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Comment by giobox | Hacker News Reader

It's not yet clear virtual locking diffs on quad motor cars will be as effective as a fully locked diff.

A software system will likely have to detect some small degree of slip before intervening to sync the wheel speed again - that slip just wouldn't occur in the first place on a fully locked setup given its impossible for one wheel to spin faster than the other. This seems to be the experience of Rivian owners who have quad motor cars with virtual locking diffs.

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That's a solvable software problem. The electric motors know how much they are spinning, they just need to be modulated quickly enough and intelligently enough.

Motors know nothing. The encoders know how much current is being input to the motor, but do not know the rotational output of the motor or the friction coefficient between the tires and the ground. All three are things you'd need to emulate a locking differential in software. You could fake the last two by having a reliable way to measure the travel of the vehicle from a source other than the wheels (GPS, downward camera, etc).

Err, sorry but the encoders don't measure current. An absolute position encoder knows the exact speed/position of the motor. By that you can extrapolate the wheel speed position which you can just match 1:1 with the one on the other side. It won't be perfect as the reaction time of the system if wheel A breaks away and starts spinning but you don't need GPS or a downward camera to mimic a locking diff.

Sorry I was trying to rewrite for clarity and said encoder instead of controller.

If you naively emulate (as Telsa surely would) a 1:1 fully locking diff in software it will suck to drive. Electronic limited slip diffs already exist, like the Jeep FD/AD systems, which will use tire rotation sensors to determine forward momentum.

Now that I think about it, the Ford is probably already doing this.

Technically, you can sometimes measure the motor position without a position sensor by measuring the back EMF (although I doubt Tesla is doing this, IIRC it's hard to get good starting torque out of this setup)

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