Saturday, November 24, 2012

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!