Sunday, March 28, 2010

79> Somethin's Buggin' Me...

I would like to offer an update on the effort toward standardizing a track design for hanging, (gondola style) PRT. I have toyed with a few variations of the Acronym “SMART” which could stand for “Small-scale Modular Rapid Transit”, or “Standards-based Modularized Automated Rail Transport”, or some variation like that. Anyway I like the idea of a calling it the “SMART” system, and it will eventually describe an architecture that can broken down into separately contractible (and maintainable) parts, such as track, vehicles, controls and stations. This is to reduce the total vendor dependency, which municipalities have so far rejected. (post 77) In the last year I have shown many iterations of track for such a system, exploring various possible uses, including indoor and outdoor freight, high speed, group transport, platoons etc. trying to ascertain the most versatile dimensions and layout.

One “fly in the ointment” that I have recently identified is the matter of slope vs. overall height, from the bottom of the cab to the top of the track. In post 16 I show a vehicle being lifted vertically by an external chain. While complete vertical lift is not absolutely necessary, and is obviously less energy efficient than simply lifting passengers instead, I can still see where this might be advantageous. In a system that depends on pervasive local access for its usefulness, the need for inexpensive, barebones stations is great. Vertical travel capability permits the combination of the minimal footprint of elevator-equipped stations with the absolute station minimalism of bus stops. I don’t want to design that out. On the other hand, this requires a swing-arm of a height that is at least half as high as the passenger compartment is long. For example, an 8’ cab hung from the middle (4’ back) needs a 4’ pivot arm. If the vehicle has an auto-style “sit-down” cab, that is, say, only four 4’ high, that is still eight feet to the bottom of the track. Although this is advantageous from the point of view of keeping curious hands out of harm’s way, the problem is that I have previously identified a general track height of about 36 inches as a good minimum size for high speed, high-performance bogies. Add this to the 8’ previously noted and now we have a minimum overall height of 11’. Can this still go into buildings? My guess is yes, much of the time, although it is close enough that it would probably often be necessary to rework some of the support structure for the floor above. It would obviously be better to be able to simply hang track between floors.

 So what is a PRT designer to do? Well for one thing, perhaps recognize that 100 mph vehicles probably wouldn’t be docking in office buildings. That would be a lot of power (and motor weight) for getting around downtown. Can a slower system translate into something that can fit into buildings better? Perhaps. 

In the pictures above, note that the wheels have the flange typical of ordinary railroad cars. Whereas this high-wear part is normally made of steel, which is too loud for PRT use, in low speed environments I believe many abrasion resistant plastics would work. (Nylon comes to mind.) The flange can be separately replaceable. Note that this allows the elimination of the upper guide wheels. I considered a tilted wheel design instead, but with this design the wheels can span the track’s slot so that the bogie cannot physically fall out of containment on a “Y” or merge, provided there is a cantilevered “frog”. This continuous support also lessens strain on the reduced guide-wheel arrangement. The wheels are smaller, in this case being about 15”, and are based on readily available 3 kW hub motors for scooters. The top speed would be under 40 mph, but with dual-bogie designs (8 wheels, like a railroad car) the speed could be greater. Also 3 kW is not a magic number. There are a few more powerful hub motors out there in the same size range and none employ water-cooling, which boosts performance greatly. I want to note that do not know the nature of the structural differences between these low voltage, high amperage motors and the high voltage low amperage motors that would be better for electrified rail (rather than battery) systems… Can anyone shed some light on this? Anyway, this bogie style could run in the track for the high-speed and GRT designs I have previously shown. This design allows the track height to be reduced down to about 24 inches, enabling greater ease of installation in buildings.

More research needs to be done, though, on the average clearance between floors. There are a number of competitive building methods, and I imagine there are strong regional differences as well. For example, steel girder buildings tend to leave long (and tall) floor support trusses in place, supporting poured concrete floors, whereas concrete based building methods usually have more columns and therefore shorter spans for supporting the floor, which may be tensioned with cables in a more unified approach. I have never really spent much time poking around above the ceiling tiles in strange buildings. Perhaps I’ll pose as an exterminator sometime… “Bugs, Mam,” I’d say looking down from the step-ladder with a grim expression, “Big Ones!…”

I guess, if push came to shove, I would favor limiting the slope angle in lieu of building accessibility. In the meantime this gives reason to question any aspect of a standardized track design that adds unnecessary height. I have begun a study to determine how much angle is possible with how much swing-arm reduction and how that would affect station design. I’ll be keepin’ busy.


afransen said...

What about a telescoping arm? Most of the time it could be 'locked' in a retracted position that would limit swing to say 45 degrees, but could be lowered (at stop or low speeds) hydraulically or whatever to accommodate steeper grades? That could cut an extra foot out, too, by my rough calculation. Limiting to 30 degree swing could yield yet another foot. I imagine such a system would add some to cost, weight, complexity, etc. but if it allows us to have our cake and eat it too, it might be justified.

Dan said...

Dan The Blogger Responds...finally.
Thanks for the thoughts, Alfransen. Yes, I have considered telescoping. In a general sense it’s like the “active suspension” on that Michelin wheel motor YouTube Video, but upside down, as far as what would be employed if we wanted real control over the swinging and pitch, instead of gravity and inertia. (the latter being subject to swinging back and forth and wind gusts) It is not a stretch to imagine the system with big enough cylinders to accomplish what you are talking about. What do you think of that Bubbles and Beams technique of coming down the pole? (It’s near the end, where they go dual-mode.) There is nothing mechanically difficult in achieving that- I could build it in my backyard…A bit thicker post and connecting arm and it could even incorporate a counterweight-pulley arrangement. I just have a feeling it might scare some people off. Would people even ride regular elevators if the cables were exposed? In their design there is no swing-arm at all, but it is notable that where they make the pickup in the field there is a lot of fencing and even a hill. That’s a huge footprint. I would like to be able to come down in a parking lot if at all possible, without making the owner provide anything besides the site. (no power required by the tenant or owner) This would greatly simplify negotiations, because there is nothing to lose, even if the store becomes vacant.

Assuming we stick with the swing-arm, is there some simple joinery that can “fold out” or hinge, or must it telescope to hold the vehicle out from the vertical guideway?

P.S. I was shy a few inches on that 4 foot estimate. In a car, part of the ease of getting in and out is because of the wheel height. If you take the wheels off of a car, so the car floor is on the ground, the seats are too low to get in and out of easily. A PRT cab is like that, and so we need higher seats, higher roofline. This makes matters worse.

P.P.S – After considering using the vehicle as the elevator, I have to say it makes a lot of sense. It only needs to have a lift of, maybe, nine feet, and with a counterweight, that’s nothing, power consumption wise. I am actively following up on the idea. Thanks again. If I hadn’t been writing a response to your comment I might have passed this over.

afransen said...

I don't know about that elevator idea that is depicted in the video. It seems to be putting a lot of trust in that mechanical connection. I don't like the idea of dual mode, anyway.

Are you suggesting that vehicles be lowered to the ground in this fashion for boarding/debarking at stations rather than having the guideway descend? Not sure I like the implications for loading time that that entails. It might be less visually intrusive though.

Dan said...

Alfransen, I like to leave capabilities intact until there is a compelling reason to give up on them. I have been looking at the concept of vertical guideways (and the associated swing-arm on the vehicles) as such case. I like it, but I’m not married to it. The fact is that the swing arms make the whole system need to be taller, and that has a price. In the last few posts I have been reminded of this in regards to overpasses and now use in typical buildings. For what? It must be remembered that adding any kind of hardware on to a whole fleet is very costly, and I have yet to make the case that vertical capability is even going to be extensively used. Very steep might be just as good 95% of the time. For the other 5%, yes, there are other means, like in the clip. (BTW, Note the crossed cables… They help with the forces on the admittedly tiny stabilizer bar.) You are right about the speed of passenger turnaround. I’ll quit here because I think I ought to frame these issues a bit better in a full post.

cmfseattle said...

could the vehicle body be rotated 90 degrees (around the vertical axis) before descent and after ascent?

chronokun said...

I'd opt for either a rotating/telescoping arm to keep the height down at stations or just living with extra height(after all new buildings constructed can factor the height into their design and it's not that likely for older buildings to retrofit every floor with PRT stations.) rather than making stations inaccessible to high-speed >=100mph pods.

an-148 said...

worth a consideration is the last comment of cmfseattle concerning rotation of 90° : the "pods" are a smaller in width than in lenght: that would reduce the lenght of the arm.
that faculty to rotate the cabin between arm and roof was already suggested in post 52 concerning the wishbone design.