Steering (Description): Description and operation

DESCRIPTION

Steering column

The steering column is secured to the body by two brackets. The column consists of two sections that telescopically collapse in the event of a frontal impact. The steering shaft also consists of two telescopic collapsible sections secured by two shear pins. The steering shaft rotates in the steering column via a ball-bearing at its upper end and two ball bearings at its base.

When the steering wheel is rotated, the turning effort of the shaft is transmitted to the pinion shaft of the the steering rack via the intermediate shaft. The intermediate shaft comprises two universal joints which counter the varying angle between steering shaft and rack; and a telescopic internal shaft which acts as a damper. This construction limits intrusion of the column in the event of a crash and also prevents suspension movement and vibration being transmitted through the steering column.

The steering column has a torque sensor, an electric motor and gearbox positioned at the lower part of the column which are integral components of the Electrical Power Assisted Steering (EPAS) system.

No parts of the steering column are serviceable and failure of any electrical or mechanical component will require column replacement.

EPAS System

The EPAS system is based on a conventional rack and pinion steering system, and also includes updated EPAS ECU software to improve steering response and characteristics and a column tilt adjustment feature.

EPAS provides variable power assistance depending on vehicle speed and driver’s steering effort. Power assistance will be most evident to the driver when maneuvering the vehicle at low speeds, this assistance reduces progressively as vehicle speed increases.

Steering motor

Steering assistance is provided by a motor mounted on the steering column. The motor is attached to a cast housing which forms part of the column and is secured with two Torx screws. A single connector connects the motor to the main harness.

The motor drives a wormwheel attached to the steering shaft to provide the power steering assistance. The level of assistance provided by the motor varies according to torque sensor output and road speed.

The EPAS ECU calculates the required torque using inputs from the torque sensor and road speed transducer, and supplies a current to the motor of the appropriate amperage to provide the necessary steering assistance.

An electrically operated clutch connects the motor drive to the column when assistance is required. If a system failure occurs, the EPAS ECU will not engage the clutch, disconnecting motor assistance to the column.

Torque sensor

Driver steering effort is measured by a torque sensor mounted on the steering column. This information is sent to the EPAS ECU, which also monitors engine and vehicle speed.

The torque sensor comprises a rotary potentiometer with two tracks. The potentiometer is connected, via a pin and bearing, to a track on a slider. The slider moves axially depending on steering torque input from the steering wheel. This axial movement is converted, via the track, into rotary movement of the potentiometer in proportion to the torque input. The potentiometer passes information to the EPAS ECU for torque input and direction.