A vehicle is driving on a country road on a cold, dull December morning. The road surface leaves a lot to be desired, and potholes and ruts make for uneven road conditions. The driver does not notice any of this. He effortlessly steers the vehicle along the route and through sharp bends. Even an obstacle which suddenly appears is effortlessly avoided by the vehicle, without setting the driver’s pulse racing too much. This special feeling of safety, comfort and effortless superiority is unmistakeable. It is inherent to a Mercedes. The developers behind it refer to this feeling as the “Mercedes-Benz driving character”.
Development is a matter of character
“We aspire to make our vehicles immediately recognisable. Not just through the hallmark Mercedes design, but also the handling,” is how Markus Riedel, Head of the “Ride & Handling” Centre, sums up his team’s main mission. “In concrete terms a Mercedes is characterised by three very pronounced traits: optimum ride comfort, high driving safety and tangible self-assurance. In every situation a Mercedes driver must have the impression that their vehicle still has reserves.” It is the job of the engineers at the Sindelfingen Mercedes-Benz Technology Centre to constantly optimise these three KPIs and develop them further for every new model platform.
At the start of every development cycle the required specification of these three top properties, supplemented by sportiness and precision, is translated into over 90 measurable physical characteristic values, including for vertical, lateral and longitudinal dynamics. Countless calculations, design variants and simulations on supercomputers are necessary before the parameters on paper can be fed into the target description, i.e. into the specification of the component parts and wheel alignment systems. “The real art of the development process is for us to reconcile these statistical values with one another to such an extent that they create a complete driving experience geared to humans,” says Markus Riedel, explaining the challenges in developing the handling characteristics. Because for all this digitisation and technology: it is the subjective perception whilst driving that is decisive.
This means that all the objective statistical values – whether calculated or measured – have to be interpreted with subjective perception. This is the only way in which the Mercedes-Benz driving character can be created.
“Modern handling is therefore developed through the triad of digital simulation on the computer, measurements on test stands and finishing touches through subjective fine-tuning with real vehicles. Each of these three instruments has its strengths, but also its limitations. The actual art is now to combine them with one another optimally.”
Feeling meets physics
To enable the checking of calculations to run efficiently, even at the early development phases in which there is as yet no actual prototype, there is the Driving Simulation Centre in Sindelfingen. In what is known as the Ride Simulator – two seats mounted next to one another on a hexapod with electrical regulators – the Mercedes-Benz chassis experts work, for example on the damping variance. With the aid of digital vehicle prototypes and the surface data of real test routes, the driver and co-driver can carry out virtual test drives and check how they find, for example, the same uneven road surface with a comfortably soft or sportily tauter suspension tuning. “With this digital transformation we save ourselves one to two coordination loops in testing with real prototypes. We have an entire collection full of virtual vehicles and driving situations in the computer which we can incorporate at the press of a button,” Markus Riedel says, explaining how the process works.
This is where it is literally possible to experience at first hand a phenomenon familiar from the realm of acoustics: the masking effect. “When you are out on the road in a normal vehicle these days, you hardly notice the exterior noises such as wind noise, because they are overlaid by the engine noise. These noises are missing from an electric vehicle – and even the airstream seems much louder,” Markus Riedel explains. It is a similar story with the various vibration frequencies: “Naturally we can try to dampen away all the vehicle’s lower-frequency body movements. But the high-frequency vibrations remain – and this is that tingling in the stomach that many people find so unpleasant.”
In the Handling or Moving-Base Driving Simulator one floor up, the focus is on the topic of safety and self-assurance. With its ball structure on a twelve-metre rail, the 360-degree screen, sound systems for reproducing the wind noise and a fast electric drive, it is not dissimilar to a flight simulator. Here, too, the data sets created by supercomputers serve to replicate driving situations which appear as real as possible to the test subjects. The difference is that they are sitting in a complete vehicle body here and can accelerate, steer and brake for themselves.
In this way even dynamic driving manoeuvres such as a double lane change or the effect that a strong lateral gust of wind has on the suspension can be studied extensively. “Many people think that in Development we just deal with steering, braking, damping, etc. But the Handling Simulator also helps us learn which tyres are suitable for which setup, or if the aerodynamics still need to be optimised,” says Markus Riedel. The big advantage of simulations is that the results are always objective and reproducible, whilst subjective assessments in the actual test vehicle can tend to vary. Plus the control units required for handling have to be configured for the Mercedes-Benz driving character in a targeted manner. We have the Hardware-in-the-Loop laboratory for configuring the control units.
Loop by loop to the right setting
The rapid increase in electronic control units, whose tasks include assuming responsibility for the drive electronics and suspension in modern vehicles, calls for a level of testing which is as thorough as that on the suspension itself. One possible way of accelerating the development of vehicle components is a process called Hardware-in-the-Loop (HiL), which has been vehemently used and developed further at Mercedes-Benz ever since the infamous moose accident test. In the HiL laboratory, real control units, such as the ESP®, are connected to a supercomputer which simulates the vehicle’s behaviour with the aid of computer models.
An example: the vehicle computer model drives through a slalom course at high speed and sends sensor data describing the vehicle behaviour to the ESP®. It reacts and in turn sends the corresponding control impulses back to the hydraulic brake, thus producing precisely the brake force required at the tyres to enable the vehicle to continue meeting the driver’s wishes. This creates a loop.
In this way countless driving manoeuvres and all the suspension control units can be tested in a secure environment over and over again for a diverse range of vehicle platforms. There are now 22 functions integrated in the ESP® control unit. They have to be successfully tested in around 900 manoeuvres. And even if every single manoeuvre has to be performed in real time, a great deal of development time can be saved by including Hardware-in-the-Loop. “We can carry out nearly 20 million computing operations in one night on a HiL test stand. With this example alone you can see just what is possible thanks to digitisation. And we can also simulate marginal manoeuvres here too, without endangering any people or vehicles,” comments Markus Riedel, explaining the value of Hardware-in-the-Loop for the development of handling.
On the test track
The test drives in the actual vehicle on real routes should of course only ever approach these limits with extreme care, if at all. In order to put the finishing touches to all the characteristic properties of the Mercedes-Benz driving character – so the ride comfort, driving safety, self-assurance, sportiness and precision – the vehicle itself is also tested and optimised. “It’s just like a good wine. You can describe it in chemical terms, but that subtle something comes from people for people. As far as we are concerned the simulations cannot replace tests in real conditions. But they do help to put the actual prototypes onto the real test track faster and with a higher maturity level,” is how Markus Riedel justifies the subsequent development phase on the test track.
“Testing on public roads, except for ride comfort, would be irresponsible,” Markus Riedel explains. “And even when it comes to optimising the springing comfort the volume of traffic or the width of the road prevents us from always driving at the same speed on the same route in order to gain comparable impressions. In addition to this, good routes for fine-tuning are disappearing because they are being asphalted smooth. We measured these routes before they were overhauled and preserved them on the computer with all their dips and bumps, grooves and potholes. And on our test tracks we have faithfully reconstructed these roads. A modern S-Class can and must be measured against its predecessors on the same stretch, for example, from simulation to testing in the actual vehicle.”
Even cobblestones were put down on a short section of the test track in Sindelfingen. On test tracks and for ride comfort on public roads, too, the pre-defined suspension settings are optimised and fine-tuned with various parameters in real conditions. Concrete motorways, for instance, present a special challenge. Here there are transverse joints at regular intervals, and when the vehicle speed is increased they cause higher-frequency agitation. The vehicles are tuned in such a way that the agitation is reduced as much as possible for the occupants.
The engineers do not like to compromise when it comes to handling: “We are dealing with technological development at the absolute limit here, and we want to optimise all the driving properties as much as possible,” says Markus Riedel. “But if ever there are any conflicts of interest, then driving safety, ride comfort and self-assurance always have priority before sportiness or precision when we make our decisions.” Depending on the vehicle platform, after years of development and countless test drives, in the end precisely the right suspension tuning is achieved that lends every single model its very special Mercedes-Benz driving character – so that drivers arrive at their destination relaxed, even after a long journey in difficult conditions.
Source: Daimler AG