Monday, October 18, 2010

106> Simple Circuits

For those of you expecting, from the last post, for me to explain my method of requiring only a byte-sized communications to perform a merge, I’m afraid I have to retract that claim. What seemed, at first glance, to be a two-bit digital message is more accurately a couple of blips in an analogue signal, because the timing of the blips is part of the message. It is a moot point, though, because from a transmission point of view it is much smaller and simpler than preparing any sized packet, the usual way of sending it in digitized form.

I want to reiterate that I am not trying to reinvent the wheel. Some variant of ordinary digital networking technology is certainly the way to go as a primary system. I believe, however, that the following examination is worth doing for a number of reasons. For one thing, it forces us to look squarely at the details of the task at hand, which gives direction to what future PRT software and network structure might look like. Secondly, it relates more directly to the kind of systems that have gained regulatory approval in the past, such as trains or people movers. It is possible that it might be simpler to gain approval for a system that is built on top of the same kind of circuit-switched system logic that older, approved systems use. Thirdly, if there will be a back-up system, it would seem logical to have something dissimilar enough to be easily and completely partitioned from the primary. Lastly, this blog has some role to play as an educational resource. Exploration of the problems of control and safety can be simplified by doing so in an allegorical way, since most readers are not well versed in modern telecommunications methodologies. All that being said… 

This illustration shows the information accumulated over a few seconds by two proximity sensors positioned at corresponding locations on the two tracks leading to an upcoming merge point. The vertical lines represent units of time and the numbered “blips” represent PRT vehicles. In an animated version of this illustration everything would be traveling right to left, so that information gathered on the left of the picture is older than the information on the right. Imagine it like a window looking out on a highway with the traffic moving right to left, and a snapshot has been taken as the fourth vehicle crosses the midpoint of the field of vision.

Here a reference voltage drops with the breaking of a beam, although other schemes could be used. In this example the vehicles mark twice, once at the front of the vehicle and once at the back. This enables the calculation of velocity. This, then, gives all of the necessary information required to determine if the sampled vehicles will have a conflict at the merge point, and what routines can be used to solve it. Additional monitoring points can be used to create a complete picture of traffic, although from any particular vehicle’s point of view very little of that picture is actually relevant. The picture above represents the “view” of vehicle 4.

Below shows the required broadcast areas for such a system. Traveling through such an area gives each vehicle a chance to construct a representation like the illustration above by “listening” to the traffic cross the sensors in real time. Each vehicle would have a different “picture.”  I would suggest that sending the data directly in such a raw form through dedicated channels, fibers or circuits might well make more sense than digitizing it on the spot and directing it through a multicast addressing protocol, especially as a back-up system. In such a scheme normal attenuation (fading of the signal as distance from the transmission source increases) could limit the communications to the relevant vehicles. Note, however, that no communication between vehicles is actually required. If they all have the same information they would, in theory, move independently in the “faith” that neighboring vehicles would be moving as expected as well. This would be highly risky but for the fact that some form of headway distance monitoring is already required as part of a collision avoidance safety system.

I dislike, as is well known, the notion of complicating the track with sensors and controllers as a general rule, and this scheme doesn’t change that. But one can never weigh any trade-off without examining both sides. This round goes to the wayside control model, although a vehicle-based equivalent is doable. For example, the vehicles themselves could “sound-off” as they pass a marker or RFID tag, and a pair of non-interfering RF channels could be used in a leaky feeder system. (I believe I have been too worried about “cross-talk” with a leaky feeder system. There should be plenty of radio bands available that can coexist on one or more cables.)

On a personal note, I am back from New Hampshire, and will get to my Email backlog very soon. Besides making a portable shower stall that snaps together like Styrofoam Legos and a solar hot water heater, I made good progress on this 14x18’ experimental structure that will be a garage/shop/storage building and a place to hang an electrical meter. (My cabin is too far from the road for electric service, but I really need my power tools)  I have been a busy boy! I wish I had cleaned the site a bit before taking the picture, but it’s all clean and partially sheathed now. By the way, what’s shown is less than $1600. into the project, including everything – labor, delivery, and materials. (including the 25 tons of gravel under the insulated, pre-plumbed slab) I should have it completely dried in for under $8. sq. ft. Now that’s sweat equity!… and my excuse for posting so infrequently over the past 7 weeks. I thought you all might like to know where I’ve been.


Andrew F said...

Hey Dan, is that structure comprised entirely of laminated 2x4s? Nails, bolts or screws? It looks like the horizontal pieces might be 1x4s.

How well would such a structure handle wind load? Seems like it needs a horizontal brace further down.

In other words, you're welcome to have an off-topic post about your structure when its complete! :)

Ryan Baker said...

I agree with you that an analog system has utility as a backup system due to unique dissimilarities from digital systems, but you also seem a little unconvinced that digital is the right primary system, so let me make the case for that.

In the application of PRT control, the primary advantage of digital over analog is error correction. Analog systems have the characteristic of degrading gradually. In some cases that can be advantageous, but with enough excess bandwidth used for redundant transmissions analyzed by error correcting algorithms, a digital system will provide error free operation at a higher level of physical signal degradation than a PRT control system could reasonably accept within the analog signal.

Redundant transmission can be used with analog signals as well, but to achieve the same amount of error detection and correction capability requires much more physical infrastructure.

Digital also tends to be more interoperable, since almost all digital signals include some kind of identifier, it's possible to translate from one format to another if necessary. Analog signaling generally requires much tighter controls to achieve interoperability. Digital can benefit from these same controls, but when they fail, the fall back is more readily available.

Lastly, I believe a digital system will better allow you to incorporate multiple redundant inputs.. such as lidar, track sensors, accelerometers, etc., and thus offer another level of error correction.

Dan said...

Dan the Blogger Responds -
Andrew, let me get to Ryan’s PRT related comment first.

I don’t disagree Ryan. If you don’t believe me, I would bring up the example of trying to accomplish a merge where it requires the smooth adjustment of a half a dozen or more vehicles. I would never try to pull that off without digitizing the data, because it would be foolhardy not to have feedback loops assuring that the maneuver was, indeed, proceeding as planned. Nor would I contemplate going much over 10 mph with any scheme I have offered so far.

I do get more attracted to circuit-switched or analog in cases of simple, super slow, off-and-on logic, however, because they can be hardwired directly with very reliable results. The transmission can incorporate quite high power if needed to keep the signal clean. The case I outlined is mostly digital, though, since the information is useless without being digested in terms of a clock and software. I’m just using the vehicle’s computer for DSP instead of putting an equivalent chipset at the sensor.

I stand by my reasons, outlined in paragraph 2, for examining these methods and expect to continue into other areas as well, such as docking maneuvers. Personally, I’m getting a lot of ideas about network structures and control software from the exercise. I would add that when we discuss some of those ideas, it will be useful to be able to refer back to the hard-wired counterparts.

Dan said...

O.K. Andrew, All about the structure… It’s all 1x3 furring strips, and yes, it’s quite flexible at the moment. I built a smaller one twenty years ago and at this stage it was like a trampoline on top. It’s been through a bunch of hurricanes since. This one’s actually a lot stiffer. You see, I have no electricity, nor a truck up there, just a car. I’m too old for much hand sawing or nailing (I prefer thin boards and small nails!) and I’m doing it all myself. So I chose a structure that makes it’s own ladder, can be added to without straining the ol’ back. All of the wood was gang-cut (into 2s, 4s and 6s before unstrapping) at the Home Depot 30 miles away and transported (during my grocery runs) INSIDE the car. The key is that everything is synergistic. Every piece of wood strengthens the others around it.

Near the bottom the roof will flare for a drip edge a few feet up, barn style, above a couple of feet of siding. The verticals for that are not on yet, nor is the inside half-wall that will support my work benches. That is why it’s only two boards thick at the bottom, where it’s bolted through the slab. The far side looks funky because my temporary horizontals are half-off making ready for (3) 2’x8’ south facing skylights. The structure will be strengthened further by sheathing and a finished inside, as each new layer of thickness makes the curves stiffer, because of the radius difference. (That’s why laminated curves are so strong.) I need to thicken the walls for insulation anyway. The end walls will actually “float” in pockets allowing some flex before arresting the movement. It hasn’t been my fastest project, but it has been fun and easy to build so far.

It may be radical, but hey! I started with geodesic domes. Wow. That was 39 years ago. I guess some kids just never grow up!