Steer-by-wire: thyssenkrupp steers innovative path in development

Steer-by-wire is on the verge of a breakthrough in production vehicles. In terms of its impact and the possibilities it opens up, the innovative technology can be described as nothing short of revolutionary. All the more so because thyssenkrupp has opted for a unique development approach.

It’s not only in connection with highly automated or even completely autonomous driving that steer-by-wire (SbW) is set to play a central role in the future. Using electrical signals to transmit steering inputs offers a wide range of possibilities in other areas as well. For example, the absence of a mechanical link between steering wheel and steered wheels allows previously undreamed-of freedom in vehicle design. Numerous vehicle types and layouts can be created on one and the same platform. Whether front left, right or center, or theoretically even from the back seat: the position from which the vehicle is steered is completely immaterial. Steer-by-wire technology allows practically every conceivable configuration. And perhaps also one or two others that are still unthinkable today.

After numerous decisive development advances in the recent past, the steering revolution is on the verge of a breakthrough in production cars. So far, the only production vehicles to feature this technology are the Infiniti Q50 sedan, launched in 2013, and the Infiniti Q60 coupe derived from it (2016). However, they cannot be considered pure steer-by-wire vehicles because they have a mechanical fallback system. The first production models relying exclusively on steer-by-wire are expected from 2023. Initially, they will probably not be produced by traditional OEMs but by start-ups and other newcomers to the automotive industry. According to conservative estimates, there will be around one million steer-by-wire vehicles on the roads worldwide by 2030.

thyssenkrupp going its own way in hardware and high-level and low-level software

The central issues with steer-by-wire are undoubtedly safety and steering feel. Both will contribute to the acceptance of this technology by OEMs and customers and accelerate the breakthrough. The way a car steers and the feedback it provides on road conditions and vehicle dynamics are key characteristics. For vehicle manufacturers, they are distinguishing features of “their” brand. The details are therefore of particular importance. An OEM’s own individual steering DNA must be replicated as perfectly as possible by a steer-by-wire system.

Due to the absence of a mechanical link between the wheels and the steering wheel, the required torque must first be calculated and then reproduced at the steering wheel. To define the required torque, various sensor values are used. Several other variables must also be estimated through calculation. “In this so-called high-level software area, we are pursuing a slightly different approach than most of our competitors,” explains Kristof Polmans, SVP Research and Advanced Engineering at thyssenkrupp’s steering division. “We’re focusing less on transmitting road-wheel forces directly to the steering wheel and more on calculating torque levels at the steering wheel independently on the basis of other values, and thus generating road feedback indirectly. That means we achieve a robust steering feel with a great deal of tuning freedom. Nevertheless, we still generate the road feedback the customer needs.”

thyssenkrupp is also pursuing a new, individual approach in the area of low-level software, i.e. actuating the required torque. It is directly related to the hardware layout and only possible with an integrated system solution. “Our thinking was: if we’re developing a completely new technology, it’s best to start with a completely blank sheet of paper,” says Polmans. So instead of building steer-by-wire technology on top of the existing infrastructure of conventional electronic power steering systems like the majority of competitors, thyssenkrupp made its mechanical solution completely independent of the existing infrastructure. For example, there is no worm gear mechanism in thyssenkrupp’s steer-by-wire solution. That would have required a torque sensor as a control instance. The large variations in friction in various areas due to manufacturing tolerances in mass production, and the unavoidable mechanical wear result in the need for a closed control loop. “Instead of this classic feedback control approach where the measured values have to be continuously rechecked, we use feedforward control,” says Polmans. “Thanks to the elimination of unnecessary electrical components and thanks to our own high- and low-level software approach, our system is more stable, more robust as well as less prone to errors and quality problems in mass production.”

Vehicle Motion Control as means of redundancy

For OEMs and vehicle manufacturers, steering systems are an integral part of a higher-level system. At thyssenkrupp, steer-by-wire is part of Vehicle Motion Control (VMC). Instead of concentrating solely on the steering domain, thyssenkrupp always looks at overall vehicle dynamics. This also has an impact on the issue of redundancy. Instead of securing functions with mechanical solutions or system duplications, thyssenkrupp uses existing components of other vehicle systems. In this way, “alternative steering functions” can be achieved, for example using the brakes, the powertrain or the components of an active chassis, all-wheel or rear-wheel steering system or active stabilizers. This integrated approach not only provides a space- and cost-saving solution, above all it solves various safety-related challenges and ensures higher availability.

Modular Research Platform as one of the keys to success

The development progress achieved in the recent past in the areas of steer-by-wire and Vehicle Motion Control can be attributed in large part to an additional development tool that thyssenkrupp has been using successfully for several years: the Modular Research Platform (MRP). The traditional development and test sequence used in the automotive sector, consisting of simulation, test rig work and road tests with a specific model or vehicle type, has various disadvantages. For example, there is not always a clear correlation between the results obtained from simulations and those from test rig work. In road tests with a specific model there is no flexibility or modularity. These disadvantages are eliminated by thyssenkrupp’s Modular Research Platform, which can recreate any kind of chassis architecture – without any restrictions in terms of packaging space or the like. The advantages it can offer in steer-by-wire development work have not gone unnoticed: thyssenkrupp is currently building three more MRPs – two of them under contracts from customers.