Power Steering System Description and Operation HPS

The hydraulic power steering pump is a constant displacement vane-type pump that provides hydraulic pressure and flow for the power steering gear. The hydraulic power steering pump is belt-driven.

The power steering fluid reservoir holds the power steering fluid and may be integral with the power steering pump or remotely located. The following locations are typical locations for the remote reservoir:

The 2 basic types of power steering gears are listed below:

In the recirculating ball system, a worm gear converts steering wheel movement to movement of a sector shaft. A pitman arm attached to the bottom of the sector shaft actually moves one tie rod and an intermediate rod move the other tie rod.

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 may contain an integral fin-type or 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 1 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.

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 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.

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).

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.

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.