipment to ensure high quality track. They would then be hauled out & lifted in place and welded to existing track using a specialized welding bogie. Another option would have a mobile plant that is able to weld the pieces together continuously, slap on the exterior and hang it on the support structures in one process. The rail roads do something similiar when doing track maintenance.

Either option would need to be able to install switches as needed.