Method of controlling a steer-by-wire steering system of a road vehicle with yaw rate control

Abstract

The invention relates to a Method of controlling a steer-by-wire steering system (1) of a road vehicle having at least two steerable road wheels, wherein two steerable road wheels are connected to each other via a rack (6) moved by a road wheel actuator (5), the method comprising the following steps a) Determining a target rack position of the rack (6) based on a steering input from a driver; b) Calculating a predicted yaw rate (10) of the road vehicle based on a predicted steering wheel angle; c) Measuring an actual yaw rate (11) and calculating a difference between the predicted yaw rate (10) and the actual yaw rate (11); d) Determining a first value for rack position control based on the difference of the actual yaw rate (11) and the predicted yaw rate (10) in a first controller (9); e) Calculating a steady state yaw rate (15) of the road vehicle based on the steering wheel angle, the velocity of the road vehicle and the lateral and longitudinal acceleration; f) Calculating a difference between the steady state yaw rate (15) and the actual yaw rate (11); g) Determining a second value for rack position control based on the difference of the actual yaw rate (11) and the predicted steady state yaw rate (15) in a second controller; h) Calculating a modified rack position request for the road wheel actuator (7) based on the sum (17) of the first and second values and the target rack position.

Description

[0001] Method of controlling a steer-by-wire steering system of a road vehicle with yaw rate control

[0002] The present invention relates to a method of controlling a steer-by-wire steering system of a road vehicle according to the preamble of claim 1 and to a steer-by-wire steering system for a road vehicle designed to carry out the method.

[0003] In steer-by-wire (SbW) systems of road vehicles, the steering wheel is decoupled from the road wheels. A road wheel actuator operates a rack that is connected to the road wheels and steers them. A steering wheel actuator, also known as a feedback actuator, is acting on the steering wheel and provides the driver with feedback at the steering wheel.

[0004] As there is no mechanical connection between the steering wheel and the road wheels, the steering feedback and the steering ratio can be freely programmed. This makes it possible to improve the control of the steering rack position (road wheel angle) by taking into account mechanical deviations and disturbances such as side wind gusts and braking on mu-split road surface to increase driver comfort.

[0005] It is an object of the present invention to provide a method for controlling a steer-by-wire steering system of a road vehicle which improves the agility of the road vehicle and the driving comfort. This object is achieved by a method of controlling a steer-by-wire steering system of a road vehicle having the features of claim 1 and by a steer-by-wire steering system for a road vehicle designed to carry out the method.

[0006] Accordingly, a method for controlling a steer-by-wire steering system of a road vehicle having at least two steerable road wheels, wherein two steerable road wheels are connected to each other via a rack moved by a road wheel actuator, the method comprising the following steps:

[0007] a) Determining a target rack position of the rack based on a steering input from a driver;

[0008] b) Calculating a predicted yaw rate of the road vehicle based on a predicted steering wheel angle;

[0009] c) Measuring an actual yaw rate and calculating a difference between the predicted yaw rate and the actual yaw rate;

[0010] d) Determining a first value for rack position control based on the difference of the actual yaw rate and the predicted yaw rate in a first controller;

[0011] e) Calculating a steady state yaw rate of the road vehicle;

[0012] f) Calculating a difference between the steady state yaw rate and the actual yaw rate;

[0013] g) Determining a second value for rack position control based on the difference of the actual yaw rate and the steady state yaw rate in a second controller;

[0014] h) Calculating a modified rack position request for the road wheel actuator based on the sum of the first and second values and the target rack position.

[0015] The method takes into account both steady state disturbances such as side wind gusts and mechanically induced strong nonlinear characteristics, thereby improving the agility of the road vehicle and the driving comfort. Preferably the steady state yaw rate is the yaw rate that is assumed in case the steering angle stays constant. For the calculation the measured yaw rate and steering wheel speed signals can be used.

[0016] To avoid unwanted signal oscillations, the first and second controller are preferably active at different times.

[0017] Preferably, an indicator is used to analyse the motion of the road vehicle body, whereby the indicator deactivates the unwanted controller in each case to ensure that only the controller suitable for the motion provides an output signal.

[0018] In step c) the predicted steering wheel angle is calculated adding to the current measured steering wheel angle 6stwthe measured steering wheel speed 6stwmultiplied with a prediction horizon dt. This way the steering wheel speed is used to calculate the steering angle that the driver wants to achieve in the future and thereby allowing to predict what yaw rate should occur in the future (predicted yaw rate). Preferably, the prediction horizon dt is 0.4 seconds.

[0019] The second controller can use a proportional term including a reverse calculation of the reference yaw rate to add a steering angle to compensate for part of the yaw rate deviation. The second controller can also integrate the yaw rate offset during a disturbance situation and then compensate the yaw angle of the vehicle.

[0020] Further, the second controller can include an integrator which is always reset to zero unless the driver is in a steady state driving situation.

[0021] To reduce noise in the signal, the first and second controller outputs are filtered independently of each other or the sum of them.

[0022] Further a steer-by-wire steering system for a road vehicle designed to carry out the above described method is provided.

[0023] An example embodiment of the present invention will now be described with reference to the drawings.

[0024] FIG. 1 is a schematic view of a steer-by-wire-steering system. FIG. 2 is a block diagram representing a method for controlling the steering system.

[0025] FIG. 1 shows a schematic drawing of a steer-by-wire steering system 1 with a steering shaft 2 connected to a steering wheel 3. There is no mechanical connection between the steering wheel 3 and the road wheels 4. A road wheel actuator 5 operates a rack 6 via a recirculating ball gear 7.

[0026] When a driver operates the steering wheel 3, the steering shaft 2 is rotated, which is detected by a shaft sensor, which is not shown in the drawings. A controller is configured or programmed to calculate an operation signal for the road wheel actuator 5 from the signal detected by the shaft sensor. By operating the road wheel actuator 5 with the operation signal, the rack 6 is moved and the wheels 4 are turned. At the same time, forces introduced in the rack 6 from the road wheels 4 are recognized by another sensor not shown in the drawings, and a feedback signal is calculated, which is applied to the steering shaft 2 by a steering wheel actuator 8, also called feedback actuator, so that the operator can recognize the feedback in the steering wheel 3.

[0027] A target vehicle body motion set by the driver at the steering wheel 3 (feedback actuator) shall give a proper reference to the road wheel actuator 4 so that the vehicle follows the target vehicle body motion.

[0028] Ideally, the control of the rack position is linked to the control of the vehicle body by a simple translation. In reality, however, the existing mechanical discrepancies show a strongly non-linear characteristic (tire dynamics, bushings, play due to wear in the system, friction, and so on), a comparison of target yaw rate and measured yaw rate can be used to eliminate the discrepancy, but in existing yaw rate controllers the target yaw rate is calculated based on the actual steering wheel position. Although it is possible to steer the vehicle in this way, as the actual target steering wheel angle is greater than the actual steering wheel angle during fast steering maneuvers, the target yaw rate is not as high as it should be, resulting in a noticeably slower response. Therefore, in the invention the road wheel actuator is being over-controlled to pursue the vehicle body motion response and align it to a predicted vehicle body motion within the vehicle's stability limit. The target vehicle body motion has two components, a steady state yaw rate reference and a predicted yaw rate reference. Without considering measurement noise, these two values would only differ if the driver changes his inputs and the steering wheel angle is therefore not constant.

[0029] The predicted yaw rate reference is used in a first part of the control system dealing with mechanical deviation and its influence while for the disturbance mitigation of side wind gusts and braking on mu-split road surface the steady state yaw rate reference is used in a second part, as it is less problematic from a signal noise perspective.

[0030] The control method includes the following steps:

[0031] • Predicting the vehicle body motion based on the driver input at the steering wheel;

[0032] • Computing a predicted yaw rate based on the steering wheel movement, as well as a steady state yaw rate;

[0033] • Comparing the predicted yaw rate and steady state yaw rate to the actual yaw rate of the vehicle;

[0034] • Using the difference to modify the rack position reference of the roadwheel actuator, so that the vehicle will follow the predicted vehicle body motion with more agility and by rejecting disturbances without the driver intervening.

[0035] The driver can still steer in a normal way but the vehicle response will be more dynamic and more comfortable.

[0036] The problems of dynamic driving and disturbance suppression are essentially divided into two separate controllers that are active at different times.

[0037] Figure 2 represents the method of controlling the steering system. The block diagram and the two controllers are explained in detail below. A first so-called fast controller 9 works on the difference between a predicted yaw rate 10 and a measured yaw rate 11. The yaw control is conducted by comparing the measured vehicle yaw rate 11 in particular obtained from an on-board inertia sensor with the predicted yaw rate 10. The predicted yaw rate is calculated using a predicted steering wheel angle 8stwPred, which equals the current measured steering wheel angle 8stwplus the measured steering wheel speed 8stwmultiplied with a prediction horizon dt

[0038] 8stwPred 8$tw+ dt * 8$w.

[0039] The prediction can either be linear (constant speed), as given above, or of first order (increased speed, using the difference between two steering wheel speed 8stwmeasurements). A linear prediction is related to less noise and is therefore preferred.

[0040] The prediction horizon dt refers to the time over which the system's future conduct is predicted. The length of the prediction horizon is a vital parameter. The prediction horizon is used to shorten the response time of the fast controller, which is accompanied by a reduction of noise in the control signal and a smaller dead zone. The fast controller then reacts more predictably when the driver makes smaller corrections at the steering wheel site. One step in the prediction horizon is therefore preferably in a range between 0.05 and 0.5 seconds, more preferably equal to 0.4 seconds.

[0041] The difference 12 between the measured yaw rate 11 and the predicted yaw rate 10 is triggering the fast controller 9 to correct the actual yaw rate and reduce the difference between the two compared values by calculating a value for modification of the rack position request to actuate the road wheel actuator.

[0042] It can be implemented that the difference is averaged over a certain time to reduce noise. Further sensor signals 13 like the vehicle speed, steering wheel angle speed and so on are used by the fast controller to calculate the value for modification of the rack position request. The values are put into a look-up table that then gives a value between 0 and 1 with which the controller output is multiplied. During normal steady state driving disturbances like side wind gusts and mu split surface condition can act on the vehicle and induce unwanted vehicle movement, this is uncomfortable for the driver and requires driver input to mitigate.

[0043] A second so-called slow controller 14 works on the difference between a steady state yaw rate 15 and the measured yaw rate 11 to account for the driving disturbances. The steady state yaw rate 15 is calculated by using a Kalman Filter. The steady state yaw rate 15 can be calculated based on the steering wheel angle, the vehicle velocity and the lateral and longitudinal acceleration of the vehicle.

[0044] The difference 16 between the measured yaw rate 11 and the steady state yaw rate 15 is triggering the fast controller 14 to correct the actual yaw rate and reduce the difference between the two compared values by calculating a value for modification of the rack position request to actuate the road wheel actuator.

[0045] It can be implemented that the difference is averaged over a certain time to reduce noise. Further sensor signals 13 like the vehicle speed, steering wheel angle speed and so on are used by the fast controller to calculate the value for modification of the rack position request.

[0046] Both controller outputs are added up 17 and are used after filtering 18 of the signal as a rack position request for actuation of the road wheel actuator.

[0047] An indicator that is not shown analysis the vehicle body motion and activates the controller that is wanted in the situation so that the two separate controllers 9, 14 are always active at different times. Both controllers 9, 14 are therefore generally switched off. The difference as to when which controller 9, 14 is active is primarily due to the movement of the steering wheel. If the steering wheel is moved beyond a certain threshold value of the steering wheel speed, the fast controller 9 is activated, otherwise the slow controller 14 is activated. The slow controller 14 can include a first concept that uses a proportional term that uses a reverse calculation of a reference yaw rate to add a steering wheel angle to compensate part of the yaw rate deviation during constant disturbance situations, such as braking on split-mu road surface and side wind gusts. The steering wheel to be added is described by the following equation:

[0048] T * ( / + EGv2)

[0049] 8f = - * C

[0050]

[0051] Jv

[0052] , wherein C is used as a tuning factor.

[0053] Further, the slow controller 14 can include a second concept. In the second concept an integrator is added that is always reset to zero unless the driver is in a constant driving situation, in this way interference of the integrator during dynamic driving is avoided. In detail a PID controller is implemented, which uses the yaw rate offset as a control signal and rack position as output. The slow controller can also integrate the yaw rate offset during a disturbance situation and then compensate the yaw angle of the vehicle.

Claims

Claims1. Method of controlling a steer-by-wire steering system (1) of a road vehicle having at least two steerable road wheels, wherein two steerable road wheels are connected to each other via a rack (6) moved by a road wheel actuator (5), the method comprising the following stepsa) Determining a target rack position of the rack (6) based on a steering input from a driver;characterized in that the method includes further steps:b) Calculating a predicted yaw rate (10) of the road vehicle based on a predicted steering wheel angle;c) Measuring an actual yaw rate (11) and calculating a difference between the predicted yaw rate (10) and the actual yaw rate (11);d) Determining a first value for rack position control based on the difference of the actual yaw rate (11) and the predicted yaw rate (10) in a first controller (9);e) Calculating a steady state yaw rate (15) of the road vehicle;f) Calculating a difference between the steady state yaw rate (15) and the actual yaw rate (11);g) Determining a second value for rack position control based on the difference of the actual yaw rate (11) and the predicted steady state yaw rate (15) in a second controller;h) Calculating a modified rack position request for the road wheel actuator (7) based on the sum (17) of the first and second values and the target rack position.

2. Method according to claim 1, characterized in that the first and second controller (9,14) are active at different times.

3. Method according to claim 2, characterized in that an indicator is used to analyse the motion of the road vehicle body and that the indicator deactivates the unwanted controller (9,14).

4. Method according to one of the preceding claims, characterized in that the predicted steering wheel angle is calculated adding to the current measured steering wheel angle (5stw) the measured steering wheel speed (5stw) multiplied with a prediction horizon (dt).

5. Method according to claim 4, characterized in that the prediction horizon (dt) is 0.4 seconds.

6. Method according to one of the preceding claims, characterized in that the second controller (14) uses a proportional term including a reverse calculation of the reference yaw rate to add a steering angle to compensate for part of the yaw rate deviation.

7. Method according to claim 1, characterized in that the second controller (14) includes an integrator which is always reset to zero unless the driver is in a steady state driving situation.

8. Method according to claim 1, characterized in that the second controller (14) integrates the yaw rate offset during a disturbance situation and then compensates the yaw angle of the vehicle.

9. Method according to claim 1, characterized in that the first and second controller (9,14) outputs are filtered (18) independently of each other or the sum of them.

10. Steer-by-wire steering system (1) for a road vehicle designed to carry out the method according to claim 1.

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