JPH06312655A - Control for automatic brake device - Google Patents

Abstract

PURPOSE:To improve controllability and stability by increasing and correct the duty ratio of compressive/decompressive control to the viscosity change in fluid with accumulator pressure or brake oil temperature in an automatic brake device for constant fluid pressure of compressive/decompressive control. CONSTITUTION:A decompressive solenoid valve 30 (40) is disposed between a master cylinder 14 and a wheel cylinder 17 in a vehicle brake pipe 15. An accumulator 29 which always accumulates high pressure is communicated with the wheel cylinder 17 through a compressive solenoid valve 32 (42). When the controller foresees a possibility of collision at an automatic brake mode, a close signal is outputted to the decompressive solenoid valve 30, and a duty control signal is outputted to the compressive solenoid valve 32. With this constitution, the lower the accumulator pressure and the brake fluid temperature detected by sensors 37, 56 are respectively, the larger the correction value is calculated to be, and at least compressive duty ratio is increased to be corrected by the correction value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control method for an automatic braking device which is used in a driving assistance system (ADA) of a vehicle such as an automobile and automatically brakes. Regarding

[0002]

2. Description of the Related Art In recent years, as a measure to dramatically improve the safety of a vehicle, a technique for avoiding a collision, that is, an operation system is automatically controlled to a safety side as compared with a technique for reducing a collision damage such as an airbag. A comprehensive driving support system (ADA / Active Drive Assist system) has been developed to avoid a collision. This ADA system automatically adjusts the control unit that recognizes the road condition, the external environment of other vehicles, obstacles, etc. three-dimensionally by the cameras mounted in front of and behind the vehicle, and the operation system of the brake, throttle, and steering automatically. It is configured to be capable of autonomous traveling by including various devices that are operated on. Then, as functions aimed at by the ADA system, a collision prevention function in the case of unsafe driving operation, and an automatic driving function acting on behalf of the driving operation so as to keep a safe inter-vehicle distance in the case of asleep etc. are considered.

Therefore, the automatic braking device of the ADA system is required to have a function of surely decelerating or stopping the vehicle in order to avoid a collision or the like. Here, in order to obtain the maximum deceleration of 1 G, it is necessary to quickly generate a brake pressure of, for example, 100 kg / cm ² . For this reason, the brake hydraulic control system has an accumulator in which the pressurizing source stores a high pressure of, for example, 150 to 180 kg / cm ² in consideration of pressure loss, and when the risk of collision is predicted, the accumulator pressure is immediately changed to the wheel. It is configured so that it can be introduced into a cylinder to apply pressure braking with good response. Further, as a means for electrically controlling the accumulator pressure or for electrically reducing the high brake pressure, a method of controlling by using an ON-OFF type pressure solenoid valve and pressure reduction solenoid valve has been proposed. ing.

Since the accumulator pressure is maintained within the predetermined range as described above, the pressure difference between the wheel cylinder and the upper limit pressure and the lower limit pressure changes. Further, since the viscosity of the brake oil changes due to the influence of temperature or the like, the hydraulic pressure gradient fluctuates even if the solenoid valve is operated by the same duty signal. That is, when the accumulator pressure is low,
When the oil viscosity is high at low temperature, the hydraulic pressure gradient becomes gentle and a delay in pressurization occurs. For this reason, it is required to compensate the fluctuation of the hydraulic pressure gradient with respect to the accumulator pressure and the oil viscosity.

Conventionally, in the pressure control in the case where a pressure source such as an accumulator is provided, the accumulator pressure is maintained within a predetermined range of an upper limit pressure and a lower limit pressure by a motor pump operation by a pressure switch or the like. Then, the solenoid valve is configured to operate at a duty ratio that is preset according to modes such as slow pressure increase and rapid pressure increase.

[0006]

By the way, in the above-mentioned prior art, since the accumulator pressure is set by the pressure switch, the accumulator pressure can be properly set in a wide range or the accumulator pressure can be set. It cannot be corrected. Since the fluctuations in accumulator pressure and oil viscosity are not taken into consideration, there is a problem that the hydraulic pressure gradient fluctuates depending on the usage state of the accumulator pressure and the temperature change of the oil, and it is not possible to always perform optimum pressurization control.

In view of the above, the present invention has an object of improving the controllability and the like by making the hydraulic pressure gradient of the pressurizing / depressurizing control constant in the automatic braking device provided with the accumulator.

[0008]

In order to achieve this object, the present invention provides a pressure reducing solenoid valve between a master cylinder and a wheel cylinder of a brake pipe of a vehicle, and as a pressurizing source, a motor pump is always driven to a predetermined pressure. Has an accumulator that accumulates accumulator pressure in the range of the lower limit pressure and the upper limit pressure.This accumulator communicates with the wheel cylinder via the pressurizing solenoid valve, and predicts the risk of collision when the automatic brake control unit is in the automatic brake mode. Then, the close signal is output to the pressure reducing solenoid valve and the duty signal is output to the pressure increasing solenoid valve. When the risk of collision disappears, the close signal is output to the pressure increasing solenoid valve and the duty signal is output to the pressure reducing solenoid valve. In the automatic braking system, the lower the accumulator pressure and the brake fluid temperature, the more The amount was large calculation, characterized by increasing correcting at least pressure duty ratio by the correction amount.

[0009]

In the present invention based on the above method, when the vehicle is in the automatic brake mode, the automatic brake control unit electrically determines the relationship between the own vehicle and the preceding vehicle. If a collision risk is predicted, the pressure solenoid valve opens with the duty signal, and the pressure is controlled so that the accumulator pressure is introduced into the wheel cylinders and the brake pressure is quickly raised, and the vehicle automatically brakes. Then the collision is avoided. When the danger of collision disappears, the pressure reducing solenoid valve is opened by the duty signal, and the brake pressure is controlled so as to reduce the brake pressure of the wheel cylinder to release the brake.

In addition, the accumulator pressure changes low,
When the oil viscosity is highly changed by the brake oil temperature and the oil does not flow easily, the pressurization duty ratio is increased and corrected by the correction amount, so that the oil flow due to the accumulator pressure becomes the same. Therefore, the hydraulic pressure gradient of the pressurization control has a constant pressurizing characteristic, and it is possible to always stably and favorably perform the automatic brake operation.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. An outline of the vehicle drive system and the brake hydraulic pressure control system will be described with reference to FIG. First, the vehicle is the left and right front wheels 1
It has L, 1R and rear wheels 2L, 2R, and the engine 3 as a drive system is connected to the left and right rear wheels 2L, 2R via the clutch 4, the transmission 5, the propeller shaft 6, the differential device 7, and the axle 8. Composed of transmission. The throttle valve 10 of the engine 3 is provided with an actuator 11 having a throttle control unit 12 so as to perform throttle control during traction control, cruise control, automatic brake control and the like.

Explaining the brake hydraulic pressure control system, a master cylinder 14 having a brake pedal 13 will be described.
As a diagonal two-system brake pipe 15, a P-system brake conduit 16 communicates with the wheel cylinders 17 of the right front wheel 1R and the left rear wheel 2L, and an S-system brake conduit 18 connects the left front wheel 1L and the right rear wheel 2R. It is communicated with the wheel cylinder 17. Therefore, the brake lines 16 and 18 for the P system and the S system
The automatic brake hydraulic unit 21 of the automatic brake device 20 is provided in the middle of the
The signal is used to automatically brake.
When the ABS device is installed, the ABS hydraulic unit is connected in series to the downstream side of the automatic brake hydraulic unit, and it is possible to prevent wheel lock during automatic brake operation.

The automatic brake hydraulic unit 21 will be described with reference to FIG. Brake piping 15 with two diagonal lines
The P-type brake pipeline 16 will be described. The P-type brake pipeline 16 is provided with a pressure reducing solenoid valve 30 and a relief valve 31. Further, as a pressurizing source 34, a pipe line 23 from the oil reservoir 22 of the master cylinder 14 communicates with an oil pump 24 so as to generate a hydraulic pressure by driving a motor 25. The discharge side conduit 26 of the oil pump 24
Communicates with the relief valve 28 via the check valve 27,
The accumulator 29 communicates with the accumulator 29 so that pressure can be accumulated, and further communicates with the P-system brake pipe 16 via a pressure solenoid valve 32. Similarly, a pressure reducing solenoid valve 40 and a relief valve 41 are also provided in the S system brake pipe 18, and a pipe line 44 branched from the pipe line 26 has a pressurizing solenoid valve 42 and communicates with the S system brake pipe line 18. .

A hydraulic pressure sensor 37 for detecting the accumulator pressure Pa is provided in the pipe line 26, and is controlled so that the motor 25 is driven at the lower limit pressure and the motor 25 is stopped at the upper limit pressure. With this motor control, the oil pump 24 is operated to constantly accumulate a high pressure in the range of the upper limit pressure and the lower limit pressure in the accumulator 29, and the accumulator pressure Pa can pressurize the brake pressure Pb with good response. A hydraulic pressure sensor 36 for detecting a pedal pressure and a hydraulic pressure sensor 33 for detecting a P system brake pressure are provided in the pipeline 16, and a hydraulic pressure sensor 43 for detecting an S system brake pressure is provided in the pipeline 18.

The pressure-reducing solenoid valves 30 and 40 are normally open, and open when the duty signal is turned off to allow normal braking action. At the same time, the pressure is reduced when the automatic brake is activated and closed when turned on. Pressure solenoid valve 3
Reference numerals 2 and 42 are normally closed, which are closed when the duty signal is OFF, opened when ON, and pressurized when the automatic brake is activated.

The automatic brake control unit 60 will be described with reference to FIG. First, CCDs that are arranged in front of and behind the vehicle to recognize road conditions and obstacles and obtain further distance images
It has a camera 50, a vehicle speed sensor 51 for detecting a vehicle speed, an automatic brake selection switch 52, and a brake switch 53 for detecting a brake operation state. The signals of these sensors and switches and the signals of the actual brake pressure Pb of the hydraulic pressure sensors 33, 43 are input to the automatic brake control unit 60 and electrically processed.

The automatic brake control unit 60 is a CCD
It has a camera 50, an automatic brake selection switch 52, and a rear-end collision prediction unit 61 to which signals from the vehicle speed sensor 51 are input. When the vehicle travels in the automatic brake mode in which the selection switch 52 is turned on, the distance between the host vehicle and the preceding vehicle and the relative speed are calculated based on the information from the CCD camera 50 and the vehicle speed V, and the risk of a rear-end collision is calculated. Predict the presence or absence of. When the danger of a rear-end collision is predicted, an approach warning signal is output to the alarm 55. Further, the throttle opening sensor 54 and the signal for predicting a rear-end collision are input to the throttle close determination unit 62, and when the rear-side collision is predicted and the throttle opening is equal to or larger than a set value, a throttle close signal is output to the throttle control unit 12.

A signal for predicting a rear-end collision is input to a required deceleration calculating section 63, and a necessary deceleration G for keeping a safe inter-vehicle distance and avoiding a rear-end collision is calculated from the inter-vehicle distance and relative speed. The required deceleration G and the signal from the brake switch 53 are input to the target brake pressure setting unit 64 to set the target brake pressure Pt according to the required deceleration G when the brake switch is OFF. The target brake pressure Pt and the actual brake pressure Pb of the hydraulic pressure sensors 33 and 43 are input to the deviation calculating section 65 to obtain the deviation ± ΔP between them. This deviation ± ΔP is input to the dead zone determination unit 66 and compared with the dead zone ± α. If the deviation ± ΔP is within the dead zone ± α, the ON / OFF output unit 67 sends an OFF signal to the pressurizing solenoid valves 32, 42. An ON signal is output to the pressure reducing solenoid valves 30 and 40 to control the brake pressure to be maintained at a predetermined level.

When the deviation ± ΔP is outside the dead zone ± α,
The deviation ± ΔP is input to the control amount calculation unit 68, pressurization or depressurization is determined by a positive or negative sign, and the control amount C corresponding to the deviation ± ΔP is determined by PD control, for example. In the case of pressurization, the ON / OFF output unit 67 outputs an ON signal to the pressure reducing solenoid valves 30 and 40. Then, the control amount C is input to the duty ratio determining unit 69 and converted into a predetermined pressurizing duty ratio Du, and this duty signal is output to the pressurizing solenoid valves 32, 42 at a predetermined time interval by the output unit 70. In the case of depressurization, the ON / OFF output section 67 outputs an OFF signal to the pressurizing solenoid valves 32 and 42.
Then, the control amount C is input to the duty ratio determining unit 71 to be converted into a predetermined pressure reducing duty ratio Dd, and this duty signal is output to the pressure reducing solenoid valves 30 and 40 at a predetermined time interval by the output unit 72 so as to output the brake pressure. Configured to control.

In the control system described above, a compensation measure when the accumulator pressure or the oil viscosity changes will be described. First, the hydraulic pressure sensor 37 for detecting the accumulator pressure Pa and the oil temperature sensor 56 for detecting the brake oil temperature Tb are provided, and these accumulator pressure Pa and brake oil temperature Tb are provided.
Is input to the correction amount calculation unit 73 of the automatic brake control unit 60 to calculate the correction amount Dr. Here, when the accumulator pressure Pa is high near the upper limit pressure and the brake oil temperature Tb is further heated by the engine or the like and the oil viscosity is low, the oil flows most easily, and in this case, the reference value Do of the shortest ON time becomes To do. Further, when the accumulator pressure Pa is low near the lower limit pressure, or when the brake oil temperature Tb is low, such as immediately after the engine is started in the winter, it becomes difficult for the oil to flow. In this case, therefore, the correction may be made longer than the reference value Do. .

Therefore, the correction coefficient Kp for the accumulator pressure Pa and the correction coefficient Kt for the brake fluid temperature Tb are set to determine the map as shown in FIG. And the correction amount D
Calculate r as follows: Dr = Do + Kp + Kt (Kp, Kt> 0, Kp <K
t) This correction amount Dr is input to the duty ratio determining unit 69,
The duty ratio Dc for the control amount C and the correction amount Dr are added. Then, the pressurizing duty ratio Du is calculated and corrected by Du = Dc + Dr. Further, in the pressure reducing control, only the correction coefficient Kt for the brake oil temperature Tb is used, and the correction amount Dr
Is calculated by Dr = Do + Kt. The correction amount Dr is input to the duty ratio determining unit 71, and the decompression duty ratio Dd is calculated by Dd = Dc + Dr and is corrected.

Next, the operation of this embodiment will be described. First, when the engine is in operation, the pressure reducing solenoid valves 30 and 40 in the brake control system are turned off and open, and the master cylinder 14 communicates with the wheel cylinders 17 of the front and rear wheels 1L, 2L ,. Therefore, when the vehicle is stopped or running,
By operating the brake pedal 13 normally, it is possible to generate the brake pressure Pb in the wheel cylinder 17 and perform braking. The pressurizing solenoid valves 32 and 42 are OF
Closed at F, the pressure source 34 is disconnected from the wheel cylinder 17. In the pressure source 34, the pump 24 is operated by the motor control by the hydraulic pressure sensor 37, and the accumulator 29 is operated.
A high accumulator pressure Pa is constantly accumulated in the.

When the driver turns on the automatic brake selection switch 52 while the vehicle is traveling, the automatic brake mode is set. Therefore, in the automatic brake control unit 60, the CCD
When the distance between the host vehicle and the preceding vehicle or the obstacle and the relative speed are detected based on the information of the camera 50 and the vehicle speed and these values are equal to or less than the set values and the danger of a rear-end collision is predicted, an approach warning is given by an alarm 55. At this time, when the driver normally operates the brake and the brake switch 53 is turned on, the automatic brake mode is released.

On the other hand, when the brake is not operated, the target brake pressure Pt is set high in accordance with the required deceleration G based on the inter-vehicle distance to the preceding vehicle and the relative speed, and the target brake pressure Pt and the actual brake system brake. The deviation ΔP from the pressure Pb is calculated, and the pressurization is determined. Therefore, as shown in FIG. 5, the pressure reducing solenoid valves 30 and 40 are immediately closed by turning on, and the wheel cylinder 17 is disconnected from the master cylinder 14. At this time, the control amount C corresponding to the deviation ΔP is calculated, and the duty ratio Dc corresponding to the control amount C is set. Further, according to the map of FIG. 4, the reference value Do and the correction coefficients Kp, K for the accumulator pressure Pa and the brake fluid temperature Tb are set.
The correction amount Dr is calculated from t, and the duty ratio Dr corresponding to the correction amount Dr is set. Then, the duty ratios Dc and Dr are added to set the pressurizing duty ratio Du, and this duty signal is applied to the pressurizing solenoid valves 32 and 42.
Then, as shown in the figure, the pulse is turned on and opened. Therefore, the accumulator pressure Pa of the pressurizing source 34 is introduced into the wheel cylinder 17 and is quickly pressurized, and therefore the brake pressure Pb follows the target brake pressure Pt with good response as shown in FIG. Feedback is controlled so that it rises at.

Here, when both the accumulator pressure Pa and the brake oil temperature Tb are high and the oil flows most easily, the correction amount Dr becomes only the reference value Do of the shortest ON time, and the pressurizing duty ratio Du is corrected to the minimum. It When oil becomes difficult to flow due to a decrease in accumulator pressure Pa or brake oil temperature Tb, the pressurization duty ratio Du is increased and corrected by the correction coefficient Kp or Kt. If both the accumulator pressure Pa and brake oil temperature Tb are low, both correction coefficients are corrected. Kp and K
By t, the pressurizing duty ratio Du is increased most and is corrected so as to increase the oil flow rate. Therefore, even when the accumulator pressure Pa or the oil viscosity changes, the accumulator pressure P
The oil flow due to a is the same, and the hydraulic pressure gradient m during pressurization control has a constant pressurizing characteristic.

The target brake pressure Pt and the brake pressure P
When b becomes substantially equal and the deviation ΔP between them enters the dead zone ± α, the pressurizing solenoid valves 32 and 42 are also turned off as shown in FIG.
Close with. Therefore, the brake pressure Pb of the wheel cylinders 17 is kept constant by the containment, and even if the brake pressure Pb fluctuates to some extent, the dead zone ± α limits the brake pressure feedback control to maintain a comfortable ride. By the above pressure control and holding control, the front and rear wheels 1L, 2
L, ... Are automatically braked, whereby the vehicle decelerates or stops at the same speed as the required deceleration G, and a rear-end collision with a preceding vehicle is avoided in advance.

When the relative distance or relative speed to the preceding vehicle increases due to the automatic brake control and the risk of a rear-end collision disappears, the target brake pressure Pt is set low. Therefore, in this case, the pressure reduction is judged, and the pressurizing solenoid valves 32 and 42 are kept OFF and closed as shown in FIG. At this time, the decompression duty ratio Dd is set by the duty ratio Dc of the control amount C corresponding to the deviation −ΔP between the target brake pressure Pt and the actual brake pressure Pb and the correction amount Dr similar to the above, and this duty signal is reduced. Solenoid valve 30, 40
Then, it is turned off like a pulse and opened as shown in the figure. Therefore, the brake pressure Pb of the wheel cylinder 17 gradually escapes to the oil reservoir 22 and is reduced with a stepwise characteristic as shown in FIG. 6 to release the brake. In this case, when the brake oil temperature Tb is low, the pressure reduction duty ratio Dd is increased and corrected by the correction coefficient Kt, so that the brake pressure P
The oil flow due to b is the same. Therefore, the hydraulic pressure gradient n in the case of pressure reduction control is also made constant, and the brake is always released in the same manner.

Another embodiment of the present invention will be described with reference to FIG. In this embodiment, the accumulator pressure Pa is input to the motor control unit 81 of the motor control unit 80.
When the brake oil temperature Tb is input to the pressure setting unit 82 and the oil temperature Tb is high, the upper and lower limit pressure range is, for example, 150 to
When the oil temperature Tb is low and the oil viscosity is high, the upper and lower pressure limits are set to, for example, 170 to ²⁰ when set to 180 kg / cm ^2.
Increase to 0 kg / cm ² . The motor control unit 81 compares the accumulator pressure Pa with the upper and lower limit pressure ranges, outputs an operation signal to the motor 25 to drive when the pressure falls below the lower limit pressure, and outputs a stop signal to stop the motor 25 when the upper limit pressure is reached. To be configured.

Therefore, when the brake fluid temperature Tb is low and the oil does not flow easily, the accumulator pressure Pa is corrected to be high and the pressure difference between the accumulator 29 and the wheel cylinder 17 becomes large. Therefore, the pressurizing solenoid valves 32, 4
With the pressurizing duty ratio Du of 2, the oil flows in the same manner, and the pressurizing characteristic has a constant hydraulic pressure gradient.

The embodiment of the present invention has been described above, but the present invention is not limited to this.

[0031]

As described above, according to the present invention,
In an automatic brake device equipped with an accumulator, the duty ratio of the pressure increase / decrease control is increased and corrected for changes in the oil viscosity due to changes in accumulator pressure and brake oil temperature, so the hydraulic pressure gradient in the pressure increase / decrease control can be made constant. it can. For this reason, the delay in pressurization when oil is difficult to flow is improved, the delay in automatic brake operation is reduced, and controllability and stability are improved. Further, when the oil flows easily, the duty ratio can be set to the shortest, so that it is possible to finely control the brake pressure by reducing the pressurization amount and the depressurization amount, and the riding comfort is improved.

According to another embodiment of the present invention, since the upper and lower limit pressures of the accumulator pressure are increased and corrected by the brake oil temperature, it is possible to compensate for the change in oil viscosity due to the brake oil temperature, and the structure is simple. Become. Since the accumulator pressure increases only when the brake oil temperature is low, oil hammering is less likely to occur.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an electric control system suitable for a control method for an automatic braking device according to the present invention.

FIG. 2 is a hydraulic circuit diagram showing a hydraulic unit of the automatic brake device.

FIG. 3 is a diagram schematically showing a vehicle drive system and a brake hydraulic pressure control system.

FIG. 4 is a diagram showing a correction map for accumulator pressure and brake fluid temperature.

FIG. 5 is a diagram showing control of pressurization, holding, and depressurization.

FIG. 6 is a diagram showing a characteristic of brake pressure.

FIG. 7 is a block diagram showing an electric control system of another embodiment of the present invention.

[Description of symbols] 14 master cylinder 15 brake pipe 17 wheel cylinder 20 automatic brake device 21 automatic brake hydraulic unit 29 accumulator 34 pressurizing source 30, 40 pressure reducing solenoid valve 32, 42 pressurizing solenoid valve 37 hydraulic sensor 56 oil temperature sensor 60 Automatic brake control unit, 65 deviation calculation unit 68 control amount calculation unit 69, 71 duty ratio determination unit 73 correction amount calculation unit

Claims (3)

[Claims] 1. A pressure reducing solenoid valve is provided between a master cylinder and a wheel cylinder of a vehicle brake pipe, and an accumulator that constantly accumulates an accumulator pressure within a predetermined lower limit pressure and upper limit pressure by driving a motor pump as a pressure source. This accumulator is connected to the wheel cylinder via the pressurizing solenoid valve, and when the automatic brake control unit predicts the risk of collision when in the automatic brake mode, it outputs a close signal to the pressure reducing solenoid valve and pressurizes it. When the duty signal is output to the solenoid valve and the danger of collision disappears, in the automatic brake device that outputs the close signal to the pressurizing solenoid valve and the duty signal to the depressurizing solenoid valve, the lower the accumulator pressure and the brake fluid temperature, Calculate a large correction amount, and use this correction amount to add at least A method for controlling an automatic brake device, comprising increasing and correcting a pressure duty ratio. 2. The correction amount is calculated by a reference value when the accumulator pressure is high and the brake oil temperature is also high, and a correction coefficient which is set in accordance with a decrease in the accumulator pressure and the brake oil temperature. The method for controlling the automatic braking device according to claim 1. 3. A decompression solenoid valve is provided between a master cylinder and a wheel cylinder of a vehicle brake pipe, and an accumulator that constantly accumulates accumulator pressure within a predetermined lower limit pressure and upper limit pressure by driving a motor pump as a pressure source. This accumulator is connected to the wheel cylinder via the pressurizing solenoid valve, and when the automatic brake control unit predicts the risk of collision when in the automatic brake mode, it outputs a close signal to the pressure reducing solenoid valve and pressurizes it. When the duty signal is output to the solenoid valve and the risk of collision disappears, in an automatic brake device that outputs a close signal to the pressurizing solenoid valve and a duty signal to the depressurizing solenoid valve, the lower the brake fluid temperature, the higher the accumulator pressure. Set the upper and lower pressure limits to a high value, and use the upper and lower pressure limits and the hydraulic sensor to detect A method for controlling an automatic braking device, characterized in that a motor pump is driven by the accumulator pressure that is output.