The Audi E-tron Brings Groundbreaking Brake-By Wire Technology to Electric Cars
Audi's ingenious new braking system is another step towards a future where the only thing connecting controls to road are a few strands of wire.
The progression of typically mechanical vehicle systems toward electronic control has been—for the average consumer, anyway—one of the less-visible technological shifts of the last 50 years. Throttle-by-wire systems that possess no mechanical link between the pedal and the engine have been around for decades, and have helped enable precision cruise control, stability control, and pre-crash safety systems that modulate engine torque regardless of what the driver’s foot is up to. Most car owners don’t know or care about such relative minutiae, and typically, the worst that can happen if it fails is that you just won’t go anywhere.
Steer-by-wire—a newer and rarer technology debuted by Infiniti in the Q50 sedan in 2014—allows for tunable feel and responsiveness with, again, no physical link between the steering wheel and the spinning rubber at the front corners. On the consumer freak-out scale between “NBD” and “WTF?” this system leans perilously close to the latter; after all, visions of steering wheels spinning cartoonishly from lock to lock with no effect could give anyone a fright. But Infiniti’s equally innovative mechanical backup system—which, at last check a year ago, had never been activated in actual use—ensures you retain control even if some digital gremlin pees on your steering sensors. No failures, no problem.
Which brings us to brake-by-wire. This idea has been lurking since the dawn of modern electrified cars in the early 2000s—most notably in the Toyota Prius, but also in GM, Ford, and Honda hybrids. It helps considerably with regenerative braking, where the motors take first crack at slowing the vehicle in order to pump electricity back into the batteries (unless the mechanical brakes are needed for more aggressive braking). Because it’s an even scarier proposition than steer-by-wire, the tech is advancing more slowly. Formula One race cars have had it since 2014, but beyond them, it’s mostly been the domain of a few hybrid models.
In fact, Audi’s new E-tron is the first fully electric vehicle to include brake-by-wire technology...and even there, it comes with an asterisk. While true brake-by-wire systems would use electronically controlled brake calipers instead of hydraulic pressure, Audi’s system is electrohydraulic. There’s still no mechanical link between pedal and pad—save for a redundant backup we’ll discuss shortly—but the actuator on the business end of the brakes remain hydraulic; it's just controlled by the computer in response to the amount of pressure the driver lays on the pedal.
This system was necessary in order for Audi to hone the entire vehicle as a unit for maximum performance and efficiency. Because the engineers wanted an ultra-fine-tuned regenerative braking system in the E-tron—it ultimately generates 30 percent of the vehicle’s range—they needed brake-by-wire to help modulate the handoff between regen and mechanical braking at the discs.
The system starts with a main brake cylinder filled with hydraulic fluid. Pressure sensors and pedal-travel sensors determine the braking force the driver is asking for; this information goes to the brake control unit, while a “simulator” pushes back on the pedal to generate the resistance drivers are used to. (That’s your brake “feel,” of course.) The system then sends this intel to another computer that controls the powertrain, including the motors and brakes. That handoff determines whether the regenerative braking can be used or if the hydraulic brakes need to be called in—the threshold being whether 0.3g of brake force is desired.
As a result, the system requires less foot pressure to generate more braking power, since the electronic actuators do the dirty work with the hydraulics at the wheels. In my first drive of the Audi E-tron in Abu Dhabi, the electrohydraulic braking was immediate and responsive, and it felt perfectly acceptable and familiar, even under harder driving. (It’s an 5,000-pound SUV, of course, so a similar system in a performance car might generate a notably different response.)
But what happens if the computerized system fails? Though unlikely, that’s also where things become even more interesting. In order to not freak people out too much, and to meet regulatory requirements, the E-Tron does have a redundant mechanical brake system. That backup is designed to do one thing—detect if the brake-by-wire system is working, and if not, instantly pop open a valve that sends the hydraulic pressure generated by the pedal directly to the mechanical brake pistons and calipers. It simply bypasses regen and the digital link to the hydraulic system downstream completely.
“So far, the system hasn’t failed in use during our testing,” says Michael Wein, the manager of the E-tron’s braking system. “If it did, it would send an alert to the driver and tell them that the system should be inspected. The most annoying result of this is that they wouldn’t have regeneration, and it would take a bit more pressure with your foot to brake—but no worse than a conventional braking system.”
One of the benefits of electrification of mechanical systems—the “by-wire-ification” of transportation—is the reduced cost and weight, and greater control over the systems. At the moment, the systems generally aren’t any lighter due to the presence of redundant backups, but eventually those will go away, and we’ll be left with a much more streamlined series of control systems. Ultimately, fully severing those physical link will represent a dramatic shift in the evolution of mobility, one that will completely reinvent the way vehicles are designed.
You only need to look to aviation for an example. Fly-by-wire systems that exists today—to varying extents in Airbus and Boeing’s most modern airliners—have completely reinvented the architecture of flight. When you no longer have mechanical links, the control inputs from whatever interface you decide to use—typically still conventional joysticks and pedals—can be programmed to do what you want. Further, because modern airplanes are so complex aerodynamically, they often deploy multiple control surfaces to execute the maneuvers they've determined the pilot is asking for. So pushing the stick to the right, for example, no longer simply activates the ailerons toward a right-hand turn; a single control input can initiate any combination of elevator, rudder, aileron, throttle, elevons, and trim controls to make that right-hand turn.
That complete divorcing of stick input and control-surface movement will play a critical role in future aeromobility, as well. Veteran helicopter manufacturer and upstart air-taxi innovator Bell is looking at the ways fly-by-wire can enable new control methodologies that may not look remotely like conventional stick-and-rudder controls. Similarly, as the steering, throttle, and braking systems in cars are fully separated from the conventional inputs we’ve all grown so accustomed to—and when they no longer need mechanical backups—we’ll have a completely clean slate to work with. Perhaps, for instance, the brake pedal isn’t the best way to slow the vehicle. Perhaps steering is better controlled with the feet, while braking and acceleration via hand controllers of some sort. Perhaps there’s a hand controller that melds steering, braking, and acceleration into a single interface, like the Mercedes-Benz F200 concept car of two decades ago. When you don’t have to do something the old way out of necessity, you’re free to find something better. Science will have to determine what that might be.
Of course, it’s absolutely possible science will find that the old way actually is the best way—that foot pedals and a steering wheel provide the best feedback and the best geometry for control. There’s a certain logic to it that makes a ton of sense, and it feels awfully right when you're at the controls of something special. Still, when you’re cranking that wheel and stabbing those pedals in your future Porsche, Ferrari, or Lamborghini, you can bet, at some point down the line, there won’t be anything connecting you to the wheels other than a few thin strands of wire.
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