Hacking ABS Controllers

[eightfifty]

All of my background is in Computer & Electrical engineering, but I have an increasing interest in (or obsession with?) automotive stuff.

Anyway, I've been wondering about ABS systems. The car I am experimenting with is a 2002 Subaru Outback, which has all-wheel-drive and open front & rear differentials. If one front wheel and one rear wheel both lose traction, the car is stuck. I'd like to build a system such that the car can still pull itself along with only one gripping wheel. I know that many newer cars have this ability without using limited-slip or lockers, and I think they implement it in a similar way to what I'm imagining.

The car has 4-wheel disc brakes and 4-wheel ABS. I should not have much of a problem implementing a microcontroller to read the output of each wheel speed sensor and detect slip.

Once a certain amount of slip is detected, I would want to apply the brakes on only the slipping wheel(s) so that the differential applies torque to the wheel(s) with traction and the car can dislodge itself.

My uncertainty is in the physical implementation of the ABS system. It only ever engages when the driver physically applies the brakes (of course!). My understanding is that it then periodically releases and reapplies this pressure while it is operating. Does it depend the braking pressure in the system being provided by the driver in order to work, or can it brake a wheel sufficiently hard on its own, without the driver having to depress the pedal?

 

[patprimmer]

The hydraulic pressure it uses comes from depressing the brake pedal. The ABS simply controls and directs that pressure.

Your basic concept is not new and is in fact the system used on tractors from before the last century by means of individual brake pedals for each rear wheel. VW beetle based off road buggies and race cars have used a modified hand brake mechanism to create a similar device on the rear wheels. You could also do this to the rear wheels by adapting your hand brake. It might be dangerous to use the ABS in this manner as it will not react in the proper way when the brakes lock if you modify it to deliberately lock wheels rather than to deliberately momentarily release them.

Regards
Pat
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[GregLocock]

To add to pat's comment.

Modern traction control/yaw control systems are self energised, that is, they will apply the brakes selectively even if you haven't hit the brake pedal.

But seriously, we can't tell you how to stuff around with the ABS (etc) codes on your car.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.