System for monitoring vehicular slip angle

Abstract: A slip angle monitoring system directly compares an ab­ solute value of a lateral acceleration ( | Y″ | ) with an ex­ perimentarily derived given value which serves as a slip criterion causing a slip angle on a vehicle. Slip angle is set at zero while the absolute value of the lateral accelera­ tion is smaller than or equal to the given value. On the other hand, when the absolute value is greater than the given value, the slip angle (ϑ₁ or ϑ₂) is derived by dividing the lateral acceleration by the longitudinal acceleration or by taking minus one power of tangent of quotient dividing the lateral acceleration by the longitudinal acceleration.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a system for monitoring slip angle at a vehicle, such as an automotive vehicle. More specifically, the invention relates to a system which can project vehicular slip angle with simple construction and a simplified process. Description of the Background Art In the modern control systems for controlling vehicular activity in an automotive vehicle, such as a suspension control system, anti-skid brake control system, power train control and so forth, a slip angle at a road wheel is one of parameter to perform precise control operation. In order to monitor wheel slip angle, Japanese Patent First (unexamined) Publication No. 62-299430 discloses a vehicular attitude control system for a four-wheel drive type vehicle, which controls wheel slip angle within a desired range. The disclosed system includes a longitudinal acceleration sensor for monitoring a longitudinal acceleration exerted on the vehicular body and a lateral acceleration sensor for monitoring a lateral acceleration exerted on the vehicle body. A wheel slippage date is derived by dividing the lateral acceleration value by the longitudinal acceleration value. The arithmetic operation is performed when judgement is made that the vehicle is traveling in a curved road or corner. Judgement, in the proposed system, is made by monitoring a steering angular displacement to detect steering angle greater than a predetermined value. The disclosure further proposes to detect the vehicular condition passing through the curved road or corner by comparing the longitudinal acceleration and the lateral acceleration. The disclosed system intends to control drifting magnitude of the four-wheel drive type vehicle for tight cornering and presenting spinning of the vehicle due to excess slip angle on the wheel. In this aspect, the disclosed system is successful to provide slip angle data for a control system which controls means for generating a yawing force for drifting. However, when the disclosed system is applied for proving slip angle data for the purpose other than drift control, the accuracy level of detection of occurrence of the lateral slip of the wheel simply monitored on the basis of the steering angle, cannot be satisfactorily high for control operation requiring higher precision level of detection of the slip angle. Namely, since the magnitude of lateral acceleration is variable depending upon the vehicle speed even at the same steering angular displacement, slip angle can be varied depending upon not only the steering angular position but also vehicular speed. When the vehicle speed is high, relatively large wheel slip angle can be created even by small magnitude steering angular displacement. On the other hand, in the latter case, when the vehicle is driven at a substantially constant speed, the longitudinal acceleration on the vehicle body is maintained substantially zero. Therefore, any magnitude of lateral acceleration may cause judgement that the vehicle is turning. Furthermore, when the vehicle is in deceleration (acceleration being negative), lateral acceleration held at zero becomes greater than the longitudinal acceleration to trigger slip angle derivation. On the other hand, while the vehicle is in decelerating or acceleration state, the longitudinal acceleration can be greater than the lateral acceleration so that the system ignore so as not to perform derivation of the wheel slip angle despite the vehicle is turning. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a system for monitoring slip angle on a vehicle. In order to accomplish aforementioned and other objects, a slip angle monitoring system, according to the present invention, directly compares an absolute value of a lateral acceleration with an experimentarily derived given value which serves as a slip criterion to cause slip angle on a vehicle. Slip angle is set at zero while the absolute value of the lateral acceleration is smaller than or equal to the given value. On the other hand, when the absolute value is greater than the given value, the slip angle is derived by dividing the lateral acceleration by the longitudinal acceleration or by taking minus one power of tangent of quotient dividing the lateral acceleration by the longitudinal acceleration. Preferably, the slip angle may be corrected on the basis of a wheel acceleration in order to incorporate wheel acceleration dependent component in the resultant value. In the practical construction, the given value may be variable depending upon a vehicle speed or the longitudinal acceleration. According to one aspect of the invention, a system for monitoring a slip angle of a vehicle comprises: a longitudinal acceleration monitoring means for monitoring a longitudinal acceleration exerted on the vehicle and producing a longitudinal acceleration indicative signal; a lateral acceleration monitoring means for monitoring a lateral acceleration exerted on the vehicle and producing a lateral acceleration indicative signal; a wheel speed sensor for monitoring a rotation speed of a vehicle wheel to produce a wheel speed indicative signal; and an arithmetic circuit for receiving said longitudinal acceleration indicative signal, said lateral acceleration indicative signal and said wheel speed indicative signal, said arithmetic circuit comparing the absolute value of said lateral acceleration indicative signal with a predetermined value and calculating a basic slip angle on the basis of values of said longitudinal acceleration indicative signal and said lateral acceleration indicative signal if the absolute value of said lateral acceleration indicative signal is greater than said predetermined value; and is characterized in that said basic slip angle is corrected with a correction factor resulting from differentiating said wheel speed indicative signal to give a slip angle when the absolute value of said lateral acceleration indicative signal is greater than said predetermined value. The arithmetic circuit may derive the correction factor by differentiating the wheel speed indicative signal. Practically, the correction factor is a wheel acceleration data to be added to the longitudinal acceleration indicative data. The arithmetic circuit derives the basic slip angle by dividing the longitudinal acceleration indicative signal by the lateral acceleration indicative signal. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description of the invention given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment, but are for explanation and understanding only. In the drawings: Fig. 1 is an illustration showing a slip angle monitoring system, which is applied to an automotive vehicle; Fig. 2 is an illustration showing slip angle in relation to the vehicle activity including vehicle traveling direction and accelerations exerted on a vehicle body; Fig. 3 is a block diagram of a slip angle monitoring system of Fig. 1 ; Fig. 4 is an illustration showing an embodiment of a slip angle monitoring system according to the present invention, which is applied to an automotive vehicle; Fig. 5 is an illustration showing slip angle in relation to the vehicle activity including vehicle traveling direction and accelerations exerted on a vehicle body; and Fig. 6 is a block diagram of the embodiment of a slip angle monitoring system of Fig. 4 ; DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Fig. 1 , a slip angle monitoring system has a longitudinal acceleration sensor 2 and a lateral acceleration sensor 3 . These longitudinal and lateral acceleration sensors 2 and 3 may be arranged on appropriate positions on the vehicle body 1 . The longitudinal acceleration sensor 2 monitors a longitudinal acceleration exerted on a vehicle body and produces a longitudinal acceleration indicative signal X'' . The direction of the longitudinal acceleration coincides with the direction of the longitudinal axis of the vehicle body. The lateral acceleration sensor 3 monitors a lateral acceleration exerted on the vehicle body and produces a lateral acceleration indicative signal Y'' . The direction of the lateral acceleration Y'' is transverse to the longitudinal axis of the vehicle body. The longitudinal and lateral acceleration sensors 2 and 3 are connected to an arithmetic circuit 4 which arithmetically derives a slip angle data. Assuming that the vehicle travels along a path represented by an arrow A , the center O of the vehicle body 1 passes a center of the path as indicated by the arrow A . During curving, a lateral acceleration as indicated by' a vector X'' and a longitudinal acceleration as indicated by a vector Y'' are exerted on the vehicle body 1 . A composite acceleration as indicated by a vector Z is the acceleration actually exerted on the vehicle body. On the other hand, vehicular speed vector V is in tangential direction of the curved path A . As seen, the composite acceleration vector Z points directions transverse to the vehicular speed vector V . When the lateral acceleration Y'' is zero, the composite acceleration Z also becomes zero. This means that the vehicular speed vector V matches the vehicular longitudinal axis and thus the vehicle is traveling on a straight path. When the lateral acceleration Y'' is substantially small but not zero, the composite acceleration vector Z is offset from the vehicular speed vector V in substantially small magnitude in angular direction. As long as the angular shift of the composite acceleration vector Z with respect to the vehicular speed vector V is maintained substantially small, such angular shift may not affect cornering force of the vehicle. The angular shift range ₀ may be variable depending upon the vehicular cornering characteristics. Therefore, the angular shift range ₀ is experimentally derived according to the cornering characteristics of the vehicle to which the preferred embodiment of the slip angle monitoring system is applied, and according to a desired precision level in monitoring the slip angle. In the practical process in deriving the slip angle ₁ , the absolute value | Y'' | of the lateral acceleration indicative signal value is compared with the reference value ₀ . If the lateral value | Y'' | is smaller than or equal to the reference value ₀ , then, judgement is made that the vehicle is traveling on substantially straight road. On the other hand, if the absolute value | Y'' | of the lateral acceleration is greater than the reference value ₀ , judgement is made that the vehicle is traveling on a curved road or turning a corner. Then, the slip angle ₁ is arithmetically calculated on the basis of the longitudinal acceleration indicative signal value X'' and the lateral acceleration indicative signal value Y'' . Namely, the slip angle ₁ can be calculated from one of the following equations:  1 = X''/Y''  1 = tan -1 (X''/Y'') Fig. 3 shows a block diagram of the slip angle monitoring system. As seen from Fig. 3 , the arithmetic circuit 4 has a divider 101 receiving the longitudinal and lateral acceleration indicative signals X'' and Y'' from the longitudinal and acceleration sensor. The divider 101 implements dividing operation to divide the longitudinal acceleration indicative data X'' by the lateral acceleration indicative data Y'' . The slip angle thus derived is fed to a circuit 102 for implementing calculation according to the equation (2). Fig. 4 shows an embodiment of the slip angle monitoring system according to the present invention. In this embodiment, a wheel speed sensor 5 is provided in addition to the system of Fig. 1 . The wheel speed sensor 5 monitors rotation speed of a vehicular wheel to produce a wheel speed indicative signal. The wheel speed indicative signal of the wheel speed sensor 5 is fed to the arithmetic circuit 4 . The arithmetic circuit 4 performs arithmetic operation to derive a wheel acceleration indicative data Vx'' on the basis of the wheel speed indicative signal. The wheel acceleration indicative data Vx'' is practically derived by differentiation of the wheel speed indicative signal. With the wheel acceleration indicative data Vx'' , a wheel acceleration dependent correction value is derived for correcting the slip angle derived on the basis of the longitudinal acceleration indicative signal value X'' and the lateral acceleration indicative signal value Y'' according to the equations of (1) and (2). In practice, a corrected slip angle ₂ is derived by the following equations:  1 = (X'' + Vx'')/Y'' and  1 = tan -1 {(X'' + Vx'')/Y''} Fig. 5 shows influence of wheel acceleration Vx'' to the slip angle derived on the basis of the longitudinal acceleration and the lateral acceleration. Namely, in the example of Fig. 5 , the vehicle travels along a curved path which curves toward right with respect to the vehicular longitudinal axis. At the shown position, the vehicle speed vector V is offset in angle from the longitudinal axis of the vehicle body at an angle  . This can be precisely derived by incorporating wheel acceleration component Vx'' as additional component for determining the actual longitudinal acceleration X 1 '' (= X'' + Vx'') which is created by the centrifugal force during cornering. Assuming the lateral acceleration is Y'' or Y₁'' , the composite acceleration Z is offset with an angle ₁ . This can be compared with the composite acceleration derived on the basis of the longitudinal acceleration indicative signal value X'' and the lateral acceleration indicative signal data Y'' . It can be appreciated that the slip angle thus derived taking the wheel acceleration dependent component, can provide higher accuracy than that of the former embodiment. Fig. 6 shows a block diagram implementing the embodiment of the slip angle monitoring system according to the present invention. As can be seen from Fig. 6 , the arithmetic circuit 4 of the shown embodiment of the slip angle monitoring system includes a discrimination circuit 201 which receives the lateral acceleration indicative signal Y'' . The discrimination circuit 201 derives an absolute value | Y'' | of the lateral acceleration indicative signal value and compares the absolute value with the given value ₀ . The discrimination circuit 201 produces a discriminator signal indicative that the absolute value of the lateral acceleration indicative signal is smaller than or equal to the given value ₀ to a circuit 202 which outputs zero level slip angle indicative signal. The discriminator circuit 201 on the other hand outputs the discriminator signal representative of the absolute value of the lateral acceleration indicative signal greater than given value ₀ to a circuit 203 . The circuit 203 performs arithmetic operation according to the equation (3) or (4) as set forth above. While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding of the invention, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention set out in the appended claims.

Claims: A system for monitoring a slip angle of a vehicle (1), comprising: a longitudinal acceleration monitoring means (2) for monitoring a longitudinal acceleration exerted on the vehicle (1) and producing a longitudinal acceleration indicative signal (X''); a lateral acceleration monitoring means (3) for monitoring a lateral acceleration exerted on the vehicle (1) and producing a lateral acceleration indicative signal (Y''); a wheel speed sensor (5) for monitoring a rotation speed of a vehicle wheel to produce a wheel speed Indicative signal (V); and an arithmetic circuit (4) for receiving said longitudinal acceleration indicative signal (X''), said lateral acceleration indicative signal (Y'') and said wheel speed indicative signal (V), said arithmetic circuit comparing the absolute value of said lateral acceleration indicative signal (Y'') with a predetermined value (₀) and calculating a basic slip angle (₁') on the basis of values of said longitudinal acceleration indicative signal (X'') and said lateral acceleration indicative signal (Y''), If the absolute value of said lateral acceleration indicative signal (Y'') is greater than said predetermined value (₀); characterized in that said basic slip angle is corrected with a correction factor ( Vx''=dVx/dt ) resulting from differentiating said wheel speed indicative signal (V) to give a slip angle (₁) when the absolute value of said lateral acceleration Indicative signal (Y'') is greater than said predetermined value (₀). A system as claimed in claim 1, characterized in that said arithmetic circuit (4) sets said slip angle (₁) to zero when the absolute value of said lateral acceleration indicative signal (Y'') is less than or equal to said predetermined value (₀). A system as claimed in claim 1 or 2, characterized in that said basic slip angle is derived by dividing said longitudinal acceleration indicative signal (X'') by said lateral acceleration indicative signal (Y''). A system as claimed in claim 3, characterized in that said arithmetic circuit (4) effects the correction by adding said correction factor to said longitudinal acceleration indicative signal (X'').

Inventors: AKATSU YOHSUKE, FUJIMURA ITARU, FUKUNAGA YUKIO, FUKUSHIMA NAOTO, SATOH MASAHARU, AKATSU, YOHSUKE, FUJIMURA, ITARU, FUKUNAGA, YUKIO, FUKUSHIMA, NAOTO, SATOH, MASAHARU
Applicants: NISSAN MOTOR, NISSAN MOTOR CO., LTD.
Application year: 1989
Main classification: B60T8
IPC codes: B60T8, B60G17, G01P15