Steer-by-wire steering system for determining the position of a steering rod

Abstract

The invention relates to a steer-by-wire steering system comprising a steering rod (10) or at least a single-wheel actuator and a sensor system (14), wherein the sensor system (14) comprises a transmitting coil (22) and at least one receiving coil (24) and a sensor, wherein the steering rod (10) comprises a planar, conductive, non-ferromagnetic target strip (12), wherein the steering rod (10) is movably mounted in the axial direction over a distance S, wherein the target strip (12) is offset from the axial direction of the steering rod (10) by an angle ΣThe sensor system (14) is arranged rotated outwards between the steering rod (10) and the sensor system (14) and is rigidly connected to the steering rod (10), wherein the transmitting coil (22) is configured to be excited by an alternating electric field signal, wherein the at least one receiving coil (24) is configured to receive an alternating magnetic field emanating from the target strip (12), and wherein the sensor is configured to measure the signals induced in the receiving coil (24).

Description

The invention relates to a steer-by-wire steering system and, more particularly, to a linear sensor for determining the position of a steering rod or a single wheel actuator.Prior ArtSteer-by-wire steering systems are an advanced technology in automotive engineering, in which the conventional mechanical connection between steering wheel and steering gear is replaced by electronic control systems. In a classic steering system, a steering rod transmits the rotational movement of the steering wheel directly to the wheels. In a steer-by-wire steering system, on the other hand, the movements of the steering wheel are detected by sensors and transmitted as electrical signals to a control computer. The latter processes the signals and controls the movement of the wheels with the aid of actuators. As a result, feedback and steering assistance can be adjusted precisely, which enables better vehicle control and individual adjustment of the steering characteristic.The basic structure of a steer-by-wire steering system comprises several essential components. First, there are sensors on the steering wheel that detect rotational angles and torque. This information is passed on to a central control computer which calculates the desired direction of travel and dynamics. The control commands are then passed on to electrical actuators, which position the wheels accordingly. Another important component is the feedback system, which provides the driver with realistic feedback on the road conditions by transmitting artificial forces back to the steering wheel.A particular problem with steer-by-wire steering systems is the determination of the exact position of the steering rod or the single wheel actuators after restarting the vehicle. While in a mechanical system the position of the steering rod is physically predetermined by its connection to the wheels, in a steer-by-wire steering system the position after the switch-on must be electronically determined. This can be challenging, since sensors and control units initially do not have precise information about the wheel position.Sensors that determine the position of the steering rod via the rotation of the steering system drive are not unambiguous. The problem of ambiguity can be solved by a counting system which counts the number of rotations of the drive and from this determines the exact position of the steering rod. However, when the vehicle is off, the counter does not function. If the steering rod moves when the vehicle is off, for example because the wheels are moved in a workshop on lifting the vehicle onto a lifting platform, the stored counter value becomes zero value since it no longer indicates the correct position of the steering rod.Therefore, it is important to use a steering rod position determining system that allows for a clear linear travel measurement.The object of the invention is thus to propose a measuring method and a corresponding sensor system for a steering system of a vehicle, with which the position determination of the steering rod or the individual wheel actuators is unambiguous.The object is achieved by the subject matters of the independent claims.In the following, the term steering rod is used synonymous with steering rods in the actual sense, i.e. for steering two wheels, or in the sense of a single wheel actuator, i.e. for steering a single wheel. The terms are therefore interchangeable.Disclosure of the InventionAccording to a first aspect of the invention, this object is achieved by a steer-by-wire steering system comprising a steering rod or at least one single wheel actuator and a sensor system.In vehicles with single-wheel steering systems, single-wheel actuators are used instead of steering rods, to which the problems described above in determining the position likewise apply. The measuring principle presented with this invention can be embodied equally with steering rods and single wheel actuators. The terms are therefore interchangeable within the scope of this invention.The sensor system comprises a transmitting coil and at least one receiver coil and a sensor. The steering rod comprises a planar conductive, nonferromagnetic target strip.The target strip is preferably firmly connected to the steering rod. It can be glued, welded or connected to the steering rod by means of a form-fit or force-fit mechanism, for example.The steering rod is mounted movably in the axial direction over a distance S, wherein the target strip rotates out of the axial direction of the steering rod by an angle α between the steering rod and the sensor system and is rigidly connected to the steering rod.The transmitting coil is configured to be excited by an alternating electric field signal. The at least one receiver coil is configured to receive an alternating magnetic field signal originating from the target strip. The sensor is configured to measure the signals induced in the receiver coil.The sensor system is preferably fixedly connected to a reference frame, in particular a steering system housing and / or the body, so that the position of the steering rod to be determined is always measured relative to the vehicle.In operation, the handlebar moves under the sensor system. During this movement, the overlap region of the sensor system with the target strip migrates within the area detectable by the sensor system. That is, depending on the position of the steering rod, the region of the target strip is located under the sensor system and in particular under the transmitting coil at a different position.The transmitting coil is excited with an alternating electric field and generates a corresponding magnetic field. This magnetic field induces eddy currents in the target tire, which in turn generate alternating magnetic fields of response. The receiver coil is configured to receive the fields generated by the target strip. It is therefore important that the target strip consists of a planar conductive but nonferromagnetic material.The excitation frequency of the transmitting coil is preferably 3 MHz to 5 MHz. The frequency should be chosen such that the movement of the target strip or the steering rod under the sensor system is comparatively slow to the excitation frequency.With only one receiver coil, a voltage amplitude induced by the response fields in the receiver coil can be detected as a response. The amplitude is dependent on the position of the part of the target strip which is located under the sensor system and in particular under the at least one receiver coil. Since this region shifts during the movement of the steering rod under the sensor system, the measured amplitude of the response signal also varies as a function of the position. From this connection, the position of the steering rod can be unambiguously determined.The alternating magnetic field response signal received by the receiver coil is demodulated. Since the parameters used, in particular the excitation frequency, the impedance of the target strip and preferably the air gap between the sensor and the target strip, are constant or almost constant, the position of the target strip under the sensor or under the receiver coil can be determined from the amplitude of the received signal.In one embodiment, a reference function may be used to determine the position of the handlebar from the signal received by the at least one receiver coil.The position determination of the steering rod in the steering system described here is thus unambiguous, whereby the invention achieves the object set by it.In one embodiment, the steering rod or the at least one single wheel actuator comprises a milled-out region, wherein the target strip is positioned in the milled-out region.The target strip is preferably arranged such that it does not protrude from the milled-out region and / or is aligned with the steering rod from the point of view of the sensor system.The positioning of the target strip in the milled-out region of the steering rod advantageously allows the steering rod with the target strip to move unimpeded under the sensor system and, if appropriate, past further components of the steering system. Particularly preferably, the target strip becomes covered within the profile of the steering rod, so that it does not project beyond the round basic shape of the steering rod in cross section.A milled-out region on the steering rod with a width of at least 10 mm has proven to be particularly advantageous. This width allows an angle α that allows a particularly accurate measurement of the position of the handlebar.In one embodiment, the handlebar has a diameter D and wherein the angle α is less than or equal to arctan(D / S).The angle α is basically to be selected as large as possible, so that the change in the position of the overlap region between sensor and target strip changes at a maximum over the movement distance S. The larger α, the smaller the measurement tolerance and thus the larger the measurement accuracy. Alpha can be positive or negative. The sign determines whether the target strip is arranged on the steering rod rotated to the left or to the right by α.However, the angle α is limited in this embodiment by the target strip not protruding beyond the steering rod, or at least not significantly protruding beyond the steering rod, from the point of view of the sensor system. In the limit, the entire width of the handlebar, i.e. its diameter D, is used. Since the target strip extends over the length of the distance S, the critical angle arctan(D / S) results therefrom.The angle α is preferably selected to be as large as possible in order to achieve the best possible sensitivity of the output signal with respect to the linear movement of the steering rod, so that the target strip at the two ends of the distance measuring region just touches the front or rear edge of the milled portion of the steering rod in this way.In one embodiment, the flat side of the target strip has a length L and a width B, wherein the windings of the transmitting coil and the windings of the at least one receiver coil run parallel to the length L and to the width B of the flat target strip.The transmitting coil generates a magnetic field which is intended to strike the target strip as perpendicularly as possible in order to induce eddy currents as strong as possible there. Ideally, the windings of the transmitting coil are therefore oriented perpendicular to the surface normal of the target strip.The induced eddy currents flow in the surface of the target strip. The charges moving in this process in turn generate magnetic fields which project from the surface of the target strip. These magnetic fields are detected by the at least one receiver coil. Due to the orientation of the magnetic fields, the windings of the at least one receiver coil are likewise parallel to the surface of the target strip and oriented.In one embodiment, the at least one receiver coil has a winding geometry with at least two periods, wherein regions with mutually opposite magnetic flux directions are arranged next to one another in each period.By arranging the regions with mutually opposite magnetic flux directions in at least two periods, edge effects, in particular measurement errors during position determination, can be compensated.In one embodiment, the target strip is spaced apart from the steering rod or the at least one single wheel actuator using a spacer, wherein the spacer consists of electrically non-conductive and / or non-ferromagnetic material.The alternating field generated by the transmitting coil may possibly induce eddy currents in the steering rod, which in turn may retroreflect magnetic fields and interfere with the position measurement. By a certain distance between the target strip and the steering rod, the current induced in the steering rod can be reduced in an advantageous manner. In order that the spacer itself does not retroreflect magnetic fields, it should be made of a non-conductive and / or non-ferromagnetic material.In one embodiment, the sensor is configured such that it can be connected to a control system for reading out the sensor data via a plug connection.Due to the design, the measurement accuracy of the sensor is susceptible to movements perpendicular to the direction of movement of the steering rod. The influence of vibrations or forces on the sensor and the entire sensor system can be reduced by decoupling external systems from the sensor system. The sensor is therefore preferably not connected directly, but rather via a plug connector to an evaluation system.In one embodiment, the sensor system is pressed against the steering rod or the at least one individual wheel actuator by a restoring element, in particular by a compression spring.The accuracy of the sensor and the position determination increases if the relative position between the sensor system and the steering rod is determined solely by the movement of the steering rod due to the steering movement. Effects such as vibrations or other external effects can be reduced when the sensor system is pressed against the handlebar. This task can be fulfilled by a restoring element.In one embodiment, the sensor system includes two receiver coils.The use of two receiver coils is a particularly preferred embodiment, since this can advantageously significantly increase the accuracy of the position determination. In a further embodiment, the receiver coils are arranged with inverted winding geometries.For evaluation with two receiver coils, the amplitude of the measured signals does not have to be used. Instead, the phase shift of the signals measured with the receiver coils and using the arctan2function can likewise be used to determine the position of the target strip under the sensor system, from which position the position of the steering rod can in turn be derived. This embodiment shows a higher robustness with respect to fluctuations and measurement errors, that they relate equally to the results of both coils and can thus be calculated out.In one embodiment, the at least one receiver coil comprises at least two windings electrically connected in series and / or the transmitter coil comprises at least two windings arranged on different layers of a circuit carrier and electrically connected in series.Advantageously, the higher number of turns has the effect that the coils can detect a higher amplitude with a small installation space. Therefore, higher numbers of turns may improve the signal-to-noise ratio and / or reduce the installation space.In one embodiment, the transmitting coil encloses the at least one receiver coil.The arrangement of the transmitting coil around the receiving coil or coils has several advantages. On the one hand, the alternating magnetic field of response emitted by the target is practically unmeasurable owing to the eddy currents outside the excited region, that is to say outside the region around the transmitting coil. On the other hand, the installation space in the interior of the transmitting coil is used and optimized in such a way that the sensor can be kept compact overall.In one embodiment, the windings of the transmitting coil have a rectangular basic shape, wherein two sides of the windings are arranged out of the axial direction of the steering rod by the angle α. Alternatively, two sides of the windings are arranged parallel to the direction of movement of the steering rod.A rectangular basic shape is a shape in which the sides enclose an angle of 90°. This does not necessarily mean that the corners must be pointed. It is possible and not contrary to the inventive concept to round off the corners of the transmitting coil.In the first alternative, the entire sensor system is preferably aligned on the target strip, so that a rectangular region of the target strip is always located within the transmitting coil. This arrangement of the sensor system can reduce a linearity error when reading out the alternating response field.In the alternative, the sensor system is oriented to the direction of movement of the handlebar. This arrangement minimizes the installation space required for the sensor system, as a result of which the steering system can be made smaller and more compact overall.In a further aspect, the invention relates to a sensor system for a steer-by-wire steering system as described above.In a further aspect, the invention relates to a method for measuring an absolute position of a steering rod in a steer-by-wire steering system as described above, the method comprising:applying an alternating electric field signal to the transmitting coil;receiving an alternating magnetic field signal with the at least one receiver coil;determining the position of the steering rod from the received alternating field signal.The position of the handlebar can be determined from the amplitude of the received signal, as already explained above.In one embodiment, the sensor system includes a second receiver coil, the method further comprising:receiving a second alternating magnetic field signal with the second receiver coil;determining an angle φ based on the alternating magnetic field signal of the first receiver coil and the alternating magnetic field signal of the second receiver coil; andwherein the position of the steering rod is also determined from the second alternating field signal and / or from the angle φ.Advantageously, the position of the steering rod can be determined more precisely by way of the angle determination than solely from the signal amplitude.In a further aspect, the invention relates to a computer program with program code for carrying out a method as described above when the computer program is executed on a computer and / or to a computer-readable data carrier with program code of a computer program for carrying out a method as described above when the computer program is executed on a computer.In a further aspect, the invention relates to a system for measuring an absolute position of a steering rod in a steer-by-wire steering system as described above, wherein the system is configured to carry out the above-mentioned method.In summary, it can be stated that the present invention specifies a steer-by-wire steering system, a sensor system for a steering system, a method for measuring the absolute position of the steering rod, a computer program, a data carrier with program code and a system for measuring the absolute position of the steering rod.The described embodiments and developments can be combined with one another as desired.Further possible embodiments, developments and implementations of the invention also include combinations of features of the invention described above or below with respect to the exemplary embodiments, which combinations are not explicitly mentioned.Brief Description of the DrawingsThe accompanying drawings are intended to provide a further understanding of the embodiments of the invention. They illustrate embodiments and, in conjunction with the description, serve to explain principles and concepts of the invention.Other embodiments and many of the advantages mentioned are evident with reference to the drawings. The elements of the drawings shown are not necessarily shown true to scale with respect to one another.The following are shown: FIG. 1 is a perspective view of a portion of a steering rod with a target strip and a sensor system; FIG. 2 shows the winding structure of an embodiment of a sensor system; and FIGS. 3a-3f schematically show two possible arrangements of the sensor system in relation to the target strip and to the direction of movement of the steering rod.In the figures of the drawings, identical reference numerals designate identical or functionally identical elements, components or components, unless indicated to the contrary.FIG. 1 shows a portion of a steer-by-wire steering system having a steering rod 10, a target strip 12, and a sensor system 14.The steering rod 10 has a milled-out region 16. the target strip 12 is arranged within the milled-out region 16, such that the target strip 12 does not protrude beyond the steering rod 10 from the perspective of the sensor system 12.The targt strip 12 is arranged obliquely in the milled-out region 16, such that the target strip 12 also extends over the distance S or the length of the milled-out region 16 over the entire width B thereof. As in a function, each position of the handlebar is assigned a value of the received alternating-field response signal.The center of the area of the target strip 12 that is under the sensor system 14 can be considered a continuously growing linear function of the position s, where s is in an interval [0, S].For each value of f(s), there is a certain value that can be determined from the alternating-field response signal. From the invertibility of these relationships, the position of the steering rod can be determined from the alternating-field response signal.The sensor system 14 determines the position of the steering rod 10 from the relative position between itself and the targing strip 12, which is especially important here for the position perpendicular to the direction of movement of the steering rod 10. For the sensor system 14, a misdetermination of this relative position can look the same as a displacement of the target strip 12 in the direction of movement of the steering rod 10. It is therefore important that the relative position between the sensor system 14 and the target strip 12 is as precise as possible. In the embodiment shown, the sensor system 14 comprises two structural measures for this purpose.The sensor system 14 is pressed against the steering rod 10 by a restoring element, which is designed here as a compression spring 18. In this case, the movement of the steering rod 10 along its longitudinal axis under the sensor system 14 should preferably not be impaired. The movement perpendicular thereto should, however, be avoided as completely as possible. This can be effected, for example, with a guide device (not shown here) or coating of the contact surfaces.The second measure for reducing the measurement error is the connection of the sensor system 14 to an evaluation, computing or control unit. In order that a cable does not transmit any tension and thus no intrinsic movement to the sensor system, the sensor can be connected to the control unit via a plug connector 20. The connector is preferably connected to the handlebar housing, but not directly to the sensor system 14. A connection of the sensor system 14 to at least one flexible flexurally and torsionally elastic cable is advantageous, so that movements are decoupled.In one embodiment, the sensor may be wirelessly coupled to a control unit, such that the connector 20 is also not necessary.FIG. 2 shows an exemplary arrangement of the coils of a sensor system according to the invention. In the illustrated embodiment, the sensor system includes a transmit coil 22 and a receive coil 24.The transmitting coil 22 here comprises three double turns, which enclose a rectangular area. This transmitting coil 22 is excited with an alternating electric field signal, so that it generates a corresponding magnetic field.The coil configuration of the receiver coil 24 is arranged within the turns of the transmitter coil 22. The turns of the receiver coil 24 are in the same plane as the turns of the transmitter coil 22, which allows the sensor system to be positioned close to the target strip, whereby the currents induced in the target strip by the transmitter coil are as large as possible, which in turn results in the greatest possible alternating-field response signal for the receiver coil 24.The turns of the receiver coil 24 describe, to an approximation, two above one another eighths, so that the magnetic flux in the adjacent eighth clusters would cancel one another out. The eighth structure repeats in two periods, so that two eighths are arranged one above the other. An extension direction 26 can thus be defined for the receiver coil 24. In an embodiment with two receiver coils 24, the extension directions 26 are preferably arranged opposite to each other. The basic shape of the periods of the receiver coils is advantageously based on a sine or cosine function.In one embodiment, the handlebar with the target strip would move under the sensor system, with the direction of movement being perpendicular to the direction of extension 26. In FIG. 2, this could therefore be from left to right or from right to left.FIGS. 3 ato 3 fschematically illustrate the movement of the sensor 14 over the target strip 12, FIGS. 3 ato 3 cillustrate a first embodiment and FIGS. 3 dto 3 fillustrate a second embodiment for arranging the sensor system 14.The illustrated embodiments differ in the arrangements or the alignments of the sensor system 14. In the upper representations 3 ato 3 c, the sensor system is aligned such that its windings are partially aligned parallel to the direction of movement of the steering rod. In the lower embodiments in FIGS. 3 dto 3 f, the windings of the transmitting coil of the sensor system 14 are oriented at the orientation of the target strip 12.In both embodiments, the sensor system 14 is rigid and non-moving. Preferably, the sensor system 14 is connected to a steering system housing or directly or indirectly to the body. The steering rod with the target strip moves in the direction of the arrow under the sensor system. Depending on the steering direction, this would be to the left or to the right in the figures.As the target strip 12 moves under the sensor system 14, the relative position of the area of the target strip 12 that is directly under the sensor system 14 changes. If the steering rod is at the right stop, cf. FIGS. 3 aand 3 d, the sensor system 14 detects the target strip 12 at the upper edge. With the movement to the left, the overlap region of the target strip 12 migrates downward. If the steering rod is at the left stop, cf. FIGS. 3 cand 3 f, the sensor system 14 detects the target strip 12 at the lower edge.By uniquely assigning the position of the overlap region of the target strip 12 with the sensor system 14 to the alternating-field response signal that the receiver coil of the sensor system 14 receives, the position of the steering rod can be uniquely determined.

Claims

Steer-by-wire steering system comprising a steering rod (10) or at least one single wheel actuator and a sensor system (14), wherein the sensor system (14) comprises a transmitting coil (22) and at least one receiver coil (24) and a sensor, wherein the steering rod (10) comprises a planar conductive, non-ferromagnetic target strip (12), wherein the steering rod (10) is mounted movably in the axial direction over a distance S, wherein the target strip (12) rotates out of the axial direction of the steering rod (10) by an angle α between the steering rod (10) and the sensor system (14) and is rigidly connected to the steering rod (10), wherein the transmitting coil (22) is configured to be excited by an alternating electric field signal, wherein the at least one receiver coil (24) is configured to receive an alternating magnetic field signal originating from the target strip (12), wherein the sensor is configured to measure the signals induced in the receiver coil (24).The steering system of claim 1, wherein the steering rod (12) or the at least one single wheel actuator comprises a milled area (16), wherein the target strip (12) is positioned in the milled area (16).Steering system according to any of the preceding claims, wherein the steering rod (10) has a diameter D and wherein the angle α is less than or equal to arctan(D / S).Steering system according to one of the preceding claims, wherein the flat side of the target strip (12) has a length L and a width B, wherein the windings of the transmitting coil (22) and the windings of the at least one receiver coil (24) run parallel to the length L and to the width B of the flat target strip (12).Steering system according to one of the preceding claims, wherein the at least one receiver coil (24) has a winding geometry with at least two periods, wherein regions with mutually opposite magnetic flux directions are arranged next to one another in each period.Steering system according to one of the preceding claims, wherein the target strip (12) is spaced apart from the steering rod (10) or the at least one single wheel actuator using a spacer, wherein the spacer consists of electrically non-conductive and / or non-ferromagnetic material.Steering system according to one of the preceding claims, wherein the sensor is designed such that it can be connected to a control system for reading out the sensor data via a plug connection.Steering system according to one of the preceding claims, wherein the sensor system (14) is pressed against the steering rod (10) by means of a restoring element, in particular by means of a compression spring (18).Steering system according to one of the preceding claims, wherein the sensor system (14) comprises two receiver coils (24).Steering system according to one of the preceding claims, wherein the transmitting coil (22) encloses the at least one receiver coil (24).Steering system according to one of the preceding claims, wherein the windings of the transmitting coil (22) have a rectangular basic shape, wherein two sides of the windings are arranged out of the axial direction of the steering rod (10) by the angle α or wherein two sides of the windings are arranged parallel to the direction of movement of the steering rod (10).A sensor system for a steer-by-wire steering system according to any one of claims 1 to 11.Method for measuring an absolute position of a steering rod in a steer-by-wire steering system according to one of Claims 1 to 11, wherein the method comprises: - applying an alternating electric field signal to the transmitting coil (22); - receiving an alternating magnetic field signal with the at least one receiver coil (24); - determining the position of the steering rod (10) from the received alternating magnetic field signal.The method according to claim 13, wherein the sensor system (14) comprises a second receiver coil (24), the method further comprising: - receiving a second alternating magnetic field signal with the second receiver coil (24); - determining an angle φ based on the alternating magnetic field signal of the first receiver coil (24) and the alternating magnetic field signal of the second receiver coil (24); and wherein the position of the handlebar (10) is further determined from the second alternating magnetic field signal and / or from the angle φ.Computer program with program code for carrying out a method according to one of Claims 13 or 14 when the computer program is executed on a computer and / or computer-readable data medium with program code of a computer program for carrying out a method according to one of Claims 13 or 14 when the computer program is executed on a computer.A system for measuring an absolute position of a steering rod in a steer-by-wire steering system according to any one of claims 1 to 11, wherein the system is configured to perform a method according to any one of claims 13 or 14.

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