Monday, July 20, 2009
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.
Posted by Dan at 10:14 PM