Electronic stability and vehicle dynamic controls certainly form the glory areas of chassis electronics but brakes and suspension as well as steering continue to evolve and increasingly use electronic controls. In some instances, the subsystems operate independently and provide improved performance over the previous mechanical or electromechanical versions. However, when these subsystems connect together, they can provide enhanced functionality well beyond the capability of the separate systems. This report will discuss some of the recent developments in chassis electronics products, the growing applications of active suspension and ongoing efforts to take active suspension/vehicle dynamics to the next level.

Combining electronics with mechanical or hydraulics is the first step toward ultimate x-by-wire control. While frequently occurring on a component or a single system basis, ultimately, the goal is linking all the vehicle's control systems together. Advancements at both the component and system levels continue to provide greater control and integration. A new electromechanical brake design provides an example of this progress.

With its electrohydraulic combi (EHC) brake, Continental AG initially combines hydraulic front axle wheel brakes with fully electric wheel brakes on the rear axle. The next step is replacing the hydraulic control on the front wheels with electronic control. According to Michael Zydek, head of Systems Development at the Electronic Brake Systems Business Unit, “By combining the technical expertise of both teams of engineers, we can now focus on optimizing the friction brake and the front axle actuator, on perfecting a combination of mechanics and electronics so as to develop an electromechanical brake which will be a permanent market feature.”

As shown in Figure 1, the electromechanical brake uses an electric motor, gearbox and spindle piston. Ongoing testing will determine whether the spindle or a wedge design will meet production requirements. Continental is convinced that a universal electronic interface makes this design well suited for integration into a global chassis control system, both on hybrid systems with dry brakes on the rear axle only and also with fully electronic braking systems.

Computer-controlled suspension systems provide improved comfort in luxury and high-end vehicles. These systems are slowly expanding and appearing in sport utility vehicles and more. Perhaps one of the most unique and widely applied systems is based on LORD Corporation's magneto-rheological (MR) fluid technology.

Instead of using electromechanical servo-valves, Delphi's MagneRide suspension system employs MR technology in its monotube shock absorbers. Using input from four suspension displacement sensors, a lateral accelerometer and a steering wheel angle sensor (Figure 2), the system controller continually adjusts the damping forces as frequently as once every millisecond. With a faster response compared to valve-based controlled suspension systems, MagneRide also offers increased damper tuning capability and a very large damping range. In some implementations, the driver can control the damping with a console-mounted two-position switch for maximum road feel and control or increased ride comfort.

The MR fluid technology uses magnetically soft particles suspended in a synthetic hydrocarbon base fluid. As shown in Figure 3, when a magnetic field is applied, the particles align in the direction of the magnetic flux increasing the resistance to the movement of the damper piston. In the unenergized mode, the randomly oriented particles allow the fluid to behave similar to a conventional damper fluid. With increased energy, the bond between the particles increases creating greater resistance and damping capability. Since changes in the damping force occur almost instantaneously, the system provides continuously variable real-time damping.

The damping force in Delphi's MagneRide suspension depends only on the power applied to the magneto-rheological fluid. Up to 1,000 adjustments can occur within a second using a peak power of only 20 watts at each of the system's four dampers. On average, a damper requires just 5 watts.

Since its initial introduction on Cadillac in the 2002 model year, the MagneRide system now appears on more than a dozen models from several OEMs. “We continue to make evolutionary changes in the product to improve performance and lower the cost,” said David Hoptry, program manager, Chassis Systems Products, Automotive Holdings Group, Delphi Corporation.

Hoptry noted that many engineers designing suspension control systems use Simulink to perform the algorithm development and an autocode generator to create the code and download it into a 32-bit floating point microcontroller. To achieve the 1 millisecond loop time in its system, Delphi engineers use a smaller fixed-point MCU and do not use autocoding so they can generate the code as efficiently as possible.

The sensors in Delphi's controlled suspension system are quite unique. “A lot of the systems out there today use accelerometers and they vary from six to eight per car,” said Hoptry. “What we use is four-wheel position sensors.” The sensors employ Hall-effect technology for non-contact sensing and high durability. Encasing the Hall-effect element in the plastic housing and mounting the moving magnet outside of the housing avoids a path for water entry without using seals that could wear out.