EPAS System

The PSCM controls the functions of the EPAS system and communicates with other modules over the HS-CAN. The PSCM determines the required assist torque and direction from the steering wheel input torque signal, vehicle speed signal and other internal measurements and then triggers the corresponding electric motor current. The EPAS mechanism consists of a torque sensor at the steering wheel input shaft, an electric motor with integrated control module, a toothed belt drive and a recirculating ball nut. None of the assist mechanism components are available separately for service.

To activate, the EPAS system requires battery voltage at the PSCM hot at all times input and ignition voltage at the PSCM ignition-run input. In addition, the system must communicate with other modules over the HS-CAN. The PSCM must receive the power mode message from the BCM in order to start operating.

The main input for calculating the level of EPAS assist is the steering torque sensor signal. Vehicle speed is also taken into consideration in order to achieve the vehicle speed dependent steering assist characteristic.

The EPAS gear uses a reversible motor to apply the steering assist. The motor is connected to the rack of the steering gear by a drive belt and recirculating ball nut. The motor is used by the PSCM to move the rack inside the steering gear housing.

The PSCM continually monitors and adjusts steering efforts based on the steering torque sensor, motor position and HS-CAN inputs to enhance the feel of the steering system. As vehicle speed increases, the amount of assist decreases to improve and enhance road feel at the steering wheel. As vehicle speed decreases, the amount of assist increases to ease vehicle maneuvering. Compensation is made to reduce the effect of pull or drift experienced when driving on roads with a high degree of camber and for the impact of wheel imbalance on steering feel.

The steering torque sensor senses the torque at the steering gear input shaft, the sensor is hard wired to the PSCM and works by measuring the relative rotation between an input and output shaft which are connected by a torsion bar. The torque sensor sends out 2 PWM signals which allows a channel to channel cross-check and an accurate correction of the neutral point.

The PSCM is self-monitoring and is capable of setting and storing Diagnostic Trouble Codes (DTCs). Depending on the DTC set, the PSCM may enter a failure mode. In addition, the PSCM may send a request to the IPC to display a message in the message center, alerting the driver of a potential EPAS concern. The warning message is sent over the HS-CAN to the BCM where it is converted to a MS-CAN message and sent to the IPC over the MS-CAN.