DaimlerChrysler AG, the company that brought us advances such as antilock brakes, acceleration skid control, and the electronic stability program, proposes a solution to an age-old automotive engineering dilemma: Do you set the suspension for a sporty, stiff ride in the interest of road safety and driving dynamics, or use softer springs and let comfort take precedence? With the mechatronic Active Body Control (ABC) suspension on the latest Mercedes CL, drivers can have the best of both worlds.
“Another interesting feature is the safety advantage that the system provides after a high-speed swerving maneuver. Whereas a car body normally continues to rock and sway for some time after sudden and very fast steering movements, Active Body Control stabilizes the body within a very short time.” says Arno Rohringer, head of spring and shock absorber development at Mercedes.
The ABC system is the latest active suspension to hit the luxury-car market. It is based on interaction between electronic, hydraulic, and mechanical components. The system switches between comfort and sport modes by means of a button on the dashboard. Rather than give a completely flat ride, the system leaves in some roll as a kinematic cue to the driver. In comfort mode, the CL rides like a sedan. In sports mode, the car limits roll for a stiffer ride more like that of a sports car.
“Cars with conventional suspension systems have to wait until after the event before limiting body movements caused by inertia, braking, and accelerating forces. By contrast, Active Body Control in the new CL detects body movements as they start to develop and corrects them immediately,” says Rohringer.
Brawn meets brains
Vibrations are generally caused by uneven roads or braking and cornering. They make a car, especially its body, pitch and roll. With ABC, body movements are physically controlled by hydraulically driven servocylinders in the suspension struts. The system reduces body vibrations to a maximum of 5 Hz.
Plungers located between the coil springs and the body apply forces to the springs in response to computer commands based on control algorithms stored in the system’s microcontrollers. The strut cylinder in effect generates different degrees of extension at the bottom of the coil spring to counteract body movements.
ABC uses a high-pressure hydraulic system with a tandem pump and a compact block located in the engine compartment. These components build up and monitor the system to maintain a constant pressure level of 200 bar. A relief valve prevents overpressurization. Valve blocks and accumulators are positioned on each axle.
The amount of pressure applied to the springs, as well as the duration of application, is based on sensor readings of body position. Thirteen sensors placed throughout the vehicle monitor leveling, acceleration, and spring position and keep two microprocessors informed of the driving situation. The microprocessors compute and issue control commands for the plungers inside the suspension struts every 10 msec. Higher-frequency vibrations of the wheels (6 to 20 Hz) are handled by passive gas-pressurized shock absorbers and coil springs.
ABC includes automatic self-leveling based on actual load conditions. The driver can also manually vary the ground clearance. A center console pushbutton lets drivers choose between two levels at low speeds. In position one, the body is raised about 25 mm and in position two, as much as 50 mm. When speed increases, the body automatically sinks back to the starting position. At speeds around 140 km/hr, the body lowers 10 mm to reduce air resistance and fuel consumption.
If the system should fail, the car’s suspension will lock in the position assumed at the time of failure. The suspension is reported to reduce body roll by up to 68% when starting from a stop, cornering, and braking, as compared to the predecessor. The sport suspension features further reduced body roll angles. With ABC, only 0.4 sec pass before the body is fully stabilized after a sudden swerving maneuver. With the previous model the body continues rocking for 0.6 sec.
ABC is not the first electronic-control suspension system from a luxury carmaker. For instance, Cadillac features a continuously variable road-sensing suspension (CVRSS), which uses computer-controlled hydraulic dampers to automatically adjust ride and handling for a range of driving conditions. Sensors monitor wheel position in relation to the body and steering angle to help anticipate corners or changes in direction so the computer can instruct the dampers to firm up for better handling. The goal of this system is to provide soft dampers most of the time for a smooth ride, but handle large bumps which can overwhelm conventional soft-damper suspension systems.
The 2000 version of CVRSS will feature updates. It will have the ability to adjust damping forces independently in each direction of wheel travel at all four corners of the car. And during aggressive cornering, the outside dampers will stiffen in compression and inside dampers will stiffen in rebound to diminish the amount of body roll mid turn. The suspension is also retuned to work in conjunction with braking to control body dip. BMW also offers a system similar to the Cadillac CVRSS. But, as the Mercedes ABC suspension does, it gives drivers the option of a sport or comfort setting.
Active suspension fends off vibration and body roll through turns
Then, bumps in the road cause the car to vibrate. As body movement starts, three acceleration sensors detect the vibrations and alert the microcomputers. At the same time, sensors on the struts signal the distances between the wheels and the body, the hydraulic cylinders’ positions in the struts, and the degree of longitudinal acceleration of the car. The two microcomputers use control algorithms to calculate a proper physical response.
As the driver steers into the turn, the adjustment variables are calculated. The hydraulic system receives the microcomputers’ commands 10 msec later. The computing process runs continuously with new commands issued every 10 msec.
When the CL reaches the crest of the turn, a precisely measured amount of hydraulic oil flows to each suspension strut. Hydraulic cylinders in the struts on the right hand side of the vehicle (outside of the turn) extend to reduce body roll. At the same time, hydraulic cylinders on the left side (inside of the turn) retract.
In the meantime, the microcomputers apply new values which take into account the unevenness of the road. Sensors on the struts monitor the movements of the cylinders continuously. The cylinders move at frequencies up to 5 Hz to counteract vibrations.
Suspension in the making
Researchers borrowed technology from the motor sport group to refine the system. A part-active suspension which focused on low-frequency body movements was placed on the Mercedes Type C 11 racing car. It did well in training, but was blocked from the race track by regulations.
The C 112 sports car study followed in 1991, adding comfort to the formula. Its partially active suspension system helped steady the car body when starting from a stop, braking, or cornering, but also gave a comfortable ride, which was not a priority with the Group C race car.