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!
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