Electric steering system comprising a sensor for determining the steering rod force

Abstract

The invention relates to an electric steering system for a motor vehicle, comprising a steering housing (1), a steering rod (2), a ball screw drive (KGT), wherein the steering rod (2) is guided in the steering housing (1) and can be moved by the ball screw drive. The ball screw drive comprises a ball screw drive bearing (3) arranged in the steering housing (1), with axially arranged spacer disks (4, 17), one on each side of the ball screw drive bearing (3). At least one measuring device (7, 13) is arranged axially between the ball screw drive bearing (3) and the steering housing (1), the measuring device (7, 13) being used to determine a steering rod force.

Description

[0001] R.410505

[0002] - 1 -

[0003] Description

[0004] title

[0005] Electric steering system with a sensor to determine the steering force.

[0006] The present invention relates to an electric steering system for a motor vehicle with a sensor for determining the steering rod force according to the preamble of claim 1.

[0007] State of the art

[0008] Electric power steering systems for motor vehicles typically feature a steering housing in which a rack is mounted for longitudinal movement. A pinion, rotatably mounted within the steering housing, engages with the teeth of the rack, causing it to move laterally. This movement, in turn, transmits the steering input via tie rods and steering knuckles, resulting in the steering of the vehicle's wheels. Alternatively, a ball screw drive can be used instead of the pinion, in which a driven ball nut moves the steering rod.

[0009] In conventional steering systems with a mechanical connection to the steering column, the steering rod has splines because the rotational movements of the steering shaft are transmitted to the rack via a pinion. In steer-by-wire steering systems, the mechanical connection to the steering column is eliminated. Therefore, steering rods in steer-by-wire systems no longer have splines for the steering pinion, but only splines or a ball screw for the steering mechanism.

[0010] In steering systems with a central actuator, i.e., a steering system with one steering drive for both steerable wheels, a tie rod is mounted at each end of the steering linkage, via which the wheels are pivoted. In steering systems with decentralized, translational actuators, i.e., individual wheel actuators, where each steerable wheel has its own steering drive, a steering linkage can be arranged at each wheel, which pivots the wheel via a tie rod. R.410505

[0011] Since this invention can be used in classic steering systems, but also in steer-by-wire steering systems with central or decentralized actuators, the following description refers to a steering rod.

[0012] In such electric steering systems, no sensor is currently used to measure the steering rod force. Instead, the steering rod force is estimated. Parameters such as the current in the motors or the steering rod position, for which values ​​from the rotor position sensor are used, are employed for this estimation. Such an estimate may not be sufficiently accurate for certain driving maneuvers. The cause of these inaccuracies is primarily internal friction effects within the steering system during slow movements. These include, for example, stick-slip effects, friction within the ball screw drive, and in the belt drive.

[0013] As described, the calculation is based, among other things, on data regarding motor dimensions, i.e., a component that, relative to the force path, is located relatively far from the point of force application. Consequently, internal steering system frictions have a significant influence on the calculated steering rod force. The object of the invention is therefore to determine the steering rod force as accurately as possible in order to, for example, improve steering feel and enable further functionalities at the vehicle level.

[0014] Advantages of the invention

[0015] The present invention relates to an electric steering system for a motor vehicle with the characterizing features of the independent claim.

[0016] In an EPSapa type electric steering system, a gear concept consisting of a toothed belt and ball screw drive (KGT) is used to convert the rotational movement of the electric motor into a linear movement of the steering rod.

[0017] The invention assumes that in the EPSapa electric steering system, the total steering force acting on the steering rod is transferred from the KGT bearing to the housing. Spacers are typically arranged axially on both sides between the KGT bearing and the housing, through which the forces are transmitted to housing elements. Depending on the direction of the acting axial force, the left or right R.410505

[0018] The spacer was compressed, reducing the axial gap. The invention proposes detecting this change in the gap using a measuring device and thus deducing the magnitude and direction of the acting axial force. Various measuring principles can be used for the measuring device.

[0019] The signals from the sensor modules are transmitted via cables to a control unit, which calculates corresponding signals to optimize the steering feel.

[0020] In a first embodiment, inductive sensor elements are used on the spacer disk. An inductive sensor element contains a coil that generates a magnetic field. The metallic components conduct the magnetic field better than the air gap. A change in the thickness of the air gap results in a measurable change in the magnetic flux.

[0021] In a spacer plate, at least two sensor elements are arranged in opposing bores. Additionally, milled grooves can be provided in the spacer plate to guide the sensor cables and to house evaluation electronics.

[0022] In a second embodiment, a capacitive sensor is used, which operates based on the change in the electrical capacitance of a capacitor. The surface of the spacer washer that is in contact with the KGT bearing can be covered with a sensor film. This sensor film is a circular ring divided into at least three segments. Thus, the sensor film is divided into several sensor sensing elements. This allows the resulting axial force components to be determined during evaluation. The sensor film can consist of a base material (e.g., a flexible PCB) and a capacitive measuring layer applied to it (e.g., with laminated copper foil).

[0023] In a third embodiment, a sensor element is used that operates on the basis of the piezoresistive effect. The structure is similar to that in the second embodiment. The difference here is that the base carrier material (e.g., flexible PCB) is coated with a substrate.

[0024] In a steering gear, both spacer discs, i.e., to the right and left of the KGT bearing, can be equipped with such sensor elements. It is also possible to arrange a sensor element on only one spacer disc. R.410505

[0025] The wheel forces are transmitted to the steering rod via tie rods. The tie rod is angled relative to the steering rod at a tie rod angle, the angle of which depends on the wheel angle and the suspension compression. This results in axial and radial components of the tie rod forces acting on the steering rod. To determine the acting axial force component using the sensor modules in the spacers, at least three sensor elements must be evenly distributed around the circumference of each spacer, for example, at 120°. By averaging the individual loads determined from the at least three sensor elements, the total acting axial load can be calculated.

[0026] When using two spacers with integrated sensors, i.e., on both sides of the KGT bearing, the minimum number of sensor elements required per spacer can be reduced to two. The two sensor elements are positioned at a 90° angle to each other on the KGT.

[0027] In a fourth embodiment, a magnet is integrated into the spacer. A high-resolution magnetic sensor is arranged on the opposite side of the housing. As the gap narrows, the magnet moves towards the magnetic sensor. These changes in gap size are evaluated by the magnetic sensor.

[0028] In a fifth embodiment, a further layer of a conductive polymer is applied to the spacer. Such conductive polymers have the property that their conductivity changes under pressure. This change in conductivity is detected, allowing the acting force to be deduced.

[0029] The invention is explained in more detail below with reference to the figures. These show:

[0030] Fig. 1 shows the KGT bearing of a steering gear of type EPSapa.

[0031] Fig. 2 shows a section of the KGT bearing with a sensor according to the invention in the first embodiment.

[0032] Fig. 3 shows the sensor according to the invention in the first embodiment.

[0033] Fig. 4 shows a spacer disc according to the invention for a KGT bearing R.410505

[0034] - 5 -

[0035] Fig. 5 shows the arrangement of sensor elements according to the first embodiment.

[0036] Fig. 6 shows a section of the KGT bearing with a sensor according to the invention, corresponding to the second embodiment.

[0037] Fig. 7 shows a sensor module according to the invention, corresponding to the second and third embodiments.

[0038] Fig. 8 shows a sensor module according to the invention, corresponding to the fourth embodiment.

[0039] Fig. 9 shows a sensor module according to the invention, corresponding to the fifth embodiment.

[0040] Figure 1 shows the ball screw bearing 3 of an EPSapa steering system. The ball screw drive, which converts the rotary motion of the motor (not shown) into a translational motion of the steering rod 2, is arranged in a housing 1. The ball screw drive comprises a ball screw bearing 3, which is axially supported on both sides by spacers 4, 17 on the housing 1. A tolerance washer 5 may also be arranged between a spacer 4 and the housing 1.

[0041] Figure 2 shows an enlarged section of the KGT bearing 3. Spacers 4 and 17 are arranged to the right and left of the KGT bearing. Tolerance washers 5 are also arranged between the housing 1 and the spacers 4 and 17. There is a gap 6 on both sides between the spacers 4 and 17 and the tolerance washers 5, which decreases depending on the applied axial force.

[0042] According to the first embodiment, a measuring device 7 is integrated in the spacer 4, which measures the size of the gap 6.

[0043] Figure 3 shows the measuring device 7, which is integrated into the spacer 4. The measuring device 7 operates inductively and has a coil 8 that generates a magnetic field. The metallic components conduct the magnetic field better than the air gap 6. A change in the thickness of the air gap 6 results in a measurable change in the magnetic flux. R.410505

[0044] - 6 -

[0045] The measuring device 7 is pressed into the spacer 4. A seal (not shown) can be used radially at the interface between the measuring device 7 and the spacer 4. A defined distance 9 ensures that the outer ring of the KGT bearing does not contact the sensor. This protects the sensor from overload.

[0046] There is also a defined distance on the other side of the measuring device 7. Under maximum load or compression of the elastomer, the right side of the spacer 4 comes into contact with the housing 1 (not shown) or the tolerance washer 5. The measuring device 7 still maintains a remaining distance to the housing 1, thus also protecting the measuring device 7 from overload.

[0047] Figure 4 shows a spacer 4 according to the invention. The sensors of the measuring device 7 are pressed into bores 10 on opposite sides, with one of the two sensors visible before assembly. The spacer 4 has a milled recess 11 with a cable exit, in which evaluation electronics can be housed. Cables (not shown) can be routed from the measuring device 7 in cable grooves 12 to the evaluation electronics in the recess 11.

[0048] Figure 5 shows an advantageous arrangement of sensors of the measuring device 7 according to the first embodiment. Since two sensors are arranged at 90° intervals from each other on the KGT, the minimum number of sensors required per spacer disk 4, 17 can be reduced to two.

[0049] Figure 6 shows a section of the KGT bearing 3 with a measuring device 13 according to the second embodiment of the invention. The measuring device 13 comprises a sensor film which is adhered to the spacer washers 4, 17.

[0050] In all versions, it is possible to provide one cable exit from the steering housing 1 for each spacer 4, 17 with measuring device 7, 13. Alternatively, both sensors of the measuring device 7, 13 can be connected to each other during pre-assembly. This is shown in Figure 7. The two sensor elements 14 are connected and have a common evaluation unit 15. A cable exit 16 leads from the evaluation unit 15 through the steering housing 1 (not shown).

[0051] In Figure 8, according to the fourth embodiment, the gap 6 is measured with a magnetic sensor 19. For this purpose, a magnet 18 is integrated into the spacer 17. R.410505

[0052] A damping disc 20 is arranged between the spacer disc 17 and the magnetic sensor 19 as an elastic buffer. The damping disc 20 prevents noise when the spacer disc 17 contacts the magnetic sensor 19.

[0053] In Figure 9, according to the fourth embodiment, the gap 6 is measured using conductive polymers. For this purpose, an elastic disk 21, which consists of electrically conductive polymers, is arranged between the spacer disk 17 and a sensor 22.

Claims

R.410505 - 8 - Claims 1. Electric steering system for a motor vehicle, comprising a steering housing (1), a steering rod (2), a ball screw drive (KGT), wherein the steering rod (2) is guided in the steering housing (1) and can be displaced by the ball screw drive; the ball screw drive comprises a KGT bearing (3) which is arranged in the steering housing (1), wherein a spacer washer (4, 17) is arranged axially on both sides of the KGT bearing (3), characterized in that at least one measuring device (7, 13) is arranged axially between the KGT bearing (3) and the steering housing (1), wherein the measuring device (7, 13) serves to determine a steering rod force.

2. Electric steering system according to claim 1, characterized in that the measuring device (7) determines the size of a gap between the KGT bearing (3) and steering housing (1).

3. Electric steering system according to claim 2, characterized in that the measuring device (7) acts inductively, wherein the measuring device (7) comprises at least one coil (8) which is integrated into the spacer disk (4, 17), wherein the measuring device (7) determines a change in the magnetic field in order to determine the size of the gap.

4. Electric steering system according to claim 2, characterized in that the measuring device (7) comprises a magnet (18) which is integrated into the spacer disk (4, 17), that the measuring device (7) comprises a magnetic sensor (19) which is arranged opposite the magnet (18), wherein the measuring device (7) determines a change in the magnetic field in order to determine the size of the gap. R.410505 - 9 - 5. Electric steering system according to one of claims 3 or 4, characterized in that at least two sensor elements (7) or two magnets (18) are integrated in a spacer disc (4, 17).

6. Electric steering system according to claim 1, characterized in that the measuring device (7) determines the pressure load between the KGT bearing (3) and steering housing (1).

7. Electric steering system according to claim 6, characterized in that the measuring device (7) acts capacitively, wherein the measuring device (7) comprises a sensor film with a capacitive measuring layer which is applied to the spacer disc (4, 17) or to the KGT bearing (3).

8. Electric steering system according to claim 6, characterized in that the measuring device (7) is based on the piezoresistive effect, wherein the measuring device (7) comprises a sensor film with a piezoresistive measuring layer which is applied to the spacer disk (4, 17) or to the KGT bearing (3).

9. Electric steering system according to claim 6, characterized in that the measuring device (7) comprises a conductive polymer which is applied to a spacer disk (4, 17).

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