Sen­sotronic Brake Con­trol

Al­most 20 years since its in­tro­duc­tion, Chris Ran­dall in­ves­ti­gates the in­no­va­tive brake-by-wire sys­tem. 

Car Mechanics (UK) - - Contents -

Not so many years ago, stop­ping a car was an es­sen­tially straight­for­ward busi­ness: discs up front, drums be­hind and per­haps ABS for an added dash of tech­nol­ogy. Then, in 2000, Mercedes­benz an­nounced it was tak­ing a more am­bi­tious ap­proach. De­vel­oped in con­junc­tion with Bosch – re­port­edly over a num­ber of years and at a cost of tens of mil­lions of pounds – the Sen­sotronic Brake Con­trol (SBC) sys­tem was un­veiled.

Hailed by its de­vel­op­ers as a mile­stone in driv­ing safety, the key dif­fer­ence with the new sys­tem was that it did away with the con­ven­tional con­nec­tion be­tween the brake pedal and the brak­ing force ap­plied at the wheels. In­stead of the driver’s press­ing of the brake pedal act­ing di­rectly on a master cylin­der, thereby gen­er­at­ing hy­draulic pres­sure that was de­liv­ered to the brakes at each wheel, SBC used sen­sors to de­ter­mine the pres­sure the driver was ap­ply­ing and the speed of that ap­pli­ca­tion. A con­trol unit then in­ter­preted the sig­nals and, via pres­sure mod­u­la­tors within the sys­tem, de­cided how much hy­draulic pres­sure should be ap­plied at each wheel. Es­sen­tially, this was brake-by-wire.

The other key el­e­ments of the SBC sys­tem were an elec­tric pump con­trolled elec­tron­i­cally, a reser­voir op­er­at­ing at 140-160 Bar and a tan­dem master cylin­der that could be em­ployed in the event of com­plete elec­tri­cal fail­ure, restor­ing con­ven­tional hy­draulic brak­ing al­beit with longer stop­ping dis­tances. The sys­tem also in­cor­po­rated a spe­cial sim­u­la­tor, the role of which was to pro­vide an ele­ment of feed­back at the pedal, al­low­ing SBC to mimic the re­sponses a driver would ex­pect to feel un­der­foot as they braked. And there was an ex­tra layer of com­plex­ity be­cause the con­trol unit also re­ceived in­put from the anti-lock and sta­bil­ity con­trol sys­tems, the re­sult of which was im­pres­sively fine con­trol of the brak­ing at each in­di­vid­ual wheel.

The first model to ben­e­fit from SBC was the R230-gen­er­a­tion SL launched in Oc­to­ber 2001. SBC would go on to be fit­ted to a wide range of Mercedes cars, from the E-class ex­ec­u­tive saloon to the SLR su­per­car de­vel­oped with Mclaren.

The com­pany was quick to ex­tol the ben­e­fits de­liv­ered by the so­phis­ti­cated ar­range­ment, not least of which was a claimed 3% re­duc­tion in the stop­ping dis­tance dur­ing an emer­gency stop from 75mph be­cause the high-pres­sure reser­voir and elec­tron­i­cally-con­trolled mod­u­la­tors en­sured max­i­mum brak­ing pres­sure was de­liv­ered much more quickly. A fur­ther ben­e­fit was that SBC could recog­nise any sud­den move from throt­tle to brake; pre­dict­ing an emer­gency, the sys­tem was able to ‘pre-fill’ by in­creas­ing brake line pres­sure, bring­ing the pads into light con­tact with the discs and min­imis­ing any de­lay in ap­ply­ing full stop­ping power.

That wasn’t all, as SBC also al­lowed Mercedes to in­tro­duce a host of new brak­ing func­tions. One was Traf­fic Jam As­sist, which al­lowed the car to be braked au­to­mat­i­cally in slow-mov­ing traf­fic. An­other was Soft-stop, which claimed that bring­ing the car to a halt in ur­ban driv­ing would be smoother, be­cause brake pres­sure was ‘feath­ered’ in the last few mo­ments. Mean­while, Drive-away As­sist pre­vented rolling back on hills. And, in wet weather, turn­ing on the wind­screen wipers meant the sys­tem gen­tly ap­plied the brakes at in­ter­vals to dry the discs.

There was more, too. Par­ent com­pany Daim­ler had car­ried out re­search show­ing that two-thirds of driv­ers were alarmed by the puls­ing of the brake pedal on Abs-equipped cars dur­ing an emer­gency stop, mean­ing they ei­ther didn’t push hard enough or were tempted to re­lease the brakes al­to­gether. With no con­nec­tion be­tween pedal and hy­draulics, SBC re­moved this puls­ing ef­fect at a stroke.

It was clever stuff, but like all such ad­vanced tech­nol­ogy it wasn’t with­out prob­lems. Com­plaints were voiced by own­ers, with the soft­ware and SBC pump sin­gled out as com­mon causes of trou­ble, lead­ing Mercedes to re­call more than a mil­lion ve­hi­cles for checks to be car­ried out. While the com­pany said prob­lems mainly af­fected high­mileage cars sub­jected to fre­quent stop­ping – city taxis were cited as an ex­am­ple and all cab­bies in Europe were of­fered a free check – buyer con­fi­dence had been dented.

So much so, that the de­ci­sion was taken to aban­don SBC for the facelifted W211 E-class in­tro­duced in 2006, al­though it would con­tinue on SLR and May­bach mod­els un­til their pro­duc­tion life­cy­cles ended. Its in­tro­duc­tion had been a brave move for Mercedes, but one that ul­ti­mately failed to pay off.

But it wasn’t the end for the con­cept of brake-by-wire, be­cause a num­ber of man­u­fac­tur­ers per­se­vered with the tech­nol­ogy, most notably Toy­ota and Lexus, both of which em­ploy it on ve­hi­cles to this day.

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.