Slip control system for motor vehicle

Abstract: A slip control system for a motor vehicle controlling slips in driving wheels comprising, a sensor for detecting a slip ratio of each driving wheel, a device for decreasing a slip of each driving wheel by controlling a driving force transmitted to the driving wheel so that the slip ratio of the driving wheel is at least equal to predetermined desired slip ratio when the slip ratio of the driving wheel is equal to the desired slip ratio, a sensor for detecting a yawing rate of the vehicle, and a sensor for detecting a steering amount of the vehicle, said slip decreasing device including correcting device for decreasing the desired slip ratio when a steering condition is detected based on amounts detected by both the yawing rate detecting sensor and the steering amount detecting sensor.

Description: BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a slip control system for a motor vehicle according to the preamble of claim 1 (EP-A-0 338 588) and in particular to a slip control system which is able to improve stability of a motor vehicle when the vehicle is in the steering operation. Description of the Related Art There has been proposed a slip control system for a motor vehicle as shown in Japanese Patent Laid-Open Publication No. 58-16948, No. 57-22948, No. 62-231836, No. 64-94029, and the like. When an excessive slip is caused between driving wheels and road surfaces during the starting and/or running operations, the slip control system decreases the excessive slip under a predetermined value to obtain driving forces which are transmitted to the road surfaces from the driving wheels. Therefore not only the starting and accelerating abilities but also the running and driving stabilities are improved. The slip control system includes an engine control system which decreases a degree of an openning of a throttle valve, delays an ignition timing, and cuts a fuel to the engine so as to decrease an excessive engine output, and/or a brake control system which increases brake fluid pressure so as to increase a braking force of driving wheels transitionally. When the vehicle is running on the low friction coefficient road surface, an excessive depression of the acceleration pedal causes an excessive slip in the driving wheels. The engine control system and/or brake control system decreases the driving torque transmitted to the driving wheels so as to decrease the excessive slip under a predtermined desired value and to obtain driving forces which are transmitted to the road surfaces from the driving wheels. The behavior characteristic of the vehicle in the slip condition of the driving wheels when the vehicle is running straight is different from that when the vehicle is steered. When the vehicle is steered, a relatively large change of the behavior of the driving wheels is caused by a relatively small slip and therefore the vehicle is in the unstable condition. However, since the abovementioned slip control system controls the driving torque transmitted to the driving wheels in accordance with substantially equal or same desired slip ratio when the vehicle is running straight and is steered, the vehicle is not able to correspond properly to the behavior change of the driving wheels in the steering operation and so that the stability of the vehicle is not obtained. From the EP-A-0 321 894 a traction control system is known, comprising a steering angle sensor. The signal from this sensor is fed to a wheel liftoff prevention means. From the EP-A-0 338 588 a driving wheel torque control device is known, comprising a steering angle sensor and a yaw motion detector. A slip condition is detected according to a slip reference value corrected by a correction value to be set according to the yaw motion condition and a reference value of the yaw motion condition. SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a slip control system for improving the stability of the vehicle in the steering operation. It is another object of the invention to provide a slip control system for improving the stability of the vehicle in the steering operation with the good controllability. These objects are achieved with a slip control system with the features of claim 1. Advantageous embodiments are given in subclaims 2 to 14. The above and other objects and features of the present invention will be apparent from the following description by taking reference with accompanying drawings employed for preferred embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a general diagram of a slip control system in accordance with the present invention. Figure 2a through 2d are schematic views showing the operation of a throttle opening control mechanism. Figure 3 a a time chart showing the outline of the slip control. Figure 4 is a block diagram showing a circuit which determines a desired slip value for the brake control operation and a desired slip value for the engine control operation to set both desired slip values of the rear wheels. Figure 5 is a diagram showing the lower limit of the throttle opening determined by the traction control system. Figure 6 is a flow chart showing a main control of the traction control operation in the slip control unit UTR. Figure 7 is a flow chart showing an engine control operation in the slip control unit UTR. Figure 8 is a flow chart showing a brake control operation in the slip control unit UTR. Figure 9 is a flow chart showing how the desired slip ratio is corrected in accordace with the first embodiment of the present invention. Figure 10 is a flow chart showing the switching operation from the right and left independent brake control to the right and left integrated brake control in accordance with the second embodiment of the present invention. Figure 11 is a block diagram of the slip control system carrying out the slip control operation in accordance with the third embodiment of the present invention. Figure 12 is a flow chart showing the procedures in the brake control operation by the slip control system in Figure 11. Figure 13 is a block diagram of the slip control system carrying out the slip control operation in accordance with another example of the third embodiment of the present invention. Figure 14 is a flow chart showing the procedures in the brake control operation by the the slip control system in Figure 13. Figure 15 is a diaguram showing the relationship between the coefficient K and the yawing rate. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1, there is shown a motor vehicle A having a slip control system in accordance with an embodiment of the present invention. The vehicle A has right and left front wheels 1FR, 1FL for driven wheels, and right and left rear wheels 1RR, 1RL for driving wheels. A driving system includes an engine 2 disposed in the front portion of the vehicle, an automatic transmission 3 connected directly to a crankshaft of the engine 2, a propeller shaft 4 connected to an outputshaft of the automatic transmission 3, a differential 5 connected to the rear end portion of the propeller shaft 4, and driving shafts 6R, 6L extending from the differential 5 to right and left directions respectively and connected to the right rear wheel 1RR and the left rear wheel 1RL respectively. A throttle valve 42 is provided in an intake path 41 of the engine 2 to control the amount of the intake air. The throttle valve 42 is connected to an acceleration pedal 43 through a throttle opening control mechanism 44 and an acceleration wire 112a. The throttle opening control mechanism 44 includes a motor 106 which decreases the opening of the throttle valve 42 transitionally. The automatic transmission 3 comprises a torque converter 11 with a lockup clutch 11A operated by hydraulic pressure and a transmission 12 with a multiple-stage transmission gear mechanism. The transmission control operation is carried out by selecting combinations of magnetization and/or demagnetization of a plurality of solenoids 13a which are incorporated in a hydraulic control circuit for the automatic transmission 3. connection and disconnection of the lockup clutch 11A is carried out by selecting magnetization or demagnetization of a solenoid 13b which is incorporated in the hydraulic control circuit. The motor vehicle A includes a brake system. The brake system includes each brake 21FR, 21FL, 21RR, 21RL provided in each wheel 1FR, 1FL, 1RR, 1RL, a tandem type master cylinder 27 connected with each caliper (brake cylinder) 22FR, 22FL of the front wheel brakes 21FR, 21FL via brake conduits 23FR, 23FL, a hydraulic booster 26 connected with each caliper 22RR, 22RL of the rear wheel brakes 21RR, 21RL via brake conduits 23RR, 23RL, and a brake pedal 25. The master cylinder 27 supplies predetermined brake fluid pressure to the front wheels 1FR, 1FL. The master cylinder 27 changes the braking force which is applied to the brake pedal 25 and boosted by the booster 26 into the brake fluid pressure and then supplies the brake fluid pressure to the calipers 22FR, 22FL through each brake conduit 23FR, 23FL connected respectively to first discharge outlet 27b and second discharge outlet 27a. The brake conduits 23FR, 23FL include relief conduits 38FR, 38FL respectively which communicate to a reservoir 31. The relief conduits 38FR, 38FL include normally closed type magnetic proportional switching valves 37FR, 37FL which work as an outlet valve of an antilock braking system (ABS). Figure 1 shows the condition that the switching valves 37FR, 37FL are switched to the closing positions. The booster 26 boosts the braking force of the brake pedal 25 and transmits it to the master cylinder 27, and supplies brake fluid pressure in a booster chamber (not shown) to the calipers 22RR, 22RL through each brake conduit 23RR, 23RL to which an accumulator (not shown) is connected. A pump 29 is connected to the booster 26 through a fluid pressure supplying conduit 28. The pump 29 supplies the fluid in a reservoir 31 at a predetermined pressure to the conduit 28, which is maintained at a predetermined line pressure by the accumulator. A return conduit 30 is connected to the booster 26 so as to return the fluid from the booster 26 to the reservoir 31. The booster chamber of the booster 26 is connected to a brake conduit 33 in which a normally opened type magnetic switching valve 34 is provided and a on-way valve 35 is provided in parallel to the switching valve 34. Figure 1 shows the condition that the switching valves 34 is switched to the opening position. The brake conduit 33 branches into brake conduits 23RR, 23RL for the rear wheels 1RR, 1RL at a junction point P. Each normally opened type magnetic proportional switching valve 36R, 36L is provided in each brake conduit 23RR, 23RL. Each brake conduit 23RR, 23RL is provided with each relief conduit 38RR, 38RL which is branched on the down stream portion of the switching valves 36R, 36L and communicates to the reservoir 31 respectively. The relief conduits 38RR, 38RL are provided with normally closed type magnetic proportional switching valves 37RR, 37RL respectively which work as an outlet valve of an antilock braking system (ABS). Figure 1 shows the condition that the switching valves 37RR, 37RL are switched to the closed positions. At the junction point P, there is provided a branch conduit 28a communicating to the conduit 28. The branch conduit 28a is provided with a normal closed type switching valve 32. Figure 1 shows the condition that the switching valve 32 is switched to the closed position. The motor vehicle A further includes a control unit UAT for the automatic transmission 3, a slip control unit UTR which includes an antilock braking control system (hereinafter called ABS control system) and a traction control system. The solenoids 13a, 13b are controlled by the control unit UAT for the automatic transmission 3. When the brakes are operated, the ABS control system maintains desired friction force between each wheel 1FR, 1FL, 1RR, 1RL and the road surface to obtain the optimal braking force for the vehicle. When the excessive slip is caused in the rear wheels 1RR, 1RL during the vehicle being running or starting the traction control system decreases the amount of excessive slip so as to obtain the desired driving force of the rear wheels 1RR, 1RL. The control unit UAT receives detected signals from sensors 61, 62 in which the sensor 61 detects the opening of the throttle valve 42 and the sensor 62 detects the vehicle speed based on the rotation number of the propeller shaft 4. In accordance with these signals and stored transmission characteristics and lockup characteristics, the control unit UAT determines the transmission and lockup conditions, and then outputs the control signals to the solenoids 13a, 13b of the automatic transmission 3 to carry out the transmission and lockup controls. To the slip control unit UTR are input various kinds of signals which are detected respectively by the throttle sensor 61, the vehicle speed sensor 62, wheel rotation speed sensors 63, 64, 65, 66 for detecting the wheel rotation speed of each wheel 1FR, 1FL, 1RR, 1RL, an acceleration opening sensor 67 for detecting the depression amount of the pedal 43, a motor rotation amount sensor 68 for detecting the rotation amount of the motor 106, a steering sensor 69 for detecting the angular steering amount of a steering wheel (not shown), a manual switch 70 for selecting control modes, a brake switch 71 for detecting the depression of the pedal 25, a G sensor 73 for detecting a reduction speed of the vehicle, a yawing rate sensor 74 for detecting a yawing rate (an angular speed around the vertical direction of the vehicle) of the vehicle, and a shift position sensor 75 for detecting shift positions. The control unit UTR further includes an input interface for receiving the signals from abovementioned sensors, a microcomputer comprising a ROM in which control programs and various kinds of control maps and the like are stored, a RAM in which various kinds of memories necessary for carrying out the control are stored, and a CPU, an output interface for outputing control signals to the control unit UAT, and control circuits for operating the valves 32, 34, 36R, 36L, 37FR, 37FL, 37RR, 37RL and the motor 106. In the control unit UTR, the ABS control system detects slip conditions of each wheel 1FR, 1FL, 1RR, 1RL based on the difference between an estimated vehicle speed and the wheel rotation speed of each wheel detected by each sensor 63, 64, 65, 66. The estimated vehicle speed is computed based on the reduction speed of the vehicle detected by the G sensor 73. When the ABS control system determines that at least one of the wheels 1FR, 1FL, 1RR, 1RL is in the lock condition, the system controls the openings of the switching valves 37FR 37FL, 37RR, 37RL and decreases the line fluid pressure in the brake conduit(s) 23FR, 23FL, 23RR, 23RL whose wheel is determined to be in the lock condition so as to cancel the lock condition. For example, in the braking operation, the ABS control system detects the excessive slip in the front wheel 1FL and then carries out a duty control on the switching valve 37FL so as to increase the opening of the switching valve 37FL so that the fluid pressure in the brake conduit 23FL and the caliper 22FL is decreased, and then the brake fluid pressure in the brake 21FL is decreased transitionally The slip in the front wheel 1FL is controlled so that the front wheel 1FL is able to obtain desired friction force against the road surface and an optimal braking force. In the control unit UTR, the traction control system carries out an engine control and a brake control. The engine control is carried out by controlling the revolution amount of the motor 106 in the throttle opening control mechanism 44 to decrease the output of the engine 2 transitionally when the excessive slip is caused in the driving wheels (the rear wheels 1RR, 1RL) at the time of the vehicle being starting or running. The brake control is carried out by controlling the valves 32, 34 in open and close positions and controlling the degree of opening of the valves 36R, 36L, 37RR, 37RL so as to control the value of the brake fluid pressure. By the engine control operation and the brake control operation, driving torques transmitted to the rear wheels 1RR, 1RL are decreased and the slips in the rear wheels 1RR, 1RL are controlled. In the engine control operation, the throttle opening control mechanism 44 decreases the degree of the opening the throttle valve 42 less than the acceleration opening of the pedal 43 to decrease the output of the engine 2. Referring to Figure 2A, the throttle opening control mechanism 44 will be described hereinafter with reference to Figures 2A ∼ 2D. The throttle opening control mechanism 44 includes an acceleration pedal side lever 112 connected to the acceleration pedal 43 through the an acceleration wire 112a, a throttle valve side lever 113 connected to the throttle valve 42 through the a throttle wire 112t, an abutting lever 114 having an abutting portion 114a for abutting on the right side of the lever 112 and an abutting portion 114b for abutting on the right side of the lever 113, a driving lever 111 provided to be driven to the left and to the right, the motor 106 driving the lever 111, and a stopper 123 restricting the leftward movement of the lever 111 within a predetermined amount. The lever 113 is biased to the right which is the direction for closing the throttle valve 42, by a return spring 121. Disposed between the lever 112 and the lever 114 is a spring 116 for biasing the abutting portion 114a to abut it on the lever 112. In the same way, disposed between the lever 113 and the lever 114 is a spring 122 for biasing the abutting portion 114b to abut it on the lever 113. The biasing force of the spring 116 is set larger than those of the spring 122 and the return spring 121. The throttle opening control mechanism 44 constructed as described above operates as follows. First, there is a condition wherein the lever 111 abuts against the stopper 123. In this condition, the levers 112, 113 and 114 are made integral under the biasing force of the springs 116 and 122 as shown in Figures 2a, 2b, so that the degree of opening of the throttle obtained, hereinafter called a throttle opening, is proportional to the degree of opening of the accelerator, hereinafter called an accelerator opening. That is, the throttle opening varies within a range of 0 ∼ 100 % while the accelerator opening varies within a range of 0 ∼ 100 %. Figure 2a shows a condition wherein the throttle opening is 0%, that is, the accelerator opening is 0 %, and Figure 2b shows a condition wherein the throttle opening is 75%, that is, the accelerator opening is 75%. In the condition of Figure 2b, there remains a distance between the levers 111 and 114, which is necessary for making the throttle opening vary from 75% to 100 %. Thus, the levers 111 and 114 abut against each other when the throttle opening becomes 100 %, that is, the accelerator opening becomes 100%. When the lever 111 is driven to the right by the motor 106 in the condition shown in Figure 2b, the lever 114 is moved to the right against the spring 116 as shown in Figure 2c. Thus, the throttle opening becomes small, while the accelerator opening remains as it is. In Figure 2c, the throttle opening is 0 %, that is, the throttle is completly closed, while the accelerator opening is 75%. In this condition, the abutting portion 112b of the lever 112 abuts against the lever 113. When the accelerator opening is made 100% in the condition shown in Figure 2c, the lever 112 is moved to the left, and accordingly the lever 113 is moved to the left by the abutting portion 112b. Thus, the throttle opening changes from 0 % shown in Figure 2c to 25 % shown in Figure 2d. As is clearly understood from the above description, even if the lever 111 becomes stuck in the condition shown in Figure 2c, the throttle valve 42 can be opened to an opening of 25 % by making the accelerator opening 100%, so that the vehicle A can be driven to a repair shop. In the brake control operation, by the switching valve 34 being switched to the full-closed position and the switching valve 32 being switched to the full-opened position, the fluid pressure suppling conduit 28 increases the line pressure in the brake conduits 23RR, 23RL, and then the line pressure is maintained by the switching valves 36R, 36L being switched to the closed position, or the line pressure is released by the switching valves 37RR, 37RL being switched to the open position. Figure 3 is a time chart which shows a relationship between the general vehicle wheel rotation characteristic of the rear wheels in the traction control operation, and the engine control operation and brake control operation by the traction control system. Figure 4 is a block diagram showing a circuit determining a desired slip value for the brake control operation and a desired slip value for the engine control operation to set both desired slip values of the rear wheels. Figure 5 is a diagram showing the lower limit of the throttle opening determined by the traction control system. Figure 3 shows the general vehicle wheel rotation characteristic of the driving wheel, for example rear left wheel 1RL, in the traction control operation. The other driving wheel or rear right wheel 1RR has substantialy same characteristic as the rear left wheel 1RL. In Figure 3, V KRL shows the wheel rotation speed of the driving wheel or the rear left wheel 1RL in the slip control operation, and V J shows the wheel rotation speed of the driven wheel or the average value of the front wheels 1FR, 1FL. And a threshold a and a threshold b are shown. The threshold a corresponds to a basic desired slip ratio SE employed in the engine control operation which controls the slip ratio of the rear wheel 1RL by controlling the opening of the throttle valve. The threshold b corresponds to a basic desired slip ratio SB employed in the brake control operation which controls the slip ratio of the rear wheel 1RL by controlling the pressure of the brake fluid. The threshold b is set as larger value than the threshold a. The slip ratio S RL of the rear wheel 1RL is calculated as follows: Slip Ratio S RL = V KRL - V J V KRL The basic desired slip ratios SE, SB are calculated respectively as follows: SE = SET V J SB = SBT V J where SET and SBT show respectivly a desired slip value for the engine control operation and a desired slip value for the brake control operation both which are set in the rear wheels 1RL, 1RR in common. The desired slip value SET for the engine control operation and the desired slip value SBT for the brake control operation are set so that the rear wheels 1RL, 1RR obtain the predetermined driving force mainly when the vehicle is running straight. Specifically the desired slip values SET and SBT are determined based on the parameters such as the maximum friction coefficient µmax of the road surface, the vehicle speed, the accelerator opening, the steering angle, and the running mode selected by the mode selecting switch 70 as shown in Figure 4. As shown in Figure 4, the control unit UTR has a map where a basic value STBO of the desired slip value SBT and a basic value STAO of the desired slip value SET are stored respectivly whose parameters are larger one of the slip ratios S RL . S RR of the respective rear wheel and the maximum friction coefficient µmax of the road surface which is estimated based on the driven wheel rotation speed V J . In the map, the basic value STBO is set as an larger value than the basic value STAO. Abovementioned desired slip values SET and SBT are obtained respectively by multiplying the basic values STAO, STBO by a correction gain KD. The correction gain KD is obtained by multiplying a gain coefficient VG by gain coefficients ACPG, STRG, MODEG. The gain coefficient VG is provided so as to obtain the stability of the vehicle in accordance with the increase of the vehicle speed and is given by a map 82 as a function of the vehicle speed. The gain coefficient ACPG is provided so as to obtain the driving force in accordance with the acceleration demand of the driver and is given by a map 83 as a function of the accelerator opening. The gain coefficient STRG is provided so as to obtain the stability of the vehicle in the steering operation and is given by a map 84 as a function of the steering angle. The gain coefficient MODEG a given by a table 85 and is manually selected from two modes, i.e. sports mode and normal mode by the driver. Referring back to Figure 3, before the time t 1 , the slip is small so that the throttle opening Tn is controlled under the basic throttle opening TH · B which is proportional to the accelerator opening by the lever 111 of the throttle opening control mechanism 44 being maintained at the position shown in Figure 2A and Figure 2B. At this time, the pressures of the brake fluid supplied to the brakes 21FR ∼ 21RL are decreased by switching valves 32, 34, 36R, 36L and 37FR ∼ 37RL being maintained at the nomal positions respectively. At the time t 1 when the driving wheel rotation speed V KRL increases to the threshold a of the basic desired slip ratio SE, the engine control operation by the slip control system is started. In the engine control operation, the traction control system in the slip control unit UTR carries out a feedforward control so as to drop the throttle opening down to a lower limit control value SM by the lever 111 of the mechanism 44 being moved forwardly by the motor 106. Referring to figure 5, the lower limit control value SM of the throttle opening is stored in a map whose parameters are the vehicle speed and the maximum friction coefficient µmax or the road surface, and is determined based on those parameters. The maximum friction coefficient µmax varies from 1 to 5 where the smallest is equal to 1 and the largest is equal to 5. When the road surface has relativly small maximum friction coefficient µmax, relativly large lower limit control value SM is provided so as to decrease the output of the engine 2 rapidly. And when the road surface has relativly large maximum friction coefficient µmax, relatively large lower limit control value SM is provided so as to prevent the vehicle from stalling caused by the overreduction of the output of the engine 2. After the throttle opening is dropped to the lower limit control value SM, the traction control system carries out a feedback control of the opening of the throttle valve 42 so that the slip ratio S RL of the rear wheel 1RL becomes the desired slip ratio SE for the engine control operation. The feedback control is carried out by controlling the rotation of the motor 106 in the mechanism 44 so that the throttle opening is controlled under TH · M as shown in Figure 3. Referring back to Figure 3, after the time t 1 , the driving wheel rotation speed V KRL of the rear wheel 1RL is still increasing against the driven wheel rotation speed V J , while the engine control is operated. At the time of t 2 , the driving wheel rotation speed V KRL increases to the threshold b of the basic desired slip ratio SB. At this time, the brake control operation of the traction control is started so that the slip ratio S RL of the rear wheel 1RL is equal to the basic desired slip ratio SB. Namely, in the brake control operation, the slip control unit UTR of the traction control system controls the switching valves 32, 34, 36RR, 36RL so that the brake fluid pressure of the brake 21RL of the rear wheel 1RL is increased to the pressure Pn RL , and then the pressure Pn RL , is maintained. After the time t 2 , both engine control and brake control are operated to decrease the slip of the rear wheel 1RL. When the driving wheel rotation speed V KRL is decreased under the threshold c of the basic desired slip ratio SB for the brake control at the time of t 3 , the slip control unit UTR stops the brake control operation by opening the switching valve 37RL and decreasing the brake flid pressure, while the engine control operation is still being carried out until the slip ratio S RL has no possibility of increasing or the acceleration opening is reduced to zero. While the driving force of the rear wheel 1RL generally has a tendency to increase up to the predetermined value with the rise of the slip ratio, the cornering force of the rear wheel 1RL generally has a tendency to decrease with the rise of the slip ratio. Therefore the vehicle comes in the unstable condition with the rise of the slip ratio. In accordance with an embodiment of the invention, respective basic desired slip ratio SE, SB of the engine control and brake control is corrected to smaller values based on the signals from the steering sensor 69 and the yawing rate sensor 74. When the yawing rate sensor 74 detects the yawing rate which is larger than predetermined value and the steering sensor 69 detects the angular steering amount which is smaller than predetermined value, or the vehcle is in relatively unstable steering condition, the basic desired slip ratios SE, SB are corrected to first desired slip ratios SE' , SB' respectively. When the yawing rate sensor 74 detects the yawing rate which is larger than predetermined value and the steering sensor 69 detects the angular steering amount which is larger than predetermined value, or the vehicle is in relatively stable steering condition, the basic desired slip ratios SE, SB are corrected respectively to second desired slip ratios SE'', SB'' which are larger than the first desired slip ratios SE', SB'. When the vehicle is in relatively unstable steering condition, the traction control operation is started relatively in the early stage at the time of the slip ratio S RL of the rear wheel 1RL being equal to the threshold a', b' of the first desired slip ratios SE', SB'. In the traction control operation, the output of the engine 2 and the brake fluid pressure are controlled so that the the slip ratio S RL is equal to the first desired slip ratios SE', SB'. The slip ratio S RL of the rear wheel 1RL is controlled in the relatively early stage , which are understood from the driving wheel rotation speed V KRL illustrated as an alternate long and short dash line in Figure 3. When the vehicle is in relatively stable steering condition, the traction control operation is started relatively more lately than the starting time in the case of the first desired slip ratios SE', SB' at the time of the slip ratio S RL of the rear wheel 1RL being equal to the threshold a'', b'' of the second desired slip ratios SE'', SB''. In the traction control operation, the output of the engine 2 and the brake fluid pressure are controlled so that the the slip ratio S RL is equal to the second desired slip ratios SE, SB''. The slip ratio S RL of the rear wheel 1RL is controlled so as to be restrained in a little bit in comparison with the case of the vehicle being running straight, which are understood from the driving wheel rotation speed V KRL '' illustrated as an alternate long and two short dashes line in Figure 3. The rear right wheel 1RR has the same wheel rotation characteristic at the time of the traction control operation as that of the rear left wheel 1RL described above and showed in Figure 3. When the friction coefficients of the road surfaces of the rear right wheel 1RR and rear left wheel 1RL are different to each other, and the slip of the rear right wheel 1RR or rear left wheel 1RL is increased in an early stage, the engine control operation is started at the time of the slip ratio S RR or S RL of the wheel 1RR or 1RL being equal to the desired slip ratio SE, SE', SE'' for the engine control. The brake control operation is carried out in the rear right wheel and rear left wheel independently. Therefore, when the slip ratio of respective rear wheel 1RR, 1RL is equal to the desired slip ratio SB, SB', SB'' for the brake control, the brake contol operation in the rear wheel whose slip ratio is equal to the desired slip ratio is started respectively. The slip control and engine control by the slip control unit UTR will be described hereinafter in detail with reference to the flow charts shown in Figures 6 through 9. Figure 6 is a flow chart showing a main control of the traction control operation in the slip control unit UTR. Figure 7 is a flow chart showing an engine control operation in the slip control unit UTR. Figure 8 is a flow chart showing a brake control operation in the slip control unit UTR. Figure 9 is a flow chart showing how the desired slip ratio is corrected. The reference letter P indecates a step in the flow charts in Figures 6 through 9. The main control of the traction control operation by the slip control unit UTR will be described with reference to Figure 6. First, the signals from the sensors such as the throttle sensor 61, the vehicle speed sensor 62, the wheel rotation speed sensors 63, 64, 65, 66, the acceleration opening sensor 67, the motor rotation amount sensor 68, the manual switch 70 for selecting slip control modes, the brake switch 71, the G sensor 73, and the yawing rate sensor 74 are input (P1). Then, at is judged whether the control unit UTR has any trouble or not (P2). If the control unit UTR has no trouble, based on the wheel rotation speed signals from the sensors 63 ∼ 66, the slip ratios S RR , S RL of the rear wheels 1RR, 1RL are calculated respectively by employing the driving wheel rotation speeds V KRR , V KRL of the driving wheels or the rear wheels 1RR, 1RL and the driven wheel rotation speed V J which is an average value of the driven wheels or the front wheels 1FR, 1FL (P3). Then, based on the signal from the acceleration opening sensor 67, it is judged whether the accelerator opening is zero or not (P4). When the accelerator opening is not zero, a slip ratio S MAX which is a larger one in the the actual slip ratios S RR , S RL of the driving wheels is compared with the basic desired slip ratio SE for the engine control. When the slip ratio S MAX is more than or equal to the basic desired slip ratio SE, the throttle opening Tn is set at the lower limit control value SM (P6, P7) and the engine control operation is carried out (P8). Referring to Figure 7, when it is judged that the slip ratio S MAX is equal to the basic desired slip ratio SE for the engine control by the rise of the slip ratio (P12), the throttle opening Tn is dropped down to the lower limit control value SM by controlling the opening of the throttle valve 42 by the motor 106 in the mechanism 44 (P13, P17). After once the opening of the throttle valve 42 is decreased to the lower limit control value SM, the throttle opening Tn is provided as the throttle opening TH · M so that the slip ratio of the rear wheels 1RR, 1RL is equal to the basic desired slip ratio SE (P15). Referring to Figure 8, while the engine control operation is carried out, the slip control unit UTR provides the basic desired slip ratio SB for the brake control operation (P9 in Figure 6, P21). Even though the engine control operation is carried out, when the slip ratios S RR , S RL of the rear wheels 1RR, 1RL increases and are equal to the basic desired slip rate SB, the brake fluid pressure Pn RR , Pn RL in the brakes 21RR, 21RL are provided so that the slip ratios S RR , S RL of the rear wheels 1RR, 1RL are equal to the basic desired slip ratio SB (P22, P23). Based on values of the brake fluid pressure Pn RR , Pn RL , the switching valves 32, 34, 36R, 36L, 37RR, 37RL are controlled. The switching valve 32 is switched to the opened position, the switching valve 34 is switched to the closed position, and the switching valves 36R, 36L, 37RR, 37RL are controlled so that the brake fluid pressure in the brakes 21RR, 21RL is equal to the abovementioned brake fluid pressure Pn RR , Pn RL (P26). Thus by controlling the the brake fluid pressure, the slip ratios S RR , S RL of the rear wheels 1RR, 1RL is decreased. When the slip ratios S RR , S RL is under the the basic desired slip ratio SB, the brake control operation for the rear wheels 1RR, 1RL is completed (P22, P25) and then the brake fluid pressure in the rear wheels 1RR, 1RL is released. Referring to Figure 9, the slip control unit corrects the basic desired slip ratios SE, SB based on the yawing rate and the angular steering amount of the vehicle. The yawing rate Φ detected by the yawing rate sensor 74 and the angular steering amount  detected by the steering sensor 69 are read (P41). When the detected yawing rate Φ is more than or equal to the predetermined value α , and the detected angular steering amount  is less than or equal to the predetermined value β, the basic desired slip ratios SE, SB are corrected respectively to the first desired slip ratios SE', SB' which are obtained by decreasing the basic desired slip ratios SE, SB to predetermined ratio (a % which is less than 100 %) (P42, P43, P44). When the detected yawing rate Φ is more than or equal to the predetermined value α , and the detected angular steering amount  is more than the predetermined value β, the basic desired slip ratios SE, SB are corrected respectively to the second desired slip ratios SE'', SB'' which is more than the first desired slip ratios SE', SB' and which are obtained by decreasing the basic desired slip ratios SE, SB to predetermined ratio (b% which is less than 100 % and more than a%) (P42, P43, P45). When the detected yawing rate Φ is less than the predetermined value α, the vehicle is substantially in the condition of running straight. Therefore, the slip control unit UTR does not correct the basic desired slip ratios SE, SB and carries out the engine control operation and the brake control operation based on the basic desired slip ratios SE, SB (P42, P46). Referring back to Figure 6, in the main control, if the control unit UTR has some troubles in which the memory devices incorporated in the control unit UTR are wrong, that is, data is not able to be read out from ROM , data read out from ROM are wrong, data are not able to be written into RAM, or data read out from RAM are wrong, and/or the slip control operation is stopped. At that time, the pressurized brake fluid pressure by the brake control operation is released , the throttle opening corresponding to the accelerator opening is obtained and further the trouble signal is sent by using lamps, buzzers or the like (P10, P11). When the acceleration pedal is released and the accelerator opening is in the closed position, the main control is completed (P12). In the embodiment mentioned above, when the slip ratios of the rear wheels 1RR, 1RL are equal to the basic desired slip ratios SE, SB in the steering operation, the slip control unit UTR controls the driving forces transmitted to the rear wheels 1RR, 1RL so that the slip ratios are less than the basic desired slip ratios SE, SB, and further corrects the basic desired slip ratios SE, SB into smaller values based on the detected values from the yawing rate sensor 74 and the steering sensor 69. Therefore, the stability of the vehicle in the steering operation can be improved by controlling the slip in the driving wheels 1RR, 1RL more than the case of the vehicle being running straight. When the yawing rate sensor 74 detects the yawing rate Φ which is more than or equal to the predetermined value, and the steering sensor 69 detects the angular steering amount  which is less than or equal to the predetermined value, the slip control unit UTR corrects respectively the basic desired slip ratios SE, SB into the first desired slip ratios SE', SB'. When the yawing rate sensor 74 detects the yawing rate Φ which is more than or equal to the predetermined value, and the steering sensor 69 detects the angular steering amount  which is more than the predetermined value, the slip control unit UTR corrects respectively the basic desired slip ratios SE, SB into the second desired slip ratios SE'', SB'' which are more than the first desired slip ratios SE' , SB' . Therefore, when the vehicle is in relatively unstable condition because the steering condition is much larger than that in which the driver steers actually, the slip of the rear wheels are controlled to extend that the stability of the vehicle can be obtained. When the vehicle is steered in a large curvature by the driver in relatively stable condition, the stability of the vehicle can be obtained with no excessive slip control operation and no loss of the drivability. Hereinafter, there is described a second embodiment of the invention. In the first embodiment mentioned above, the brake control operation is carried out in the rear right wheel and rear left wheel independently. However, in the second embodiment, when the vehicle is in steering operation, the brake control operation is switched to the select-low type right and left integrated brake control operation in which the the rear right wheel and rear left wheel are controlled equally based on the rear wheel contacting the road surface whose friction coefficient is lower than the other. So that, in the steering operation, the vehicle can obtain the stability much more than the case of the right and left independent control operation in the first embodiment. In the select-low type right and left integrated brake control operation, when either one of the slip ratios S RR , S RL of the rear wheels 1RR, 1RL is equal to the desired slip ratio SB, the brake fluid presure in the rear wheels 1RR, 1RL is increased equally to the pressure Pn so that the slip ratio is less than the desired slip ratio. For example, when the slip ratio S RL of the rear left wheel 1RL is equal to the desired slip ratio SB, the traction control system controls the switching valves 32, 34, 36R, 36L, 37RR, 37RL to increase the brake fluid pressure of the both rear wheels 1RR, 1RL to the pressure Pn and maintain the pressure Pn so that the slip ratio of the rear left wheel 1RL is less than the desired slip ratio SB. Referring to Figure 10, there is shown a flow chart showing the switching operation from the right and left independent brake control to the right and left integrated brake control. When the traction control system carried out the right and left independent brake control in the brake control operation (P51), and the yawing rate Φ which is larger than the predetermined value γ is detected by the yawing rate sensor 74, the brake control operation is switched from the right and left independent brake control operation to the right and left integrated brake control operation (P52, P54). At this time, when the accelerator opening which is larger than the predetermined value δ is detected by the accelerator opening sensor 67 because the driver depresses the pedal in a large degree, the brake control operation is not switched to the right and left integrated brake control operation but is continued in the right and left independent brake control operation so as to obtain the good drivability (P53). The vehicle can obtain the good acceleration performance by maintaining the driving force of the driving wheel whose slip is smaller. In accordance with the second embodiment of the invention, because the driving forces of the rear right and left wheels 1RR, 1RL are decreased equally in the steering operation, the slips in the rear wheels are controlled with no difference between the rear right and left wheels so that the vehicle can obtain the stability in the steering operation. Further, when the slip ratio of the rear wheel 1RR or 1RL contacting the road surface whose friction coefficient is relatively low is equal to the desired slip ratio in the early stage, the brake fluid pressure is increased equally to the pressure Pn by the select- low type right and left integrated brake control operation. So that the slips in the rear wheels are decreased to less than the desired slip ratio rapidly and the stability of the vehicle can be improved. Further, when the driver demands the acceleration of the vehicle, the brake control operation is not switched to the right and left integrated brake control operation but is continued in the right and left independent brake control operation so that the vehicle can obtain the good drivability. In accordance with the second embodiment of the invention, instead of the select-low type right and left brake control, other type right and left brake control may be employed in which the brake fluid pressure in the rear wheels 1RR, 1RL are controlled equally based on the average value of the slip ratios S RR , S RL in the rear wheels. Hereinafter, there is described a third embodiment of the invention referring to Figures 11 through 15. Figure 11 shows a block diagram of the slip control system carrying out the slip control operation in accordance with the third embodiment of the invention. Figure 12 is a flow chart showing the procedures in the brake control operation by the slip control system in Figure 11. In the third embodiment of the invention, when the vehicle is in the steering operation and in the engine brake operation which is an example included in the low accelerator opening conditions, the desired slip ratio SB is controlled to be increased more than the normal operation so as to decrease the brake control gain. Referring to Figure 11, the engine brake condition detecting means 100 includes the accelerator opening sensor 67, the shift position sensor 75 and the slip control unit UTR. The steering condition detecting means 200 includes the yawing rate sensor 74 and the slip control unit UTR. The brake force control gain control means 300 and the brake force control means 400 includes the slip control unit UTR. In the normal condition, the brake control operation is carried out based on the desired slip ratio SB for the brake control. When both the engine brake condition and the steering condition are detected by the engine brake condition detecting means 100 and the steering condition detecting means 200, the brake force control gain control means 300 provides the desired slip ratio SB 1 (shown in Figure 3) which is larger than the desired slip ratio SB employed in the normal condition, and further the brake force control means 400 carries out the brake control operation based on the desired slip ratio SB 1 . Referring to Figure 12, there is described a flow chart showing the procedures in the brake control operation by the slip control system in Figure 11. First, the accelerator opening, the shift position, and the yawing rate are input to the slip control unit UTR (P61). The slip control unit UTR judges whether the accelerator opening is decreased less than the predetermined value within the predetermined time and whether the shift position is more than or equal to 3rd position (P62, P63). When the answers are YES in both steps P62, P63, the vehicle is in the engine brake condition. When the answer is NO in either step of P62, P63, the vehicle is in non engine brake condition. Namely, the condition in which the accelerator opening is decreased less than the predetermined value within the predetermined time and further the shift position is more than or equal to 3rd position means the condition in which the acceleration pedal is released rapidly in the high speed operation and thereby the engine brake is working. Negative condition mentioned above means the condition in which the engine brake is not working. Next, the slip control unit UTR judges whether the yawing rate is more than or equal to the predetermined value (P64). When the yawing rate is more than or equal to the predetermined value, the vehicle is in the steering condition. When the yawing rate is less than the predetermined value, the vehicle is not in the steering condition. Because the vehicle is in the normal condition when the answer is NO in either step of P62, P63, P64, the brake control operation is carried out by the brake force control means 400 based on the desired slip ratio SB for the brake control (P65). Because the vehicle is in the engine brake condition and in the steering condition when all answers are YES in the steps of P62, P63, P64, there is provided the desired slip ratio SB 1 ( SB 1 = SB × 1.2 ) which is larger than the desired slip ratio SB (P66) and the brake control operation is carried out by the brake force control means 400 based on the desired slip ratio SB 1 (P67). Figure 13 shows a block diagram of the slip control system carrying out the slip control operation in accordance with another example of the third embodiment of the invention. Figure 14 is a flow chart showing the procedures in the brake control operation by the slip control system in Figure 13. Referring to Figure 13, the slip control system further includes a yawing rate detecting means 500 with the yawing rate sensor 74 in comparison with the system shown in Figure 11. Specifically the yawing rate detecting means 500 includes the yawing rate sensor 74, therefore it shares one part of the brake force control means 400. In the embodiment, when the desired slip ratio SB 1 is provided by the brake force control gain control means 300 in the engine brake condition and in the steering condition, the larger the yawing rate is or the more rapidly the steering operation is carried out, the larger desired slip ratio SB 1 is provided. Referring to Figure 14, there is described a flow chart showing the procedures in the brake control operation by the slip control system in Figure 13. The brake control operation in Figure 14 is substantially the same as the brake control operation in Figure 12 where how to provide the desired slip ratio SB 1 in the engine brake condition and in the steering condition is different. Namely, when all answers are YES in the steps of P69, P70, P71, the coefficient K is read in from the map in accordance with the amount of the yawing rate (P73), and the desired slip ratio SB 1 ( SB 1 = SB× K ) is determined by using the coefficient K (P74). Figure 15 is a diagram showing the relationship between the coefficient K and the yawing rate. The larger the yawing rate is, the larger the coefficient K is or the larger the desired slip ratio SB 1 . In the third embodiment of the invention mentioned above, the engine brake condition may be detected by other factors instead of the accelerator opening and the shift position. The steering condition may be detected by other factors such as the angular steering amount instead of the yawing rate. In the third embodiment, the desired slip ratio SB 1 is increased so as to decrease the brake force control gain. The decrease of the brake force control gain corresponds to the decrease of the brake force. Therefore there may be employed to decrease the amount of the brake fluid in the brake control operation. In the third embodiment, the steering condition may include the rapid steering condition in which the yawing rate is larger than the predetermined value. In the third embodiment, when the vehicle is in the steering condition and in the small accelerator opening condition, the brake force control gain in the brake control operation is decreased, or the brake force in the driving wheels are decreased, so that the vehicle can be prevented from the decrease of the grip force of the driving wheels which is caused by the traction control and can improve the stability. Further, in accordance with the third embodiment, by the brake force control gain being decreased based on the amount of the yawing rate, the vehicle can obtain a large grip force even in the rapid steering condition because the more rapidly the steering operation is carried out, the more the brake force is decreased, and therefore can improve the stability much more. Moreover, in the embodiments of the invention, the yawing rate of the vehicle may be detected by both the steering amount detected by the steering sensor and the vehicle speed detected by the speed sensor. The vehicle with no ABS control system may be applied to the invention. In the engine control operation, the engine output may be decreased by the variable control of the ignition timing and/or the fuel cut of the engine. Further, the combination of this control operation in which the engine output is decreased by the variable control of the ignition timing and/or the fuel cut of the engine and the control operation in which the engine output is decreased based on the throttle opening described in the above embodiment may be employed. The control of the throttle valve may be carried out by the stepping motor instead of the throttle opening control mechanism. The slip ratio may be employed by other definitions which correspond to the slip ratio substantially.

Claims: A slip control system for a motor vehicle (A) controlling slips in driving wheels (1RR, 1RL) comprising: means (UTR, 32, 34, 36R, 36L, 37RR, 37RL, 106) for decreasing a slip of each driving wheel (1RR, 1RL) by controlling a driving force transmitted to the driving wheel (1RR, 1RL) so that the slip ratio (S RR , S RL ) of the driving wheel (1RR, 1RL) is at least equal to a predetermined desired slip ratio (S E , S B ); means (74; 69, 63, 64) for detecting a yawing rate of the vehicle (A); and means (69) for detecting a steering amount of the vehicle (A); wherein said slip decreasing means (UTR, 32, 34, 36R, 36L, 37RR, 37RL, 106) includes correcting means for decreasing the desired slip ratio (S E , S B ) when a steering condition is detected based on amounts detected by both the yawing rate detecting means (74; 69, 63, 64) and the steering amount detecting means (69); wherein said slip decreasing means (UTR) carries out a feed back control of the driving force effective at the driving wheel (1RL, 1RR) so that the slip ratio (S RR , S RL ) of said driving wheel (1RR, 1RL) becomes the desired slip ratio (S E , S B ); characterized in that there are provided means (65, 66, 73) for detecting a slip ratio of each driving wheel (1RR, 1RL); said correcting means store a predetermined yawing rate amount value (α) and a predetermined steering amount value (β) with the condition that with yawing rates less than the predetermined yawing rate value (α) the vehicle is regarded to run straight, and calculate the desired slip ratio (S E , S B ) in direct dependence on a slip value (SET, SBT) regarding the conditions when the vehicle is running straight; said correcting means decreases the desired slip ratio (S E , S B ) by a first correction amount (S E - S E ', S B - S B ') when the yawing rate  which is more than or equal to said predetermined value (α) is detected by the yawing rate detecting means (74; 69, 63, 64) and the steering amount  which is less than or equal to said predetermined value (β) is detected by the steering amount detecting means (69), and said correcting means decreases the desired slip ratio by a second correction amount (S E - S E '', S B - S B '') which is smaller than the first correction amount (S E - S E ', S B - S B ') when the yawing rate  which is more than or equal to said predetermined value (α) is detected by the yawing rate detecting means (74; 69, 63, 64) and the steering amount  which is more than said predetermined value (β) is detected by the steering amount detecting means (69); the slip control system comprising means (UTR, 82 - 84) for setting and adapting the desired slip ratio (S E , S B ) inversely related to the vehicle speed. A slip control system for a motor vehicle in accordance with claim 1, said slip decreasing means comprising: brake control means (UTR,32,34,36R, 36L,37RR,37RL) for decreasing a slip of each driving wheel independently by controlling brake fluid pressure in each driving wheel (1RR,1RL), said brake control means (UTR,32,34, 36L,36R,37RR,37RL) controlling the brake fluid pressure in each driving wheel (1RR,1RL) equally based on the desired slip ratio (S E ,S B ) of each driving wheel (1RR,1RL) when the yawing rate which is more than or equal to predetermined value is detected by the yawing rate detecting means (74;69,63,64). A slip control system for a motor vehicle in accordance with claim 1 or 2 in which said yawing rate detecting means includes a yawing rate detecting sensor (74). A slip control system for a motor vehicle in accordance with claim 1 or 2 in which said yawing rate detecting means includes a steering sensor (69) and a vehicle speed sensor (63,64). A slip control system for a motor vehicle in accordance with claim 1 or 2 in which said system further comprises an ABS control system (UTR). A slip control system for a motor vehicle in accordance with claim 1 in which said slip decreasing means includes brake control means (UTR, 32,34,36R,36L,37RR,37RL) for controlling a driving force of the driving wheel (1RR,1RL) and engine control means (UTR,106) for controlling an output of an engine (2). A slip control system for a motor vehicle in accordance with claim 6 in which said engine control means (UTR,106) decreases the output of the engine (2) by controlling an opening of a throttle valve (42). A slip control system for a motor vehicle in accordance with claim 7 in which said enging control means includes a throttle opening control mechanism (106) which controls the opening of the throttle valve (42). A slip control system for a motor vehicle in accordance with claim 2 in which said brake control means (UTR,32,34,36R,36L,37RR,37RL) decreases the slip of each driving wheel (1RR,1RL) equally based on the driving wheel contacting on a road surface whose friction coefficient is smaller than that of another road surface. A slip control system for a motor vehicle in accordance with claim 2 in which said brake control means (UTR,32,34,36R,36L,37RR,37RL) decreases the slip of each driving wheel (1RR,1RL) equally based on an average slip ratio of both driving wheels. A slip control system for a motor vehicle in accordance with any combination of claims 1 - 10, further comprising: means (17) for detecting a small accelerator opening condition of the vehicle (A); and means (300) for decreasing a brake force control gain in the brake force control means (400) when both the steering condition and the small accelerator opening condition are detected by the steering condition detecting means (69) and small accelerator opening condition detecting means (67,75). A slip control system for a motor vehicle in accordance with claim 11, wherein: the larger the yawing rate output from the yawing rate detecting means (74;69,63,64), the more the said brake force control gain decreasing means (300) reduces the brake force control gain. A slip control system for a motor vehicle in accordance with claim 11 in which said small accelerator opening condition detecting means (67,75) is means for detecting an engine brake condition of the vehicle, and in which said small accelerator opening condition is an engine brake condition. A slip control system for a motor vehicle in accordance with claim 13 in which said engine brake condition detecting means includes a sensor (67) for detecting an accelerator opening and a sensor (75) for detecting a shift position.

Inventors: KAGEYAMA FUMIO, KAWAMURA MAKOTO, NOBUMOTO KAZUTOSHI, OKAZAKI HARUKI, SONE AKIRA, TSUYAMA TOSHIAKI, KAGEYAMA, FUMIO, KAWAMURA, MAKOTO, NOBUMOTO, KAZUTOSHI, OKAZAKI, HARUKI, SONE, AKIRA, TSUYAMA, TOSHIAKI
Applicants: MAZDA MOTOR, MAZDA MOTOR CORPORATION
Application year: 1991
Main classification: B60K28
IPC codes: B60T8