Thursday, July 16, 2009

42> Design Time!

A few posts back I revealed what I consider to be the most logical track design. Notably absent was any description of the drive units that would inhabit this track, so here is what I’ve got. I know that the parts as shown are held together by flimsy, non-braced plates that extend into areas where they do no good, among other issues. It is, however, mostly a drawing to identify and re-route interferences. (Areas where multiple parts are expected to occupy the same space) It is also about as far as I have gotten in “SketchUp”, Google’s free 3D drafting software. It is not ready to post for you, the readers, to work with yet, because I didn’t break it into editable components. This exercise has, however, revealed how stunningly simple the mechanics are, with “wheel-motors” as the propulsion, since there is no need for power transmission. The wheels just turn, with up to 20 hp each.

The system I envision uses the basic eight-wheel architecture of an ordinary railroad car. The first illustration is of one half of a one half, to show the details that would otherwise be hidden. Red arrows indicate the rotary to linear (cam) movement that controls the steering. The second illustration shows how a mirror image assembly completes the (half) unit. The plates in yellow are the structural elements from which a PRT “pod” would hang. It would be suspended between a pair of the double units pictured. Such an architecture would enable very tight station maneuvering in 3D, while distributing weight over a large area, (for the cheapest track) The four-wheel (half) units, as pictured, could be used as is, in factories or distribution centers, or even on the grid, (for light delivery) with weight loads in the 40-170 kg. range.

This final illustration shows a cam mechanism that could be used to alternately raise and lower he steering guide wheels. Two servomotors operate the camshaft jointly, yet either can operate it separately, because of the ratcheting mechanisms. If either motor fails, the unit can operate normally yet the failure is immediately detected by the encoders.


prophet666 said...

came across your blog through blogger found it very informative

Bengt Gustafsson said...

I really like the ratchet mechanism. It solves the problem with having to drag a malfunctioning (and maybe locked) motor around.

One concern is with neutral position. Do you intend to set the mechanism in neutral between switches? If so, are you aware that with one motor out of order you need to do a 3/4 turn instead of 1/4 to engage the desired switch wheel?

Dan said...

Dan the Blogger Responds:

Prophet666- Thanks for stopping by. It’s just my little way to try to make the world a better place, one reader at a time.

Hi Bengt-
You know, I almost put the cam assembly in a different post, because I thought someone might raise the question you pose. This is especially true because, due to my slow learning curve in SketchUp, I could not show the doubled assembly with one set of guide wheels up and one down, as I would have liked.

The fact is that I would probably keep the set of guide wheels up that would make the vehicle exit as the default position. This would seem to be prudent from a safety point of view. There is one trade off, however, that I will get back to at the end. But first there is one detail missing in this discussion, one that will eventually show up in an illustration, and that is the following:

I would like to have ALL wheel contact surfaces separated from the “box beam” framework by cushioning. As I have mentioned, I like to push the envelope, performance wise, and then dial back from there. This includes shooting for a silent, smooth ride, even at higher speeds. I am somewhat worried about sympathetic vibrations occurring between certain frequency emanations from the vehicles and certain lengths of truss or box beam. Furthermore, adjustable rubber-mounted running surfaces offer precision positioning and better “finger-jointing” for expansion control. I would point out that these steering guide wheel surfaces do not need to be continuous. They are needed for switching only, and the motor unit is fully and accurately constrained without them. Therefore, I made the decision to have the steering guide wheel running surfaces “taper-in” to contact. That is to say that with the guide wheel up in position, and the track meets the wheel, instead of vise-versa.

The result of this arrangement is that there are no forces at all on the switching process, because it switches into empty space, well ahead of where the steering guide wheel running surfaces begin. The track tapers out to meet the already positioned guide wheel and exerts a controlled pressure at an exact right angle to the travel of the height controlling assembly. The cam, once in the upward position, is in its strongest defense against being forced to reposition, leverage wise. I envision using stepper motors for their positional holding power, perhaps even with a gear head. The use of steppers here also greatly simplifies the quarter and three quarter turns you mention. This brings me back to that drawback I mentioned earlier.

One problem with not having continuous steering guide wheel running surfaces is that when contact is made, the wheel must go from zero to full rpm in an instant. This is, of course, a much greater problem at higher speeds than low. Under the above scenario, this “instant spin” situation would occur at every fork and merge in the track, even if the vehicle is simply going straight. There is, however, in my design at least, a section of widening track associated with every fork that requires the steering guide wheels to be engaged. Having a neutral position seems risky because it is the only way a vehicle could conceivably go neither right nor left but rather hit the middle instead, resulting in a collision with an immovable object. Therefore, I am not looking into a neutral position at this time. I am, however, looking for a good way to have the steering guide wheels “pre-spinning” at the proper rpm.