IT8922645A1 - ANTI-LOCK BRAKING SYSTEM FOR VEHICLES - Google Patents

Description

"ANTI-LOCK BRAKING SYSTEM FOR VEHICLES"

SUMMARY

The present invention relates to a vehicle anti-lock brake control system that does not require the use of an auxiliary hydraulic pump and, in the preferred embodiment, is adapted to control only the rear brakes of a four-wheeled vehicle. The control system includes a normally open solenoid-actuated isolation valve connected from the master cylinder and rear wheel cables and a normally closed solenoid-actuated relief valve connected between the rear wheel brakes and a fluid accumulator. A computer control unit is connected to monitor rear wheel speed and deceleration and, during vehicle braking, functions to control the application of hydraulic pressure to the rear brakes through the isolation and relief valves in order to correct deviations in wheel speed and prevent brake lockup. The computer control unit is adapted to detect a change in the road surface from low to high friction coefficient, which occurs during a deviation in wheel speed. of the wheels, and subsequently applies additional pressure to be applied to the rear brakes by selectively opening the isolation valve. In addition, the control unit is designed to detect when one of the rear wheels remains locked after anti-lock measures have been taken. In this case, the standard operating parameters that control the reapplication of additional pressure are modified.

DESCRIPTION

The present invention relates generally to anti-lock braking systems for vehicles and, more particularly, to an anti-lock braking system which does not require the use of an auxiliary hydraulic pump and which is adapted to selectively control the application of hydraulic fluid pressure to selected brakes to prevent lockup of the associated wheels.

Braking a vehicle in a controlled manner in adverse conditions such as rain, snow, or ice generally requires precise application of the brakes by the vehicle driver. In these conditions, or in panic stopping situations, a driver will often apply excessive brake pressure, causing the wheels to lock up in such a way that excessive slippage occurs between the wheels and the road surface. Wheel-lock conditions can lead to a loss of directional stability and, possibly, uncontrolled vehicle spins.

In an ongoing effort to improve vehicle operating safety, many companies have become interested in developing anti-lock braking systems. While these systems typically control the braking of each braked wheel on a vehicle, some systems have been developed to control the braking of only a portion of the braked wheels. Examples of prior art anti-lock braking systems are described in U.S. Patent Nos. 3,515,440; 3,731,979;

3,870,376 and 3,880,474

Prior art anti-lock braking systems generally include a central control unit for monitoring the speed and deceleration of the controlled wheels. When the vehicle brakes are applied and the control unit detects an incipient wheel lock condition, the central control unit operates to control or command the application of hydraulic pressure through control valve means to the associated brakes to prevent lockup of the controlled wheels. Typically, the anti-lock braking system includes means for cyclically releasing and reapplying pressure to the associated brakes to limit wheel slip to a safe level while continuing to produce adequate braking torque to decelerate the vehicle as desired by the driver. In these systems, the means for reapplying pressure generally consists of a separate hydraulic power source. An example of an anti-lock braking system not requiring the use of a separate hydraulic pump is described in U.S. Patent 4,418,966.

Despite the tremendous advantages an anti-lock braking system can provide in bringing a vehicle to a controlled stop under adverse braking conditions, few vehicles are actually equipped with such systems or control systems. One of the main reasons for this is that the control units and associated valves for such systems are quite sophisticated and expensive, and are therefore typically found only on more expensive vehicles.

The present invention relates to improved control features for an anti-lock vehicle brake control system that does not require the use of a separate hydraulic pump and, in the preferred embodiment, is used solely to control the brake pressure at the rear wheels of a four-wheeled vehicle. The anti-lock brake system monitors the speed and deceleration of the rear wheels and, during braking of a vehicle, operates to control the application of hydraulic pressure to the vehicle's rear brakes through an anti-lock control valve to prevent the associated wheels from locking. When excessive deceleration of the rear wheels is detected during stopping of a vehicle, this indicates that an initial deviation in wheel speed has occurred and that the rear wheels are approaching an incipient lockup condition. The control then closes an isolation valve to maintain rear brake pressure at a relatively constant level. If, after the isolation valve has been closed, the difference between the rear wheel speed and a speed ramp value is greater than the value set by the control valve, the brake pressure will be maintained at a relatively constant level. If the theoretical calculated slip threshold exceeds a predetermined slip threshold, and the deceleration of the rear wheels exceeds a predetermined amount, then an exhaust valve can be selectively opened to reduce pressure to the rear brakes and correct the initial wheel speed deviation.

After the first cycle of wheel speed offset, and depending on the various operating parameters monitored by the system, the control unit may enter a selected one of several separate application modes to apply additional pressure to the rear brakes by selectively opening the isolation valve. Five such separate reapplication modes are described in the aforementioned U.S. patent application Ser. No. 063,361. Since the anti-lock brake control system of the present invention does not utilize a separate hydraulic pump, instances in which additional pressure must be delivered to the rear brakes must be carefully monitored and controlled since the supply of pressurized fluid that can be used to augment the pressure is extremely limited compared to systems utilizing a separate pump. This application specifically relates to several improvements in the system described in the aforementioned U.S. patent application Ser. No. 063,361. In particular, the control unit of the present invention is capable of applying additional pressure to the rear brakes. This invention provides a means for detecting a change in road surface from a low to a high coefficient of friction or "mu" that occurs during a deviation in the speed of the controlled wheels. In accordance with the present invention, the control means is first adapted to determine a reference deceleration value representing the average wheel deceleration over the wheel speed shift cycle. Subsequently, after the wheel speed shift has been corrected, the control means is adapted to determine the average wheel deceleration over a specified period of time beginning at the end of the shift cycle. If this wheel deceleration is greater than the indicated deceleration by a predetermined amount, this indicates a change in road surface conditions from a low coefficient of friction or "mu" surface to a higher coefficient of friction surface during the shift cycle. The increase in deceleration is a consequence of the additional braking provided by the front wheels when the vehicle travels over a higher coefficient of friction surface. high which wheels are either uncontrolled or are under the control of a separate anti-lock system. In these cases, rear brake pressure can generally be increased to provide additional braking without causing the brakes to lock. This is accomplished by momentarily opening the isolation valve. The present invention also relates to means for detecting when one of the two rear wheels remains completely or partially locked after anti-lock control measures have been taken to correct a deviation in wheel speed. Such a situation may occur either on a low friction surface or on a differential (high/low) friction surface. After such anti-lock control has been attempted to correct a deviation in wheel speed, and the control means has determined that at least one of the controlled wheel brakes remains at least partially locked, the control means is adapted to modify the predetermined set of operating parameters that normally control the reapplication of additional pressure to the brakes. These parameters are modified to eliminate or reduce the normal reapplication of pressure. The foregoing, as well as other features and advantages of the present invention, will be readily apparent to those skilled in the art upon reading the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. 1 is a schematic diagram illustrating a vehicle anti-lock braking system utilizing control features of the present invention; FIGS. 2a and 2b are waveform diagrams generally illustrating the improved operation of the anti-lock braking system under two different braking situations; and FIGS. 3a and 3b are flow diagrams illustrating the manner in which the anti-lock braking control system of FIG. 1 achieves the operating characteristics shown in FIGS. 2a and 2b. It should be noted at the outset of this specification that, although the control system is described herein for use with an anti-lock braking system adapted to control only the rear wheels of a four-wheeled vehicle, the control system (and parts thereof) may be used in conjunction with a vehicle anti-lock braking system. The anti-lock braking system 10 may also be employed in four-wheel anti-lock braking systems. Referring to the drawings, FIG. 1 shows a schematic diagram of a vehicle anti-lock braking system 10 that includes the control features of the present invention. The anti-lock braking system 10 is particularly adapted to monitor and control the braking of a predetermined number of wheels of a multi-wheeled vehicle having at least one braked wheel that is not controlled by the anti-lock braking control system. For example, as illustrated in FIG. 1, the anti-lock braking system may be used to control the braking of the rear wheels of a four-wheeled vehicle in which the front brakes of the vehicle are not controlled by the anti-lock braking system. Such a system is particularly desirable for a vehicle such as a small truck, for example, in which the weight supported by the rear wheels may vary greatly due to the wide range of payloads the truck may carry and the split between front brake pressure and rear brake pressure may vary. be difficult to control. As shown in FIG. 1, the anti-lock brake system is installed on a vehicle having a hydraulic brake system consisting of a brake pedal 12 connected to operate a dual-reservoir master cylinder 14. When the vehicle operator depresses brake pedal 12, master cylinder 14 supplies pressurized hydraulic fluid from a front reservoir 14a through a hydraulic line 16a and a rear reservoir 14b through a hydraulic line 16b to a conventional combining or proportioning valve 18. The combining or proportioning valve 18 includes a first outlet line 18a adapted to supply hydraulic fluid at a first predetermined pressure to operate the vehicle's front brakes 19a and 19b and a second outlet line 18b that supplies fluid at a second predetermined pressure to operate the vehicle's rear brakes 20a and 20b. Unless otherwise shown in the drawings, the combination valve 18 is typically equipped with an integral differential pressure switch to detect a predetermined pressure difference between the fluid in conduits 16a and 16b, which difference is indicative of a partial failure. The anti-lock braking system utilizes a control valve 21 to selectively control the application of pressure to the rear brakes 20a and 20b when the system is in the anti-lock mode. The valve 21 may be of the type described in U.S. Patent Nos. 4,668,023 and 4,673,226, both of which are incorporated herein by reference. More specifically, the control valve 21 includes a normally open isolation valve 22 connected between conduit 18b and a conduit 24 that supplies pressurized braking fluid to the rear brakes 20a and 20b. As will be discussed, the isolation valve 22 is solenoid operated and is actuated by a solenoid. closed in the event that incipient rear wheel lockup is detected to maintain the pressure in conduit 24 at a relatively constant level and thereby prevent further increases in pressure in conduit 18b from being delivered to conduit 24. ;Furthermore, valve 21 includes a normally closed relief valve 26 connected between conduit 24 and a conduit 27 which is connected to a fluid accumulator 28. Accumulator 28 includes a variable volume fluid reservoir 28a for containing hydraulic fluid which is maintained at a slightly elevated pressure by a sliding piston 28b biased by a spring 28c. More specifically, spring 28c maintains the fluid in the accumulator at a pressure slightly higher than the unactivated pressure of the fluid in conduit 24. As will be discussed, when isolation valve 22 is used, the If the brake pedal 12 has been closed and the pressure retained in line 24 continues to cause excessive rear wheel slip, the relief valve 26 selectively opens to direct fluid into the accumulator 28 to reduce the pressure in line 24 and prevent lockup of the rear brakes. After the brake pedal 12 has been released, the isolation valve 22 is opened and the pressurized fluid in the accumulator 28 can be returned to line 24 through a check valve 29. Alternatively, the check valve 29 can be eliminated and the relief valve 26 can be opened momentarily after the brake pedal has been released to return fluid in the accumulator 28 to line 24. A check valve 31 is connected through the isolation valve 22 between lines 18b and 24 and provides fluid flow from line 24 to line 18b when the pressure in line 24 is greater than the pressure in line 18b. Therefore, when the brake pedal is released and the isolation valve is open, the increased pressure in conduit 24 can propagate to conduit 18b through both the open isolation valve 22 and the check valve 31. The operation of the isolation valve 22 and the exhaust valve 26 is controlled by a computer control module or unit 30. The isolation valve 22 and the exhaust valve 26 are solenoid-operated valves having solenoids 22a and 26a which may be connected to the computer control module via electrical lines 32 and 34, respectively. In order to determine whether the vehicle operator is braking the vehicle, the computer control unit 30 is connected to a brake light switch 36 via a line 38 to control or monitor whether the brake pedal 12 is depressed. The computer control module or unit 30 is controlled by a computer control module or unit 30. The control module 30 is also connected via a line 42 to a single speed sensor 40 that monitors the average speed of the vehicle's rear wheels by sensing the rotation of the rear differential ring gear (not shown). In addition to monitoring the position of the brake pedal 12 via the brake light switch 36 and the speed of the rear wheels via the speed sensor 40, the computer control module 30 is connected to a differential pressure switch 44 via a line 46. The switch 44 provides two separate functions. First, when the system is operating in the anti-lock mode, the switch 44 is used to monitor the differential pressure across the isolation valve. Second, when the system is not in the anti-lock mode and the vehicle is in a normal braking mode, the switch 44 is used to monitor the condition of the exhaust valve. To perform its first function, the differential pressure switch 44 is used to monitor the condition of the exhaust valve. The differential pressure switch is connected to monitor the pressure difference between the fluid in lines 18b and 24 and is designed to close contacts 44a to ground line 46 when the pressure in line 18b is greater than the pressure in line 24. When the system is in the anti-lock braking mode and contacts 44a of the differential pressure switch are closed, this indicates that the isolation valve has closed and the pressure in line 18b is greater than the pressure in line 24, and when contacts 44a of the pressure switch are open, this indicates that the pressure in line 18b is equal to or has fallen below the pressure in line 24. In cases where contacts 44a have closed, and then opened, while brake light switch 36 remained activated, this indicates that the pressure in line 18b has fallen below the pressure in line 24. This term refers to a situation in which the operator has initially applied a relatively high braking force to the brake pedal to cause the system to enter an anti-lock mode and close the isolation valve to prevent the rear wheels from locking, and has subsequently reduced the braking force on the pedal without necessarily fully releasing the pedal. It is in this situation that it is desirable to release the anti-lock mode and return the braking system to its normal operating mode. Therefore, if the system is in the anti-lock mode and the computer control unit or module 30 detects that the differential pressure switch contacts 44a were at one point closed, but are now open, and the brake pedal is still depressed, the system will return to the normal braking mode. Typically, some hysteresis is present. This is associated with the operation of the differential pressure switch 44, whereby the pressure switch contacts 44a do not oscillate between a closed and open position when the pressure in line 18b remains relatively equal to the pressure in line 24. When the brake system is not operating in the anti-lock mode, the differential pressure switch 44 is wired to monitor the condition of the valve seat of the exhaust valve 26 by monitoring the fluid pressure in the accumulator 28. At this point, the accumulator 28 should be empty, and the normally closed exhaust valve 26 should prevent pressurized fluid in line 24 from being delivered to the accumulator. However, in the event that fluid leaks at the exhaust valve seat into the accumulator, the increase in pressure in line 27 will be detected by the pressure switch 44 closing the pressure switch contacts 44a to signal the control module 30 of a potentially defective exhaust valve. As shown in FIG. 1, the differential pressure switch 44 may be provided with an integral resistor 44b connected in parallel with the switch contacts 44a. Resistor 44b typically has a relatively high resistance value, such as 10k ohms. Resistor 44b enables computer control module 30 to monitor the continuity of line 46 when the system is not in the anti-lock mode and contacts 44a are open. In such cases, line 46 should provide a signal path to ground through resistor 44b. Control module 30 is connected to a brake failure warning light 48 which is activated in the event that a failure in the anti-lock braking system is detected. Fundamentally, the anti-lock braking system of the present invention monitors vehicle speed and the vehicle's ability to maintain a safe operating condition. and deceleration of the rear wheels and, during vehicle braking, functions to control the application of hydraulic pressure to the vehicle's rear brakes through control valve 21 in order to prevent a brake lockup condition. In the event that excessive rear wheel deceleration is detected, indicating that a wheel speed deviation has occurred and the rear brakes are approaching a lockup position, control module 30 closes isolation valve 22 to maintain pressure in line 24 at a relatively constant level. If, after isolation valve 22 has closed, the difference between the actual rear wheel speed and the calculated theoretical speed ramp value exceeds a predetermined slip threshold, and the rear wheel deceleration exceeds a predetermined amount, then exhaust valve 26 may be released. The brake line 24 may be selectively opened to reduce pressure in conduit 24 and correct the incipient lock-up condition. In some cases, after an incipient lock-up condition has been corrected, it is desirable to reapply additional pressure to the rear brakes to increase rear wheel braking. For example, as described in the aforementioned U.S. Patent Application Ser. No. 063,361, it has been found desirable to automatically apply additional pressure after the first wheel speed offset cycle to cause a second wheel speed offset. This automatic reapplication of pressure ensures that the maximum controlled pressure is supplied to the rear brake during an anti-lock stop. Additionally, after the second wheel speed offset cycle, the system monitors changes in rear wheel deceleration to detect instances where the vehicle is moving from a road surface such as ice, where the coefficient of friction (mu) between the vehicle and the road surface is high. A relatively low-pressure surface (low-mu surface) may be compared to a road surface such as concrete, where the coefficient of friction between the vehicle and the road surface is relatively high (high-mu surface). In these cases, as the vehicle's front wheels contact the higher-mu surface, the uncontrolled front brakes will cause an increase in vehicle deceleration as the vehicle moves from the low-mu surface to the high-mu surface. Under these conditions, the pressure retained in line 24 on the rear brakes can generally be increased to provide additional braking without causing a rear brake lockup condition. This is accomplished by momentarily opening isolation valve 22 to allow the higher-pressure fluid in line 18b to be supplied to line 24. Due to the continued braking force of the driver's hand on the vehicle's brake pedal under a hard braking condition, the pressure in line 18b will be increased. generally greater than the pressure in conduit 24. ;Other reapplication modes, such as an end-of-stop reapplication mode that automatically reapplies additional pressure at or near the end of a vehicle stop, are described in detail in the aforementioned U.S. patent application Ser. No. 063,361. ;Referring now to FIGS. 2a and 2b, there are waveform diagrams that will be used to describe the improved operation of the anti-lock braking system under certain braking situations. Specifically, FIG. 2a illustrates the operation of the anti-lock braking control system when the vehicle begins braking on a relatively low-friction surface, enters the anti-lock braking control mode, and then moves onto a relatively high-friction surface during a second wheel speed deviation. FIG. 2b illustrates the operation of the anti-lock braking system when the vehicle is braked on a surface of varying mu or a surface of relatively low mu where one of the rear wheels remains locked after anti-lock control measures have been taken. It will be understood that the waveform diagrams of FIGS. 2a and 2b and the flow diagrams of FIGS. 3a and 3b represent only a portion of the control characteristics of the anti-lock system and that a more complete description of the anti-lock system can be found in the aforementioned U.S. patent application Ser. No. 063,361. Referring now to FIG. 2a, the actual speed of the rear wheels is indicated by an upper curve 50 while the actual braking or brake pressure of the rear wheels is indicated by a lower curve 51. The operation of the isolation valve 22 is represented by waveform 52 while the operation of the exhaust valve 26 is represented by waveform 52. The vehicle is initially moving along a relatively low-mu surface such as snow or ice at a velocity V without the vehicle brakes being applied. At this time, the isolation valve 22 is open while the exhaust valve 26 is closed. At time t, the driver initiates a sudden stop of the vehicle by fully depressing the brake pedal to rapidly increase the braking pressure in line 24 and decelerate the vehicle. Initially, the braking pressure in line 24 is not sufficient to cause any rear wheel lockup. However, shortly after time t, the rear wheel velocity begins to slip relative to the actual vehicle velocity (represented by the dashed curve 55) causing the rear wheels to enter an initial wheel velocity offset cycle 50a. As shown in Figure 2a, after time t, the rear wheel velocity drops to 0. The actual wheel speed 50 begins to decrease relative to the actual vehicle speed 55 as indicated by portion 50b. Before entering wheel speed offset cycle 50a, the computer control will have calculated a theoretical speed ramp (not shown) that represents the speed at which the vehicle would be moving if it were decelerated at a predetermined maximum rate or degree, e.g., 1.0 g. The computer control module also continues to monitor the rear wheel deceleration and the actual rear wheel speed relative to the calculated speed ramp value. When the rear wheel deceleration reaches a predetermined value (e.g., 1.3 g) at time t2, the computer control module generates a signal to close the isolation valve and maintain the pressure in line 24 at a constant level P, while the uncontrolled rear brake pressure follows curve 51a. When the difference between the rear wheel speed and the actual rear wheel speed is reached, the computer control module generates a signal to close the isolation valve and maintain the pressure in line 24 at a constant level P, while the uncontrolled rear brake pressure follows curve 51a. If the actual wheel speed 50 is exceeded and the calculated speed ramp value exceeds a predetermined slip threshold St, this indicates that a predetermined slip is occurring between the actual wheel speed and the vehicle speed. In FIG. 2a, the slip threshold is exceeded and the rear wheel deceleration exceeds a predetermined amount at time t3. At this time, the actual wheel speed will have decreased sufficiently that it is desirable to selectively reduce pressure to the rear wheels in an attempt to return the actual wheel speed to the vehicle speed. Therefore, the computer control module generates one or more exhaust pulses 53a beginning at time t to selectively reduce the rear brake pressure to a level P. The particular number of exhaust pulses generated will vary depending on the particular control valve used and the amount of pressure reduction that occurs for each pulse. It will be understood that the uniform width and spacing between a series of exhaust pulses is shown for simplicity and that, as described in U.S. Pat. No. 4,673,226, it may be desirable to control the width and spacing between exhaust pulses to achieve a desired pressure reduction curve. Generally, exhaust pulses 53a are terminated approximately at a time when the actual wheel deceleration has decreased below the predetermined amount. After the actual wheel speed "accelerates" along the portion 40c and returns to the actual speed to complete the first cycle 50a of offsetting the wheel speed and correcting the slip condition, it has been found desirable to selectively and automatically reapply additional pressure to the rear brakes in order to ensure that the maximum controlled pressure is applied to the rear brakes. How is this accomplished? As depicted in the isolation valve waveform 52, a series of automatic reapplication pulses 52a are generated at time t after the end of the rotational acceleration portion 50c to selectively increase the rear brake pressure to a level P. Typically, the number of automatic reapplication pulses 52a will vary depending on the particular control valve used. Pulses 52a are generated until a second wheel speed offset cycle 50e is initiated. As with the blow-off pulses, the width and spacing between successive reapplication pulses may be varied to control the pressure application curve. Once the slip threshold has been exceeded in the second wheel speed offset cycle, one or more blow-off pulses 53b are generated (at time t^) to correct the slip condition and reduce the rear brake pressure to a level P. Thereafter, until the uncontrolled pressure 51a supplied by the vehicle driver is reached, the pressure is applied to the rear brakes. above the pressure P, the antilock braking system will maintain the pressure at the level P as long as the road surface friction rxazx remains relatively constant. Typically, the pressure P will be higher than the pressure P. In FIG. 2a, the vehicle moves from a low-friction surface to a high-friction surface during the second wheel speed cycle 50e at a time t. The present invention specifically relates to a computer control module capable of detecting such a change in road surface conditions during a wheel speed cycle. In accordance with the present invention, such a change is detected by first calculating the average wheel deceleration during the time period of the second wheel speed cycle 50e, which, in FIG. 2a, is represented by the time period T. By measuring this time period, and detecting the wheel speed, the vehicle is able to detect the change in road surface conditions during a wheel speed cycle. V and V2 of the rear wheels at the beginning and end of the time interval T, the average deceleration can be calculated by determining the difference between V and V and dividing this difference by the ground velocity T. Subsequently, the computer control module is capable of operating* to measure the deceleration of the rear wheels within a second time period T immediately following the time period T. The average wheel deceleration during the time period T can be calculated in a similar manner using the rear wheel velocity values V and V at the beginning and end of the time period T. If it is determined that the average wheel deceleration during the time period T is greater than the average wheel deceleration during the time period T by a predetermined amount, then this indicates a situation in which it is likely that the vehicle moved from a low-friction surface to a high-friction surface during the second 50e cycle of wheel velocity deviation. How is this determined? As mentioned previously, this increase in deceleration is due to the increased braking provided by the front wheels as the vehicle travels on the higher friction surface. In such cases, the braking pressure at the rear wheels can generally be increased without causing the rear brakes to lock up. Therefore, at time t, the computer control module will generate additional reapply pulses 52b to increase the pressure to a level P and cause a third wheel speed offset cycle at 50f. The control module then generates one or more exhaust pulses 53c at time t to reduce the pressure to a level P and correct the third wheel speed offset. In the event that the change from a low-friction surface to a high-friction surface occurs after the end of the second wheel speed offset cycle 50e, the computer control module will generate one or more exhaust pulses 53c at time t to reduce the pressure to a level P and correct the third wheel speed offset. of the wheels}such variation in road surface conditions can be detected in the manner described in the above-mentioned U.S. patent application serial number 063,361,

Referring now to FIG. 3a, there is a simplified flowchart that can be used to summarize the operation of the anti-lock braking system illustrated in FIG. 2a. In FIG. 3a, a processing function 60 represents "normal" braking conditions, in which the anti-lock braking system has not been activated and the vehicle's braking pressure is under complete control of the vehicle driver. During these braking conditions, the anti-lock braking system of the present invention continues to monitor the average speed of the rear wheels and calculate the level of deceleration and slip and compare these values to predetermined anti-lock actuation thresholds. This comparison function is represented by a decision point 61 where, if no wheel speed deviation has been detected, the system branches to NO and remains in a normal braking mode. However, in the event that a first wheel speed deviation has been detected, the system will return to the normal braking mode. of the wheels, the system branches to YES and enters a processing function 62 in which the anti-lock control mode is activated. At this point, the isolation valve 22 is closed (at time t) to initially maintain the pressure at level P and then the normally closed relief valve 26 is selectively opened (at time t) to selectively reduce the pressure to level P and correct the initial wheel speed deviation. From this point, the program enters a processing function

processing function 63 wherein, at time t, additional pressure is applied via application pulses 52a to raise the pressure to level P and cause the second wheel speed deviation cycle 50e. Subsequently, a processing function 64 is performed wherein the exhaust valve is operated via exhaust pulse 53b to selectively reduce the pressure to level P thereby correcting the second wheel speed deviation.

After the second wheel speed deviation has been corrected, the program enters a processing function at 65 where the average wheel deceleration (R) that occurs during the second wheel speed deviation cycle (time period T) is calculated. Next, the program enters a processing function 66 and calculates the average wheel deceleration (A) within a second predetermined time period T immediately following time period T. The average wheel deceleration R is then compared to the average wheel deceleration A at a decision point 66. If the wheel deceleration A D is greater than the reference wheel deceleration by a predetermined amount (X), this indicates a situation where there is a probability that the vehicle has moved from a low-mu surface to a high-mu surface during the second wheel speed deviation. The program then branches to YES and enters a processing function 68 where additional reapply pulses 52b are generated to increase pressure to a level P and cause the third wheel speed offset 50f. The third wheel speed offset is then corrected in processing function 69 by generating one or more dump pulses 53c to reduce pressure to a level P and correct the third wheel speed offset. From this point, or if the wheel deceleration does not exceed the reference wheel deceleration by a predetermined amount, the program enters or initiates a processing function 70 that maintains the rear brake pressure at a relatively constant level unless predetermined operating parameters are met as described in U.S. Patent Application Ser. No. 063,361.11 which would require additional rear brake pressure control.

The present invention also relates to a novel solution for determining whether one of the two rear wheels of the vehicle remains locked after anti-lock control measures have attempted to correct a wheel speed deviation. This particular feature of the present invention is particularly desirable in anti-lock control systems where a single speed sensor is used to provide a speed signal indicating the average speed between two or more wheels of a vehicle. This is commonly the case in rear-wheel drive vehicles where a rear center differential is used to split torque between the left and right rear wheels of the vehicle. In these situations, a single speed sensor, such as that shown in Figure 1, is used to monitor the speed of a differential component rotating at the average speed of the two rear wheels.

It has been found that under certain braking conditions, particularly when braking the vehicle on a relatively low-friction surface or when braking the vehicle on a differential-friction surface having a relatively low-friction surface on one side of the vehicle and a relatively high-friction surface on the other side of the vehicle, correction of the slip condition of a locked wheel can be difficult. However, in these situations, if it is possible to detect that one of the rear wheels has remained locked after the anti-lock control module has attempted to correct the wheel runout, then steps can be taken to assist in freeing or disengaging the locked wheel. For example, in accordance with the present invention, when it has been determined that one of the rear wheels remains locked after an attempt to correct a wheel speed runout, the normally standard predetermined operating parameters that can cause the wheel to be released or disengaged are modified. The additional pressure to be applied to the rear brakes is modified such that no additional reapply pulses, or a smaller number of reapply pulses, are subsequently generated. It has also been found desirable to modify the end-of-stop routine described in the aforementioned U.S. Patent Application Serial Number 063,361 such that the end-of-stop speed ramp decays to half its normal rate.

Referring now to FIG. 2b and FIG. 3b, the manner in which the present invention detects such a locked rear wheel will now be discussed. As depicted in FIG. 3b, a processing function 70 represents "normal" braking conditions in which the anti-lock braking system has not been activated and the vehicle's brake pressure is under complete control of the vehicle driver. During such braking conditions, the anti-lock braking system of the present invention continues to monitor the average speed of the rear wheels and calculate the level of deceleration and slip to determine if a wheel speed slip condition is present requiring the system to initiate the anti-lock mode. More specifically, if wheel speed slip is detected at a decision point 71, the program branches to YES and enters or initiates a processing function 72 in which the average speed of the rear wheels is detected. The average speed (VR ) of the rear wheels at the onset (time To ) of a wheel offset 73 shown in FIG. 2b is stored. Next, the program enters a processing function 74, where the anti-lock control mode is activated in an attempt to correct the wheel speed offset 73. However, because one of the rear wheels remains locked, the average speed of the rear wheels is stabilized along a ramp 75 that is midway between a vehicle speed curve 76 and a zero speed level 77. In this situation, one of the rear wheels is turning at vehicle speed while the other rear wheel is locked.

In order to detect such a condition, the program enters a processing function 78 in which a vehicle speed ramp is determined within a predetermined time period T, as depicted in Figure 2b. Next, a processing function 79 is executed in which the speed ramp is extrapolated backwards (as depicted in Figure 2b by a dotted line 80) to determine a calculated wheel speed (V) at time T. This calculated vehicle speed is then compared (at a decision point 81) with a predetermined percentage of the average reference wheel speed VR. In Figure 2b, this percentage is depicted as 2/3 of the reference speed V. In the event that the calculated speed is less than 2/3 of the reference speed, this is an indication that one of the two rear wheels is locked or at least partially locked. In such cases, a lock is applied. It has been found desirable to modify any additional reapplication modes. Therefore, the program branches from a decision point 81 to YES and initiates a processing function 82 in which the predetermined operating parameters controlling the reapplication modes are modified as previously stated. However, in the event that the calculated speed is greater than 2/3 of the target speed, the program branches to NO and enters or initiates a processing function 83 in which anti-lock control is continued with the standard operating parameters for the reapplication modes. The anti-lock control system of the present invention has been illustrated and described in a manner that is considered to represent its preferred embodiment. However, it will be understood that the features and operation of the present invention may be modified without departing from the spirit or scope of the appended claims. For example, although the control system has been described for use in association with a pumpless anti-lock braking system to control only the rear wheels of a vehicle, the features

that the previously described operating systems or portions thereof may be included in an anti-lock braking system comprising a hydraulic pump and designed to control the braking pressure at all four wheels of the vehicle.

Claims (13)

CLAIM'S 1. In a wheeled vehicle having means for supplying pressurized braking fluid to actuate the associated wheel brakes of the vehicle, a braking control system or system for controlling the application of pressurized braking fluid to at least one selected wheel brake to control braking of the associated wheel, said control system comprising: normally open isolation valve means connected between the power means and the selected wheel brake; normally closed exhaust valve means?connected between the selected wheel brake and a fluid reservoir; and control means connected to operate said isolation valve means and said exhaust valve means, and said control means including means for sensing deceleration of the associated wheel, and for detecting deviations in wheel speed of the associated wheel relative to the actual speed of the vehicle, said control means being adapted to close said isolation valve means to maintain fluid pressure to the selected wheel brake at a relatively constant level after a deviation in wheel speed of the associated wheel has been detected, said control means being adapted to selectively open said exhaust valve means after said isolation valve means has been closed to permit fluid to flow into said fluid reservoir to selectively reduce pressure to the selected wheel brake to correct said deviation in wheel speed of the associated wheel, said control means being adapted to determine a reference deceleration value representing the average deceleration of the wheel during one cycle of said deviation in wheel speed of the associated wheel. of wheel and said control means being adapted to reapply additional pressure to the selected brake in the event that the actual average deceleration of the wheel within a predetermined period of time following said cycle is greater than said reference deceleration value by a predetermined amount. 2. A control system as defined in claim 1 wherein said wheel speed deviation is a second wheel speed deviation and wherein prior to said second wheel speed deviation, a first wheel speed deviation of the associated wheel occurs and said control means is adapted to close said isolation valve means to maintain the fluid pressure to the selected wheel brake at a relatively constant level after said first wheel speed deviation has been detected, and said control means is then adapted to selectively open said exhaust valve means after said isolation valve means has been closed during said first wheel speed deviation to permit fluid to flow into said fluid reservoir to selectively reduce the fluid pressure to the selected wheel brake and correct said first wheel speed deviation, said control means being adapted to selectively open said isolation valve means after said first wheel speed deviation is detected. been corrected to selectively increase the pressure to cause said second deviation in wheel speed. 3. The control system of claim 2 wherein said control means reapplies additional pressure after said second wheel speed deviation has been corrected to cause a third wheel speed deviation and wherein said control means selectively opens said relief valve means to reduce fluid pressure to correct said third wheel speed deviation. 4. The control system of claim 1 wherein said control means is adapted to determine said reference deceleration value as a function of a first vehicle speed value representing the speed of the associated wheel immediately prior to said wheel speed offset cycle and a second vehicle speed value representing the speed of the associated wheel immediately following said wheel speed offset cycle. 5. A control system as claimed in claim 1, wherein said predetermined time period begins at the end of said cycle. 6. In a wheeled vehicle having means for delivering pressurized braking fluid to actuate the associated wheel brakes of the vehicle, a braking control system for controlling the application of pressurized braking fluid to at least two selected wheel brakes to control the braking of at least two selected wheels associated with said control system comprising: normally open isolation valve means?connected between the power supply means and the selected wheel brake; normally closed exhaust valve means, connected between the selected wheel brake and a fluid reservoir; speed sensor means to generate a signal representing the average speed of the selected wheels; control means connected to operate said isolation valve means and said exhaust valve means, said control means including means for detecting deviations in the wheel speed of the selected wheels relative to the actual speed of the vehicle, said control means being adapted to close said isolation valve means to maintain the fluid pressure at the brakes of the selected wheels at a relatively constant level after a deviation in the speed of the associated wheels has been detected, said control means being adapted to selectively open said exhaust valve means after said isolation valve means has been closed to permit fluid to flow into said fluid reservoir to selectively reduce the fluid pressure at the brakes of the selected wheels in an attempt to correct said wheel speed deviation, said control means being responsive to a predetermined set of operating parameters to selectively open said isolation valve means after said wheel speed deviation has been corrected to selectively increase the pressure at the brakes of the selected wheels, said control means including means for determining when at least one of said selected wheel brakes remains at least partially locked after attempted braking to accommodate said wheel speed deviation, said control means being adapted to modify the predetermined voltage of operating parameters in the event that at least one of said selected wheel brakes becomes at least partially locked. 7. The control system of claim 6 wherein said control means includes means for determining the actual speed of the vehicle and said control means determines when at least one of said selected wheel brakes becomes at least partially locked by comparing the average speed of the selected wheels with the actual speed of the vehicle. 8. The control system of claim 6 wherein said control means determines an actual average wheel speed of the selected wheels at a predetermined time immediately prior to said wheel speed deviation and thereby determines a wheel speed ramp within a predetermined time period after said wheel speed deviation has apparently been corrected, and wherein said control means is adapted to extrapolate said wheel speed ramp back to said predetermined time to determine a calculated average wheel speed at said predetermined time or moment, said control means being adapted to modify said predetermined set of operating parameters in the event that said calculated average wheel speed is less than said actual average wheel speed by a predetermined amount. 9. A control system or system according to claim 6, wherein said speed sensing means consists of a single speed sensor. 10. In a wheeled vehicle having means for supplying pressurized braking fluid to actuate the associated wheel brakes of the vehicle, a braking control system or arrangement for controlling the application of pressurized braking fluid to at least two selected wheel brakes to control the braking of at least two selected wheels associated with said control system comprising: valve means connected between the power supply means and the selected wheel brakes; speed sensing means for generating a signal representative of the average speed of the selected wheels; and control means connected to actuate said valve means, said control means including means for sensing selected wheel speed deviations relative to the actual speed of the vehicle, said control means being adapted to actuate said valve means to selectively reduce fluid pressure to the brakes of the selected wheels and attempt to correct said wheel speed deviation, said control means being responsive to a predetermined set of operating parameters to actuate said valve means after said wheel speed deviation has been corrected to selectively increase pressure to the selected wheel brakes, said control means including means for determining when at least one of said selected wheel brakes remains partially locked after attempting to correct said wheel speed deviation, said control means being adapted to modify said predetermined set of operating parameters in the event that at least one of said selected wheel brakes remains at least partially locked. 11. The control system of claim 1 wherein said control means includes means for determining the actual speed of the vehicle and said control means determines when at least one of said selected wheel brakes remains at least partially locked by comparing the average speed of the selected wheels with the actual speed of the vehicle. 12. The control system of claim 1 wherein said control means determines an actual average wheel speed of the selected wheels at a predetermined time immediately prior to said wheel speed deviation, and thereby determines a wheel speed ramp within a predetermined time period after said wheel speed deviation has apparently been corrected, and wherein said control means is adapted to extrapolate said speed ramp backward to said predetermined time to determine a calculated average wheel speed at said predetermined time, said control means being adapted to modify said predetermined set of operating parameters in the event that said calculated average wheel speed is less than said actual average wheel speed by a predetermined amount. 13. A control system or system according to claim 10, wherein said speed sensing means consists of a single speed sensor.