Variable Effort Steering System Description and Operation

The Variable Effort Steering (VES) system, or MAGNASTEER II® varies the amount of effort required to steer the vehicle as vehicle speed changes or lateral acceleration occurs. At low speeds, the system provides minimal steering effort for easy turning and parking maneuvers. At high speeds, the system provides firmer steering (road feel) and directional stability. When the system senses lateral acceleration, steering becomes firmer to reduce oversteering. The Electronic Brake Control Module (EBCM) controls a bi-directional magnetic rotary actuator located in the steering rack and pinion. The EBCM varies the steering assist by adjusting the current flow through the actuator. The actuator adjusts the amount of power steering assist to achieve a given level of effort to steer the vehicle. The VES system accomplishes this by adding or subtracting torque on the input shaft to the rack and pinion. The main component of the system is an electromagnetic actuator, which consists of a multiple-pole ring-style permanent magnet, a pole piece, and an electromagnetic coil assembly. The VES system uses the Antilock Brake System (ABS) wheel speed sensor inputs to determine vehicle speed. When the EBCM senses vehicle speed, it commands a current to the actuator that is most appropriate for each speed. The system also uses inputs such as Handwheel position, wheelbase, understeer coefficient and steering ratio to calculate lateral acceleration. The EBCM commands current from negative two amps to positive three amps to the actuator, which is polarized. At low speeds, a negative current is commanded, which assists steering. At medium speeds, no current is commanded and steering is assisted by hydraulics only. At high speeds, a positive current is commanded, which creates steering resistance. Ignition voltage and ground are provided through the EBCM. The EBCM has the ability to detect malfunctions in the actuator or the circuitry to the actuator. Any malfunctions detected will cause the system to ramp to zero amps and steering will be assisted by hydraulics only.