Saturday, April 18, 2009

28> An “Ah-Hah!” Moment



In the post dated 3/22, I posed the question about what to do with an unbalanced load. I got the answer that I didn’t want, that the cabin couldn’t hang freely, but rather needed an active mechanical positioning system to correct the problem. Unfortunately, the beauty of the gondola design is that it minimizes errant G forces within the cabin by free hanging, so having an active system to correct for “level” would seem to be counter-productive. I think I have solved the problem. First let’s look at a little picture (above) I did to explain how to stop a passenger compartment from swinging too freely. If a vehicle swings too freely a turn or a gust of wind could get it swinging back and forth like a swing set. Pictured are two hydraulic/pneumatic cylinders tapped to work in both directions. (Sorry they look like car shocks, personal clip-art)
The cylinders are connected by hydraulic fluid lines with valves. When the valves are fully open, the “gondola” swings freely. When fully closed it doesn’t swing at all. The valves can then be adjusted for best performance. My guess is that a small bubble of air in the system will enhance performance. Note that two pair of such cylinders are needed, one for front to back swing control and one for side-to-side control. These would be integrated with any other suspension system.

Here is the “Ah-Hah!” part. The unbalanced load creates a difference between the (loaded) cabin position and the position that the cabin should take from (unoccupied) gravity. But this gravity is not necessarily down. It is whatever the forces of momentum make it. Therefore a simple “out of level” detector will work continuously during a journey, because it won’t just detect down, it will detect gravitational force and an active system meant to detect a severely unbalanced load can be added to a free-swinging system. It can straighten out a weight-tipped cabin while still allowing the cabin to swing.

Here’s how it works. A simple tilt detector like this
mercury switch (similar to those found in a pinball machine) determines that the cabin is tilted and the motor (pictured in blue) activates a screw jack to adjust the level. This process is purposely a bit slow, say 3 seconds. During a trip, the cabin will swing somewhat freely. (dampened by the hydraulic system) Any variation from the cabin floor being perpendicular to gravity or G forces will activate the motor, which will contribute to G force mitigation in a minor way during velocity changes and banking, but will principally keep the “gondola” level during constant velocity straight-aways and during “docking.” I have to say that this is a very immature, out of proportion design, posted as a conceptual drawing only.

4 comments:

akauppi said...

I will be blogging from the PRT@LHR conference during this week here:
http://openduck.blogspot.com

Please place any questions you might have as comments on that blog.

Mr_Grant said...

What is the weight of one set of load balancers? Is the purple thing a hydraulic fluid tank? This opens the design to criticism that it will leak on whatever is below the guideway (see 'system droppings' at rebuttal to Light Rail Now)

Do you plan a redundant "B" set that take over if the first "A" set fails? Will the "A" set completely disengage so as not to interfere with operation of "B"?

Are you keeping a weight budget for your vehicle and bogey? I am seeing the advantage of a supported design, where in corners you just bank the track.

Dan said...

Dan the Blogger Responds- Hi akauppi, I'll be interested..check your inbox..
Mr. Grant, Good points, all. First, you really need to click and maybe enlage it further to get the detail. The “purple” thing is supposed to be blue, a motor connected to a screw jack. These should not fail unless the power fails. As I have mentioned before (can’t remember which post) the screw jack is what powers the flaps on commercial jets, (as well as the landing gear, rudders, etc. It is much more reliable than hydraulics. I have a friend who is retrofitting the tilting floors for flight simulators and Disney rides from hydaulics to screw jacks for that exact reason. Good screw jacks actually have ball bearings between the screw and jack part, all hardened steel, etc. Very reliable.
The hydraulic part is the motion dampener, and your point is well taken. I too have run the banked track thing over in my head more than once. I certainly do not count it out.. What I propose is functionally similar to any commercial door closer, except double cylinder. I have yet to see one leak. It is also similar to a car brake, and I have seen them leak, plenty of times, but on old, worked on cars. Anyway the cowling around the motor unit can be made to catch any possible fluid leak.
Please understand, if this all seems complex it isn’t because the vehicle hangs, it’s because we are talking high comfort and high performance. Requiring elevated stations, (and elevators for the disabled) large turning radii, (requiring purchase of right-of –way on super expensive corner real estate) and speeds of only 30 mph are formulas to kill PRT before it gets started. Also banked corners have an optimal pitch per speed. I hate having a “one speed fits all” approach as a starter
I guess it all comes down to a maintainence schedule. Do they come in once a week or once per 1000 miles? I would assume there would be a list of points to check that could be done in 5 minutes, with a more careful check done on a longer cycle. On a still longer cycle would be rebuilding and replacing, just like an aircraft. I think as little as 7 cents a mile could provide this service, once the maintenance infrastructure is in place.
The matter of weight… I’m guessing under 50 lbs. for everything related to tilt control. As for safety, the tilt control is really more about comfort than safety. Swinging side to side like a pendulum after a fast, sharp turn should be avoided, and not by only going slow around large radii turns, as has been the case in previous designs.

Bengt Gustafsson said...

Note that with a broken tilt mechanism (stuck at an odd angle) you may not be able to dock to station platforms.