Steering system with active compensation for road disturbances

A vehicle includes an adaptive front steering system including an assist motor that provides an overlay angle superimposed on a steering wheel angle. External torque to the assist motor is estimated by a controller and evaluated to determine whether it is the result of an external disturbance. If so, the controller evaluates whether the external disturbance is a road disturbance, such as by determining that road wheel speed varied from the vehicle speed or that an impact site is present in outputs of external sensors of the vehicle. If a road disturbance is determined to have occurred, compliance may be introduced in order to reduce the transmission of torque to the steering wheel. Compliance may be introduced by adjusting current to the assist motor, a target angle to the assist motor, reducing gain in the control system for the assist motor, or shorting leads of the assist motor.

BACKGROUND

Field of the Invention

This invention relates to power steering systems and, more particularly to adaptive power steering systems.

Background of the Invention

Active steering systems may continuously and intelligently alter the relationship between a driver's steering inputs at the steering wheel and the angle of the steered road wheels (i.e., the road wheel angle (RWA)) of the vehicle. An active steering system, for example, varies the degree that the road wheels turn in response to rotation of the steering wheel via a mechatronic auxiliary system. At lower speeds, this technology may reduce the amount that the steering wheel must be turned (improving performance in situations such as parking and other urban area traffic maneuvers), and at higher speeds, the system helps to prevent increased steering responsiveness resulting from increased speeds (providing improved direction stability).

Examples of active steering systems include, but are not limited to, a steer-by-wire system, in which there is no mechanical connection between the steering wheel and the steering of the road wheels, and a superimposed steering system, in which the steering angle that is generated at the steering wheel by the driver is superimposed on a steering angle generated by a servo motor, in a superimposition gear mechanism, and the sum of the two steering angles is transmitted to the steering of the road wheels. Such active steering systems provide various advantages, including, for example, the ability to change the angle of the steered road wheels without the intervention being directly perceptible to the driver.

The system and methods disclosed herein provide an improved approach for implementing an active steering system in order to deal with road disturbances that may transmit torque to the driver's hands.

Although the following detailed description makes reference to illustrative embodiments, many alternatives, modifications, and variations thereof will be apparent to those skilled in the art. Accordingly, it is intended that the claimed subject matter be viewed broadly.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings. However, these various exemplary embodiments are not intended to limit the disclosure. To the contrary, the disclosure is intended to cover alternatives, modifications, and equivalents.

An active steering system, such as, for example, Active Front Steering (AFS) provides an electronically controlled superimposition of an angle to the steering wheel angle provided by the driver of the vehicle. The amount of this superimposed angle (“hereinafter the superimposition angle”) is determined by the AFS system based on the steering state of the steering system.

As used herein, the “steering state” refers to some or all of the steering wheel angle, steering wheel rotational direction, steering wheel angle rate of change, road wheel angle, road wheel angle rate of change, vehicle speed, vehicle acceleration, vehicle lateral acceleration, and yaw rate at a particular point in time (i.e., the steering situation created by the driver of the vehicle at a given point in time).

Various exemplary embodiments of the present disclosure contemplate using various vehicle sensors to provide signals to a controller, and the controller then utilizes the signals to both determine the steering state and to detect and counter disturbances. The vehicle sensors may include existing vehicle sensors when available, such as, for example, active steering sensors, such as, for example AFS sensor(s), antilock brake system (ABS) sensor(s), steering wheel angle sensors, steering wheel velocity sensors, wheel speed sensors, vehicle speed sensors, and an actuator angle sensor, a pinion angle sensor, or any combination thereof. Because most existing vehicles already contain the above sensors, certain embodiments of the present disclosure contemplate using only existing vehicle sensors. Embodiments of the present disclosure also contemplate systems and methods that include and utilize additional sensors as needed to provide the signal inputs used in the systems and methods of the present disclosure.

Turning now to the drawings,FIG. 1shows the structure of an exemplary embodiment of a system20for correcting steering offsets in accordance with the present disclosure. A motor vehicle1includes an active steering system, such as, for example, an Active Front Steering (AFS) system (shown by individually labeled components). In use, a driver of the motor vehicle1may control the direction of motion of the motor vehicle1via a steering wheel2, thereby deflecting (i.e., turning) the steering wheel2by an amount equal to a steering wheel angle3. The AFS system, may include, for example, a superimposition gear mechanism4, which superimposes a superimposition angle6(i.e., an overlay angle6) on the steering wheel angle3via an actuator, such as, for example, an electric motor5. A resulting compensated steering angle7, which includes the steering wheel angle3and the superimposition angle6, moves a steering gear8, which moves two of the wheels9-1,9-2,9-3,9-4into a desired position (i.e., the desired RWA). As shown in the embodiment ofFIG. 1, in a vehicle having front axle steering, the compensated steering angle7will cause the steering gear8to move the front wheels9-1,9-2(i.e., the steered road wheels) of the vehicle1. The steering gear8may include an assist motor or hydraulic system that provides assistance to change the angle of the road wheels9-1,9-2in accordance with the compensated steering angle7.

In various embodiments of the present disclosure, the system20may include one or more preexisting vehicle sensors embodied within various subsystems of the vehicle1, and a controller12that is configured to receive signals from the sensors, determine a steering state based on the signals.

In various embodiments, for example, the active steering system includes a steering wheel angle sensor10that may detect the steering wheel angle3for determination of a desired road wheel angle (RWA). In particular, the active steering system superimposes an angle on the steering wheel angle3in order to achieve a desired RWA as known in the art.

The system20may further include various additional sensors used to detect and compensate for road disturbances. For example, an actuator angle sensor15may detect the superimposition angle6provided by the electric motor5. The sensors may further include a pinion angle sensor11, a vehicle speed sensor13, wheel speed sensors14associated with each of the front wheels9-1,9-2of the vehicle1and/or a steering wheel velocity sensor16. Such sensors may be present in various additional subsystems of the vehicle1, including, for example, the anti-lock braking system (ABS, not shown).

As described below, the torque generated by the electric motor5may be used in order to detect and compensate for external disturbances to steered wheels9-1,9-2. In some embodiments, torque may be estimated from current through the motor5. Accordingly, a current sensor17may measure this current and provide an output indicating this current to the controller12. The current sensor17may be housed within the electric motor5or be placed elsewhere within the vehicle1.

Those of ordinary skill in the art will understand that the system20illustrated inFIG. 1is exemplary only and intended to illustrate one embodiment of the present disclosure. Accordingly, systems and vehicles encompassing such systems in accordance with the present disclosure may have various types, numbers and/or configurations of actuators, controllers, and/or sensors without departing from the scope of the present disclosure and claims. For example, although the system20illustrated and described with reference toFIG. 1includes an AFS system, embodiments of the present disclosure contemplate damping road disturbances in conjunction with any type and/or configuration of active steering system.

As shown inFIG. 1, the controller12receives signals from some or all of the steering wheel angle sensor10, vehicle speed sensor13, wheel speed sensors14, actuator angle sensor15, steering wheel velocity sensor16, and current sensor17. The controller12may include, for example, an existing vehicle controller such as the Electronic Control Unit (ECU) of the vehicle1, or a dedicated controller, or control may be distributed among more than one vehicle controller, as would be understood by one ordinarily skilled in the art.

Referring toFIG. 2, the illustrated method200illustrates a basic approach for reducing transmission of road disturbances to the steering wheel2and the driver's hands. In response to detecting202a disturbance from the road, such as from a pot hole, debris, curb, etc., additional compliance is introduced204into the system20. In particular, inasmuch as there is a decoupling between the steering wheel2and the road wheels9-1,9-2, the superimposition angle6and torque provided by the electric motor5may be controlled in order to reduce the transmission of disturbances to the steering wheel2.

The manner in which the road disturbances are detected202may be according to the methods of some or all ofFIGS. 3 through 6.

Introducing204compliance may be performed in various ways. In one example, the controller12alters the target angle for the electric motor5. For example, if the electric motor5is rotating clockwise and an external torque is detected that also causes a clockwise rotation, the controller may adjust the target angle for the motor5to require reduction in the clockwise rotation. In another example, if the motor5is rotating clockwise and a road disturbance is detected the causes counter-clockwise rotation, the controller may adjust the target angle for the motor5to require an increase in clockwise rotation.

In some embodiments, the controller12may introduce compliance by adjusting current to the motor5. For example, where a road disturbance results in the motor5generating more torque to reach a target angle, e.g., the disturbance urges the road wheels in an opposite direction than the movement induced by motor torque, the current to the motor5may be increased. Where a disturbance results in the motor5generating less torque, e.g. urges the road wheels in a same direction as the motor torque, the current to the motor5may be decreased.

In yet another embodiment, inducing compliance may include reducing a gain of the controller12in response to a road disturbance thereby making the steering system more compliant. By doing this, the effect of spikes in torque due to road disturbance are reduced, but the vehicle1still follows the desired trajectory. In particular, the function by which the superimposition angle6is determined as a function of steering wheel angle3may be modified such that the superimposition angle6will be reduced in response to a detected road disturbance as compared to the superimposition angle6determined by the controller12in the absence of the detected road disturbance for a given steering state.

In yet another embodiment, damping of road disturbances may be achieved by shorting leads of the electric motor5. In particular, for a three-phase motor, shorting the leads will result in damping of torque imposed on the motor5that increases with the speed at which the motor5is rotated.

In some embodiments, one or more of these approaches may be used according to different circumstances. For example, the controller12may select among one of the above approaches, or select a combination of two or more of the above approaches, according to a magnitude of a detected road disturbance.

Referring toFIG. 3, the illustrated method300may be used to estimate external torque exerted on the steering system20, such as by detecting the external torque applied to the motor5. In practice, the controller12controls the superimposition angle6by applying a torque to the motor5, e.g. by supplying an amount of current or providing a torque control signal that instructs the motor to generate a given amount of torque. In some embodiments, the controller12calculates this torque as a function of a difference between an actual motor angle, a target motor angle (e.g. to achieve the desired superimposition angle), and motor speed. When an external torque is applied to the steering system20, the difference between actual and target angle becomes larger till a torque equilibrium between external torque, motor torque, and motor acceleration is reached. As a result of this phenomenon, it is possible to estimate how large the external torque input is and also if the torque input is induced by the driver or the road wheels. The method300provides an approach for determining the external torque on the steering system20.FIGS. 4 through 7illustrate approaches for processing the external torque to determine whether it is caused by a road disturbance such that additional compliance should be induced in the steering system.

The method300may include measuring302current drawn by the electric motor5, such as by using the current sensor17or some other means. The measured current may be used as an estimate of torque output by the motor. For example, the motor5may be measured under various loads and input currents to derive a function approximating torque as a function of input current and motor angular velocity. Speed of the motor5may be derived from outputs of the actuator angle sensor15.

The method300may include estimating304friction. Friction is a generally constant force in the steering system20. Accordingly, it may be determined experimentally for the steering system20for a variety of situations (vehicle speed, steering wheel angular velocity, steering wheel angle, etc.). An estimate of friction may then be obtained based on the current values for these parameters and the experimentally determined relationship between these parameters and friction. Alternatively, friction may be determined based on measures of current vehicle operating conditions.

Friction can, for example, be estimated by using a friction model (e.g. Coulomb model, Stribeck model). The parameters required for these models can either be estimated online or offline. Another way to estimate friction is to observe the system behavior during specific operating conditions, where the external torque is known (e.g. system start-up, End of line calibration).

The method300may further include measuring306an angular position of the electric motor5, such as from an output of the actuator angle sensor15. Outputs of the actuator angle sensor15over time may be used to calculate308the speed of the motor and calculate310its acceleration. Alternatively, one or more separate sensors may sense one or both of these values directly.

An external torque may then be calculated312according to the values determined at steps302,304,308, and310. For example, external torque may be calculated as:
TE=I*aM+D*WM−TF+TM,
where TEis the external torque, I is the rotational inertia of the motor, aMis the acceleration of the motor, WMis the angular velocity of the motor, D represents the damping in the system (e.g., viscous friction), TFis the torque due to friction, the TMis the torque exerted by the motor5as estimated from motor current.

In some embodiments, a less precise estimate of external torque may be used that may still provide acceptable results. For example, friction and inertia may be ignored or the external torque may simply be approximated as the torque of the motor as estimated from the current drawn by the motor.

Referring toFIG. 4, the illustrated method400may be used to detect disturbances caused by impacts on the road wheels9-1,9-2. The method400may include determining402the external torque applied to the steering system20or the external torque as applied to the electric motor5. For example, the external torque may be calculated according to the method300ofFIG. 3.

The method400may further include comparing404the external torque to an expected external torque for the current steering state. Specifically, for a given steering state, the torque on the electric motor5may be consistent. For example, the torque exerted on the electric motor5by the steering wheel and the torque transmitted to the electric motor5from the road wheels9-1,9-2for a given steering state may be known by experimental measurements under for a variety of steering states. For example, a curve fit outputting an approximate external torque as a function of some or all of these parameters defining the steering state may be determined.

The method400may include evaluating406whether the difference between the external torque of step402and the expected torque of step404indicate a disturbance. For example, if the external torque exceeds the expected torque by some threshold amount, which may be threshold dependent. At slow speeds, a driver often makes sharp turns of the steering wheel2, whereas at high speeds adjustments are smaller and slower, since large movements would result in loss of control. Accordingly, the difference may be compared to a threshold amount that varies, e.g. decreases, based on velocity.

If step406indicates a disturbance, the method400may include evaluating408whether the disturbance was generated by the driver or by an impact to the road wheels9-1,9-2. Examples of methods for determining whether a disturbance is due to a road disturbance are described below with respect toFIGS. 5 through 7.

If the disturbance is not found408to be driver generated, the compliance is introduced410as described above with respect to step204ofFIG. 2.

If no disturbance is indicated406or the disturbance is found408to be driver generated, then compliance is not introduced410and the AFS operates in a conventional manner. Likewise, if no disturbance is indicated406or the disturbance is found408to be driver generated, any compliance introduced410in a previous iteration of the method400may be removed. In some embodiments, the additional compliance may result in a deviation between a desired relationship between the road wheel angle and the steering wheel angle. Accordingly, this deviation may be reversed gradually in order to not affect the stability of the vehicle1. Removing this deviation may include implementing the methods disclosed in U.S. application Ser. No. 14/558,427 filed Feb. 27, 2014, and entitled SYSTEMS AND METHODS FOR CORRECTING STEERING OFFSETS, which is hereby incorporated herein by reference in its entirety.

FIGS. 5 through 7illustrate approaches for estimating whether a detected disturbance is driver generated or is the result of a road disturbance. The approaches ofFIGS. 5 through 7may be used separately or in combination. For example, if any one of the approaches ofFIGS. 5 through 7indicates a road disturbance rather than a driver input, a road disturbance may be determined to be occurring.

Referring specifically toFIG. 5, the illustrated method500may make use of signal outputs from sensors14mounted on or near the road-wheels9-1,9-2, such as on the steering knuckle. Sensors14may detect such parameters as wheel speed, steering knuckle acceleration (e.g., due to road impacts), or other parameters.

The method500may include receiving502the speed of the vehicle1, such as from vehicle speed sensor13and receiving504wheel speeds for the road-wheels9-1,9-2from the sensors14. Step504may further include receiving an acceleration measurement from sensors14, e.g. a vertical acceleration of the wheels9-1,9-2.

The method500may include evaluating506whether the speed of step502and sensor outputs of step504indicate slip. In particular, where the wheel speed of step504for one of the road wheels9-1,9-2is slower or faster than the vehicle speed of step502by some threshold amount, then a road disturbance may be determined508to be indicated. Likewise, if the speed of one wheel is found506to differ from the speed of the other wheel by a threshold amount above that due to the current turning radius of the vehicle1, then a road disturbance may be found508to be indicated. Where vertical acceleration of the road wheels9-1,9-2is measured, a value of vertical acceleration in one or both wheels found506to be above a threshold may be determined508to indicate a road disturbance.

Referring toFIG. 6, while also referring toFIG. 7, a method600may include detecting road disturbances using one or more forward facing cameras702a,702bor other sensors704a,704bmounted to a vehicle700including the controller12and a steering system20as described above. For example, sensors704a,704bmay include RADAR (radio detection and ranging) sensors, LIDAR (light detection and ranging) sensors, SONAR (sound navigation and ranging) sensors, ultrasonic sensors, and the like.

The method600may include receiving602outputs of the sensing devices702a,702b,704a,704band analyzing604outputs of the sensing devices702a,702b,704a,704bfor features indicating wheel impact sites. For example, the controller12may identify a ground plane in sensor data, holes or projections706in that ground plane. Holes or projections lying on the trajectories708a,708bof the road wheels9-1,9-2may be identified during the analyzing steps604. If a hole or projection706is identified that is found606to be likely to impact with the road wheels9-1,9-2, then a disturbance detected immediately following detection may be determined608to be a road disturbance, e.g. if detected within a threshold of a time required for the road wheels9-1,9-2to arrive at the location of the detected hole or projection706.