WO2001096159A1 - Method and device for adjusting the braking effect of a vehicle - Google Patents

Abstract

The invention relates to the adjustment of the braking effect of a vehicle comprising at least two wheels located right and left on an axle, in relation to the direction of travel. The wheels can roll on a roadway with different friction values. The braking effect is adjusted jointly on the two wheels. Two operating modes are provided for this purpose and can be selected according to the friction values present on the wheels and/or according to the vehicle speed and/or according to the wheel rotation behaviour. In a first selectable operating mode, the select-low mode mentioned above, the joint adjustment process is activated in accordance with the movement behaviour of the wheel with the low friction value that is detected. In a second selectable operating mode, the select-high mode mentioned above, the joint adjustment process is activated in accordance with the movement behaviour of the wheel with the higher friction value that is detected. A deceleration value representing the longitudinal deceleration of the vehicle is also determined. The invention is characterised in that the second operating mode, the select-high mode mentioned above, is only selected when the deceleration value falls below a threshold value, which can be predetermined.

Description

Method and device for adjusting the braking effect in a vehicle

State of the art

The invention relates to a method and a device for adjusting the braking effect in a vehicle with the features of the independent claims.

Possibilities are known from the prior art for counteracting the tendency of a braked wheel to lock, for example by manipulating the braking force on each wheel in the sense of individual regulation. For such an individual regulation, however, a possibility must be provided for each wheel to carry out the braking action on this wheel individually or separately from the other wheels. In the case of a hydraulically acting brake, such systems provide a control channel with the corresponding hydraulic components for each individually controlled wheel brake. As an example of anti-lock control systems, reference should be made to Bosch Technical Reports, Volume 7 (1980) Issue 2.

In addition to such braking systems, it is also known to jointly regulate or control the braking effect on several wheels of a vehicle. From DE 22 43 260 C2 01/96159

- 2 -

(corresponds to US Pat. No. 3,907,378), it is known, for example, to jointly adjust the braking effect on the wheel brakes of the wheels of an axle in order to prevent them from locking. Since the wheels of an axle can roll on parts of the road with very different road / tire friction coefficients, it must necessarily be specified which of the wheels to be controlled together specifies the common braking effect. In this context, it is known to provide two operating modes for a common setting of the braking effect on the two wheels of an axle.

In one operating mode, the so-called select-low mode, the common setting of the braking effect, in general the braking pressure, is carried out depending on the wheel with the lower wheel speed. If the vehicle moves on a lane with different lane / tire friction values on the vehicle sides (μ-split condition), the brake pressure in select-low mode usually becomes dependent on the wheel with the lesser lane / tire -Friction value made. In the select-low mode on an axle, the brake pressure on the wheel brakes of this axle is set such that the wheel with the smaller road surface / tire friction coefficient of both wheels, the low wheel, is operated in optimal slip. This prevents both wheels from locking, which results in high cornering. However, the wheel with the larger road surface / tire coefficient of friction, the high wheel, is braked. This has the disadvantage that, in the case of a μ-split condition, only braking according to the lower road surface / tire friction coefficient can be achieved.

In the so-called select high mode, the common setting of the braking effect, generally the braking pressure, is carried out depending on the wheel with the higher wheel speed. If the vehicle is moving on a road with different road surface / tire friction coefficients on the vehicle sides (μ-split condition), the brake pressure in select high mode is therefore generally dependent on the wheel with the higher road surface / tire friction coefficient; in select high mode on one axle, the brake pressure is thus transferred to the

Wheel brakes on this axle are set in such a way that the wheel with the greater road surface / tire coefficient of friction is operated by both wheels in optimum slip. This ensures that if there is a μ-split condition, the wheel with the lower road surface / tire friction coefficient is over-braked and possibly even blocked and then can absorb only little or no cornering forces. Locking the low wheel can also damage the tires. The advantage of select high mode is that the vehicle has a relatively short braking distance.

DE 22 43 260 C2 shows that the time period during which the switch to the select high mode is made dependent on the vehicle speed and / or on the vehicle deceleration in such a way that the time period increases as the vehicle speed or vehicle deceleration decreases ,

In addition, it is known that in a so-called 2-channel ABS (braking effects on the front and rear axles are regulated together) or 3-channel ABS (braking effects on the rear axle are regulated together, on the front axle there is an individual adjustment of the Braking effect possible) the braking effect on the rear axle is set according to the select-low mode.

DE 197-52 807 AI shows a system for adjusting the braking effect in a vehicle with the features of the preambles of the independent claims. If there is friction Differences in value that can be specified to a lesser extent, the choice of an operating mode occurs depending on the extent of the existing friction values. As an alternative or in addition to this, it can be provided that when there are differences in the coefficient of friction which can be predetermined to a greater extent, the selection of an operating mode is made as a function of the detected vehicle speed.

As mentioned, to save costs both in anti-lock control systems (ABS) and in electronic brake systems (EBS), system configurations are known which carry out the brake pressure control on wheels of individual axles with only a single-channel pressure modulator. Here, the two wheels of an axle can be monitored for a blocking tendency according to the control method Select-High or Select-Low. In a third mode (Select-Smart), the select-low operating mode is now selected for brake control on a homogeneous road, while the select-high operating mode is set for brake control under a μ-split condition (inhomogeneous road). The Select Smart mode has the advantages of the two types of control listed above.

For the detection of μ-split road conditions, it can be used that there is a different wheel slip behavior on an inhomogeneous road. During the ABS control process, you can see larger and earlier drops in wheel speed on the wheels that are on the road with the lower coefficient of friction. The different brake pressures on the wheels, which move according to the different coefficients of friction, are also a criterion for the existence of a μ-split road condition. For this purpose, the brake pressure differences on the wheels in question can either be sampled by temporarily switching from select low to select high mode or with associated axles be formed with individual Radre.gelung by pressure calculation, for example ^'by a simulation.

However, a disadvantage of the Select Smart mode is that the detection of a μ-split road condition and thus the switching to the Select High mode can also be incorrectly carried out on a road with a homogeneous coefficient of friction. For example, the criteria described for switching from select-low to select-high operation can also occur when cornering or when there is a vehicle load that is distributed very differently on the vehicle sides or also with wheel brakes that act differently on one side or pull obliquely or pull on one side. However, if the switch from select-low to select-high operation is wrong, especially on a grippy lane, the cost savings due to the saved pressure modulator can be used up several times with just a single braking operation due to tire damage his.

In addition to the common setting of the braking action on the wheels of an axle described above, a common setting of the braking action on the wheels on one side of the vehicle with axles lying close together is also possible.

This is the case, for example, in the case of three-axle commercial vehicles in which the wheel brake control of the two rear axles is combined on one side and is monitored with only one brake pressure modulator in each case. Even with two- or three-axle trailers, the brake control of the wheels of axles that are close to each other is often summarized page by page. Since a friction coefficient difference (μ-split) acting on the sides of the vehicle has no negative effect on braking, the normal control mode is the same for side-by-side and therefore the same Track-running wheel pairs mostly the select-low operating mode already described, in which the joint setting of the braking effect, in general the braking pressure, is carried out depending on the wheel with the lower wheel speed. This normal control mode can also be given in the case of a page-wise combination of three-axle wheel assemblies, where, as a rule, only the movement behavior of the wheels of the first and third axles is sensed and the wheels of the middle axle are indirectly monitored.

As with any select-low control, underbraking can occur, namely when there are braking force differences on the two sensed wheels and combined into a control circuit, or if they occur over time. Since, as already mentioned, with a select-low control the common brake pressure is always adjusted to the wheel with the lowest wheel speed, the second sensed wheel and possibly also the unsensitive and carried wheel are braked to the same extent as the braking force differences or there are also load differences between the two sensed wheels.

Braking can occur, for example, if one of the two closely spaced and sensed axles is designed as a lift axle and the weight is relieved in the suffered condition, but the wheels themselves still touch the ground and thus wheel speed signals are also measured. Since there is practically no load weight on this incorrectly lifted axle, the second axle, which now carries the full weight, will be completely braked in a select-low mode.

To avoid the aforementioned disadvantages, the switching condition according to the invention is proposed. Advantages of the invention.

As mentioned, the invention is based in a first embodiment on a system for adjusting the braking effect in a vehicle with at least two wheels arranged on the right and left of an axle with respect to the direction of travel. The wheels can roll on a road surface with different coefficients of friction. The adjustment of the braking effect on the two wheels takes place jointly, two operating modes being provided for this purpose, which can be selected depending on the friction coefficients present on the wheels and / or depending on the vehicle speed. In a first selectable operating mode, the select-low operation mentioned above, the common setting is dependent on the detected one

Movement behavior of the wheel with the lower coefficient of friction. In a second selectable operating mode, the select-high operation mentioned above, the common setting takes place depending on the detected movement behavior of the wheel with the higher coefficient of friction. Furthermore, a deceleration value representing the vehicle longitudinal deceleration is recorded.

The essence of the invention is that the second operating mode, the select high operation mentioned above, is only selected when the delay value falls below a predefinable threshold value. It is therefore proposed according to the invention that the switchover from select low to select high is only enabled or permitted in the event of vehicle decelerations below a specific threshold value.

The invention is based on the consideration that when braking on a road with inhomogeneous coefficients of friction (μ-split condition), even with an individual duel independent wheel control (individual control) only certain maximum values for the vehicle longitudinal deceleration can be achieved. For example, on a road with an icy edge of the road (coefficient of friction μ «0.1) and a dry middle of the road (coefficient of friction μ * 0.6) there is a maximum deceleration of a = 0.35g (g is the Fall acceleration 9.81 / sec ^) achievable. If the value a> 0.35g is already reached when the vehicle is braked without switching from select-low to select-high, this can generally only be a road with high grip or high coefficient of friction. In order to avoid the disadvantages mentioned at the outset (for example costly tire damage), according to the invention the changeover from the select-low to select-high control mode will be blocked.

A second embodiment of the idea of the invention is based on the joint setting of the braking action on the wheels on one side of the vehicle described at the beginning. In this variant, two operating modes are also provided, which can be selected at least as a function of the detected rotational speed of the at least two wheels. This also happens - in a first selectable operating mode (Select Low, SL) the common setting depends on the movement behavior of the wheel with the lowest rotational speed and - in a second selectable operating mode (Select High, SH) the common setting depends on the movement behavior of the greatest rotational speed of the wheel.

Furthermore, a deceleration value representative of the vehicle longitudinal deceleration is recorded. - y -

In this variant of the invention, too, the core of the idea is that the second operating mode is selected at least as a function of the delay value falling below a predefinable threshold value. The problem of under-braking in the setting of the braking effect summarized page by page is solved by the second variant of the invention. Such an asymmetrical braking state is identified in the same way as in the control on an asymmetrical grip on the roadway according to the first variant of the invention. The monitoring of the switchover logic with release of the switchover only below a certain threshold value is carried out in the same way as for the axis-wise summary of the braking action setting.

In this variant, too, the advantage of the invention is that, in the case of side-by-side wheel control with monitoring of the changeover logic from select-low to select-high, underbrakes below a certain deceleration value and tire damage above the deceleration value are avoided.

The changeover monitoring according to the invention is both in anti-lock control systems (ABS) and in electrical ones

Brake systems (EBS) can be used, irrespective of whether the vehicle systems are braked by compressed air or hydraulically braked. The invention is applicable to both commercial vehicles and passenger vehicles.

In an advantageous embodiment of the invention, provision is made for a wheel to represent the extent of the differences in the coefficients of friction between the at least two wheels arranged on the right and left of an axle with respect to the direction of travel - IU -

Value is determined. The two operating modes are then selected depending on the value.

Furthermore, it can be provided that the value, which represents the extent of the differences in the coefficients of friction, is determined depending on the detected rotational movements of the wheels. In particular, it is provided that the changes in the rotational movements of the wheels over time are determined and the value is determined as a function of the changes over time.

In a further advantageous embodiment of the invention it is provided that the detection of the changeover mode is based on the different slip behavior of the two vehicle wheels running on different coefficients of friction. This is advantageous because it can be much easier to monitor the two sensed wheels for different slip behavior than to determine the respective wheel deceleration. In addition, the wheel deceleration is more dependent on the respective braking pressure than on the actual coefficient of friction. It is therefore proposed that

a speed variable representing the longitudinal vehicle speed (reference speed) is determined, in particular depending on the rotational movements of the wheels,

- The rotational movements of the wheels are compared with the speed variable and

- The changeover to the second operating mode (SH) is determined depending on the comparison.

In addition, it can be provided that

- A wheel slip representing the wheels

Slip size is determined depending on the rotary movements and the speed size, and - The second operating mode (select high, SH) is only selected if the wheel slip of the wheel with the lower coefficient of friction exceeds a predefinable value.

The deceleration value is preferably determined as a function of the detected rotational movements of the wheels. In this case, it is provided in particular that, depending on the detected rotational movements of the wheels, a reference speed is formed which represents the longitudinal vehicle speed. The delay value can then be inferred from the change in the reference speed. The reference speed can be filtered over time. A difference value is then formed between the filtered and the unfiltered reference speed and the deceleration value is formed depending on the difference value.

Further advantageous embodiments of the invention can be found in the subclaims.

drawing

The drawing shows schematically a block diagram of a 2-channel brake system based on FIG. FIG. 2 shows the control unit controlling the brake system as a block diagram, FIGS. 3 and 4 disclosing details of the control

embodiment

In the following, the invention will be described in detail using an exemplary embodiment. Here, the invention is to be illustrated by way of example using a hydraulically operating 2-channel ABS. However, it should be expressly mentioned that the idea according to the invention is not limited to hydraulic brake systems. The hydraulic vehicle brake system according to the block diagram shown in FIG. 1 has a two-circuit master brake cylinder 117 with a reservoir 104 and with a brake pedal 106, which is connected to the brake booster 105 via a pedal rod. The two brake circuits HZ2 and HZ1 are assigned to the brakes 120hl and 120hr or 120vl and 120vr of the rear and front axles of a vehicle.

The brake circuit HZ1 includes a main brake line 108 starting from the master brake cylinder 117, which leads to the anti-lock device 10, and a wheel brake line 118v, which starts from the anti-lock device 10 and leads to the wheel brakes 120vl and 120vr of the front axle. In a similar manner, the brake circuit HZ2 includes a main brake line 107 and a wheel brake line 118h, which leads to the wheel brakes 120hl and 120hr of the rear axle.

The anti-lock device 10 has two valves MV1 and MV2 116 and 115, which are installed in series with two check valves 114 and 113. There are also two return pumps RFP1 and RFP2 112 and 111, which have a common drive motor, not shown. To reduce pressure pulsations, the return flow takes place through two damper chambers with downstream throttles D2 and Dl (109, 110) into the main brake lines 107, 108.

Valves 115 and 116 are designed as 2/2-way valves which can be controlled by means of electromagnets (control signals AI and A2) and which are open when electromagnets are de-energized. Depending on the position of the solenoid valves 115 and 116, the brake pressure can be built up or reduced jointly for the wheel brakes of the front axle on the one hand and jointly for the wheel brakes of the rear axle on the other hand. - i J -

The reference numerals 130hl, 130hr, 130vl and 130vr designate wheel speed sensors which are known per se and which detect the rotational speed of the corresponding wheels and transmit them to the control unit 140 as signals Nhl, Nhr, Nvl and Nvr. Depending on these input signals, the control unit 140 calculates the control signals AI and A2 for controlling the solenoid valves 115 and 116.

The more precise functioning of the control unit 140 is to be illustrated with reference to FIG. 2.

In the control unit 140, the wheel speed signals Nhl, Nhr, Nvl and Nvr, in addition to the block 1401 to be described below, are fed to the block 1403. Here, a reference speed Vref representing the longitudinal vehicle speed is first formed from the wheel speed signals Nhl, Nhr, Nvl and Nvr. For the brake control, this can be done in the simplest way by using the largest wheel speed as the reference speed Vref. Alternatively, the second largest wheel speed can be used for the reference speed Vref; in this case it is taken into account that the vehicle can be equipped with an emergency wheel with a smaller wheel radius.

In block 1403, the brake slip for each wheel is determined from the reference speed Vref and the wheel speed. An instability value is formed from the wheel slip thus determined and the wheel deceleration, taking into account further variables, if necessary. The braking stability of each wheel can be concluded from the size of this instability value. In the case of the individual control mentioned at the outset, the brake pressures in each wheel brake would be controlled in such a way that a stable course of the wheel movement is achieved, that is to say in particular no excessive brake slip occurs. Depending on the detection of a risk of wheel locking on at least one of the wheels, the control unit 140 switches on the drive motor (not shown) and thereby starts the return pumps RFP1 and

RFP2. Furthermore, the control device 140 controls at least that of the valves MV1 / MV2 into the closed position, on the associated wheel of which there is a risk of wheel locking. In this way, due to the closed valve MV1 / 2 by the operating return pump RFPl / 2, pressure medium is conveyed back from the wheel brake cylinder to the master brake cylinder 117. As a result, the brake pressure within the wheel brake cylinder decreases, the braking effect decreases and the risk of locking is reduced. If the risk of wheel locking is sufficiently reduced, the corresponding valve MVl / 2 is opened so that it returns to its starting position, with the result that a pressure difference between the master brake cylinder and the wheel brake cylinder that is present as a result of the previous reduction in brake pressure becomes smaller.

For the sake of simplicity, only the first variant of the invention (axially summarizing the brake setting) is described in detail below.

As only a common brake pressure control of the two axles is possible due to the limited hydraulic design, the brake pressure on the axles is either set in such a way that the stability of the wheel with the smaller roadway / tire is either in select-low mode - Friction value or in Selct-High mode the stability of the wheel with the larger road surface / tire friction value is in the foreground. In order to distinguish which of the two modes is to be set on which axis, the two signals (SL / SH) _VA and (SL / SH) _{HA are} transmitted from block 1403 Block 1402 fed. These signals can take on two values, depending on whether

on the front axle the select low mode (signal Shγ ^) r - on the front axle the select high mode (signal SH _VA ), on the rear axle the select low mode (signal SL ^) or on the rear axle of the Select high mode (signal SH _HA )

should be set.

To determine which operating mode is currently to be set, the wheel speed signals Nhl, Nhr, Nvl and Nvr are fed to block 1401. In block 1401, the wheel deceleration values are calculated by differentiating. Since the maximum possible wheel deceleration is a direct measure of the current coefficient of friction between the tire and the road, the maximum wheel deceleration of the right and left wheels can be used to determine the coefficient of friction μ _r for the right half of the vehicle and the coefficient of friction μτ_ for the left half of the vehicle. These values are fed to block 1402.

As mentioned at the beginning, it can also be used for the detection of μ-split road conditions that there is a different wheel slip behavior on an inhomogeneous road. During the ABS control process, you can see larger and earlier drops in wheel speed on the wheels that are on the road with the lower coefficient of friction. The different brake pressures on the wheels, which move on the different coefficients of friction, are also a criterion for the existence of a μ-split road condition. For this purpose, the brake pressure differences on the wheels in question can be sensed either by temporarily switching from select low to select high operation are or with associated axles with individual wheel control by pressure calculation.

Parts of block 1402 are shown in more detail in FIG.

The friction values μ _r and μτ_ determined in block 1401 are compared with one another in block 1402, whereupon depending on the comparison result, that is to say depending on the size of the friction values and, if appropriate, depending on the size of the current vehicle speed Vref

- The switching signal (SL / SHJy ^ for switching from select low to select high on the wheel brakes of the front axle or

- The switching signal (SL / SH) ^^ for switching from select low to select high on the wheel brakes of the rear axle

be formed. These signals are sent through the switches

14024 and 14025 fed to block 1403.

The vehicle reference speed Vref formed in block 1403 is differentiated in block 14022 from the vehicle longitudinal acceleration al. The vehicle longitudinal acceleration a1 is compared in block 14023 with a predefinable threshold value SW. As already described, the threshold value can be, for example, 0.35 g (g is the acceleration due to gravity).

Depending on whether the vehicle's longitudinal acceleration is al

If the threshold SW exceeds or falls below, the signal U _on / ₀ ff assumes the value U _on or U ₀ ff. Depending on the signal U _on / ₀ ff, the switches 14024 and 14025 are closed or opened. If the longitudinal vehicle acceleration al exceeds the threshold value SW, the switches U ₀ f open the switches 14024 and 14025 or prevent the switches from closing; This means that the switch commands (SL / SH) _VA and (SL / SH) H _A cannot be carried out, a switch to the high-selector mode is prevented. Such a switchover is only permitted if the vehicle longitudinal acceleration a1 falls below the threshold value SW by the signal U _on '.

As an alternative to the differentiation 14022 shown in FIG. 3, the vehicle longitudinal acceleration can also be determined, as can be seen in FIG. 4. Here, the vehicle reference speed Vref is filtered by a PT _] _ filter (block 14026). To determine the longitudinal vehicle acceleration al, the unfiltered vehicle reference speed Vref is subtracted from the filtered signal in the addition stage 14027.

Claims

- 1 «-Ϊ0 claims 1. Method for setting the braking effect in a vehicle with at least two wheels (120ij) arranged on the right and left of an axis with respect to the direction of travel 15 with a different coefficient of friction (μ _r , μτ_) Can roll off the road, and with a common setting (115, 116) of the braking effect on the two wheels with two operating modes (SL, SH), which depend on the existing friction coefficients on the wheels and / or depending on the driving 20 tool speeds can be selected, whereby in a first selectable operating mode (select low, SL) the common setting depending on the detected movement behavior of the wheel with the lower coefficient of friction and in a second selectable operating mode (select high, SH) the common input 25 position depending on the detected movement behavior of the wheel with the higher coefficient of friction, whereby a deceleration value (al) representing the vehicle longitudinal deceleration is recorded, characterized in that the second operating mode is only selected when the deceleration 30 value. (Al) falls below a predefinable threshold value (SW). 2. Method for adjusting the braking effect in a vehicle with at least two with respect to the direction of travel 35 vehicles on one side with wheels (120ij) a common setting (115, 116) of the braking action on the two wheels with two operating modes (SL, SH) which can be selected at least as a function of the detected rotational speed of the at least two wheels, wherein - in a first selectable operating mode (select low, SL) the common setting depends on the movement behavior of the wheel with the lowest rotational speed, in a second selectable operating mode (select high, SH), the common setting is dependent on the movement behavior of the wheel with the greatest rotational speed, - A deceleration value (al) representing the vehicle longitudinal deceleration is detected, and - The second operating mode is selected at least depending on the deceleration value (al) falling below a predefinable threshold value (SW). 3. The method according to claim 1, characterized in that a value representing the extent of the friction coefficient differences between the at least two wheels (120ij) arranged on the right and left of an axle with respect to the direction of travel is determined and the two operating modes (SL, SH) are dependent be chosen by the value. 4. The method according to claim 3, characterized in that - The value is determined as a function of detected rotary movements (Nij) of the wheels (12.0ij), it being provided in particular that the changes in time of the rotary movements (Nij) of the wheels (120ij) are determined and the value as a function of the changes over time is determined and / or - The value is determined depending on the detected rotational movements (Nij) of the wheels (120ij), it being provided in particular that - u - a speed variable representing the longitudinal vehicle speed is determined, in particular depending on the rotary movements (Nij) of the wheels (120ij), - the rotational movements (Nij) of the wheels (120ij) are compared with the speed variable and - the value is determined depending on the comparison, 5. The method according to claim 1, characterized in that the rotational movements (Nij) of at least two wheels (120ij) are recorded, a speed variable representing the longitudinal vehicle speed is determined, in particular depending on the rotary movements (Nij) of the wheels (120ij), a slip size representing the wheel slip is determined as a function of the rotary movements (Nij) and the speed size, and - The second operating mode (select high, SH) is only selected if the wheel slip of the wheel with the lower coefficient of friction exceeds a predefinable value. 6. The method according to claim 1 or 2, characterized in that the deceleration value (al) is determined depending on the detected rotary movements (Nij) of the wheels (120ij), it being provided in particular that a reference speed (depending on the detected rotary movements (Nij) Vref) is formed, which represents the longitudinal vehicle speed, and the change in the reference speed (Vref) is used to infer the deceleration value (al), in particular providing that the reference speed (Vref) is temporally filtered and a difference value between the filtered and the unfiltered Reference speed (Vref) is formed and the deceleration value (al) is formed depending on the difference value. 7. Device for adjusting the braking effect in a vehicle with at least two wheels (120ij) arranged on the right and left of an axle with respect to the direction of travel, which can roll on a road surface having different friction values (μ _r , μτ_), and with a common setting (115, 116) of the braking effect on the two wheels with two operating modes (SL, SH), which can be selected depending on the existing friction values on the wheels and / or depending on the vehicle speed, wherein in a first selectable operating mode (select low, SL ) the common setting depending on the detected movement behavior of the wheel with the lower coefficient of friction and in a second selectable _, operating mode (select high, SH) the common setting is dependent on the detected movement behavior of the wheel with the higher coefficient of friction, a Deceleration value (al) representing vehicle longitudinal deceleration is detected, characterized in that means are provided by means of which the second operating mode is only selected if the deceleration value (al) falls below a predefinable threshold value (SW). 8. Device for setting the braking action in a vehicle with at least two wheels (120ij) arranged on one side with respect to the direction of travel of the vehicle with a common setting (115, 116) of the braking action on the two wheels with two operating modes (SL, SH) which are selectable at least depending on the detected rotational speed of the at least two wheels, wherein in a first selectable operating mode (select low, SL) the common setting is dependent on the movement behavior of the wheel with the lowest rotational speed, - in a second selectable operating mode (select high, SH) the common setting depending on the movement behavior of the wheel with the greatest rotational speed occurs, - A deceleration value (al) representing the vehicle longitudinal deceleration is detected, and - The second operating mode is selected at least depending on the deceleration value (al) falling below a predefinable threshold value (SW). 9. The device according to claim 7, characterized in that a value representing the extent of the friction coefficient differences between the at least two wheels (120ij) arranged on the right and left of an axle with respect to the direction of travel is determined and the two operating modes (SL, SH) are dependent be chosen by the value. 10. The device according to claim 9, characterized in that - The value is determined depending on the detected rotational movements (Nij) of the wheels (120ij), it being provided in particular that the changes in time of the rotary movements (Nij) of the wheels (120ij) are determined and the value is determined depending on the changes in time will and / or - The value is determined depending on the detected rotational movements (Nij) of the wheels (120ij), it being provided in particular that - A speed variable representing the longitudinal vehicle speed, in particular depending on the rotational movements (Nij) of the wheels (120ij), is determined, - The rotational movements (Nij) of the wheels (120ij) is compared with the speed variable and - The value is determined depending on the comparison. 11. The device according to claim 7, characterized in that - J - the rotational movements (Nij) of at least two wheels (120ij) are recorded, , a speed variable representing the longitudinal vehicle speed is determined, in particular depending on the 5 rotary movements (Nij) of the wheels (120ij), - A slip size representing the wheel slip is determined depending on the rotary movements (Nij) and the speed variable, and - We only select the second operating mode (select high, SH) gel if the wheel slip of the wheel with the lower coefficient of friction exceeds a predefinable value. 12. The device according to claim 7 or 8, characterized in that the delay value (al) depending on detected 15 rotary movements (Nij) of the wheels (120ij) is determined, it being provided in particular that depending on the detected rotary movements. (Nij) a reference speed (Vref) is formed, which represents the longitudinal vehicle speed, and from the change in the reference speed 20 (Vref) is inferred about the delay value (al), it being provided in particular that the reference speed (Vref) is temporally filtered and a difference value is formed between the filtered and the unfiltered reference speed (Vref) and the delay value 25 (al) is formed depending on the difference value.