TECHNICAL FIELD

The subject matter described herein, in general, relates to a braking system and in particular relates to a method of controlling slip of a vehicle with an anti-lock braking system.

BACKGROUND

In a speeding vehicle, sudden application of brakes can cause locking of wheels. This sudden locking of wheels can cause slipping of the wheels, resulting in a loss of control over the vehicle. Typically, to avoid wheel slips, an anti-lock braking system (ABS) is used.

The ABS can be implemented in various ways for maintaining stability of the vehicle. In one such way, angular speed of each of the wheels of the vehicle is determined using wheel speed sensors, for example, variable reluctance type wheel speed sensor, optical wheel speed sensors, etc.. The stability of the vehicle is controlled based on a comparison of the determined wheel speed values with predetermined wheel reference speed values.

Another method uses dynamic adjustment of the wheel reference speed values with respect to the vehicular speed values measured by speed measuring devices, for example, magnetic velocity sensors, ultrasonic linear position sensors, etc. There also exist methods in which the ABS is controlled by comparing predetermined wheel reference acceleration values with acceleration or deceleration inputs received from an accelerometer. Some methods use both the wheel speed values and the acceleration/deceleration values to calculate the slip and to limit the maximum and minimum values of the slip.

However, the effectiveness of braking using the above methods, in implementing anti-lock braking systems for stable braking, is not consistent. Such inconsistent braking may lead to vehicular slip, particularly when the road condition changes from dry to wet and slippery or the braking torque applied at the wheels is significantly high as compared to the frictional torque offered by the road.

SUMMARY

This summary is provided to introduce a method for controlling slip of a vehicle through an anti-lock braking system, which is further described below in the Detailed Description.

In one embodiment, the slip of the vehicle is controlled through an anti-lock braking (ABS) system based on the rate of change of deceleration of the vehicle and the knowledge of an instantaneous or current state of brakes. The rate of change of deceleration is determined by a processing means by differentiating the deceleration with respect to time. Deceleration can be measured using an accelerometer, which is usually mounted on the body of the vehicle. It will be appreciated by a person skilled in the art that various other methods, that are known in the art, can also be used for the purpose. For example, deceleration values may be obtained by using inputs from velocity sensors. The state of the brakes at any instant can be determined by using various types of state indicators known in the art. Use of the rate of change of deceleration allows the ABS to respond quickly, and, hence, enhances consistency and braking effectiveness of the system.

In case sensors detect zero acceleration (i.e., constant velocity ) for the vehicle for more than a threshold period of time, the brakes are automatically applied for an instant. In one implementation, the accelerometer is used to detect whether the vehicle is stationary or moving at zero acceleration. If the vehicle is moving at zero acceleration, then the momentary application of brakes causes deceleration. This change in deceleration that is positive in nature is monitored and the current brake state is maintained till the rate of change of deceleration remains positive.

In another embodiment, constant vehicle velocity, or zero velocity, corresponding to a stationary vehicle, can be detected using velocity sensors. In an implementation, a computed value of the slip is used by the braking system for stable braking. In said implementation, wheel speeds are detected using wheel speed sensors and the wheel speeds are used as an input, along with the vehicle velocity, for the calculation of slip of the vehicle.

These and other features, aspects, and advantages of the present subject matter will become better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects and advantages of the present invention will now be described in connection with preferred embodiments of the invention, in reference to the accompanying drawings. The illustrated embodiments, however, are merely examples and are not intended to limit the invention. The drawings include the following figures.

Fig. 1 illustrates an exemplary anti-lock braking system for a vehicle.

Fig. 2 illustrates slip-friction characteristics for a road-wheel interface.

Fig. 3 illustrates the effect of braking on the rate of change of friction coefficients at the front and rear wheels of the vehicle.

Fig. 4 illustrates an exemplary method for implementing the anti-lock braking system as described in Fig. 1.

DETAILED DESCRIPTION

Systems and methods for controlling an anti-lock braking system in a vehicle, for providing better control to a user of the vehicle during a braking event, are described. In one embodiment, the anti-lock braking system (ABS) takes into account the rate of change of deceleration and the knowledge of a current state of brakes for preventing vehicular slip. The rate of change of deceleration depends on the rate of change of friction coefficient. The rate of change of friction coefficient is further dependent on the rate of change of slip of the vehicle in relation to a slip-friction characteristic curve. For determining the rate of change of deceleration and the instantaneous state of the brakes, multiple sensors such as wheel speed sensors, linear velocity sensors, an accelerometer, brake sensors, etc. are used.

As and when the brakes are applied by the user, the rate of change of deceleration is monitored. For all positive values of the rate of change of deceleration, the current state of brakes is maintained by the controlling unit. If the rate of change of deceleration drops to a negative value, then it is indicative that at least the front wheel or the rear wheel is not achieving stable braking. Subsequently, the front brake is released automatically in an attempt to effect a non-negative rate of change of deceleration. The current rate of change of deceleration is, then, monitored. If the rate of change of deceleration is zero, the brakes are applied momentarily to effect a positive rate of change of deceleration. When the rate of change of deceleration becomes positive, then the brake status is maintained. However, if the rate of change of deceleration continues to be negative, the front brake is applied and the rear brake is released automatically and the rate of change of deceleration is monitored. This application and release of the front and rear brakes is repeated till the rate of change of deceleration assumes a positive value. Hence, the brake status to be achieved by the ABS is realized by the knowledge of the current state of the brakes and the rate of change of deceleration.

Fig.l illustrates an exemplary anti-lock braking system 100 for achieving stable braking in a vehicle. The system 100 can be used in a variety of automobiles, such as two wheelers, three wheelers, four wheelers, etc. The system 100 prevents wheels from locking during application of brakes suddenly by a user of the vehicle. In case the user applies the brakes hard, for example, during a panic situation, the anti-lock braking system (ABS) 100 is invoked for controlled application of the brakes to obtain stable braking.

In one embodiment, the system 100 is implemented in a two wheeler and it utilizes sensors, such as an accelerometer 102-1, a wheel speed sensor 102-2, and a state indicator 102-3 to determine various control variables governing brake actuators of the ABS 100. The control variables may include deceleration of the vehicle, linear velocity of the vehicle and that of its wheels, the instantaneous state of the brakes, etc. It will be understood that various other types of sensors known in the art, in addition to the ones mentioned above, and in different combinations, can be used to enhance the performance of the system 100 described herein.

The accelerometer 102-1 measures acceleration and deceleration of the vehicle. In another embodiment, the system 100 may use sensors, for example, linear velocity sensors, to obtain linear velocity for the calculation of acceleration and deceleration of the vehicle. The wheel speed sensor 102-2 is used to obtain the linear equivalent of the angular speed of the wheels. The state indicator 102-3 determines the current state of the brakes. In one implementation, the state indicator 102-3 can be a pressure sensor when a hydraulic braking system is used. In said implementation, the state indicator 102-3 measures the pressure in brake hoses and determines the current state of the brakes based upon such measurement.

In another implementation, the state indicator 102-3 obtains values of control variables stored in a microcontroller at an earlier instance and uses the control variables to determine the current state of the brakes. In said implementation, a control unit 104 can be used to process signals received from the accelerometer 102-1, the wheel speed sensor 102-2, and the state indicator 102-3. After processing the received signals, an output signal can be sent from the control unit 104 to a brakes controller 106. Based on the received output signal, the brakes controller 106 either applies or releases the brakes to control slip. The brakes controller 106 changes the brake state in relation to the current brake state in such a way that the wheels utilize the maximum possible frictional force available at a road-wheel interface, for braking, without allowing the wheels to lock up. This is done by changing the state of brakes so that the value of slip is maintained within an optimal range. This principle can be understood more clearly by referring to slip-friction characteristic graph 200 of fig. 2.

Fig. 2 illustrates a graph 200 representing a slip-friction characteristic curve 201 for a road-wheel interface. The graph 200 shows a change in vehicle slip on X axis and a corresponding change in friction coefficient of the road-wheel interface on Y axis. Typically, the friction coefficient of the interface increases with slip till it reaches a peak value and decreases afterwards. Also quintessentially, slip increases when the brake is applied and decreases when the brake is released.

The curve 201 indicates all possible operating points of the wheel at a particular road-wheel interface. For example, an operating point 202 represents vehicle slip and friction coefficient at a given instant of time. Further, the graph 200 includes two regions, namely, regions 204 and 206, which are to the left and right of the peak 208 of the curve 201, respectively. Arrows 210-1 and 210-2 in the direction of increasing vehicle slip values represent application of the brake by the brakes controller 106 and arrows 212-1 and 212-2 in the direction of decreasing vehicle slip values represent release of the brake by the brakes controller 106.

In the region 204, the slip-friction characteristic curve 201 having a positive slope represents an increase in the friction coefficient of the interface when there is an increase in slip due to the application of the brake. This increase can be witnessed until the peak 208 is reached. The peak 208 of the curve 201 denotes a critical slip value 214 at which the friction coefficient at the road-wheel interface is maximum. In region 206, a descending curve 201 with a negative slope, originating from the peak 208, represents a decrease in the friction coefficient with further increase in slip of the vehicle.

When the brakes are applied by the user, the friction coefficient varies with the slip value, depending upon the region 204 or 206 in which the operating point 202 falls. If the operating point 202 lies in the region 204, the friction coefficient increases with an increase in the slip value and if the operating point 202 is in the region 206, the friction coefficient decreases for an increasing value of slip.

For a given period of time, when the operating point 202 is in the region 204, if the brake is released, then the slip decreases and the operating point 202 moves in the direction of the arrow 212-1. In this case, the friction coefficient at the road-wheel interfaces decreases. On the other hand, when the brake is applied in this time period, the slip value increases indicating a positive rate of change of slip. In this case, the operating point 202 moves in the direction of the arrow 210-1 and the friction coefficient at the road-wheel interfaces increases, showing a positive rate of change.
For a given period of time when the operating point 202 is in the region 206, if the brake is released, then the slip decreases. In this case, the operating point 202 moves in the direction of the arrow 212-2 and the friction coefficient increases. On the contrary, if the brake is applied in this time period, the slip value increases and the operating point 202 moves in the direction of the arrow 210-2. Consequently, the friction coefficient decreases.

If at any point in time, the operating point 202 lies at the origin of the graph 200, a no-slip condition is reached. This no-slip condition maybe accounted for in two ways, either the vehicle is moving at a constant velocity or is stationary.

The rate of change of the friction coefficient at the road-wheel interface can be represented as the product of the slope of the slip-friction coefficient characteristic curve 201 at the operating point 202 and the rate of change of slip by the following equation:

where r indicates the rate of change of the corresponding variable, fi represents the friction coefficient, X. represents the slip value, and 5 represents a derivative operator.

Thus, it is observed that the rate of change of the friction coefficient is positive for the operating point 202 moving towards the peak 208 of the curve 201. The positive values for the rate of change of the friction coefficients occur in two cases, first, when the brake is applied and the operating point 202 is in the region 204; and second, when the brake is released and the operating point 202 is in the region 206. In the first case, the slip is smaller than the critical slip 214, while in the second case, the slip is larger than the critical slip 214. Further, a negative rate of change of the friction coefficient can be witnessed in two cases. First, when the operating point 202 is in the region 206 and the brake is applied causing an increase in the slip and an increase in the tendency of the wheels to lock. Second, when the operating point 202 is in region 204 and the brake is released causing a decrease in the slip but without any effective braking of the wheel. In both the cases, when the rate of change of deceleration is negative, an undesired braking performance is the outcome.

At a particular operating point, where the slip corresponds to the maximum possible friction at the interface, i.e., at the peak 208 of the curve 201, the slope of the curve is zero. This implies that the rate of change of the friction coefficient is zero, according to the above mentioned equation (1), and the friction coefficient at the interface is maximum.

If the dynamics of the vehicle in a braking event are observed, one can notice certain forces, referred as normal forces, acting on the wheels. Also, the normal force acting on the front wheel is higher than that on the rear wheel due to a virtual force that acts on the center of gravity of the vehicle during deceleration. Hence, there is an uneven distribution of the normal forces between the wheels. Also, the total friction force acting on the vehicle is the sum of the products of the friction coefficient and the normal force acting at each of the wheels. Thus, the deceleration of the vehicle is obtained as a function of the friction coefficient at each wheel. On differentiation by the control unit 104, the rate of change of deceleration is obtained as a linear combination of the rates of change of friction coefficient and proportionality coefficient of each of the wheels. This can be shown by a relation such as mentioned below :

where subscript 1 and 2 indicate front brake and rear brake respectively, r indicates the rate of change of the corresponding variables, d indicates deceleration of the vehicle, p indicates friction coefficient at the road-wheel interface, and f1(µ1,µ2) and f2(µ1,µ2) indicate proportionality coefficients. Hence, it is apparent that a complexity arises due to the contribution of the front and rear wheels to the rate of change of deceleration.

In operation, to avoid wheel lock and to achieve maximum stable braking performance for all road conditions, the rate of change of the friction coefficient at each wheel is maintained as positive. As the rate of change of deceleration is a linear combination of the rate of change of the friction coefficients at each of the wheels, braking control for avoiding the wheel lock and to achieve maximum stability can be achieved. For this, the rate of change of deceleration of the vehicle is maintained at positive values. A positive value of the rate of change of deceleration that is close to zero value corresponds to an operating point that is closest to the peak 208 of the slip-friction coefficient characteristics curve 201. If the operating point 202 moves from the peak 208 either to the regions 204 or 206 by a small amount, stable braking performance can still be achieved. Thus, stable braking performance corresponds to a condition when the operating point 202 of each of the wheels is in proximity to the peak 208 of the curve 201.

Further, the wheel speed sensor 102-2 may provide inputs that may be used to calculate the slip and then limit the slip of individual wheels to a predetermined range to account for the near-constant value of the friction coefficient near the peak of the curve 208. For a different road-wheel interface where there is no change in the friction coefficient or deceleration with an increase in slip beyond the peak 208, i.e., the slip-friction curve 201 is flat after the peak 208, there is still a need to limit the slip of individual wheels. In one embodiment, the value of the slip is controlled within the minimum and maximum values of slip typical to the given road-tire interface to avoid instability. If the slip of any wheel increases beyond a predetermined value, the brake for that particular wheel is released. On the other hand, if the slip of a wheel decreases below the predetermined value, then the brake for that wheel is applied to bring the slip down to lower values, within the predetermined range.

Fig. 3 illustrates an effect of brake states on the rate of change of friction coefficients at front and rear wheels of a vehicle, respectively. Table 300 represents the events taking place after the brakes are applied by a user. The events of application and the release of brakes refer to the application and release of the front and rear brakes by the anti-lock braking system 100.

Table 300 has four rows 302-1, 302-2, 302-3 and 302-4 and nine columns 304-1, 304-2, ..., 304-9. Each row is further sub-divided into two rows. The top sub-row and the bottom sub-row depict the rate of change of the friction coefficients at the front wheel and the rear wheel, respectively.

In the sub-rows, '+' sign depicts an increase in the rate of change of the friction coefficient and '-' sign depicts a decrease in the rate of change of the friction coefficient, as a result of the application or release of the front or rear brake by the anti-lock braking system 100. The table serves to construe the combined effect of the rear and front brake on overall braking.

Table 306 illustrates four states, namely, SI, S2, S3, S4, stated in table 300, of the front and rear brakes. Expressions ON and OFF imply applied and released states of the brake, respectively. Table 308 illustrates operating points from LI to L9 on the slip-friction characteristics curve 201, as used in table 300, for the front and rear wheels. The tables 306 and 308 aid as a key to interpret table 300.

Further, in table 300, the top and bottom sub-rows depict the effect of a brake state and an operating point location on the rate of change of friction coefficients at the front and rear wheels. For example, the cell corresponding to an operating condition S3-L2 in table 300 refers to a brake state S3 from table 306 and an operating point L2 from table 308. In this operating condition, the front brake is ON and the rear brake is OFF. Also, the operating point of the front wheel lies in the region 204 and the operating point of the rear wheel lies in the region 206. The overall effect on the rate of change of friction coefficients is inferred by comparing the readings for the current state of brakes with the previous state of brakes. The readings for comparison are taken from table 300 considering that the normal force is greater at the front wheel than at the rear wheel. Hence, the braking achieved at the front wheel is greater than that at the rear wheel. Also, it should be noted that the rate of change of deceleration is a linear combination of the rates of change of the friction coefficients at each wheel.

Table 300 materializes by taking into consideration the effect of the state of brakes, applied or released, on the slip.

The invention can be more clearly understood with reference to the tables 300, 306, and 308 from the following example. The example mentioned below is only intended for understanding and is not intended to limit the scope of the invention When the user suddenly applies brakes, the brakes controller 106 switches both the front and the rear brake ON, corresponding to the brake state SI in table 306. The accelerometer 102-1 measures the deceleration of the vehicle and the control unit 104 computes the rate of change of deceleration. In another embodiment, a linear velocity sensor provides inputs to the control unit 104 which further computes the deceleration of the vehicle and the rate of change of deceleration. When the rate of deceleration is positive during the state SI, the operating point of the wheels may be located at LI. With time, the rate of deceleration becomes negative while the brakes are in the state SI due to shifting of the operating point to either of the locations L2, L3, L4, etc. Therefore, the operating condition may also change to either S1-L2, S1-L3, or S1-L4. As soon as a negative value of the rate of change of deceleration is obtained by the control unit 104, a signal is sent to the brakes controller 106 by control unit 104 to switch OFF the front brake. This corresponds to the state S2 in table 306. When the brakes controller 106 toggles the brake state from SI to S2, the control unit 104, further, computes and monitors the current rate of change of deceleration.

For the purpose of understanding, consider an example, where the operating point shifts from LI to L3, when the rate of change of deceleration becomes negative. This means that before the brake state is toggled, the operating condition changes from Sl-Ll to S1-L3. On toggling the brake state to S2, the operating condition changes from S1-L3 to S2-L3. It may be construed from table 300, that the rate of change of deceleration corresponding to S1-L3 (previous state) is smaller than that corresponding to S2-L3 (current state). In corporeal terms, when the brake state is toggled to S2 and if the rate of change of deceleration increases substantially and becomes positive, then it means that the operating point had shifted from LI to L3.

In another example, the operating point shifts from LI to L2, when the rate of change of deceleration becomes negative. It means that before the brake state was toggled, the operating condition had changed from Sl-Ll to S1-L2. On toggling the brake state to S2, the operating conditions change from S1-L2 to S2-L2. In this case, the rate of change of deceleration further decreases and is negative even on toggling the state from SI to S2. Again, it may be construed from table 300, that the rate of change of deceleration corresponding to S1-L2 (previous state) is greater than that corresponding to S2-L2. (current state). The control unit 104, then, signals the brakes controller 106 to switch ON the front brake again and achieves the previous state of brakes SI. The brakes controller 106 then turns the rear brake OFF (state S3) and the operating condition now becomes S3-L2. The control unit 104 monitors the current rate of change of deceleration and as there is a positive increase in the rate of change of deceleration, the control unit 104 maintains the same state of the brakes. This can also be seen from table 300 that the rate of change of deceleration for S3-L2 (final state) is greater than that for S2-L2 or S1-L2.

In yet another example, assume that the operating point shifts from LI to L4 before the brake state was toggled, .i.e. the operating condition changes from SI-LI to S1-L4. On toggling the brake state to S2, the operating conditions change from S1-L4 to S2-L4. In this case, the control unit 104 senses only a slight positive increase in the rate of change of deceleration from the previous state of brakes SI. Now the brakes controller 106 does not change the brake state to the previous state. Rather, in this case, the brakes controller 106 toggles the rear brake OFF too, which corresponds to the brake state S4 and an operating condition S4-L4. The control unit 104 monitors the current rate of change of deceleration and as the rate of change of deceleration increases positively, the controller 106 maintains the same state of brakes.

In case the braking event starts with either the operating points of the front and rear wheels at the origin of the curve 201 or the operating point of either of wheels at the origin, then the control unit 104 monitors the rate of change of deceleration. For any negative value of the rate of change of deceleration, the brake states are toggled again, in the same way as discussed above, till a positive value is achieved.

Fig. 4 illustrates an exemplary method for implementing an anti-lock braking system 100 of Fig. 1. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method, or an alternative method. Additionally, individual blocks may be deleted from the method without departing from the spirit and scope of the subject matter described herein. Furthermore, the method can be implemented in any vehicle without departing from the scope of the invention. The following description is presented with reference to fig. 1, 2, and 3. However, the following description can also be understood without considering the mentioned figures.

At block 402, brakes are applied on the wheels of the vehicle by a user. The working of the anti-lock braking system 100 involves application and release of the brakes by the brakes controller 106, which comes into action only when the brakes are applied by the user.

At block 404, the rate of change of deceleration and linear velocity of the wheels are determined. For example, the rate of change of deceleration is calculated by the control unit 104 by differentiating a deceleration value with respect to time. Further, the linear velocity of the vehicle can be determined from the deceleration value. The deceleration can be measured using the accelerometer 102-1, which can be mounted on the body of the vehicle. In one embodiment, value of slip of the vehicle is computed by the control unit 104. It may be understood that for the purpose of computation of slip of the vehicle, the control unit 104 may employ any method known in the art. In another embodiment, linear velocity sensors may be used to obtain the linear velocity of the vehicle, which may be further used to determine deceleration, and which may also be used to determine value of slip of the vehicle. The deceleration value may be, still further, used to determine the rate of change of deceleration.

Further, the current state of the brakes is determined by using a state indicator 102-3. In one implementation, the state indicator 102-3 can be a pressure sensor that measures the pressure in the brake hoses. In another implementation, the state indicator 102-3 can be a device that measures the movement of a brake wire. In yet another implementation, the state indicator 102-3 can provide values of control variables to the control unit 104 for determination of the current state of the brakes.

At block 406, the control unit 104 checks whether the value of deceleration is zero or not. The control unit 104 receives a signal corresponding to zero deceleration value when the brakes are in a released condition, during the braking event.

Block 408 is invoked in case the control unit 104 senses a zero deceleration. In this case, during the brake application event, the brakes controller 106 applies the brakes momentarily to check whether the vehicle is moving with constant velocity or is stationary. In one embodiment, a linear velocity sensor may be used to check whether the vehicle is stationary or moving at a constant speed, and in case the vehicle is moving at a constant speed, the brakes are applied momentarily. When the brakes are applied momentarily, the vehicle starts decelerating. In one implementation, in case of zero deceleration, the value of slip computed at the block 404 is used to achieve stable braking. This is done by limiting the value of slip within a predetermined range, which corresponds to a range of maximum values of friction force offered by the road to the wheel. The value of slip is maintained between this optimal range of values by toggling the brake states, in relation to wheel speed inputs.

In case zero deceleration is not determined by the control unit 104, block 410 is invoked. At the block 410, the control unit 104 monitors if the values of the rate of change of deceleration are positive.

Block 412 is invoked in case the rate of change of deceleration is positive. The brakes controller 106 then maintains the same brake state as the previous state, as determined at the block 404, to achieve maximum stable braking without locking of the wheels.

Block 414 is invoked when a negative rate of change of deceleration is determined. In this case, the brakes controller 106 toggles the brake state to a new state in relation to the previous state of brakes to achieve maximum stable and controlled braking. The parameters determined at the block 404 help the brakes controller 106 to limit the operating point 202 of the wheels near the peak 208 of the slip-friction characteristic curve 201. In this way, the vehicle is decelerated under stable braking conditions.

When brakes are suddenly applied by the user, the vehicle starts to decelerate rapidly and will keep on decelerating until the brakes are released or the vehicle comes to a halt. Thus initially, slip of the vehicle increases.

The increase in slip is controlled by the friction offered by the road, i.e., the friction coefficient of the road-wheel interface. On continuous application of the brakes by the brakes controller 106, the vehicle would reach a critical value of slip where friction offered by the road is maximum. At this critical value, the slip of the vehicle is very small and the friction coefficient at the road-wheel interface is large. On further application of the brakes, the slip increases to a value greater than the critical value of the slip, while the friction offered by the road and the deceleration of the vehicle decrease. In other words, the slip increases such that the operating point 202 of wheels largely deviates from the peak 208 and enters the region-206. At this point, the brakes are released to avoid further increase in slipping.

Consequently, the front brake and/or the rear brake is released by the anti-lock braking system 100 according to the requirement for stable braking. This is performed to bring the operating point 202 of the individual wheels back to the vicinity of the peak 208. Thus a condition of anti-locked wheels is maintained.

If brakes are continuously released, during the event of braking, deceleration of the vehicle decreases such that the operating point 202 of the individual wheels deviates away from the peak 208 towards the point of origin resulting in ineffective braking. In such a case, the brakes are momentarily applied by the brakes controller 106 to bring the operating point 202 in the vicinity of the peak 208 of the slip-friction characteristic curve 201. Also, depending on the rate of change of deceleration, the brakes may be toggled or maintained to the current state to achieve stable braking performance. In this way, it is ensured that the operating point 202 of individual wheels remain in the vicinity of the peak 208 of the slip-friction characteristic curve 201.

Therefore, by controlling the application and release of the brakes, the operating point 202 of individual wheels can be limited to the vicinity of the peak 208 of slip-friction characteristic curve 201.

This sequential application and release of the brakes controls the slip of the wheels for stable braking.

The above-described methods and systems describe anti-locking of wheels of a vehicle for stable braking. Although the subject matter has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are also possible. As such, the spirit and scope of the appended subject matter should not be limited to the description of the preferred embodiment contained therein.

I/We claim:

1. A method for controlling an anti-lock braking system (100) comprising:

determining a rate of change of deceleration of a vehicle;

determining a current state of a front brake and a rear brake; and

adjusting the front brake or the rear brake based on the rate of change of deceleration and the current state of the brakes to obtain a non-negative rate of change of deceleration.

2. The method as claimed in claim 1, wherein the adjusting comprises:

adjusting the front brake based on the rate of change of deceleration and the current state of the front brake;

determining a current rate of change of deceleration; and

adjusting the rear brake based on the current rate of change of deceleration and the current state of the rear brake.

3. The method as claimed in claim 1, wherein determining the rate of change of deceleration of the vehicle includes computing the rate of change of deceleration from deceleration signals or velocity signals.

4. The method as claimed in claim 3, further comprising actuating the front brake and the rear brake momentarily when the deceleration of the vehicle is zero or when the vehicle has a constant velocity.

5. The method as claimed in claim 3 further comprising calculating a slip of the vehicle from the deceleration signals or velocity signals.

6. The method as claimed in claim 5, further comprising controlling the slip of the vehicle within a predetermined range when the deceleration of the vehicle is zero or when the vehicle has a constant velocity.

7. An anti-lock braking system (100) in a vehicle comprising:
a state indicator (102-3) for determining a current state of brakes of the vehicle to provide state signals; and

a control unit (104) that receives the state signals, and deceleration signals, or velocity signals;

characterized in that

the control unit (104) determines a rate of change of deceleration from the deceleration signals, or velocity signals, and generates output signals based on the rate of change of deceleration and the state signals; and

a brakes controller (106) that controls braking of the vehicle in response to the output signals to provide a non-negative rate of change of deceleration.

8. The anti-lock braking system (100) as claimed in claim 7, further comprising an accelerometer (102-1) that generates the deceleration signals, or a velocity sensor that generates the velocity signals.

9. The anti-lock braking system (100) as claimed in claim 7, wherein the output signals trigger the brakes controller (106) to actuate the brakes momentarily when the deceleration of the vehicle is zero or when the vehicle has a constant velocity.

10. The anti-lock braking system (100) as claimed in claim 7 further comprising a wheel speed sensor (102-2) that measures angular speed of wheels of the vehicle and generates angular wheel speed signals.

11. The anti-lock braking system (100) as claimed in claim 10, wherein the angular wheel speed signals are used by the control unit (104) to compute a slip of the vehicle, which is maintained within a predetermined range by the brakes controller (106) when the deceleration of the vehicle is zero or when the vehicle has a constant velocity.

12. The anti-lock braking system (100) as claimed in claim 7, wherein the brakes controller (106) controls braking in a hydraulic braking system or a brake-by-wire braking system.

13. A vehicle comprising the anti-lock braking system (100) as claimed in any of the claims 7to 12.