Interesting to see HJ's DT column definition of Sensotronic Brake Control (SBC) on the new Mercedes SL series as 'brake-by-wire'. As someone who's spent years developing software (including embedded solutions such as this), I'm sure that MB have got some sort of fail safe behaviour built into the system. Problem with electronic systems can be that when they fail they do so totally and without warning, unlike a good old mechanical solution. With something like a braking solution I'd worry about this!
Anybody out there (HJ?) know what MB have in place to detect/handle faults? As he points out, with a microprocessor controlled pressure simulator to provide pedal feel this is obviously a sophisticated system.
How do backroomers feel about having a computer apply their brakes for them?
Rich
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How do they feel about travelling in fly-by-wire aircraft? I hope they've borrowed some of the safety technology from that industry - there is an aerospace division of Daimler is there not?
Regards
John S
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Good point John, I guess that critical aircraft systems have a fair degree of duplication, but that always comes at a (high) price. I'm just intrigued as to how MB have done it and what is the perceived added value of such a system over traditional mechanical/hydraulic braking (if they've designed in complete redundancy then I guess that a cost saving will not be one of them).
I'm hoping that HJ has the press pack for this announcement and can enlighten us as to its advantages etc...
BTW - Personally speaking, I wouldn't feel comfortable travelling in a fly by wire aircraft that I'd written the software for unless there was an override of some sort.
Rich
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IIRC, critical systems on aircraft are triplicated.
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"IIRC, critical systems on aircraft are triplicated."
True, I also recall overhearing a commercail pilot radio comm'n as follows.
"Just like to remind the ground crew that our Widget is up the spout. So is the back up, so we're operating on the second backup and we don't have a third back up. It looks as though the original fault was reported on dd/mm/yy ( a date much much earlier!!!)
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Yes computers are anything but reliable in my electrical engineering experience, i wonder what transducer they use to transmit the braking signal to the electronics in the first place, this i believe will be the first to fail, is it a potentiometer? These fail on a regular basics in our works even the highest quality military spec.
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If everything was to aircraft standards most people would probably feel happy, (altho I remember meeting a commercial airline pilot 10 years ago who had hair raising stories about early fly by wire systems, the aircraft equivalent of the blue screen of death and the use of the reset button. He had refused to crosstrain at that time).
However the garage industry cannot pretend to maintain to aircraft standards ( inc recent experience of the inability of MB to diagnose a simple fault electronics after 5 days......). And what happens when a Fast Fit monkey gets involved?
At least in a plane you have the 3rd dimension to play with, and large amounts of vertical and horizontal separation. By definition car brake systems are generally only used at critical moments. Just look at how often F1 cars seem to suffer immobilising terminal electronics failure, and these are not in bargain basement conditions.
With an long electronics and software background I would not be happy with a system developed by anybody I know in the elctronics industry (albeit with very little personal experience of airline systems).
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Perhaps the MB failsafe is the brakes locking on, thus rendering the car undriveable?
Terry :o)
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Well, I can now answer my own questions! There's a Daimler-Benz paper on SBC at autoweb.autospeed.com/A_0759/P_1/feature.html which is very readable. Seems that SBC is actually the marrying together of all the vehicles braking systems (ABS, Traction Control and standard braking) under microprocessor control. The chief advantage is increased braking efficiency and there is a traditional tandem hydraulic system as a backup. Quote follows:
"By contrast, in the Mercedes-Benz Sensotronic Brake Control, a large number of mechanical components are simply replaced by electronics. The brake booster will not be needed in future either. Instead sensors gauge the pressure inside the master brake cylinder as well as the speed with which the brake pedal is operated, and pass these data to the SBC computer in the form of electric impulses. To provide the driver with the familiar
brake feel, engineers have developed a special simulator which is linked to the tandem master cylinder and which moves the pedal using spring force and hydraulics. In other words: during braking, the actuation unit is completely disconnected from the rest of the system and serves the sole purpose of recording any given brake command. Only in the event of a major fault or power failure does SBC automatically use the services of the tandem master cylinder and instantly establishes a direct hydraulic link
between the brake pedal and the front wheel brakes in order to decelerate the car safely."
System seems to offer loads of sophistication as you might expect. Check out the predictive pre-filling of the braking system, the "Soft-Stop Function", "Dry-Braking Function", "Traffic-Jam Assist" and "Drive-Away Assist".
I know that if you can afford a motor with a system like this installed then you probably don't worry too much about servicing costs, but I do shudder to think of the cost of maintaining/repairing this type of
electronic gubbins, especially once these cars hit the used market.
Point your browsers at autoweb.autospeed.com/A_0759/P_1/feature.html for the whole article.
Regards
Rich
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"Only in the event of a major fault or power failure does SBC automatically use the services of the tandem master cylinder and instantly establishes a direct hydraulic link between the brake pedal and the front wheel brakes in order to decelerate the car safely."
Now this does worry me!
If the the control system has to 'establish a direct hydraulic link' under fault conditions! It sounds that it has to be proactive once it has failed.
Where are all the aircraft and control engineers who could spring to the defence of this (potential) nightmare?
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At this time there is a legal requirement for a fall-back system in case of electronic/hydraulic/?????? failure.There are also developments in steer-by-wire.
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Not too tough to workout the failsafe mode... one way would be for all the fancy electronics to work over the first bit of pedal travel and let the last 2 in of travel work the front brakes as normal... in standard use that last bit of travel would never be seen but in a failure it would always be there...
recall single circuit brakes in the 'old days' one tiny leak and no brakes... and a mad dash for the emergency brake... (as it was in those days.... it was NOT called a parking brake!) I was taught to use it to stop the car from normal speeds about once a month to make sure it was working well... on my 1948 jeep you need to use it with care to avoid locking up the rears.
Randolph Lee
Nantucket Island, U.S.A.
I live in my own little world, but it's ok, they know me
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Great thread!
A high proportion of these cars will be sold to the wealthy in litigation happy USA. MB better have it right as one catastrophe involving brake failure will be enough to bankrupt even Mercedes Benz.
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I was taught to use it to stop the car from normal speeds about once a month to make sure it was working well...
I use mine on every corner. ;-)
--
Parp, Parp!
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This could, however, be as dangerous (high speed busy motorway) as the brakes not working at all-just depends what situation you're in I guess.
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The post above was referring to this earlier post:
"Perhaps the MB failsafe is the brakes locking on, thus rendering the car undriveable?"
Put it in the wrong place-sorry!
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