Wednesday, May 29, 2013
Let’s start with a bit more about minimal systems. In my recent post, “For Example,” I experimented with the limits of capacity within the confines of a minimum of track (4 miles) and (4) stations. The reason such exploration is necessary is because for PRT to be adopted by a city as a viable mode of transportation, it must first pass the litmus test of being the cheapest solution to an immediate problem. Bureaucracies aren’t known for being far-sighted or visionary, and individual decision makers need to cover their butts every step of the way. They are, after all, custodians of taxpayer money. Therefore the fact that PRT should be set up as a distributed system with many small stations really doesn’t matter. To these people, at this stage, any small station is one that should be eliminated to save a few bucks. Their mindset is essentially reactionary - “If it ain’t broke, don’t fix it!” Widening roads is their first instinct, not making a pedestrian-friendly city. The exercise in “For Example” imagined stations with mythic levels of pedestrian traffic, just to examine the logistics. What is not so imaginary, though, is that only moving great numbers of people will get the attention or funding that a project needs to get off the ground. ULTra’s Amritsar project, I think, is case in point. With up to 100,000 visitors per day and only 7 huge stations, it is not at all what most PRT pioneers envisioned. Yet that was what got enough traction in the real world to actually get funding.
Even without the timid bureaucracy, there is a basic problem with a small, starter PRT network - there are liable to be too few destinations available to attract a steady stream of walk-up customers to any given station. Or put the opposite way, there are too few stations feeding any given destination. Either way you look at it, it is the network effect in reverse. It’s the internet with only a dozen web sites. In addition there is a matter of distance. For track to pay for itself it needs to be used. If the stations are far apart, there will need to be more vehicles on route to pay those bills. In the “For Example” post there were vehicles leaving four stations at one per two seconds at each station to keep the track at full capacity. Suppose we had twenty stations over a similar four mile loop. That is still one vehicle leaving each station every ten seconds if you expect to have the track at full capacity. Now where can you find that kind of pedestrian density for all twenty stations? Luckily, with proper engineering, both track and stations can be made cheap enough to break even at far less than full capacity, and vehicles ought to be mainly amortized by mileage, so there is still hope. But the problem of small networks is still real, nevertheless.
So PRT badly needs a strategy to “get a foot in the door” so at least one network can grow to reach critical mass. Otherwise it is very hard to see how this “chicken or the egg” dilemma can be broken. Trying to ensure that every possible situational roadblock has been addressed, I have designed ultra-small open-air stations, ultra-high capacity stations, track that can turn any corner, vehicles that can even drop down vertically … you name it. But the best strategy is to have some kind of a way for a transit authority to ease into PRT - to “dip their toes in,” so to speak. This must involve some kind of modest starter network that doubles as being clearly the best, most cost effective way to solve some big, immediate mobility problem.
One very insightful reader commented that a good place for PRT to start might be a city (or cities) straddling rivers or other bodies of water. I must say that this is the best suggestion that I have heard in quite some time. Let’s examine it.
In such cases a good percentage of the mobility problems might be for lack of bridges or good approaches to them. Bridges, like roads in general, must be engineered to handle bumper-to-bumper cement trucks, so they are vastly more sturdy and expensive than they need to be to accommodate a steady stream of puny humans in evenly spaced, featherweight pods. A bridge for PRT vehicles would be absolutely tiny - in both stature and cost - even while being capable of delivering people at a prodigious rate. But the bridge itself is just the beginning. On either side of the bridge there must be approaches to it.
These days to justify a new bridge, you pretty much need to be building it as part of a freeway. When is the last time you saw a city inaugurate a brand new two lane bridge? Consider the devastating effect that steering that much traffic towards a new crossing point would do to the riverfront real estate on each side. Also, it’s not just on the banks. Unless there are already riverfront highways on both sides, such roads will cut neighborhoods in half, bring noise, and, of course, will be terribly expensive and disruptive to build.
Adding a PRT bridge would require no new roads, no demolition. Transit planners would have a totally unique opportunity to create a painless shortcut between the most important destinations on each side. Also, cities often make riverfront property into parks because of the possibility of flooding. In many cases these are important tourist destinations, with nearby businesses catering to pedestrians as well as drive-up customers. Joining such car-optional areas on either side of a river would be a boon for such destinations. Car-free zones are highly symbiotic with PRT, since every additional loop extends that “cars not required” status further. Add a zoo, theaters, museums or a shopping mall, and the system gets real gravity. A group of destinations becomes the destination itself, once it is all walkable. And let’s not forget, apartments and condos on “The Riverwalk” would then follow, and those folks could live quite well without a car. Real estate developers love this kind of stuff. One deal leads to another as the whole area goes “upscale.”
One twist I would like to mention in passing is the possibility of using old bridges. I recently heard of a case in Louisville, Kentucky where they saved the old “Big Four” railroad bridge across the Ohio River, and converted it to pedestrian use. This reminds me of the “Rails to Trails” movement, where they convert unused railroad lines to “hike & bike” trails. I’ve often wondered if they would consider sharing their trails (bridges and all) with a nice, quiet and green PRT system!
Anyway, in a nutshell, the premise is this; Building a shortcut between important destinations on either side of a body of water may be too expensive and disruptive to contemplate for ordinary road or rail. PRT, however, being lightweight and elevated, can make the overland portion with the least disruption, and requires a bridge costing only a fraction as much. To top it off, bodies of water typically have parks along their banks that could use PRT service as well. These factors might combine to create the potential for an inexpensive starter PRT system that can still attract sufficient ridership to turn a profit.
Could this be the “foot in the door” that we’ve been looking for? It certainly seems to me that PRT developers would do themselves a big favor by having the resources in place to accomplish such a project.
Friday, May 10, 2013
Here is a one way to deal with the troubling specter of people being trapped in hanging pods. First I have to say, though, that such a thing could never happen; (at least not with the SMART system being designed here!) The vehicles all have robust battery backup, can go into reverse if the track is broken, and can be made to push or tow, and the individual vehicles are not tied to a central control that can create a cascading failure. Furthermore, with only four moving parts in the drivetrain (those being the wheels) and manual override controls on board, the odds of ever needing to evacuate are infinitesimally small. Nevertheless, there are those who might feel more comfortable with such a system in place anyway. This is for them.
The support pole on the left has the system folded up and ready to deploy. On the right it is ready for people to exit the vehicle. The system should be able to be powered from the vehicle’s backup battery. Not visible is the full winch and cable layout, which would work sort of like an automatic Venetian blind. Unlike most designs in this blog, this is truly an “artist’s rendering.” That is to say that it has not really been engineered with any degree of specificity. Still, I think it demonstrates that a crude elevator can be folded up against the track and that the system can be pretty minimal. Commodity winches are widely available and inexpensive, and would be more than adequate for emergency use, so providing the system should not raise track costs significantly. I would not expect they would be used on every pole or even every other pole, but rather, perhaps, every tenth one. There would need to be a protocol for moving emptied vehicles out of the way, however.
And speaking of only four moving parts, let me change subjects and give an update on my little motor project from a couple of posts ago. Before going on my summer break, I did, finally, get it running, although not yet with any sophisticated stepper motor controllers. So instead of ramping voltages smoothly, I am just dumbly turning electromagnets on an off. But hey! - At least it works! Also, my purpose here is to use this to build a working scale-model of a SMART style PRT system. I am happy to report that the motor will work fine for that, including having sufficient power for climbing vertically. I have to admit, though, that it is very, very hard to build 3 more just like it, when I already have vastly improved designs in mind that I am dying to try!
I also want announce the creation of a YouTube channel… It's the "openprtspecschannel"! My sole video is of the motor in action, although I will tell you beforehand that if you just want to skip the technical stuff and just watch it run its demo program, just go to around the 5.5 minute mark. It’s not a very professional presentation, with a couple of obvious mistakes, but for a spur of the moment, “first take” it will do just fine. I just wish that I had explained a bit more about what hub motors are really, really good for, which is hollow-track PRT. To summarize a couple of points that I failed to include:
Since PRT track should be thin as possible, there is precious little room for extra hardware. Hub motors neatly solve this problem. They are also (potentially) extremely efficient, having evolved as the most competitive method for powering solar race cars, for example. The problem of trustworthy traction (that might seem to indicate a linear motor as a better choice) is not really an issue in an enclosed environment, not just because the track is dry, but also because it is easy to clamp onto the track’s interior surfaces in any emergency. But there is more to it than that… Hub motors need not preclude linear motors. There is no reason not to combine the two. Indeed, there are potential advantages. Since hub motors are direct-drive and essentially linear motors rapped into a circle, this common DNA would seem to indicate that a single control signal could easily synchronize both. Such a hybrid could have enhanced acceleration and braking even with a smaller “flat spot” on the wheels. (traction area) This means harder, rounder tires for more efficiency and durability. Linear motors can react with permanent magnets, surfaces induced to be magnetic, or other electromagnets. Face to face LIMs, for example, are essentially twice as strong. Thus, for limited areas coming in and out of stations, a small, onboard LIM could pack a real punch combined with LIMs in the track. Away from the station, it could play a role in centering the bogie, taking pressure off of centering guidance wheels, increasing their life. It could even help steer. This is too complicated for early iterations, but it is nice to know that the option exists for some future time when people demand better and better performance. After all, with a cabin that is designed to cancel G forces, extremely rapid and nimble performance may come to be expected.
Lastly, one problem with motors in general is heat dissipation. This is especially true of LIMs, since they have no moving parts. Consequently manufacturers include ports to pump water through them. Hub motors, (at least “pancake style” ones) on the other hand, being thin but of large diameter, spread the heat out over a large area while using that large diameter to deliver great natural leverage.(torque) It is extremely easy to vent such a motor to “pump” air past the coils while still having only a single moving part…the wheel itself. And to reiterate one point from previous posts… Linear motors can have a problem with maintaining close proximity between the motor and the magnetized surface, especially when that space is between a vehicle and its track. Rotary motors can inherently have rotors and stators nearly touching for minimum waste of magnetic flux.
More torque. Direct drive. More efficient. Air cooled. Require NO extra space… Easily integrated with supplemental linear motors. This is why I think axial-flux, (pancake) style hub motors have a great future in PRT!