Control method for a rear-wheel steering system

Abstract

A control method for a rear-wheel steering system, the control method comprising:detecting a failure of at least one of four wheel speed sensors (S1, S2, S3, S4) by a controller (1) based on output values ​​of the wheel speed sensors (S1, S2, S3, S4);estimating a vehicle speed using output values ​​of remaining normal wheel speed sensors, except for an output value of an abnormal wheel speed sensor in which the failure is detected when the controller (1) detects the failure in the abnormal wheel speed sensor;andcontrolling the steering of rear wheels by the controller (1) based on the estimated vehicle speed,wherein, in estimating a vehicle speed, when at least one abnormal wheel speed sensor of the four wheel speed sensors (S1, S2, S3, S4) is detected, an output value of the abnormal wheel speed sensor is replaced by the output value of at least one normal wheel speed sensor, wherein, when there are a plurality of normal wheel speed sensors, the output value of the abnormal wheel speed sensor is replaced by an average value of output values ​​of the plurality of normal wheel speed sensors.;

Description

BACKGROUNDFIELD OF DISCLOSUREThe present disclosure relates to a control method of a rear wheel steering system in which a sudden change in the amount of rear wheel steering control is prevented when a fault occurs in a wheel speed sensor.DESCRIPTION OF RELATED ARTIn a four-wheel steering system, at low speed, the rear wheel steering angle is controlled to a phase opposite to the front wheel steering angle to reduce the turning circle, and at high speed, the rear wheel steering angle is controlled to the same phase as the front wheel steering angle, thereby improving the steering stability.That is, by mounting an actuator such as a hydraulic device, a motor and a reduction gear for rear wheel steering angle control, the rear wheels of a vehicle are steered accordingly depending on the running state of the vehicle, thereby improving the running performance of the vehicle.In addition, in the conventional rear wheel steering apparatus, the vehicle speed is estimated based on the average value of the wheel speeds measured by the four wheel speed sensors. The rear wheels are steered and controlled according to the estimated vehicle speed.However, if there is a failure in the signal output of the wheel speed sensor or if there is a rapid braking / acceleration operation, then the estimated vehicle speed can be changed rapidly when a particular wheel is locked or slipping. When the vehicle speed is suddenly changed, the amount of rear wheel steering also changes rapidly, whereby a difference may occur in rear wheel control and stable rear wheel control cannot be performed.From DE 10 2015 110 965 A1, a vehicle control system is known having a first and a second brake module and having a plurality of wheel speed sensors, which are communicatively connected to the first brake control module; in this case, the first module is programmed to receive wheel speed data from the wheel speed sensors and the second module is programmed to process the wheel speed data in the event of a fault in the first module. Upon detecting a fault, the first module is programmed to use the wheel speed data to calculate at least one wheel speed, provide the at least one wheel speed to the second module, and receive wheel speed data from less than all wheel speed sensors. It is provided that a further module calculates an average speed of driven wheels and provides the average speed of the driven wheels to the second module.DE 10 2005 053 965 A1 also discloses a vehicle in which wheel speed sensors, which are respectively arranged on drive wheels and idler wheels, record rotational speeds of the corresponding wheels. An ECU includes a wheel speed calculation section that calculates wheel speeds of the drive wheels and the idle wheels based on the detected rotational speeds, and an abnormality detection section that detects abnormality of the wheel speed sensors based on the calculated wheel speeds. The abnormality detecting portion detects the abnormality of the wheel speed sensors based on: a first wheel speed which is at least one of the wheel speeds of the drive wheels and the idle wheels that are substantially zero; at least one of the wheel speeds except for the first wheel speed that is greater than a prescribed threshold; and an average wheel speed of the idle wheels that is greater than an average wheel speed of the drive wheels.The foregoing is intended merely to aid in understanding the background of the present disclosure. The foregoing is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those of ordinary skill in the art.OVERVIEWAccordingly, the present disclosure is made in consideration of the above-mentioned problems that will be apparent from the related art. It is the object of the present disclosure to provide a control method for a rear wheel steering system in which a sudden change in the amount of rear wheel steering control is prevented when a failure occurs in a wheel speed sensor.The above object is achieved by a control method for a rear wheel steering system having the features of claim 1 or a control method for a rear wheel steering system having the features of claim 10. The control method according to the present invention includes detecting, by a controller, a failure in at least one of four wheel speed sensors based on output values of the wheel speed sensors. The method includes estimating a vehicle speed using output values of remaining normal wheel speed sensors except an output value of an abnormal wheel speed sensor at which a fault is detected when the controller detects a fault in the wheel speed sensor. The method includes controlling steering of rear wheels by the controller based on the estimated vehicle speed. In estimating a vehicle speed, when at least one abnormal wheel speed sensor of the four wheel speed sensors is detected, an output value of the abnormal wheel speed sensor is replaced with the output value of at least one normal wheel speed sensor.When the wheel acceleration output from the wheel speed sensors is equal to or greater than a target value in the detection of a failure, it may be determined that a failure has occurred.In the estimation of a vehicle speed, when two abnormal wheel speed sensors of the four wheel speed sensors are detected, output values of the abnormal wheel speed sensors may be replaced with an average value of the output values of the remaining two normal wheel speed sensors.When three abnormal wheel speed sensors of the four wheel speed sensors are detected in the estimation of a vehicle speed, output values of the abnormal wheel speed sensors may be replaced with an output value from the remaining normal wheel speed sensor.After estimating a vehicle speed, the at least one abnormal wheel speed sensor may be diagnosed as normal when a state in which an absolute value of a difference between the output value of the abnormal wheel speed sensor and the output value of the at least one normal wheel speed sensor or the average value of the output values of the plurality of normal wheel speed sensors is equal to or less than a target value is maintained for a predetermined time.In estimating a vehicle speed when the vehicle is accelerated by an accelerator operation, the vehicle speed may be estimated based on a minimum value among all output values of the wheel speed sensors.In estimating a vehicle speed when the vehicle is braked by operating the brake pedal, the vehicle speed may be estimated based on a maximum value among all output values of the wheel speed sensors.In estimating the vehicle speed, when accurately detecting one abnormal wheel speed sensor among the four wheel speed sensors, the output value of the abnormal wheel speed sensor may be replaced with the output value of two normal wheel speed sensors that do not include the wheel speed sensor diagonally opposed to the abnormal wheel speed sensor.In the estimation of a vehicle speed, when a vehicle dynamic control system is operated, the vehicle speed may be estimated based on an average value of two output values except a minimum value and a maximum value among all output values of the wheel speed sensors.According to the present disclosure, the above-described control method is advantageous in that when a failure is detected in the wheel speeds of the wheel speed sensors based on the wheel speed of the normal wheel speed sensor, the abnormal wheel speed is replaced to estimate the vehicle speed. Thus, it is possible to estimate the vehicle speed more accurately than the conventional result of the vehicle speed estimation. Thereby, a sudden change in the amount of rear wheel steering is prevented, thereby avoiding the occurrence of a feeling of difference due to the rear wheel steering.BRIEF DESCRIPTION OF THE DRAWINGSThe above and other objects, features, and other advantages of the present disclosure will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which they are incorporated. In the drawings, there are shown: FIG. 1 is a schematic diagram illustrating a configuration of a rear wheel steering of a vehicle according to the present disclosure; FIGS. 2A and 2B are flowcharts sequentially illustrating a control process for the rear wheel steering system according to the present disclosure; and FIG. 3 is a graph illustrating comparison results of the vehicle speed estimation according to the present disclosure with conventional results of the vehicle speed estimation.DETAILED DESCRIPTIONHereinafter, an embodiment of the present disclosure will be described in detail with reference to the accompanying drawings.A control method of a rear wheel steering system of the present disclosure, when an error occurs in the signal output from the four wheel speed sensors S 1, S 2, S 3, and S 4, carries out estimation of the vehicle speed using a normal wheel speed sensor except for an abnormal wheel speed sensor. Sudden change in the control amount of a rear wheel steering is thus prevented. The control method includes detecting a fault, estimating a vehicle speed, and controlling the steering.Referring to FIGS. 1, 2A, and 2B, in order to be more specific to the present disclosure, a controller 1 may first detect, in the step of detecting a failure, a failure in the wheel speed sensors S 1, S 2, S 3, and S 4 based on output values of the wheel speed sensors S 1, S 2, S 3, and S 4.For example, when the wheel acceleration output from the wheel speed sensors S 1, S 2, S 3, and S 4 is equal to or greater than a target value, it is determined that an error has occurred. Thereby, the abnormal wheel speed sensor can be detected.In the vehicle speed estimating step, when the controller 1 detects a failure in the wheel speed sensors S 1, S 2, S 3, and S 4, a vehicle speed may be estimated using output values of the remaining normal wheel speed sensors except an output value of an abnormal wheel speed sensor at which a failure is detected.Specifically, in the vehicle speed estimating step, when an abnormal wheel speed sensor of the four wheel speed sensors S 1, S 2, S 3, and S 4 is detected, an output value of the abnormal wheel speed sensor is replaced with an average value of the output values of two normal wheel speed sensors except for a remaining normal wheel speed sensor disposed at a position diagonal to the abnormal wheel speed sensor.Further, in the steering control step, the controller 1 may control the rear wheels based on the estimated vehicle speed.In other words, when a failure has occurred in one wheel speed sensor out of the four wheel speed sensors S 1, S 2, S 3, and S 4, only the output values of two normal wheel speed sensors are used without using the average value of the output values of three normal wheel speed sensors. This is because there is a slight difference in wheel speed between the front and rear wheels, and there is a difference in wheel speed between the left and right wheels when turning.Accordingly, for example, it can be recognized that an error has occurred in the wheel speed, which is the output value, of the wheel speed sensor S 1 of the front left wheel FL. When the abnormal wheel speed is replaced with the average value of the wheel speeds measured by the front right wheel FR, the rear left wheel RL, and the rear right wheel RR, which are normal wheel speed sensors S 2, S 3, and S 4, a rear wheel other than the front wheel is included in the calculation, and thus the abnormal wheel speed is replaced with the wheel speed value that is more weighted on the speed of the rear wheel.In order that the wheel speed replacing the wheel speed of the abnormal wheel speed sensor S 1 has the same weight as the front, rear, left and right wheels, the wheel speed of the right rear wheel RR which is at the normal wheel speed but is disposed at a position diagonal to the abnormal wheel speed sensor is excluded. The wheel speed of the front left wheel FL, which is the abnormal wheel speed, is also excluded. The average value of the output values of the remaining two normal right front wheels FR and left rear wheels RL, which replaces the wheel speed of the abnormal wheel speed sensor, is calculated, thereby enabling more accurate estimation of the vehicle speed.In addition, in estimation of a vehicle speed, when two abnormal wheel speed sensors of the four wheel speed sensors S 1, S 2, S 3, and S 4 are detected, an output value of the abnormal wheel speed sensor is replaced with an average value of the output values of the remaining two normal wheel speed sensors.For example, when the errors are detected in the wheel speed sensors S 1 and S 2 of the FL and the FR, the wheel speed of the FL and the FR is replaced with the average value of the wheel speed of the wheel speed sensors S 3 and S 4 of the RL and the RR.In addition, in the estimation of a vehicle speed, when three abnormal wheel speed sensors out of the four wheel speed sensors S 1, S 2, S 3, and S 4 are detected, the output value of the abnormal wheel speed sensor may be replaced with the output value of the remaining one normal wheel speed sensor.For example, when the errors are detected in the wheel speed sensors S 1, S 2, and S 3 of the FL, FR, and RL, the wheel speed of the FL, FR, and RL at which the errors have occurred is replaced with the wheel speed of the wheel speed sensor S 4 of the RR.Further, after estimating a vehicle speed, the abnormal wheel speed sensor may be diagnosed as normal when a state in which an absolute value of a difference between the output value of the abnormal wheel speed sensor and the output value of the normal wheel speed sensor is equal to or less than a target value is maintained for a predetermined time.For example, the abnormal wheel speed sensor is diagnosed as being normal when the absolute value of the value obtained by subtracting the wheel speed of the normal wheel speed sensor from the wheel speed of the abnormal wheel speed sensor is equal to or less than a predetermined value, and this state is maintained for 500 ms or longer.In the estimation of a vehicle speed, the vehicle speed when the vehicle is accelerated by an accelerator operation may be estimated based on the minimum value among all the output values of the wheel speed sensors S 1, S 2, S 3, and S 4.In other words, when an accelerator pedal sensor (APS) 3 determines that the accelerator pedal is being pressed, since the probability of occurrence of a spinning wheel is high, the vehicle speed is estimated based on the lowest wheel speed among the wheel speeds of the four wheel speed sensors S 1, S 2, S 3, and S 4.Further, in estimating a vehicle speed when the vehicle is braked by operating the brake pedal, the vehicle speed may be estimated based on a maximum value among all output values of the wheel speed sensors S 1, S 2, S 3, and S 4.In other words, when a brake pedal sensor (BPS) 5 determines that the brake pedal is being operated, since the probability of occurrence of wheel lock is high, the vehicle speed can be estimated based on the highest wheel speed among the wheel speeds of the four wheel speed sensors S 1, S 2, S 3, and S 4.Further, in estimating a vehicle speed when a vehicle dynamics control system is operated, the vehicle speed may be estimated based on an average value of two output values except for the minimum value and the maximum value among all the output values of the wheel speed sensors S 1, S 2, S 3, and S 4.For example, the driving dynamics control system may be an electronic stability control (ESC) 7. When the ESC 7 is turned on, the vehicle speed is estimated based on the average value of the second and third wheel speeds of the wheel speeds of the wheel speed sensors S 1, S 2, S 3, and S 4.Referring to FIGS. 2A and 2B, these figures help to sequentially explain the control process for the rear wheel steering system of the present disclosure. By receiving the wheel speed outputted from each of the wheel speed sensors S1, S2, S3, and S4, the wheel acceleration is detected (S10). Then, it is determined whether the wheel speed sensors S 1, S 2, S 3, and S 4 have the detected wheel acceleration greater than A (S 20).As a result of the determination in step S20, if there is an abnormal wheel speed sensor having the wheel acceleration greater than A, the wheel speed of the abnormal wheel speed sensor is replaced with the average value of the wheel speeds of the remaining two normal wheel speed sensors which do not include the abnormal wheel speed sensor and the normal wheel speed sensor disposed at a position diagonal to the abnormal wheel speed sensor (S30).Moreover, as a result of the determination in S 20, when there are two abnormal wheel speed sensors having a wheel acceleration greater than A, the wheel speed of the abnormal wheel speed sensor is replaced with the average value of the wheel speeds of the two normal wheel speed sensors (S 40).Further, as a result of the determination in S 20, when there are three abnormal wheel speed sensors having a wheel acceleration greater than A, the wheel speed of the abnormal wheel speed sensor is replaced with the wheel speed of a normal wheel speed sensor (S 50).Further, as a result of the determination in S 20, when there are four abnormal wheel speed sensors having a wheel acceleration greater than A, the wheel speed of the abnormal wheel speed sensor is replaced with the vehicle speed calculated by an engine management system (EMS) 9 (S 60).Subsequently, it is determined whether the absolute value of the value obtained by subtracting the wheel speed of the normal wheel speed sensor from the wheel speed of the abnormal wheel speed sensor is equal to or less than B and this state is maintained for 500 ms or more (S 70). When the above condition is satisfied, the abnormal wheel speed sensor is diagnosed as normal, and the wheel speed of the normal wheel speed sensor is used for estimating the vehicle speed (S 80).In addition, after S 70, it is determined whether the vehicle is accelerated by an accelerator pedal operation (S 90). When it is determined that the acceleration is being performed by the accelerator pedal, the minimum value is selected among the wheel speeds of the four wheel speed sensors S 1, S 2, S 3, and S 4 (S 100). The vehicle speed is finally estimated based on the selected value (S 150).On the other hand, when the vehicle is not accelerated, it is determined whether the vehicle is decelerated by operation of the brake pedal (S 110). When it is determined that the deceleration is performed by the brake pedal, the maximum value is selected among the wheel speeds of the four wheel speed sensors S 1, S 2, S 3, and S 4 (S 120). The vehicle speed is finally estimated based on the selected value (S 150).On the contrary, when the acceleration or deceleration operation is not performed, it is determined whether the braking is performed according to the ESC 7 operation (S 130). When it is determined that the braking situation occurs, the average value of the remaining two wheel speeds is obtained except for the minimum value and the maximum value among the wheel speeds of the four wheel speed sensors S 1, S 2, S 3, and S 4 (S 140). The vehicle speed is finally estimated based on the obtained average value (S 150).Subsequently, the rear wheels are finally steered and controlled based on the estimated vehicle speed (S 160) (S 160).As described above, according to the present disclosure, when an error in the wheel speeds of the wheel speed sensors S 1, S 2, S 3, and S 4 based on the wheel speed of the normal wheel speed sensor replaces the abnormal wheel speed to estimate the vehicle speed. As illustrated in FIG. 3, this makes it possible to estimate the vehicle speed more accurately than the conventional result of the vehicle speed estimation. Sudden change in the amount of rear wheel steering is thus prevented, thereby preventing the occurrence of a feeling of difference due to the rear wheel steering.

Claims

A control method for a rear wheel steering system, the control method comprising: detecting, by a controller (1), a failure of at least one of four wheel speed sensors (S1, S2, S3, S4) based on output values of the wheel speed sensors (S1, S2, S3, S4); estimating a vehicle speed using output values of remaining normal wheel speed sensors except an output value of an abnormal wheel speed sensor in which the failure is detected when the controller (1) detects the failure in the abnormal wheel speed sensor; controlling steering of rear wheels by the controller (1) based on the estimated vehicle speed, wherein in estimating a vehicle speed, when at least one abnormal wheel speed sensor of the four wheel speed sensors (S1, S2, S3, S4) is detected, an output value of the abnormal wheel speed sensor is replaced with the output value of at least one normal wheel speed sensor, wherein in the presence of a plurality of normal wheel speed sensors, the output value of the abnormal wheel speed sensor is replaced with an average value of output values of the plurality of normal wheel speed sensors.The control method according to claim 1, characterized in that when a failure is detected, when the wheel acceleration output from the wheel speed sensors (S1, S2, S3, S4) is equal to or greater than a target value, it is determined that a failure has occurred.The control method according to claim 1, characterized in that in estimating a vehicle speed, when two abnormal wheel speed sensors of the four wheel speed sensors (S1, S2, S3, S4) are detected, output values of the abnormal wheel speed sensors are replaced with an average value of the output values of remaining two normal wheel speed sensors.The control method according to claim 1, characterized in that in estimating a vehicle speed, when three abnormal wheel speed sensors of the four wheel speed sensors (S1, S2, S3, S4) are detected, output values of the abnormal wheel speed sensors are replaced with an output value from the remaining normal wheel speed sensor.The control method according to claim 1, characterized in that after estimating a vehicle speed, the at least one abnormal wheel speed sensor is diagnosed as normal when a state in which an absolute value of each difference between the output value of the abnormal wheel speed sensor and the output value of the at least one normal wheel speed sensor or the average value of the output values of the plurality of normal wheel speed sensors is equal to or less than a target value is maintained for a predetermined time.The control method according to claim 1, characterized in that in estimating a vehicle speed when the vehicle is accelerated by an accelerator pedal operation, the vehicle speed is estimated based on a minimum value among all output values of the wheel speed sensors (S1, S2, S3, S4).The control method according to claim 1, characterized in that in estimating a vehicle speed when the vehicle is braked by an operation of the brake pedal, the vehicle speed is estimated based on a maximum value among all output values of the wheel speed sensors (S1, S2, S3, S4).The control method according to claim 1, characterized in that in estimating a vehicle speed when a vehicle dynamic control system is operated, the vehicle speed is estimated based on an average value of two output values except a minimum value and a maximum value among all output values of the wheel speed sensors (S1, S2, S3, S4).The control method according to claim 1, characterized in that in the estimation of the vehicle speed, when accurately detecting one abnormal wheel speed sensor among the four wheel speed sensors (S1, S2, S3, S4), the output value of the abnormal wheel speed sensor is replaced with the output value of two normal wheel speed sensors that do not include the wheel speed sensor diagonally opposed to the abnormal wheel speed sensor.A control method for a rear wheel steering system of a vehicle, the control method comprising: detecting, by a controller (1), at least one abnormal wheel speed sensor among four wheel speed sensors as an error based on output values of the wheel speed sensors; determining, when the controller (1) detects the error, an estimated speed value by estimating a vehicle speed using output values of remaining normal wheel speed sensors except an output value of the abnormal wheel speed sensor at which the error is detected; controlling steering of rear wheels of the vehicle by the controller (1) based on the estimated vehicle speed, wherein in estimating a vehicle speed, when accurately detecting one abnormal wheel speed sensor among the four wheel speed sensors, an output value of the one abnormal wheel speed sensor is replaced with an average value of the output values of two normal wheel speed sensors except a remaining normal wheel speed sensor disposed at a position diagonal to the abnormal wheel speed sensor on the vehicle.

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