The bypass of the hydraulic circuit and direct steering boost with the aid of an electric motor (electric power steering system ) has additional advantages in terms of weight and engine bay space compared with electro-hydraulic steering, because of the omission of all the hydraulic components. Other advantages are obtained through more variations of the steering boost because of the purely electrical signal processing.
The electrical servo unit can be installed on the steering column , pinion or gear rack . The steering axle loads and maximum gear rack forces are, depending on the particular arrangement, about 650 kg and 6000 N, 850 kg and 8000 N or 1300 kg and 10 000 N.
The systems only have limited power because the current is limited by an operating voltage of 12 V. They are of interest though for smaller vehicles. In this class of vehicles in particular, electric power steering systems
show their advantages, not least because of the small amount of energy required. The introduction of the increased voltage of 42 V will make the use of electrical power steering systems and wheel brakes much easier.
above Figure. shows the steering system of the Opel Corsa (1997) with electric power steering. It is a system with steering-tube transmission, i.e. the intermediate spindle transmits the whole of the torque resulting from the steering wheel force and servo boost. Due to the more direct steering transmission, this torque is clearly higher than in a comparable manual steering system, something which must be taken into consideration when deciding on the size of the components which control performance.
the method of operation of the servo assembly (EPAS system by NSK) becomes clear: a plastic worm gear 7 is applied to the steering tube 2. This is engaged by the worm 6, which in its turn is connected to the shaft of the servomotor 5. Steering-wheel torque generates a torsional movement of the torsion bar (concealed by the sliding sleeve 3). The steering tube area is axially grooved above the torsion bar and spindle-shaped below.
As the spindle rises, the sliding sleeve makes an axial movement on the steering tube proportional to the torsion of the torsion bar. This axial movement is transmitted to the rotary potentiometer 4 via a tap. Corresponding to a group of curves, the servo boost is determined from the steering-wheel torque and driving speed signals and the servomotor 5 controlled accordingly.