JPH0524527A - Slip controller for vehicle - Google Patents

Abstract

PURPOSE:To carry out brake control from the start in the constitution in which a pneumatic booster is used as a driving source for generating brake hydraulic pressure. CONSTITUTION:A pneumatic type booster 11 is interposed between a brake pedal 12 and a master cylinder 8, and a subvalve device 38 for selecting a negative pressure feeding state and an atmosphere feeding state is connected in a negative pressure feeding passage 37 for the pneumatic booster, in order to generate a brake hydraulic pressure in the master cylinder by operating the pneumatic booster, even in the case where the brake pedal is not stepped on. A master cylinder brake hydraulic pressure is generated by operating the pnueumatic booster by selection-operating the subvalve device through the brake control. Taking account of the operation delay of the booster, the slip control for the driving wheel by the brake control, i.e., the control for the hydraulic pressure adjusting valves 15L and 15R is started after the lapse of a prescribed period from the start of the operation of the booster.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vehicle slip control device for preventing excessive slip of driving wheels on a road surface.

[0002]

2. Description of the Related Art Various slip control devices (traction control devices) have been proposed which prevent excessive slippage of driving wheels on a road surface during acceleration of a vehicle and thereby satisfy acceleration and vehicle stability. ing.

The slip control is performed by reducing the torque applied to the drive wheels, and therefore the brake control for applying a brake force to the drive wheels and the engine for reducing the torque (output) generated by the engine. Control is performed. In both the brake control and the engine control, the feedback control is generally performed so that the actual slip value of the driving wheels on the road surface becomes a predetermined target value.

JP-A-63-166 discloses that slip control is performed by both brake control and engine control.
Japanese Patent No. 649 discloses that only engine control is performed when the slip value of the drive wheels is small, and both engine control and brake control are performed when the slip value of the drive wheels becomes large. Has been done. In addition, when the slip control is performed by the brake control,
Japanese Patent No. 5344 discloses a device for operating a pneumatic booster interposed between a brake pedal and a master cylinder during slip control in order to generate a brake hydraulic pressure for brake control. ing.

[0005]

When the brake control for slip control is performed and the brake hydraulic pressure is generated in the master cylinder by using the pneumatic booster, the brake control is performed. It turned out that the initial control was not performed well. After pursuing such a cause,
It was found that there was a considerable response delay from the start of operation of the pneumatic booster to the generation of the brake hydraulic pressure in the master cylinder.

Therefore, the object of the present invention is to provide good brake control from the beginning, assuming that the brake hydraulic pressure for slip control is generated by utilizing the boosting action of the pneumatic booster. It is an object of the present invention to provide a slip control device for a vehicle, which is capable of performing the above.

[0007]

To achieve the above object, the present invention has the following configuration. That is, when the master cylinder that serves as a source of the brake fluid pressure for the drive wheel brake, the first chamber and the second chamber defined by the movable partition, and the brake operating member are operated. A main valve device for switching a gas pressure supply mode so as to generate a pressure difference between the first chamber and the second chamber, and receiving a pressure difference between the first chamber and the second chamber; A pneumatic booster for transmitting the driving force of the movable partition to be driven to the master cylinder, and an air pressure between the first chamber and the second chamber in a state where the brake operating member is not operated. A sub-valve device for switching the gas pressure supply mode so as to cause a difference, a hydraulic pressure adjusting means for adjusting the brake hydraulic pressure supplied from the master cylinder to the drive wheel brake, and a road surface of the drive wheel. The slip detection means for detecting the slip value for the In a state where the booster has been activated by,
A brake for controlling the hydraulic pressure adjusting means so that the slip value detected by the slip detecting means becomes a predetermined target value.
And a control means for controlling the hydraulic pressure adjusting means by the brake control means is set to be performed after a lapse of a predetermined delay period from the start of operation of the auxiliary valve device. It has such a configuration.

[0008]

According to the present invention, when the brake fluid pressure generated in the master cylinder is delayed after the booster is activated, that is, when the brake fluid pressure is stabilized in a state corresponding to the activation of the booster. Since the brake control is performed in the above step, the brake control can be favorably performed from the beginning.

The operation start timing of the booster is as follows:
The value can be set as appropriate at a time when the slip control by the control does not become extremely slow. For example, the slip value of the driving wheel can be selected as the start threshold value for setting the operation start timing.

Further, the slip control is performed by the engine control for controlling the torque generated by the engine, and the engine control is started earlier than the brake control to start the operation of the booster. It can be set as the same time as the start time.

The delay period may be set, for example, by a timer, but when slip control is performed by the engine control in order to avoid torque absorption from the drive wheels that causes fuel consumption deterioration due to brake control as much as possible. Can be set as after the drive wheels begin to exhibit deceleration. In this case, considering that the left and right drive wheels can be controlled independently, which is an advantage of the brake control,
The start of the brake control can be limited to when the difference between the slip values of the left and right driving wheels is a predetermined value or more.

As the hydraulic pressure adjusting means, a hydraulic pressure adjusting valve interposed in a brake hydraulic pressure passage between the master cylinder and the drive wheel brake is used to delicately control the brake hydraulic pressure. It is preferable for doing so.

Embodiments of the present invention will be described below with reference to the accompanying drawings. In FIG. 1, 1FL is the left front wheel, 1FR
Is a right front wheel, 1RL is a left rear wheel, and 1RR is a right rear wheel. The engine 2 is horizontally mounted on the front of the vehicle body, and the torque generated by the engine 2 is transmitted to the clutch 3, the transmission 4, and the differential gear 5, and then the left front shaft is passed through the left drive shaft 6L. 1FL and is transmitted to the right front wheel 1FR via the right drive shaft 6R. In this way, the vehicle has front wheels 1
FL and 1FR are used as driving wheels, and rear wheels 1RL and 1RR are used as front wheels and are driven wheels.

Brake 7FR to 7R mounted on each wheel
R is a hydraulic disc brake.
Further, the master cylinder 8 as a source of the brake fluid pressure is generated.
Is a tandem type having two discharge ports 8a and 8b. The break pipe 13 extending from one discharge port 8a of the master cylinder 8 is branched into two in the middle,
The branch pipe 13F is connected to the left front wheel brake 7FL (the wheel cylinder mounted in the caliper), and the branch pipe 13R is connected to the right rear wheel brake 7RR.
The branch pipe 14 extending from the other discharge port 8b of the master cylinder 8 is also branched into two, the branch pipe 14F is connected to the right front wheel brake 7FR, and the branch pipe 14R is connected to the left rear wheel brake 7RL. It is connected.

Branch pipe 13 for front wheels, that is, for drive wheels
Electromagnetic hydraulic pressure adjusting valve 15L or 1 is provided on F and 14F.
5R is connected, and an electromagnetic opening / closing valve 16L or 16R is connected to the branch pipes 13R and 14R for the rear wheels. The hydraulic pressure adjusting valves 15L and 15R are the brake 7FL,
The brake fluid pressure supply from the master cylinder 8 to the 7FR and the brake fluid pressures of the brakes 7FL and 7FR are provided in the pipe 2
The mode of switching to the reservoir tanks 22L and 22R via 1L and 21R is switched. The brake liquid in the reservoir tank 21L is returned by the pump 23L to the pipe 13 via the pipe 25L to which the check valve 24L is connected. Similarly, the brake liquid in the reservoir tank 22R is pumped by the pump 23L.
By R, the check valve 24R is returned to the pipe 14 via the pipe 25R to which the check valve 24R is connected.

The stepping force on the brake pedal 12 is
It is transmitted to the master cylinder 8 via a booster or brake booster 11. This booster 11 is basically the same as a known vacuum booster, but during slip control, as will be described later, even if the brake pedal is not stepped on, a boosting operation is performed. Is configured to do.

The booster 11 has a case 31 fixed to the vehicle body and the master cylinder 8, and the inside of the case 31 is composed of a diaphragm 32 and a valve body 33 fixed thereto. A chamber 34 and a second chamber 35 are defined. Negative pressure (for example, intake negative pressure of the engine 2) is constantly supplied to the first chamber 34, and when the brake pedal is not depressed, the second chamber 35 is communicated with the first chamber 34 and -The operation of the star 11 is stopped. When the brake pedal 12 is depressed,
Atmospheric pressure is supplied to the second chamber 35, whereby the diaphragm 32 is displaced forward together with the valve body 33 to perform a boosting function.

The switching between the negative pressure supply and the atmospheric pressure supply to the second chamber 35 is basically performed by a valve device mounted inside the valve body 33. This valve body 33
The part will be described with reference to FIG.

First, the valve body 33 has a power piston 41 fixed to the diaphragm 32, and a recess 41a formed in the power piston 41 has a recess 41a formed therein. And are fitted. The output shaft 43 serves as an input shaft of the master cylinder 8. A valve plunger 45 is attached in the valve body 33 at the tip of the input shaft 44 connected to the brake pedal 12. A vacuum valve 46 is arranged behind the valve plunger 45.

The power piston 41 has a pressure introducing passage 50.
The pressure introducing passage 50 is always communicated with the space X formed around the valve plunger 45. This space X is always communicated with the second chamber 35. A valve seat 47, on which the vacuum valve 46 is seated, is formed at the end of the pressure introducing passage 50 that opens toward the space X. In addition, the vacuum valve 46 is a valve plunger 45.
The valve seat 45a formed at the rear end is also seated on and off.

In the above structure, it is assumed that a negative pressure is being introduced into the pressure introducing passage 50. In this state, when the brake pedal 12 is not depressed, the vacuum valve 46 is seated on the valve seat 45a by the urging force of the springs 48 and 49 in the state of FIG. 2, but is separated from the valve seat 47. ing. Therefore, the pressure introducing passage 5
The negative pressure from 0 is introduced into the second chamber 35 through the space X, and the boosting action is not performed.

When the brake pedal 12 is depressed,
The input shaft 44 and therefore the valve plunger 45 are moved forward (moved leftward in the figure). During this forward movement, the vacuum valve 46
Is first seated on the valve seat 47, and the space X and the pressure introducing passage 50 are
Communication with the vacuum valve 46 and then the valve seat 45
a is separated. By separating the vacuum valve 46 and the valve seat 45a, the atmospheric pressure from the rear of the valve body 33 is introduced into the space X, and the second chamber 35 becomes the atmospheric pressure. As a result, the diaphragm 32 is displaced forward together with the valve body 33, and as a result, the output shaft 43 moves forward and a boosting action is performed. The brake reaction force from the master cylinder 8 is transmitted to the valve plunger 45 and thus the brake pedal 12 via the reaction disc 42.
When the stepping operation force of the brake pedal 12 is released, the return spring 36 (see FIG. 1) returns to the state of FIG. 2 to prepare for the next boosting action. The parts described above are the same as the known vacuum booster, but the members 45, (45a) 46, 47 constitute the main valve device in the present invention.

In the present embodiment, for slip control, the state of introducing the negative pressure of the first chamber 34 and the state of introducing the atmospheric pressure into the pressure introducing passage 50 are switched. That is, the first chamber 34 and the pressure introducing passage 50 are connected via the pipe 37, and the pipe 37 is connected to the three-way electromagnetic switching valve 38 (see FIG. 1). This switching valve 38
Causes the pressure introducing passage 50 to communicate with the first chamber 34 during demagnetization, and introduces atmospheric pressure into the pressure introducing passage 50 during excitation.
When the switching valve 38 is excited and atmospheric pressure is introduced into the pressure introducing passage 50, the space X, and thus the second chamber 35, becomes
Even if the brake pedal 12 is not depressed, the atmospheric pressure is reached, and as a result, the boosting action is performed to generate the brake hydraulic pressure in the master cylinder 8. The switching valve 38 constitutes the auxiliary valve device of the present invention.

FIG. 3 schematically shows a control system, in which U is a control unit constructed by using a microcomputer.
The signals from the sensors or switches S1 to S7 are input. The sensors S1 to S4 detect the rotation speeds of the wheels 1FL to 1RR. The switch S5 is an accelerator switch that is turned on when the accelerator pedal 10 is fully closed. Switches S6 and S7 are not
It is operated when the key pedal 12 is depressed, and for example, one switch is a normally open type and the other is a normally closed type. Further, although the control unit U outputs to each device shown in FIG. 3, reference numeral 9 is a torque adjusting means for adjusting the torque generated by the engine 2. The torque adjusting means 9 adjusts the intake air amount,
Alternatively, the generated torque is adjusted by combining the number of fuel cut cylinders and the ignition timing adjustment.

Next, an outline of slip control is shown in FIG.
The explanation will be made by also referring to. In FIG. 4, the left drive wheel 1
An example is shown in which the right drive wheel 1FR is not slipped and the right drive wheel 1FR is largely slipped.

Engine Control First, engine control starts with the left and right front wheels 1FL, 1FR.
Of the respective slip values, the larger slip value becomes greater than or equal to a predetermined start threshold value (t in FIG. 4).
1 time point). The engine control is performed by feedback controlling the torque adjusting means 9 so that the actual slip value becomes the engine target value (first target value) STE. The engine control is stopped when the accelerator is fully closed or when the actual slip value reaches the control continuation threshold value SC (smaller than the first target value) for a predetermined time or longer (Fig. 4 from t6 to t7).

Break Control The start of the brake control is when all of the following conditions are satisfied. The first start condition is that the engine is being controlled. The second start condition is the left and right drive wheels 1FL, 1F.
This means that the difference between the actual slip values of R has become equal to or greater than the predetermined value (t2 in FIG. 4). The third start condition is that the vehicle speed is equal to or lower than the predetermined first vehicle speed V1. The fourth start condition is that a predetermined delay time described below has elapsed.

Prior to the start of the brake control, a response delay is expected, and at the same time when the engine control is started, the switching valve 38 is started.
Is boosted, the booster 11 is put into the boosting action state, the hydraulic pressure adjusting valves 15L and 15R are in the relief position, and the opening / closing valves 16L and 16R are closed. Then, after exciting the switching valve 38, when the actual slip value shows a predetermined deceleration (the time until the predetermined deceleration is shown is the delay time, between t2 and t3 in FIG. 4). , Blur
Control is started.

For the brake control, the left and right driving wheels 1FL, 1F are used.
By independently controlling the feedback of the hydraulic pressure adjusting valves 15L and 15R so that the actual slip value becomes the brake target value (second target value) STB (> STE). Is performed (duty control).

The break control is stopped when any one of the following conditions is satisfied. The first stop condition is when the engine control is stopped. The second stop condition is when the vehicle speed becomes a high vehicle speed equal to or higher than a predetermined second vehicle speed V2 (V2> V1). The third stop condition is that the control signals for the left and right hydraulic pressure adjustment valves 15L and 15R both indicate a reduced pressure and for a predetermined time (500 mse in the embodiment).
c) It is when it continues (t4 to t5 in FIG. 4).

The fourth stop condition is the left and right brakes 7F.
This is when the brake fluid pressures of L and 7FR both become zero. The fifth stop condition is when the depression of the brake pedal 12 is detected by one of the switches S6 and S7 (the brake pedal 12 is depressed by the switches S6 and S7). If that is detected, the start of the brake control is prohibited).

When the brake control is stopped, the switching valve 38 is operated as long as the engine control is performed, the hydraulic pressure adjusting valves 15L and 15R are in the relief position, and the opening / closing valve 1 is opened.
6L and 16R are closed (the same state as the standby state until the start of the brake control). The switching valve 38 is demagnetized when the engine control is stopped or when the brake pedal 12 is depressed.

Next, a control example of the present invention will be described with reference to the flow charts of FIG. 5 and subsequent figures. In the following description, P indicates a step. FIG. 5 shows the overall flow. First, at P1, signals from respective sensors are input, and then at P2, the rotational speeds WSRL and WSRR of the left and right drive wheels 1FL, 1FR are averaged, The vehicle speed VS is calculated.

At P3, the first target value STE for engine control and the second target value STB for brake control are determined in a known manner (STB> STE). In P4,
The slip value SFL of the left driving wheel 1FL is the rotational speed W
It is calculated by subtracting the vehicle speed VS from SFL, and similarly, the slip value SFR of the right drive wheel 1FR is calculated by subtracting the vehicle speed VS from its rotation speed WSFR.

In the processes of P5 to P7, the larger slip value of the slip values SFL and SFR of the left and right drive wheels is set as the slip value SA for engine control. In P8, it is determined whether or not the slip flag is currently 1, and when the slip flag is 1, it means that the slip control is being performed (at least the engine control is being performed). When the determination in P8 is NO, after the start determination of engine control, which will be described later, is made in P9,
At 0, the end determination described later is made.

When the determination in P8 is YES, the engine control is performed in P11, and then in P12.
It is determined whether or not the break flag is 1, and when the break flag is 1, it means that the break control is in progress. When the determination in P12 is NO, after the brake start determination described below is performed in P13, the determination in P10 is performed.
Move to. If the determination in P12 is YES, P
After the brake control is performed at 14, the process proceeds to P10.

FIG. 6 shows the contents of P9 in FIG. First,
At P21, the actual slip value SA for the engine
Is greater than or equal to a predetermined value indicating a predetermined start threshold value. If the determination in P21 is NO, it means that the slip control is unnecessary, and therefore the routine is returned to P1 as it is. When the determination in P21 is YES, the slip flag is set to 1 in P22, the engine control is started in P23, and the brake control start is prepared in P24. Specifically, in the preparation at P24, the switching valve 38 is excited to move the master cylinder 8 to the brake.
To generate hydraulic pressure, and to adjust the hydraulic pressure adjustment valves 15L, 1
Setting 5R to the relief position and opening / closing valves 16L and 16
To close R.

FIG. 7 shows the contents of P13 in FIG. First, at P31, it is judged if the vehicle speed VS is equal to or lower than the first vehicle speed V1. If YES in the determination in P31, the rotational speed difference DNL between the left and right drive wheels 1FL and 1FR is calculated in P32, and then in P33.
It is determined whether this difference DNL is greater than or equal to a predetermined value.
If YES in the determination in P33, in P34,
It is determined whether or not a predetermined delay time has elapsed since the above difference occurred (t2 to t3 in FIG. 4). If the determination in P34 is YES, the brake flag is set to 1 in P35, and then in P36, the hydraulic pressure adjusting valve 15L,
The brake control is started by controlling 15R. NO if the name is P31, P33, or P34
In case of, it ends as it is.

FIG. 8 shows the contents of P10 in FIG. First, in P41, it is determined whether or not the accelerator is fully closed. When the determination in P41 is YES, P42
At S43, after the slip control, that is, the engine control and the brake control are both stopped, each flag is reset to 0 at P43.

If NO in P41, it is determined in P45 whether the actual engine control slip value SA is less than or equal to the end threshold SC. This P45
If the determination is YES, it is determined in P46 whether or not the state in which SA is equal to or less than the termination threshold SC continues for 1000 msec. YES in the determination of P46
In the case of, it shifts to P42 and the slip control is stopped.

When the determination in P45 or P46 is NO, it is determined in P47 whether the vehicle speed VS is equal to or higher than the second vehicle speed V2. When the determination in P47 is YES, it is determined again in P48. The brake control is stopped while gradually reducing the brake fluid pressure to prevent slippage, and then the brake flag is reset to 0 at P49. The break control is gradually stopped by reducing the pressure reducing signal to the hydraulic pressure adjusting valves 15L and 15R at predetermined time intervals, but the degree of pressure reduction at each time becomes large. Restrictions are added to prevent it from passing.

When the determination in P47 is NO, it is determined in P50 whether or not the brake hydraulic pressures of the left and right drive wheel brakes 7FL and 7FR have both become zero. This P50
If the determination is YES, the brake control is stopped in P51 and then the process proceeds to P49. The estimation of the brake fluid pressure can be indirectly obtained by various methods already proposed, but a sensor for directly detecting the brake fluid pressure can be used.

When the determination in P50 is NO, it is determined in P52 whether or not the control signals to the hydraulic pressure adjusting valves 15L and 15R both indicate a reduction in pressure.
If the determination is YES, it is determined whether or not the state where both the pressure reduction signals are 0 continues for 500 msec. If the determination in P53 is YES, the brake control is stopped in P51. When the determination in P52 or P53 is NO, the process ends as it is (continuation of engine control and brake control).

Here, the break switch S6 or S
On the other hand, when it is detected that the brake pedal 12 is depressed, the brake control is forcibly stopped by the interrupt process for the flow chart.

An anti-lock brake device is often attached to prevent the wheels from locking when the brake pedal 12 is depressed. In this case, the slip control described above is normally performed. When failing to stop, the anti-lock brake control is performed as it is,
At the time of failure in which the anti-lock break control is not normally performed, the control unit UA for the anti-lock break control outputs a fail signal to the control unit U, as shown in FIG. It is preferable to inhibit slip control.

Although the embodiment has been described above, the slip value is calculated not as the difference between the driving wheel and the driven wheel but as a ratio, for example, as the driving wheel speed / driven wheel speed, or (driving wheel speed-driven wheel speed). ) / Driven wheel speed can also be shown.
Further, as the brake hydraulic pressure center, that is, the hydraulic pressure adjusting means, without using the hydraulic pressure adjusting valves 15L and 15R separately,
The switching valve 38 may also be used as the liquid pressure adjusting valve (for example, the pressure difference between the first chamber 34 and the second chamber 35 is adjusted by performing the duty control of the switching valve 38). Furthermore, in order to generate a pressure difference between the first chamber 34 and the second chamber 35, the relationship between the negative pressure and the atmospheric pressure is not limited, and, for example, the negative pressure and the compressed air, the atmospheric pressure and the compressed air, and the like. A pressure difference may be generated depending on the relationship.

[Brief description of drawings]

FIG. 1 is an overall system diagram showing an example of a vehicle to which the present invention is applied.

FIG. 2 is a cross-sectional view showing a main part of a brake booster.

FIG. 3 is a diagram showing a relationship between inputs and outputs to a control unit.

FIG. 4 is a time chart schematically showing a control example of the present invention.

FIG. 5 is a flowchart showing a control example of the present invention.

FIG. 6 is a flow chart showing a control example of the present invention.

FIG. 7 is a flowchart showing a control example of the present invention.

FIG. 8 is a flowchart showing a control example of the present invention.

[Explanation of symbols]

1FL, 1FR Front wheels (driving wheels) 1RL, 1RR Rear wheel (driven wheel) 2 engine 7FL-7RR Break 8 master cylinder 9 Torque adjusting means 11 Break Buster (Booster) 12 brake pedal 15L, 15R Liquid pressure adjusting valve 16L, 16R open / close valve 34 First Room 35 Room 2 32, 41 movable partition 37, 50 Negative pressure supply route to main unit 38 Switching valve (sub valve device) 45, 46, 47 Main valve device U control unit

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hiroaki Sakamoto             3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda             Within the corporation (72) Inventor Haruki Yata             3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda             Within the corporation

Claims (6)

[Claims] 1. A master cylinder, which is a source of a brake fluid pressure generation for a drive wheel brake, a first chamber and a second chamber defined by a movable partition, and a brake operating member are operated. And a main valve device for switching the gas pressure supply mode so as to generate an atmospheric pressure difference between the first chamber and the second chamber, and a pressure difference between the first chamber and the second chamber. A pneumatic booster that receives the driving force of the movable partition wall and transmits the driving force to the master cylinder; and, in a state where the brake operating member is not operated, the first chamber and the second chamber. A sub-valve device for switching the gas pressure supply mode so as to generate a pressure difference between them; a hydraulic pressure adjusting means for adjusting the brake hydraulic pressure supplied from the master cylinder to the drive wheel brake; Slip detecting means for detecting a slip value with respect to the road surface of the wheel; And a brake control means for controlling the hydraulic pressure adjusting means so that the slip value detected by the slip detecting means becomes a predetermined target value under the condition that the booster is operated. , And the blur
The vehicle slip control device, wherein the control of the hydraulic pressure adjusting device by the control device is set to be performed after a predetermined delay period has elapsed from the start of operation of the sub valve device. 2. The hydraulic pressure adjusting means according to claim 1, wherein the hydraulic pressure adjusting means is a hydraulic pressure adjusting valve connected to a brake hydraulic pressure path from the master cylinder to the drive wheel brake, and during the delay. The hydraulic pressure adjusting valve is maintained in a reduced pressure state for releasing the brake hydraulic pressure of the drive wheel brake. 3. The engine control means according to claim 1, further comprising engine control means for controlling a torque generated by the engine so that a slip value detected by the slip detection means becomes a predetermined engine target value. The operation start time is set to the control start time of the engine control means. 4. The delay period according to claim 3, wherein the delay period is set from the start of control by the engine control means to the start of deceleration of the drive wheels. 5. The negative pressure according to claim 1, wherein a negative pressure is constantly introduced into the first chamber, and the main valve device selectively selects the second chamber as one of the first chamber and the atmosphere. Configured as a switching valve that communicates with the. 6. The sub valve device according to claim 5, wherein the auxiliary valve device is configured as a switching valve that switches between a negative pressure supply mode and an atmospheric supply mode for a negative pressure supply path to the main valve device.