WO2003011669A1

WO2003011669A1 - Method for analyzing the efficiency of wheel brakes in a brake system for a motor vehicle with at least two-axles

Info

Publication number: WO2003011669A1
Application number: PCT/EP2002/008363
Authority: WO
Inventors: Gerald Kunz; Timo Gurzawski
Original assignee: Continental Teves Ag & Co. Ohg
Priority date: 2001-07-27
Filing date: 2002-07-26
Publication date: 2003-02-13

Abstract

The invention is based on the problem that individual wheel brakes must be intact especially in a 'brake-by-wire' braking system so that the required braking force is actually realized. In order to determine whether all wheel brakes are intact, the invention provides for a test cycle, wherein the same total braking torque is applied by distributing said torque differently in the wheel brakes of the front and rear axles. Deceleration of the vehicle should show no changes if all wheel brakes are intact. If changes do occur, it must then be assumed that one of the wheel brakes whose braking torque has been reduced is defective.

Description

Method for analyzing the effectiveness of wheel brakes in a brake system for a motor vehicle with at least two axles

The invention relates to a method for analyzing the effectiveness of wheel brakes, which can be actuated by means of an external force, of a brake system for a motor vehicle with at least two axles, each wheel brake transmitting a specific braking torque to its associated wheel of the vehicle on the basis of a manipulated variable, which is between the tires of the respective vehicle Causing braking force acting on the wheel and the roadway, and the manipulated variables being determined individually for the wheel or axle in relation to the measured actual deceleration, taking into account a target deceleration.

Brake systems in which the wheel brakes are actuated by an external force, the size of the external force and thus the braking torque achieved on the wheels being determined by a manipulated variable which is derived from the driver's desire for a specific target deceleration, are also called "brakes -by-wire "brake systems. There is a theoretically justifiable relationship between the manipulated variable and the resulting vehicle deceleration, in which parameters of the braking system, the vehicle mass and the road inclination are included. The correct operation of such a brake system depends crucially on the fact that the theoretically expected vehicle deceleration actually occurs with a certain manipulated variable. If there are deviations, this can have various reasons: The vehicle mass can be increased due to payload, braking can take place on a road inclined in the direction of travel or the parameters of the braking system may have changed due to a defect in the brakes that either none of the Control variable corresponding braking torque is generated or, due to internal resistance, the braking force does not correspond to the braking torque. So z. B. in a hydraulic brake system in which the brake pressure forms the manipulated variable, a sufficient pressure can be exerted on a jamming brake piston, but because of the clamping it cannot exert a corresponding application force on a brake disc rotating with the wheel.

DE 197 26 116 describes a brake system according to the preamble of claim 1, in which the load state of the vehicle is determined by comparing the target deceleration with the actual deceleration and the braking force is then adapted to the actual vehicle weight.

The same applies to the brake system according to DE 197 55 112. In this system, the inclination of the roadway is inferred from the comparison of the target deceleration with the actual deceleration.

WO 99/24300 is concerned with a method for optimizing an electronic brake force distribution.

The procedures described always assume that the brake system is intact and working properly. However, if a wheel brake is defective, the procedures presented in the documents can lead to unintended results. In the brake system according to DE 197 26 116, the external force would be continuously increased, to compensate for a supposed payload, which in turn can damage the external power plant.

The invention is therefore based on the object of presenting a method in a “brake-by-wire” brake system with which defects in the wheel brakes can be determined and with which the defective wheel brake can be identified if necessary.

To achieve the object, the invention provides that in a first step the braking torques on the front and rear axles are set in a basic distribution and the associated basic deceleration is measured and registered, that in a second step the braking torques on the front and the rear axle in a test distribution that deviates from the basic distribution, and the associated test delay is measured and registered, the sum of the braking torques on the front and rear axles being the same in both steps, and in a third step the basic deceleration and the test deceleration being compared with one another. will be.

With this test cycle, a characteristic value is determined, namely the difference between the basic delay and the test delay, which permits a large number of conclusions which make it possible to initiate appropriate countermeasures.

A first possible result can be that the difference between the two vehicle decelerations is different from zero. If the difference is significant, it inevitably means that at least one wheel brake is defective. In the second step, only braking torques are shifted from one axis to the other. which should not have any influence on the vehicle deceleration which occurs if all wheel brakes are intact and the radius of the wheels on the front and rear axles are the same. For the vehicles where this is not the case, the proportion to be shifted must be corrected in the ratio of the radius. In addition, the braking torque introduced into the wheel brakes must not exceed the maximum friction force that can be transmitted between the tire and the road surface, which would result in the wheel locking, so that the manipulated variable describing the braking torque and the braking force are no longer linearly correlated.

The method also makes it possible to determine the defective wheel brake. If the test deceleration is greater than the basic deceleration, it can be concluded that the wheel brakes on the axle are defective and the braking torque has been reduced.

Since the front axle wheel brakes normally transmit higher braking forces due to the changing axle load distribution when braking, a defect is more likely to be expected. It is therefore advisable to reduce the braking torque on the wheel brakes on the front axle and increase the braking torque on the wheel brakes on the rear axle in the test cycle, so that if there is a difference between the basic deceleration and the test deceleration, a defective wheel brake on the front axle can be concluded immediately ,

A shift in the braking torque from the wheels on one side of the vehicle to the wheels on the other side could be used in a corresponding manner to precisely locate a defective wheel brake. However, such a shift leads to a moment around the vertical axis of the vehicle, which causes the vehicle to skid can. As a rule, however, it is sufficient to know that any of the wheel brakes is defective, since all the wheel brakes will be checked when a workshop check is due.

The test cycle described above can of course be carried out at any time with braking initiated by the driver. However, it is advisable to do this only if the target deceleration is not reached, which means that the steps following the first step need only be initiated if the measured basic deceleration is based on a theoretically expected vehicle deceleration, i.e. the target deceleration, differs.

If it turns out in the test cycle that the difference is zero or almost zero, this can only mean that the payload has increased (or a payload has been removed again) or the braking is taking place on an inclined roadway. In this case, the manipulated variables for the wheel brakes on both axles are corrected by a factor of the same amount until the actual deceleration corresponds to the target deceleration.

This correction is maintained at least until a new deviation of the actual deceleration from the target deceleration occurs.

The invention is to be explained in more detail below on the basis of an exemplary embodiment. To show:

Fig. 1 is a schematic representation of a two-axle vehicle with wheel brakes on the front and rear axles and with a vehicle deceleration sensor and Fig. 2 is a flowchart for describing the test cycle

FIG. 1 schematically shows a four-wheel vehicle 1 with a front and a rear axle 2, 3. The individual wheels are provided with wheel brakes 4, 5 with which a braking torque can be exerted on the wheels. The wheel brakes are connected to a usually hydraulic external power system 6. A manipulated variable determines the extent to which pressurized pressure medium is supplied to the individual wheel brakes, as a result of which the application force and thus the braking torque is determined. The pressure depicts the manipulated variable linearly.

A pedal 7 with a path simulator 8 is also available to the driver. The manner in which the pedal 7 is actuated can be used to infer the target deceleration with which the driver wishes to brake the vehicle. It is therefore often referred to as a delay in request.

Based on this target deceleration, the manipulated variables are determined and the wheel brakes are controlled accordingly. It should be taken into account that due to the shifting of the axle load during braking, the wheel brakes of the front axle must be supplied with a higher braking torque than that of the rear axle in order to obtain the same adhesion utilization on the wheels of the front and rear axles (this is the ratio of braking force to load on the respective wheel).

In order to optimize the brake force distribution in "Brake-by ~ Wire" brake systems, it is important to know why a deceleration request is not achieved with the set braking torques: To design the brake force distribution, the weight of the vehicle ready to drive is added. reasons and assumed that the brake system is in a technically perfect condition. The desired deceleration is determined during braking using the measured pedal travel and a characteristic curve ^{^} stored in an electronic controller.

To achieve this vehicle deceleration, a braking torque must be built up via the wheel brakes. This braking torque acts via the tire radius at the point of contact between the tire and the road as braking force. If the load state of the vehicle and the state of the brake system correspond to the design basis, the vehicle is braked with an actual deceleration that corresponds to the desired deceleration desired by the driver. The actual deceleration can be determined by an acceleration sensor or vehicle deceleration sensor 9 or from the course of the wheel speeds. In this sense, wheel sensors for determining the peripheral speed also represent a vehicle deceleration sensor.

Furthermore, a certain vehicle deceleration is also assigned a certain proportion (Phi) of the braking force of the rear axle in the total braking force via a characteristic curve stored in the electronic controller. When braking, Phi follows the ideal distribution of braking force and becomes smaller as the vehicle deceleration increases, which means that the wheels on the rear axle contribute less to the total deceleration in relation to the wheels on the front axle as braking increases.

If the determined vehicle deceleration does not match the driver's deceleration request, there are several possible reasons:

1. The vehicle is loaded. 2. The vehicle brakes on an uphill or downhill gradient.

3. The vehicle's wheel brakes are in poor technical condition.

To correctly distribute the braking force and to compensate for a poor braking system, it is necessary to know the reason for the vehicle deceleration not being achieved. To do this, proceed as follows according to the diagram in FIG. 2:

If the vehicle reaches the desired deceleration (decision diamond 10), the brake system is intact. The process ends by determining that the system is OK (i. 0.) (action 11). If the actual deceleration does not reach the target deceleration, the share of the rear axle braking force in the total braking force is increased in accordance with the dynamic axle load distribution (action 12).

If the vehicle is loaded and the brake system is not defective, the vehicle deceleration will not change due to the change in the brake force distribution (junction 13). The procedure recognized the loading of the vehicle or a lane inclination as the reason for the desired delay not being achieved (action 14).

If the vehicle is not loaded and the condition of a front axle brake has deteriorated due to heavy use, the greater use of the rear brake will enable more braking force to be achieved on the entire vehicle and the actual deceleration will increase (junction 15). The amount of braking torque that is transmitted from the front axle to the rear axle can be converted entirely into braking force there. On the front axle, this amount would be due to poor radio tion of the brake can only be partially converted into braking force. The braking force that can be realized on the entire vehicle therefore increases. The procedure recognized the poor technical condition of the vehicle's braking system for the desired deceleration not being achieved (Action 16).

Example:

A vehicle needs in the design state with intact brake system for the. Vehicle deceleration of one g (= 9.81m / s ² ) has a braking torque of 10,000 numbers. When fully loaded, the vehicle requires a braking torque of 15,000 numbers for the same vehicle deceleration.

Case 1:

The vehicle is fully loaded, the driver wants to decelerate with 0.5g:

The vehicle needs 0.5g * 15,000 Nm / g = 7,500 Nm to achieve the desired vehicle deceleration. However, since the pedal characteristic curve is designed for the unladen vehicle (10,000 Nm / g), the driver must request 0.75 g here.

With this desired deceleration, for example, a share of the rear axle in the total braking force of 0.3 would be ideal.

This results in a braking torque of: 0.75g * (10,000 Nm / g) * (1 - 0.3) = 5,250 Nm on the front axle and a braking torque of: 0.75g * (10,000 Nm / g) * 0.3 = 2,250 Nm.

The total moment is: 5,250 Nm + 2,250 Nm = 7,500 Nirr.

For example, if Phi is increased to 0.4 in order to determine the cause of the difference between the delivered (target) and achieved (actual) vehicle deceleration, the distribution of the braking force changes as follows:

The braking torque on the front axle is: 0.75g * (10,000 Nm / g) * (1 - 0.4) = 4,500 Nm.

A braking torque of:

0.75g * (10,000 Nm / g) * 0.4 = 3,000 Nm.

The total moment is:

4,500 Nm + 3,000 Nm = 7,500 Nm.

The increase in Phi has only changed the braking force distribution. The vehicle deceleration does not change because the deductible braking torque does not change. The payload of the vehicle is thus recognized as the cause of the desired delay not being achieved.

Case 2;

The condition of the front brake has deteriorated, so that only about 50% of the requested braking torque can be transmitted here, for example. The vehicle is not loaded and the driver must now also request 0.75g vehicle deceleration to achieve the desired 0.5g vehicle deceleration:

Θ ^' , 75g * (10,000 Nm / g) * (1 - 0.3) * (50%) = 2,625 Nm can now be realized on the front axle; on the rear axle will continue

0.75g * (10,000 Nm / g) * 0.3 = 2,250 Nm reduced.

The sum of the braking torques is thus: ■ 2,625 Nm + 2,250 Nm = 4,875 N:

In this case, if Phi is increased again to 0.4, the front axle has a braking torque of

0.75g * (10,000 Nm / g) * (1 - 0.4) * (50%) = 2,250 Nm at the rear axle a braking torque of

0.75g * (10,000 Nm / g) * 0.4 = 3,000 Nm. The vehicle deceleration is now due to the increase in phi to 0.4

2,250 Nm + 3,000 Nm = 5,250 Nm available.

The vehicle deceleration will increase. Wear of the brake system is recognized as the cause of the desired deceleration not being achieved.

The method thus recognizes the poor technical condition of a brake system. With this information, the "brake-by-wire" system can behave accordingly and continue to enable safe braking or inform the driver about the condition of the brakes.

Claims

Claims:

1. A method for analyzing the effectiveness of wheel brakes in a brake system for a motor vehicle with at least two axles, each wheel brake transmitting a specific braking torque to the associated wheel of the vehicle on the basis of a manipulated variable, which causes a braking force acting between the tire of the respective wheel and the road, and wherein the manipulated variable wheel or axle are determined based on the measured vehicle deceleration taking into account a target deceleration, characterized in that in a first step the braking torques on the front and rear axles are set in a basic distribution and the associated ones. Basic deceleration is measured and registered, that in a second step the braking torques on the front and rear axles are set in a test distribution that deviates from the basic distribution and the associated test deceleration is measured and registered, in both steps the sum of the braking torques on the front and Rear axle is the same, and that in a third step the basic deceleration and the test deceleration are compared with each other.

2. The method according to claim 1, characterized in that if a difference is found between the two vehicle decelerations, a defect in one of the wheel brakes is concluded.

3. The method according to claim 2, characterized in that in the event that the test delay is greater than. the basic delay is due to a defect in the Wheel brakes of the axle is closed, the braking torque has been reduced.

A method according to claim 1, characterized in that the steps following the first step are initiated when the measured basic deceleration deviates from a theoretically expected vehicle deceleration.

Method according to Claim 4, ^' characterized in that, in the event that the basic deceleration corresponds to the test deceleration, the manipulated variables for the wheel brakes on both axles are corrected by a factor to the same extent until the actual deceleration corresponds to the target deceleration.

A method according to claim 5, characterized in that the correction is maintained at least until a renewed deviation of the actual deceleration from the target deceleration occurs.