Friday, June 26, 2009

39> Progress on the Track

Here is what my track design has evolved into. Note that the bottom (shown on previous designs) has been removed. I think that it is important to simplify the problem by separating the track into its two functions. The first function is to provide the surfaces upon which the PRT vehicle may roll and steer. The second function is to span the distance between supports. Separating the two makes sense, because the supporting, spanning role is site and weight dependent. Inside of a building, for example, a very minimal track could be hung from the building structure itself at short intervals, and headroom might be at a premium over ability to span.

In this illustration a minimal track design is shown in black, and the “wrap around” area in blue is where a supporting truss, soundproofing and outer skin can be. In the latest iteration, both the drive wheel contact area support and the top middle beam (where the steering guide wheels contact) are designed to have some variability in size, at least for the time being, as weight requirements and preferable fabrication techniques will play a role in this decision. It may be most practical, for example, to have these as separately fabricated modular parts, as they all would include rubber mounted running surfaces and some machining.

Optimum internal dimensions are 30” x 20” range, (75-77cm x 50-52cm) with drive wheels of under 21” x 7.5” wide. ( 53cm dia. by 19cm) The track dimensions allow guide wheel sizes of about 7” x 2.5” (178 mm x 64mm)

Advantages of this track include being able to handle multiple weight classes, (final specs should allow variable steel thickness) being able to accommodate vehicle speeds in excess of 60 mph (100 km/h) without requiring excessive guide wheel speeds (under 3000 rpm) being easy to fabricate, including turns and forks, with standard stock and flame cut steel (and minimal machining) ability to accommodate various propulsion means including wheel motors and LIMS, being able to accommodate slopes up to and including vertical, being able accommodate extremely tight turning radii, (with preferred drive train/motor configurations) being sized to allow use within buildings with typical ceiling heights, having a standard exterior profile which may be incorporated into a variety of truss or suspension structures.

Tradeoffs- The only tradeoff I have been required to make is size. If I were to limit speed and weight substantially, it could be made a bit smaller, but there are diminishing returns for the following reasons. Greater height allows more leverage against being twisted by inertial forces, longer wearing drive wheels, and it contributes to spanning stiffness, as well as allowing more flexibility in drive unit design. Greater width allows larger guide wheels, which then last longer because they have more, wear area and slower speeds, and equally contributes to stiffness and design flexibility.

I welcome your comments.

Friday, June 19, 2009

38> The 16th Rule

I was considering my response to alert reader cmfseattle’s comment on my June 7th post when I got to thinking about this addition to his comment. “Rules of engineering” (NIH) and what I was about to write seemed to warrant a post of it’s own and so here it is:

J Edward Anderson, for those who don’t know, is sort of the “grand elder statesman” of PRT. He holds patents, has written books, countless papers, and currently heads up PRT International. One of his papers is “15 Rules of Engineering”, and rule number 9 is “Recognize and Avoid NIH (Not Invented Here)”

So am I just re-inventing the wheel? A quick look at PRT patents would tend to support that case. Here is just one sample illustration. Look familiar?

Well here is my defense. Dr. Anderson left out one rule, one that I will call, “Think Super,” and it goes something like this.

All designs come up against natural constraints such as the laws of physics, social preferences, budgets, time, etc. All designs also carry the limitations implicit in the definition of project itself. Dan’s sixteenth rule of engineering would caution against accepting such restraints without being absolutely sure that there is no simple way to work around them. For example, how big should a PRT vehicle be? Answer. Somewhere between microscopic and celestial, until some factor forces constraint. I know what you’re thinking… (OK, not really…) “If it’s called “Personal Rapid Transit” It should be sized for its purpose, say big enough for 4 adults.” By that logic, it should be sized for one and one only. After all it says “personal”. But are we not designing an automated parcel delivery system where the parcels are people? If all else is equal why exclude the possibility of delivering anything? Now before someone starts writing about the downside of cargo delivery, understand that this is just an example. The downsides that that writer would list would be the constraints I have spoken about.
It is an unfortunate side effect of the profession that engineers are tasked with creating a design from decision-makers with time and budget constraints of their own. I know few engineers with the guts to really think “outside-the-box” in the critical initial stages of a project. Limiting the objectives of a task limits the work involved and speeds completion. That’s sound business practice in most cases but it leaves improvement for later models, making for slow, evolutionary change.
So why re-invent PRT? Because all of the designs I have seen are constrained, not by what is possible, but by what is expected. For example, 95% of PRT is track. It’s the permanent part. Yet it seems to me that precious little time has been spent considering the final form and function of this potentially enormous investment. To my knowledge, I am the only one (or at least one of precious few) suggesting designing-in the capability for carrying modernized street lighting and utilities or having a configuration that could be adopted for use in a warehouse. If functionality can be designed in with no additional cost, why not?

Near my camp in New Hampshire there is bike trail utilizing the remnants of a railroad track that went all of the way to Boston. It was built, however, for smaller trains than are standard today, with narrower track and bridges. Its present use speaks for itself. How did this standard get on the wrong side of history? How do we avoid making the same mistake? In a discussion about an existing PRT design I was reminded that vehicles need not corner quickly because it would buffet the passengers too much. What about a trip to the hospital or freight delivery at 3 am? Or repositioning empty vehicles? I was reminded that all of the vehicles travel at the same speed. Why? I will remind the reader that for most of the history of PRT, control without crashing was the issue. I think we’re moving to a place where the cars can have the intelligence to follow a much wider menu of directives.

So this is my philosophy on designing a PRT system. How fast? Lightning fast. How steep? Vertical. How tight the turns? On a dime. I say, let’s design SUPER PRT first and then back off from there, as required by current constraints, rather than putting time, thought and money into designs that perpetuate limitations simply to expedite a business model. Don't get me wrong. I have nothing but respect for the people trying to bring this technology to market. I just want to prevent track coming down in 20 years because better, newer systems and new uses require a slightly different design.

Lastly I would like to point out that I am endeavoring to create a set of standards first, not a set of blueprints. As I envision it, these standards would be useful for future designers, inventors, contractors and their customers as a means of simplifying navigation in a sea of complex functional concepts. Prioritizing the above-mentioned constraints inevitably leads to differing opinions on design options, and so a natural branching occurs. Such a branching has already occurred regarding PRT vehicles which hang and those that don’t. Have we ever really defined the trunk from which these branches emanate? Or are we just going to let it be defined by Wikipedia or Webster and design from that?

Friday, June 12, 2009

37> Coming …Soon?

At the very beginning of this blog, I envisioned the possibility and intention of collaboratively designing a PRT system, and I have taken a step in that direction. The first problem was the fact that not everybody who could make a valuable contribution to a design has access to, or knows how to use, AutoCAD, or even a vector based drawing program. (Or a decent paint program for that matter)
Alert reader and frequent commenter akauppi, when asked about this matter, suggested Inkscape for a Drawing program and Acorn for a paint program, both free to download. Apparently Acorn is only for Macs, but I have found what I consider to be a great, free paint program in Paint.Net, which, by supporting layers of variable transparency, allows on-screen positioning of separately created parts. The most exciting to me, however, are the tools provided by Google. Besides hosting this blog and my email account and analytics, they give away a very competent 3D design program called SketchUp, which I used to draw the second illustration of the last post. But there’s still more. Google also hosts space and tools for project collaboration. Although they are intended for code development, there is no reason why they can’t be used for the design software listed above. They even include tools for revision control and a wiki. So coming soon, you’ll be able to modify my designs and post those revisions. But I have to warn you, I know very, very little about SVN (look it up in Wikipedia. I had to) and Sketchup takes time to learn as well.. Meanwhile, a simple question was asked about my last post. What’s so special about the layout of those wheels? (Refer to the illustrations from the last post) well, if akauppi, doesn’t get it, I guess I better explain for all.
A good design begins, foremost, with a good understanding of what you’re trying to do and what you have to work with. In the case of all of those wheels, vs. the expected forces exerted on them, it is geometry. Move the wheels up or down, forward or backward, and the performance changes. (I would like to note, however, that these illustrations are consistent with PML’s wheel-motors and my scheme for climbing steep slopes) And then there is the track (which, because track is reproduced into infinity, is really, really important to get right)
I could write a few paragraphs on every dimension and every angle, but have not, because I recognize that I have attracted many readers who are not engineering oriented, and this is a good thing, because we’re not designing transportation for engineers. This blog has attracted a group of very thoughtful contributors, and I feel confident that the core design issues are being dealt with in a forum that will eventually yield superior results to the “top-down” approach that commercial enterprises have to use. I want to urge patience, however, because good designs take a long time, even for teams of full-timers.

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.