Monday, February 23, 2009

20> Podcar Control and Encoders

I’ve been wanting to start a conversation about PRT control for some time, so I thought I would start with this primer on motor control.

I don’t know a whole lot about brushless motors, other than that brushes are the means regular motors use to time the advance the magnetic fields ahead of the advancing rotor. With modern electronics, however, motors do not require mechanical means to achieve this timing. Triggering the magnets electronically offers great precision. Instead of “full-on, full-off”, magnetic forces can be ramped up or down; the motor’s rotation can be advanced or reversed, even held frozen.

Whereas in theory one could always know just where the shaft is, rotation-wise, under load the actual rotations or speed could differ from what is expected. One answer is the optical encoder.

My thanks to for the logic diagram and shaft encoder pic. I just glanced at the site, but it looks educational. Sorry about the resolution. Click on the image to enlarge it.

The key is the C shaped “electric-eye”. (optocoupler) As holes in the disk align with the beam, a logic pulse is created. In the third picture, I depict how the same principle could be used to inform a PRT system on a pod-car’s position (and speed) along the track.

Some thoughts:
1. The track can be fitted with the PRT equivalent to traffic signs. Unlike drivers, PRT vehicles will follow their instructions to the letter. “Signs” in the track can trigger precise deceleration rates for upcoming turns, acceleration rates for merging, etc.
2. These “traffic signs” can be dynamic, reflecting real-time data.
3. The track itself should also receive data from the PRT vehicles. Track segments can “know” their traffic counts, for example, to upgrade or downgrade their availability factor, for routing decisions. It would seem that the process of merging would be the result of direct cross talk between cars and track.

There’s a lot more to be said about the division of control responsibilities, so I’ll save that for a separate post.

Saturday, February 7, 2009

19> A More "Mature" PRT Track Design

Here is a section of a more mature PRT track design. I envision this with a skin to keep out the elements. Not shown in the picture is the soundproofing, except for the vibration absorbing material shown in purple, which isolates track vibration from the truss structure. The circular holes are for utility conduits. I have not figured out the cost or even the weight yet, but I must say it is dirt-cheap. Greater height and steel gauge enables longer spans. Note that the design does not show a great strength against sideways forces such as very high winds. A horizontally triangulated truss can cap the structure to address this, but it probably should be in removable sections, so as to provide access to the conduits.

In the traditional business model, a big company would keep the details of the track secret or proprietary, even though it could provide lots of local jobs and that could be a selling point. The integration of utility lines and street lighting could also be of interest to local companies, but again, if the PRT company wants to keep complete control, this is less likely to be in the cards.

The design obviously has to be made approvable by structural engineers for anticipated weight and span, but after that any structural steel fab shop can whip the stuff out. Any city has a dozen such firms. Let’s remember that city managers like to “bring home the bacon”. Also, once the track (at least the structural aspect) is free to be outsourced to locals, and city officials can get bids that they believe in, those in charge will be forced to justify the much greater expense of every subsequent road expansion project against the backdrop of this simple, outsourcable alternative. That would seem to be a battle that PRT can win.