GB2323454A

GB2323454A - Torque distribution control in a vehicle

Info

Publication number: GB2323454A
Application number: GB9813549A
Authority: GB
Inventors: Koji Matsuno
Original assignee: Fuji Jukogyo KK; Fuji Heavy Industries Ltd
Current assignee: Subaru Corp
Priority date: 1994-06-27
Filing date: 1995-06-26
Publication date: 1998-09-23
Anticipated expiration: 2015-06-26
Also published as: GB9813549D0; GB2323454B

Abstract

A four-wheel drive vehicle is controlled to improve the steering and road holding of a vehicle in a marginal condition when at least some of the vehicle wheels begin to slip during a turn. To achieve this the vehicle has a yaw rate sensor 17, a lateral acceleration sensor 18, a steering angle sensor 16 and a vehicle speed sensor 15 signals from the sensors are delivered to a control unit 20 where the yaw rate and lateral acceleration are calculated and the difference between the calculated rates and the sensed rates is used to select values of front and rear wheel cornering power (Kf,Kr). Using the steering angle and the vehicle speed a target angular velocity is calculated using calculating means 21. Together with the cornering powers a target directional angular velocity is calculated using means 24. From the directional angular velocity and the target directional angular velocity is calculated a correction coefficient (Ke) which is used to control a torque distribution means.

Description

A DRIVING FORCE CONTROL SYSTEM AND A METHOD OF CONTROLLING THE DRIVING FORCE OF A VEHICLE The present invention relates to a control system for controlling the driving force of a vehicle so as to trace its projected course in a marginal region.

This is a division of GB9512954.0 (GB-2290633-A). The subject matter relating to controlling the engine power by controlling the fuel injector of an engine is claimed alone and in combination with the control of torque distribution between the front and rear wheels in the parent application.

Generally, in a situation where tire grip is sufficiently secure, the behaviour of a vehicle during a turning acceleration can be analysed according to a vehicular motion model. In this state within a small lateral slip angle, for example up to around 3 degrees, the vehicle turns smoothly with a lateral movement corresponding to a yawing of the vehicle. However, in a running condition where tire grip reaches the limit of adhesion, such as running on a road with a surface having low friction coefficient, the vehicular motion model can not be applied by itself. Therefore, the relationship between the yawing and the lateral movement of the vehicle is not established. In this marginal state, for example in the case of a front wheel drive vehicle, the vehicle shows such an awkward behaviour that it does not follow the desired path but drifts out due to side slip of the front wheels when the driving force is too large.

In the driving force control techniques it is important to secure stability and manoeuvrability in the marginal state on the low friction coefficient road surface.

With respect to the technique for controlling vehicular behaviour in the marginal region where a tire characteristic can not be approximated to a linear expression there is a technique disclosed in Japanese Patent Application A-4-179207.

This Patent application proposes a technique in which the cornering powers of front and rear wheels are determined according to the differences between a target yaw rate and an actual yaw rate and between a target lateral acceleration and an actual lateral acceleration. A slip angle of the vehicle body is estimated based on the equations of motion parametrising these cornering powers by forecasting the slip angle at the marginal region. Thus the driving force is controlled according to the torque distribution ratio corresponding to the estimated slip angle.

However, in the prior art control system described above, since the driving force is controlled based on an estimated slip angle of the vehicle body there is a disadvantage that the control system is ineffective in the case where the vehicle drifts out regardless of the position of the steering wheel.

The present invention is intended to obviate the disadvantage of the known driving force control system. It is an object of the present invention to provide a driving force control system for a vehicle capable of following the intended course of the vehicle under any driving conditions.

Accordingly the present invention provides a driving force control system in a vehicle having a fuel injected engine and torque distribution control means to control the distribution of torque between front and rear wheels, comprising: a vehicle yaw rate sensor for detecting an actual yaw rate, a vehicle lateral acceleration sensor for detecting an actual lateral acceleration, a vehicle steering angle sensor for detecting the steering angle, a vehicle speed sensor, a target directional angular velocity determining means responsive to the steering angle and the vehicle speed to determine a target yaw rate, a directional angular velocity determining means responsive to said steering angle and said vehicle speed to calculate a calculated yaw rate and a calculated lateral acceleration and further responsive to a front wheel cornering power and a rear wheel cornering power to calculate a directional angular velocity of the vehicle, a deviation calculating means for calculating; a deviation of the calculated yaw rate from the actual yaw rate and a deviation of the calculated lateral acceleration from the actual lateral acceleration, a tire characteristic control means for estimating the front and rear wheel cornering power based on the yaw rate deviation and the lateral acceleration deviation, correction coefficient generating means responsive to the target directional angular velocity and the directional angular velocity to generate a correction coefficient for application to the torque distribution control means so that the engine torque is distributed to maintain control of the vehicle.

The directional angular velocity calculating means calculates a yaw rate and a lateral acceleration and a directional angular velocity by solving equations of motion of the vehicle. The parameters necessary for solving the equations of motion are supplied from a steering angle sensor and a vehicle speed sensor. The cornering powers of the front and rear wheels are also used as parameters to calculate the directional angular velocity.

Further according to the present invention there is provided a method of controlling the driving force in a vehicle having a fuel injected engine and torque distribution control means to control the distribution of torque between front and rear wheels, comprising the steps of: detecting an actual yaw rate, detecting an actual lateral acceleration, detecting the steering angle, detecting the vehicle speed sensor, calculating a calculated yaw rate and calculating a calculated lateral acceleration according to the steering angle and the vehicle speed calculating a directional angular velocity of the vehicle in response to the steering angle, the vehicle speed and cornering powers of the front and rear wheels, determining a target directional angular velocity of the vehicle according to the steering angle and the vehicle speed, calculating a deviation of the calculated yaw rate from the actual yaw rate and the deviation of the calculated lateral acceleration from the actual lateral acceleration, estimating the front and rear wheel cornering power based on the yaw rate deviation and the lateral acceleration deviation, generating a correction coefficient in response to the target directional angular velocity and the directional angular velocity, applying the correction coefficient to the torque distribution control means so that the engine torque is distributed to maintain control of the vehicle.

The deviation calculating means calculates a deviation of the calculated yaw rate from the actual yaw rate and a deviation of the calculated lateral acceleration from the actual lateral acceleration. The actual yaw rate and lateral acceleration data are detected by a yaw rate sensor and a lateral acceleration sensor respectively and supplied to the deviation calculating means.

The object of the tire characteristic control means is to estimate a cornering power of the front and rear wheels based on the yaw rate deviation and lateral acceleration deviation.

The target directional angular velocity determining means determines a target directional angular velocity by using the equations of motion parametrising the steering angle, the vehicle speed and a predetermined cornering power of the front and rear wheels. These predetermined cornering powers are obtained from the equivalent cornering powers of a tire on the road with a high friction coefficient of road surface.

The correction coefficient generating means acts to generate a correction coefficient for correcting the torque distribution between the front and rear wheels. The correction coefficient is determined according to the degree of difference between the directional angular velocity and the target directional angular velocity.

In the present invention the steering behaviour of the vehicle is measured using the parameters of calculated yaw. rate calculated lateral acceleration, and directional angular velocity determined from the actual yaw rate and actual lateral acceleration. When the vehicle shows a marginal behaviour such as a drift-out during a turn on the low friction coefficient road, based on the deviation between the calculated yaw rate and the actual yaw rate and the deviation between the calculated lateral acceleration and the actual lateral acceleration, the cornering powers of front and rear wheels are estimated with high accuracy according to the state of the marginal behaviour of the vehicle.

In the directional angular velocity calculating means, the directional angular velocity is estimated according to the deviation of the vehicle path from its intended course in the marginal behaviour of the vehicle. In the target directional angular velocity determining means the target directional angular velocity is determined based on the steering angle and the vehicle speed on the basis of the cornering characteristic of the vehicle on the high friction coefficient road and in the correction coefficient generating means, the correction coefficient for reducing the engine power is determined according to the degree of the deviation of the directional angular velocity from the target directional angular velocity, that is, according to the degree of the vehicular deviation from its intended course.

An embodiment of a driving force control system and a method of controlling the driving force of a vehicle embodying the present invention will now be described, by way of example, with reference to the accompanying drawings; in which: Fig. 1 is a block diagram showing an embodiment of a driving force control system for a vehicle according to the present invention; Fig. 2 is a schematic diagram showing a vehicle in which is installed a driving force control system according to the present invention; Fig. 3 is a diagram showing a motion of a vehicle and a two-wheel vehicle model; Fig. 4 is a diagram showing the relationship between a correction coefficient of engine output and a deviation ratio of an angular velocity of course direction from a target angular velocity of course direction; and Fig. 5 is a diagram showing a trace brought about by a vehicle turning a circle.

Referring now to Fig. 2, numeral 1 denotes a vehicle having an engine 2 installed at the front. The engine 2 is connected with a transmission 4 through a clutch 3. A drive shaft 5 of the transmission 4 is connected with the front wheels 8 through a differential apparatus 6 and drive shafts 7 so as to transmit power to the front wheels 8 and the rear wheels 9. As a fuel injector 11 is provided on the engine 2 downstream of a throttle valve 10 in the intake system of the engine 2. To control the output power of the engine a fuel injection signal Tp may be transmitted from a fuel injection control apparatus 12 to the fuel injector 11.

The vehicle 1 has a vehicle speed sensor 15 for detecting vehicle speed V, a steering angle sensor 16 for detecting steering angle b f, a yaw rate sensor 17 for detecting actual yaw rate T' and a lateral acceleration sensor 18 for detecting actual lateral acceleration Go'. The signals from these miscellaneous sensors are input to a control unit 20 and processed therein.

Before a specific description of the electronic control system is given, it will be helpful in understanding the present invention to describe the basic principle of the control. It is well known that a turning performance or coursetraceability of a vehicle is largely dependent upon a change in the coefficient of friction of road surface. When the friction coefficient of the road surface is low around the limit of tire grip, the lateral force of the tire is lowered according to the theory of a circle of friction. If this lowering of lateral force of tire is deemed as a lowering of cornering powers Kf, Kr of front and rear wheels, a vehicular motion model of a linear region can be extended to apply to a marginal region.

That is to say, a calculated yaw rate T and a calculated lateral acceleration Gy are calculated by solving equations of vehicular motion based on a steering angle & f and a vehicle speed V and then a deviation AT of the calculated yaw rate T from an actual yaw rate T' and a deviation AG of the lateral acceleration Gy from an actual lateral acceleration Gy' are calculated respectively. Based on these deviations A and AG, the cornering powers Kf, Kr of front and rear wheels can be estimated with high accuracy according to vehicle behaviour in the marginal region. Details of these calculation processes are described in Japanese Patent Application A-4-179207.

Among variables of state indicating a coursetraceability of the vehicle 1, there is an angular velocity of course direction u (hereinafter referred to as directional angular velocity u. The directional angular velocity u is a value obtained by differentiating an angle of course direction e according to a trace of the vehicle 1 turning with a radius R, as shown in Fig. 5 and is expressed as a function of an inverse number 1 / R of the turning radius R, the vehicle speed V, the lateral acceleration Gy and the yaw rate T.

Accordingly, the value of the directional angular velocity u does not change so long as the vehicle follows the reference circle, even if a change occurs in the behaviour of the vehicle during turning on a low friction coefficient road. However, once the vehicle gets out of the reference circle, i.e., when the vehicle goes away outwardly, that value is reduced and when it goes away inwardly, the value is increased. That is to say, the change of the directional angular velocity u indicates a state of deviation from the course of the vehicle. Thus, the course-traceability of the vehicle can be improved by watching the directional angular velocity u.

Next, it will be described how the directional angular velocity u is calculated.

First, referring to Figs. 3(a) and 3(b), the movement of the point P of a centre of gravity will be described. Here, a coordinate system fixed to ground is designated as X-Y and the one fixed to the vehicle is designated as x-y. Angles around a vertical axis are defined positive, if anticlockwise and negative, if clockwise. Assuming that the vehicle travels at constant speed, a velocity vector of the point P is expressed as follows: R = ua + vb (1) where R; a position vector at the point P in the coordinate plane X-Y, a; a unit vector in the x direction, b; a unit vector in the y direction, u; a velocity component in x direction, v; and a velocity component in y direction.

An acceleration vector of the point P is expressed as follows: R = ua + ua + vb + vb (2) Where #a, #b are deviation amounts for a At second and 7 denotes a yaw rate of the vehicle, since Aa = TAtb, b = ##ta, the velocity vector of a, b is expressed as follows: a = Tb, b = - Ta (3) Therefore, the acceleration vector of the point P is as follows: R = (u - vn)a + (v + uT)b (4) Since the vehicle travels at the constant speed, a vehicle speed V is constant, In this case, the motion of the point P is expressed as follows using a side slip angle ( is small) u = Vcos =, V, v = Using = V u = Vsin = -V v = Vcos = vp (5) Substitution of the equations (5) into the equation (4) gives the following equation: R = -V( + #) a + V( + T)b (6) Further, if the side slip angle is small, the equation (6) is expressed as follows: R - V( + That is to say, the centre of gravity of the vehicle P can be regarded to have an acceleration whose direction is perpendicular to the direction in which the vehicle travels and the acceleration is determined by the vehicle speed V, the change of side slip angle and the yaw rate T. Hence, the lateral acceleration Gy and the directional angular velocity u are calculated according to the following equation (8).

Gy = V( + T ) = Vu By use of a two wheel vehicle model shown in Fig. 3(c), an equation of motion when the vehicle turns around a fixed circle will be described.

Following equations whose variables are the side slip angle and the yaw rate T are established.

mV( + T) = Yf + Yr (9) Iz = LfYf - LrYr (10) Where m; a vehicle mass, V; a vehicle speed, I; an inertia of yaw moment, Yf, Yr; a cornering force of front and rear wheels respectively and Lf, Lr; a distance from the centre of gravity to the centre of front and rear wheels respectively.

In the region where the cornering forces Yf, Yr can be treated as being in a linear relationship with the tire slip angles pf, Pr, using the equivalent cornering powers Kf, Kr, the cornering forces Yf, Yr are expressed as Yf = 2Kf f, Yr = 2KrBr. Substituting these into the equations (9) and (10) , the following equations are obtained.

mV( + T) = 2Kf(Ff- -LfT/V)+2Kr(6r- +Lrn/V) (11) IT = 2LfKf(Ff- -LfT/V)+2LrKr(br- +LrT/V) (12) where bf; a steering angle.

By using these three equations (8), (11) and (12), the yaw rate T, the lateral acceleration Gy and the directional angular velocity u are calculated.

Based on the above mentioned basic principle, the control system as shown in Fig. 1 will be described.

The control unit 20 includes directional angular velocity calculating means 21 to which the steering angle df, the vehicle speed V and the estimated cornering powers Kf, Kr of front and rear wheels are input. These parameters constitutes an adaptive observation system according to the adaptive control theory. The important thing of this invention is that the yaw rate 7, the lateral acceleration Gy and the directional angular velocity u are calculated by applying the above mentioned equations of motion, (8), (11) and (12) extended to the marginal region. Further, the yaw rate 7 and the lateral acceleration Gy calculated in the directional angular velocity calculating means 21 are transmitted to deviation calculating means 22 wherein deviations A7 and AG are calculated by subtracting the actual yaw rate T' and actual lateral acceleration Gy' from the calculated yaw rate T and calculated lateral acceleration Gy respectively.

The deviation AT T of the yaw rate and the deviation AG of the lateral acceleration are input to tire characteristic control means in which the cornering powers Kf, Kr of front and rear wheels are estimated based on these deviations AT and AG.

That is to say, here, in a case where the actual lateral acceleration Gy is decreased and AG is positive, since it is judged that the vehicle is drifting out or spinning in the marginal area, both the cornering powers Kf and Kr should be reduced. On the other hand, in a case where AG is negative, since it is judged that the vehicle is in tuck-in, both Kf and Kr should be increased. In a case where the actual yaw rate 7 is reduced and AT is positive, judging that the vehicle is drifting out, the cornering power Kf of the front wheels should be reduced and Kr of the rear wheels should be increased. In a case where the actual yaw rate T is increased and AT T is negative, judging that the vehicle is spinning, Kf of the front wheels should be increased and Kr of the rear wheels should be reduced. How the cornering powers Kf, Kr are corrected according to the state of both deviations AT, AG is summarised in the following Table 1: Table 1 Kf Kr Reduce Reduce If vG > 0 Increase Increase If AG c 0 Reduce Increase If AT 7 > 0 Increase Reduce If A7 c < O The cornering powers Kf, Kr corresponding to drift-out or spinning of the vehicle in the marginal region are determined correctly every moment by reducing or increasing the cornering powers previously obtained by a predetermined increment according to the Table 1.

Further, the control unit 20 has target directional angular velocity determining means 24 for determing a target directional angular velocity #0 corresponding to the steering angle bf and the vehicle speed V of the moment. That is to say, the target directional angular velocity VO is determined by using the aforementioned equations of motion (8), (11) and (12). When solving these equations, the cornering powers Kf, Kr of the parameters thereof are assumed to be a constant value on the high friction coefficient road respectively.

The directional angular velocity v and the target directional angular velocity VO are input to correction coefficient generating means 25 wherein the state of deviation of the vehicle from its intended course is judged from the difference between the calculated directional angular velocity and the target directional angular velocity V0 and a control signal for controlling the engine power may be outputted therefrom, namely a correction coefficient Ke for correcting the engine power is calculated. The correction coefficient Ke is determined for example as shown in Fig. 4, as a function of the deviation ratio e, where e = (u - u0)/v0.

Specifically in this embodiment, when the vehicle trace (path) comes outside of its intended course, u becomes smaller than vO(v < vO) and the deviation ratio e becomes negative.

When the vehicle trace comes inside of its intended course, v becomes larger than vO(V > vO) and the deviation ratio e becomes positive. When the deviation ratio exceeds 20% on the positive or negative sides, the correction coefficient Ke is so determined as being reduced according to an increase or decrease of the deviation ratio. The signal of the correction coefficient Ke may be output to the fuel injection control apparatus 12 for correcting the fuel injection amount Tp. That is to say, if Ke is 1.0, no correction is made on the fuel injection amount Tp, and if Ke becomes 0, the fuel injection amount Tp is corrected to the minimum value for example.

When the vehicle is operated, the engine 2 supplies power to the front wheels 8 through the transmission 4 and the differential 6. Supplied power is controlled by controlling the fuel injection amount Tp of the fuel injector 11.

Then, in the directional angular velocity calculating means 21 of the control unit 20, the yaw rate J, the lateral acceleration Gy and the directional angular velocity v are calculated and further, in the deviation calculating means 22, the deviations A7 T and AG are calculated based on those calculated yaw rate T and lateral acceleration Gy and based on those detected actual yaw rate T' and actual lateral acceleration Gy' In the tire characteristic control means 23, the cornering powers Kf, Kr of front and rear wheels 8, 9 are estimated respectively according to the control method based on the adaptive control theory. Finally, in the directional angular velocity calculating means 21, the directional angular velocity u is calculated and the state of deviation of the vehicle from its intended course is always watched.

When the vehicle runs on a road with a dry surface, i.e., a high friction coefficient road, the grip of the tire is sufficient, the calculated yaw rate T and lateral acceleration Gy approximately coincide with the actual yaw rate T' and lateral acceleration Gy'. Because of this, the cornering powers Kf, Kr which are estimated in the tire characteristic control means 23 are those which the tire possesses originally and therefore the directional angular velocity v coincides with the target directional angular velocity VO approximately.

Consequently, the correction coefficient Ke for correcting the engine power becomes 1.0 according to the map shown in Fig. 4 and thus the engine power is not corrected.

On the other hand, when the vehicle makes a turn with acceleration on a low friction coefficient road, the side force of the front drive wheel 8 becomes small and as a result the vehicle gets in the situation where a side slip first occurs on the front wheel side. When actually the front wheel 8 exceeds a grip limit of tire and slips outward in the lateral direction, as illustrated in Fig. 5, the vehicle trace n travels outside of the target course m. At this moment, both deviations AG and A7 T become positive due to decreases of the actual lateral acceleration Gy' and the actual lateral acceleration T' and as a result of this the cornering powers Kf, Kr of front and rear wheels are corrected so as to give a larger decrease to the cornering power Kf of the front wheel.

As a result of this, the directional angular velocity V calculated in the directional angular velocity calculating means 21 becomes a small number abruptly responding to the situation of the vehicle, "drift out", on the low friction coefficient road. Further, when the deviation ratio e of the directional angular velocity u from the target directional angular velocity vO exceeds -20%, the correction coefficient Ke is determined to be less than 1.0, to reduce engine power, according to the map shown in Fig. 4.

Consequently, the fuel injection amount Tp of the fuel injector 11 is reduced by this correction coefficient ke and due to this the drive force of the front wheel 8 is reduced.

Then, the side force of tire on the front wheel 8 is increased with a decrease of the drive force and consequently the side slip of the front wheel 8 is restrained. Thus, the vehicle 1 is prevented from drifting out and the vehicle trace n is corrected so as to coincide with the target course m.

When the vehicle 1 is prevented from drifting out, the actual lateral acceleration Gy' is restored and the actual yaw rate T' is increased. Then, the calculated directional angular velocity ! is also increased. Further, engine power is gradually returned to the original state as the deviation ratio e is decreased. Thus, the vehicle 1 can be smoothly turned so as not to deviate from the trace determined based on the turning characteristic on the high friction coefficient road even at the marginal region on the low friction coefficient road.

On the other hand, in the case of the rear wheel drive vehicle, when the vehicle makes a turn with acceleration on the low friction coefficient road, a lateral slip takes place at the rear wheel first and the vehicle 1 comes near spinning.

when the vehicle trace n travels inside of the target course m and it starts spinning, the directional angular velocity v is calculated in the same manner as mentioned before. Further, when the deviation ratio e exceeds +20%, the correction coefficient Ke acts on the fuel injection control apparatus 12 so as to reduce engine power, whereby the vehicle can make a smooth turn even at the marginal region on the low friction coefficient road.

In the case of the four wheel drive vehicle capable of distributing torque between front and rear wheels, the correction coefficient Ke is used for varying a ratio of torque distribution between front and rear wheels. For example, when the vehicle drifts out, the correction coefficient Ke may be changed to distribute greater torque to the rear wheel than to the front wheel and when it spins, the coefficient Ke may be changed so as to distribute larger torque to the front wheel than to the rear wheel.

In summary, the driving force control system according to the present invention comprises directional angular velocity calculating means for calculating a directional angular velocity which indicates the state of deviation of the vehicle from its intended course, deviation calculating

Claims

CLAIMS 1. A driving force control system in a vehicle having a fuel injected engine, torque distribution control means to control the distribution of torque between front and rear wheels, comprising: a vehicle yaw rate sensor for detecting an actual yaw rate, a vehicle lateral acceleration sensor for detecting an actual lateral acceleration, a vehicle steering angle sensor for detecting the steering angle, a vehicle speed sensor, a target directional angular velocity determining means responsive to the steering angle and the vehicle speed to determine a target yaw rate, a directional angular velocity determining means responsive to said steering angle and said vehicle speed to calculate a calculated yaw rate and a calculated lateral acceleration and further responsive to a front wheel cornering power and a rear wheel cornering power to calculate a directional angular velocity of the vehicle, a deviation calculating means for calculating; a deviation of the calculated yaw rate from the actual yaw rate and a deviation of the calculated lateral acceleration from having a fuel injected engine and torque distribution control means to control the distribution of torque between front and rear wheels, comprising the steps of: detecting an actual yaw rate, detecting an actual lateral acceleration, detecting the steering angle, detecting the vehicle speed sensor, calculating a calculated yaw rate and calculating a calculated lateral acceleration according to the steering angle and the vehicle speed calculating a directional angular velocity of the vehicle in response to the steering angle, the vehicle speed and cornering powers of the front and rear wheels, determining a target directional angular velocity of the vehicle according to the steering angle and the vehicle speed, calculating a deviation of the calculated yaw rate from the actual yaw rate and the deviation of the calculated lateral acceleration from the actual lateral acceleration, estimating the front and rear wheel cornering power based on the yaw rate deviation and the lateral acceleration deviation, generating a correction coefficient in response to the target directional angular velocity and the directional angular velocity, applying the correction coefficient to the torque distribution control means so that the engine torque is distributed to maintain control of the vehicle.

6. A method according to claim 5 wherein comprising the step of determining the characteristics of cornering power from.

the tire characterlstics on a road with a high friction coefficient.

7. A method according to one of claims 6 or 7 comprising the step of obtaining a deviation ratio by diving the deviation of said directional angular velocity and sad target directional angular velocity by said target directional angular velocity and setting the torque distribution equal to the deviation ratio.

8. A diving force control method as herein described.