Sunday, February 7, 2010


I want to talk a bit about brakes. I am afraid that much of this has been previously discussed in the comments section, but not every reader follows the comments, and so I have to include it. First let’s clear up a little something about LIMs. (Linear Induction Motors) Proponents will point out that they make traction irrelevant. Icy surfaces will have no effect on braking. It’s like a tractor beam. I will abbreviate my reasons for leaning away from them by saying this: A motor’s efficiency depends on the close proximity of rotor and stator because magnets in very close proximity have more push and pull. With a LIM this close proximity is the between track and car, and that is very hard to precisely maintain, especially on curves. I believe a system needs to be adaptable to the space available to it. If using LIMs means not being able to corner tightly to conform to a city’s layout, I will lean toward other direct drive techniques. This is partly because I don’t see the braking issue as unsolvable.

First point. Ice. If a system has track that is directly exposed to the elements anything but LIM propulsion would seem to be problematic, especially at higher speeds. With a hanging system though, the track may be expected to remain dry except for condensation. If the running surface is to be rubber-mounted for sound and expansion reasons, however, its thermal mass is so low that it can easily and cheaply be kept at or above ambient air temperature, with a low wattage resistive wire, so that condensation would not form. Let me be clear. This is not about melting ice or evaporating moisture. Condensation will only occur if moist air is warmer then the track surface, such as if warm ocean fog is contacting track that is still cold from the night before. There are several points worth considering. One is that all activity by vehicles using the track will generate heat, and lot’s of it. Actually getting rid of the heat in (in a hanging system) would seem to be the greater problem most of the time. A second thought is that if there was a condition promoting condensation, care should be taken to design the track so that water will not drip onto the running surfaces.

About smooth surfaces – One thing that has been pointed out is that smooth metallic running surfaces do not provide much traction in the first place. I would offer this. There is only so much braking that you want to subject a passenger to, except for emergencies. I submit that on a smooth dry surface there will be sufficient traction to reach the braking limits that would be acceptable from this comfort standpoint. So the traction issue is a safety/extraordinary event issue that will never happen anyway.

All brakes, including linear and rotary motor magnetic braking systems, will be overwhelmed by being undersized, and they will all certainly be undersized to handle “brick wall” stops instantly, traction or no traction. For this reason a back-up system is needed. The obvious solution is to directly engage the track with brake shoes in some manner. Such a system would not be used for routine braking, lest there be wear to the track over time. But for emergencies, extreme braking power is quite feasible.

This brings up another issue. Passenger restraints. I personally favor a padded waist restraint bar if it can be incorporated gracefully. I would like to discourage movement about the cabin, so that it (the cabin) can hang semi-freely, rather than have this motion be 100% simulated. We don’t need kids intentionally “rocking-the-boat” so-to-speak.

There is another safety feature that is sometimes mentioned which bears consideration. There is no reason to supply power to the track directly behind any vehicle. Having no power would seem to be a pretty good defense against malfunctions of headway distances. I can’t say that I have worked out the details, but it is worth noting that if there is no electrical draw, switching on and off can be repeated with very little wear. (no sparking, etc.) I would like to see what inventive minds could do with the concept.

I do not think it is unreasonable to consider that each vehicle should have several rangefinders to determine how it is spaced between others. Such devices are commonplace these days. They come with cameras and are showing up on cars as a way to help drivers not back into things. With a homogonous fleet and guaranteed vehicle-to-vehicle alignment, I think self-spacing and impending-collision detection systems are very doable.

With a hanging system, safety should be almost a none-issue. In post 43 I illustrate how the bogies from which the cab hangs can be spaced to prevent those cabs from hitting each other, and how the swinging action can help absorb shock. Another idea would be a sort of airbag idea for bogies, shown below.
In the event of an immanent collision, the cylinder would quickly extend, filled by gases created by a measured explosive charge. Collapsing is much more difficult, however, as the gases are trapped in the cylinder until they leak out. Such devices could be both forward and backward facing, so they would hit each other in an impact. The U-shaped ends are fitted with a strap or cable and are designed accommodate misalignment, especially on curves. As they compress, resistance would increase. This is akin to safety “crumple zones” in cars but much more controllable. A key advantage to such a system is the relatively long length of the combined compressive strokes. Adding length reduces the G-forces acting on the passengers.

This is coupled with the forward swinging motion of the “gondola” shown in post 43. The preferred deceleration profile would gradually increase G-forces that swing the cabin forward, so that at the time of impact passengers would be being pushed into their seats rather than out of them, as would be the case with any bottom supported vehicle. This, coupled with the methods mentioned above, should create a system that can be demonstrated to be extremely safe, despite smooth track and fairly hard wheels, which are desirable from a mechanical efficiency point of view.

1 comment:

Ryan Baker said...

I've read quite a lot of your comparisons between wheel braking vs. LIMs, but the argument I've never seen you address is this. How do you deal effectively with the numerous factors that make wheel breaking force unpredictable?

Are you just assuming that no vehicle is ever braking at maximum force and as such can always apply a little more force is it has slightly worn tires or some other condition that causes it to brake with less force than expected?

Maybe the whole accidental engagement of brakes or locked wheels scenario isn't rational, but I'm thinking that even though it wouldn't cause a PRT system to become less safe than a road it would add considerable risk if you didn't address it via LIMs or something similar or via larger headways. I think the possibility of needing larger headways would justify the cost of the LIMs.