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!

 

20> Podcar Control and Encoders

I’ve been wanting to start a conversation about PRT control for some time, so I thought I would start with this primer on motor control.

I don’t know a whole lot about brushless motors, other than that brushes are the means regular motors use to time the advance the magnetic fields ahead of the advancing rotor. With modern electronics, however, motors do not require mechanical means to achieve this timing. Triggering the magnets electronically offers great precision. Instead of “full-on, full-off”, magnetic forces can be ramped up or down; the motor’s rotation can be advanced or reversed, even held frozen.

Whereas in theory one could always know just where the shaft is, rotation-wise, under load the actual rotations or speed could differ from what is expected. One answer is the optical encoder.

My thanks to ikalogic.com for the logic diagram and shaft encoder pic. I just glanced at the site, but it looks educational. Sorry about the resolution. Click on the image to enlarge it.

The key is the C shaped “electric-eye”. (optocoupler) As holes in the disk align with the beam, a logic pulse is created. In the third picture, I depict how the same principle could be used to inform a PRT system on a pod-car’s position (and speed) along the track.

Some thoughts:
1. The track can be fitted with the PRT equivalent to traffic signs. Unlike drivers, PRT vehicles will follow their instructions to the letter. “Signs” in the track can trigger precise deceleration rates for upcoming turns, acceleration rates for merging, etc.
2. These “traffic signs” can be dynamic, reflecting real-time data.
3. The track itself should also receive data from the PRT vehicles. Track segments can “know” their traffic counts, for example, to upgrade or downgrade their availability factor, for routing decisions. It would seem that the process of merging would be the result of direct cross talk between cars and track.

There’s a lot more to be said about the division of control responsibilities, so I’ll save that for a separate post.

18> PRT Motor-in-the-Wheel Design

This illustration shows how a motor can be mounted inside of a PRT “Pod-Car” wheel.
I was first inspired by a design from an electric “concept car” where the wheels were actually motors. The idea is simple. If you plug in a motor but hold the shaft, it will spin, (winding up the cord until it unplugs itself). So why not feed the cord through the shaft somehow? That way the motor itself would be a wheel, instead of mounting a wheel on the shaft? (Or more likely, a sprocket, gear or pulley) It turns out this idea is actually widely used. A Google search revealed that the concept is commonly employed to move conveyor belts, (drum motors) and to electrify bicycles in China (hub motors) and in fans, especially computer and ceiling fans.

Despite the obvious simplicity (and efficiency) of the idea, apparently the motors don’t perform well in terms of torque, as compared with a much higher speed motor fitted with a reduction gear. Also they are not exactly cheap or easy to find, so I bit the bullet and researched some more conventional motors.

My switching design calls for hard wheels, so I put in a bunch of them to have the traction to climb steep smooth slopes. I don’t really know how many is really optimum, at this point, but I checked out some “NEMA” standard sized brushless motors, and found that the NEMA 42 standard comes with sufficient torque that if each or the wheels had a motor within they could pull (together) with between 300 and 800 lbs. of forward power, while having a top speed of 60 mph. (Sorry, younger and euro readers, I am not natively metric and feeling lazy) Obviously, reducing the top speed increases torque proportionately, but it also worthy to note just how much of a trade-off is required between the weight, the steepness of the climbs and the top speed. There is no free lunch. For example a combined vehicle/payload weight of 1600 lbs. will probably not make a 45-degree climb.

It is also worth noting that the separate motors solve the fact that my system has no differential gears. For those who do not know what a differential gear is, here is a brief explanation – On car, when turning a corner, the outside wheels have further to travel than the inside wheels. Therefore they have to revolve faster, making more revolutions than the inside wheels. The differential gear accommodates this, preventing one or both wheels from slipping on the pavement, which would create wear and partial loss of control. Even though the right and left wheels in my PRT design are fairly close together, the same phenomena would apply to a lesser extent. With separate right and left motors, however, the RPMs of the wheels can be precisely controlled to actually create the appropriate steering forces.

15> The "Podcar" and the PRT Dream

First, I HATE the term “Podcar” or “pod”. Unfortunately, Since the PRT thing in Ithaca last summer, it seems that a lot of “PRT” Googling results in this awful term. Are people who ride “pods” considered “Pod-People”? (Anyone remember that movie?) Anyway, I use it here in the title solely for the cynical reason that it might bring in some (Googled “POD”) readers, and some of them might become contributors to the cause. I would like to reiterate that cause now, as many people might tend to scan the site, without realizing it’s purpose, which is buried in the end of the “About this Blog” section
The ultimate dream would be to realize the world’s first large-scale open-source mechanical, software, legal, environmental and social engineering project, with a full set of blueprints, contracts, position papers, environmental studies, etc. provided by the masses, for the benefit of humanity and the planet. A “Wikipedia” of how to build a truly 21st century transportation system with local talent and resources, with a hefty “how –to” section on every possible situation. A framework by which primitive “model T” iterations of PRT are skipped, leading to successful and ever more efficient and beneficial implementations worldwide.
But on a much more humble scale, a bunch of us are “second-guessing” each other, on the topic of the day, or week, and our thoughts are being archived in the “cloud.” Anything we say may provoke a design breakthrough, or cut off some bone-headed approach that would only be a setback. In other words, it’s all good, as long as we get enough contributors to be a real resource that can, at least, be found.
I am not free to work toward this full time, but it is my passion. I wish I could post daily, but I can’t. I have many more ideas than I have posted so far, but I have to meter them out. I don’t want the best ideas buried too deep. So to any readers out there- Please post your thoughts about anything I write or any comments you read. Please spread the word! At some point I will start an email campaign to known advocacy groups and academia, but I need a blog gushing with comments, arguments, and ideas. If we, the people, reach a definable consensus, that is powerful stuff. It is the stuff of dreams.
Last but not least . Enjoy the illustration. I would like to note, however, the motor units shown don’t represent any particular design based on inner parts, size of the inner workings or anything else, for that matter. It’s hard enough just making them look shiny. But sometimes a picture says a thousand words, (and one blog address). I wanted to get this out sooner rather than later.