Power Steering System Description and Operation with EPS
The Electric Power Steering (EPS) system reduces the amount of effort needed to steer the vehicle. The system uses the power steering control module (PSCM), torque sensor, power steering motor rotational sensor, power steering motor, discrete battery voltage supply circuit, and the GMLAN serial data circuit to perform the system functions. The PSCM and power steering motor are each serviced separately from the steering column assembly. However, the motor rotational sensor is serviced as part of the power steering motor, and the torque sensor is serviced as part of the steering column assembly. The PSCM also monitors vehicle speed from the engine control module (ECM) via the GMLAN serial data circuit. 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.
Torque Sensor
The 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 column shaft, the torque sensor's 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 column 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.
EPS Motor
The 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 column housing and assists steering through a worm and reduction gear attached to the steering column shaft. A DC/DC converter is applied to the battery voltage input within the PSCM to boost the pulse width modulated (PWM) motor drive circuit. The motor is a brushless, 3-phase motor with a rated maximum phase current of 35 amps-rms.
Power Steering Control Module (PSCM)
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).
Power Steering System Description and Operation with HPS
The hydraulic power steering system consists of the following components:
| • | The hydraulic power steering pump is a belt-driven, constant displacement vane-type pump that provides hydraulic pressure and flow for the power steering gear. |
| • | The power steering fluid reservoir holds the power steering fluid and is remote to the power steering pump. |
| • | The power steering gear is a rack and pinion system. In the rack and pinion system, the rack and the pinion are the 2 components that convert steering wheel rotation to lateral movement. The steering shaft is attached to the pinion in the steering gear. The pinion rotates with the steering wheel. Gear teeth on the pinion mesh with the gear teeth on the rack. The rotating pinion moves the rack from side to side. The lateral action of the rack pushes and pulls the tie rods in order to change the direction of the vehicle's front wheels. |
| • | The power steering pressure hose connects the power steering pump union fitting to the power steering gear and allows pressurized power steering fluid to flow from the pump to the gear. |
| • | The power steering return hose returns fluid from the power steering gear back to the power steering fluid reservoir. The power steering return line contains a line-type power steering fluid cooler. |
In a typical power steering system, a pump generates hydraulic pressure, causing fluid to flow, via the pressure hose, to the steering gear valve assembly. The steering gear valve assembly regulates the incoming fluid to the right and left chambers in order to assist in right and left turns.
Turning the steering wheel activates the valve assembly, which applies greater fluid pressure and flow to one side of the steering gear piston, and lower pressure and flow to the other side of the piston. The pressure assists the movement of the gear piston. Tie rods transfer this force to the front wheels, which turn the vehicle right or left.
