Method for controlling the front wheel steer angle.

0001The present invention relates to a method for controlling the front wheel steer angle which can improve the driving stability of a vehicle through improvement of the response properties of the vehicle with respect to the steering wheel input angle.

0002As well known, the response property of the yaw rate r of a conventional vehicle, whose front wheel steer angle δ is controlled in proportion to the steering wheel input angle δ_sw by the driver, can be expressed by the following transfer function:<maths id="math0001" num=""><img file="EP0309293A2_D0001.tif" /></maths> where<maths id="math0002" num=""><img file="EP0309293A2_D0002.tif" /></maths><maths id="math0003" num=""><img file="EP0309293A2_D0003.tif" /></maths><maths id="math0004" num=""><img file="EP0309293A2_D0004.tif" /></maths><maths id="math0005" num=""><img file="EP0309293A2_D0005.tif" /></maths><maths id="math0006" num=""><img file="EP0309293A2_D0006.tif" /></maths>

0003The symbols used here are defined as follows: <ul id="ul0001" list-style="none"><li>Gí(s) : transfer function of the yaw rate r with respect to the front wheel steer angle δ_f</li><li>n : steering gear ratio ( δ_sw= n 8_f)</li><li>Vo : vehicle speed</li><li>ℓ: wheel base</li><li>K : stability factor</li><li>T_r: advance time constant of the yaw rate</li><li>ζ : damping coefficient</li><li>ω_n : natural frequency</li><li>C_f: front wheel cornering power (combination of right and left wheels)</li><li>C_r: rear wheel cornering power (combination of right and left wheels)</li><li>a : distance between the front axle and the center of gravity</li><li>b : distance between the rear axle and the center of gravity</li><li>m : mass of the vehicle</li><li>I : yaw rate moment of inertia of the vehicle</li></ul>

0004Equation (1) disregards the contribution from the rolling motion and assumes that the motion of the vehicle consists of two degrees of freedom of motion or the yawing rotational motion and the lateral parallel motion. However, it is possible to make Equation (1) to be substantially exact in most cases even when the influence from the rolling motion cannot be neglected, by appropriately modifying the coefficients in the various terms of Equations (2) through (6).

0005From Equations (5) and (6), one can see that the natural frequency On of the yawing motion of the vehicle becomes lower and the damping coefficient becomes less as the vehicle speed increases. Therefore, at high speed, the response delay between the steering wheel input by the driver and the onset of the yawing motion of the vehicle becomes so great and the damping of the yawing motion of the vehicle becomes so small that the driver experiences a certain difficulty in steering the vehicle along a prescribed course, and this difficulty increases with the increasing vehicle speed.

0006In view of such problems of conventional vehicles, a primary object of the present invention to improve the damping property of the yawing motion of the vehicle.

0007A second object of the present invention is to eliminate the undesirable time delay between the steering wheel input and the yawing motion of the vehicle.

0008A third object of the present invention is to improve the damping property of the yawing motion of the vehicle and/or to eliminate the undesirable time delay between the steering wheel input and the yawing motion of the vehicle simply through appropriate control of the front wheel steer angle with respect to the steering wheel input.

0009Viewed from one aspect the invention provides a method for controlling the front wheel steer angle in a front wheel steering system which controls the front wheel steer angle 8_f as a sum of a steering wheel input angle feed-forward term f₁ which is based on the steering wheel input angle s_sw, and a yaw rate feed-back term f₂(r) which is based on a detected value of the yaw rate r, or according to the following formula; δ_f=f₁(δ_sw) + f₂(r) (7) wherein: the transfer function of the yaw rate feed-back term f₂(r) with respect to the yaw rate value r is given by<maths id="math0007" num=""><img file="EP0309293A2_D0007.tif" /></maths> (where s is the Laplace variable and <img file="EP0309293A2_D0008.tif" /> {} denotes Laplace transformation).

0010Thus, by appropriately determining the values of Ti and T_d, the damping and time delay properties of the yaw response of the vehicle can be controlled at will. In particular, if Ti = Tr, the damping property may be controlled without affecting the other properties of the yaw response of the vehicle. Further, by controlling the front wheel steer angle by accounting for not only the proportional term of the steering wheel input angle but also the derivative term which is proportional to the steering wheel input angular speed, in the feed-forward control of the front wheel steer angle in relation with the steering operation by the driver, the delay property of the yaw response of the vehicle can be improved.

0011Since the dynamic yaw response of the vehicle is dependent on the vehicle speed, by varying Ti and T_d according to the vehicle speed Vo, a particularly favorable response can be obtained.

0012According to a preferred embodiment of the present invention, the steering wheel input angle feed-forward term f₁(δ_sw) is given as a sum of a proportional term and a derivative term or as<maths id="math0008" num=""><img file="EP0309293A2_D0009.tif" /></maths>

0013Particularly when<maths id="math0009" num=""><img file="EP0309293A2_D0010.tif" /></maths> and<maths id="math0010" num=""><img file="EP0309293A2_D0011.tif" /></maths> the yaw response of the vehicle becomes proportional to the steering wheel input, and there is no delay therebetween.

0014An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: <ul id="ul0002" list-style="none"><li>Figures 1 through 6 are block diagrams for showing the principle of the method for controlling the front wheel steer angle according to the present invention;</li><li>Figure 7 is a schematic perspective view of a structure for implementing the method according to the present invention; and</li><li>Figure 8 is an illustrative view showing an embodiment of the addition mechanism incorporated in the steering gear box.</li></ul>

DETAILED DESCRIPTION OF THE DRAWINGS

[control mode I]

Increase in the damping coefficient due to yaw rate feed-back

0015The feed-back control of the yaw rate r is performed by using a wash-out filter as shown in Figure 1.<maths id="math0011" num=""><img file="EP0309293A2_D0012.tif" /></maths><maths id="math0012" num=""><img file="EP0309293A2_D0013.tif" /></maths><maths id="math0013" num=""><img file="EP0309293A2_D0014.tif" /></maths><maths id="math0014" num=""><img file="EP0309293A2_D0015.tif" /></maths>

0016By eliminating <maths id="math0015" num=""><img file="EP0309293A2_D0016.tif" /></maths> from Equations (15) and (16),<maths id="math0016" num=""><img file="EP0309293A2_D0017.tif" /></maths><maths id="math0017" num=""><img file="EP0309293A2_D0018.tif" /></maths>

0017Here, by setting Ti = T_r and rearranging the above equation, the yaw rate transfer function G₂(s) with respect to the steering wheel input angle 8 _sw is given by<maths id="math0018" num=""><img file="EP0309293A2_D0019.tif" /></maths>

0018On the other hand, since the yaw rate transfer function of a conventional front wheel steering vehicle without any yaw rate feed-back is, from Equation (1), given by G₂(0) = (1/n)G₁(0) the following relationship can be derived as follows:<maths id="math0019" num=""><img file="EP0309293A2_D0020.tif" /></maths>

0019The difference between Equations (17) and (17') is found only in the s-term in the denominator, and it means that the yaw rate feed-back adds T_d to the coefficient <maths id="math0020" num=""><img file="EP0309293A2_D0021.tif" /></maths>. In other words, by performing the yaw rate feed-back given in Figure 1 and setting the parameter T₁ = T_r, the coefficient of the s-term or the damping term in the denominator of Equation (17), which gives the characteristic equation for the yaw rate, increases by T_d. The other terms of Equation (17) are identical to those of Equation (17').

0020In short, as shown in Figure 1, by performing a feed-back through a wash-out filter such as the one expressed by T_ds/(1 + T_rs), the damping of the yaw response of the vehicle can be controlled at will without changing the natural frequency On and the advance time constant T_r of the yaw rate.

[control method II]

0021Feed-forward control which adds a derivative term based on the steering wheel angular velocity to the front wheel steer angle, in addition to the steering wheel input angle

0022As shown in Figure 2, the front wheel steer angle 8_f with respect to the steering wheel input angle δ_sw is dynamically controlled according to the transfer function (1 + T_2S), instead of controlling the same simply with a fixed gear ratio (1/n). The coefficient T₂ of the s-term is generally known as the differentiation time. From the previous discussion, Figures 2 and 3 are equivalent to each other.

0023By combining the blocks arranged in series in Figure 3 into one, Figure 4 is obtained. In other words, the yaw rate r against the steering wheel input angle δ_sw is given by

0024<maths id="math0021" num=""><img file="EP0309293A2_D0022.tif" /></maths>Here, by selecting the time constants T₂ and T_d such that<maths id="math0022" num=""><img file="EP0309293A2_D0023.tif" /></maths>and<maths id="math0023" num=""><img file="EP0309293A2_D0024.tif" /></maths>

0025Equation (18) can be simplified into<maths id="math0024" num=""><img file="EP0309293A2_D0025.tif" /></maths> and only the proportional term remains, with the result that the transfer function given in Figure 4 can be reduced into the one shown in Figure 5.

0026The parameters T₂ and T_d which satisfy Equations (19) and (20) can be readily found as follows:<maths id="math0025" num=""><img file="EP0309293A2_D0026.tif" /></maths><maths id="math0026" num=""><img file="EP0309293A2_D0027.tif" /></maths>

0027By rewriting Equations (22) and (23) using Equations (4) through (6), one obtains<maths id="math0027" num=""><img file="EP0309293A2_D0028.tif" /></maths><maths id="math0028" num=""><img file="EP0309293A2_D0029.tif" /></maths>

0028In others words, T₂ and T_d both becomes functions of the vehicle speed Vo only. The other terms are constants which are all uniquely determined by the specifications of the vehicle.

0029In the actual process of control, as shown in Figure 6, the vehicle speed Vo is detected and the parameters T₂ and T_d are computed according to Equations (24) and (25). Then, the front wheel steer angle 8_f is controlled as a process of addition and subtraction with respect to the steering wheel input angle 8 _sw, according to the steering wheel input angle δ_sw and the yaw rate r.

0030According to this control method, the response of the yaw rate r becomes free from time delay with respect to the steering wheel input angle δ_sw and the behavior of the vehicle becomes highly predictable with the result that the driving of the vehicle is much simplified for the driver.

0031Figure 7 shows a specific structure for implementing the above described method.

0032The information, such as the vehicle speed, the steering wheel input angle, the front wheel steer angle and so on, detected by a vehicle speed sensor 1, a steering wheel input angular speed sensor 2, a steer angle sensor 5, a yaw rate gyro 6 and so on, is supplied to a computer 6 which realizes the previously mentioned transfer function through a digital process, and performs the required steering operation by supplying to a motor 3 a certain drive signal as an addition or subtraction with respect to the steering wheel input angle. The steering angle wheel angular speed sensor 2 may conveniently consist of an angular speed sensor, such as a tacho-generator which can directly measure the angular speed, but may also consist of an angle sensor, such as a potentiometer or a rotary encoder, whose output is to be differentiated by the computer. The addition and subtraction process for the steering wheel input angle is performed by an addition mechanism 10, as shown in Figure 8, which is incorporated in the steering gear box 4. This addition mechanism comprises an input shaft 11 which is connected to the steering shaft, a plurality of planetary gears 13 pivotally supported by a carrier 14 which is in turn securely attached to the input shaft. The planetary gears 13 mesh with a sun gear 15 as well as with a fixed internal gear 12. A plurality of different planetary gears 16 are pivotally supported by a carrier 17 which is fixedly attached to an output shaft 20 for undergoing an angular motion proportional to the steer angle of the front wheels, and mesh with a rotatably supported internal gear 18, as well as with the common sun gear 15. The pinion gear 19 which is fixedly attached to the output shaft of the motor 3 meshes with an outer gear formed around the internal gear 18.

0033In this mechanism, when the motor 3 is not turning, the rotation of the input shaft 11 is transmitted to the output shaft 20 at one-to-one ratio by way of the planetary gears 13, the sun gear 15 and the planetary gears 16.

0034On the other hand, as the motor 3 turns, the internal gear 18 turns accordingly, and the resulting rotation of the planetary gears 16 adds to the rotation of the output shaft 20.

0035In the above described embodiment, attention was directed to the output response of the yaw rate, but it is equally possible to direct the attention to the lateral acceleration of the vehicle as the output response. Thus, the time delay of the response of the yaw rate r with respect to the steering wheel input angle δ_sw is eliminated, and the behavior of the vehicle becomes highly predictable so that a substantial advantage is obtained in reducing the burden on the driver.