Bosch Brake Control Pad Replaces the Conventional Brake Pedal

Brake-by-wire technology debuted with the 2001 Mercedes-Benz SL, and the technology has not exactly covered itself in glory during the intervening years. Parking at the lunch stop during the media drive for the Mercedes-AMG SLR McLaren in South Africa, I nearly rear-ended the car ahead of me because the Bosch-supplied Sensotronic brake computer system interpreted steady brake pedal pressure as “keep rolling forward at the same speed.”

A conventional brake system brings the car to a smooth stop under consistent brake pressure, as the car loses inertia. Alfa Romeo had better success when it debuted a Continental-supplied brake-by-wire system for the Giulia and Stelvio in 2018. Though it still felt a bit artificial, the system understood steady brake pressure as a request to stop gradually.

Honda’s Acura NSX hybrid-electric sports car endured a fraught gestation, but along the way, the company tuned the brake-by-wire calibration to perfection. Even though sports car drivers place a premium on finesse and feel, the NSX’s fake brakes provided the experience customers wanted.

Alas, more recently, Ferrari Purosangue’s execution of brake-by-wire falls short of Acura’s, reminding us that just because a technology has been correctly developed in some quarters, others may still need some time to catch up. In the case of the Purosangue, the initial application of brake pressure produces unexpectedly little result, prompting the driver to press harder on the pedal. That induces more aggressive stopping than the driver sought, so slowing the Purosangue can be jerkier than the driver would want.

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The Bosch Brake Control Pad sits where the brake pedal would be, but it is fixed in place and does not move. BOSCH

One benefit of brake-by-wire systems is that, because there is no connection between the brake’s hydraulic system and the driver’s foot, there is no pedal vibration when the anti-lock brake system activates on brake-by-wire systems. Some drivers are spooked by the pedal vibration and reduce pressure on the pedal to make it stop, extending stopping distances in emergencies.

Driverless cars will, by definition, brake by wire, as the computer will directly control the brake system without driver participation. We can expect more cars to move to brake-by-wire in the meantime because these systems give driver assistance systems more direct control of the car’s dynamics.

The challenge will be to provide drivers with something familiar while using the system. Bosch’s new Brake Control Pad system is different from a conventional brake pedal, which travels a distance to physically displace hydraulic fluid in the master cylinder. This movement and the pressure that causes it are familiar to drivers and are the parameters that brake-by-wire computers are struggling to translate into the intended braking force.

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The Bosch control pad takes the pedal movement out of the system, with a fixed pad that the driver pushes against to request a stop. More pressure equals more brake force. During my test drive of the Tesla prototype equipped with the Bosch system, I found the brake control pad to be surprisingly easy and intuitive to use.

The brake control pad sends electric signals to this electrically actuated master cylinder. DAN CARNEY

The pad responds predictably to pressure, and Bosch engineers walked me through three different levels of sensitivity. All three levels seemed acceptable, though the middle setting seemed the most similar to modern brakes.

While the high setting was sensitive, it was easy to brake smoothly in regular driving. Where it revealed itself was sudden braking, as when responding to a surprise like a squirrel in the road, when it applied more brake force than intended.

When a carmaker deploys the Bosch Brake Control Pad, it would surely be wise for that company’s dealers to ensure that buyers are introduced to the technology so that they know it may feel a little different than they are used to. But if they don’t, odds are that most drivers will not notice a difference.

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The Bosch system uses the Brake Control Pad to send signals to an electrically actuated master cylinder that is plumbed into a conventional brake system. Brake-by-wire proponents, especially those on the manufacturing side, look forward to the opportunity to employ electrically actuated calipers, with no hydraulics in the car at all. This would let carmakers eliminate messy hydraulic fluid from their factories.

But for now, Bosch’s incremental approach is to use familiar, proven hardware in the system, while replacing only the traditional mechanical brake pedal. Hydraulic-free brake by wire will come later. If it is in a driverless, autonomous vehicle, then any differences in brake performance from the use of electric actuators won’t concern anyone but the engineers tasked with calibrating the system.