ADAPTIVE MODE ANTI-LOCK BRAKE CONTROLLER

Abstract: A wheel lock control apparatus is described which utilizes both independent and select low modes of wheel lock controlled braking. When braking on a surface in which the coefficients of friction between the two sides of the vehicle are substantially different or when the vehicle is undergoing severe steering manoeuvres while braking, the wheel lock control apparatus utilizes a select low mode of braking for the vehicle rear wheels so as to maintain vehicle stability. However, when the four wheels are being braked on a substantially uniform coefficient of friction surface, the wheel lock control apparatus utilizes an independent mode of braking for each of the wheels of the vehicle so as to minimize the vehicle braking distance.

Description: This invention relates to wheel lock control apparatus for anti-lock braking of vehicle wheel brakes. When the brakes of a vehicle are applied, a tyre torque between the wheel and the road surface is generated that is dependent upon various parameters including the road surface condition and the amount of slip between the wheel and the road surface. For a given road surface, the tyre torque between the wheel and the road surface increases with increasing wheel slip values to a peak tyre torque occurring at a critical wheel slip value. As the value of wheel slip increases beyond the critical wheel slip value, the tyre torque between the wheel and the road surface decreases. Stable braking results when the wheel slip value is equal to or less than the critical wheel slip value. However, when the wheel slip value becomes greater than the critical wheel slip value, braking becomes unstable resulting in sudden wheel lockup, reduced vehicle stopping distance and a deterioration in the lateral stability of the vehicle. Numerous wheel lock control apparatus have been proposed to prevent the wheels from locking up while being braked. These apparatus generally prevent a wheel from locking by controlling the applied brake pressure when an incipient wheel lockup condition is sensed so as to maintain substantially the maximum possible tyre torque between the tyre and road surface while at the same time preventing the wheel from operating in the unstable braking region. Some of the known wheel lock control apparatus utilize an "independent" mode of braking wherein each of the front and rear vehicle wheels are controlled independently so as to establish the maximum possible tyre torque at each wheel during wheel lock controlled braking. By so maximizing the tyre torque at each wheel, the stopping distance of the vehicle is minimized. However, under certain conditions, this mode of operation can lead to reduced vehicle stability. The conditions for this occurrence are either grossly different coefficients of friction between right and left sides of the vehicle, or severe steering manoeuvres while braking. Other forms of known wheel lock control apparatus utilize what is referred to as the "select low" mode of wheel lock controlled braking which provides for improved vehicle stability and steerability when the vehicle is being braked on a split coefficient of friction surface. In this form of wheel lock control apparatus, the front brakes are typically controlled independently as above described but the rear wheel brakes are controlled such that the brakes of the rear wheel being braked on a high coefficient of friction surface is regulated in response to the conditions of the rear wheel being braked on a lower coefficient of friction surface. While this mode of braking increases the vehicle stopping distance, vehicle stability and steerability is improved. For example the lateral friction force of the rear wheel being braked on the high coefficient of friction surface is increased since it is being controlled to a low slip value. Prior art arrangements can be found in US-A-4,451,096 and US-A-4,439,832. The present invention provides for an improved wheel lock control apparatus which utilizes both independent and select low modes of wheel lock controlled braking. A wheel lock control apparatus and a method of operation thereof, are characterised by the features specified in the characterising portions of claims 1 and 4 respectively. When braking on a surface in which the coefficients of friction between the two sides of the vehicle are substantially different or when the vehicle is undergoing severe steering manoeuvres while braking, the wheel lock control apparatus of this invention utilizes a select low mode of braking for the vehicle rear wheels so as to maintain vehicle stability. However, when the four wheels are being braked on a substantially uniform coefficient of friction surface, the wheel lock control apparatus of this invention utilizes an independent mode of braking for each of the wheels of the vehicle so as to minimize the vehicle braking distance. In the embodiment hereinafter described, a difference in the coefficients of friction between the right and left sides of the vehicle or a severe steering manoeuvre while braking is sensed based on the difference in the determined tyre torque of the front wheels while braking. The present invention will now be described by way of example with reference to the following description of a preferred embodiment, and the accompanying drawings, in which:- Figure 1 is a diagram illustrating the brake friction coefficient between a wheel and a road surface as a function of the percentage slip between the wheel and road surface for two road surface conditions; Figure 2 is a general diagram of the wheel lock control apparatus for controlling the brakes in accordance with the principles of this invention; Figure 3 is a longitudinal cross-sectional view of the actuator of Figure 2 for modulating the brake pressure to prevent wheel lockup; Figure 4 is a diagram of the electronic controller of Figure 2 that is responsive to brake apparatus parameters for controlling the brake pressure to inhibit wheel lockup in accordance with the principles of this invention; and Figures 5 to 9 are diagrams illustrating the operation of the electronic controller of Figure 4. A wheel under the influence of braking has two major torques acting on it: brake torque and tyre torque. Brake torque arises from the application of brake pressure through the brake mechanism and tyre torque is generated by the friction of the tyre-road interface as wheel slip occurs. Brake torque T b is assumed to be proportional to brake pressure P b with a known brake gain K b and is defined by the expression T b = P b K b . (1) Tyre torque T t is related to the brake friction coefficient µ between the tyre and the road surface, the normal load N on the tyre and the wheel rolling radius R and is defined by the expression T t = µNR. (2) For the free body consisting of the brake, wheel, and tyre, the equation of motion is I w ω + T b - T t = 0 (3) where I w is the wheel moment of inertia and ω ˙ is the wheel angular acceleration. When the difference between the tyre torque T t and the brake torque T b is positive, the wheel accelerates; and when negative, the wheel decelerates. Combining equations (l) and (3), tyre torque T t is defined as T t = I w ω + P b K b . (4) As can be seen, the tyre torque can be calculated from values that are either known or can be measured. The wheel moment of inertia I w and the brake gain K b are known values, the value of brake pressure P b can be measured and ω ˙ can be determined by differentiating the value of wheel speed ω which can be measured. The brake friction coefficient term µ of the tyre torque T t is a nonlinear function of the magnitude of slip between the wheel and the road surface during braking and is dependent upon the road surface condition. Figure l illustrates the brake friction coefficient µ as a function of percentage-wheel slip for two road surface conditions. For a given road surface, it can be seen that as wheel slip is increased in response to increased brake torque T b , the brake friction coefficient µ and therefore the tyre torque T t increases until a critical wheel slip value at which the brake friction coefficient and the tyre torque are at a maximum. A further increase in wheel slip results in a decrease in the tyre torque T t due to a decrease in the brake friction coefficient µ and high wheel deceleration values. The maximum tyre torque resulting in a maximum braking effort for a given road surface is achieved when the brake torque T b produces the critical wheel slip value. When the braking effort produces a wheel slip exceeding the critical wheel slip value, the braking operation becomes unstable and typically results in sudden wheel lockup which in turn results in increased stopping distance and a deterioration in the steering and lateral stability of the vehicle. In general, the wheel lock control apparatus identifies the value of the braking pressure P b that produces the maximum tyre torque T t . This is accomplished by continuously calculating the tyre torque value T t of equation (4) during braking. Any time the calculated value is larger than any previously calculated value, the value of the tyre torque T t and the braking pressure P b is stored so that the maximum tyre torque and brake pressure producing it are known. When an incipient wheel lockup is detected, the brake pressure P b is dumped to allow the wheel speed to recover and the brake pressure is thereafter reapplied to the stored value to establish a braking condition in which the wheel slip is substantially at the critical wheel slip value for the existing road surface condition. This results in substantially the maximum possible tyre torque T t and minimum stopping distance for the road surface condition. A general overview of the wheel lock control apparatus is illustrated in Figure 2. The wheel lock control apparatus for the brake of a single wheel is illustrated, it being understood that the brakes of the remaining wheels of the vehicle are similarly controlled. A standard wheel brake 10 for a wheel 11 is actuated by controlled hydraulic pressure from one of two sources. The primary source is a motor driven actuator 12 and the secondary source is a standard master cylinder 14 controlled directly by the vehicle brake pedal 16. An electromagnetic valve 18 which is normally open is energized when the motor driven actuator 12 is operative to control the hydraulic pressure to the wheel brake l0 so as to decouple the master cylinder 14 and vehicle brake pedal 16 from the hydraulic pressure output of the motor driven actuator 12. When the electromagnetic valve 18 is deenergized, the hydraulic pressure to wheel brake l0 may be modulated directly by the vehicle brake pedal 16 and master cylinder 14. The electromagnetic valve 18 is deenergized only during limited vehicle operating conditions such as low vehicle speed or during failed conditions of the primary hydraulic pressure source to permit brake pressure modulation by the master cylinder 14. An electronic controller 20 is responsive to the outputs of a brake pedal force sensor 22 providing a signal that is a measure of the operator applied brake pedal force F, a wheel speed sensor 24 that provides a signal that is a measure of wheel speed ω, and a pressure sensor 26 that provides a signal that is a measure of the (hydraulic) brake pressure P b applied to the wheel brake l0 from the master cylinder 14 or the motor driven actuator 12. The electronic controller 20 is responsive to those signals to (a) energize the electromagnetic valve 18 when the wheel speed ω exceeds a value corresponding to a low vehicle speed such as 3 mph, (b) control the motor driven actuator 12 so as to apply a (hydraulic) brake pressure P b to the wheel brake l0 that is proportional to the brake pedal force F times a gain constant G for providing power assist during normal braking conditions, and (c) limit the brake pressure P b applied to the wheel brake l0 for wheel lock control. Referring to Figure 3, the motor driven actuator 12 in the preferred embodiment includes a DC torque motor 28 whose output shaft drives an input gear 30 which in turn rotatably drives an output gear 32. The drive member 34 of a ball screw actuator is secured for rotation with the output gear 32. The drive member 34 engages and axially positions a driven member 36 of the ball screw actuator. The driven member 36 drives a piston 38 to control the hydraulic pressure output of the motor driven actuator 12. The torque output of the DC torque motor 28 is translated to a directly related hydraulic brake pressure P b output of the motor driven actuator 12 that is applied to the wheel brake l0. As illustrated in Figure 4, the electronic controller 20 takes the form of a digital computer 40 and a motor control circuit 41. The digital computer 40 is standard in form and includes a central processing unit (CPU) which executes an operating program (computer program) permanently stored in a read-only memory (ROM) which also stores tables and constants utilized in controlling the hydraulic pressure input to the wheel brake l0. Contained within the CPU are conventional counters, registers, accumulators, flag flip flops, etc. along with a clock which provides a high frequency clock signal. The digital computer 40 also includes a random access memory (RAM) into which data may be temporarily stored and from which data may be read at various address locations determined in accordance with the computer program stored in the ROM. A power control unit (PCU) receives battery voltage and provides regulated power to the various operating circuits in the electronic controller 20. The digital computer 40 further includes an input/output circuit (I/O) that in turn includes a discrete output section controlled by the CPU to provide a control signal to the electromagnetic valve 18. In controlling the wheel brake l0, the digital computer 40 outputs a digital signal to the motor control circuit 41 via the I/O representing a desired value of the hydraulic brake pressure. The motor control circuit 41 converts the digital signal representing the desired brake pressure to an analogue signal which is compared with the actual measured value of the brake pressure P b . By standard closed loop adjustment that may include both proportional and integral terms, the DC torque motor 28 current is controlled so that the actual measured brake pressure P b is made equal to the desired brake pressure. The I/O also includes an input counter section which receives a pulse output from the wheel speed sensor 24 having a frequency representing wheel speed ω. Wheel speed ω is then determined by counting clock pulses between wheel speed pulses. An analogue-to-digital unit (ADU) is included which provides for the measurement of analogue signals. The analogue signals representing conditions upon which the hydraulic brake pressure P b to the wheel brake l0 is based are supplied to the ADU. In the present embodiment, those signals include the brake pressure value P b from the pressure sensor 26 and the output of the brake pedal force sensor 22 providing a measure of the brake pedal force F. The analogue signals are sampled and converted under the control of the CPU and stored in ROM designated RAM memory locations. Referring to Figure 5, when power is first applied to the apparatus such as when the vehicle ignition switch is rotated to its "on" position, the computer program is initiated at point 42 and then proceeds to a step 44 where the digital computer 40 provides for initialization. For example, at this step 44 initial values stored in the ROM are entered into ROM designated RAM memory locations and counters, flags and timers are initialized. After the initialization step 44, the computer program proceeds to a step 46 where the computer program conditions the electronic controller 20 to allow interrupts to occur and then to a background loop step 48 which is continuously repeated. This background loop may include, for example, diagnostic routines. In the preferred embodiment of this invention, an interrupt is provided by the CPU at 5 millisecond intervals. Following each interrupt, the execution of the background loop step 48 is interrupted and the routines for establishing the hydraulic brake pressure P b to each of the front and rear wheel brakes are executed. Control of the front wheel brakes of the vehicle will first be described. The front wheel brakes are controlled by the electronic controller 20 independently in identical manner by identical computer program routines. Figures 6-8 combined illustrate the routines executed by the electronic controller 20 in controlling the hydraulic brake pressure P b to the wheel brake of one of the front wheels. Referring to Figure 6, the five millisecond interrupt routine is illustrated. This interrupt routine is entered at point 50 and proceeds to a step 52 where the last measured wheel speed ω is saved and the new values of the wheel speed ω, brake pedal force F and brake pressure P b to the wheel brake l0 are read and stored in ROM designated RAM memory locations. Next, the computer program proceeds to a decision step 54 where it is determined whether or not the operator is commanding brake application. The wheel brakes l0 are considered applied if the value of the brake pedal force F is greater than zero. If the wheel brakes l0 are not applied, the computer program proceeds to a step 56 where a brake pressure command value P c for the wheel brake l0 is set equal to zero. Also at this step 56, the speed of the vehicle as represented by the speed ω v of a hypothetical unbraked wheel is set equal to the wheel speed ω measured at step 52. Since the wheel brakes l0 are not applied, the wheel slip is substantially at zero so that the actual and hypothetical wheel speeds can be equated. From step 56, the computer program proceeds to a step 58 where a D-flag (represented by the state of a flip-flop or a RAM memory location) is reset to condition the computer program to execute an identification routine (illustrated in Figure 7) which identifies the brake pressure producing the critical wheel slip value and therefore the maximum possible braking effort and which establishes the identified brake pressure following the sensing of an incipient wheel lockup condition. As will be described, the D-flag is set following the sensing of an incipient wheel lockup condition to condition the computer program to execute a dump routine (illustrated in Figure 8) to release the brake pressure P b and allow the wheel speed ω to recover. Also at step 58, the maximum allowable brake pressure P m is set equal to a calibration constant K p such as 10342 kPa (1500 psi) and a RAM memory location storing the value of the maximum calculated tyre torque T tm is set equal to zero. Thereafter, the computer program exits interrupt routine for the respective wheel. The foregoing steps 52 to 58 are continuously repeated at 5 millisecond intervals as long as the vehicle operator does not command brake application. However, when a brake pedal force F is applied to the vehicle brake pedal 16, the computer program proceeds from decision step 54 to a series of steps that provide an estimation of the value of vehicle speed ω v as represented by the speed of a hypothetical unbraked wheel. It is noted that the initial value of ω v was set equal to the actual wheel speed ω at step 56 prior to operation of the vehicle brake pedal 16. This series of steps begins at step 59 where the rate of change in wheel speed (wheel angular acceleration) ω ˙ is determined from the old value of wheel speed saved at step 52 and the new value stored at step 52. The determined rate of change of wheel speed ω ˙ is then compared with a constant deceleration of lg at decision step 60. The lg deceleration value represents the maximum possible vehicle deceleration. When wheel deceleration is less than lg, it is assumed that the vehicle is decelerating at the same rate as the wheel 11. If, however, the wheel deceleration exceeds lg, it is assumed that the vehicle deceleration remains at the maximum value of lg. If the wheel deceleration is less than or equal to lg, the computer program proceeds from decision step 60 to a decision step 62 where ω ˙ is compared to zero. If the comparison indicates wheel deceleration, the computer program proceeds to step 64 where the rate of change of vehicle speed ω ˙ v is set equal to the actual measured rate of change of wheel speed ω ˙. If, however, the comparison at decision step 62 indicates no change in wheel speed ω or wheel angular acceleration ω ˙ , the computer program proceeds to a step 66 where the rate of change of vehicle speed ω ˙ v is set equal to zero. Returning to decision step 60, if it is determined that the wheel deceleration is lg or greater, the computer program proceeds to a step 68 where ω ˙ v is set equal to the maximum possible vehicle deceleration of lg. From the respective steps 64, 66 or 68, the computer program proceeds to a step 70 where vehicle speed ω v is estimated. This estimation is based on an initial value of vehicle speed ω v-1 determined during the previous execution of the interrupt routine and the rate of change of vehicle speed ω ˙ v , determined at step 64, 66 or 68 over the five millisecond interval Δt between interrupt periods. From step 70, the computer program proceeds to decision step 72 where the actual wheel speed ω measured at step 52 is compared to the vehicle speed ω v determined at step 70. If the wheel speed ω is equal to or greater than the vehicle speed ω v (which cannot occur during braking of the wheel), the value of vehicle speed is corrected at step 74 by setting the vehicle speed ω v equal to wheel speed ω and the initial vehicle speed ω v-1 to be used at step 70 in the next execution of the interrupt routine is set equal to wheel speed ω. If at decision step 72 the wheel speed ω is determined to be less than the vehicle speed ω v , the computer program proceeds to a step 76 where the initial vehicle speed ω v-1 to be used at step 70 during the next execution of the interrupt routine set equal to the value of vehicle speed determined at step 70. Following step 74 or step 76, the computer program proceeds to a decision step 78 where the vehicle speed ω v is compared to a calibration constant such as 4.8 kph (3 mph). If the vehicle speed is less than 4.8 kph (3 mph), the computer program proceeds to a step 80 where the brake pressure command value P c is set equal to the value of the brake pedal force F times a gain constant G for providing power assisted braking. Thereafter, the computer program proceeds to a step 82 where the electromagnetic valve 18 of Figure 2 is deenergized and then to the step 58 previously described. If the vehicle speed ω v , is greater than 4.8 kph (3 mph), the computer program proceeds from decision step 78 to step 84 where the electromagnetic valve 18 is energized to decouple the master cylinder 14 from the motor driven actuator 12. Brake application is thereafter provided solely via the motor driven actuator 12 as controlled by the electronic controller 20. From step 84, the computer program proceeds to a decision step 86 where the state of the D-flag is sampled. If the D-flag is reset to condition the computer program to execute the identify routine, the computer program proceeds to a step 88 where the identify routine is executed. If decision step 86 determines that the D-flag is set, the computer program is conditioned to execute a dump routine, and the computer program proceeds to a step 90 where the dump routine is executed. During this dump routine, the pressure to the wheel brake l0 is released to allow the speed of the wheel 11 to recover from an incipient lockup condition. Following the steps 88 or 90, the computer program exits the interrupt routine for the respective wheel. The computer program executes an interrupt routine as described for each of the remaining vehicle wheels following each 5 millisecond interrupt after which the program returns to the background loop step 48 of Figure 5. Referring to Figure 7, the identify routine of step 88 of Figure 6 is illustrated. This identify routine (A) provides for power assisted braking, (B) identifies the brake pressure producing the critical wheel slip corresponding to the maximum possible tyre torque between the tyre and the road surface, (C) senses an incipient wheel lockup condition and conditions the computer program to execute the dump routine to allow wheel recovery from the lockup condition and (D) reestablishes the brake pressure to substantially the identified pressure producing the critical wheel slip value. The identify routine is entered at point 92 and proceeds to a step 94 where the value of the tyre torque T t is calculated in accordance with the equation (4) from the wheel deceleration ω ˙ determined at step 59, the brake pressure P b measured at step 52 and the known values of wheel moment of inertia I w and brake gain K b . From step 94, the computer program proceeds to decision step 96 and step 98 that function to identify the brake pressure producing the maximum value of tyre torque and to determine the decrease in tyre torque from the peak value that represents an incipient wheel lockup condition. At decision step 96, the tyre torque T t calculated at step 94 is compared with the maximum previously calculated value of tyre torque T tm stored in memory. If the value calculated at step 94 is greater than the stored value T tm , the computer program proceeds to a step 98 where the stored value T tm is set equal to the larger value calculated at step 94, a stored value of brake pressure P bm representing the brake pressure corresponding in time to the stored maximum calculated value of tyre torque T tm is set equal to the brake pressure P b measured at step 52 and a stored value of the decrease in tyre torque T DEL from the stored peak value T tm that represents an incipient wheel lockup condition is updated. In this embodiment, T DEL is a predetermined percentage of the maximum calculated tyre torque T tm . Accordingly, the value of T DEL stored at step 98 is set equal to T tm /K DEL where K DEL is a calibration constant establishing the percentage drop in tyre torque as the wheel slip exceeds the critical wheel slip value that represents an impending wheel lockup condition. For illustration purposes only, K DEL may be 4.0 establishing a 25% decrease in tyre torque T t . The foregoing sequence of steps 96 and 98 are repeated with each execution of the identify routine as long as the tyre torque is increasing. If decision step 96 should determine that the calculated value of tyre torque T t is less than the stored maximum calculated value T tm , step 98 is bypassed. This will occur when the brake pressure P b results in a wheel slip that exceeds the critical wheel slip value which in turn results in a decrease in the tyre torque. The stored value of brake pressure P bm then represents the brake pressure establishing the critical wheel slip value and therefore the maximum braking effort and the stored value of T DEL is the decrease in tyre torque representing an incipient wheel lockup condition. The present invention utilizes the stored values of P bm for each of the front wheels to detect braking on a split coefficient of friction surface wherein the left and right wheels of the vehicle are being braked on surfaces having different coefficients of friction or to detect a severe steering manoeuvre while braking. This is based on the fact that the stored value of P bm for each of the front wheels will be substantially equal to one another when both wheels are at their respective critical wheel slip values when being braked on surfaces having substantially equal coefficients of friction and in the absence of a severe steering manoeuvre. A difference in the stored values of P bm for each of the front wheels is indicative of the braking of the front wheels on a split coefficient of friction surface with the magnitude of the difference representing the difference between the coefficients of friction of the surfaces on the left and right sides of the vehicle. The computer program next determines whether or not an incipient wheel lock condition exists. At decision step 99 the ratio ω/ω v is compared with a reference value S L above which stable braking takes place. In one embodiment, S L may equal 0.92 representing 8% wheel slip. A ratio less than S L indicates a potential for unstable braking. Particularly, if the wheel slip exceeds the value represented by S L and the wheel is decelerating, a decrease in the tyre torque T t to a value below the stored maximum calculated tyre torque T tm by an amount equal to T DEL is a result of wheel slip exceeding the critical wheel slip value as the wheel 11 decelerates toward a lockup condition. If decision step 99 determines that a potential exists for unstable braking, the computer program proceeds to determine if an incipient wheel lockup condition exists based on the decrease in the tyre torque from the peak value (if the wheel is decelerating) or based on the magnitude of wheel slip. Decision step 100 determines if wheel acceleration is negative. If negative the computer program proceeds to decision step 101 to determine if the tyre torque T t calculated at step 94 is less than the maximum calculated tyre torque T tm stored at step 98 by the value T DEL or greater. If the tyre torque T t has not decreased from the peak value by the value T DEL , representing stable braking based on this parameter or if wheel acceleration is not less than 0 as determined at decision step 100, the computer program proceeds to decision step 102 where the ratio ω/ω v is compared with a reference value S m (such as 0.7) which represents a wheel slip value that exceeds the largest possible critical wheel slip value for any road surface condition. A ratio less than S m indicates that braking has become unstable and an incipient wheel lockup condition exists. If either of the decision steps 99 and 102 indicates a stable braking condition, the computer program proceeds to a decision step 104 where the value of the operator requested brake pressure that is equal to the applied brake pedal force F times the gain constant G is compared with a maximum allowable brake pressure P m . If the product is less than the maximum value, the computer program proceeds to a step 106 where the brake pressure command value P c is adjusted toward the operator requested pressure in accordance with a first order lag filter equation to provide power assisted braking. Thereafter, the computer program exits the identify routine and returns to the background loop step 48. If at decision step 104 it is determined that the operator requested brake pressure is greater than the maximum allowable brake pressure P m , the computer program proceeds to a pressure ramp routine where, through repeated executions of the identify routine, the maximum allowable brake pressure P m and the brake pressure command value P c are ramped up at rates dependent upon the tyre-road interface condition until decision step 104 detects that the maximum allowable brake pressure P m has become greater than the operator requested pressure or, if the operator requested brake pressure results in an unstable braking condition, until the brake pressure command value P c results in an incipient wheel lockup condition at which time the brake pressure establishing the critical wheel slip value has been identified by the steps 96 and 98 as well as the value of T DEL to be used at decision step 101 in determining whether or not an incipient wheel lockup condition exists. The brake pressure identified is used to reestablish the commanded brake pressure after the wheel recovers from the incipient lockup condition. The result of the ramping of the brake pressure is a periodic low frequency reidentification of the brake pressure producing the critical wheel slip value. The routine for ramping the brake pressure begins at a decision step 108 where the value of a time t₁ in a RAM timing register is compared to zero. The initial value of time t₁ establishes a delay in the ramping of the brake pressure command value P c . Thereafter, the time t₁ functions in establishing the ramp rate. If the time t₁ is greater than zero, the computer program proceeds to a step 110 where the time t₁ is decremented. Thereafter, at step 112, the computer program proceeds to adjust the brake pressure command value P c toward a predetermined fraction FRAC of the maximum allowable brake pressure P m in accordance with the filter equation P c = (Z p .P co ) + (Z z .P m .FRAC) (5) where Z p and Z z are values established as will be described based on the value of the stored value of brake pressure P bm so that P c is ramped at a rate dependent upon the road-tyre friction coefficient and P co is the prior value of P c . The time constant of this expression is generally small so that the brake pressure P b is quickly ramped toward the maximum allowable brake pressure P m . By setting the maximum allowable brake pressure P m to the stored value of brake pressure P bm after an incipient wheel lockup condition is sensed (as will be described), the commanded pressure established upon repeated executions of step 112 will be the predetermined fraction FRAC of the pressure producing the critical wheel slip. In one embodiment, FRAC is 0.9 so that the resultant brake pressure produces substantially the critical wheel slip value. As long as an incipient wheel lock condition is not detected and the operator requested brake pressure is greater than the maximum allowable brake pressure P m , the steps 108 to 112 are repeated at the five millisecond interrupt interval until t₁ has been decremented to zero. After t₁ has been decremented to zero, the computer program proceeds from decision step 108 to decision step 114 where the time t₂ in a RAM timing register is compared to zero. If the time t₂ is greater than zero, the computer program proceeds to a step 116 where the time t₂ is decremented. Following step 116 or decision step 114, the computer program proceeds to a step 118 where the maximum allowable brake pressure P m is incremented and the time t₁ is set equal to K n (t₂+l) , where K n is a calibration constant. Thereafter, the steps 114 to 118 will be bypassed upon repeated executions of the identify routine until t₁ is again decremented to zero. From this it can be seen that the maximum allowable brake pressure P m is periodically incremented at intervals determined by K n and t₂. When t₂ is decremented to zero, the maximum allowable brake pressure P m is incremented with each K n executions of the identify routine. The initial value of t₂ is based on the stored value of brake pressure P bm as will be described so that the maximum allowable brake pressure P m and therefore the brake pressure command value P c is ramped at a rate dependent upon the tyre-road friction coefficient. Following step 118, the computer program proceeds to step 112 where the brake pressure command value P c is again set as previously described. Repeated executions of the foregoing steps function to increase the brake pressure command value P c exponentially. This increase will be continued until (A) an incipient wheel lock condition is forced so as to force a reidentification of the brake pressure producing the critical wheel slip value via the steps 96 and 98 or (B) the operator requested brake pressure becomes less than the maximum allowable brake pressure P m . If the brake pressure command value P c is increased to a point resulting in the wheel slip value becoming greater than the critical wheel slip value, the wheels then quickly approach a lockup condition. This incipient wheel lock condition is detected at decision step 101 or decision step 102. When the incipient wheel lockup condition is detected, the stored value of brake pressure P bm in memory at that time is the brake pressure producing the critical wheel slip value and therefore the maximum possible tyre torque. After a wheel lockup condition has been sensed, the computer program proceeds to a decision step 120 where the time t₂ is compared with a time t k1 . As will be seen, these two values will be equal only if a wheel lockup condition is sensed within a predetermined constant time period t k2 (such as 500ms) after the brake pressure is reestablished after recovery from an incipient wheel lockup condition. A wheel lockup occurring within this period after reapplication of the brake pressure implies the application of an unstable brake pressure producing an incipient wheel lockup condition. If this condition exists, the computer program proceeds to a decision step 122 where the stored value of brake pressure P bm , stored at step 98 and identified as the brake pressure establishing the critical wheel slip value, is compared with the brake pressure command value P c which resulted in the incipient wheel lockup condition. If greater, the computer program proceeds to a step 124 where the stored value of brake pressure P bm is corrected to the brake pressure command value P c . This condition represents an error in the calculation of the tyre torque either through changes in the brake line coefficients or errors in various constants used in the determination of the calculation of the tyre torque T t . Since the brake pressure producing the critical wheel slip value can never be greater than the brake pressure command value P c that resulted in an incipient wheel lock condition, the stored value of brake pressure P bm is reduced to the value of P c causing the incipient wheel lock condition. From decision step 120 if the time t₂ is not equal to t k1 , from decision step 122 if P bm is less than P c , or from step 124, the computer program proceeds to a step 125 where the value of t k1 is set equal to k t1 .(l-P bm /K p ) where k t1 is a calibration constant and k p is limit of the brake pressure as described with respect to step 58. From the above expression, it can be seen that t k1 varies inversely with the stored value of brake pressure P bm producing the maximum braking effort. As will be seen, this results in a rate of increase in the brake pressure via the steps 114, 116 and 118 that varies directly with the maximum calculated tyre torque T tm stored at step 98. At step 126, the values of Z p and Z z to be used in the filter equation (5) in step 112 are established. Z p is set equal to the expression (K z -P bm /K p )/K z where K z is a calibration constant. In one embodiment, K z was selected to be 5.0 resulting in Z p being equal to approximately 0.8 when braking on a surface having a high coefficient. Z z is set equal to l-Zp. As can be seen, the values of Z p and Z z are dependent upon the stored value of brake pressure P bm producing the maximum calculated tyre torque T tm such that the filter equation (5) has a time constant that decreases with increasing values of P bm . This results in a more rapid application of brake pressure for road surfaces having a higher coefficient of friction. At step 127, the D-flag is set to condition the computer program to execute the dump routine and certain initial conditions for reapplication of brake pressure are established. The initial conditions include setting the maximum allowable brake pressure P m equal to the stored value of brake pressure P bm (the brake pressure identified as producing the critical wheel slip value), setting the time t₁ equal to the constant t k2 and setting the time t₂ equal to the value t k1 previously described which makes the initial value of t₂ dependent upon P bm to control the rate of increase of P m as a function of the road surface condition as previously described. The computer program next proceeds to a step 128 where the dump routine is executed. Thereafter, during executions of the 5ms interrupt routine of Figure 6, the identify routine is bypassed via the decision step 86 and the dump routine of step 90 is executed until the D-flag is again reset. The dump routine executed at step 128 of the identify routine of Figure 7 and at step 90 of the interrupt routine of Figure 6 is illustrated in Figure 8. This dump routine is entered at point 130 and proceeds to decision step 131 where wheel slip represented by the ratio of wheel speed ω to the speed ω v , of the hypothetical unbraked wheel (vehicle speed) is compared to a constant S k representing wheel speed approaching vehicle speed. S k may be, for example, 0.92 representing a wheel slip of 8 percent. If the ratio is less than S k , the computer program proceeds to a decision step 132 where wheel angular acceleration ω ˙ is compared with a low value ω ˙ L such as a value representing lg. If the wheel speed ω has not yet begun to accelerate at this level in its recovery from the incipient lockup condition, the computer program proceeds to a step 134 where the brake pressure command value P c is set to zero to allow the wheel speed ω to recover from the incipient wheel lockup and toward vehicle speed ω v . From step 134, the computer program compares at decision step 136 the time t R that the brake pressure has been dumped with a maximum allowable time K R beyond which the brake pressure is to be reapplied even if recovery from the lockup condition has not been detected. If the time period K R has not been exceeded, the time t R is incremented at step 138 and the computer program returns to the background loop step 48 of Figure 5. Returning to decision step 132, if the wheel angular acceleration ω ˙ has exceeded the low value ω ˙ L , the brake pressure command value P c is set at step 140 equal to the then existing brake pressure P b to effect a hold of the brake pressure until wheel speed recovery is detected. At decision step 142, the present wheel angular acceleration ω ˙ t is compared to the previous wheel angular acceleration ω ˙ t-1 . If wheel acceleration is increasing indicating that the wheel slip is still decreasing toward the critical wheel slip value, the computer program proceeds to the decision step 136 previously described. If decision step 131 detects wheel speed recovery based on wheel slip decreasing to a value below that represented by S k or if decision step 142 detects that the wheel slip is less than the critical wheel slip value represented by a decrease in wheel acceleration or if decision step 136 detects a brake pressure dump duration exceeding K R , the computer program proceeds to a step 144 where the D-flag is reset to condition the computer program to execute the identify routine of Figure 7. Also at this step 144, the maximum calculated tyre torque T tm is set to zero so that the identify routine is conditioned to reidentify the brake pressure establishing the critical wheel slip value, the hypothetical unbraked wheel speed ω v is set equal to the last measured wheel speed ω and the time t R is reset. The computer program then exits the dump routine of Figure 8 and returns to the background loop step 48. During the following executions of the interrupt routine of Figure 6 at the 5 millisecond interrupt intervals, the computer program executes the identify routine at step 88 until the D-flag is again set at step 127 after an incipient wheel lockup condition is sensed. Control of the rear wheel brakes of the vehicle will now be described. The rear wheels are each controlled by identical computer program routines. The routine for controlling the hydraulic brake pressure to the brake of a rear wheel is the same as the above described routine for a front wheel brake except for the modification to the identify routine of Figure 7 as illustrated in Figure 9. The added computer program steps of Figure 9 to the identify routine provide for the control of the operating mode of the rear wheel brakes in accordance with the principles of this invention. Particularly, the routine senses when the vehicle wheels are being braked on a substantially uniform coefficient of friction surface and controls the braking of the rear wheel in an independent mode so as to minimize the vehicle stopping distance and senses when the coefficient of friction between the two sides of the vehicle are substantially different or when the vehicle is undergoing severe steering manoeuvres while braking and controls the braking of the rear wheels in a select low mode where both wheels are jointly controlled based on the rear wheel being braked on the lowest coefficient of friction surface. For purposes of differentiating the parameters associated with the four vehicle wheels, the designations lf, rf, lr and rr identifying the left front, right front, left rear and right rear wheels, respectively, will be added to the subscripts of the parameters previously referred to with respect to Figures 6-8. Referring to Figure 9, after the step 112 or the step 128 previously described, the computer program for controlling a rear wheel brake proceeds to a decision step 146 where the stored value of brake pressure P bmlf stored at step 98 in the identify routine controlling the left front wheel brake is compared with the value of P bmrf stored at step 98 in the identify routine controlling the right front wheel brake. These values will be substantially equal if the two front wheels are being braked on substantially equal coefficient of friction surfaces in the absence of a severe steering manoeuvre but will be unequal by an amount related to the difference in the coefficient of friction between the left and right sides of the vehicle or by an amount related to the severity of a steering manoeuvre while braking. Other parameters, such as T tmlf and T tmrf may also be utilized to indicate braking on a split coefficient of friction surface or a severe steering manoeuvre while braking. The stored values of brake pressure P bmlf and P bmrf will be substantially equal when the vehicle is being braked on a split coefficient of friction surface or while undergoing a severe steering manoeuvre until the wheel slip of the front wheel on the lowest coefficient of friction surface or on the inside of a turn during a severe steering manoeuvre exceeds the critical wheel slip value at which time its maximum calculated tyre torque T tm and the corresponding stored value of brake pressure P bm is at its peak. The stored value of brake pressure P bm of the front wheel on the highest coefficient of friction surface or on the outside of a turn during a severe steering manoeuvre will continue to increase as the maximum calculated tyre torque increases. Therefore, a rear wheel having a lower stored value of brake pressure P bm (or T tm ) is indicative of the wheel being braked on a road surface having a lower coefficient of friction surface or having a lower maximum calculated tyre torque due to a turning manoeuvre. The magnitude of the difference is indicative of the difference in the coefficients of friction or the degree or a severe steering manoeuvre while braking. If P bmlf is equal or greater than P bmrf , the computer program proceeds to a step 148 where P bmrf is subtracted from P bmlf to obtain the difference ΔP. If P bmrf is less than P bmlf the computer program proceeds from decision step 146 to a step 150 where P bmlf is subtracted from P bmrf to obtain the difference ΔP. As previously indicated, the value of ΔP is an indicator of the difference in the coefficient of friction between the left and right sides of the vehicle or an indicator of a severe steering manoeuvre. Steps 146 to 150 define sensing means for sensing a difference in the coefficients of friction of a road surface on the right and left sides of a vehicle. From step 148 or 150, the computer program proceeds to a decision step 152 where ΔP is compared with a calibration constant K D . A value of ΔP equal to or greater than K D represents a large difference in the coefficients of friction between the left and right sides of the vehicle or represents a severe steering manoeuvre of the vehicle while braking, both of which could result in reduced vehicle stability. If this condition exists, the computer program proceeds to a decision step 154 where the stored value of brake pressure P bmlr stored at step 98 in the routine controlling the left rear wheel is compared with the value of P bmrr stored at step 98 in the routine controlling the right rear wheel to determine which rear wheel is being braked on the highest coefficient of friction surface. In general, P bmlr and P bmrr will be substantially equal until the slip of the rear wheel on the lowest coefficient of friction surface exceeds the critical wheel slip value at which time the corresponding stored value of brake pressure P bm is at its peak. The stored value of brake pressure P bm of the rear wheel on the highest coefficient of friction surface will continue to increase as the determined value of tyre torque increases. Therefore, a rear wheel having a lower stored value of brake pressure P bm is indicative of the wheel being braked on a road surface having a lower coefficient of friction surface. If P bmlr is greater than P bmrr indicating that the left rear wheel is being braked on the higher coefficient of friction surface, the computer program proceeds to a step 156 where the brake pressure command value P clr to the left rear brake is set equal to the value of the brake pressure command value P crr to the right rear brake determined at step 112, 134 or 140 of the routine controlling the right rear brake. If at decision step 154 P bmrr is determined to be greater than P bmlr indicating that the right rear wheel is being braked on the higher coefficient of friction surface, the computer program proceeds to a step 158 where the brake pressure command value P crr to the right rear brake is set equal to the value of the brake pressure command value P clr to the left rear brake determined at step 112, 134 or 140 of the routine controlling the left rear brake. From steps 156 or 158, the computer program exits the identify routine. Through the steps 152, 154, 156 and 158, the pressure applied to the rear wheels during wheel lock controlled braking are controlled in the select low mode where the braking pressure applied to the rear wheel being braked on the highest coefficient of friction surface is controlled to the pressure established for the rear wheel being braked on the lowest coefficient of friction surface. However, if at decision step 152 it is determined that the coefficients of friction between the sides of the vehicle are not greatly different, the computer program exits the identify routine and the rear wheels are braked in an independent mode in the same manner as the front wheels so as to minimize the vehicle braking distance. The electronic controller 20 therefore defines control means having an independent braking mode and a select low braking mode. Further, steps 152 to 158 define setting means for setting the electronic controller in either the independent braking mode, or in the select low braking mode.

Claims: A wheel lock control apparatus for a vehicle having wheels (11) including right and left front wheels and right and left rear wheels for transporting the vehicle over a roadway surface having varying coefficients of friction, each wheel having a wheel brake (10) responsive to a brake pressure for braking the wheel, the wheel lock control apparatus comprising sensing means (146-150) for sensing a difference in the coefficients of friction of the roadway surface on the right and left sides of the vehicle; and control means (20) for controlling the brake pressure to the wheel brake of each wheel to prevent the wheel from locking, the control means having a select low braking mode wherein the brake pressure to the wheel brakes of both the right and left rear wheels are controlled to the controlled brake pressure to the wheel brake of the right or left rear wheel that is on the side of the vehicle at which the road surface has the lowest coefficient of friction; characterised by the control means (20) also having an independent braking mode wherein the brake pressure to the wheel brake (10) of each wheel (11) is controlled independently; and by the wheel lock control apparatus comprising setting means (152-158) for setting the control means (20) in the independent braking mode when the sensed difference in the coefficients of friction of the road surface on the right and left sides of the vehicle is less than a predetermined value and setting the control means in the select low braking mode when the sensed difference in the coefficients of friction of the road surface of the left and right sides of the vehicle is greater than the predetermined value. A wheel lock control apparatus according to claim 1 wherein the sensing means includes responsive means (146) responsive to respective controlled brake pressures of the wheel brakes of the front wheels. A wheel lock control apparatus according to claim 1 or claim 2, comprising means for each wheel for (A) determining the maximum tyre torque between the tyre and the road surface during the application of brake pressure, (B) storing the value of brake pressure corresponding in time to the determined maximum tyre torque following each application of brake pressure, (C) detecting an incipient wheel lockup condition of the wheel, (D) dumping the brake pressure to allow wheel speed to recover, and then (E) reapplying a brake pressure following wheel speed recovery at a value substantially equal to the last stored value of brake pressure; wherein the sensing means includes determining means (150,152) for determining the difference in the stored brake pressures for the front wheels; and the setting means includes comparing means (154) for comparing the values of the stored brake pressures for the rear wheels; and means (152) responsive to a determined difference in the values of the stored brake pressures for the front wheels greater than a predetermined value for setting the brake pressure applied to the wheel brake (10) of the rear wheel having the highest stored value of brake pressure to a value equal to the value of the brake pressure applied to the rear wheel having the lowest stored value of brake pressure. A method of controlling the brake pressure applied to the wheel brakes (10) of the left and right rear wheels (11) of a vehicle to prevent lockup of the rear wheels during braking on a roadway surface, comprising the step of sensing (146-150) the difference in the coefficients of friction between the left and right sides of the vehicle and further comprising the steps of independently controlling the brake pressures applied to the wheel brakes of the right and left rear wheels so as to prevent wheel lockup of the rear wheels when the sensed difference in the coefficients of friction between the left and right sides of the vehicle is less than a predetermined amount; and controlling the brake pressure applied to the brakes of the right and left rear wheels to a common value that prevents wheel lockup of the rear wheel on the side of the vehicle at which the road surface has the lowest coefficient of friction surface when the sensed difference in the coefficients of friction between the left and right sides of the vehicle is greater than a predetermined value.

Inventors: KADE, ALEXANDER (NMI), LEPPEK, KEVIN GERARD
Applicants: GENERAL MOTORS CORPORATION
Application year: 1987
Main classification: B60T8
IPC codes: B60T8