Sunday, February 17, 2013
Back at Christmas, I posted about having purchased a hub motor in the form of a cheap ceiling fan. That acquisition marked a first tentative step toward building an experimental scale model of the SMART (Suspended Multi-axis Automated Rail Transport) PRT platform. I thought an update might be in order. After great deliberation, I decided that the fan motor would be just too hard to modify in the way that I wanted. It was a little on the big side anyway. (1/4 scale) What I opted for instead was to completely start from scratch. After all, what I really want is an ironless axial flux hub motor and they don’t make such a beast. Sure, there are a few small “hub motors” out there, but they are generally of the radial flux design. (Requires too wide of a wheel) And these days any motor that fits into a wheel’s hub is called a “hub motor” even though the motor might be stationary instead of turning with the wheel. Some even have internal gearing, so they are not even one-piece devices. These variations will only confuse everything and everybody. In the end it turns out that the same constraints drive both the actual design and the scaled down version. A taller, skinnier and shorter bogie simply allows a more compact track. It seemed better to clarify these relationships with a model that is as faithful to the full scale version as possible. Here is what I have so far, mounted on a demonstration stand. I’ll dissect it for you in a moment, but first let me tell you one more reason why I am going to all of the trouble.
Ever see the video of that old tape by Aerospace Corp? That is more than a good primer on PRT. That model is one very cool toy…There are still thousands of model railroad enthusiasts out there who get great satisfaction building similar stuff, not to mention robot buffs. Could model “railbots” (not to be confused with the game) ever gain a following? After all, a working model of the SMART PRT platform would certainly be equally fun to watch and vehicle avoidance and routing strategies would offer programmers something truly challenging to chew on.
Every year they hold competitions to race ridiculously impractical and expensive solar vehicles across the desert - the only place where they reliably work - even in the daytime - while teams of enthusiastic high-school kids build robots that can only push balls around. What if these people had something a bit more practical to work on? Beyond that, what could be cooler than a “train set” where the “cars” can go in any direction, even straight up? And talk about educational! This cross between model railroading and robotics would be wonderful for our high schools and colleges. How about a race where the object is to carry full glasses of water through a 3D network without spilling a drop or touching another vehicle?
In the course of looking for components, I ran into all kinds of hobbyist robot motors – but no gearless, direct-drive systems. Maybe there’s a market for such things… perhaps enough to where I could get some much needed help refining the control electronics and software, or to where someone might even start offering something like this in kit form, or even a whole set. I can see it now - Lego Mindstorm's new “Mobot Racer” kits. Every kid should have one!
I have seen model trains chugging around restaurants and malls. I’d love to donate a “railbot” set for the cafeteria at Google. That would get them thinking! I’ll bet they would be delivering food to the tables in no time. Finally I would add that the I have spent a lot of time considering ways to make SMART PRT track affordable, and these attributes play out in the planned 1/6th scale as well. Making homemade track should prove both easy and very cheap. But making these little guys fast as slot cars yet able to climb straight up without a bogey body fat with gears takes some slick conversion of electromagnetic force. Hence my little project.
Here is all you need, propulsion-wise. This is all made with hand tools and stuff like lamp parts, PVC pipe caps … nothing more exotic than motor wire and magnets. The shaft and winding assembly (left) is simply sandwiched between the two magnet arrays (bottom) and capped with the piece shown on top.
Below is the heart of the motor. On the right are bare windings and on the left I am holding similar windings cast in a polyester resin disc. I embedded aluminum window screen on both sides, both for strength and for a heat-sinking purposes. In larger scale production the windings would be captive in an aluminum disc designed for the purpose. I produced the windings by taking 24’ (4.2 ohm) lengths of wire and winding them around a spool (washer/square nut/washer arrangement) spun by a drill… about 6 minutes each.
The basic principle is this. There are 16 windings sandwiched between 20 rotating magnet sets, which creates 64 positions per revolution where 4 magnets and 4 electromagnets can align. (They form a cross pattern). The individual windings are series connected in four cross-shaped groups. For example, windings at the north, east, south and west positions are connected together with a single set of leads. The magnets alternate in polarity and so do the individual winding leads. Sorry – some people will actually WANT these details…
The bottom line is that each winding set can be electrified to attract nearby magnets and when they reach dead center the winding reverses and repels that magnet instead. Meanwhile other magnets are at play, attracting or repelling hard because they are all close to, but not on, their own dead center positions. Each group of coils reverses polarity 16 times per revolution. To run the motor (in the most obvious fashion) requires 4 separate, consecutively timed square waves, so that makes 64 total polarity changes per revolution. There are probably all kinds of ways to tweak those waveforms for more speed with less heat… The way I outlined is “full on” for maximum torque. Two sequenced bipolar stepper motor drivers should run this nicely and enable micro-stepping, which would both smooth out the wave form and allow the coils a bit of lower voltage time to cool. Not that I would use necessarily use this like a stepper motor, although it certainly can be. Since the magnets are of alternating polarity, a Hall effect sensor place almost anywhere will trigger 20 times per revolution, so servomotor style feedback will be accurate to that (18 degree) fraction of a turn as well, as is. Between these two forms of position determination and markers in the track, our little bogie will never be lost – even by fraction of an inch.
Now for the hard part. The analog electronics and software programming. Unfortunately I’m not trained in either, or this little puppy would be going through its paces right now. Lately I’m up to my ears in Googled articles about “H bridges” and transistor saturation. At least I have plenty of holding torque and I’m continuing my education!
Posted by Dan at 4:02 PM
Saturday, February 9, 2013
When I first started this blog, I did so with the conviction that what PRT needed was some sort of standardization – a way to allow PRT to be a joint venture between specialized companies doing what they do best, instead of some little startup trying to be ten companies at once. That way a depth of resources could be brought to play that could never be matched by any other method, except, perhaps, direct government subsidies.
Times have changed and things are changing faster by the minute. This was spelled out to me, yet again, as I was writing my last post on forward-compatibility and extensibility. In that post I outlined a “last mile” strategy, and it involved slower, bare-bones loops within loops – almost like big long circular driveways. This represents an important call… A call for a plurality of speeds. (I have called for high speed routes as well.) This requires a degree of control complexity that seemed pretty far-fetched just a couple of years ago.
A related topic involves vehicle size. Many see the utility of using the track for other purposes beside strictly passenger service. Freight could be delivered, for example, straight into buildings and even into offices or down store isles with vehicles specifically designed for the task. Many will also agree that there might be some cases when passenger vehicles of differing sizes would be appropriate. That means that these two qualities (multi-speed and multi-sized) would require a track that would support such configurations. A system designed to be forward-compatible and extensible into the relatively distant future should, therefore, ideally incorporate such possibilities in a current track design, even if such features are not part of the current system iteration.
One problem with getting the most out of the infrastructure that this track represents is the question of ownership and maintenance. Back in the day, PRT was thought of as one big machine, presumably operated by a specialized team stationed at the terminals of some giant mainframe. Now, with distributed computing, cloud based sharing, dirt cheap sensors and the rest, there is real question as to how much will really need to be done centrally. After all, if you give Google’s automated cars their own lane and program them to pick you up, taxi-style, haven’t you just created a PRT system that is essentially like ULTra? The key point, it seems to me, is vehicle autonomy. If positionally-aware, constantly communicating vehicles can take current traffic data and pick (and report) their own routes, and they can be made to establish safe headways and merge safely, then managing the system would potentially be much simpler than what is currently done by law enforcement for our roadways. Taking the intelligence out of the track, it appears, has benefits. Is “dumb” track the most forward-compatible and extensible choice? Quite likely, I think. It is also cheaper and therefore more extendible in terms of coverage, and is less “breakable” as a system. Thank goodness for technological advances!
It may even be the case that it is easier to get through regulation hurdles with autonomous vehicles on dumb track. After all, it’s hard to argue that a tracked environment is somehow more dangerous than the unpredictability and traction-dependence of city streets, where driverless automobiles would go. Also the specter (or assumption) of larger, multi-passenger vehicles seem absent from the safety discussions at present. I don’t believe Google’s robocars have had to pass any APM (Automated People Mover) standards and they certainly haven’t been subject to the arcane rules designed for trains, something that has been of arguable concern in PRT circles.
Let’s bring standards back into the discussion. As I said in the opening, I have long been an advocate of open-source standards for PRT, so that a system can be made by a multiplicity of specialized companies. But I am also very wary of setting standards too soon, as they could rope the whole effort into one that favors an inferior design or soon-to-be obsolete technology. One important reason to establish them, though, has to do with the PRT business model.
From a point of view of doing the most for society and the wellbeing of the planet, the best model would be for the track to be open to all qualified vehicles…any and all meeting certain standards. There is every reason to want to take as many vehicles off of the roads as possible, and so there is good reason to make the track as versatile as possible in that regard. This brings us right back to extensibility and forward compatibility. Note that I said “model” and not “business model.” From the “PRT business” point of view, designing for the distant future makes no sense. Any PRT startup has no choice but to begin in the least complicated way possible. In terms of overall long-term business growth, though, it makes great sense. What is good for society and the planet also unleashes all sorts of potential business models that cannot exist today. Maybe tiny delivery “bots” will bring you your pizza. Wouldn’t that beat the current paradigm of using a person, fossil fuel, and a multi-ton vehicle to do the job? The point is that extensibility and forward-compatibility are only useful to a business insofar as they relate to their best financial interest in the relatively near future, and therefore their designs will reflect this, even if those designs do not reflect the best interest of the environment, traffic mitigation, or other such benefits.
I envision a business model where a PRT company serves as a general contractor and business partner with a city, but whose role gradually diminishes with time. In such a scenario the revenue stream and responsibilities gradually shift to the city, although obviously the PRT company could always be kept on for whatever role is agreed upon. The important point here is that the PRT company’s exclusive rights to the track should have an endpoint. This would seem attractive to the city, since it does not have to grant never-ending monopoly status to a company that may or may not live up to expectations.
This is where design extensibility and forward-compatibility meet open standards, business viability, as well as societal and ecological priorities. Although it makes no sense for a PRT provider to design a track that can, someday, more efficiently deliver pizza, mail or bags of cement, it makes more sense to the eventual owner, the city. The city’s needs are much more closely attuned those environmental, societal and macro-economic problems that PRT has such great potential to solve.
This mismatch between a PRT provider’s needs and a city’s needs directly influence the desirability of PRT as a meaningful alternative to other transportation solutions. If PRT is to represent a prudent investment in the future, there should never be a possibility of having to scrap miles of track because of aging vehicles or an obsolete operating system, or a bankrupt PRT company. Every component should be able to be updated – forever. Perhaps this is why, although rail-type PRT offers numerous advantages over pavement based systems, it is the latter that is making the most inroads. An elevated mini-roadway may not be what I would prefer to have going down my street but it is clearly an extensible choice. It could be used for bicycles, scooters, golf carts, pedestrians, manned electric taxis, whatever. This represents a safer investment for city planners, even as it caps routing choices and speeds.
A standardized PRT platform that is easily adapted and upgraded would similarly represent a safer choice, if the various parts of the system were sufficiently simple. The owners (the city) should preferably have a system where they could simply take bids for repair, replacement, expansion, etc. The main danger is that such a standard will be a bad one.
It seems to me that building a good standard should be a lot like building a sound structure… Start with a sure foundation. This can start with a wish-list. The two listed above - to allow multiple speeds and multiple vehicle types, are neither required nor advantageous in early iterations, and so without the altruistic guidance of forward-looking standards such qualities could take many decades to see the light of day. The same could be said of multi-axis capabilities, guideways that can carry street lighting and utilities, and other qualities that might be on our list. But if we really want PRT to make a dent in the miserably obsolete transportation status quo, we need to look far ahead and plot a course.
Standards can be very exact and legally precise or vague but definitional - a “standard” putter, for instance. The latter, consensus-driven definition always precedes anything more exact, and there is not a great deal of consensus on PRT design. But what can we generally agree on in terms of PRT’s eventual role? About the objectives that are important to the environment, the society, the economy, etc.? What qualities will create the potential for a pervasive system? What qualities will eventually permit the most efficient management of the system?