75> Happy Tails to You....

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. 

74> Tilted Design, Motorcycle Tires

Here is a logical offshoot of the last tilt-wheel design that I posted. It is still far from finished, lacking controllers, backup battery, brakes, etc. What is interesting about this design is that it is scaled to use standard motorcycle tires, which are speed and weight rated and specifically designed to take wear and pressure on the sidewalls. Weights ratings go to the 1750 lb. (794 kg) range and speed ratings are in excess of 175 mph. (282 k/h)

I am increasing worried, however, that perhaps one track doesn’t work for all applications. This design ties the interior dimensions to about 36” x 24”. (914 x 610 mm)  That’s not bad for long spans but is kind of awkward in buildings, under bridges, or in tunnels. While the swing-arm on the top of my designs should allow natural g-force correction and extremely steep slopes, it, too, adds to the height of the finished system. The result is very much taller than, for instance, the Skyweb Express system. 

In my brief exploration of underpasses in last week’s post, I was confronted by the disadvantage of having a tall system height (track plus vehicle) in this situation. I am not sure how many underpasses have room for both pedestrians and a fenced off Podcar lane and this troubles me. The Skyweb Express model not only doesn’t have this problem but it also appears that I may have underestimated Skyweb’s maneuverability somewhat. There is also the matter that I have several tweaks in mind that would greatly enhance their design (as I understand it) in this regard.

I remember in the old days I thought it would be pretty cool to be able to retrofit one road lane into four PRT lanes, packed two high and two across. One problem with that vision, however, is the need to keep people away from the track for safety reasons. This need becomes more pronounced with every incremental increase in system speed, so it really needs to be an integral part of the infrastructure from the inception. The Ultra system uses the inelegant chain-link fence strategy. Some bottom-supported systems (Vectus, Skyweb Express) use the strategy of keeping the track elevated and preventing access with the station design. Captive bogie hanging systems (such as I have been designing) have the clear advantage here, but it still seems prudent to keep the bottom slot out of reach even though it could, in theory, be very narrow, precluding, for example, insertion of an arm. How high is “out of reach?” Having given up on a combined track/vehicle height of under eight feet, (2.4 m) my next main worry is being able to fit between floors of a typical building. I will save the protracted discussion, though, of station design or optimal track height for a time when I have some illustrations prepared. Suffice it to say track height is not without consequence, so the large wheels may come at a cost. 

I have to say, though, that this design passes the “smell test” in that it appears to be a design that would be at home at very high speeds, which, of course, can be reduced. This is much better than starting with a design that has requires modifications to go fast. I also don’t see any inherent maneuverability problems in terms of relatively tight turning radii. Be forewarned though, that this design will take a while. Shown is an old version. Currently the frame (yellow) has been completely scrapped.   Simplify! Simplify! Simplify! 

73> Overwhelmed by an Underpass

I couple of weeks ago I was asked to quantify how tightly I thought a PRT vehicle would need to turn. In the case of PRT, the vertical turning radius must also be considered. (going into, cresting, or coming out of a slope) I have always considered that these radii must be fairly tight, but I had not really examined how and why I drew this conclusion.

I started my inquiry with a Google Maps starting with a place I have gotten stuck in traffic in the past. 

 

It is a place where an older road leading out of town intersected with a highway “loop” around it. Then time passed, and they built a mall and widened both roads a bunch of times. Now it’s a nightmare, but with good restaurants. Sound familiar? How would the various PRT systems compare as a solution?  The bottom picture shows the view looking east from “point B” and the smaller inset (point A) shows a small open-air bus stop, typical of the southern U.S.

The bus stop is included because it is part of the problem more than a solution.

On a typical afternoon, the traffic builds sufficiently so that it takes at least three full traffic light cycles for a bus to even reach this stop. There it blocks traffic further as passengers board - unless it just happens to reach the stop in synch with the rest of traffic stopping as well. Going straight, it blocks vehicles that might be able to turn right on the red light. If buses didn’t run through this intersection it would be better for everyone - especially the bus passengers, (who have to spend nearly ten minutes on this one intersection) but the alternative routes are nearly as bad. It seems obvious that the bus routes through here are very expensive to operate, and there is no place to fit light rail. The east/west road is as wide as will fit, and the loop was just widened - again. (Note the five-lane feeder road.)

PRT could cut through this mess like a hot knife through butter. I guess my main question is whether going over forty feet (12m) high to clear a raised highway is acceptable. As a devotee of all things futuristic, I personally have no problem with it, but will it sell at city hall? If the answer is no, it exemplifies a lot of what I have been saying about sharp curves, steep slopes, variable speeds and hanging vehicles. Hanging podcars, as I envision them, would have no problem going either over or under. The concept of a uniform line speed is challenged, however, because the PRT vehicles would need to bunch up and slow down to make the abrupt elevation drop and/or turns. I guess a two second headway would be about all we could get through per track, though.  If going over is the thing to do, the stations may need to be moved back from the intersection quite a ways, depending on how steep of a slope the system is designed for.  

I Googled around a bit, looking for other examples and found this. It is Main Street, Houston, TX as it passes under I-45. Pictured are the tracks of the new light rail.  I guess the car traffic has been largely diverted to other roads. On the face of it, it looks like they are squandering enough space to move a heck of a lot of people. Again note the need for PRT to go over or under. Actually there is virtually no way out of downtown Houston that avoids this overpass dilemma. Then there is there is loop 610 farther out, with the same thing…  And then there is the outer loop. Anyway, the situation is the same in lots of cities across the globe, so systems ought to be designed to handle the situation gracefully, whether it’s going over an underpass, or under an overpass! ;o)

72> STOP THAT!

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.