JP3071221B2 - Hydraulic active suspension - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a suspension for a vehicle such as an automobile, and more particularly to a hydraulic active suspension.

[0002]

2. Description of the Related Art As one of hydraulic active suspensions for vehicles such as automobiles, for example, Japanese Patent Application Laid-Open No. 61-19 / 1986.
As described in Japanese Patent No. 3907, an actuator is provided corresponding to each wheel to supply and discharge working fluid to and from the working fluid chamber, thereby increasing and decreasing the vehicle height of a corresponding portion. A hydraulic active suspension including a working fluid supply / discharge unit that supplies and discharges fluid, a longitudinal acceleration detection unit that detects a longitudinal acceleration of the vehicle body, and a control device that controls the working fluid supply / discharge unit in accordance with the longitudinal acceleration of the vehicle body is provided. Already known.

According to such an active suspension, since the supply and discharge of the working fluid to and from the working fluid chamber is controlled in accordance with the longitudinal acceleration of the vehicle body, the vehicle height at a portion corresponding to each wheel is increased or decreased. Pitching such as nose dive and squat of the vehicle body can be positively controlled.

[0004]

However, in the conventional active suspension as described above, the working fluid supply / discharge means is controlled based on the longitudinal acceleration of the vehicle even when the vehicle is stopped, so that the working fluid supply / discharge means is controlled immediately before the vehicle stops. If so-called brake release for reducing the brake oil pressure is not performed, there is a problem that a nose dive and a squat of the vehicle body are repeatedly generated several times.

[0005] Such phenomena occur because (1) control based on the longitudinal acceleration of the vehicle body tends to diverge in the vicinity of several Hz due to the response delay of the control device and the fluid circuit of the active suspension, and (2) The longitudinal acceleration different from the true longitudinal acceleration (differential value of the vehicle speed) of the vehicle body due to the longitudinal compliance of the vehicle is detected, and the working fluid supply / discharge means is controlled based on the detected longitudinal acceleration. It is considered that hunting occurs in the pitching control of the vehicle body due to a factor.

The present invention has been made in view of the above-mentioned problems in a conventional hydraulic active suspension, and has an effect that a nose dive and a squat of a vehicle body are repeatedly generated when braking of a vehicle is stopped without performing a so-called brake release operation. It is an object of the present invention to provide an improved fluid-type active suspension that can be suppressed as much as possible.

[0007]

SUMMARY OF THE INVENTION According to the present invention, as described above, according to the present invention, a working fluid is supplied to and discharged from a working fluid chamber provided for each wheel so that the vehicle height of a corresponding portion can be reduced. Actuator for increasing / decreasing, working fluid supply / discharge means for supplying / discharging working fluid to / from the working fluid chamber, and vehicle speed detecting means
And a braking detection means for detecting a braking state of the vehicle, a longitudinal acceleration detecting means for detecting the longitudinal acceleration of the vehicle body, and a control device for controlling the working fluid supply / discharge means in accordance with the longitudinal acceleration of the vehicle body, The control device operates when the vehicle is in a braking state and
When the vehicle speed is equal to or lower than the reference value, the forward / rearward acceleration of the vehicle is set to 0 assuming that the forward direction of the vehicle is the positive direction of the longitudinal acceleration of the vehicle.
When the vehicle exceeds the braking force, the fluid-type active suspension is configured to control the working fluid supply / discharge means by regarding the longitudinal acceleration of the vehicle body as 0 at least until the braking state of the vehicle is released.

[0008]

Generally, if the forward direction of the vehicle is the positive direction of the longitudinal acceleration of the vehicle body, when the vehicle is in a braking state and the vehicle is decelerated by braking, the longitudinal acceleration of the vehicle body detected by the longitudinal acceleration detecting means is Since the value becomes a negative value, if the longitudinal acceleration of the vehicle body becomes a positive value when the vehicle is in the braking state, it is considered that the longitudinal acceleration of the vehicle body is hunting due to the pitching of the vehicle body. Also braking the vehicle
The pitching of the vehicle body at the time is as fast as when the vehicle stops.
Occurs in very small situations. According to the configuration described above, when the longitudinal acceleration of the vehicle body exceeds 0 when the vehicle is in the braking state and the vehicle speed is equal to or less than the reference value, that is, when the longitudinal acceleration of the vehicle body hunts, Until the braking state of the vehicle is released, the longitudinal acceleration of the vehicle body is regarded as zero and the working fluid supply / discharge means is controlled.

Therefore, even if the longitudinal acceleration of the vehicle body detected by the longitudinal acceleration detecting means is hunted during braking of the vehicle, the control of the working fluid supply / discharge means based on the hunting longitudinal acceleration is avoided. the controlled based on longitudinal acceleration fluid supplying and discharging means for hunting (1)
In addition, it is possible to effectively prevent the nose dive and squat of the vehicle body from being repeatedly generated due to the factors of (2) .

The present invention will be described in detail below with reference to the accompanying drawings.

[0011]

FIG. 1 is a schematic configuration diagram showing a fluid circuit of one embodiment of a hydraulic active suspension according to the present invention.

In FIG. 1, reference numeral 10 denotes a reservoir for storing oil as a working fluid. One end of the connection passage 12 and one end of the working fluid discharge passage 14 are connected to the reservoir 10. The other end of the connection passage 12 is connected to a suction side of a pump 18 driven by the engine 16.
The pump 18 is a variable displacement pump in the illustrated embodiment, and one end of a working fluid supply passage 20 is connected to its discharge side. The other end of the working fluid supply passage 20 and the other end of the working fluid discharge passage 14 are connected to the P port and the R port of a pilot control type 3 port 3 position switching type switching control valve 24 of the pressure control valve 22, respectively. I have. A check valve 15 is provided in the middle of each of the working fluid discharge passages 14 on the pressure control valve 22 side of the communication connection portion 14 a with the working fluid discharge passage from the other wheel. Only the flow of the working fluid from the control valve 22 toward the reservoir 10 is allowed.

The pressure control valve 22 includes a switching control valve 24, a connection passage 26 for connecting the working fluid supply passage 20 to the reservoir 10, and a fixed throttle 28 and a variable throttle 30 provided in the middle of the passage. ing. A of the switching control valve 24
The connection passage 32 is connected to the port. The switching control valve 24 is a spool valve that takes in the pressure Pp in the passage 26 between the fixed throttle 28 and the variable throttle 30 and Pa in the connection passage 32 as pilot pressure. When the pressure Pp is higher than the pressure Pa, the port P To a switching position 24a for connecting the port and the port A, and when the pressures Pp and Pa are equal to each other, a switching position 2a for cutting off the communication of all the ports.
4b, and when Pp is lower than the pressure Pa, the switching position is switched to the switching position 24c where the port R and the port A are connected. The variable throttle 30 changes the effective passage cross-sectional area of the throttle by controlling the current supplied to the solenoid, thereby changing the pressure Pp in cooperation with the fixed throttle 28.

The other end of the connection passage 32 is connected to a working fluid chamber 38 of an actuator 36 provided corresponding to a wheel. As shown, the actuator 36 is a type of cylinder piston device, which is disposed between a suspension member (not shown) supporting the wheels and the vehicle body, and supplies and discharges a working fluid to and from a working fluid chamber 38. As a result, the vehicle height of the corresponding part is increased or decreased. The gas-liquid spring device 4 is provided in the working fluid chamber 38 by a passage 40.
2 is connected, and a throttle 44 is provided in the middle of the passage 40. Thus, the gas-liquid spring device 42 acts as a suspension spring or an auxiliary suspension spring, and the throttle 44 generates a damping force.

In the middle of the connection passage 32, a shutoff valve 46 is provided. The shut-off valve 46 is a pilot pressure control device 48
, The valve is opened when the pilot pressure Pc exceeds a predetermined valve opening value, and is closed when the pilot pressure falls below a predetermined valve closing value. The pilot pressure control device 48 includes a connection passage 50 that connects the working fluid supply passage 20 and the reservoir 10 in communication.
And a fixed throttle 52 and a variable throttle 54 provided in the middle of the passage so that the pressure between the fixed throttle and the variable throttle is supplied to the shut-off valve 46 as the pilot pressure Pc.

A check valve 58 that allows only the flow of the working fluid from the filter 56 and the pump 18 to the pressure control valve 22 is provided in the working fluid supply passage 20. An accumulator 60 is connected to the working fluid supply passage 20 downstream of the check valve 58.

The check valve 15, the pressure control valve 22, the connection passage 32, the throttle 44, the shutoff valve 46, the actuator 36, the gas-liquid spring device 42 and the like are provided corresponding to each wheel.
In FIG. 2, the pressure control valves corresponding to the right front wheel, the left front wheel, the right rear wheel, and the left rear wheel are 22fr, 22fl, 22rr, respectively.
22rl.

The pressure control valve 22 is controlled by an electric control device 66 shown in FIG. The electric control device 66 includes a microcomputer 68. The microcomputer 68 may have a general configuration as shown in FIG. 2, and includes a central processing unit (CPU) 70 and a read only memory (ROM) 72.
, A random access memory (RAM) 74, an input port device 76, and an output port device 78, which are connected to each other by a common bus 80.

The longitudinal acceleration Gy of the vehicle body is detected by the longitudinal acceleration sensor 62 not shown in FIG.
Signal indicating that the forward direction of the vehicle is positive, a vehicle speed sensor 6
3, the signal indicating the vehicle speed V and the brake switch (BKS)
W) A signal indicating whether or not the brake switch is on is input from 64, and a signal related to the running state of the vehicle is input from a group of sensors 65 such as a vehicle height sensor and a lateral acceleration sensor. I have. Input port device 7
6 appropriately processes the signal inputted thereto,
The CPU 70 outputs a processed signal to the CPU and the RAM 74 in accordance with an instruction from the CPU 70 based on a program stored in the CPU 70. The ROM 72 stores the control flow shown in FIGS. The output port device 78 outputs a control signal to the variable throttle 54 of the pilot pressure control device 48 via a drive circuit 84 in accordance with an instruction from the CPU 70, and outputs the control signals to the pressure control valves 22fr and 22fr via drive circuits 86 to 92.
Control signals are output to the variable apertures corresponding to rl, 22rr, and 22rl.

Next, the operation of the illustrated embodiment will be described with reference to the flowcharts shown in FIGS.

The control according to the flowchart of FIG. 3 is started by closing an ignition switch (not shown). 4 and 5, the flag Fst relates to whether or not the vehicle is in a braking state, 1 indicates that the vehicle is in a braking state, and the flag Fh is the longitudinal acceleration of the vehicle body. Gy relates to whether or not the vehicle is in a hunting state, 1 indicates that the longitudinal acceleration is in a hunting state, flag Fa relates to whether or not the braking state of the vehicle has been released, and 1 denotes braking. Indicates that the state has been released.

In step 100, the pilot pressure P
c is gradually increased, whereby the shut-off valve 46 is gradually opened until it is fully opened. If necessary, details of such control are described in, for example, Japanese Patent Application No. 2-1998 filed by the same applicant as the present applicant.
See No. 83.

In step 200, a signal indicating the longitudinal acceleration Gy of the vehicle body detected by the longitudinal acceleration sensor 62, a signal indicating the vehicle speed V detected by the vehicle speed sensor 63, and whether the brake switch 64 is on. Then, a signal indicating the vehicle running state, such as the vehicle height and the lateral acceleration of the vehicle body at the portion corresponding to each wheel detected by the group of sensors 65, is read.

In step 300, as will be described later with reference to the flowcharts shown in FIGS.
The longitudinal acceleration AGy of the control vehicle body used for the active calculation in the next step 400 is calculated based on the actual longitudinal acceleration Gy of the vehicle body.

In step 400, step 200
The active calculation is performed based on the signal read in at step 300 and the longitudinal acceleration AGy calculated at step 300. That is, the pressure control valve 22fr for controlling the target pressure of the working fluid chamber of each actuator for controlling the posture of the vehicle body and the riding comfort of the vehicle according to the running state of the vehicle and the pressure in each working fluid chamber to the target pressure. , 22fl, 22rr, 22rl
The control current supplied to the solenoid of the variable throttle is calculated, and thereafter, the routine proceeds to step 500.

This active calculation does not constitute the gist of the present invention, and is carried out in an arbitrary manner as long as the pressure control valve is controlled in accordance with the longitudinal acceleration of the vehicle so as to reduce or prevent pitching of the vehicle. For example, JP-A-2-17540 filed by the same applicant as the present applicant.
This may be performed as described in US Pat.

In step 500, step 400
The pressure control valve 22fr, 2 based on the control current calculated in
By controlling 2fl, 22rr, and 22rl, the pressure in the working fluid chamber of each actuator is controlled, whereby active control such as vehicle body attitude control and vehicle ride comfort control is executed.

In step 600, it is determined whether or not an ignition switch (IG) (not shown) is off. If it is determined that the IG is not off, step 20 is executed. Then, when it is determined that the IG is off, the control according to the illustrated flowchart ends.

In the above-described step 300, that is, in the first step 302 in the routine for calculating the longitudinal acceleration AGy of the vehicle body for control, it is determined whether or not the flag Fst is 1, and Fst = 1. When it is determined that Fst = 1, the process proceeds to step 320. When it is determined that Fst = 1, the process proceeds to step 304.

In step 304, it is determined whether or not the actual longitudinal acceleration Gy of the vehicle body is equal to or less than G ₀ (a negative constant close to 0), and it is determined that Gy ≦ G ₀ is not satisfied. proceeds to step 316 when it is performed, when the determination indicating that the Gy ≦ G ₀ has been performed, the process proceeds to step 306.

In step 306, when the vehicle speed V is V
It is determined whether or not V is equal to or less than ₀ (a positive constant close to 0). When it is determined that V ≦ V ₀ is not satisfied, the process proceeds to step 316, and the determination that V ≦ V ₀ is determined. If so, go to step 308.

In step 308, it is determined whether or not the brake switch is on. If it is determined that the brake switch is not on, the process proceeds to step 316, where the brake switch is turned on. If it is determined that the switch is on, the process proceeds to step 310.

Thus, in steps 304 to 308, it is determined whether or not the longitudinal acceleration and the vehicle speed of the vehicle body are respectively equal to or less than the corresponding reference values and the brake is operated, that is, whether or not the vehicle is in a braking state. Done.

At step 310, the count value Cst of the counter relating to the time of the braking state of the vehicle is incremented by one, and thereafter the routine proceeds to step 312.

In step 312, the count value Cst
Is determined to be greater than or equal to N ₁ (a positive integer),
Proceeds to step 318 when the has been made determination to the effect not Cst ≧ N _1, discrimination to the effect that Cst ≧ N _1, i.e., when the vehicle is the determination of the effect on the braking state is carried out at _least once N to step 314 move on.

In step 314, the flag Fst is set to 1
, And then the process proceeds to 316.

In step 316, the count value Cst
Is reset to 0, and then the process proceeds to 318.

In step 318, the longitudinal acceleration AGy for control is set to the actual longitudinal acceleration Gy, and thereafter the routine proceeds to step 400 in FIG.

In step 320, the flag Fh is set to 1
Is determined, the process proceeds to step 326 when it is determined that Fh = 1, and the process proceeds to step 322 when it is determined that Fh is not 1.

In step 322, it is determined whether or not the actual longitudinal acceleration Gy exceeds 0, that is, the actual longitudinal acceleration of the vehicle body is a positive value even though the vehicle is in the braking state. Therefore, it is determined whether or not hunting has occurred. When it is determined that Gy> 0, the process proceeds to step 318, and when it is determined that Gy> 0, the process proceeds to step 324.

In step 324, the flag Fh is set to 1
, And then the process proceeds to step 342.

In step 326, the flag Fa is set to 1
Is determined, that is, whether or not the braking state of the vehicle is released. When it is determined that Fa = 1, the process proceeds to step 344, and the determination that Fa = 1 is not performed. If so, go to step 328.

In step 328, the count value C of the counter relating to the time after the braking state of the vehicle is released is set.
a is incremented by one, and then the process proceeds to step 330.

In step 330, the count value Ca is calculated.
Is determined to be greater than or equal to N ₂ (positive integer),
When it is determined that Ca ≧ N ₂ is not satisfied, the process proceeds to step 342, and when it is determined that Ca ≧ N ₂ , the process proceeds to step 332.

In step 332, the count value Ca is calculated.
Is decremented by one, and then the process proceeds to step 334. Note that this step requires time after the determination of yes in step 330 is made until the determination of yes is made in the next step 334, so that the count value Ca
Is for avoiding becoming excessive, and may be omitted.

In step 334, it is determined whether or not the brake switch has been switched off, that is, whether or not the operation of the brake has been released, and it is determined that the brake switch has not been switched off. If the determination is made that the brake switch has been switched off, the process proceeds to step 342.
Proceed to.

In step 336, the longitudinal acceleration AGy (= 0) for control and the actual longitudinal acceleration Gy are obtained.
Is set to Gy, and then the routine proceeds to step 338.

In step 338, the flag Fa is set to 1
, And then the process proceeds to step 340.

In step 340, the count value Ca is calculated.
Is reset to 0, and then the process proceeds to step 342.

In step 342, the longitudinal acceleration AGy for control is set to 0, and thereafter, in step 4 in FIG.
Go to 00.

[0051] Te is At a step 344, is performed absolute value of whether or not D ₀ (small positive constant) following determination of the deviation Df of the longitudinal acceleration, | Df | determine the effect that ≦ D ₀ When is performed, the process proceeds to step 354, where | Df | ≦ D
_When it is determined that it is not ₀ , step 346 is executed.
Proceed to.

In step 346, it is determined whether or not the deviation Df is positive. If it is determined that Df> 0, the process proceeds to step 350, where it is determined that Df> 0. When the determination is made, the process proceeds to step 348.

In step 348, the deviation Df is set to Dc.
(Small positive constant) The value is decremented, and thereafter, the flow proceeds to step 352.

In step 350, the deviation Df is set to Dc.
The value is incremented, and the process proceeds to step 352.

In step 352, the longitudinal acceleration AGy for control is set to the sum of the actual longitudinal acceleration Gy and the deviation Df, and thereafter the routine proceeds to step 400 in FIG.

Thus, in steps 344 to 352, control is performed so that the longitudinal acceleration AGy for control after the braking state of the vehicle is released gradually becomes an actual longitudinal acceleration Gy from zero.

In step 354, all the flags F
st, Fh, and Fa are reset to 0, and thereafter, the flow proceeds to step 356.

In step 356, the longitudinal acceleration AGy for control is set to the actual longitudinal acceleration Gy, and thereafter the routine proceeds to step 400 in FIG.

Thus, according to the illustrated embodiment, in steps 304 to 308, the vehicle is in the braking state and the vehicle speed is reduced.
It is determined whether or not the vehicle speed is equal to or less than the reference value . In these steps, the vehicle is in the braking state and the vehicle speed is equal to or less than the reference value.
When a determination is that takes place is, steps 320 and 32
It is determined whether or not hunting has occurred in the longitudinal acceleration of the vehicle body by determining whether or not the longitudinal acceleration Gy of the vehicle body detected in step 2 is positive. When a determination is that the hunting before <br/> after acceleration has occurred due to the pitching in the vehicle body during braking of the vehicle At a these steps are performed, the braking state of the vehicle At a step 326 to 334 Until it is determined that the acceleration has been released, the longitudinal acceleration sensor 6
The longitudinal acceleration AGy of the vehicle body for control is set to 0 regardless of the actual value of the longitudinal acceleration Gy of the vehicle body detected by 2.

FIG. 6 is a time chart showing the operation of the illustrated embodiment during braking of the vehicle. In FIG. 6, t ₁
Shows the time when occurred hunting longitudinal acceleration Gy, t ₂ denotes the time when the acceleration is started, t ₃ indicates the time when the brake switch is switched off.

Therefore, if hunting occurs in the longitudinal acceleration of the vehicle body when the vehicle is in the braking state, the pressure control valve 22 is controlled assuming that the longitudinal acceleration of the vehicle body is 0 until the braking state is released. The pressure in the working liquid chamber 38 of the actuator 36 is controlled based on the longitudinal acceleration of the hunting vehicle body, and the occurrence of repeated nose dives and squats of the vehicle body due to this is effectively suppressed.

FIGS. 7 and 8 are graphs showing specific examples of the longitudinal acceleration Gy of the vehicle body and the change of the pitch angle of the vehicle body in the illustrated embodiment and the conventional active suspension, respectively. From the comparison of these figures, it can be seen that in the conventional active suspension, the back-and-forth swing of the vehicle body is two times.
It can be seen that, in the illustrated embodiment, the vehicle body swings back and forth in the front-rear direction only once, whereas half-turn occurs.

According to the embodiment shown in FIG.
When it is determined in steps 6-334 that the braking state of the vehicle has been released, in steps 344-352, the longitudinal acceleration AGy of the control vehicle body gradually decreases from 0 to the actual vehicle longitudinal acceleration Gy. Therefore, unnatural pitching of the vehicle body, for example, when the vehicle resumes acceleration after the braking state of the vehicle is released, is reliably prevented.

In the above-described embodiment, the means for controlling the pressure in the working fluid chamber of each actuator is a pressure control valve, but this means may be a flow control valve.

Although the present invention has been described in detail with reference to specific embodiments, the present invention is not limited to such embodiments, and various other embodiments may be made within the scope of the present invention. The possibilities will be clear to the skilled person.

[0066]

As is apparent from the above description, according to the present invention, the vehicle is in the braking state and the vehicle speed is lower than the reference value.
When the front direction of the vehicle is the positive direction of the longitudinal acceleration of the vehicle body when it is below, if the longitudinal acceleration of the vehicle exceeds 0 and becomes a positive value, in other words, the pitching of the vehicle body once occurs
At least until the vehicle is released from the braking state, the working fluid supply / discharge means is controlled by assuming the longitudinal acceleration of the vehicle to be zero, so that the longitudinal acceleration of the vehicle detected by the longitudinal acceleration detecting means during braking of the vehicle is controlled. Even if the acceleration is hunting, the working fluid supply / discharge means is prevented from being controlled based on the hunting longitudinal acceleration, whereby the working fluid supply / discharge means is controlled based on the hunting longitudinal acceleration. Nose dive and squat can be effectively prevented from occurring repeatedly.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a fluid circuit of one embodiment of a hydraulic active suspension according to the present invention.

FIG. 2 is a block diagram showing the electric control device of the embodiment shown in FIG.

FIG. 3 is a flowchart showing a general flow of control achieved by the electric control device shown in FIG. 2;

FIG. 4 is a flowchart showing a part of a calculation routine of a longitudinal acceleration of a control vehicle body during a general flow shown in FIG. 3;

FIG. 5 is a flowchart showing a remaining part of a routine for calculating the longitudinal acceleration of the control vehicle body during the general flow shown in FIG. 3;

FIG. 6 is a time chart showing the operation of the illustrated embodiment.

FIG. 7 is a graph showing a specific example of changes in the longitudinal acceleration of the vehicle body and changes in the pitch angle of the vehicle body in the hydraulic active suspension of the present invention.

FIG. 8 is a graph showing specific examples of changes in the longitudinal acceleration of the vehicle body and changes in the pitch angle of the vehicle body in the conventional hydraulic active suspension.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Reservoir 16 ... Engine 18 ... Pump 20 ... Working fluid supply path 22 ... Pressure control valve 32 ... Connection passage 36 ... Actuator 38 ... Working fluid chamber 42 ... Gas-liquid spring device 44 ... Restrictor 46 ... Shut-off valve 48 ... Pilot pressure control Device 60 Accumulator 62 Front-rear acceleration sensor 63 Vehicle speed sensor 64 Brake switch 65 A group of sensors 66 Electric controller 68 Microcomputer 70 CPU 72 ROM 74 RAM 76 Input port device 78 Output port apparatus

Claims (1)

(57) [Claims] An actuator provided for each wheel to supply / discharge a working fluid to / from a working fluid chamber to increase / decrease a vehicle height at a corresponding portion, and to supply / discharge the working fluid to / from the working fluid chamber. Working fluid supply / discharge means and vehicle speed detection means
And a braking detection means for detecting a braking state of the vehicle, a longitudinal acceleration detecting means for detecting the longitudinal acceleration of the vehicle body, and a control device for controlling the working fluid supply / discharge means in accordance with the longitudinal acceleration of the vehicle body, The control device operates when the vehicle is in a braking state and
When the vehicle speed is equal to or lower than the reference value, the forward / rearward acceleration of the vehicle is set to 0 assuming that the forward direction of the vehicle is the positive direction of the longitudinal acceleration of the vehicle.
A hydraulic active suspension configured to control the working fluid supply / discharge means by assuming the longitudinal acceleration of the vehicle to be zero at least until the braking state of the vehicle is released.