EPAS System

The PSCM controls the functions of the EPAS system and communicates with other modules over the HS-CAN.

To activate, the EPAS system needs to be connected to battery voltage at the hot at all times input and at the ignition/run input to the PSCM. In addition, the system must communicate with other modules over the HS-CAN. The PSCM must receive the engine status signal from the PCM to be set into operation mode.

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

The steering shaft is composed of an input shaft and an output shaft. The two parts of the steering shaft are connected to one another via a torsion bar. When the driver turns the steering wheel, torsion occurs in the steering shaft. The amount of torsion depends on the frictional force between the tires and the road surface. The torsion is increased by the use of the torsion bar, causing relative movement between the input and output shafts. This relative movement is detected by a steering shaft torque sensor and transmitted to the PSCM. The relative movement is the main parameter for calculating the control current for the EPAS motor and thus for the power assisting force. The PSCM uses a reversible motor to apply the steering assist.

The PSCM continually monitors and adjusts steering efforts based on the 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 that can be experienced when driving on roads with a high degree of camber. Also compensation for the impact of wheel imbalance on steering feel is made up to a predetermined threshold.

The steering shaft torque sensor senses the torque at the steering wheel. It 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 IPC.