Friday, May 10, 2013

156> Escaping to the Movies

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!

Anyway, get some popcorn and enjoy the movie!  Also, there are a few other axial flux and hub motor projects on YouTube, some quite impressive… Check ‘em out!

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