There is a problem that I have yet to solve with the gondola-like design I have proposed for PRT vehicle design. It is the problem of balancing the load. When two heavy people sit together, if the “pod” simply hangs by gravity alone, it will be very far out of level.
Let me back up and state the advantages of the design. The first is turning speed. If an ordinary vehicle makes a sharp turn at high speed, it creates sideways G-forces. The answer to this has always been to avoid such turns by designing the road with only gradual curves. (Consider the real estate consumed by a “cloverleaf”) Sometimes the roads are banked somewhat. The problem with gradual curves is that they are very unwieldy (design-wise) in an urban environment. With city streets generally designed with sharp corners, gradual curves have to use the airspace above valuable corner real estate. Right-of-way issues will be untenable. Highly banked track has drawbacks too, such as being banked only for a certain optimum speed, and adding expense to every curve and corner network-wide.
The second advantage is a quicker acceleration and deceleration rate. Any driver who has owned a compact car or a car with bad brakes or has tried to save gas by hardly pushing the petals knows that limiting acceleration and deceleration too much is a recipe for late arrival. If the name of the game is passenger throughput both acceleration and deceleration should be robust enough that having the vehicle be able to rock forward and back to minimize G forces would be desirable.
The third is steep slopes. In order to descend to street level without blocking driveways, or even just for versatility in a hilly town, handling steep slopes is a must.
Then there is the issue of passenger comfort and safety. The gondola design creates a means by which all G-force is diverted downward, toward the floor of the vehicle. This greatly enhances passenger comfort, safety and saves a lot of otherwise spilled coffee.
So the unsolved question is this; How do we keep the advantages of a free-hanging vehicle while not having it tilt from an uneven load? Note that making the vehicle wheelchair friendly tends to mean other seating is further from the center of gravity, making the problem worse.
So that’s the problem. If you have an idea, please post it, if it involves a picture, email it to me and I will post it for you.
10 comments:
Dan the Blogger laments...
I don't know why this guy is rendering blue...Seems like a problem with the site. Double clicking gives the correct colors..
I'll try to fix it later...
Don't forget that motion sickness emanates from a discrepancy between the visual and gravitational notion of "down". While banking so that the G-vector pushes the person down in the chair is good when flying at a high altitude this is just as there is no visual reference. For tilting trains no more than 50% reduction of sideways forces can be accepted before too many passengers get nauseous.
The "joint" must be under active control, not passive.
MIST-ER proposes this, see #10 at mist-er.com/faq.html
Suspending the car is attractive because the cars can swing in turns, but I believe that Ed Anderson long ago demonstrated that this design is not optimum. Putting the car above the track may not look as cool, but it saves a lot in cost, weight, and complication. Banking the track for turns is not a major problem, since the cars run at constant speed.
Dan the blogger responds-
Hope you don’t mind if I call you THMB. Can’t say I like the name.. Anyway, It was Dr. Anderson’s failure to ever sell a PRT system that led me to start this blog. It’s not about swingin’ and looking cool. It’s about buying right of way for clipped corners, (Ed Anderson’s system had a minimum turning radius of 50 ft) the cost of raised disabled compliant stations, the visual impact of a Anderson’s LARGE track only 19 ft overhead, and other issues. Please read “Pods Just Gotta Hang,” my 11/09/08 post.
Hi Dan,
I've been reading your blog with some interest the last few weeks & I've enjoyed your take on PRT design.
While I agree with you on the issue of large turning radius - I couldnt really call Dr Anderson's track design "large". If memory serves, it was 36" high by 18" wide.
Mebbe you're thinking of the Raytheon design.
Dan the blogger responds..again..
Bengt, you are right, and I would have responded sooner, but I had to think about it, and never posted. I don't get motion sick, so I design for people like me. I'm stepping back now, however, and reconsidering a bunch of assumptions. thanks. P.S. I put Beamways on my link list.
Mr. Grant, (if or when you're out there) do you know about any "non-Raytheon" Anderson designs? (I've always wondered if it wasn't the "suits" from Raytheon that are responsible for a lot of went wrong with PRT in the 90's. BTW, "active control" is not what I was hoping for.The last thing I want is to have to design the thing to be an up-side-down Segway..I was rather hoping for some hydraulic or spring solution. (but I'm still stuck)
Anonymous-yes I am thinking of the Raytheon design, Posted earlier. (8/09/08) I am unaware of any Pre-Raytheon Anderson track designs. Thanks for your support!
As you probably know, Mist-er does this by not allowing the gondola to hang freely, but instead being supported by two support points (two corners of their triangle-profiled track). This essentially makes the construction stable and resistant to wind or off-center balancing. So they say.
[ Actually, the Mist-er construction only gives the stability for sideways, not for back-forward axis. So the issue remains. ]
Another approach can be to "so what"; let it hang the way people would sit. It's their fault, then. :) In general, people are rather adaptive creatures and we've even learned to climb in and out of cars.
Dan the Blogger Responds..
Akauppi, I know you're European, (Finland, right?) so you are well qualified answer this question. What's the general standard for access for disabled, (wheelchair bound) travelers? By U.S. law, we have to have each vehicle and boarding point wheelchair accessible. On a PRT "pod" that means a space in the center of the vehicle, and that means non-disabled riders sit further back or forward, creating the tipping problem. Do you have similar statutes?
Superelevation solves the problem of high speed turns. Since pods ought to all be travelling at the same speed along any given stretch of track, and there is no "lane changing" involved as with regular cars, superelevation shouldn't pose any safety issues, at least with steel wheel on steel rail vehicles. Unfortunately, it looks like most PRT designs today use rubber tires... but most of them are designed for relatively low-speed operation anyway, so the issue of high-speed turns is relatively moot for them.
As for slopes, rack railways have a solution for this. I think in general it's better to use ideas for locomotion that come from rail rather than automobiles.
I don't see the advantage for acceleration/deceleration. There ought to be better ways of stopping the vehicle that don't involve shaking up its contents somewhat violently. In particular, one could simply give the vehicle enough off-line track at each station to come to a stop or slow by inherent rolling resistance, or resistance added to the portions of off-line track approaching stations.
The only real advantage I see to hanging cars is a smaller guideway. But there are probably ways to make a minimalist guideway for top-riding cars, particularly if you use steel wheel on steel rail. Many conventional railway bridges, after all, are pretty minimalist considering the bulky machines they carry.
There is a really simple solution to all this hanging gondola stability stuff, which is to hang it from more than one contact point with the guideway. Two points give you front-back stability; four points full stability. One point may supporting more of the weight than the others, but this is basically the best you can do for stability with a gondola craft.
Whatever you do, there is going to be a certain upper speed limit imposed on gondola craft because of tension on the supports. This is a problem for those of us who want to see PRT function as more than just a low-speed urban shuttle.
And FWIW, I think putting the car on top of the track looks plenty cool :-P. It's having it hang below reminiscent of certain giggle-worthy things that makes it look fogey.
Post a Comment