Sunday, March 20, 2011
Well, it is “back to the drawing board.” In the course of the continuing debate about dual mode I have started to lean more and more toward the idea that the shape and functionality of the vehicle is secondary to bogie function and design. If the control system is shifted to the bogies, then the vehicles can almost be viewed as simple containers. As such, the main concern is how much they weigh and little else.
Of course there must be some degree of control from the passenger compartment. An emergency “abort trip” command comes to mind. It wasn’t previously so clear to me, though, where the computers and communications equipment would primarily reside and why. In post 56 I raised the possibility of autonomous “engines” that could live within the track and be called upon to boost the speeds of otherwise slower PRT vehicles. Clearly this would require command and control that is sometimes free from the vehicle below. Now I am contemplating taking this idea to its logical conclusion, which is to have a mobile, standardized “skyhook” that can latch onto a passenger compartment. Primary communications and driving functions would be from the bogie, which is only networked to the cab. (Please bear with the simplistic nature of the “hook,” as shown in the illustration. There is a lot to consider design-wise, and I haven’t gotten very far.)
The approach enables multiple, concurrent business models. For example, privately owned vehicles could “hitch a ride” right along with public PRT vehicles. Freight vehicles could be little more than containers with an RFID tag. It also enables some promising schemes that can only be accomplished via privately owned, dual mode vehicles. Since one or two seat vehicles could be robust enough for some general road use without being overly heavy, they could play an important role in a transportation mix. Such small vehicles would not pass ADA compliance rules for public transportation, yet clearly should be encouraged for environmental and energy efficiency reasons. With this scheme they could be developed and sold by vehicle makers directly to individuals.
In another model there could be multiple taxi or limousine fleets. I particularly like the idea of separate business entities competing for the most comfortable fleet of vehicles. “Fit and finish” issues have always been a weakness in public transportation, since there is little competition in the field. By putting the brains into the bogies we simplify the challenge of creating a great, aesthetically pleasing and ergonomic passenger vehicle. This is no simple matter. Modern cars employ assembly lines many miles long to assemble tens of thousands of parts. Since there is so much to it, why not ensure that this part of the project is completely within the core competency of a wide and competitive field of companies?
I do not mean to totally confuse the PRT world with endless choices. But the “last mile” problem is real and not going away, and I doubt the notions of dual mode or private ownership will either. From a design point of view it is a question of “Why not?”
Ultimately, the obvious business model is one of collecting a fee for using the track and the auto-navigating bogies within it. The “chicken and egg” problem would seem to mean that the service would start out as purely public transit, meaning the cabs are “rented” as well. The company responsible for this service would have to keep vehicles clean and in good working order. I would think the cab interior, save the seat cushions, would be bare-bones, of hard, scrubbable materials. A taxi or limousine company, on the other hand, would pay for bogies only, at a discount, and then charge passengers a premium price for riding in cabs fitted for more comfort. Rigorous passenger screening or even memberships would minimize vandalism of the amenities required for a truly luxurious ride.
Privately owned vehicles can be introduced even if they are not dual mode. “Pods” could be centrally garaged, for a fee, and made to arrive at any station upon request. (Failure of the owner himself to arrive on time would have to result in a penalty charge.) Eventually privately owned and garaged dual mode vehicles might appear, but I question how they would compare with regular cars, which themselves might be automated at that point. After all, if that is the case they could simply drop you at a station and go back home. An automated taxi could be waiting for you at your destination. (No, robocars alone CANNOT replace PRT, which is specifically designed for 3D, non-stop travel. Robocars will never be able to get across a city as fast unless they sprout wings!)
But back to the sky hook. A quick look at the illustration above makes another point about the design we have been developing. That is that the swing-arm itself is a fairly complex gizmo, which is a bit troublesome. Seeing it without the vehicle, though, gives a clue about who might want to make it. Below is a gantry robot. I am very glad these things are getting much cheaper, although they are not exactly following Moore’s Law.
In fact, here is a clip of the machine in action. Doesn’t it seem like this system would be a lot more useful if it were untethered? Such an arm connected to a bogie would clearly need to clamp the track for precision positioning, but other than that..
Finally, when I say back to the drawing board, I mean it. Below is an example of how the three-wheel design from the last post might play out. Such a design can “land” on a flat surface like an airplane. The back wheel can be jacked up to pivot the front down creating a front-loading boarding ramp. The wheel size would be dependent on the anticipated use. Were it to remain permanently attached to the bogey, they would be very small. For dual mode they would be replaced with larger, motorized ones, and the maximum passenger weight would have to be restricted accordingly. I figure I might as well share it, because I plan to shift my focus back to the bogey for a while. Chances are it will end up buried deep in the dustbin that is my hard drive, at least in its current form.
Sunday, March 6, 2011
I want to express a few more thoughts on the subject that I raised in the last post, that being using something along the lines of the EN-V as a dual mode PRT vehicle. The first conclusion that I have come to is that the ability to balance on two wheels is not really that advantageous for PRT. This is especially true in countries that have the equivalent of the US’s “Americans with Disabilities Act.” ADA requirements for wheelchair accessibility mean vehicles need to be longer than the very short configuration that the self-balancing hardware was meant to enable.
I realize now that what enthused me most about the EN-V was not the self-balancing capability but rather the maneuverability afforded by the side mounted, independently engageable drive-wheels. Actually, steering in this way is not at all new. Inspired by aviation design, geodesic dome pioneer R Buckminster Fuller developed and prototyped what he called the “Dymaxion Car” back in the early 1930’s to address some of the same weight and efficiency issues that we are concerned with today. This mammoth eleven passenger vehicle got 22 mpg!
I remember reading anecdotes about amazed onlookers staring wide-eyed as the vehicle made a U-turn and parallel parked into an impossibly small spot in a single motion by using the full 90 degree pivoting ability of its single rear wheel. The same geometry is widely used today in the form of “piggyback” forklifts, primarily for the maneuverability it affords. This rear-wheel steering concept should not be confused with the many other “reverse trike” designs out there that have front wheel steering. Front wheel steering is undoubtedly better for roadworthiness at higher speeds but the space and position requirements tend to highjack a vehicle’s design more than Bucky’s layout.
Those following the comments section of the last post have already heard my opinions on the dangers of adding anything more than the most modest weight gains to a vehicle. I still believe that the concept of cheap light track should be the primary design consideration because the main advantages of PRT only really manifest themselves within a network. Configurations of simple loops or figure eights would be better served by GRT, shuttles, etc. If dual mode capabilities compromise this priority… Well, in Texas we call that “Lettin’ the tail wag the dog.” There needs to be some limit to how much weight that ground travel capability imposes on the system design, and that limit is a painfully small amount.
I think it is time to consider the matter though, because certain aspects of the whole system design may be contingent on the results. For example, having wheels on the sides strongly suggests having a front-loading door, and that influences station architecture. Any ground clearance creates an elevation that must be navigable by wheelchair. Seating changes could influence weight limits and distribution, possibly changing bogey design. So here are a few thoughts.
The case for 3 or 4 wheels is not completely clear. With a four legged table, if you remove a leg it may or may not balance on the remaining three. In any case it won’t immediately crash over. A four wheeled vehicle behaves similarly when going over a pothole. The momentary removal of support has little effect. This is not so with fewer wheels. At least with the two in-line wheels of a motorcycle you can steer around bumps. Not so true with trikes or Segways. This leads to the design choice (for 2-3 wheeled vehicles) of larger diameter wheels that can better span dips. This can also be accomplished by wider or double wheels but this adds weight quickly. For four wheelers, the wheels can be smaller, but only with all four wheels being highly steerable can you match the maneuverability of those two side mounted wheels.
None of these limitations bode well for the goal of speedy, comfortable, or long range dual mode. The question becomes one of how much hardware one is willing to haul around everywhere. Way back in Post 50, I brought up the idea of a drive-by-wire skateboard approach to address the problem of carrying this deadweight. Obviously this is a complicated solution, but one that completely addresses issues like larger batteries, robust suspension, etc. There is a tipping point where carrying around integrated dual-mode hardware becomes impractical weight-wise and the skateboard becomes the better choice. In my opinion this threshold is reached well before the vehicle is roadworthy. This is not to say that seldom-trafficked residential streets wouldn’t be drivable, just that busy streets are not safe for such vehicles just as they aren’t safe for golf carts.
If you preclude busy streets, with their crazy drivers and potholes, and assume that trips will be fairly short, then the PRT vehicle could carry around the needed hardware without too much extra weight. The emergency battery could be split between the bogie and the cab, and that cab portion could be sufficient for short trips. If travel is mostly on paths specially paved for the purpose, suspension requirements are minimal. If the trips are short, minimal tires will last an adequately long time. If trips are short, speed is not a concern, so motors can be small and light. If speed is not a concern, wheel and load balance geometries can be used that would be less than satisfactory for ordinary driving, such as the Dymaxion car design.
In conclusion, I think the best balance may be in the old Dymaxian car design, with a clamshell front door. Two large diameter (but thin) wheels (think dirt bike) with “in wheel” motors would fit into a pair of skinny wheel wells. In the rear would be an external (but shrouded) steering wheel. Some means would be needed to rock the vehicle forward to facilitate entry for wheelchairs. The wheels could be designed as modular, removable components and vehicles without wheels might coexist within the system. I would shoot for under 50 kg of total added weight. I was hoping to include some preliminary illustrations, but these things take a lot of time.