Traction control apparatus for automotive vehicle

Abstract: A traction control apparatus for carrying out a traction control process of an automotive vehicle (20) to control a driving force of an engine (21) by adjusting control a throttle position of a throttle valve (41). The apparatus includes: a feedback control part (11) for performing a feedback control process to adjust the throttle position of the throttle valve based on a slip ratio of driving wheels of the vehicle, so as to prevent the vehicle from undergoing excessive slippage at the driving wheels when the vehicle is accelerating; an inhibiting part (12) for inhibiting the performing of the feedback control process when the traction control process is started, and for setting the throttle position of the throttle valve to a first value; a first control part (13) for enabling the inhibiting part (12) to inhibit the performing of the feedback control process until a slip ratio convergence is detected; and a second control part (14) for setting the throttle position of the throttle valve to a second value smaller than the first value when the driving wheel slip ratio is not converging after the throttle position has been changed to the first value at the start of the traction control process.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention generally relates to a traction control apparatus for an automotive vehicle, and more particularly to a traction control apparatus according to the preamble of the patent claim 1. (2) Description of the Related Art When an automotive vehicle is accelerating on a snowy road, it is likely that the driving force, transmitted from the engine to the driving wheel of the vehicle, is greater than the maximum friction force between the wheel and the road. Since the maximum friction force between the wheel and the snowy road is relatively low, the vehicle undergoes excessive slippage at the driving wheels when it is accelerating on the snowy road. If the driving wheels slip, the driving force acting in the direction of vehicle movement and the side force acting in a direction lateral to the vehicle movement are excessively lowered. As a result, the vehicle acceleration is lowered and the vehicle running condition becomes unstable. In order to eliminate the above problem, there is a known traction control device for automotive vehicles. In the known traction control device, a slip ratio is determined based on a speed difference between the driving wheel speed and the vehicle running speed, and the driving force produced by the engine is controlled such that the slip ratio is adjusted to be a given target slip ratio. The document EP-A-386792 discloses a traction control device of the kind described above. This document teaches that the known traction control apparatus has a first control means for subjecting a torque adjusting means to feedback control so as to allow a slip value of driven wheels to reach a predetermined target value in response to the output from a slip value detecting means, a second control means for reducing the actual torque to be applied to the driven wheels temporarily by a predetermined decrement and a third control means for reducing the torque to be applied to the driven wheels gradually when the slip value detected by the slip value detecting means does not reach a second judgement value set as a value smaller than the first judgement value even if a predetermined period of time elapses after start of control by the second control means. This arrangement has the following disadvantages: The document EP-A-386792 teaches inhibiting a feedback control process at the start of a traction control process and controlling the throttle opening angle of a sub-throttle valve to a predetermined target value (the lower limit value SM). Furthermore, in the apparatus of this document a road friction coefficient is estimated at the start of the traction control process wherein the target value is determined based on the estimated coefficient. Thus, the throttle valve is closed to the determined target value at the start of the traction control process. However, a road friction coefficient which is estimated when the slip control is not yet performed is not accurate. It may occur that the estimated road friction coefficient is smaller than the actual road friction coefficient. In this case the throttle valve is controlled at the start of the traction control process so that the throttle valve is closed to a too small opening angle. The ability of acceleration of the vehicle in which the apparatus of the document EP-A-386792 is installed becomes poor in the above-mentioned case. Furthermore, the throttle valve in this apparatus in not opened unless the feedback control process is performed again. A recovery of the throttle opening angle of the throttle valve to an adequate opening angle will, therefore, be delayed. It is an object of the present invention to provide a traction control apparatus for a vehicle which controls the throttle opening angle more sensitive and accurate. This object is achieved by a traction control apparatus having the technical features according to the patent claim 1. The traction control apparatus of the present invention tins includes feed-back control means for performing a feed-back control process to adjust the throttle position of the throttle valve based on a slip ratio of driving wheels of the vehicle so as to prevent the vehicle from undergoing excessive slippage at the driving wheels when the vehicle is accelerating, inhibiting means for inhibiting the performing of the feed-back control process of said feed-back control means when the traction control process is started, and for setting the throttle position of the throttle valve to a first value, and first control means for detecting whether or not the driving wheel slip ratio is converging, and for enabling said inhibiting means to inhibit the performing of the feed-back control process until said slip ratio convergence is detected and second control means (14) for setting the throttle position of the throttle valve to a second value smaller than the first value when said first control means detects that the driving wheel slip ratio is not converging after the throttle position of the throttle valve has been changed to the first value at the start of the traction control process. The first value is predetermined according to a high-road-friction throttle position curve stored in a memory of said traction control apparatus and said second value is predetermined according to a low-road-friction throttle position curve stored in said memory. Other objects and further features of the present invention will be more apparent from the following detailed description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a block diagram showing a preferred embodiment of a traction control apparatus according to the present invention; FIG.2 is a system diagram showing an automotive vehicle to which the traction control apparatus shown in FIG.1 is applied; FIGS.3A, 3B and 3C are a flowchart for explaining a traction control process performed by a control unit of the automotive vehicle shown in FIG.2; FIGS.4A and 4B are tables showing conditions and calculations used to determine a target speed of driving wheels when the traction control process is performed; FIG.5 is a chart showing three types of throttle position curves for defining a road-friction-dependent throttle position value according to an engine speed, which value is used in the traction control process; and FIGS.6A through 6E are timing charts for explaining the operation of the traction control apparatus according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT A description will now be given of a preferred embodiment of a traction control apparatus according to the present invention. FIG.1 shows a traction control apparatus for carrying out a traction control process of an automotive vehicle in which a throttle position of a throttle valve is controlled based on a driving wheel slip ratio. In the traction control apparatus shown in FIG.1, a feedback control part 11 performs a feedback control process to adjust the throttle position of the throttle valve based on a slip ratio of driving wheels of the vehicle, so as to prevent the automotive vehicle from undergoing excessive slippage of the driving wheels when the vehicle is accelerating. An inhibiting part 12 inhibits the performing of the feedback control process when the traction control process is started, and sets the throttle position to a first value. A first control part 13 detects whether or not the driving wheel slip ratio is converging, and enables the inhibiting part 12 to inhibit the performing of the feedback control process until the slip ratio convergence is detected. A second control part 14 sets the throttle position to a second value smaller than the first value when the driving wheel slip ratio is detected as not converging after the throttle position has been changed to the first value at the start of the traction control process. In the traction control apparatus shown in FIG.1, a time counter 17 is coupled to the second control part 14 described above. This time counter 17 measures a time period between a time the throttle position of the throttle valve is set to the second value and a time it is detected that the driving wheel slip ratio is converging after the setting of the throttle position to the second value. When it is detected that the driving wheel slip ratio is converging after the throttle position of the throttle valve was changed to the second value, the second control part 14 sets the throttle position to a third value which is predetermined in accordance with the time period measured by the time counter 17. In the traction control apparatus shown in FIG.1, a slip ratio determining part 15 is coupled to the above described feedback control part 11 and first control part 13. This slip ratio determining part 15 determines a driving wheel slip ratio based on a speed difference between a target driving wheel speed and a driving wheel speed. The target driving wheel speed is derived from a road condition, and the driving wheel speed is the average of driving wheel speeds indicated by output signals of speed sensors on the driving wheels of the vehicle. In the above described apparatus, a target speed setting part 16 is coupled to the first control part 13. The target speed setting part 16 sets a target throttle position of the throttle valve based on a previous target throttle position and a correction amount, so that the throttle valve is operated by the target throttle position to control the driving force of the driving wheels. Next, a description will be given, with reference to FIG.2, of an automotive vehicle to which the traction control apparatus shown in FIG.1 is applied. FIG 2 schematically shows a rear-wheel-driven automotive vehicle 20 with an engine 21, an electronic control unit (ECU) 22, a microcomputer 23, and an automatic transmission 24. The automotive vehicle 20 has front wheels 25 and 26 which are driven wheels, and has rear wheels 27 and 28 which are driving wheels. An output shaft of the engine 21 is coupled to a torque converter of the automatic transmission 24, and driving power of the engine 21 is transmitted from the automatic transmission 24 to the rear wheels 27 and 28 via a drive shaft 29 so that the vehicle 20 is propelled and moved with the driving force of the rear wheels 27 and 28. In the automotive vehicle 20 shown in FIG.2, wheel speed sensors 30 and 31 are provided at the front wheels 25 and 26 so as to sense the rotational speed of each of the front wheels, and wheel speed sensors 32 and 33 are provided at the rear wheels 27 and 28 so as to sense the rotational speed of the rear wheels. An engine speed sensor 34 is mounted on the engine 21 so as to sense the rotational speed of the crankshaft of the engine 21. This rotational speed is called an engine speed. A signal indicating the engine speed sensed by the engine speed sensor 34 is supplied to both the ECU 22 and the microcomputer 23. Signals indicating the wheel speeds sensed by the wheel speed sensors 30-33 are respectively supplied to the microcomputer 23. An intake passage of the engine 21 is made up of an intake pipe 35, a surge tank 36, and an intake manifold 37, and this intake passage communicates with a combustion chamber 38 of the engine 21 so that intake air is fed into the engine 21 in a direction indicated by an arrow in FIG.2. Within the intake pipe 35, a main throttle valve 40 and a sub-throttle valve 41 upstream of the main throttle valve 40 are provided. A throttle position (or a throttle opening) of the main throttle valve 40 is adjusted in response to the position of an accelerator pedal 39 set by a vehicle operator, in order to control the flow of intake air. Independently of the operation of the main throttle valve 40, a throttle position of the sub-throttle valve 41 is adjusted by a throttle valve actuator 42 such as a stepping motor. This throttle valve actuator 42 is operated by a control signal supplied from the microcomputer 23, so as to adjust the throttle position of the sub-throttle valve 41. As shown in FIG.2, a throttle position sensor 43 is mounted so as to sense the throttle position of the main throttle valve 40, and outputs a signal indicating the throttle position of the valve 40 to both the ECU 22 and the microcomputer 23. Also, a throttle valve sensor 44 is mounted so as to sense the throttle position of the sub-throttle valve 41, and outputs a signal indicating the throttle position of the valve 41 to both the ECU 22 and the microcomputer 23. The ECU 22 is made up of a digital computer, and carries out a fuel injection control process and an ignition timing control process. A fuel injection valve 45 is provided in the intake manifold 37 so as to inject fuel into the engine 21 in accordance with a fuel injection time. In order to determine the fuel injection time, the ECU 22 carries out the fuel injection control process in accordance with the signals supplied from the engine speed sensor 34, the throttle position sensors 43, 44, and other sensors (not shown in FIG.2). A spark plug 46 is provided in the combustion chamber 38. In order to determine an ignition timing of the spark plug 46, the ECU 22 also carries out the ignition timing control process in accordance with the signals supplied from the engine speed sensor 34, the throttle position sensors 43, 44, and other sensors (not shown). Moreover, the ECU 22 carries out a shifting control process for the automatic transmission 24 in accordance with various signals including: the throttle position signal supplied from the main throttle sensor 43; a shift position signal supplied from the automatic transmission 24; a vehicle speed signal supplied from a vehicle speed sensor (not shown); and a control signal supplied from the microcomputer 23. This shifting control process is carried out in order to determine a shift position of the automatic transmission 24 which is appropriate for the vehicle running conditions. In the automotive vehicle shown in FIG.2, a traction control process according to the present invention (which will be described later) is carried out by the microcomputer 23. The engine speed signal from the engine speed sensor 34, the wheel speed signals from the wheel speed sensors 30-33, and the throttle position signals from the throttle position sensors 43 and 44 are input to the microcomputer 23. In accordance with the input signals, the microcomputer 23 carries out the traction control process to control the output torque of the engine 21. A drive signal indicating the result of the traction control process is output to the throttle valve actuator 42, and the throttle position of the sub-throttle valve 41 is adjusted by the actuator 42. The output torque of the engine 21 is thus controlled when the traction control process is performed. Signals supplied from the microcomputer 23 to the ECU 22 include: a TRC signal indicating whether the traction control process is being performed, a fuel cut signal indicating whether the fuel injection valve 45 is operated under the fuel cut condition, and an ignition delay signal indicating whether the ignition timing of the spark plug 46 is delayed. In the automotive vehicle 20 shown in FIG.2, the drive torque of the engine 21 during the traction control process is controlled by setting the throttle position of the sub-throttle valve 41. More specifically, in the traction control apparatus of the present invention, a throttle position Ts of the sub-throttle valve 41 is determined in accordance with a slip ratio dV with respect to the rear wheels 27 and 28 (the driving wheels), as follows. Ts(n)= Ts(n-1) + (dTs/dt) To (dTs/dt)= K1 dV + K2 K3 dG In the above formula (1): "Ts(n)" denotes a target throttle position of the sub-throttle valve 41 to be determined; "Ts(n-1)" denotes a previously determined throttle position of the sub-throttle valve 41; and "To" denotes a time period on which the traction control process is repeated. In the above formula (2): "K1", "K2" and "K3" are coefficients having positive values; "dV" denotes a speed difference between a target driving wheel speed Vt3 (determined from a vehicle running speed or driven wheel speed) and the actual driving wheel speed Vd; and "dG" denotes a difference between the changing rate of the vehicle speed Vto and the changing rate of the driving wheel speed Vd. In the traction control apparatus shown in FIG.1, the driving wheel slip ratio is defined as the speed difference dV (= Vt3 - Vd) . The difference "dG" is indicated by its absolute value as in the equation: dG= | (dVto/dt) - (dVd/dt) | . Next, a description will be given, with reference to FIGS.3A through 3C, of a traction control process performed by the traction control apparatus according to the present invention. FIGS.3A through 3C show the traction control process performed by the traction control apparatus of the present invention. This process is repeatedly performed by the microcomputer 23 of the vehicle in FIG.2 every 12 milli-seconds (ms). In the traction control process in FIG.3A, step 100 detects whether or not the execution conditions for starting the traction control process are satisfied. The execution conditions include: 1) the throttle position Tm of the main throttle valve 40 is not set to the minimum opening position, and 2) none of the relevant sensors mentioned above are malfunctioning. If any of the execution conditions are not satisfied, step 140 shown in FIG.3C is performed and the traction control process ends. In step 140, flags FS and CEND (described below) are reset to zero, and the throttle position Ts of the sub-throttle valve 41 is set to a maximum throttle position Tsmax. If the execution conditions are satisfied in step 100, the reading of speed parameters from the relevant sensors and the associated calculations are performed by the microcomputer 23 in steps 102-107. Step 102 sets a vehicle speed Vto by taking the average of a right-hand front wheel speed Vfr and a left-hand front wheel speed Vfl respectively indicated by the wheel speed signals output from the sensors 30 and 31, in accordance with the equation: Vto= (Vfr + Vfl)/2 . Step 103 sets a target speed Vt3 of the rear wheels 27 and 28 (the driving wheels) based on the value of the vehicle speed Vto in step 102. The target speed Vt3 of the driving wheels is determined in accordance with the conditions and calculations indicated in FIG.4A, as follows. 1) when Vto ≤ 5 km/h, Vt3= (Vto + Knn1) km/h , 2) when 5 km/h < Vto ≤ 40 km/h, Vt3 is set to the greater one of (Vto + Knn2) and (5 km/h + Knn1), 3) when Vto > 40 km/h, Vt3 is set to the greater one of (Vto + S Vto) and (40 km/h + Knn2), where Knn1 and Knn2 are given coefficients, S denotes a target slip ratio. The coefficients Knn1 and Knn2 and the target slip ratio S are preset to be either one of two values as indicated in FIG.4B. If the road friction force is relatively low, these coefficients Knn1, Knn2 and S are preset to the values at the left-hand column of the table in FIG.4B. If the road friction force is relatively high, these coefficients Knn1, Knn2 and S are preset to the values at the right-hand column of the table. The coefficient "µo" indicated in FIG.4B is derived from the friction force "µ" according to the following equation: µ= [(mf + mr)/mr] a/gm where "mf" is the weight of the front wheels 25 and 26, "mr" is the weight of the rear wheels 27 and 28, "a" is the vehicle acceleration, and "gm" is the acceleration due to gravity. The vehicle acceleration "a" in the equation (3) is calculated from the changing rate of the rotational speeds Vfl and Vfr of the front wheels 25 and 26 indicated by the signals output from the sensors 30 and 31. Alternatively, the vehicle acceleration "a" is derived from a signal output by an acceleration sensor (not shown) provided in the automotive vehicle. It should be noted that the target speed Vt3 of the driving wheels is set to be a value greater than the vehicle speed Vto, such that the vehicle undergoes a desired slippage. When the vehicle speed Vto is relatively low, the target slip ratio S (= (Vt3 - Vto)/Vto) has a relatively large value, which ensures increased vehicle acceleration. When the vehicle speed Vto is relatively high, the target slip ratio S has a relatively small value. The traction control with respect to the side force is preferentially performed rather than the traction control with respect to the drive force, which ensures increased straight line stability of the vehicle. The value of the target speed Vt3 of the driving wheels set in step 103 corresponds to the value of the target slip ratio S. After step 103 is performed, step 106 sets a start speed Vtb at which a throttle position feedback control process (which will be described later) is started. When a driving wheel speed is increased to this start speed Vtb, the throttle position feedback control process is started. The start speed Vtb in step 103 is determined in accordance with the equation: Vtb= Vt3 + B (B: a given coefficient) For the sake of simplicity, the coefficient B is preset in accordance with the vehicle speed and the road condition. According to the equation (4), the start speed Vtb is set to a value greater than the target speed Vt3, thus preventing the frequency of the traction control process from being high. After step 106 is performed, step 107 sets a driving wheel speed Vd by taking the average of a right-hand rear wheel speed Vdr and a left-hand rear wheel speed Vdl respectively indicated by the signals output from the wheel speed sensors 32 and 33, in accordance with the equation: Vd= (Vdr + Vdl)/2 . Step 108 detects whether or not a throttle position control flag FS is equal to 1. Since the initial value of the flag FS is equal to 0, the answer to step 108 is, initially, negative. If FS= 0, step 109 is performed. However, the throttle position control flag FS will be subsequently set to 1. If FS= 1, step 120 in FIG.3B is performed, which step will be described later. Step 109 detects whether or not the driving wheel speed Vd is greater than the start speed Vtb. If Vd ≤ Vtb, the driving wheel speed does not reach the start speed at which the throttle position feedback control process is to be started. Then, step 140 in FIG.3C is performed and the traction control process ends. In step 140, the flags FS and CEND are reset to zero and the throttle position Ts of the sub-throttle valve is set to the maximum throttle position Tsmax. If the answer to step 109 is affirmative (Vd > Vtb), the driving wheel speed does reach the start speed. Steps 110 and 111 are performed, and then the traction control process ends. Step 110 sets the flag FS to 1 (FS <-- 1). Step 111 sets the throttle position Ts of the sub-throttle valve 41 to an initial throttle position f(NE) (Ts <-- f(NE)). FIG.5 shows three types of throttle position curves for defining a road-friction-dependent throttle position value in accordance with an engine speed. A throttle position value, obtained from one of the three curves in accordance with the engine speed, is used by the traction control apparatus of the present invention to set the throttle position of the sub-throttle valve 41. Each throttle position value of the three curves as a function of the engine speed is stored in a memory of the microcomputer 23. A high-road-friction throttle position curve THNESH in FIG.5 defines a throttle position value as a function of the engine speed when the road friction force is relatively high. A low-road-friction throttle position curve THNESL in FIG.5 defines a throttle position value as a function of the engine speed when the road friction force is relatively low. A middle-road-friction throttle position curve THNESM in FIG.5 defines a throttle position value as a function of the engine speed when the road friction force is intermediate. The initial throttle position f(NE) in step 111 is defined from a value of the high-road-friction throttle position curve THNESH in FIG.5 which value is defined as a function of the engine speed NE. The engine speed NE is indicated by the signal output from the engine speed sensor 34. In the automotive vehicle shown in FIG.2, the sub-throttle valve 41 is always set to the maximum throttle position while the throttle position feedback control process is not performed. If the throttle position feedback control process is started when the sub-throttle valve 41 is at the maximum throttle position, it will take a certain time until the vehicle response to a change in the throttle position of the sub-throttle valve 41 becomes sensitive. In order to obtain a sensitive response quickly after the throttle position feedback control process is started, the sub-throttle valve 41 is set to the initial throttle position f(NE) in step 111. If the answer to step 108 is affirmative (FS= 1), step 120 in FIG.3B is performed. Step 120 detects whether or not a feedback enable flag FBAR is equal to 1. Since the initial value of the feedback enable flag FBAR is equal to 0, the answer to step 120 is, initially, negative. If FBAR= 0, step 121 is performed. Step 121 detects whether or not an initial throttle position change flag FTCHG is equal to 1. Since the initial value of the initial throttle position change flag FTCHG is equal to 0, the answer to step 121 is, initially, negative. If FTCHG= 0, step 122 is performed. Step 122 detects whether or not the throttle position of the sub-throttle valve 41 indicated by the signal output from the throttle position sensor 44 in FIG.2 reaches the initial throttle position Ts (=f(NE)) previously set in step 111. If the answer to step 122 is negative, the traction control process ends. If the answer to step 122 is affirmative, step 123 is performed. Step 123 detects whether or not the driving wheel slip ratio dV (= (Vt3 - Vd)) is smaller than a prescribed reference value K4. In step 123, it is determined whether or not the driving wheel slip ratio is converging due to the road friction force. The driving wheel slip ratio dV is indicated by the difference between the target driving wheel speed Vt3 (step 103) and the driving wheel speed Vd (step 107). If the slip ratio dV is smaller than the reference value K4 (dV < K4), it is determined that the driving wheel slip ratio is converging. Then, step 124 sets the feedback enable flag FBAR to 1. After step 124 is performed, the throttle position feedback control procedure (steps 133-139 in FIG.3C) is started. If step 123 detects that the slip ratio dV is not smaller than the reference value K4 (dV ≥ K4), it is determined that the driving wheel slip ratio is not still converging due to a relatively low road friction force, and step 125 is performed. In step 125, the initial throttle position Ts of the sub-throttle valve 41 is changed to 1/4 of a value of the low-road-friction throttle position curve THNESL in FIG.5, and the initial throttle position change flag FTCHG is set to 1. In step 125, a value of the low-road-friction throttle position curve THNESL according to the engine speed NE indicated by the output signal of the engine speed sensor 34 is retrieved from the memory of the microcomputer 23. It should be noted that the throttle position (the second value) set in step 125 is smaller than the throttle position (the first value) set in step 111. After step 125 is performed, step 126 resets a convergence time counter CTSLP to zero, and the traction control process ends. If the answer to step 121 is affirmative (FTCHG= 1; the initial throttle position has been changed), step 127 increments the convergence time counter CTSLP (CTSLP <-- CTSLP + 1). Step 128 detects whether or not the driving wheel slip ratio dV is smaller than the reference value K4. If the answer to step 128 is negative (dV ≥ K4), it is determined that the driving wheel slip ratio is not converging, and then the traction control process ends. If the answer to step 128 is affirmative (dV < K4), it is determined that the driving wheel slip ratio is converging, and then step 129 is performed. Step 129 changes the throttle position Ts of the sub-throttle valve 41 to a value according to the value of the convergence time counter CTSLP. The value of the convergence time counter CTSLP at this time indicates a time period from the reaching of the initial throttle position f(NE) (step 122) to the detection of the driving wheel slip ratio convergence (step 128). Thus, the convergence time counter value varies depending on the road condition in which the vehicle is running. More specifically, in step 129, the throttle position Ts is changed to a value of the high-road-friction throttle position curve THNESH based on the engine speed NE when the convergence time counter CTSLP is smaller than or equal to 96 ms (= 12 ms x 8). When the convergence time counter CTSLP is between 96 ms and 204 ms (= 12 ms x 17), the throttle position Ts is changed to a value of the middle-road-friction throttle position curve THNESM based on the engine speed NE. When the convergence time counter CTSLP is between 204 ms and 504 ms (= 12 ms x 42), the throttle position Ts is changed to 1/2 of a value of the curve THNESM based on the engine speed NE. When the convergence time counter CTSLP is greater than 504 ms, the throttle position Ts is changed to 1/5 of a value of the curve THNESM based on the engine speed NE. After step 129 is performed, step 130 sets the feedback enable flag FBAR to 1 so that the execution of the throttle position feedback control process is allowed. After step 130 is performed, the step 126 described above is performed to reset the convergence time counter CTSLP to zero. Then, the traction control process ends. If the answer to step 120 is affirmative (TBAR= 1) or after step 124 is performed (dV < K4), steps 133 and 134 shown in FIG.3C (the throttle position feedback control procedure) are performed. Step 133 sets a correction amount dTs which is used to determine a target throttle position Ts(n) of the sub-throttle valve 41. This correction amount dTs is indicated by the second term "(dTs/dt) To" of the equation (1) above. Step 134 sets the target throttle position Ts(n) of the sub-throttle valve 41 based on the previously set target throttle position Ts(n-1) and the correction amount dTs. The values Ts(n-1) and dTs are stored in the memory of the microcomputer 23. In step 134, the target throttle position Ts(n) of the sub-throttle valve 41 is determined in accordance with the equation: Ts(n)= Ts(n-1) + dTs . A signal indicating the thus determined target throttle position Ts is output by the microcomputer 23 to operate the throttle valve actuator 42. Therefore, the throttle position of the sub-throttle valve 41 is set to an appropriate value. The throttle position feedback control procedure described above is finished after it is confirmed that three requirement conditions in step 135, 136, and 137-139 are satisfied. Step 135 detects whether or not the throttle position Ts of the sub-throttle valve 41 is greater than a throttle position Tm of the main throttle valve 40. If the answer to step 135 is negative, step 139 is performed, and then the traction control process ends. Step 139 resets a time counter CEND to zero. If the answer to step 135 is affirmative, step 136 is performed. Step 136 detects whether or not the absolute value of the slip ratio dV (= the speed difference (Vt3 - Vd) is less than a prescribed value "d". If the answer to step 136 is negative, the above step 139 is performed, and then this process ends. If the answer to step 136 is affirmative, step 137 is performed. Step 137 increments the time counter CEND (CEND <-- CEND + 1). After step 137 is performed, step 138 detects whether or not the time counter CEND is greater than a prescribed time D. If the time counter CEND is greater than the prescribed time D, it is determined that a time period during which the vehicle is operating under the requirement conditions of Ts > Tm and dV < d is greater than the prescribed time D. If the answer to step 138 is affirmative, all the requirement conditions are satisfied, and the above step 140 is performed, then the traction control process ends. If the answer to step 138 is negative, then the traction control process ends and no other steps are performed. The value d in step 116 and the time D in step 118 are predetermined depending on the type of the vehicle to which the traction control apparatus of the present invention is applied. When all the three requirement conditions of step 135-139 are satisfied, it is determined that the vehicle does not undergo driving wheel slippage although the sub-throttle valve 41 is opened more widely than the main throttle valve 40. Thus, in the above described traction control process, the throttle position feedback control procedure is completed at this time. Next, a description will be given, with reference to FIGS.6A through 6E, of the operation of the traction control apparatus according to the present invention. FIG.6A shows changes in the throttle positions of the main throttle valve 40 and the sub-throttle valve 41 when the vehicle is accelerating. In FIG.6A, the change in the throttle position of the main throttle valve 40 is indicated by a solid line Ia, and the change in the throttle position of the sub-throttle valve 41 is indicated by a solid line IIa. FIG.6D shows a change of the throttle position control flag FS. When the main throttle valve 40 is set to the maximum throttle position as indicated by the line Ia in FIG.6A and the driving wheel speed Vd is greater than the start speed Vtb, the flag FS is set to 1 as indicated in FIG.6D. In response to the setting of the flag FS, the throttle position Ts of the sub-throttle valve 41 is changed to the initial throttle position f(NE) (= THNESH) indicated by a time point A on the line IIa in FIG.6A. After the time point A, if the slip ratio dV is detected as being smaller than the reference value K4, the execution of the throttle position feedback control process is allowed because it is determined that the driving wheel slip ratio is converging. The throttle position Ts of the sub-throttle valve 41 is gradually increasing as indicated by a dotted line IIb in FIG.6A. Conversely, after the time point A, if the driving wheel speed indicated by a solid line IV in FIG.6C is greater than the driven wheel speed (or the vehicle speed) indicated by a solid line III in FIG.6C, the slip ratio dV is not smaller than the reference value K4 (dV ≥ K4). It is then determined that the driving wheel slip ratio is not converging. The execution of the throttle position feedback control process is inhibited, and the throttle position Ts of the sub-throttle valve 41 at this time is changed to 1/4 of a value of the low-road-friction throttle position curve THNESL (shown in FIG.5). The change in the throttle position Ts is indicated by a solid line IIc in FIG.6A. FIG.6B shows a change of the convergence time counter CTSLP. After the throttle position Ts is changed to 1/4 of the value of the curve THNESL, the convergence time counter CTSLP is being incremented during the time period between the time point A and a time point B (in FIG.6C), as indicated in FIG.6B. When it is detected at the time point B that the slip ratio dV is smaller than the reference value K4 because the speed difference between the driving wheel speed and the vehicle speed is decreased, the throttle position Ts of the sub-throttle valve 41 is changed to a value based on the value of the incremented counter CTSLP at that time. FIG.6E shows a change of the feedback enable flag FBAR. At the same time as the throttle position Ts is changed at the time point B, the feedback enable flag FBAR is set to 1, so as to allow the execution of the throttle position feedback control procedure. In the above described embodiment, during a time period from the start of the traction control process to the detection of the slip ratio convergence: dV < K4, the performing of the throttle position feedback control procedure is inhibited. Thus, it is possible to prevent the sub-throttle valve 41 from being closed to an excessively small throttle position due to the performing of the throttle position feedback control procedure. If the slip ratio is detected as not converging when the throttle position Ts of the sub-throttle valve has reached the value derived from the high-road-friction throttle position curve THNESH, the throttle position Ts is changed to the value derived from the low-road-friction throttle position curve THNESL (which value is smaller than the THNESH value). Thus, it is possible to reduce the time from the start of the traction control process to the detection of the slip ratio convergence during which the driving wheel slip ratio is relatively great. The throttle position Ts of the sub-throttle valve 41 is changed to a value in accordance with the convergence time counter CTSLP value. This counter value indicates the time for the convergence of the driving wheel slippage. Thus, the throttle position Ts of the sub-throttle valve 41 is changed to a value appropriate for the road condition according to the present invention.

Claims: A traction control apparatus for carrying out a traction control process of an automotive vehicle (20) in which a throttle position of a throttle valve is controlled based on a driving wheel slip ratio, which apparatus includes feed-back control means (11) for performing a feed-back control process to adjust the throttle position of the throttle valve based on a slip ratio of driving wheels of the vehicle so as to prevent the vehicle from undergoing excessive slippage at the driving wheels when the vehicle is accelerating, inhibiting means (12) for inhibiting the performing of the feed-back control process of said feed-back control means when the traction control process is started, and for setting the throttle position of the throttle valve to a first value, and first control means (13) for detecting whether or not the driving wheel slip ratio is converging, and for enabling said inhibiting means (12) to inhibit the performing of the feed-back control process until said slip ratio convergence is detected and second control means (14) for setting the throttle position of the throttle valve to a second value smaller than the first value when said first control means (13) detects that the driving wheel slip ratio is not converging after the throttle position of the throttle valve has been changed to the first value at the start of the traction control process. characterized in that the first value is predetermined according to a high-road-friction throttle position curve stored in a memory of said traction control apparatus and said second value is predetermined according to a low-road-friction throttle position curve stored in said memory. A traction control apparatus according to claim 1, characterized in that said first control means (13) determines whether or not a speed difference between a target driving wheel speed and a driving wheel speed is smaller than a prescribed reference value, so as to detect whether or not the driving wheel slip ratio is converging, said target driving wheel speed being derived from a road condition, and said driving wheel speed being indicated by signals output from driving wheel speed sensors. A traction control apparatus according to claim 2, characterized in that said first control means (13) detects that the driving wheel slip ratio is converging if the speed difference is smaller than the reference value, and detects that the driving wheel slip ratio is not converging if the speed difference is not smaller than the reference value. A traction control apparatus according to claim 1, characterized in that said first control means (13) detects whether or not the driving wheel slip ratio is converging after the throttle position of the throttle valve has been changed to the first value. A traction control apparatus according to claim 1, characterized in that said first control means (13) detects whether or not the driving wheel slip ratio is converging after the throttle position of the throttle valve has been changed to the second value. A traction control apparatus according to claim 1, characterized in that said second control means (14) sets the throttle position of the throttle valve to a third value when said first control means (13) detects that the driving wheel slip ratio is converging after the throttle position has been changed to the second value at the start of the traction control process, said third value being predetermined in accordance with a time period between a time the throttle position is set to the second value and a time said slip ratio convergence is detected. A traction control apparatus according to claim 1, characterized in that said apparatus further comprises time counter means (17) coupled to said second control means (14) for measuring a time period between a time the throttle position of the throttle valve is set to the second value and a time it is detected that the driving wheel slip ratio is converging after the setting of the throttle position to the second value. A traction control apparatus according to claim 1, characterized in that said apparatus further comprises slip ratio determining means (15), coupled to said feedback control means (11) and to said first control means (13), for determining a driving wheel slip ratio based on a speed difference between a target driving wheel speed and a driving wheel speed, said target driving wheel speed being derived from a road condition, and said driving wheel speed being the average of driving wheel speeds indicated by output signals of speed sensors on the driving wheels of the vehicle. A traction control apparatus according to claim 1, characterized in that said apparatus further comprises target speed setting means (16), coupled to said first control means (13), for setting a target throttle position of the throttle valve based on a previous target throttle position and a correction amount, so that the throttle valve is operated by the thus determined target throttle position in order to control the driving force of the driving wheels. A traction control apparatus according to claim 1, wherein said vehicle (20) is provided with a main throttle valve (40) being operated in accordance with a position, set by a vehicle operator, of an accelerator pedal (39), and with a sub-throttle valve (41) being operated by a throttle valve actuator in accordance with a target throttle position.

Inventors: YAGI YOSHIFUMI, YAGI, YOSHIFUMI, Yagi, Yoshifumi, c/o Toyota Jidosha K. K.
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1993
Main classification: B60K28
IPC codes: B60K28, F02D29