Sunday, February 28, 2010
A problem inherent in any automatic transportation system is how to avoid collisions, specifically a following vehicle hitting a leading vehicle from behind. Two factors that make PRT susceptible to rear-end collisions are the relatively tight spacing between vehicles and any autonomy given to these vehicles.
It’s my nature to want to push the envelope in terms of spacing, autonomy and speed, so naturally I’m trying to figure out how to do it safely.
Historically PRT designs have had a set line-speed. The only variation was to drop back or increment forward by a set amount, and to have a slower speed around off-line stations. This makes keeping vehicle spacing very easy, and simpler in terms of getting a product to market. With self-banking, hanging vehicles, however, the only limit to the speed is what the passenger can handle. If a uniform speed is used, it must be one that is comfortable for passengers who are older, or less adventurous, or prone to motion sickness. The whole system gets slowed down for a small percent of potential passengers. If we are serious about creating a system that can make a profit and also compete with cars, I think we need to go as fast as is practical and comfortable for each passenger who is traveling. There may be a timid passenger in the way, but there may not. There is no system that will be full 24 hours per day, especially on the edges of the network, and optimal travel in these instances deserves consideration too. There is no rule against fast or slow lanes, passing lanes are anything else borrowed from the automotive transport world. One other point is that in tight spaces turning radii might be advantageously decreased, requiring a deceleration and acceleration. This all requires a much more robust and autonomous control philosophy than what is out there today because introduces a degree of chaos to the system, and managing that may be a challenge. Anyway, control is a very complex issue, and needs to be broken down into a number of posts, and this one’s purpose is to explore just one small aspect, that being not rear-ending a vehicle.
This discussion wouldn’t be complete without mentioning the California’s PATH program. In an interesting 1997 demonstration, a tightly packed convoy of 8 Buick LeSabres traveled under automatic control at highway speeds while maintaining close headways within 10 cm. This was done with autonomous vehicles communicating with each other and with proximity sensors. There was no central control.
Because of safety issues, there needs to be extreme redundancy in any system that controls the spacing between vehicles. One idea is to create graded zones that follow each vehicle like a comet’s tail, allowing a following vehicle to ascertain the distance to the lead vehicle. The idea may be visualized as in the picture below. Each following vehicle can have behaviors that are conditional upon these zones. The colors, of course, are just for the benefit of you, the reader. How the zones are created and detected is up for grabs.
The lead vehicle must create these zones and the following vehicle must be able to detect and react to whichever zone it is entering. While a proximity-measuring sensor would seem to do the same thing, I am interested in methods that are rely the simplest physics possible, to compliment the onboard sensors. These could be coupled with equally simple fail-safe systems. What if, for example, brakes were only NOT engaged when outside of the close range (red) zone? (Let’s assume no actual contact platooning for the moment) What about power? What if no power is available to vehicles in yellow through red? It is at least worth pondering.
To create such zones, there needs to be a way to influence something along the track that can transmit the information along to the upcoming vehicle. That information needs to change in amplitude or frequency as the lead vehicle gets further away. One thought is some sort of wave-guide. For example an acrylic rod can act as a fiber-optic transmitter. Notches sawn at intervals will light up brightly, but the effect will decrease with distance from the source. Measure the brightness at such a notch and you can tell the distance from the source. The same can be done with sound or radio waves.
Here is another very simple principle that ought to work.
This works on the principle of voltage drop through lengths of wire, although actual resisters, as shown, increase the effect. The down side is that I employ an actual electrified rail to transmit the voltage to the following vehicle. Anyway, it can be seen that voltage sent by vehicle A will incrementally decrease as it gets further away. Vehicle B can measure the voltage and know its distance from vehicle A. The diodes make the electricity only flow backwards in respect to the direction of traffic. The segmented rail would create metronomic breaks in the transmission, giving an accurate means to measure velocity.
I know. The idea of the electrified rail is impractical. Anything that is done to the track needs to be very inexpensive. Track based transmission, however, is immune to “line-of- sight” issues, so vehicles can “see” each other around curves. The other problem with all of these schemes as well as vehicle based signaling, (which would probably be the primary system, but that’s got to be a whole different post) is that a failure in the signal generation or pickup emulates the absence of a leading vehicle. A broken down vehicle must not become invisible. Ideally the whole thing needs to be reversed, so that the weaker the signal, the closer the proximity. That would be like the air brakes on trucks and trains. If there’s a failure, they engage, not the other way around.