Electric steering system for a motor vehicle

Abstract

An electric steering system for a commercial vehicle, comprising a worm and roller steering system (9) in which a globoidal worm (5) engages with a toothed roller (6), the toothed roller (6) being arranged on a steering roller shaft (7), the steering roller shaft (7) transmitting a pivoting movement to a steering column lever (12). In the steering system, at least one servo unit, consisting of an electric motor (3) and a helical gear unit (4), is rotaryally coupled to the globoidal worm (5) of the worm and roller steering system (9).

Description

The invention relates to an electric steering system for a motor vehicle. State of the art Previous purely mechanical steering systems no longer met the requirements. Heavier loads on the front axle and wider tires necessitated higher steering performance. Furthermore, driver comfort had to be improved so that steering was easy despite the weight of the vehicles. Therefore, a hydraulic power steering system was added. For commercial vehicles, the hydraulic power steering was implemented as a ball-nut hydraulic steering system. A steering pump provides the hydraulic pressure. A rotary valve controls the oil distribution. The rotational movement of the steering wheel is first transferred to a worm gear, within whose coils lies an endless ball chain. The worm gear is located in a movable piston, which has internal grooves into which the balls engage. In this way, the rotation of the worm gear is converted by the balls into an axial movement of the piston; depending on the direction of rotation, the piston moves up or down. The piston transmits these movements, via gear teeth on its outer surface, to the segment shaft, which also has corresponding gear teeth and is thus set into a pivoting motion. This pivoting motion is used to swivel the wheels via the steering column lever mounted on the segment shaft. Hydraulic power steering can be implemented in a ball-nut hydraulic steering system by assisting the upward and downward movement of the piston with appropriate oil pressure. The piston already forms two pressure chambers. One is located outside the piston in the space where the segment shaft is also located, and the other is located inside the piston where the worm gear engages with the piston. To enable additional assistance functions, such as lane keeping assist, in commercial vehicles, an electric motor was added to the ball nut hydraulic steering system during further development. This allows steering interventions to be carried out independently of the driver. With a fully electric steering system, automated driving functions up to SAE Level 4 are possible. Hydraulic assistance is no longer necessary. In light commercial vehicles, an electric motor can act on the ball nut gearbox via a helical gear unit. In heavier commercial vehicles, two or more electric motors with helical gear units can be connected to the ball nut gearbox. In such concepts, the ball nut drive of the ball nut hydraulic steering system is reused, but without the pump, seals and hydraulic pressure. The piston remains a component of the ball-nut steering system, even though it is no longer functionally necessary. The piston stroke requires installation space between the input and output shafts, the so-called R-dimension. This leads to installation problems in the vehicle, as the steering system is too close to the cabin floor. Furthermore, the friction conditions in the ball-nut steering system are disadvantageous. Friction occurs particularly at the ball recirculation and at the piston in conjunction with the segmented shaft and the guide in the housing. The object of the invention is therefore to provide an electric steering system for commercial vehicles that avoids the aforementioned disadvantages. In particular, the steering system should have a low R-value and lower friction, and thus a better efficiency. This problem is solved by the subject matter of claim 1. Disclosure of the invention The present invention provides an electric steering gear for a motor vehicle. The basic steering mechanism is a worm and roller steering system. For electric power steering, at least one electric motor is connected to the worm and roller steering system via a helical gear unit. The electric motor and the helical gear unit together form a servo unit. The worm and roller steering system features a small distance between the input and output shafts, resulting in a low R-value. In this system, a globoidal worm engages with a toothed roller in the steering roller head. This steering mechanism allows the function of converting torque and direction to be achieved with a significantly lower R-value. Because of the rotating toothed roller in the guide roller head, a significantly higher efficiency can be achieved compared to a pure worm gear. Friction is considerably lower, as friction only occurs in the bearings of the worm and roller head, and between the worm and toothed roller. Recent years have shown that globoidal screws can be manufactured well on today's multi-axis machines. Advantageous embodiments and further developments are set out in the dependent claims. In a first embodiment for lighter vehicles, it may be sufficient if a servo unit acts on the worm gear steering. In a further embodiment for heavier vehicles, two servo units can be connected to the steering gear. Depending on the available installation space in the vehicle, one motor can be positioned on each side, i.e., above and below the globoidal worm gear. It is also possible to arrange both servo units on one side, either above or below the worm gear steering system. The worm and roller steering system can be connected above the steering shaft, which in turn connects a steering handle to the steering gear. A sensor system is also provided between the steering gear and the steering shaft, which measures the steering angle and steering torque by detecting the rotation of a torsion bar. In a steer-by-wire system, there is no mechanical connection to the steering handle. A control unit receives the steering signals and controls the servo units accordingly, with the servo units then having to provide the entire steering force at the steering gear. Brief description of the drawings Figure 1 shows an electric steering system according to the invention. Figure 2 shows an enlarged view of the worm gear steering system. Figure 1 shows an electric steering system according to the invention with a worm and roller steering mechanism 9. The driver inputs their steering input via a steering handle 1. The rotational movement is transmitted via a steering shaft 10 to an input shaft 2 of the steering gear. An electric motor 3 is arranged above the worm and roller steering mechanism 9, which acts on the worm and roller steering mechanism 9 via a helical gear 4. Figure 2 shows an enlarged view of the worm gear steering system 9. A globoidal worm 5 is driven via the intermediate shaft 8. The steering roller 6, which is mounted in the steering shaft head, engages with the worm 5. The steering roller shaft 7 protrudes from the housing 11 and has a conical splined section at its end for receiving the steering column lever 12. The swivel caster 6 can be designed as a two-tooth caster or as a three-tooth caster.

Claims

Electric steering system for a commercial vehicle, comprising a worm gear steering system (9) in which a globoidal worm (5) engages with a toothed roller (6), wherein the toothed roller (6) is arranged on a steering roller shaft (7), wherein the steering roller shaft (7) transmits a pivoting movement to a steering column lever (12), characterized in that at least one servo unit, consisting of an electric motor (3) and a helical gear unit (4), is rotaryally coupled to the globoidal worm (5) of the worm gear steering system (9). Electric steering system according to claim 1, characterized in that a servo unit is arranged above and below the worm gear steering (9). Electric steering system according to claim 1, characterized in that two servo units are arranged above or below the worm gear steering (9). Electric steering system according to one of claims 1 to 3, characterized in that the worm gear steering (9) is connected to a steering handle (1) via a steering shaft (10), wherein the electric steering system comprises a sensor system which measures the steering angle and steering torque via the rotation of a torsion bar. Electric steering system according to one of claims 1 to 3, characterized in that the electric steering system is a steer-by-wire steering system.

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