Sunday, June 7, 2009


Remember how I said I was all fired up and ready to design a PRT drive unit around the PML “motor wheels?” Well despite being in the deep woods without any meaningful communications or electricity (and a garage to build up by the road), I have nonetheless managed a bit of progress.
Here’s a drawing that shows the basic structure I have been working on. There are 5 drive wheels which are self-turning “wheel-motors.” The figure on the left shows how many wheels it takes to do the job (although 3-wheel sets may be substituted for 4-wheel sets on turning and guide wheels, with minor loss of stability, just as a three-legged table or a 3-wheeled car is possible but not as stable). Note that half of them become inactive in the process of switching tracks (3rd figure). In the second and third figures the red “right turn” wheels are in the upward, engaged position, allowing all of the wheels on the green “left turn” side to disengage. The ability for cars to do the switching themselves, instead of having to build many switches in the tracks (like a railroad) is pretty much a standard feature of all modern PRT designs. Keeping all of the wheels aiming parallel to the track even as the track turns sharply is the challenge, although such tight cornering would only be for very low speeds anyway. Nonetheless, any good designer would want to reduce such frictional losses and associated wear and I am no exception. If the wheels seem very bunched together it is because I originally drew this as part of a 2 assembly set, much the way train cars have two separately pivotable wheel assemblies per car. These assemblies, connected by a universal joint, would enable extreme flexibility in track layout including those very tight turns I referred to earlier.

Addendum – I wrote and drew that post while still up in the woods of New England, and have since spent some time at RIT (Rochester Institute of Technology) hooked up to broadband, so I have had a chance to further my education (via online video tutorials) on what I consider to be a pretty exciting development, a free 3D modeling program from Google. So here is the extent of my abilities so far. Here I have experimented by using the “3-wheel sets” that I referred to above. In this one the green wheels are in the engaged position and the red ones are down.


Anonymous said...

Why don't you join the transport-innovators list?

Anonymous said...

So what exactly is new or unique with your wheel layout?

Bengt Gustafsson said...

Your design is very similar to one of the first patented PRT solutions, from 1972:

Dan said...

Welcome, Ian. I guess you’re referring to the “Google groups” discussions being hosted by ATRA.. I’ll keep an eye on it but don’t expect me to jump in too often. I’ve got more on my plate than I can handle already. If you see something I really should know about, give me a heads up. Thanks.

Akauppi, I’m still grappling with that essay you provided. It dovetails with the comments from resonance (last post) in most interesting ways. As far as the design is concerned I am not really attempting to create some unique, patentable, design just yet, more hoping to elicit responses like this one from Bengt. This month I have lived in the woods for 26 days without electricity, traveled 4000 miles by plane, 360 by bus and 1500 by car, visiting 8 states. I have poured a concrete foundation, but only after I put down in pipes for both forced air and hydronic heat, as well as plumbing and under-slab electrical conduit. The last picture was created with a program I had never heard of 5 weeks ago, so I’ve been teaching myself that. I could go on, but I’m sure you get the picture. That’s why I rely on guys like you to point me in the right direction, not to mention to enrich the content of this blog.

Bengt, thanks. I guess I should do a bit more research before investing too much effort if I don’t want to waste time being stuck on a problem someone else has already solved. Those pictures are so close, it’s eerie. I guess I’ll sit back and study for a while.

Anonymous said...

You can use my name, it's Asko.

Nice to meet You. :)

cmfseattle said...

Your switching mechanism doesn't show how it will be actuated. Anderson emphasized that the point of rotation was very important.

"The upper switch arms are designed so that a plane passing through the center of the switch wheels and perpen¬ dicular to the traction surface of the switch wheels will pass through the pivot point of the upper arms. Therefore, the switch arms are self-correcting in that the switch wheels cannot exert a twisting torque on the pivot points of switch arms."


Those patents are now in the public domain.

A lot of engineering problems (differential thermal expansion, ability to adjust the running surfaces, torsional stiffness and avoiding "slope discontinuity," just to name a few) have already been worked out for you.

Transit Systems Theory
15 Rules of Engineering Design ("NIH" syndrome)
Rebuttal to Parsons-Brinckerhoff (Guideway Design)

Dan said...

Dan The Blogger Responds-
Asko, I use “akauppi” so as to not confuse new readers. Same reason I call myself "Dan the Blogger"

Thanks cmfseattle, I’ll look these over. More importantly, my readers will look them over too. A switching mechanism hasn’t been shown due partly to lack of time on my part but mostly the fact that I have been more concerned with more “global” issues, like track dimensions, gross weight, turning radius, horse power, etc. I will say that although I have previously shown a system where the guide wheels slide within a sleeve, I have various designs on my computer using the rocker-arm system. I must say this Anderson design is clever. By moving the pivot point to achieve zero twisting torque, (itself no big deal) he creates a “bistable switch” which allows the guide wheels snap, by spring, into either position but not in between. His geometry holds the guide wheel in place by the vary forces exerted on it. This scheme would seem to preclude the possibility of a guide wheel assembly being damaged by trying to grind into engagement if the system were misaligned. On the other hand, this is a neat solution to an event that must never happen in the first place. If the guide wheels fail to engage, it is a represents a catastrophic system failure. The “position lock” he avoids can be as simple as the ordinary locking that takes place when something gets screwed into position. When you jack up a car with a screw jack, it doesn’t unscrew because of the car’s weight. Gearhead servo motors can equally do the job, balanced geometry or not.

I must say that this solution is reminiscent of the many clever mechanical solutions of an era without cheap yet sophisticated sensors, servos, or microprocessor controlled actuators. That being said, it also seems like a promising scheme for a “slave” set of guide wheels, wherein, for example, a lower, servo driven wheel set causes the upper (Anderson style) wheel set to “snap” into position as well, thereby eliminating the possibility of any kind of positional conflict between sets of guide wheels, while not requiring a rock hard, flex free (yet long) linkage between the sets, since the Anderson design has that self-correcting quality. I do wish it wasn’t such a space hog.

I finally just got a chance to see the look at that rebuttal PDF. That is interesting stuff. I see why I have gotten flack for confusing taxi 2000 and Raytheon. I wish I had the weight breakdown they referred to. I am still curious about the actual weight/performance characteristics of a professionally designed LIM propulsion system.

Anyway, thanks for the input. It’ll be a while before I digest it all…

P.S. I noticed you included the 15 rules of engineering, and singled out NIH syndrome. If you think that I have that, you are probably right to a degree, especially since I love designing and hate studying. There is, however, one other good excuse. I generally try to design from absolutes backwards so as not to preclude possible functionality, as explained on a post that may or may not be up at this time.