HARRISON BOUDAKIN
BRAKE-BY-WIRE...OR NOT?
THE DIFFICULT BIRTH OF THE ELECTRO-HYDRAULIC BRAKING SYSTEM, AND WHAT IT'S DONE TO THE IDEA OF FULL BRAKE-BY-WIRE TECHNOLOGY
It’s about 7:45pm. At the wheel of your 2003 Mercedes-Benz E-Class, you briskly turn out of the side street and join a main road. Four wide lanes point straight downhill, gradually steepening towards the set of lights at the T-intersection below. You notice the traffic building up ahead, instinctively squeezing on the brake pedal to slow up well in time before the junction.
Then, without warning, an blaring alarm sounds inside the cabin and the instrument panel flashes an urgent message “Long Stop Brake Failure Stop Car!”
The trouble is, you can’t. The pedal suddenly goes soft and its travel increases dramatically as if it’s not connected to anything. In a panic, you stand on the pedal, desperately trying to obey the message and stop before the queue of traffic ahead. But the car is barely slowing, so you press even harder on the spongy brake pedal pedal and keep it there. Nothing…nothing… then, gradually, the brakes start to bite, and the E-Class eventually careens to a halt as your steer it away from the traffic up onto the empty footpath. Shaken and scared, you climb out of the car, alarms still beeping in the cabin. “SBC Brake Failure. Stop Immediately.” In the dark, rainy night, you stand on the kerb next to your new Mercedes and wonder what the hell just happened.
Between 2001 and 2005, this was the kind of nightmarish scenario that some of Mercedes-Benz’s most faithful customers came face to face with. Piloting their SL, CLS and E-Classes, drivers would suddenly find the much-vaunted Sensotronic electro-hydraulic brakes switching to a failsafe back-up mode, forcing them to wrestle their car to a halt with what felt like barely any braking power.
For the loyal customers of a brand that once claimed its models were “Engineered like no other car” this was stunning. Facing a string of failures and thousands of shocked customers, pretty soon Mercedes-Benz found itself recalling all 1.3 million cars equipped with the supposed technological marvel, SBC: Sensotronic Brake Control.
Mercedes-Benz spent nine years and 147.7 million Euros co-developing the Sensotronic electro-hydraulic brakes with German engineering giant, Bosch. When it launched on the R230 SL in 2001, SBC was heralded as the future of braking technology, and was the residue of Mercedes’ decision to develop a whole raft of electronic innovations in a effort to establish itself as the industry’s technological leader.
The theory behind SBC was very promising. Essentially it was the world’s first production brake-by-wire system, eliminating the mechanical connection between the brake pedal and the hydraulic master cylinder. Pressing the brake pedal instead activated a complex network of sensors that electronically relayed the amount of brake input to a central control unit, which then administered the precise hydraulic pressure to each individual brake calliper. Brake ‘feel’ was simulated by a spring arrangement inside the pedal mechanism, and a mechanical back-up was retained in the event of an electrical glitch.
Mercedes-Benz and Bosch claimed this electronic design brought with it a number of critical safety advantages. Because the pedal itself was physically separate from the high-pressure hydraulic master and slave cylinders, the computer had much finer and faster control over the braking system. If the driver suddenly lifted their foot off the accelerator, for example, SBC would automatically ‘pre-load’ the brakes so they could react instantly if an emergency stop was required. SBC could also gently tap the brakes periodically in wet conditions to dry the discs. And during hard stops, Sensotronic eliminated the off-putting pedal vibration normally caused by ABS, meaning drivers could confidently keep their foot planted on the pedal during an emergency stop.
However, to achieve these breakthroughs, SBC required a fiendishly complex array of electronic components, many of which the company had little experience with. In fact, Mercedes later admitted that most of the technological precedents SBC was designed around were in fact adopted wholesale from the consumer electronics industry. Trouble was, these electronics proved relentlessly unreliable when subjected to the harsh operating conditions vehicles create for components, which wouldn't have been a problem if Mercedes-Benz had tested them thoroughly enough during R&D - but they didn't. Wiring harnesses jiggled loose or deteriorated, and the software that controlled the hydraulic accumulator (below) at the centre of the system was intermittent and glitchy. Any one of these faults would trigger the mechanical fail-safe that deactivated SBC and reverted to back up hydraulics - hence, the heart-in-mouth moment when the pedal went long and soft, with the system providing just enough stopping power to bring the car to a halt.
Despite recalling every SBC-equipped car to implement software and hardware fixes, in 2006 Mercedes-Benz took the bold step and axed the system altogether. The move was a huge blow to the future of electro-hydraulic braking technology. With no other customers for its system, Bosch admitted that using electronics to transmit driver inputs into a standard hydraulic braking system was a technological dead-end. The main issue was that increasingly sophisticated ABS and ESP technology, which use an electronic regulator between the master and slave cylinders to control braking to each wheel independently, meant manufacturers could integrate most of the benefits of SBC without the cost and complexity of the electro-hydraulics.
Bosch Sensotronic was also heralded as a major step towards full by-wire braking technology, which would forgo hydraulics altogether. In these systems, sophisticated electronics would send signals from the brake pedal directly to an electro-mechanical controller at each wheel, which would drive the callipers electrically. Such a concept was thought to be critical to the industry developing fully-integrated vehicle electronics architectures, which would speed up the introduction of fully-autonomous driving technology. But the very public failure of SBC meant development of these systems slowed dramatically, and as yet not one car manufacturer has fitted a full by-wire braking system to a production car. And only select few manufacturers continue to use electro-hydraulic braking - predominantly in hybrid models such as Chevrolet’s Volt, where it allows engineers to create a more progressive pedal feel when blending hydraulic and regenerative braking systems. So thus far, only by-wire throttle control has found significant traction in the industry - by-wire steering set-ups have been introduced but have found little success, and full by-wire braking remains elusive.
There is a school of thought that says by-wire controls will soon emerge into the mainstream as we hurtle towards the autonomous driving future. But Mercedes-Benz’s SBC failure in the 2000s certainly offers some evidence that there are some mechanical technologies that are so proven, so effective and so reliable, that replacing them with full electronics is going to require a serious leap of faith for both manufacturers and drivers, and yet more expensive development and testing.
It will happen - we’re just not there yet.
Harrison Boudakin
29 August 2016