Monday, July 20, 2009

43> Crash Tests, Anyone?

Here is a picture I drew to illustrate a point about PRT safety.
Apparently there is a doctrine which dictates how far apart vehicles must be spaced for safe stopping in case an accident. This is evidently the distant relative of some old train spacing rule. It is apparently also the basis for at least one “expert” to say that PRT can never be cost effective, because it can never have sufficient capacity on a given track to be cost effective in the city, nor fast enough to be viable in the suburbs. This argument is ridiculous on so many levels I won’t even dignify it with debate, except to spend a moment undercutting its premise, which is that PRT vehicles traveling at 45 mph, (72.5 k/h) need 3 seconds of headway to be safe. (Can you imagine auto traffic being required to maintain this kind of distance? And half of the drivers are dialing phones!) An interesting article on this subject can be found here.

Anyway, the first line of defense is, in my design at least, the eight wheels of ABS style braking power which is applied magnetically (generating electricity in the process) Linear motors are theoretically unsurpassable in this magnetic braking, as they need no wheel traction, although I believe either is system is more than adequate.

The second, emergency only, safety braking feature is a telescoping hydraulic bumper, which converts the telescoping motion into activation of brake pads which can directly grab the track itself. Such a system can be designed to make contact only possible inside the track – The cabs themselves can never collide. This would also work with rail-on-the-bottom, PRT international or Taxi 2000 style systems. In disk brakes, the “disk” is simply a circular fin that is squeezed by hydraulic brake “calipers.” In the hypothetical PRT emergency, there are plenty of fins to grab onto which are stationary parts of the track.

The third line of defense, which, admittedly, is only applicable to the system I endorse, and to a lesser extent is possible with other hanging systems like Beamways or Mister, is that shock to the passenger is absorbed by the “free-hanging” nature of the cab design. (Actually the cabs do not actually “free-hang” because of anti-sway hardware, but come pretty close)

The illustration shows how the stationary vehicle, struck from behind, has its motor unit jammed forward, yet the passenger compartment hasn’t caught up. This is cushion from whiplash for the occupants of the stationary first vehicle.

The striking vehicle, in so far as other braking methods have failed, as a last resort, lifts it’s own weight, absorbing that many lbs. (kilos) to dissipate the last of the energy stored in the vehicle’s momentum. The passengers, like children on swing set, are lifted and pressed into their seats until reaching a natural or, in a worst case, cushioned, apogee, and then swung back down. This is just one more unappreciated advantage of a hanging system.


Bengt Gustafsson said...

I think you are overly optimistic on several points. One is that the free swinging of the cabin would be able to take up the kinetic energy of the moving cabin if the truck stops. The kinetic energy of the cabin would likely be enough to send the cabin spinning. As this is not possible nor desired it will have to be stopped, at which time the passengers will feel a high G load. Simultaneously the forces on mechanism that blocks the rotation gets extremely high, and these forces are of course transmitted to the drive truck and from there to the guideway, posts and foundations.

Another mistake is to say that many wheels make for safer braking. The redundancy of the brake mechanism is of course increased, but the available braking power is not changed due to the N in F = uN is subdivided into the available number of wheels.

In reality we don't want to brake fiercely, but instead precisely controlled. When you can control the deceleration precisely you can have short headways without crashing.

cmfseattle said...

you should change the name to "Dan's hanging PRT spec project."

have you run the numbers on those question mark-shaped support posts yet?

Dan said...

Hi Bengt,
I was mostly thinking about that “brick wall” stop article, not actual braking practice. In reality the swinging part would never happen, nor would the other two emergency measures. Never-the-less, if a crash test was ever required, I think the 2 emergency systems, employed in a coordinated way together with the swinging aspect, would leave the test dummy in pretty good shape, even in a fairly high speed crash. I envision pressure-actuated valves as a way to apply breaking forces just shy of throwing the passenger. You are right about the whole carriage wanting to spin. There would be a very narrow and slow speed range where the weight would cancel the remaining momentum. Actually the drawing is misleading because I wanted to have the “in track” bumpers shown as longer on the front and back, compared to the middle (impact) area where they would be compressed. I have been very busy this week, and haven’t had time to edit the picture. (Or post for that matter) I completely agree that the speed should be controlled with great precision. When the rpm’s of the wheels are controlled by the motor controller, the term “braking” is a bit misleading in the first place.
About multiple wheel braking, I have been somewhat torn between wheels with traction and longer wearing, harder (solid) wheels which would be more efficient. (The harder wheels have minimal contact with the track) The multiplicity of wheels makes the harder type more viable. The traction issue is, of course, a major weakness of wheel motors as compared with LIMs, and I do worry about possible frozen condensation.
Anyway, again, I do not envision any of the systems described to be used except to create a perception of safety in the minds of those who might not trust a system without all kinds of redundancy in the “emergency brake” department. After all, these people may not be engineers, but they can kill a project on safety concerns anyway, or maybe add a bunch of test track time.

cmfseattle, I suppose that is a fair criticism. I was reading an interview with Ed Anderson and he said something to the effect that he had met, I think, seven people who had independently invented PRT without knowing such a thing existed. I guess I would be number eight. I just tend to “invent” with a bit more depth of detail then most.

Actually, I am trying to buy a bit of time here, as I do not exactly have swarms of competing drawings in my inbox. Readership is steadily increasing, however, and hopefully there will be specific, mechanically feasible contributions at some point. Meanwhile I am just throwing out ideas to see what sticks. Some are better than others.

Anonymous said...

I'd be number nine, then. :)

Keep on doing the posts. Problem that cmfseattle raised is obvious. But the either extreme; anyone throwing anything in the air, with no common ground would not work, either. Maybe this is the open PRT spec (hanging)?

I'm using your writing and the comments of others as proof points on checking everything is thought of in "my" plan.

To be successful, it takes more than a plan. It takes a business plan and means to implement one.

Blériot XI was the first commercially successful airplane, in 1905. We are living similar times, with the PRTs.

cmfseattle said...

43- [honestly, not trying to fill up the Most Recent Comments list] one thing not mentioned, until i saw it in anderson's latest paper, is that braking through wheels will leave material on the running surfaces.

Overcoming Headway Limitations in Personal Rapid Transit Systems

also, the first time i've seen him suggest radar-like systems as a fallback.

afransen said...

That seems like a fairly minor challenge. There could be automated maintenance vehicles that either scrape, polish, or use solvents to remove any build-up of wheel material from the track. I can't imagine this being necessary more frequently than once a month, but would obviously be driven by levels of traffic.

Dan said...

43> Dan The Blogger Responds to cmfseattle and alfranen’s comments regarding the new J. Edward Anderson paper – Thanks gentlemen..

Dr. Anderson is not just writing a paper here. He is also trying to sell his particular system’s design attributes. That includes promoting the benefits of LIM propulsion and braking. Unfortunately he can be a bit misleading in his writings.

First, one alert reader recently reminded me of a term I hadn’t heard in a couple of years – “stiction.” That is when the friction between two objects is sufficient to stop movement altogether. Contrary to what Anderson would have you believe, mechanical breaking is not based on friction with the road but rather friction against the brake discs or drums.
When you apply the brakes in a car, there is friction against the brake shoe, but stiction against the road. No appreciable friction or rubber. When you LOCK your brakes, on the other hand, the brake shoes have the stiction and the tire/road has the friction. Now you leave the skid mark. That’s what ABS brakes do –keep stiction with the road, not the brake shoes.

Dr. Anderson makes his comparisons with automobiles and what Raytheon wanted to do, not with most modern proposals, which almost universally promote regenerative, magnetic brakes, which work exactly like his LIMs but in rotary form. It is true that the wheels must maintain stiction with the guideway, but he fails to mention that the skidding problems he refers to were largely the result of designs where the track cannot be kept clean and dry. In other words, LIMs are needed most when there are icy spots on your track. Also missing is the fact magnetic braking is actually a very precise rotational velocity control that all but ensures that skidding won’t take place, (a kind of ABS system) or the fact that any decent headway control should prevent all but the most modest deceleration in the first place. In other words the whole discussion is, IMHO, somewhat of a red herring.

PRT International apparently has some proprietary technology to help boost the performance of the LIMs. My worry is that anyone who buys such a system is then “locked in” to that company exclusively. That’s a hard sell, if you ask me, no matter how great the technology.

chronokun said...

"This argument is ridiculous on so many levels I won’t even dignify it with debate, except to spend a moment undercutting its premise, which is that PRT vehicles traveling at 45 mph, (72.5 k/h) need 3 seconds of headway to be safe. (Can you imagine auto traffic being required to maintain this kind of distance?"

ummm 3 second following rule rule for driving in wet weather... I guess this road rule doesn't exist where you live... even so I am amazed this seems so absurd to you...