The Dynamic Sedan

A damping system has to deal with a lot of situations. Uneven tracks inevitably lead to direct impact, which is cushioned by the suspension. If a wheel rolls over an obstacle, it lifts, compressing a spring between the steering knuckle and the car body. Then the spring decompresses. The damper's job is to attenuate the resulting vibration as quickly as possible. Aside from direct impact, vehicles have to contend with motive forces, braking forces and centrifugal forces. These forces associated with driving dynamics can prompt the car body to move in the direction of three spatial axes (see diagram below). The challenge lies in the fact that different driving situations each demand a specific damping force. Minimizing body movement on curves requires high damping force. Driving over slightly uneven surfaces only requires a little. And safe contact between the tires and the ground requires moderate damping force. “A conventional damper can only ever reach a compromise between sporty and comfortable, albeit a good compromise,” says Dr. Christoph Elbers, Vice President Car Chassis Technology Development at ZF.

ZF's semi-active Continuous Damping Control (CDC) system solved the problem of these conflicting objectives decades ago. Our semi-active damping system went into volume production for passenger cars in 1997. Using the signals from two wheel and body sensors for each spatial axis, plus additional information such as vehicle speed, lateral acceleration and steering angle stored in a control unit, the system continuously monitors driving conditions and calculates the optimum damping force for each wheel. The damper has either one or two electronically controlled solenoid valves that can change the oil flow in milliseconds to adjust the damping hardness. ZF chassis expert Dr. Elbers explains: “sMOTION takes things a step further. Using the basic design of our tried-and-tested CDC, we developed a damper that can actively pull up or push down the wheel.”

Until now, vehicle body excitation was tolerated or even desirable to a certain degree so as to give drivers perceptible feedback on conditions. But in highly automated or autonomous vehicles, all the occupants become passengers. And in that scenario, every vibration, impact, roll or pitch simply stops people from reading or working. This causes dispositions known as motion sickness, making these activities impossible,” says Dr. Elbers. “This effect is amplified by innovative seat arrangements in which driver and passengers may no longer have a view in the direction of travel.” A remedy can only be found in an active chassis. sMOTION is a ZF solution for the future that is already improving driving comfort today. Practically undisturbed by the driving process, passengers can spend their travel time working or relaxing.

Dr Achim Thomä, technical project manager for sMOTION, found himself and his team confronted with an unusual challenge. How could they convey the system's effectiveness if it stopped excitation so unobtrusively that passengers could barely feel a thing? The solution was simple. For demonstration purposes, Sven Greger now uses a tablet to control the dampers on a stationary vehicle: individually, in pairs, or all four at once. As he taps the screen, the van lifts and lowers, pitches, rolls and shakes like a rodeo horse, at which point the engineers in the seats have every reason to cling to the handles despite the fact the car is not going anywhere!