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X-by-Wire: When the Electronics Take Sole Control

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ZF has the largest portfolio of by-wire technologies in the industry. What are the advantages when the electronics control the brakes, steering and other systems in the car – with no mechanical connection to the driver?
Andreas Neemann,
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Andreas Neemann wrote his first ZF text in 2001 about 6HP transmissions. Since then, the automotive writer has filled many publications for internal and external readers, showcasing his passion for the Group's more complex subjects.
Steering or braking a passenger car took real muscle power a few decades ago. In the 1980s, life became easier for drivers thanks to electrohydraulic assistants. It was only with the advent of power steering and brake boosters that the force required for steering and decelerating was reduced. Later, other systems increased safety: ABS (anti-lock braking system) and ESC (electronic stability control) guaranteed that braking did not compromise directional or vehicle stability.

One thing that never changed was the ever-present direct mechanical connection between the steering wheel and the front wheels on the one hand and between the brake pedal and the service brake on the other. This connection to the person behind the wheel was in a way the last fallback position for safety. Should all the electronic or hydraulic systems fail, then this connection allowed the driver to continue steering or braking using sheer muscle power.

Trends Promote by-Wire Solutions

Trends Promote by-Wire Solutions

This era is now coming to an end. Soon, electronic control pulses plus actuators will be the only things moving steering systems and brakes, i.e., by wire. While the steering wheel and brake pedal will still exist, they will function like a joystick, i.e., control units the driver simply uses to trigger the electronic pulses.
Such by-wire systems are not uncharted technological territory. Since they include many other systems apart from brakes and steering, they are known as x-by-wire. Not every system had a mechanical connection to the driver. ZF's AKC (Active Kinematics Control) rear axle steering, for example, has always been controlled entirely by software; the same applies to the semi-active CDC (Continuous Damping Control) damping system, the active roll stabilization system or air suspension. All these systems increased safety, comfort or driving dynamics.
Electric mobility and autonomous driving are two unstoppable trends that are now powering the uptake of by-wire systems for braking and steering.

"In automated driving from Level 2 and above, the computer takes control of the vehicle's longitudinal and lateral dynamics – so the computer must also have full access to the steering system and the brakes. By-wire systems are much better suited to pure software control than systems that still require mechanical intervention," explains Manfred Meyer, Head of Engineering Division Chassis Solutions.
The decoupling of mechanics and electronics also enables new functions that drivers will appreciate. One example is a steer-by-wire steering system that ZF will put into volume production from 2023. "Thanks to the software control, we can make the steering ratio completely variable depending on the driving situation – unlike the fixed steering ratios used for a mechanical connection between the steering wheel and the front wheels," says Meyer. The same steering system is therefore very agile and sensitive when driving dynamically along a country road. But when parking, it only takes a few turns of the steering wheel to reach the maximum steering angle. This is not simply a comfort feature. The technology could also reduce the large number of "minor accidents" that tend to occur while parking as a result of drivers misjudging the steering angle.
"Thanks to the software control, we can make the steering ratio completely variable depending on the driving situation – unlike the fixed steering ratios used for a mechanical connection between the steering wheel and the front wheels."
Manfred Meyer, Head of Engineering Division Chassis Solutions

Generate Power When Braking

Generate Power When Braking

Automotive manufacturers also benefit from by-wire technology. Installing steer-by-wire steering systems reduces the assembly effort needed to build today's right-and left-hand drive versions of the same passenger car model. And additional design freedom exists for future passenger car models that support highly automated and autonomous driving according to Level 3 and higher ex works. Steer-by-wire supports innovative steering wheel designs, allowing the steering wheel to be completely folded or pushed to one side. Drivers then have more space when the computer steers the vehicle and they want to focus on other things. "Such flexible designs cannot be implemented using a conventional steering system with a steering rod," says Meyer.
By-wire solutions also offer advantages for the brakes – especially for the new era of electric mobility. Electric vehicles can decelerate in two ways – using the service brake or the electric motor's generator mode. In generator mode, electrical energy is fed back into the batteries, i.e., recuperated. "With brake-by-wire solutions, we can optimally design brake management to deliver maximum recuperation," says Meyer. "All in all, this increases the range, prevents wear of the service brake and, in turn, reduces particulate matter."

Absolute Fault Tolerance With Steering System and Brakes

Absolute Fault Tolerance With Steering System and Brakes

An essential prerequisite for all by-wire systems is intrinsic safety: The steering system and the brake in the passenger car must never fail; the vehicle must remain controllable even if some part of the electronics fails. Since by-wire systems have no mechanical connection to the driver as the ultimate fallback position, the electronic control units must be designed redundantly, i.e., duplicated in the vehicle together with their control electronics. The control software must accordingly foresee the scenarios where the backup system is used. "Ensuring such fault tolerance is anything but simple – a large part of our development resources is devoted to this goal when developing steer-by-wire and brake-by-wire," says Meyer. These comprehensive safety architectures for by-wire technologies required for brakes and steering systems encompass the supplier structure for purchased parts through to the issue of cybersecurity – because, logically, nobody else must be able to "hack" a car and operate the steering and brakes apart from the driver. "There aren't many companies that can manage this complexity. This is one reason why, apart from ZF, there are now only a handful of steer-by-wire and brake-by-wire systems ready for volume production on the market," Meyer concludes.

Networking Bolsters Safety

Networking Bolsters Safety

However, by-wire technology also boosts safety. Even with steer-by-wire on its own, imperceptible short steering movements can stabilize a vehicle before another system such as the ESC "anti-skid brake" intervenes. Drivers and passengers do not notice any of this. A control network with other systems can enhance safety further, especially because x-by-wire systems offer optimal networking possibilities. "Engineers have virtually infinite number of ways of controlling longitudinal, transverse and vertical dynamics to improve driving stability, driving safety and ride comfort," says Dr. Christoph Elbers, Head of Engineering Vehicle Motion Control at ZF. "This is not just about which actuators intervene and control the vehicle at a critical moment. Active by-wire systems can specifically define a vehicle's physical handling characteristics." One example is dynamic wheel load distribution between the front and rear axle. It has a significant influence on the contact surface of the tires with the road and so determines how a vehicle reaches the limits of performance. The design of an active roll stabilization gives the x-by-wire systems with their proactive control more ways of influencing handling. "While drivers do not notice how the systems control the vehicle, they still perceive the behavior of their vehicle as pleasantly dynamic and more comfortable," says Elbers. In the era of highly automated and autonomous driving, such a control network can be tailored uncompromisingly to comfort. This is necessary because other activities then distract passengers and they no longer look out of the window. In such situations, a fairly "bumpy" ride – due to poor roads or sudden braking and acceleration maneuvers – causes nausea.
So X-by-wire systems might well be one reason why being in a car is still enjoyable although we are no longer driving.