The present invention relates to a torque sensor unit having the features of the preamble of claim 1, to an electromechanical power steering system for a motor vehicle having the torque sensor unit, and to a method for determining a torque introduced into an upper steering shaft of a motor vehicle steering system, having the features of the preamble of claim 14.
Torque sensors are used in a motor vehicle to measure the torque introduced into the steering wheel by a driver. Currently used torque sensors are magnetic sensors whose measurement can be very easily disrupted by external magnetic fields. Motor vehicles in future, and to a certain extent already now, will be operated completely or partially electrically, which can bring about high external field effect measurements as a result of cables which conduct high current and are frequently located in the vicinity of the steering system. Furthermore, currently used magnetic sensors have a low level of accuracy.
The object of the present invention is therefore to specify a torque sensor, which has increased accuracy and a reduced effect of an existing magnetic interference field on the determination of the torque value.
This object is achieved by a torque sensor unit having the features of claim 1 and a method for determining a torque having the features of claim 14.
Accordingly, a torque sensor unit for measuring a torque introduced into an upper steering shaft of a motor vehicle is provided, wherein the upper steering shaft can be connected to a lower steering shaft via a torsion bar, wherein the torque sensor unit has two inductive sensors, wherein a first inductive sensor can be connected to the upper steering shaft in order to measure the rotary position of the upper steering shaft, and a second inductive sensor can be connected to the lower steering shaft in order to measure the rotary position of the lower steering shaft, and wherein an evaluation unit is designed to process the signals of the two inductive sensors, and to calculate the torque therefrom by means of the angle difference which is present between the rotary positions of the two steering shafts. The inductive sensor system on which the torque sensor is based is a contactless sensor technology with a short range, which permits conductive objects in the presence of dust, dirt, oil and moisture to be sensed cost-effectively and with high resolution, which makes the sensor extremely reliable.
An inductive sensor preferably has in each case a carrier plate, which can be connected in a rotationally fixed fashion to the corresponding steering shaft, and a circuit board, which is spatially fixed with respect to the carrier plate, wherein at least one electrically conductive track is arranged on the carrier plate, and a sensing device with two coils, which are part of a resonant circuit, is arranged on the circuit board, and wherein the sensing device is designed to sense the at least one electrically conductive track in order to generate an angle-dependent sensor signal during the rotational movement of the corresponding steering shaft.
It is preferred that the at least one electrically conductive track is closed on itself and extends around the center point of the carrier plate.
The at least one electrically conductive track preferably has a wave pattern which permits absolute angles to be determined over a revolution of the steering shaft.
In one embodiment, a single electrically conductive track is provided per sensor, said track being sensed by the two corresponding coils, wherein the two coils are arranged at an angle of 90 degrees with respect to one another. In this case, the sensing device preferably has an electronic control unit, which is configured to determine the rotational angle of the steering shaft by means of a CORDIC algorithm.
It is advantageous if the circuit board is arranged asymmetrically with respect to the center of the steering shaft, since this refinement permits a particularly compact design.
It is preferred that the at least one electrically conductive track is formed from copper.
There can also be provision that the two coils are configured to be used independently of one another. This permits, for example, the revolutions of the respective steering shaft to be counted or a sector to be detected.
In one preferred embodiment, the coils of the first inductive sensor lie in the longitudinal direction on one side of the sensing devices, and the coils of the second inductive sensor lie on the other side. It is therefore possible to ensure that the inductive sensors detect a signal which has as little interference as possible. It is advantageous here if in each case an electromagnetic shield is provided on the side of the sensing devices, which faces away from the coils, said shield ensuring that the coils of the respective sensing device read out only the assigned track. In addition, there can be provision that the sensing devices of the two inductive sensors preferably lie on opposite sides of the torsion bar in the circumferential direction, in order to minimize interference further.
In addition, an electromechanical power steering system for a motor vehicle is provided, comprising an upper steering shaft which is connected to a steering wheel, and a lower steering shaft which is connected to the upper steering shaft via a torsion bar, a torque sensor unit described above and an electric motor for assisting a steering movement, introduced into the steering wheel by a driver, as a function of the torque measured by the torque sensor unit.
Furthermore, a method for determining a torque introduced into an upper steering shaft of a motor vehicle steering system is provided, wherein the upper steering shaft is connected to a lower steering shaft via a torsion bar, and a first inductive sensor is connected to the upper steering shaft in order to measure the rotary position of the upper steering shaft, and a second inductive sensor is connected to the lower steering shaft in order to measure the rotary position of the lower steering shaft, wherein the method comprises the following steps:
An inductive sensor preferably has in each case a carrier plate which is connected in a rotationally fixed fashion to the corresponding steering shaft, and a circuit board which is spatially fixed with respect to the carrier plate, wherein at least one electrically conductive track is arranged on the carrier plate, and a sensing device with two coils, which are part of a resonant circuit, is arranged on the circuit board, wherein the coils sense at least one electrically conductive track, which rotates with the corresponding steering shaft, extends around the respective steering shaft and is closed on itself, in that a change in a resonant frequency of the resonant circuit is detected.
It is preferred that the at least one electrically conductive track has a wave pattern which permits absolute angles to be determined over a revolution of the steering shaft.
In one advantageous embodiment, in each case a single electrically conductive track is provided which is sensed by two coils, wherein the two coils are arranged at an angle of 90 degrees with respect to one another, and the rotational angle of the corresponding steering shaft is determined from the two coil signals by means of a CORDIC algorithm.
A preferred embodiment of the invention is explained in more detail below with reference to the drawings. Identical or functionally identical components are provided with the same reference symbols in all the figures here. In the drawings:
The first inductive sensor 13 has a first carrier plate 15 which is connected in a rotationally fixed fashion to the upper steering shaft 3, and a first stationary sensing device 16 which is associated therewith and is arranged on a first circuit board 18 which is connected to a first electronic control unit 17. The first carrier plate 15 has a track 19 made of an electrically conductive material, preferably copper. The track 19 is closed on itself and does not have a start or an end. The pattern of the track 19 is preferably a wave pattern which has bent triangular shapes which extend around the center point of the first carrier plate 15. The wave pattern has wave peaks and wave troughs and repeats periodically. The pattern of the track 19 is not formed concentrically with respect to the upper steering shaft 3. It is embodied in such a way that absolute angles can therefore be determined over a revolution of the shaft.
Two coils 80,81 of the first sensing device 16 are arranged on the first circuit board 18. The first circuit board 18 is preferably embodied as a PCB (printed circuit board) and has all the electronic components, in particular an evaluation circuit and the coils 80,81. The first circuit board 18 with the coils 80,81 is located directly under the first copper track 19. The first circuit board 18 is not arranged concentrically with respect to the central axis of the upper steering shaft 3.
The rotational angle of the upper steering shaft 3 is estimated by the first inductive sensor 13 in which the copper track 19 on the first carrier plate 15 is monitored. The first coils 80,81 are parts of a resonant circuit. They 80,81 generate a high-frequency magnetic field. If the track 19 is moved in the magnetic field, an induction current starts to flow owing to the electromagnetic induction. The resonant frequency of the resonant circuit changes on the basis of the mutual inductive coupling. If a non-ferrous metal object, such as for example the copper track, approaches, the resonant frequency of the electrical resonant circuit increases. The mutual inductive coupling therefore changes if the copper track 19 moves away over the coils 80,81. The first sensor 13 monitors the movement of the conductive track 19 with the first carrier plate 15 and/or the rotating upper steering shaft 3 and as a result calculates an absolute angular position. Two coils 80,81 are sufficient to calculate the angle when they are arranged at 90 degrees with respect to one another. The outputting of the two coils 80,81 is, in the case of the triangular pattern described above, a sine signal and a cosine signal. The calculation of the angle is based on the Coordinate Rotation Digital Computer (CORDIC) algorithm according to the industry standard. This algorithm permits elementary trigonometric and hyperbolic functions to be calculated efficiently with almost exclusive use of high-speed operations such as additions and multiplications to the power of two.
The second inductive sensor 14 has the same components as the first inductive sensor 13 and the same method of functioning. The components of the second inductive sensor 14 are characterized by struck through reference symbols of the first inductive sensor 13.
In this context, the first sensing device 16 and the second sensing device 16′ are arranged within the first carrier plate 15 and the second carrier plate 15′. The coils of the sensing devices 80,81,80′,81′ therefore lie in the longitudinal direction on opposite sides of the sensing device 16,16′ or of the electronic control units 17,17′. On the respective other side of the electronic control unit 17,17′ an electromagnetic shield 20,20′ is provided which ensures that the coils of the respective sensing device 80,81,80′,81′ read out only the assigned track 19,19′ and are not subject to interference in the process. The sensing devices 16,16′ therefore also lie preferably on opposite sides of the torsion bar 12 in the circumferential direction.
The torque which acts on the upper steering shaft 3 is calculated from the angle difference between the angles measured by the two inductive sensors 13,14:
TSWT=c*δ, where c is the spring constant of the torsion bar and δ is the angle difference.
A plurality of circuit boards, each with two coils, can be used in order to permit a high redundancy as well as capability of electronic fault compensation (fault orientation, mechanical faults). The coil pairs can be arranged in pairs on separate PCBs or on a common PCB.
The two inductive sensors 13,14 can be used independently of one another, for example to count the revolutions of the steering shafts or to detect a sector. However, they can also be used together, for example in a steering angle sensor with a reduction gear mechanism which functions according to the Nonius principle.
Filing Document | Filing Date | Country | Kind |
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PCT/EP2019/074351 | 9/12/2019 | WO | 00 |