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Old 11-07-2014, 11:39 AM   #1296 (permalink)
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Originally Posted by e*clipse View Post
The important thing here is that it is NOT a speed control issue. This makes the whole thing significantly different than attempting to match the speed of two independant motors. In fact, from a controls perspective, speed control requires another complete control PID loop. So with only torque control the motors won't be fighting you like tank steering. If things go really awry, one motor may produce 25% more torque than the other, but this is much different than the fight that could happen when one motor tries to go 25% faster than the other. In that case, the control loop may command FULL torque until the speeds match, or negative torque on one motor with increased torque on the opposite - which could have awful results like you are warning about.

I too am leary about putting too much computer (code) between the driver and control. This is why I'm trying for a simple analog circuit with a bypass switch. The bypass switch would make the commanded TORQUE from each motor identical.

I'm not completely convinced locker differentials are a simple solution. Putting one in an MGR would be a very expensive custom job.

When I was growing up, my dad had a Chevy Malibu with a "limited slip" differential. We went skiing every opportunity, and I can say my dad's excellent driving made up for the differential's weird behavior. If one wheel slipped, it would clutch and force all the torque to the opposite wheel, which could slip because it all off a sudden got all the torque. Then it would clutch to the opposite side. Back & forth, fishtailing down the road because of the "help" from the differential. According to a friend with lots of off-road experience one needs to be very careful with locker choice, because the on-off behavior is pretty common.
First I should say that I do not yet understand enough about electric motors and there respective behaviors. I have been a gear head far longer and only have those examples to fall back on with only a few years actually hands on with electric cars. If you were to put two ICE's back to back and run there outputs through a gear reduction and then out to one wheel each you could effectively steer the car by throttle alone. It may not work like a tank where one gear set has a brake to stop it while the other runs on but it would work to an extent.

Rather than mess with code for two controllers with motors driving two separate wheels I would simply connect both sides together with a differential. Front to back would not be too much of a problem as long as the front wheels were setup to outrun the back by a little.

By “simple” I mean gears, electronics and code are not simple to me, they are more like voodoo. There is a device that is called The Gleason Torsen. Gleason's Impossible Differential I know at least three people who run these and have nothing bad to say about them.

My old 69 Ford has a Detroit locker in the rear and a power lock in the front. When I get on the throttle hard I may as well have two solid axle drives. With a 460 the usual behavior on dry roads is a thunk and then clicks when I go round corners. It has never caused any handling problems and in snow and rain you can not even hear it when one or the other dog clutches overruns and disengages. Mostly used for camping and off road fun I have driven it for over 20 years.

When I get hold of one of those MGR's I will disassemble it and take the spool to my buddies shop and see what kind of traction device will work in it. He runs NW differential which specializes in running gear repair and up grades.

Yes it is true and that can happen but what a lot of people did not know with the limited slip differentials is that they “required” a special additive to keep the components from chattering and causing that problem. The other differential device Chevy used worked with a set of spinning weights. When one axle over ran the other it caused the spiders to lock. I never did like driving those because they did weird things.

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