JP2000033861A - Method and apparatus for controlling vehicle brake device - Google Patents

Abstract

(57) [Problem] To make it possible to determine the pressing force of a brake lining to a disk even when there is no wheel movement while a vehicle is stopped. A brake control is performed based on a target brake torque provided by operating a brake pedal and a measured actual torque within a range of a brake torque control circuit. In operating situations in which the measured brake torque cannot reliably represent the actual applied brake torque, the calculated brake torque actual value is switched and the brake torque control is performed based on the calculated brake torque actual value. Will be

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and a device for controlling a vehicle brake device.

[0002]

2. Description of the Related Art In modern control systems for vehicle brake systems, control of the value representing the braking action, for example brake torque or braking force, is used in various ways. In electromechanical disc brakes or drum brakes, in which the tightening force is formed by an electric motor or similar setting element without the intervention of hydraulic or pneumatic elements, the brake torque or Controlling the braking force has proven to be appropriate. The brake torque control for an electromechanical vehicle brake system is described, for example, in DE-A-19
No. 5,374,664.

[0003] The use of such brake torque control in electromechanical braking systems presents a problem when the driver's wishes are translated into operation at the wheels corresponding to the wishes. During travel, the driver gives the brake pedal a request for deceleration, while at rest, the driver desires to control the pressure applied to the disc by the brake lining with the same settings. Desired deceleration can be adequately dealt with by brake torque control, but during stoppage, the wheel does not move, so that the wheel pressure sensor cannot determine the pressing force.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to be able to determine the pressure of the brake lining against the disc even when there is no wheel movement while the vehicle is stopped.

[0005]

An object of the present invention is to provide an electric operation type setting device provided in each wheel brake, and the electric operation type setting device is provided within a control range of a braking torque generated in the wheel brake or a generated braking force. In the method for controlling a wheel brake system, the braking torque or the braking force is measured as a function of a braking request provided by the driver, the measured value for the braking torque or the braking force is at least one which does not represent an actual value. In one operating range, the control is performed on the basis of at least a measured value representing the stroke of the brake lining.

[0006] The above object is also achieved in that each wheel brake is provided with an electrically operated setting device, and the electrically operated setting device is provided by a driver within a control range of a braking torque generated in the wheel brake or a generated braking force. In a method of controlling a wheel brake system, which is controlled as a function of the braking wish, in which the braking torque or the braking force is measured, only one control system is used for the wheel brakes in all operating ranges, The problem is solved by a control method of a vehicle brake device according to the present invention, which has only a brake torque value or a brake force value as an input value.

[0007] The object is also to provide an electrically operated setting device for each wheel brake, a brake torque control device or a braking force for operating the electrically operated setting device as a function of the desired brake setting by the accelerator pedal and the measured actual value. A control device for a vehicle brake device comprising a control device, wherein the switching device is provided in at least one operating range in which the measured value for the braking torque or the braking force does not represent an actual value;
The control is switched to a control based on a measured value representing the stroke of the brake lining.

The above object is also achieved by providing an electrically operated setting device for each wheel brake, wherein the electrically operated setting device is provided by a driver within a control range of the braking torque generated in the wheel brake or the generated braking force. In the control of the wheel brake system, which is controlled as a function of the braking wish, in which the braking torque or the braking force is measured, only one control system is used for the wheel brakes in all operating ranges. This problem is solved by the control device for a vehicle brake device according to the present invention, which has only a brake torque value or a brake force value as an input value.

The control of the value representing the pressure of the brake lining, independently of the vehicle speed, enables the control of the vehicle brake system to correspond to the driver's wishes in all driving ranges.

A value representing the setting of the brake lining, such as the value of the angle of rotation of the motor for the electromechanical brake, is used, so that this value is controlled in response to the driver's wishes when the speed limit is exceeded. This is particularly advantageous. Therefore, since such a value, particularly the motor rotation angle, has a fixed relationship with the brake tightening force and therefore the lining pressure force by the characteristic curve,
This control is particularly advantageous.

Furthermore, it is advantageous for the electromechanical brake device to be provided with a motor rotation angle measuring device. It is particularly advantageous that only one control device is used despite controlling two different values in the various operating ranges. In this way, non-linear control characteristics, which have a significant effect on the ride quality at braking and at very low speeds, are avoided in switching from one value to another.

An appropriate identification algorithm is used for the current characteristic curve of the brake, in which abrupt switching is avoided and a gradual transition from brake torque control to position control or rotation angle control takes place. It is particularly advantageous.

Furthermore, it is particularly advantageous that only one control device is used for the exact setting of the air gap between the brake lining and the brake disc when the brake is not being operated. Since the value of the air gap cannot be determined by the brake torque sensor, it is advantageous in this case also for the air gap to be set on the basis of the stroke value or the angle value. Since the motor rotation angle has a linear relationship with the stroke of the brake lining, it is advantageous here to use the rotation angle of the rotor of the electric servomotor.

It is particularly advantageous that not only the air gap but also the braking torque and the crimping force during standstill are controlled by one and the same control device. Since the braking force sensor cannot provide any signal for the air gap setting, it is advantageous to use the solution described below for controlling the braking force (crimp force) with respect to the air gap setting.

[0015] Since the control arrangement is constructed in a modular manner, it is particularly advantageous that the components of the control device can be replaced and exchanged at any time independently of the other components.

Only one control device is used for setting the air gap, controlling the braking force when the vehicle is stopped, and controlling the braking force while the vehicle is running, so that switching between a plurality of control devices is not necessary. This is particularly advantageous. As a result, no nonlinear effect occurs at the time of switching.
For example, it is effectively avoided that the enlargement of the integral part of the control unit which is not activated when the control unit is switched on leads to unintended high accelerations of the servomotor.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. FIG. 1 shows an overall block circuit diagram of a brake device with an electromechanical tightening device for a brake in the example of a wheel pair. This wheel pair may be arranged on one axis of the vehicle or at a diagonal position. In this case, reference numeral 10 indicates a brake pedal of the vehicle. The driver's braking wish is measured by angle measurement, stroke measurement and / or force measurement via the sensor device 12 and via the line 14 the electronic control device 16
Supplied to In an advantageous embodiment, the control device comprises a control unit divided in a distributed manner. At least a part of the sensor device 12 and the electronic control device 16 are redundantly configured. Electronic control device 1
6 operates motors 22 and 24 using the H-bridge last stage via output lines 18 and 20, for example, with pulse width modulated voltage signals. In an advantageous embodiment, a commutated DC motor is used. The motor is part of the brake setting devices 26 and 28. The rotational motion of this motor is transmitted to the transmission stages 58 and 60 provided on the downstream side.
Is converted into a displacement movement for moving the brake linings 30 and 32. The brake lining is guided in brake calipers 34 and 36 and acts on brake discs 38 and 40 of wheels 1 and 2.
In a preferred embodiment, an electrically actuated spring-force brake is provided at the same time, by means of which the brake setting device can be locked in its current position, so that the motor can be switched off without current. . At this time, the position of the brake setting device is maintained without consuming energy.

On each wheel, force or torque sensors 42 and 44 are used, the signals of which are transmitted via the measuring lines 46 and 48 to the electronic control unit 16.
Supplied to In one embodiment, these sensors measure the axial support of the setting device during the braking process, and this axial support thus forms a measure for the normal force acting on the brake disc. This variant is referred to below as force measurement. Therefore, the braking force is understood as the force with which the brake shoe is pressed against the brake disk or the brake drum. In another variant, the radial bearing capacity of the brake lining is measured, thereby forming the frictional force generated in the brake disc or a measure for that frictional torque. This measurement, as well as the use of a direct torque sensor, is referred to below as torque measurement. Further sensor 5
The wheel speeds are measured via 0 and 52 and transmitted to the controller 16 via input lines 54 and 56. In addition, angle sensors 62 and 64 are provided, the signals of which are supplied to the controller 16 via lines 66 and 68. In a preferred embodiment, these angle sensors are Hall sensors, which measure, for example, the rotation of the motor of the brake setting device and generate a large number of pulses per rotation, the number of these pulses moving. A measure for the angle and thus the stroke traveled. In other embodiments, other sensors for stroke or angle measurement (eg, inductive sensors, potentiometers, etc.) are used.

The desired values for the individual wheel brakes or wheel brake groups are determined from the brake demand measured in the electronic control unit 16 in accordance with the previously programmed characteristic curve. These target values correspond, for example, to the braking torque or the braking force to be set for the wheels or wheel pairs, which values are in particular a function of the axle load distribution of the vehicle. A control deviation is determined from the desired value, possibly determined for each individual wheel, by comparison with the actual value of the braking force or braking torque measured at sensors 42 and 44, the control deviation being, for example, a time-discrete PID controller. In the form of a control algorithm. The control settings are used for operating the motor, in which case corresponding operating signals are output via lines 18 and 20.

As described above, the brake tightening force during traveling is certainly controlled by the driver's brake desire being optimally converted by the brake torque control. However, the tightening force when the vehicle is stopped or when the speed is extremely small is controlled. In addition, the setting of the air gap cannot be formed on the basis of the measured brake torque, since in these operating states the brake torque outputs no signal or outputs a signal that does not correspond in practice. . Therefore,
FIG. 2 shows a control arrangement in which the brake control is performed by a single control in all three operating ranges. In this case, the execution algorithm shown in FIG. 2 shows the cooperation of the programs or program parts shown as individual blocks, which execute the functions respectively described within the scope of the computer program.

The operation angle β of the brake pedal is measured by the sensor 12 shown in FIG. Desired pedal stroke-
The brake torque set value Mvo is obtained from the operation angle of the brake pedal via a characteristic curve 100 representing the brake torque characteristic.
r is determined. In the preferred embodiment, the characteristic curve 100, the desired pedal stroke-brake torque characteristic, is provided such that a negative brake torque setting is established within the range of release of the brake pedal.
In this case, the characteristic curve itself can be formed in any desired manner, depending on the embodiment, so that the penetration point at the operating angle 0 does not necessarily have to be represented by a finite gradient. The negative brake torque set value at the brake pedal operation angle 0, that is, the negative brake torque set value when the brake pedal is released, means a set value of a predetermined gap setting.

However, even during traveling, a negative brake torque is generated in reverse. Since negative measurements and settings are not allowed to have more than one meaning,
By providing the measured brake torque signal Mist 'by means of a value formation not shown in FIG. 2, sensor detection of negative brake torque due to reversal is eliminated.

First, the braking process during running of the vehicle will be observed. This means that the tightening force of the brake is set within the range of the brake torque control. Brake torque target value Mvo determined by characteristic curve 100
r is directly supplied to the control device 102 as the brake torque target value Msoll. Here, the control device 102
Includes a control algorithm, in which case in the preferred embodiment a control algorithm including a proportional part, an integral part and a derivative part will obviously be suitable. The control device 102 sets the target value Msoll and the actual torque M
The set value Ustell is calculated by the execution control algorithm as a function of ist, and the set value Ustell is output to the control target 104, that is, the electrically operated brake setting device. The brake setting device 104 includes a measuring device for measuring the rotation angle φ, particularly the rotation angle of the rotor of the electric motor, and a sensor for measuring the brake torque Mist ′. The measured actual brake torque is fed back to the control device 102 via the switching element 106 as the actual brake torque Mist. Switching element 106
Is located at the position shown in the drawing, that is, the brake torque value measured when the vehicle speed exceeds a predetermined limit value, for example, several km / h, is set as the actual brake torque value.
To supply. In this case, the control unit 102 brings the actual braking torque closer to the desired braking torque value and thus to the driver's wish, whereby the feedback-coupled braking torque control is performed as a function of the driver's braking wish.

The control device is of a modular type. This is shown particularly clearly in the control unit 102 and has only an instantaneous interface for coupling with its input side periphery. The controllers themselves include only control algorithms without control-dominant logic, so that each controller can incorporate the appropriate set of parameters into the system without changing other program parts.

The running speed V is determined in a known manner on the basis of at least one wheel speed. The traveling speed V is supplied to a threshold switching element 108, and the threshold switching element 1
08 checks whether it is below a predetermined limit value.
If the running speed exceeds this limit, i.e., if the running speed is high, the switching element 106 is in the position shown, while if the running speed is below the limit, the switching element 106 is shown by a corresponding signal. Switch to a position that is not In this position, the switching element 106 supplies the calculated actual torque Mist ″ to the control device as the actual torque Mist.

If the running speed falls below the speed limit, there is a risk that the vehicle will stop before the next scanning step.
In this case, the brake torque signal Mist 'will no longer be correlated with the tightening force of the wheel brakes.
In order to avoid uncontrollable clamping forces of the actuator in this range, the feedback connection of the brake torque signal is interrupted (switching of the switching element 106) and at least the torque signal determined from the rotation angle of the electric motor is Feedback coupled to the control unit. The characteristic curve 1
In the characteristic curve 110, the rotation angle of the electric motor is converted into a brake torque Mist ", and the brake torque Mist" is supplied to the control device 102 as the actual torque Mist when the speed falls below the speed limit.
In this case, the rotation angle Δφ is used as the rotation angle, and the rotation angle Δφ is
4 is determined by subtracting a predetermined rotation angle φ0 indicating a zero angle existing when the brake lining separates from the disk or the drum from the rotation angle φ measured at 4. The difference between these two values forms the absolute rotation angle Δφ, which is supplied to the characteristic curve 110. In one embodiment, the zero rotation angle φ0 is provided from the memory cell 114 or is determined by comparing the rotation angle measurement present at that time with the stored value to determine when to release the brake.

When the vehicle is stopped or when the speed is lower than the speed limit, the setting of the wheel brake tightening force is performed by the same brake torque control device 102 that is operating even during traveling.
Is performed based on the estimated brake torque value determined by the measured rotation angle, instead of based on the measured brake torque value.

In another embodiment, a stroke measurement (displacement stroke of the brake lining) is used instead of the rotation angle measurement, for which the above description applies.

The third operating range in which the controller 102 operates is air gap setting. If a condition exists for the air gap setting, the switching element 116 is closed. Switching element 116
Is closed when the corresponding signal from AND combination 118 is present. The AND combination 118 causes the rotation angle Δφ to fall below a predetermined limit value checked in the threshold value switching element 120 and at the same time the brake torque set value Mvor falls below the limit value checked in the threshold value switching element 122. Sometimes outputs a signal. In the preferred embodiment, the latter limit is zero, ie there must be a negative torque demand of the driver. If the brake torque target signal is negative and the motor rotation angle φ is in the range of the zero angle φ0, the target torque Mvor via the switching element 116
Is supplied with the component M1 which is coupled via the characteristic curve 124 to the rotation angle Δφ. This characteristic curve is not necessarily shown in FIG.
Is not a straight line as shown in FIG. In a preferred embodiment, this characteristic curve is a point mirror image around the zero angle φ0 of the scissor-shaped characteristic curve in the third quadrant and in the first quadrant is the scissor-shaped characteristic curve itself (see characteristic curve 110). . If there is a condition for setting the air gap, the target torque is changed by the motor rotation angle and is exactly zero when the controlled motor rotation angle corresponds to the desired air gap. If the rotation angle is too large (positive Δφ), the target torque is reduced as a result of the supply of M1, so that a negative control deviation at the inlet of the control device 102 forms a negative output voltage, and thus brakes the brake lining. Pull away from disc or brake drum. If the rotation angle is too small, ie negative Δφ, the reverse process takes place, in which case the control device 102 outputs a set value which brings the brake lining closer to the brake disc or brake drum.

Problems in setting the air gap also arise in a control device in which the braking force (pressing force) is controlled. The method described above is used in such devices as well.

In summary, not only the brake torque control during running (for example, brake force control) but also the control and gap setting during stoppage of the vehicle are the same.
It can be said that it is performed.

In a preferred embodiment, there is a control for each electrically operated wheel brake. In this case, the setpoint is supplied to all control devices, possibly corrected for each individual wheel. The above measures are taken at each wheel brake.

In an advantageous embodiment, a negative setpoint value is not used to activate the air gap setting, but instead a differential brake torque setpoint signal is used. The air gap setting is performed when the differential brake torque target signal Mvor becomes negative, that is, when the brake pedal is returned,
That is, the switching element 116 is closed. This avoids giving too large air gap as a result of offset compensation with torque sensor error when the brake lining has already left the brake disc without engaging the air gap algorithm at the positive target brake torque setting. It has the advantage of being done.

In another advantageous embodiment, other criteria for setting the air gap are provided (for example, a released brake pedal, etc.). Elements 118-122 are not used in this case.

In this connection, it is particularly important to detect the stop performed in the threshold stage 108, and particularly to specify the limit value. In torque detection, there is no signal in the case of a stop and in the case of a flat road surface. Therefore, a stop must be reliably detected. If this stop was not detected correctly, it would result in a premature stop, i.e. the speed analysis did not detect the stop yet but the vehicle had already stopped,
The control will cause the brake to be fully tightened. In the case of a rolling vehicle and if the stop is erroneously detected by the speed analysis, the braking operation is controlled in an open loop or closed loop only via the motor rotation angle, so that the ride quality is poor. Uncomfortable braking can occur at very low speeds, but does not cause unwanted tightening. The minimum speed that effectively avoids the above situation is given by the maximum deceleration of the vehicle,
It is determined as follows.

Detection of any small speed within the time limit is not possible. Given a given hardware (toothed disk for measuring the speed pulse) and the assumed maximum deceleration of the vehicle, a minimum speed is given, at which time the time for measuring this speed is determined by the actual vehicle speed. The minimum speed is formed by the maximum deceleration, which is less than or at least equal to the time elapsed until the stop. With this minimum speed taken into account, the vehicle does not stop already before the speed analysis detects that it is below the limit. Time T elapsed before stopping at maximum deceleration AMAX and minimum initial speed VMIN
GRENZ is calculated as follows.

[0037]

## EQU1 ## TGRENZ = VMIN / AMAX Therefore, the following equation holds for the minimum speed that can be measured within the time limit T.

[0038]

## EQU2 ## VMIN = .DELTA.S / TGGRENZ (.DELTA.S = stroke interval) Therefore, the following equation is given for the minimum speed at which the control must be switched from the closed loop control to the open loop control.

[0039]

VMIN = AMAX * ΔS An example for the minimum speed on a disk with 100 teeth and a gear circumference of 2 meters is as follows.

[0040]

VMIN = 1.61 km / h FIG. 3 shows another embodiment of the control configuration. In this case, the same reference numerals are given to blocks whose functions correspond to the blocks in FIG. Only the determination of the characteristic curve 110 differs, in the embodiment shown in FIG.
0 is fixedly provided as a function of the rotation angle. This can be uncomfortable in switching control devices in some few embodiments. Therefore,
As a supplementary form, the evaluation value of the auxiliary value of the characteristic curve 110 as a function of the rotation angle φ and the measured actual torque Mist ′ can be evaluated by a known evaluation method in the evaluation device 200 in addition to the zero angle φ0 within the range of the evaluation device. Suggested. In the evaluation device 200, an evaluation parameter h of the brake caliper is determined to form the characteristic curve 110. These parameters are always determined during operation, so that the characteristic curve 110 is always adapted. Therefore, the characteristics of the characteristic curve 110 correspond to the current characteristics of the brake caliper. When switched by the switching element 106, no sudden change in the actual brake torque value occurs accordingly. Similarly, the slope of the brake torque signal at the time of switching remains constant. As a result, the riding comfort is improved.

In addition to being used in electromechanical braking systems, the method described above is also used in other electrically controlled braking systems, such as electro-hydraulic or electro-pneumatic braking systems, in which case within the range of the wheel brakes The rotation angle as well as the brake torque are measured by a measuring technique.

[Brief description of the drawings]

FIG. 1 is an overall block circuit diagram of a brake device provided with an electromechanical tightening device for a brake in an example of a wheel pair.

FIG. 2 shows a control of a braking device with an electrically operated clamping device, in which the clamping force during travel, the clamping force at stopping and the gap are set by a single control device. FIG. 2 is a configuration diagram of a first embodiment of the device.

FIG. 3 shows a control of a braking device with an electrically operated clamping device, in which the clamping force during running, the clamping force at stopping and the gap are set by a single control device. FIG. 6 is a configuration diagram of a second embodiment of the device.

[Explanation of symbols]

 Reference Signs List 10 brake pedal 12 sensor device (brake pedal operation angle) 16,102 control device 22,24 electric motor 26,28,104 brake setting device 30,32 brake lining 34,36 brake caliper 38,40 brake disc 42,44 force sensor ( Torque sensor) 50, 52 Wheel speed sensor 58, 60 Transmission stage 62, 64 Angle sensor 100, 110, 124 Characteristic curve 104 Brake setting device (control target) 106, 116 Switching element 108, 120, 122 Threshold switching element 112 Coupling stage 114 Memory cell 118 AND coupling 200 Evaluation device

Claims (16)

[Claims] An electric operable setting device is provided for each wheel brake, and the electric operable setting device is provided with a brake request applied by a driver within a control range of a braking torque generated in the wheel brake or a generated braking force. In a method for controlling a wheel brake system, the braking torque or the braking force is measured as a function, the braking torque or the braking force is measured in at least one operating range in which the measured values for the braking torque or the braking force do not represent actual values. A control method for a vehicle brake device, which is performed based on a measured value representing a stroke of a brake lining. 2. An electric operable setting device is provided for each wheel brake, and the electric operable setting device is provided with a brake request applied by a driver within a control range of a braking torque generated in the wheel brake or a generated braking force. In the method of controlling a wheel brake device, which is controlled as a function, in which the braking torque or the braking force is measured, only one control device is used for the wheel brakes in all operating ranges and this control device is used as input value. A method for controlling a vehicle brake device, comprising only a brake torque value or a brake force value. 3. The driving condition for setting the air gap is any one of a stop of the vehicle, a low speed range and a driving range when the brake pedal is released, or any combination thereof. The method according to claim 1. 4. The control device according to claim 1, wherein the same control device is used for the braking torque control or the braking force control during driving and for the control in at least one operating range. The method described in. 5. A value representing a stroke of the brake lining is measured and a calculated actual braking torque or a calculated actual braking force in at least one operating range is determined based on the value representing the stroke of the brake lining. A method according to any one of claims 1 to 4, characterized in that: 6. The method according to claim 5, wherein the rotation value of the rotor of the motor of the electrically operated wheel brake setting device is measured as a stroke value. 7. The method according to claim 1, wherein if the predetermined travel speed limit is not reached, the measured braking torque actual value or the calculated actual value instead of the measured braking force actual value is supplied to the control unit. A method according to any one of claims 1 to 6. 8. The calculated brake torque actual value or the calculated brake force actual value is adapted by means of a predetermined characteristic curve on the basis of the measured stroke value or the measured angle value, possibly within the scope of parameter evaluation. 8. The method according to claim 1, wherein the characteristic is determined by a characteristic curve.
The method according to any one of the preceding claims. 9. The driver's desire is defined by a characteristic curve as a function of the brake pedal deflection, which characteristic provides a negative braking torque or a negative braking force in the range of release of the brake pedal. A method according to any one of claims 1 to 8. 10. The method according to claim 9, wherein an engagement with the control circuit takes place in a negative brake desired setting so that the predetermined gap is controlled for the gap setting. 11. The engagement according to claim 10, wherein the torque value or the force value is determined based on a stroke value by a brake torque target value or a characteristic curve for adjusting the brake force target value. The described method. 12. The brake torque control or the brake force control for setting the air gap is performed by a changed target value and by a calculated actual torque or a calculated actual force. the method of. 13. The method according to claim 1, wherein a stroke measurement which lies within a range of a negative torque setting or a force setting and a brake lining release is used as a condition for the air gap setting. The method described in. 14. The method as claimed in claim 1, wherein a zero value is provided for the stroke signal, and the measured stroke signal is evaluated using the zero value. 15. An electrically operated setting device for each wheel brake, a brake torque control device or a braking force control device for operating said electrically operated setting device as a function of a desired brake setting by an accelerator pedal and a measured actual value. Control means for a vehicle brake system comprising switching means, which indicate the stroke of the brake lining in at least one operating range in which the measured values for the braking torque or the braking force do not represent actual values. A control device for a vehicle brake device, wherein control is switched to control based on a measured value. 16. An electric operable setting device is provided for each wheel brake, and the electric operable setting device is provided with a brake request applied by a driver within a control range of a braking torque generated in the wheel brake or a generated braking force. In the control device of the wheel brake device, which is controlled as a function, in which the braking torque or the braking force is measured, only one control device is used for the wheel brake in all operating ranges and this control device is used as input value. A control device for a vehicle brake device having only a brake torque value or a brake force value.