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Don't assume it's not redundant
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 22 Mar 2002 14:55:52 Share Post by Email
In Reply to: Re: Sensonic Brake Control, brake by wire., John Davies, Fri, 22 Mar 2002 11:43:05
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Just because there is only one electronics box doesn't mean there isn't electrical redundancy.

Think about this - First, Mercedes has access to some of the best engineering talent in the world. Second, they have lots of lawyers, just like any other big company. Do you really think they'll spend tens of millions of dollars on developing a new braking system, and put it out there without considering (1) failure modes, and (2) lawsuits?

Mercedes saw what happened to Audi when the flap over unintended acceleration happened.

As someone that works in aircraft electronic flight control and engine control systems, I've had some experience in system redundancy and failure modes. An electrical system can be made just as reliable, possibly more so, than a hydraulic one. There are a bunch of single-point failures in today's hydraulic braking systems - it's just that the failure incidence is very low. But those systems are heavy, expensive, and require maintenance - fluid changes, etc. And we all know what replacing ABS components cost.
Airbad systems have redundant sensors, even a back-up power supply in case the battery is damaged in the accident. Hydraulic systems use accumulators to supply boost even if the engine is off.

I'm going to go out onto a limb and assume that everyone at Mercedes didn't suddenly become stupid, and remain so for the however many years it took to develop. And that all their suppliers became equally stupid. I'll bet that the brake pedal sensor is dual redundant at minimum. I'll bet there are dual electronic paths, with backup power in case of total power loss.

If the wiring and power is dual redundant from the sensor to the actuator on the brake caliper, that is a higher level of redundancy than in present hydraulic systems. Right now, the only true dual redundancy is that you have two braking systems, each working on two wheels. Lose one, and you have half your braking capability. And the linkage from the pedal into the master cylinder, and the boost system, are all single-string- one failure and you're out. With dual electrical, depending on design, you could have a failure and still have full braking at all four wheels.

You have a computer controlling your brakes now. The ABS system is computer controlled. But the rest of the 'computer' is made up of valves and passages, and instead of electrons it's hydraulic fluid. Contaminated fluid, dirt clogging passages, busted hoses, all can cause a failure.



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