Tuesday, September 22, 2009

51> A New Kind of Motor

I want to say a bit more about motors. It has been my contention that linear motors are no longer the way to go for PRT, or at least won’t be for long. In a nutshell, when PRT was first developed, computing and networking were at their infancy and there was no choice but to control all vehicles from a single “mainframe” computer. Linear motors were ideal for that, since they were able to take commands from the track itself, and eliminated all of the moving parts but the wheels, and made vehicles immune to traction problems, even on ice. A “no-brainer”. The tradeoff was efficiency, both in use of energy and materials. (in the old days of motors driving chains, gears or belts there actually wasn’t any tradeoff at all) These http://www.blogger.com/img/blank.gifdays, however, enormous computing power can be cheaply employed on every vehicle, and increasingly, those energy wasting belts, chains and transmissions are being replaced by motors catering to the specific torque and speed needs of the machine. In other words, direct-drive alternatives are the coming thing. In PRT, direct drive means that the only moving parts are the wheels and this doesn’t change. These days there are low RPM, high-torque motors that are essentially like a linear motor but in a lighter, more compact, more energy efficient package. Like a linear motor, a direct drive motor can have regenerative magnetic braking. The one trade-off is that such a vehicle relies on traction being maintained between the wheels and the track. In this aspect linear motors are still superior. Therefore the following solution is only for reasonably clean and dry environments. (Shrouded track) Emergency stops, (i.e. There is skidding) can, if need be, be accomplished by clamping the track.

So that is the background, and to regular followers of this blog, that is all old news, as is my love affair with wheel motors. (motors wherein the axle stays stationary and the motor itself revolves) So here is how all that background ties in.

I got to thinking about what specifically was so good about wheel motors and realized a couple of things. First and foremost is that it is a direct drive solution. Secondly, it is the form. Wheel motors get hi-torque and high positional resolution from having large diameters, with many magnetic poles. Generic motors generally get more torque from being made longer, a shape which is not well suited to PRT. I therefore (with the discontinuation of a number of wheel motors from PML) started looking into alternatives with search words like “flat motors,” “pancake motors” “high-torque” etc., and found a kind of motor which was new to me, the “frameless torque motor,” Illustrated below. Although it’s inner ring (rotor) turns instead of the outer, like a wheel motor, it can be housed in such a way that it will still provide true direct drive. One advantage to this motor is that the PRT designer can use readily available, standardized, replaceable automotive bearings.

I am a bit disappointed by the upper speed of the air-cooled models, however, as I was with PML’s wheel motors. A quick look at my drive unit of July 16th shows relatively large drive wheels, which, quite frankly, crowd the track, which is already taller than I would have preferred. (I would like the track and vehicle to be able fit in a building between typical floors, if possible, and, in my wildest dreams, two high under a highway overpass) Slow rotation necessitates large wheels to get speed. For example, to achieve a speed of 60 mph, (96k/h) a 14” (356mm) wheel must turn at 1440 rpm. This has been sort of a sweet spot for me, design wise, because I doubt that it would be beneficial to make the drive wheels much bigger or smaller, and I doubt any initial PRT iterations would go any faster.

The wheel diameter and motor speed information, together with presumed assumptions of adequate acceleration, form a sort of triad, which can be adjusted to size the motor and track. (In the June 16th post I showed how the track size could be decoupled from the truss size; The track itself need not be self-supporting over long spans) The track is, of course, the big kahuna of PRT design. Little pods may come and go, but the utility of the track design will be debated for generations if that track design becomes the de facto standard. It can be said, therefore, that it would be silly to base track dimensions on some arbitrary motor specifications. After all, motors can be custom wound. But, hey, wouldn’t it be nice to base track specifications on readily available parts?

To the minority of readers who are even remotely interested in form factors, torque or speed ratings of motors, I would refer you to http://www.etel.ch/torque_motors

Finely a bit of personal news. As of tomorrow morning at 4:30 AM, I will be on route to New Hampshire, (USA) to close up my cabin for the winter, and take in the magnificent fall foliage. There is no internet, or even electricity to speak of, since I am a half mile from the nearest telephone pole. I can plug in at the town library, though, and hope to post at least once in the next few weeks. So “bear” with me, if I am slow to comment or return Emails. I’ll try and post a picture
- Dan The Blogger

1 comment:

an-148 said...

in addition to the "difficult to mantain the gap" LIM's, there is another linear motor that is on the market: LSM (worth a look at ????): http://www.magnemotion.com/lsm-technology/LSM_Issues.cfm