Power Steering System (EPS)

Power Steering System Description and Operation (EPS)

The 42-volt rack & pinion electric power steering (EPS) system reduces the amount of effort needed to steer the vehicle using the power steering control module (PSCM) to control the power steering motor to maneuver the steering gear. The PSCM also uses a combination of the torque sensor, motor rotational sensor, 42-volt circuit, battery voltage circuit and serial data circuit to perform the system functions. The PSCM monitors vehicle speed from the engine control module (ECM) via the serial data circuit to determine the amount of steering assist needed to steer the vehicle. At low speeds more assist is provided for easy turning during parking maneuvers. At higher speeds less assist is provided for improved road feel and directional stability.

The EPS system consists of the following components:

* The PSCM
* The power steering motor
* The power steering motor rotational sensor
* The torque sensor
* The steering gear (rack and pinion)
* The accessory power module (APM)

The PSCM and power steering motor are both serviced separately. The power steering motor is serviced separately from the steering gear; however the power steering motor rotational sensor is serviced as part of the steering motor. The torque sensor is integrated with the steering gear pinion and is serviced as part of the steering gear.

Power Steering Control Module

The accessory power module (APM) provides a shielded, 42-volt circuit to the power steering control module (PSCM) to drive the power steering motor. The PSCM also uses the 12-volt battery circuit to power the module electronics. The normal operation voltage range the APM provides to the PSCM is between 40-44 volts. The APM is not capable of handling more than 58 volts. The dynamic brake circuit of the PSCM is used to keep the reverse motor voltage of the electric power steering system below 58 volts, and will start to consume the surplus voltage if the 42-volt circuit rises in a range between 50-58 volts.

The APM is also monitored by the hybrid control module (HCM) to detect overheating. If overheating of the APM is detected, the HCM sends a serial data message to the PSCM via the serial data gateway module as the power output to the PSCM is reduced until the APM returns to a normal operating temperature. The PSCM will reduce power assist according to the reduced output power provided by the APM, and will return to full steering assist when the APM returns to normal operating temperature.

The PSCM uses a combination of commanded motor voltage and current levels to calculate an estimated power steering system temperature. Neither the PSCM nor the power steering motor are designed to handle around 60-amps continuously and will enter into overload protection mode if the system is exposed to exclusive static steering conditions. If the steering wheel is turned to its maximum rotation point and held at this position for an extended period of time, the PSCM will reduce the amount of current commanded to the power steering motor, which reduces the amount of steering assist as well as system temperature. The PSCM has the ability to detect malfunctions within the power steering system. Any malfunction detected will cause the SERVICE POWER STEERING message to be displayed on the driver information center (DIC).

Torque Sensor

The power steering control module (PSCM) uses a combination of the torque sensor, motor rotational sensor, vehicle speed, and calculated system temperature inputs to determine the amount of assist needed. As the steering wheel is turned and torsional twist is applied to the steering shaft, the torque sensors input and output shaft sinusoidal voltage signals are processed by the PSCM to detect and calculate the steering torque. Additionally, the difference between the angle of the steering output and input shafts detected by the torque sensor and the sinusoidal voltage signals of the motor rotational sensor are both processed by the PSCM to detect and calculate the steering wheel angle.

Power Steering Motor

The power steering control module (PSCM) responds to the change in the sinusoidal voltage signals of the torque and motor rotational sensors by commanding current to the power steering motor. The motor is attached to the base of the steering gear housing and assists the steering gear pinion to maneuver the rack from left to right according to turning the steering wheel. The 42-volt circuit from the APM supplies power to the pulse width modulated (PWM) motor drive circuit. The motor is a brushless, 3 phase motor with a rated maximum phase current of 40-amps rms.