JPH0974605A - Device and method for controlling regenerative braking of electric vehicle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the regeneration energy. SOLUTION: A charging upper-limit power Pbatt is determined according to the SOC and temperature of a battery (107), a regeneration torque upper-limit value Tmax (108) is obtained by diving it by the motor speed, and comparing the result with a motor rating a regeneration torque target value Treg (109) is obtained by. By feeding back a battery voltage, a regeneration torque target value Treg is compensated (110) and is subtracted from a requested braking torque, thus obtaining a hydraulic torque target value Thyd (111). A regeneration torque and a hydraulic torque are controlled according to the regeneration torque Treg and the hydraulic torque target value Thyd (102 and 103).

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a regenerative braking control device and method mounted on an electric vehicle.

[0002]

2. Description of the Related Art When charging a secondary battery such as a lead battery or a NiMH battery, it is necessary to avoid overcharging. If a remarkable overcharge state is reached, gas is generated inside the battery of this type due to decomposition of the electrolytic solution. This gas evolution
This leads to shortened battery life. On the other hand, in an electric vehicle such as an electric vehicle that uses a motor to generate propulsive force, regenerative braking is used as one of control methods, in which a battery is charged with braking energy collected by a motor for running the vehicle. Japanese Patent Laid-Open No. 5-
In the device disclosed in Japanese Patent No. 161215, the upper limit value of the regenerative braking force (torque) is limited according to the depth of charge (DOD) or state of charge (SOC) of the battery and the temperature, so that the battery Overcharge is preferably prevented.

[0003]

However, in Japanese Unexamined Patent Publication No. Hei 5-161215, the upper limit value of the regenerative braking force is uniformly limited according to the DOD and SOC of the battery, so that there is actually room for charging. However, there is a problem that regenerative braking force is limited to prevent overcharging, and therefore regenerative braking is not fully utilized to improve the energy efficiency of the vehicle. It was The present invention has been made to solve such a problem, and by introducing a new element to regenerative braking force limitation for preventing overcharge, battery overcharge accompanying regenerative braking is introduced. It is an object of the present invention to appropriately prevent regenerative braking and at the same time to make maximum use of regenerative braking for improving the energy efficiency of the vehicle. Another object of the present invention is to make it possible to deal with changes and variations in battery characteristics over time. The present invention
Further, another object of the present invention is to make it possible to accurately control the total braking force including the regenerative braking force with the required braking force as a target.

[0004]

In order to achieve such an object, the first structure of the present invention is mounted on an electric vehicle equipped with a rechargeable battery and a motor driven by the discharge output of the battery. In the regenerative braking control device that controls the regenerative braking force generated by the motor according to the control target that is sequentially determined according to the required braking force, the state of the battery (for example, SO
C and temperature) and the number of rotations of the motor, means for determining the charging power upper limit value based on the state of the battery, and the upper limit of the control target of the regenerative braking force to the charging power upper limit value and the number of rotations of the motor. And means for restricting based on the above. As described above, in the present configuration, the charging power upper limit value is determined according to the state of the battery, and the regenerative braking upper limit value is determined based on the rotation speed of the motor, which is an element that influences the charging power together with the regenerative braking force, and the charging power upper limit value. The power control target is limited to the upper limit. Therefore, unlike the configuration of JP-A-5-161215, which uniformly limits the regenerative braking force regardless of the number of rotations of the motor, the regenerative braking can be utilized to the maximum for improving the energy efficiency of the vehicle. Needless to say, overcharging of the battery due to regenerative braking is also suitably prevented.

A second configuration of the present invention is, in the first configuration, means for detecting the voltage of the battery and a control target of the regenerative braking force when the voltage of the battery exceeds a predetermined maximum allowable value. Means and are provided. Therefore, although the upper limit of the control target of the regenerative braking force is limited based on the charging power upper limit value and the rotation speed of the motor, the battery voltage has a predetermined maximum allowable value due to changes or variations in the characteristics of the battery over time. Even when it exceeds the limit, in the present configuration, the control target of the regenerative braking force is reduced, so that the overcharge of the battery is appropriately prevented.

According to a third aspect of the present invention, in the first or second aspect, means for setting a value obtained by subtracting the control target of the regenerative braking force from the required braking force as the control target of the fluid pressure braking force, and the above electric A means for increasing / decreasing the fluid pressure braking force acting on the drive wheels of the vehicle according to its control target (for example, a valve for increasing or decreasing the fluid pressure acting on the drive wheels) is provided. In this configuration, therefore, the required braking force exceeds the upper limit of the control target of the regenerative braking force,
The reduction amount of the regenerative braking force resulting from the reduction of the control target in the above configuration is compensated by the fluid pressure braking force. As a result, the total braking force including the regenerative braking force and the fluid pressure braking force is accurately controlled with the required braking force as a target, despite the limitation or fluctuation of the regenerative braking force.

According to a fourth aspect of the present invention, in a regenerative braking control method executed in an electric vehicle equipped with a rechargeable battery and a motor driven by the discharge output of the battery, the state of the battery and the rotation of the motor are controlled. Detecting the number, determining the charging power upper limit value based on the state of the battery,
While limiting the upper limit based on the charging power upper limit value and the number of rotations of the motor, sequentially determining the control target of the regenerative braking force according to the required braking force, and the regenerative braking force generated by the motor according to the control target. And a controlling step. A fifth configuration of the present invention, in the fourth configuration, a step of detecting the voltage of the battery, a step of reducing the control target of the regenerative braking force when the voltage of the battery exceeds a predetermined maximum allowable value, It is characterized by having. A sixth configuration of the present invention is, in the fourth or fifth configuration, setting a value obtained by subtracting a control target of the regenerative braking force from a required braking force as a control target of the fluid pressure braking force, and driving the electric vehicle. And a step of increasing / decreasing the fluid pressure braking force acting on the wheel according to the control target thereof. According to these configurations, the regenerative braking control methods suitable for the first to third configurations are realized.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows the system configuration of an electric vehicle according to an embodiment of the present invention. In this system, a three-phase AC motor is used as the motor 16 for running the vehicle, and the motor 16 is driven by the discharge output of the battery 18 supplied via the inverter 17. Motor EC
The U (electronic control unit) 19 determines the control target of the power running torque according to the depression amount of the accelerator pedal and the shift position, while detecting the rotation speed wm of the motor 16, and determines the determined power running torque target value and the detected motor. The control target of the motor current is obtained based on the rotation speed wm, and the power conversion operation by the inverter 17 is controlled according to the obtained motor current target value.
As a result, the power running torque corresponding to the amount of depression of the accelerator pedal is output from the motor 16. When executing the above procedure, the motor ECU 19 refers to the SOC, temperature, voltage, etc. of the battery 18 detected by the battery ECU 20, and corrects the power running torque target value as necessary.

The vehicle shown in FIG. 1 employs hydraulic braking as braking means (more generally, fluid pressure braking using an incompressible fluid).
And equipped with regenerative braking. First, the hydraulic piping for hydraulic braking is from the master cylinder 1 to the front side wheel cylinder 3 via the front side pressure increasing valve 5 and to the rear side wheel cylinder 4 via the rear side pressure increasing valve 6.
, Respectively. The master cylinder 1 generates hydraulic pressure according to the amount of depression of the brake pedal by the vehicle operator. Further, the pressure increasing valves 5 and 6 are provided in the regenerative ECU 21.
It opens and closes according to the command from. When the pressure increasing valve 5 is opened while the pressure reducing valve 7 to be described later is closed, braking oil is introduced from the master cylinder 1 to the wheel cylinder 3,
As the hydraulic pressure in the master cylinder 1 increases, the hydraulic pressure in the wheel cylinders 3 also increases (pressure increase). On the contrary, if the pressure increasing valve 5 is closed while the pressure reducing valve 7 is closed, the introduction of the braking oil from the master cylinder 1 to the wheel cylinder 3 is blocked, so that the wheel cylinder is increased regardless of the increase in the hydraulic pressure in the master cylinder 1. The hydraulic pressure of 3 is maintained. The pressure increasing valve 6 operates in the same manner. The wheel cylinders 3 and 4 apply hydraulic braking torque (hereinafter referred to as hydraulic torque) to the front wheels or the rear wheels, respectively. Although this figure shows an example of a front-wheel drive vehicle, the present invention can be applied to a rear-wheel drive vehicle and a four-wheel drive vehicle.

The hydraulic piping for hydraulic braking further reaches the reservoir tank 13 from the wheel cylinder 3 or 4 through the front or rear pressure reducing valve 7 or 8, and further from the reservoir tank 13 to the hydraulic pump 12 and the check valve. Fluid tank 1 via 11 and switching valve 15
4 or master cylinder 1. The pressure reducing valves 7 and 8 are opened and closed according to a command from the regenerative ECU 21. When the pressure increasing valve 5 is closed and the pressure reducing valve 7 is opened, the braking oil is discharged from the wheel cylinder 3 to the reservoir tank 13, so that the hydraulic pressure of the wheel cylinder 3 decreases regardless of the hydraulic pressure generated in the master cylinder 1 ( Decompression). The hydraulic pump 12 sends the braking oil stored in the reservoir tank 13 to the switching valve 15 side in response to a command from the regenerative ECU 21. The switching valve 15 allows the brake fluid sent from the hydraulic pump 12 to be supplied to the fluid tank 14.
Whether to discharge to the master cylinder 1 or to introduce into the master cylinder 1 is switched according to a command from the regenerative ECU 21. The check valve 11 prevents the brake fluid from being introduced from the switching valve 15 side to the hydraulic pump 12 side.

The regenerative ECU 21 uses the pressure sensor 2 for the oil pressure in the master cylinder 1 and the pressure sensor 9 for the oil pressure in the wheel cylinder 3 to cause the wheel cylinder 4 to rotate.
The hydraulic pressure in each is detected by the pressure sensor 10. The regenerative ECU 21 uses the detection result to determine the target value of the braking torque (hereinafter referred to as regenerative torque) and the target value of the hydraulic torque due to regeneration, and supplies the determined regenerative torque target value to the motor ECU 19 to perform regenerative braking. Meanwhile, the pressure increasing valves 5 and 6 and the pressure reducing valves 7 and 8 are controlled according to the determined hydraulic torque target value. Regenerative ECU 2
1 also prevents bottoming of the master cylinder 1 (insufficiency of braking oil) by appropriately controlling the hydraulic pump 12 and the switching valve 15.

FIG. 2 shows a flow of operation of the regenerative ECU 21. Immediately after starting the operation, the regenerative ECU 21 first executes initialization processing such as setting of internal parameters and then (1
00), it is determined whether or not the brake pedal is depressed (101). This determination can be realized as a determination as to whether or not the hydraulic pressure detected by the pressure sensor 2 exceeds a predetermined value. Of course, it may be determined that the brake pedal is depressed by turning on a brake switch (not shown) attached to the brake pedal. When the brake pedal is not depressed, the regenerative ECU 21 sets both the regenerative torque target value Treg and the hydraulic torque target value Thyd to 0 (102), and then gives the set regenerative torque target value Treg to the motor ECU 19 ( 10
3) At the same time, the valves 5 to 8 are driven according to the set hydraulic torque target value Thyd (104). Regenerative ECU 21
At that time, if necessary, the hydraulic pump 12 and the switching valve 1
5 is driven. When the key switch is turned off by the vehicle operator (105), the regenerative ECU 21 executes a predetermined ending process (106) and ends the operation. Until the key switch is turned off, the regenerative ECU 21 repeats the operation after step 101.

When the brake pedal is depressed, the regenerative ECU
21 is the SO of the battery 18 detected by the battery ECU 20.
The charging upper limit power Pbatt is determined by referring to the built-in Pbatt table at C and temperature (107). The regenerative ECU 21 calculates the following formula based on the determined charging upper limit power Pbatt and the power rating Pmot of the motor 16.

## EQU1 ## Pmin = Min (Pbatt, Pmot)
(However, Min is a minimum value function) The power constraint value Pmin is obtained by executing the following equation, and based on the motor rotation speed wm detected by the motor ECU 19,

## EQU2 ## By executing the calculation of Tmax = Pmin / wm, the regenerative torque upper limit value Tmax
Is determined (108). The regenerative ECU 21 obtains the required braking torque K · Pmc by multiplying the oil pressure Pmc of the master cylinder 1 detected by the pressure sensor 2 by the oil pressure / torque conversion coefficient K, and the obtained required braking torque K · Pmc.
Based on the regenerative torque upper limit value Tmax determined as

## EQU3 ## By executing the calculation of Treg = Min (K.Pmc, Tmax), the regenerative torque target value Treg
Is determined (109). The regenerative ECU 21 is a battery ECU
When the voltage of the battery 18 detected at 20 exceeds a predetermined maximum allowable voltage, the value ΔV obtained by subtracting the latter from the former is converted into torque, and the following formula is used based on the value ΔT obtained by the conversion.

## EQU4 ## By executing the calculation of Treg = Treg-.DELTA.T, the regenerative torque target value Treg
Is corrected (110). The regenerative ECU 21 uses the following formula

[Equation 5] The hydraulic torque target value Thy is calculated by executing the calculation Thyd = K · Pmc-Treg.
Find d (111). The operation of the regenerative ECU 21 thereafter shifts to step 103.

3 to 7 illustrate the operating principle and advantages of this embodiment. The focus of the present embodiment is that the voltage of the battery 18 should not exceed the maximum allowable voltage in order to avoid overcharge, and as much braking energy as possible is regenerated from the motor 16 to the battery 18. In order to do so, it is necessary to increase the regenerative torque as much as possible, there is a correlation between the instantaneous charging power of the battery 18 and the voltage, and the instantaneous charging power of the battery 18 is a function of the motor rotation speed wm and the regenerative torque. Therefore, in order to regenerate as much braking energy as possible and avoid overcharging, it is sufficient to control the regenerative torque so that the voltage of the battery 18 is as high as possible without exceeding the maximum allowable voltage. ,.

First, the voltage of the battery 18 responds to a change in charging current according to a first-order delay response characteristic (or a characteristic similar to this) (see FIG. 3). Therefore, the voltage of the battery 18 stabilizes when a time of, for example, about 5 seconds elapses from the change of the charging current, and the instantaneous charging power of the battery 18 given as the product of the charging current of the battery 18 and the voltage also stabilizes at that time. In order to prevent the battery 18 from being overcharged, the voltage after stabilization does not exceed the maximum allowable voltage, and thus the instantaneous charging power after stabilization is equivalent to the maximum allowable voltage (that is, the above-described charging upper limit power Pbatt). Should not exceed. On the other hand, the instantaneous charging power of the battery 18 is the output torque of the motor 16 (regenerative torque in this case) and the motor rotation speed w.
It is also determined by the product of m and m (see Fig. 4). Therefore, when the overcharge of the battery 18 is prevented, after the charge upper limit power Pbatt corresponding to the maximum allowable voltage is determined according to the SOC of the battery 18,
If the regenerative torque target value Treg is determined based on the charging upper limit power Pbatt and the motor rotation speed wm, the regenerative torque can be further maximized within the range in which the overcharge of the battery 18 can be prevented, and eventually Maximum energy can be regenerated to the battery 18. Pba shown in FIG.
The tt table shows the charging upper limit power P corresponding to the maximum allowable voltage.
It is an example of a table that can be used in step 107 to determine the batt. In this table, the S of the battery 18
The OC and the temperature are associated with the charging upper limit power Pbatt. In a normal secondary battery, there is no room for charging as the SOC increases or the temperature decreases. Therefore, the table shown in FIG. 3 shows that the charging upper limit power Pbat increases as the SOC increases.
It is designed so that t decreases and increases with increasing temperature. It should be noted that an amount such as the specific gravity of the electrolytic solution of the battery 18 may be used instead of the SOC.

FIG. 6 and FIG. 7 show changes in the distribution of braking force between hydraulic pressure and regeneration with the progress of braking. Here, as the motor 16, the upper limit value of the regenerative torque is restricted by the motor rating Pmot in the region where the motor rotation speed wm is high, and the upper limit value of the regenerative torque is restricted by the motor maximum regenerative torque in the region where the motor rotation speed wm is low, A motor having a torque rotation speed characteristic in which regenerative torque output is prohibited in a region where the motor rotation speed wm is extremely low is assumed.

When the brake pedal is depressed while the motor 16 is rotating at a high speed as shown in FIG. 6, Treg = K.Pmc, until the required braking torque K.Pmc reaches the regenerative torque upper limit value Tmax. Thyd = 0, and the required braking torque K · Pmc is covered only by regeneration. In this state, the regenerative ECU 21 closes the pressure increasing valve 5 and the pressure reducing valve 7. After that, the required braking torque K ・ Pm
When c exceeds the regenerative torque upper limit value Tmax, Treg =
Tmin and Thyd = K · Pmc−Tmin, and a part of the required braking torque K · Pmc begins to be covered by hydraulic pressure. In this state, the regenerative ECU 21 opens the pressure increasing valve 5 and closes the pressure reducing valve 7 while K · Pmc-Tmin is increasing. On the contrary, while the K · Pmc-Tmin is decreasing, the regenerative ECU 21 controls the pressure increasing valve 5 and the pressure reducing valve 7.
open. When the required braking torque K · Pmc falls below the regenerative torque upper limit value Tmax as the motor speed wm decreases, Treg = K · Pmc and Thyd = 0 again. After that, when the motor rotation speed wm reaches an extremely low region, Tr
eg = 0, Thyd = K · Pmc, and the required braking torque K · Pmc is covered only by the hydraulic pressure. As is clear from such a braking process, in the present embodiment,
Since the hydraulic pressure of the wheel cylinders 3 can be reduced, the total braking pressure of the hydraulic pressure and the regenerative torque is always the required braking torque K, even though the regenerative torque target value Treg is limited according to the regenerative torque upper limit value Tmax. Can match Pmc. Further, since the hydraulic pressure of the wheel cylinder 4 can be reduced in accordance with the reduction of the hydraulic pressure of the wheel cylinder 3, the braking force distribution between the front and rear wheels can always be made a good value.

Further, as shown in FIG. 7, when the voltage of the battery 18 temporarily exceeds the maximum allowable voltage due to variations in characteristics and changes with time, the regenerative torque is increased by the operation of step 110 described above. Feedback will be provided to compensate for this. Therefore, the situation where the voltage of the battery 18 exceeds the maximum allowable voltage will not last long. That is, the battery 18
It is possible to prevent overcharging regardless of variations in characteristics of the battery and changes over time. Further, since the voltage feedback in step 110 is executed after limiting the regenerative torque target value Treg in accordance with the regenerative torque upper limit value Tmax, the above-mentioned ΔV is a small value, and therefore the voltage feedback gain in step 110 is small. can do. As a result, the control system related to voltage feedback can be stabilized. Further, although the regenerative torque fluctuates due to the voltage feedback, the hydraulic torque fluctuates to compensate for this, so that the required braking torque K · Pmc can always be realized.

[0020]

As described above, according to the first and fourth structures of the present invention, the upper limit value of the charging power is determined according to the state of the battery, and the motor is an element that influences the charging power together with the regenerative braking force. Based on the number of revolutions and the charging power upper limit,
Since the control target of the regenerative braking force is limited to the upper limit,
It is possible to maximize the use of regenerative braking to improve the energy efficiency of the vehicle while preferably preventing overcharging of the battery due to regenerative braking.

According to the second and fifth configurations of the present invention, the control target of the regenerative braking force is reduced when the voltage of the battery exceeds a predetermined maximum allowable value. Even when the upper limit of the control target of the regenerative braking force is limited based on the number of revolutions of the battery, even if the battery voltage exceeds the predetermined maximum allowable value due to the change or variation in the characteristics of the battery over time, It is possible to preferably prevent overcharge of the battery.

According to the third and sixth configurations of the present invention, according to the value obtained by subtracting the control target of the regenerative braking force from the required braking force,
Since the liquid pressure braking force is controlled to increase / decrease, the required braking force exceeds the upper limit value of the control target of the regenerative braking force,
The reduction amount of the regenerative braking force resulting from the reduction of the control target in the above configuration can be compensated by the fluid pressure braking force, and the total braking force including the regenerative braking force and the fluid pressure braking force is limited to the regenerative braking force. Despite the fluctuation, the required braking force can be accurately controlled with the target.

[0023]

[Addendum] The present invention can also be understood as the following configurations.

A seventh structure of the present invention is the third or sixth structure, wherein when the control target of the fluid pressure braking force is increased,
Braking from a liquid pressure generating member (for example, a master cylinder) that generates a fluid pressure corresponding to the target braking force to a fluid pressure acting member (for example, a wheel cylinder) that applies fluid pressure to the drive wheels of the electric vehicle, for example, by a pressure increasing valve. When the control target of the fluid pressure braking force is reduced by introducing the incompressible fluid for use in the fluid, the incompressible fluid for braking is discharged from the fluid pressure acting member to a predetermined tank member (for example, a reservoir tank) by, for example, a pressure reducing valve, When the fluid pressure generating member requests the introduction of the incompressible fluid for braking, the compressive fluid for braking stored in the tank member after being discharged from the fluid pressure acting member is applied to the fluid pressure generating member by, for example, a pump. It is characterized by introducing. According to this configuration, the braking incompressible fluid can be effectively used, and the bottoming of the braking incompressible fluid in the fluid pressure generating member can be preferably prevented.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an electric vehicle according to an embodiment of the present invention.

FIG. 2 is a flowchart showing a flow of operations of the regenerative ECU 2.

FIG. 3 is a timing chart showing changes in battery charging current and voltage.

FIG. 4 is a torque rotation speed characteristic diagram showing a regenerative torque upper limit value.

FIG. 5 is a conceptual diagram showing a Pbatt table.

FIG. 6 is a diagram showing a change in hydraulic pressure / regenerative braking force distribution with progress of braking together with a torque rotation speed characteristic.

FIG. 7 is a timing chart showing changes in hydraulic pressure / regenerative braking force distribution as braking progresses.

[Explanation of symbols]

1 master cylinder, 2, 9, 10 pressure sensor, 3,
4 wheel cylinders, 5, 6 booster valves, 7, 8
Pressure reducing valve, 12 hydraulic pump, 13 reservoir tank, 14 fluid tank, 15 switching valve, 16
Motor, 17 Inverter, 18 Battery, 19 Motor ECU (Electronic Control Unit), 20 Battery ECU, 21
Regenerative ECU, Pbatt charge upper limit power, Tmax
Regenerative torque upper limit value, Treg Regenerative torque target value, T
hyd Hydraulic torque target value.

Claims (6)

[Claims] 1. A regenerative braking system that is mounted on an electric vehicle equipped with a chargeable / dischargeable battery and a motor driven by the discharge output of the battery and that is generated by the motor according to a control target that is sequentially determined according to a required braking force. In a regenerative braking control device that controls power, means for detecting the state of the battery and the number of rotations of the motor, means for determining the charging power upper limit value based on the state of the battery, and charging the upper limit of the regenerative braking force control target. A means for limiting the electric power based on the upper limit value of the electric power and the number of revolutions of the motor, and a regenerative braking control device. 2. The regenerative braking control device according to claim 1, further comprising means for detecting a voltage of the battery, and means for reducing a control target of the regenerative braking force when the voltage of the battery exceeds a predetermined maximum allowable value. A regenerative braking control device comprising: 3. The regenerative braking control device according to claim 1, further comprising means for setting a value obtained by subtracting a control target of the regenerative braking force from a required braking force as a control target of the fluid pressure braking force, and driving the electric vehicle. A regenerative braking control device comprising: means for increasing / decreasing a fluid pressure braking force acting on a wheel according to its control target. 4. A regenerative braking control method executed in an electric vehicle including a chargeable / dischargeable battery and a motor driven by the discharge output of the battery, the step of detecting the state of the battery and the rotation speed of the motor, A step of determining the charging power upper limit value based on the state of the battery, and a step of sequentially determining the control target of the regenerative braking force according to the required braking force while limiting the upper limit based on the charging power upper limit value and the motor rotation speed. And a step of controlling the regenerative braking force generated by the motor according to the control target thereof, and a regenerative braking control method. 5. The regenerative braking control method according to claim 4, wherein the step of detecting the voltage of the battery, and the step of reducing the control target of the regenerative braking force when the voltage of the battery exceeds a predetermined maximum allowable value. A regenerative braking control method comprising: 6. The regenerative braking control method according to claim 4, wherein the control target of the fluid pressure braking force is set to a value obtained by subtracting the control target of the regenerative braking force from the required braking force, and driving of the electric vehicle. A regenerative braking control device comprising: a step of increasing / decreasing a fluid pressure braking force acting on a wheel according to a control target thereof.