DE102011015696B4 - Method for setting at least one vehicle-specific friction value, method for adjusting at least one vehicle-specific friction value and device for adjusting at least one vehicle-specific friction value - Google Patents

Abstract

Method for setting at least one initial vehicle-specific friction value (RW _l ), the at least one initial friction value (RW _l ) representing friction in a steering system of a motor vehicle, the steering system comprising at least one rack (8), and
a vehicle-specific displacement force is determined, the displacement force being determined as a function of a curve of a force required to displace the rack (8) over an entire displacement path of the rack (8),
characterized in that
Before the steering system is put into operation, the vehicle-specific displacement force is determined, with the displacement force being determined as the mean value of the curve of the force required to move the rack or as a maximum force of the curve of the force to move the rack, with the at least one initial Friction value (RW _l ) is determined.

Description

The invention relates to a method for setting at least one vehicle-specific friction value according to the preamble of claim 1 and a method and a device for adjusting at least one vehicle-specific friction value according to the preamble of claim 2 or according to the preamble of claim 10.

So-called power steering systems are often used in modern motor vehicles. Such steering systems include an actuator for generating a support torque, which can apply this support torque to a rack or a steering column of the steering system. A height of the support moment is z. B. depending on a z. B. by a motor vehicle driver by means of a steering handle, z. B. a steering wheel, applied manual torque determined.

In 1 a typical structure of an electro-mechanical operating steering system of a motor vehicle is shown, which has a steering wheel 1 that is connected via a first section 2 of a steering rod 13 by means of one or more universal joints 7 to a second section 3 of the steering rod 13. The steering rod 13 transmits a moment applied by a motor vehicle driver of the motor vehicle to the steering wheel 1 to a pinion 6 which engages in a toothed rack 8 which is arranged horizontally to the axis of the motor vehicle between steered wheels 11 . The pinion 6 can also be formed by any other transmission means, e.g. B. a worm shaft. Each steered wheel 11 is capable of rotating about a vertical axis of rotation A upon linear movement of the rack 8 , the steered wheel 11 being driven by a linkage 10 via the rack 8 .

The steering system also includes a servomotor 9, the output torque of which is controlled by an electronic control unit 12, which transmits a target value signal S of a support torque to the servomotor 9 using data technology. The output torque of the servomotor 9 is transmitted to the toothed rack 8 and thus to the wheels 11 by means of a drive shaft of the servomotor 9 (not shown). Due to the considerable torques to be transmitted, the drive shaft of the servo motor 9 usually acts on the toothed rack 8 via a ball gear 14 (not shown in detail), in particular via a ball screw drive.

The output shaft of the servomotor 9 is thus mechanically connected to the steering rod 13 via the ball gear 14, the rack 8 and the pinion 6. However, the mechanical connection between the output shaft and the steering rod 13 can also be direct, in that the output shaft acts directly on the steering rod 13 via a suitable gear. The output shaft of the servomotor 9 supports the steering angle of the steering wheel 1 by exerting an auxiliary torque on the steering rod 13 by means of the above-mentioned mechanical devices, which torque depends directly on the output torque of the servomotor 9 and consequently on the setpoint signals S.

The control unit 12 is usually constructed in such a way that, due to the incoming input signals, e.g. B. the manual torque DM and / or the angle of rotation DW of the handlebar 13, the amount of the exerted by the servomotor 9 auxiliary torque and transmits the corresponding setpoint S to the servomotor data technology. With the help of suitable tables stored in control unit 12, the auxiliary torque is generally determined in such a way that an auxiliary torque to be applied by servomotor 9 is determined as a function of the calculated difference in the angle of rotation and a steering angle of wheels 11 measured by steering angle sensor 15 . This auxiliary torque is selected to be large enough that, with regard to the total torque to be applied to actuate the wheels 11, a residual torque remains on the steering wheel 1, which the motor vehicle driver can easily control. As a rule, the auxiliary torque will also depend on variables that have an influence on the steering torque of the wheels 11, such as rotational angular velocity, temperature, driving condition of the vehicle, road conditions and other boundary conditions.

Furthermore, special forms of electromechanically operating steering systems have become known, in which the mechanical connection between the steering wheel 1 and the rack 8 is separated. In these so-called steer-by-wire steering systems, the forces and moments to be applied by the steering wheel 1 to the rack 8 must be simulated by appropriate electrically operated devices, although the other basic principles described above are retained.

In the steering systems described, a motor control variable for the servo motor 9 is determined by multiplying a manual torque DM acting on a steering wheel by a specific servo control amplification factor. This factor, which may also depend on other state variables, is predetermined based on tests carried out on a test vehicle in order to provide a desired servo characteristic. It is desirable here that, at a specific hand moment, which is detected by a torque sensor coupled to the steering wheel 1 4 is detected, a defined steering torque is exerted on the steering wheels 11 .

In practice, however, the steering torque actually acting on the wheels can deviate considerably from the steering torque desired via the servo characteristics. There can be a number of reasons for this, such as a change in the inertia of the system or the properties of the servomotor 9. However, a major reason is usually the friction of the system. Electromechanical steering systems have an inherent friction that results from the connection of the electric servomotor 9 and a reduction gear coupled to it as a rule, as well as the friction of the pinion on the rack 8 and the slide bearing of the rack 8 itself. Since the friction effects are generally not linear, this also results in a non-linear dependency of a movement of the wheels on the output torque generated by the servomotor 9 . The size of the influence of friction must not be underestimated. In the worst case, the friction torque can increase by up to 30% (see US 2004/0138797 A1 , [0029]) of the torque delivered by the servomotor 9. Furthermore, the discontinuities and slip-stick effects caused by non-linearities of the friction are disadvantageous for the steering feel.

A further deviation from a friction determined by means of a model can result from manufacturing errors and deviating operating data of the assemblies used. If such assemblies have a friction that is significantly greater than measured in the vehicle and assumed for the series, then part of the output torque of the electric motor 9 is used to compensate for the friction, which can lead to a spongy feeling when the steering wheel 1 is operated . A friction assumed for the series steering system can also turn out to be incorrect due to assembly errors, for example during repair work. For example, the reduction gear comprises a worm drive and a worm wheel with a comparatively high reduction ratio. In order for the reduction gear to prevent or significantly reduce resetting and rattling noises, the worm drive is pressed strongly against the worm wheel. This leads to relatively large changes in friction with assembly errors, because the amount of friction between the worm drive and the worm wheel pressed against each other is large and has a significant influence on the change in servo characteristics.

In summary, it can be said that the influence of friction is detrimental to the steering feel, can lead to significant discontinuities between the required target torque and the steering torque actually exerted on the wheels, can adversely affect the accuracy of the steering and sensitive steering control Steering, as is necessary, for example, when the wheels are automatically returned to the central position, can impede. In addition, the friction can change over time, for example by reducing the friction due to grinding in the system or increasing it due to contamination. Changes in the material condition or the repair or replacement of a part of the system can also lead to subsequent changes in friction here.

the DE 10 2008 021 848 A1 discloses a method for accounting for static and dynamic friction in a static and dynamic friction system. Here, a friction model is determined, which describes the frictional behavior of all steering systems in a series of steering systems in an initial state. Furthermore, to determine a current friction level of the steering system, the friction assigned to a defined speed of a moving part in the model is determined, with the current actually existing friction at the defined speed being measured, with the difference between the determined and the actually measured friction being the current Friction difference is determined, wherein further to determine the current friction of the steering system at a speed deviating from the defined speed of the moving part to the friction resulting from the model at this deviating speed, the determined current friction difference is added as an offset.

The disadvantage here is the relatively complex determination of a friction model and the determination of only a single friction model, which is used as a basis for all steering systems in a series.

The showing the preamble of claim 1 DE 10 2008 021 862 A1 relates to a method in which the fluctuations in a steering torque or the acting forces that result when the pinion engages in the rack are determined in a suitable form and fluctuations in the power transmission are eliminated or at least reduced depending on a currently applicable steering angle of the wheels.

the U.S. 2003/006088 A1 relates to an electric power steering system for a vehicle. The electric power steering includes a steering gear with a rack and a housing. The rack is linearly movable relative to the housing to turn steerable wheels of the vehicle. Of the Movement of the rack relative to the housing is opposed by friction. An electric motor is connected to the rack of the steering gear.

The showing the preamble of claim 2 and claim 10 respectively DE 102 064 74 A1 relates to a method for detecting friction in an electric or electromechanical steering device of a motor vehicle.

the DE 10 2008 021 848 A1 relates to a method and system, in particular a power steering system for motor vehicles. Such steering systems usually include a control loop whose controlled variable is dependent at least on the steering wheel torque. The default functions stored in a control unit are used to calculate the target value of the supporting torque as a function of input variables from the control unit, in particular the steering wheel torque.

the US2004/138797 A1 relates generally to friction compensation in a steering system for a vehicle, and more particularly to friction compensation in a steer-by-wire system using model-based and non-model-based friction compensation methods. The technical problem arises of creating a method for setting at least one vehicle-specific friction value and a method and a device for adjusting at least one vehicle-specific friction value, which enables the determination and setting of the friction value that is adapted as precisely as possible to a specific vehicle or a specific steering system, as well as a enable easy-to-implement adaptation of this vehicle-specific friction value.

The technical problem is solved by the objects with the features of claims 1, 2 and 10. Further advantageous configurations of the invention result from the dependent claims.

The invention relates to a method for setting at least one initial vehicle or steering-specific friction value according to the preamble of claim 1, the at least one friction value representing or characterizing friction in a steering system of a motor vehicle. The steering system can in particular be a so-called servo-steering system in which an actuator, e.g. B. an electromechanical or a hydraulic actuator, a support torque to support a steering maneuver of a motor vehicle driver generated. The steering system comprises at least one rack, previously described in 1 was described. The at least one friction value can represent or characterize static and/or dynamic friction in the steering system. The at least one friction value can e.g. B. be a parameter of a model of friction in the steering system. A first model can e.g. B. as $$\begin{array}{l}\mathrm{\tau =}{\text{T}}_{\text{c}}\text{sign}\left(\text{v}\right)\text{v}\ne \text{0}\\ \mathrm{\tau =}{\text{T}}_{\text{s}}\text{v}=\text{0}\end{array}$$

beschrieben werden, wobei b einen Koeffizienten einer viskosen Reibung bezeichnet. Tc, Ts und b sind hierbei Parameter der jeweiligen Modelle.where τ denotes the coefficient of friction, Tc denotes an amount of so-called Coulomb friction, Ts denotes an amount of breakaway force, and v denotes a speed of the rack. The friction can also be calculated using another model $$\begin{array}{l}\mathrm{\tau =}{\text{T}}_{\text{c}}\text{sign}\left(\text{v}\right)+\text{b*vv}\ne \text{0}\\ \mathrm{\tau =}{\text{T}}_{\text{s}}\text{v}=\text{0}\end{array}$$

will be described, where b denotes a coefficient of viscous friction. Tc, Ts and b are parameters of the respective models.

For example, the at least one vehicle-specific friction value can therefore denote the amount of Coulomb friction. Another vehicle-specific friction value can refer to the coefficient of viscous friction. Another vehicle-specific friction value can indicate the magnitude of the breakaway torque. The at least one vehicle-specific friction value is characteristic of the static, sliding and/or rolling friction present in the steering system, in particular for the frictional forces acting on the rack.

The at least one vehicle-specific friction value can also be a steering speed-dependent friction value. The vehicle-specific friction value is a function of a movement speed of a rack of the steering system z. B. during a steering maneuver. The at least one vehicle-specific friction value can also be dependent on a steering direction. In this case, the at least one vehicle-specific friction value is dependent, for example, on a left-hand steering maneuver or a right-hand steering maneuver and the displacement direction of the steering rack of the steering system that is dependent thereon.

Depending on the at least one vehicle-specific friction value z. B. the in 1 shown control unit 12 determine the level of a required support torque and generate an input signal for an actuator, in particular an electromechanical servo motor. In particular, a support torque can be increased as a function of the at least one vehicle-specific friction value such that that the friction in the steering system is compensated during a steering maneuver, i.e. balanced.

According to the invention, a vehicle-specific displacement force is determined before the steering system is put into operation. Here, a steering-specific displacement force is determined for each steering system, and not just for a single steering system in a series of steering systems. The displacement force is determined as a function of a curve of a force necessary for displacement of the toothed rack over an entire displacement path of the toothed rack. For this purpose, z. B. directly after assembly of the steering system, a force can be applied to the rack so that the rack is moved once over its entire possible adjustment path. The force required for this can, for. B. be detected and stored by a force sensor. According to the invention, the displacement force is determined as the mean value of the course of the force required to displace the toothed rack. Alternatively, the displacement force is determined as a maximum force of the course of the force for displacement of the rack. The displacement force can be determined and stored, for example, at the end of the assembly of a steering system and before delivery to a vehicle manufacturing plant. Displacement force can also be determined after the steering system has been installed in the motor vehicle.

Furthermore, the at least one initial vehicle-specific friction value is determined as a function of the displacement force. In particular, the at least one initial vehicle-specific friction value can be equal to the displacement force. Several vehicle-specific friction values, e.g. B. the parameters listed above can be determined by evaluating a friction model. Of course, the at least one initial vehicle-specific friction value can also be determined from several curves of a force required to adjust the rack, in particular one for moving the rack back and forth several times over its entire adjustment path.

The proposed method advantageously results in the at least one initial vehicle-specific friction value being determined individually for a steering system of a specific motor vehicle, thereby enabling more precise and reliable compensation of the friction for this motor vehicle. In particular, this avoids inaccuracies that arise when a friction in a steering system of a motor vehicle deviates from the reference friction, which was determined as a representative friction for this series of steering systems.

A method for adjusting at least one vehicle-specific friction value is also proposed. The vehicle-specific friction value can have been determined using the previously described method for setting at least one initial vehicle-specific friction value. However, it can also be a reference value determined for a representative steering system of a series of steering systems as a friction value applicable to all steering systems of this series. The at least one vehicle-specific friction value can be specified as previously described. The at least one friction value represents or characterizes friction in a steering system of a motor vehicle, the steering system comprising at least one toothed rack.

According to the invention, the at least one vehicle-specific friction value is adjusted at least as a function of how often at least one predetermined adjustment position of the rack is reached. An adjustment position denotes a specific position of the toothed rack along the adjustment path of the toothed rack. In particular, an adjustment position is related to a neutral position of the rack, the neutral position of the rack corresponding to a straight-ahead position of steerable wheels of the motor vehicle, the steerable wheels being mechanically coupled to the rack. The adjustment position can be specified in terms of size as a function of this neutral position by a deflection or displacement with respect to this neutral position. For example, an adjustment position can be described as x=20mm, where the rack has been adjusted or shifted 20mm in a predetermined direction from the neutral position. An adjustment position of x=−20 mm consequently corresponds to a deflection or displacement of the toothed rack by 20 mm from the neutral position in a direction opposite to the first direction. A sign of the adjustment position thus designates a direction of displacement and thus a steering direction.

The proposed method advantageously results in the at least one vehicle-specific friction value being able to adapt to a friction of the steering system that changes over time. For example, the at least one vehicle-specific friction value can be adapted to reduced friction as a result of grinding in the steering system. It can be assumed here that with increasing frequency of reaching a predetermined adjustment position, grinding in or running-in of the steering system also increases, as a result of which the amount of frictional forces is generally reduced.

An adjustment position of the rack can be z. B. be detected by a position sensor.

In a preferred embodiment, however, an adjustment position of the rack is determined as a function of a steering angle of a steering column, the steering column being mechanically connected to the rack. A transmission ratio of a gear arranged between the rack and the steering column can be previously known, so that an adjustment position of the rack can be calculated from a steering angle of the steering column, which is usually detected with an existing steering angle sensor, by means of a simple conversion.

Alternatively, it is conceivable to determine the adjustment position of a toothed rack as a function of a rotor position of a rotor of an electromechanical servomotor, the rotor being mechanically coupled to the toothed rack by means of a mechanical transmission. An adjustment position of the toothed rack can thus be determined as a function of a rotor position and a previously known transmission ratio of a transmission of the mechanical transmission between the rotor of the servomotor and the toothed rack.

Another alternative is to determine the adjustment position as a function of a pinion angle. The pinion angle describes a position of a steering pinion, which forms a mechanical coupling between the steering column and the rack. For example, the adjustment position can be determined from the pinion angle by means of a previously known transmission ratio between the pinion angle and the adjustment position. The pinion angle can, for example, be detected by means of a suitable sensor and/or determined as a function of the previously mentioned rotor position, for example assuming a predetermined transmission ratio between the rotor position and the pinion angle.

It is also possible to express an adjustment position of the toothed rack as a pinion angle, e.g. as a function of the previously known transmission ratio between the pinion angle and the adjustment position, and to carry out the method according to the invention on the basis of a pinion angle instead of an adjustment position. Here, for example, predetermined minimum and maximum adjustment positions or displacements can also be expressed as predetermined minimum and maximum pinion angles.

In this way, an adjustment position of the toothed rack can be advantageously determined by means of sensors already present in the steering system, thereby reducing additional development and component costs and reducing the space required for the elements required to carry out the method according to the invention.

In a further embodiment, the vehicle-specific friction value, in particular an amount of the vehicle-specific friction value, is reduced with increasing frequency of reaching the at least one predetermined adjustment position of the steering rack. In this way, a grinding-in or run-in behavior of a steering system is advantageously taken into account, with a frictional force of the steering system decreasing with increasing frequency of reaching a predetermined adjustment position.

In a further embodiment, the vehicle-specific friction value or a portion of the vehicle-specific friction value is multiplied by a run-in factor, with the run-in factor being reduced with increasing frequency of reaching the at least one predetermined adjustment position of the steering rack. It is conceivable that the vehicle-specific friction value is multiplied by a run-in factor of size 1 if the rack has not yet reached the at least one predetermined adjustment position, for example because a steering of the motor vehicle has not yet been actuated. With increasing frequency of reaching the at least one predetermined adjustment position, ie with increasing frequency of actuation of the steering system, the run-in factor can be reduced from the value 1. In this case, a decrease or reduction can have a linear progression with increasing frequency. However, the reduction preferably exhibits an exponentially falling profile with an increasing frequency of reaching the at least one predetermined adjustment position. A reduction in the at least one running-in factor can be limited to a predetermined minimum of the running-in factor, for example to a minimum of 0.5. As a result, it is advantageously taken into account that a steering system will always have a certain amount of friction, even if it is operated very frequently. Furthermore, the proposed reduction and multiplication with the run-in factor results in a reduction in the friction value that is easy to implement as a function of an increasing frequency of reaching the at least one predetermined adjustment position.

In a further embodiment, several predetermined adjustment positions are defined over an entire adjustment path of the toothed rack, with a counter value being incremented by a position-specific value assigned to the respective adjustment position when the toothed rack assumes or reaches the respective adjustment position. The vehicle-specific friction value is steep corresponding counter value. For example, the predetermined adjustment positions can delimit equal intervals of adjustment paths. For example, predetermined adjustment positions can be arranged at predetermined equal distances, for example every 5, 10 or 20 mm, along the adjustment path. If the entire adjustment path of the rack includes, for example, an interval of -80 mm to 80 mm, then predetermined adjustment positions are defined, for example, at positions x=-80 mm, -70 mm, . . . 70 mm, 80 mm.

Of course, the predetermined adjustment positions can also delimit intervals of adjustment paths that are not of the same size. In this case, predetermined adjustment positions can be arranged with predetermined unequal distances along the adjustment path. For example, more predetermined adjustment positions can be defined in a predetermined interval around the neutral position, for example in an interval of -30 mm to 30 mm, than in the remaining intervals of -80 mm to -30 mm and 30 mm to 80 mm. It should be noted here that the values described for the entire adjustment path and the values of the predetermined adjustment positions are given purely as examples; values that deviate from these are also possible.

This advantageously results in the simplest possible implementation of a determination of a frequency of reaching a predetermined adjustment position and an adaptation of the vehicle-specific friction value to the frequency determined in this way.

In a further embodiment, the position-specific values assigned to the respective adjustment positions increase as the absolute value of the respective adjustment position increases. For example, a first value can be assigned to a predetermined adjustment position that has a small amount, and a second value can be assigned to another predetermined adjustment position that has a larger amount, the second value being greater than the first value. In this way, it is advantageously taken into account that adjustment positions which are high in terms of magnitude and which correspond, for example, to large steering angles, are reached less frequently than adjustment positions which are smaller in terms of magnitude. If high adjustment positions are reached in this way from the top down, this reaching contributes more to the grinding in or running-in of the entire steering system than reaching lower adjustment positions in terms of amount several times. This advantageously achieves a better adaptation of the at least one vehicle-specific friction value to a running-in behavior or grinding-in behavior of the steering system.

In a further embodiment, the frequency with which an adjustment position is reached is weighted as a function of a rack force required to reach the adjustment position. A necessary rack force can be detected using a suitable force sensor. It is also conceivable to determine a necessary rack force as a function of a steering torque which is detected by a steering torque sensor and is applied to a steering column. If high rack forces are necessary to reach a predetermined adjustment position, it can be assumed that high frictional forces are present. In this case, the frequency should preferably be weighted in such a way that the at least one vehicle-specific friction value is not excessively reduced when the predetermined adjustment position is reached. For example, a frequency can be multiplied by a rack force-dependent factor, with the rack force-dependent factor increasing as the rack force for reaching the adjustment position decreases.

In a preferred embodiment, a counter value is incremented by a position-specific value assigned to a predetermined adjustment position whenever the rack assumes the respective adjustment position, with the position-specific value assigned to the respective adjustment position being weighted with a rack force required to reach the adjustment position, the vehicle-specific Friction value is reduced as the count increases. In particular, the position-specific value assigned to the respective adjustment position can be reduced as the rack force increases.

This advantageously results in an adaptation of the at least one vehicle-specific friction value being better adapted to the friction conditions actually present in the steering system. In particular, it is not exclusively assumed that a friction value decreases as the frequency with which predetermined adjustment positions are reached increases. By considering the rack force z. B. can also be achieved that the at least one predetermined friction value z. B. due to an existing pollution is increased. In this case, the frequency, in particular the count value, can also be reduced by the weighting using the rack force, which brings about an increase in the at least one vehicle-specific friction value. For this purpose, a weighting factor can be negative, for example.

Also proposed is a device for adapting at least one vehicle-specific schen friction value according to the preamble of claim 10. The at least one friction value represents or characterizes a friction in a steering system of a motor vehicle. The steering system includes at least one rack. The device according to the invention comprises at least one evaluation device and one memory device. According to the invention, the frequency of reaching at least one predetermined adjustment position of the rack can be determined by means of the evaluation device and the at least one vehicle-specific friction value stored in the memory device can be adjusted at least as a function of the frequency of reaching.

A device for setting at least one initial vehicle-specific friction value is also proposed. A steering system includes at least one rack. The device comprises at least one device for detecting a vehicle-specific displacement force and an evaluation device. By means of the device for detecting a vehicle-specific displacement force, for example a force sensor, a force necessary for displacing the toothed rack along its displacement path can be detected or determined. The initial vehicle-specific friction value can then be determined by means of the evaluation device as a function of the course of this force over an entire adjustment path of the steering rack. This can then be stored, for example, in a memory device.

• 1 eine schematische Darstellung eines Lenksystems (Stand der Technik),
• 2 eine erste Zuordnung von Werten zu vorbestimmten Lenkwinkeln,
• 3 eine weitere Zuordnung von Werten zu vorbestimmten Lenkwinkeln,
• 4 ein schematisches Flussdiagramm eines erfindungsgemäßen Verfahrens zur Anpassung mindestens eines fahrzeugspezifischen Reibungswertes,
• 5 ein weiteres schematisches Flussdiagramm eines Verfahrens zur Anpassung mindestens eines fahrzeugspezifischen Reibungswertes,
• 6 ein schematisches Blockschaltbild einer Vorrichtung zur Anpassung mindestens eines fahrzeugspezifischen Reibungswertes und
• 7 ein schematisches Flussdiagramm eines Verfahrens zur Einstellung mindestens eines initialen fahrzeugspezifischen Reibungswertes.

The invention is explained in more detail using an exemplary embodiment. The figures show:

• 1 a schematic representation of a steering system (prior art),
• 2 a first assignment of values to predetermined steering angles,
• 3 a further assignment of values to predetermined steering angles,
• 4 a schematic flowchart of a method according to the invention for adjusting at least one vehicle-specific friction value,
• 5 a further schematic flowchart of a method for adjusting at least one vehicle-specific friction value,
• 6 a schematic block diagram of a device for adjusting at least one vehicle-specific friction value and
• 7 a schematic flowchart of a method for setting at least one initial vehicle-specific friction value.

In the following, the same reference symbols designate elements with the same or similar technical properties.

1 shows a schematic overview of an electromechanical steering system, which was initially explained in more detail.

In 2 a first assignment of position-specific values W _P to predetermined steering angles LW is shown. The position-specific values W _P always become a current counter value ZW (see 4 ) added if a steering column 13 (see 1 ) reaches the predetermined steering angle LW. Here, a predetermined adjustment position of a rack 8 (see 1 ) represented by the steering angle LW. A predetermined steering angle LW thus corresponds to a predetermined adjustment position of the rack 8. In 2 it is shown that predetermined steering angles LW delimit equal intervals of steering angles, namely intervals of 10°. Predetermined steering angles LW are thus distributed at equal intervals of 10° in a permissible range of steering angles, with the permissible range of steering angles corresponding to an entire displacement path of the toothed rack 8 . A steering angle of 0° designates a neutral position, which corresponds to a straight-ahead driving position of steerable wheels 11 (see 1 ) is equivalent to. Thus, the counter value ZW is always increased by the in 2 shown position-specific value W _P = 5 is incremented if the steering column 13 is deflected by 10°, 20°, etc. The values W _P =5 and the predetermined steering angles LW are selected here by way of example. Of course, a different distribution of predetermined steering angles over an overall permissible range of steering angles, which is 2 represented as a range from -520° to 520° is possible. Values different from W _P =5 can also be assigned in predetermined steering angles LW.

In 3 a further assignment of values W _P to predetermined steering angles LW is shown. In this case, the predetermined steering angles LW delimit intervals of different sizes. Specifically, predetermined steering angles LW are set only at the steering angles -520°, -100°, -20°, 20°, 100°, 520°. At the same time, it is shown that a steering angle-specific value W _P increases as the amount of the predetermined steering angle LW increases.

berechnet. Sodann wird zum zweiten Schritt S2 zurückgekehrt.In 4 a schematic flowchart of a method according to the invention for adapting at least one vehicle-specific friction value RW is shown. In a first step S1, a counter value ZW is initialized, e.g. B. with the value 0. At the same time in the first step S1 Vehicle-specific friction value RW initialized, for example with an initial friction value RW _I. In a second step S2, a deflection of a rack 8 (see 1 ) detected, for example by detecting a steering angle LW (see 2 ). In a third step S3, it is checked whether a steering column 13 (see 1 ) has reached a predetermined steering angle LW. If this is not the case, a return is made to the second step S2. If the steering column 13 has reached a predetermined steering angle LW, the counter value ZW is incremented by a steering-angle-specific value W _P assigned to this predetermined steering angle LW (fourth step S4). In a fifth step _S5 , a new friction value RW is calculated based on z. B. the following calculation rule $$\text{RW}={\text{RW}}_{\text{l}}*0.5+0.5*{\text{RW}}_{\text{l}}*{\text{ex}}^{\left(-\text{between}\right)}$$

calculated. Then it returns to the second step S2.

In 5 Another schematic flowchart of a method for adjusting at least one vehicle-specific friction value RW is shown. The first, second and third steps S1, S2, S3 correspond to the first, second and third steps S1, S2, S3 of FIG 4 illustrated procedure. In an intermediate step S3a, if a steering column 13 (see 1 ) has reached a predetermined steering angle LW, a rack force F _z required to achieve this steering angle LW is determined. For example, this rack force Fz from a force sensor 20 (see 6 ) are detected, which generates an output signal as a function of a force F applied to a toothed rack 8. In a fourth step S4*, the counter value is then incremented by a weighted value W _P , a weighting being determined as a function f(Fz) as a function of the rack force Fz detected. In a fifth step S5 is analogous to 4 an updated friction value RW is determined.

In 6 is a schematic block diagram of a device for adapting at least one vehicle-specific friction value RW (see 4 ) shown. The device comprises at least one evaluation device 21 and one memory device 22. The evaluation device 21 can be the control unit 12 (see FIG 1 ) or be part of it. The evaluation device 21 is connected in terms of data technology to a force sensor 20 for detecting a force F, the force sensor detecting a rack force Fz. The evaluation device 21 is also connected to a steering angle sensor 23 for detecting a steering angle LW. The evaluation device 21 is also connected to a steering torque sensor 24 for detecting a manual torque DM. The evaluation device 21 can be one of the 4 or in 5 Carry out the method shown and determine a setpoint signal for a servo motor 9 as a function of a friction value RW and the manual torque DM.

In 7 a schematic flow chart of a method for setting at least one initial vehicle-specific friction value RW _l is shown. Here, in a first step S11, a rack 8 (see 1 ) adjusted over an entire displacement of the rack 8. In a second step S12, the necessary displacement force z. B. can correspond to the rack force Fz over the entire adjustment, which can be represented by a steering angle profile, determined. In a third step S13, an initial friction value RW ₁ is determined as the maximum of the force F _z determined in the second step S12.

Reference List

1

steering wheel

2

first section

3

second part

6

pinion

7

universal joint

8th

rack

9

servo motor

11

Wheels

12

control unit

13

steering rod, steering column

14

ball gear

15

steering angle sensor

20

force sensor

21

evaluation device

22

storage facility

24

steering torque sensor

Lw

predetermined steering angle

WP

value

between

count value

RW

friction value

RWL

initial friction value

vehicle

rack force

f

power

S1

first step

S2

second step

S3

Third step

S4

fourth step

S4*

fourth step

S3a

intermediate step

S5

fifth step

S11

first step

S12

second step

S13

Third step

DM

hand moment

DW

angle of rotation

Claims (10)

Method for setting at least one initial vehicle-specific friction value (RW _l ), the at least one initial friction value (RW _l ) representing friction in a steering system of a motor vehicle, the steering system comprising at least one rack (8), and a vehicle-specific displacement force being determined, wherein the displacement force is determined as a function of a curve of a force necessary for displacement of the rack (8) over an entire displacement path of the rack (8), characterized in that the vehicle-specific displacement force is determined before the steering system is put into operation, the displacement force being the mean value of the The course of the force required to shift the rack or as a maximum force of the course of the force to shift the rack is determined, with the at least one initial friction value (RW ₁ ) being determined as a function of the shifting force. Method for adapting at least one vehicle-specific friction value (RW), the at least one friction value (RW) representing friction in a steering system of a motor vehicle, the steering system comprising at least one rack (8), characterized in that the at least one vehicle-specific friction value (RW ) is adjusted at least depending on a frequency of reaching at least one predetermined adjustment position of the rack (8). procedure after claim 2 , characterized in that an adjustment position of the rack (8) is determined as a function of a steering angle (LW) of a steering column (13), the steering column (13) being mechanically connected to the rack (8). procedure after claim 2 or 3 , characterized in that the vehicle-specific friction value (RW) is reduced with increasing frequency of reaching the at least one predetermined adjustment position of the rack (8). procedure after claim 4 , characterized in that the vehicle-specific friction value (RW) or a portion of the vehicle-specific friction value (RW) is multiplied by a run-in factor, the run-in factor being reduced with increasing frequency of reaching the at least one predetermined adjustment position of the rack (8). Procedure according to one of claims 2 until 5 , characterized in that several predetermined adjustment positions are defined over an entire adjustment path of the toothed rack (8), a counter value ( _ZW ) always being incremented by a position-specific value (WP ) assigned to the respective adjustment position when the toothed rack (8) occupies the respective adjustment position, with the vehicle-specific friction value (RW) being reduced as the count value (ZW) increases. procedure after claim 6 , characterized in that the position-specific values (W _P ) assigned to the respective adjustment positions increase as the absolute value of the adjustment position increases. Procedure according to one of claims 2 until 7 , characterized in that the frequency with which an adjustment position is reached is weighted as a function of a rack force (F _Z ) required to reach the adjustment position. procedure after claim 8 , characterized in that a counter value ( _ZW ) is always incremented by a position-specific value (WP ) assigned to an adjustment position when the toothed rack (8) assumes the respective adjustment position, the position-specific value ( _WP ) assigned to the respective adjustment position also being incremented a rack force (F _Z ) required to reach the adjustment position, with the vehicle-specific friction value (RW) being reduced as the count value (ZW) increases. Device for adapting at least one vehicle-specific friction value (RW), the at least one friction value (RW) representing friction in a steering system of a motor vehicle, the steering system comprising at least one rack (8), the device having at least one evaluation device (21) and a Storage device (22), characterized in that by means of the evaluation device (21) a frequency of reaching at least one predetermined adjustment position of the toothed rack (8) is determined and the at least one vehicle-specific friction value (RW) stored in the storage device (22) is adjusted at least as a function of the frequency with which it is reached.

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