148> Off-the-Shelf PRT

I was thinking, the other day, about this blog and how badly it needs a facelift, and I had a realization.  The original purpose of the site has essentially been realized.  No, I did not succeed in getting a cadre of engineers to anoint and consecrate a set of standardized dimensions under the alter of “open source.”  Nonetheless these last few posts represent what is pretty much the closing of a chapter, design-wise.  After a rather exhaustive assessment of a variety of issues, it can be said, in most instances, that there is a clear, best way to accomplish a system with the kind of capabilities I have been advocating.  I had originally hoped to create some standards for PRT, so that the business would not require a single company to be the expert in vehicle making, station building, track building, route planning , system maintenance, traffic management software, etc., etc.  What we have, actually, is a pretty good start in that direction, at least for this one type of PRT.  In my last post I outlined a bunch of “pearls of wisdom” that, if followed, outline how the capabilities of a suspended PRT system may be greatly extended.

These principles guide track design and therefore bogie design.  Since there is nobody else really trying to push the performance envelope for suspended systems, I guess I’m sort of creating the basis for such standards as I go.  What I am advocating is an open standards approach to what I call a “SMART” network. (Suspended Multi-axis Automated Rail Transport) My vision is to create the cheapest, fastest, least intrusive, most versatile, method for “air-lifting” a load from any point A to any point B without actually flying.   

At some time in the near future, this site will no longer be about getting recruits to design a better PRT system, but rather about refining and promoting those design decisions that resulted from the work already posted.  It took a very long time to do, and there are still plenty of details to nail down, but longtime readers of this site have seen the other parts of this system and know that the current work on the bogie is akin to shaping a keystone – the final piece that must fit in an arrangement of pieces that have been fashioned just for it.  Once the essential geometry is set, and the capabilities and limitations are known, it’s detail time… time to design a prototype in earnest.  Once again, my apologies to anyone who has just found this site.  I’m sure this bogie (which doesn’t even show hardware for hanging a vehicle) must be mystifying.  I will soon put the pieces together into a unified system. I promise! 

Not that there isn’t a LOT more work to be done here, on this model.  Each piece needs to be examined for redundancies, interferences, extra weight and manufacturability.  This will take days or weeks, not hours.  Still, I think the general design demonstrates that, with the right geometry, extraordinary capabilities can be achieved with a modicum of inexpensive parts.  I have had to sacrifice almost nothing in terms of speed, turning radius or climbing, which are over 100 mph, under six feet, and any angle up to 90 degrees, (straight up) respectively.

One thing that is lacking in many PRT and dual mode proposals is a practical way forward.  Often concepts are presented that are so early in the research and development stage that only a physicist can tell if they are even feasible, let alone lucrative.  Although parts of any complex machine become more specialized over time, presenting it with too many of these one-of-a-kind components too early tends to condemn an otherwise good concept to a life on the drawing board.  Since business realities demand that commercial incentive surpasses developmental risk, a budget oriented “proof of concept” design is an invaluable first step. 

 

With this in mind, this bogie design (which is a more evolved embodiment of the ideas expressed in the last post) uses “off-the-shelf” components where possible.  The motors for this model are dimensioned from the 7000 watt hub motors from Kelly Controls.  Using four adds up to a bit over 39 hp.  The upper steering guide wheels are hub motor driven scooter wheels and tires. (13”hubs, from the same source, with Pirelli Diablo tires)  The steering guide wheel is designed for continuous contact and the hub motor can be sized up to 6kw, for an additional 8hp.  Although this would compare favorably with other systems out there, it would still be a bit underpowered for commuting.  Luckily, the main drive wheels are standard low profile automobile hubs and tires (215/35-18) and so the motors can be readily swapped with higher power ones, even up to the monster (80hp per wheel) Protean motors, which would enable performance that would put most sports cars to shame.  For climbing standard sprockets are used, drilled for lug nut extensions.  The design features truck style emergency air brakes that clamp the track.  There are dual (self-diagnostic enabled) steering guide servos that work together but can work singly as long as there is power from either the track or the onboard battery.

In this design the steering guide wheels are not on rocker arms, like in the earlier design.  Although the rocker design does seem to offer smoother engagement, this supposed advantage assumes a continuous rail to engage upon.  If the steering guide wheels are positioned first, with the contact rails being tapered to make contact after that, this is smoother still.

About the upper steering guide wheels: First, the matter of wear.  After all, they are soft rubber, relatively small, constantly engaged, and contacting at an angle.  I would first note that tires for scooters and other two wheeled vehicles have heavy sidewalls so that riders can lean into turns, which is an extreme torture test compared to pushing into the smooth steel of the “diamond” guides.  With the new (counter-rotating) lower guide wheel geometry and wider drive wheels the forces exerted on the upper wheels is minimal.  Also, engagement between the wheel and guides need not be continuous, or at least not under significant pressure.  I envision the contact being primarily on the crown of the tire most of time but, unlike scooter use, there is no driver, second rider, or vehicle weight on them.  A good finished design will enable the whole wheel to be swapped so that the tires can be changed on a bench instead of on the vehicle. Lastly, the tires are relatively cheap and specialized wheels will eventually evolve.  These should be good for well over ten thousand miles as they are.   All tires should probably have ribbed or foam reinforcement inserts, giving them the ability to run properly even if deflated.  After all, tires are hollow primarily as a cushion against uneven road surfaces, which, of course, does not apply here.

About the emergency brakes – This aspect has had the least amount of thought at this point, but I wanted to explore where the hardware would fit, so I put a crude system in place.  The idea is that the lower brake shoes engage first, pulling the bogie into the track and compressing the tires a bit before the upper shoes make contact.  We don’t want to have the brakes make the tires lose traction. The brakes are spring activated, and disengaged by compressed air.  Another concern is clamping only on one or the other side of the track on switches.  As it stands, such a one-sided drag might tend to pull the bogie off course.  I am still mulling that one over.  The geometry enables some kind of bumper activated system and/or a passenger activated one well.

I know it is very difficult to understand a system from a few pictures, but there is only so much that is worthwhile to show at this point.  An exploded view would be helpful, but most of the parts will still be evolving for a while yet.  Another issue to consider is that the ordinary way such parts are made on an industrial scale is by punching and stamping.  These processes are used to cut out a shape from metal sheet and form it into a 3D shape, which stiffens it in the prescribed manner.  This requires huge machines and matching heavy steel male and female surfaces to squeeze the plate between, often at high temperatures.  Such a process is impractical for small scale production, so we are left with making everything out of plate and profiled stock, such as tubing or angle steel.  This compromises proportions and weight tremendously, and is one reason why I think that such manufacturing should be separate from the PRT business per se.  In any case, any designs herein will be constrained by processes that can be done on a small scale, often by hand, but hopefully with shapes also suitable for mass production.  I can say from experience that, at a certain point, it is better to simply start making the thing, because the prints inevitably prove themselves short-sighted, and every change creates a ripple effect.

Well, that’s it for now, but for this closing thought.  On Thanksgiving morning, on I-10, (the principle southern road across the US) there was a massive pile-up due to fog.  Over a hundred cars and trucks were involved, scores were injured and two died.  The highway was closed for nine hours.  This is a system that seems unworthy of the times, if you ask me.  We live in a world awash in cheap sensors and amazing computing power. Yet all of that heavy machinery was being controlled by people blinded by fog and in too much of a hurry to slow down.  This is a systemic problem that needs a systemic solution. We need an app for that!

 

147> Pearls of Wisdom

Well, I said I was going to start from scratch, and that is what I am doing. Sorry for the delay in getting a post out. There were some minor family medical emergencies. And even in the best of times, a total redesign involves a lot of time chasing (and drawing) ideas that turn into dead ends. My apologies also go out to any new readers, since it has been some time since I have even shown an actual PRT vehicle. I have been preoccupied with the bogie from which such a vehicle would hang, to the exclusion of everything else. So if you have been scratching your head wondering what you were looking at, now you know. But fear not, dear readers, for I return bearing pearls of wisdom… Well, OK… Let’s just call it food for thought…

One of the most important concepts that I have learned since the inception of this blog (and the many designs and redesigns within) is that it is counterproductive to design a single track profile that is supposed to handle every type of situation. It may not be the case with every type of PRT track, but for track that is contained within elevated beams to be both sheltered and yet have a minimal profile, a single track style is, I believe, a profound mistake. A uniform track profile limits the curves to larger radii, both vertically and horizontally, and limits the angle of ascent and descent. It even limits speed. The result is a system that is less flexible and therefore less able to meet a city’s needs. PRT should not be restricted in terms of routes or performance – Let’s leave that role for light rail!

The instinctual response to performance limitations is to try to design the bogie to allow the most capability within a given track profile. It is quite counterintuitive to have the track change in midcourse as a first choice, but many of the track’s contact surfaces involved in switching, climbing or tight turns are simply not needed in straight sections of track. Why go to the expense of including them throughout?

A prime example is the “cog” method of handling steep slopes. This is my preferred method for climbing with PRT, and it is noteworthy that once so engaged, none of the regular track surfaces are needed. The track becomes nothing more than an enclosed pair of racks and some guides (orange) to keep the teeth engaged. Note that the counter-rotating guide wheels are positioned quite closely beneath the drive wheels. Whereas they would ordinarily hit the underside of the running surfaces when the track curves sharply up or down, the fact is that such abrupt changes in pitch can only happen at very low speeds and a rack could be used instead of the running surfaces in those instances anyway. Therefore the interference between those guide wheels and the running surfaces is remedied by simply removing that feature of the track, not redesigning the bogie.

In the process of trying to achieve higher speeds, I was confronted with the fact that the small surface area of the various guide wheels would tend to wear out rather quickly unless they were quite hard, which is a recipe for a noisy system. (Yes, I know “hard” does not necessarily mean durable, but I am referring here to ordinary, reasonably inexpensive wheel materials here.) The preferred small profile of the beam that would contain them (and restrict their size) seemed to indicate that the system could be either noisy and fast or quiet and slow, unless, of course, one wanted to replace the wheels every couple of thousand miles. But much of the problem wasn’t really a problem at all; the answer lies in disengaging the steering guide wheels when not in use – not by retracting them, but by simply having the track’s engaging surface end.

It is tempting, when trying to enable tight turns, to assume the bogie must be short. After all, our experience in the automotive world has taught us that smaller vehicles are more maneuverable… right? Actually that is a false analogy. With cars and trucks the road or parking spot width is generally limited by surrounding real estate. In a beam-style PRT system, the width of the beam is kept as small as possible primarily for aesthetic and cost reasons, neither of which have much to do with short little low-speed sections of track for particular, close quarters maneuvers. If the bogie doesn’t fit in the beam because of the tightness of a curve, an alternative is to simply widen the beam in those spots. This point, in particular, has taken a while to sink in with me. While there are some other performance reasons to keep the bogie short, the very limited space between the front and rear wheels in my designs has made it a challenge to cram in the steering and emergency brake components. Below is a snapshot of a slightly longer bogie that has enough space for an upper set of counter-rotating guide wheels. A single, pivoting upper steering guide wheel shares the axle. Even this slight lengthening of the bogie would greatly limit the turning radius were the track to have a minimal, uniform profile.

As a final example, there is the matter of air resistance within the track enclosure. Track that is meant for slower speeds is generally also the track placed where people would find it most intrusive, and so it should be as skinny as possible, and indeed can be, because aerodynamic drag at neighborhood speeds is a very minor issue. High speed track, say running along freeways, could be fatter, allowing the bogie to slip through the enclosed air more easily. A fatter track profile has far superior geometry for spanning long distances, and this will probably offset any apparent increase in material costs of such fat track. Again, one-size-fits-all is not the best solution.

It may seem complicated to have track with multiple configurations, but such track would be factory made anyway. Therefore there can be a fixed menu of standard track sections that would be brought in by truck and basically dropped into place. True, seldom used profiles might cost a bit more, but at least the system itself can be adapted to all cityscapes, station types, and storage/staging solutions with minimal engineering effort. Such a scheme also makes sense in view of improvements in design and technology over time. Making the track offer the widest possible range of bogie design choices bodes well for the evolution of better, more capable vehicles over time. Even though these bogies are designed to essentially last forever, that does not mean that they might not be resold on a secondary market or repurposed for freight, opening the way for next generation designs. This won’t be the case if the bogie is so spatially constrained that there is no room for design variation.

In the early days of this blog, I thought the first step in designing a superior PRT system was to design the perfect track/bogie combination by studying the problem from an end-view. Now I have come to see that the best way is to study (in 3D) how to overcome the various limits first and come up with track/bogie for those, and then design for straight runs from there. So for those of you designing your own systems at home, I hope this helps. If you need me, I’ll be at the drawing board.









 

146> Big Wheels

Anyone who has followed this blog at all (especially recently) knows that I have been obsessing about speed and guide wheel size.  It all started when I realized, some time ago, that the bogie’s guide wheels, used for steering and just to keep the bogie centered on the track, were of a diameter that is inconsistent with the goals automotive speeds, and minimum maintenance, noise, and track profile size.

 Once you start thinking about commuting speeds and distances, you really need to be thinking in terms of parts that will last for tens of thousands of miles and while still being economical to replace. 

In this pursuit I have identified several key insights.  One is that the movable steering guide wheels must always engage inwardly, and so they can engage the track from its exterior.  This means they can be of diameters that would be difficult to fit otherwise.  This is, however, made less important by another key insight, namely that the duty cycle of steering guide wheels is extremely short-lived.  Switching tracks represents a very small percentage of the time that a vehicle is in operation.  In other words, steering guide wheels to not have a diameter problem, since they are infrequently used.  Another key is that the track surfaces that actually engage the various guide wheels need not be continuous.  Therefore guide wheels can be disengaged when not in use.  This leaves the constant-duty guide wheels which center the bogie on the track as the only ones with the size problem.

The centering guide wheels can face either inwardly or outwardly, but if engaging inwardly they must disengage for every fork – this is a compromise that is unnecessary if we put them on the inside of the track, engaging outwardly.  This, then, presents a problem with space because symmetrically opposed wheel sets each only get one half of the track’s internal width.  If that width is, say, 24 inches, the wheels must have a diameter not exceeding 12,(305mm) and that is pushing the limits a bit.  Sure, there are miracle plastics, but they are not exactly cheap.  Yes, it is close, and somewhat of a judgment call, but experience has taught me to design things better than they need to be.  A faster system, skinnier track, or a little less vibration/noise are all laudable goals.

Following this logic then, one way forward is to break from the symmetrical, mirror-image paradigm of ordinary vehicles so that these outwardly facing guide wheels can be nearly as big as the track is wide. I first proposed such non-symmetry way back in ’09, in post 56. The stacked guide wheels shown above are another, more maneuverable approach.

One footnote for those new to the site… if you have stuck around this far…  For simplicity’s sake there is a lot missing from the illustrations, so here’s a bit of explanation. This is the wheel geometry for a bogie from which a PRT passenger compartment may be suspended.  It is missing almost everything but the various wheels and surfaces under discussion.  The bogie is designed to switch tracks without requiring any movable track parts, as is typical in PRT designs.  The drive wheels are self-turning via hub motors.  

By stacking the guide wheels as shown above, they can be much larger than previous designs, and so allow those faster speeds, less noise/vibration, more infrequent and cheaper replacement, smaller track or any combination of these attributes.  I would go for a little of all of the above.  The results should be speeds up to 85 mph (187 km/h) on cheap semi-hard rubber or ordinary urethane (replaced every other time the drive wheel tires are replaced) with an interior track width of under 20 inches. (500 mm)

This design takes full advantage of the basic observations I have mentioned regarding these matters, namely, the fact that guide wheels used for steering are only used for a fraction of the time while centering guide wheels are in constant duty.  Therefore steering guide wheels can be a fraction of the size of centering guide wheels.  The track itself has completely different engaging surfaces for different purposes.  In the image above I have made the discontinuous elements of the track that are used in switching only dark blue. 

In this illustration it can be seen that no steering guide wheels need to be engaged away from switching points. The upper steering guide wheel’s duty is replaced by a larger fixed wheel held captive by a pair of diamond guides. Here we rely on the fact that the pressure on these guides is very low, or else the urethane sidewalls of this wheel would be the first to go… alternatively a pair of inwardly angled wheels could be used if the front and back wheels are separated enough to allow the space.  I want to study tight track curvature applications more before committing to this. 

This pic shows the track’s divergence at a “Y”, with the bogie being fully supported while traveling in a “half-track” mode. Unfortunately, the upper steering guide wheel is nearly hidden, but it is engaging the left pipe.

Finally, here is how we could further increase our speed for still longer range travel. For high-speed travel where there are no interchanges, ALL of the regular guide wheels could disengage by having their

 engagement surfaces end. Instead, all centering would come from two sets of exterior, retractable wheels, as shown the diamond track of post 141, In this scenario these wheels would retract far in advance of any interchanges so speed through interchanges would be limited to (roughly) the regular top speed. This is, of course, well down the road. An upgrade path!

One last observation: The sprockets on the main wheels that I show are for climbing, not for drive chains. This detail… where the bogie can be fully supported by “pinion” gears driving over a guiding “rack,” means that the floor (drive surface) of the track can be removed, as well as any other internal surface feature that would restrict maneuverability. Thus the bogie can turn extremely tightly, including up or down, without the guide wheels hitting anything, so long as it isn’t too long.