The Ups and Downs of Chassis Electronics

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.

COMPONENTS AND SYSTEMS

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.

SEMI-ACTIVE SUSPENSION

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.

The mandatory requirement in the U.S. for Electronic Stability Control (ESC) on all vehicles by 2012 provides an improved starting point for any chassis control. With its Vehicle Dynamics Management (VDM) product line, Bosch interfaces to other chassis systems beyond its Electronic Stability Control (ESC) and can control these systems, even though Bosch does not manufacture steering or suspension systems. Figure 4 shows the impact of the VDM approach.

“Going forward, we have this (ESC) technology in the vehicle,” said Kay Stepper, director of marketing, Robert Bosch GmbH. “We have the expensive pieces already in the vehicle and we are trying to go forward and make better use of the existing hardware by networking the systems.”

Dynamic wheel torque control is a function that is possible today. Also called wheel torque vectoring, this approach uses information from the ESC and its associated sensors. This can improve the agility and responsiveness of the vehicle. Stepper noted that there are basically two versions of dynamic wheel torque control. One uses an active differential that can either be on the front or rear axle. This is a more costly version but also serves as a benchmark for capability. The second version, Dynamic Wheel Torque Control by Brake (DWT-B), performs a similar function but uses the brake system to implement it.

The system actively builds brake pressure on a specific corner to induce yaw into the vehicle when the vehicle does not follow the driver's input as determined by the steering angle sensor. This kind of functionality is beyond the normal capability of ESC but is possible with the existing hardware. Bosch's goal is a network that includes all systems and components involved in the vehicle's motion. In the future, a Vehicle Motion Management (VMM) system will allow common access to all relevant sensors and actuators.

The BMW X5 and X6 vehicles use today's capability in the Integrated Chassis Management (ICM). Under a variety of driving conditions, the ICM interacts with the xDrive, Dynamic Stability Control (DSC) and Dynamic Performance Control actuators and Active Steering. Using capability created by the networking of these systems, the DSC, BMW's electronic stability control, only intervenes when absolutely necessary but improved vehicle dynamics occur under all driving conditions.

In the research area, engineers are taking suspension systems to the next level. A car company and a supplier demonstrate technologies we can expect to see on production vehicles in the future.

In its F700 research vehicle, Daimler implements a chassis control system called PRE-SCAN that adds a laser scanner to monitor the road in front of the vehicle to the Active Body Control (ABC) chassis, a system already used in production Mercedes-Benz S, SL and CL-class vehicles. According to Hans-Georg Metzler, head of Assistance Systems and Chassis at Daimler's Group Research, “The vehicle not only reacts in a highly sensitive manner to irregularities in the road surface but actually looks ahead for potential hazards.”

Integrated into the LED headlight assembly, two laser sensors form a light detection and ranging (LIDAR) system to transmit pulsed laser beams in the infrared range onto the road and detect reflected light from the road. By measuring the time between pulse transmission and reflection, the system calculates the distance to irregularities in the road surface. The PRE-CAN system uses this data as well as input from other vehicle sensors and systems to apply the appropriate amount of damping to each wheel. Sensors include a range of acceleration sensors, a pressure sensor in the hydraulic system and a level sensor on each control arm. The chassis system can correct and stabilize a rolling body in a fraction of a second.

Bose Corporation has developed a fully automatic suspension system using a linear motor and power amplifier at all four-wheel suspension points and control algorithms. In this system (Figure 5), the motors retract and extend fast enough to counter the effects of bumps and potholes. In addition, the motor has sufficient strength to adjust for rolling and pitching during aggressive driving maneuvers.

Regenerative power amplifiers in the system are based on the switching technologies used in well-known Bose audio products with a unique energy-saving feature. Using the weight of the vehicle to return from an extended position, the linear electromagnetic motors act as generators and return power to the amplifiers. With this approach, the system's power consumption is reduced to less than a third of the power of a typical vehicle's air-conditioner system. As a result, the system can operate using standard 12 V power, unlike previous systems that would have required 42 V.

While fully active suspension systems have yet to appear on a production vehicle, advanced concepts are an integral part of drive-by-wire vehicles. In vehicles, such as GM's Autonomy, higher-voltage power systems ease the system design. Ongoing research and improvements in electronic control at both the product and system level promise smoother, even more comfortable chassis performance in future vehicles.

Randy Frank is president of Randy Frank & Associates Ltd., a technical marketing consulting firm based in Scottsdale, AZ. He is an SAE and IEEE Fellow and has been involved in automotive electronics for more than 25 years. He can be reached at [email protected].