A slow active hydraulic suspension system based on a reversing valve and a control method thereof

Abstract

The application discloses a slow active hydraulic suspension system based on a reversing valve and a control method thereof, which comprises a hydraulic system, an air spring system, vehicle-mounted sensors and an ECU; the hydraulic system comprises a one-way pump, a two-position four-way electromagnetic valve, an adjustable damping valve and a hydraulic cylinder, the hydraulic cylinder is fixed on a wheel, a hydraulic cylinder piston rod is fixed on a vehicle body, the adjustable damping valve is installed between a hydraulic cylinder upper cavity and a first accumulator, a hydraulic cylinder lower cavity is communicated with a second accumulator, the first accumulator and the second accumulator are connected with the two-position four-way electromagnetic valve respectively, the two-position four-way electromagnetic valve is communicated with the one-way pump to form a hydraulic system loop; the air spring system comprises an air spring, an air pump and a motor, the air spring is connected with the vehicle body and the hydraulic cylinder, the motor is connected with the air pump through a first clutch, and the air pump is connected with the air spring; the hydraulic system and the air spring system share the motor, and the motor is further connected with the one-way pump through a second clutch.

Description

Technical Field

[0001] This invention belongs to the field of passenger vehicle suspension technology. More specifically, this invention relates to a slow active hydraulic suspension system and control method based on a directional valve. Background Technology

[0002] The suspension system is a general term for all force-transmitting connection devices between the vehicle body and the wheels. Its function is to transmit the vertical, longitudinal, and lateral reaction forces exerted on the wheels by the road surface, as well as the torques caused by these reaction forces, to the vehicle body to ensure the normal driving of the car.

[0003] The stiffness and damping of traditional passive suspensions are determined based on experience or optimization design methods. The stiffness and damping of the suspension cannot be adjusted during vehicle operation. Therefore, it can only achieve optimal performance under specific road conditions and driving speeds, resulting in poor adaptability.

[0004] Traditional passive suspension systems for passenger vehicles primarily use coil springs as elastic elements, preventing the vehicle from adjusting its height according to driving conditions. This significantly limits the vehicle's passability, ride comfort, and safety. In contrast, air spring systems allow for convenient height adjustment.

[0005] The stiffness and damping of active suspension can be dynamically adjusted according to changes in driving conditions, so that the suspension system is always in the best damping state. Therefore, active suspension greatly improves the ride comfort and handling stability of the vehicle.

[0006] Existing slow active hydraulic suspension systems based on bidirectional hydraulic pumps mainly suffer from problems such as high cost and difficulty in selecting components. Summary of the Invention

[0007] To address the aforementioned problems in the existing technology, this invention provides a slow active hydraulic suspension system and control method based on a directional valve, which simplifies the suspension structure, reduces suspension costs, expands the selection range of hydraulic pumps and motors, and improves the integration of the suspension system.

[0008] The objective of this invention is achieved through the following technical solution, as shown in the accompanying drawings:

[0009] A slow-active hydraulic suspension system based on a directional control valve includes a hydraulic system, an air spring system, on-board sensors, and an ECU. The hydraulic system includes a bidirectional adjustable damping valve 1, an upper accumulator 2, a two-position four-way directional control valve 3, a one-way hydraulic pump 11, a second clutch 10, a one-way motor 9, a lower accumulator 13, an overflow valve 14, and a hydraulic cylinder 17. The lower end of the hydraulic cylinder 17 is fixed to the control arm of the wheel 15, and the piston rod of the hydraulic cylinder 17 is fixed to the underside of the vehicle body 19. The upper and lower chambers of the hydraulic cylinder are respectively connected to the two ports of the two-position four-way directional control valve 3. The bidirectional adjustable damping valve 1 and the upper accumulator are installed between the upper chamber of the hydraulic cylinder 17 and the two-position four-way directional control valve 3. The lower accumulator 13 is installed between the lower chamber of the hydraulic cylinder 17 and the two-position four-way directional control valve 3. The two-position four-way directional control valve 3 is connected to the one-way hydraulic pump 11 to form a hydraulic system circuit. The air spring system includes an air spring... The system includes a spring 18, an air spring pressure reducing valve 4, an air pump 5, an air dryer 6, an air filter 7, a first clutch 8, and a one-way motor 9. The upper end of the air spring 18 is fixed to the lower side of the vehicle body 19, and the lower end is fixed to the upper end of the hydraulic cylinder 17, thus integrating the air spring 18 and the hydraulic cylinder 17 into one unit. The one-way motor 9 drives the air pump 5 through the first clutch 8, and the air pump 5 is connected to the air spring 18 through a pneumatic pipeline. The hydraulic system and the air spring system share the one-way motor 9, and the one-way motor 9 also drives the one-way hydraulic pump 11 of the hydraulic system through the second clutch 10. The ECU is also connected to the bidirectional adjustable damping valve 1, the two-position four-way reversing valve 3, the air spring pressure reducing valve 4, the motor 9, the front axle displacement sensor 20, the acceleration sensor 21, and the rear axle displacement sensor 22 to control the various working modes of the slow active hydraulic suspension system.

[0010] Preferably, the hydraulic cylinder 17 is connected in parallel with the relief valve 14.

[0011] Preferably, the air inlet of the air pump 5 is connected in sequence to the air dryer 6 and the air filter 7.

[0012] Preferably, the vehicle-mounted sensors include a front axle displacement sensor 20, an acceleration sensor 21, and a rear axle displacement sensor 22, with the front axle displacement sensor 20 and the rear axle displacement sensor 22 respectively mounted on the upper side of the front and rear axles of the vehicle.

[0013] This invention also provides a control method for a slow active hydraulic suspension system based on a directional valve, including the following control modes:

[0014] Passive suspension mode: When the motor's response frequency range is exceeded, the invention enters passive suspension mode, where air springs, upper and lower accumulators buffer the impact of uneven road surfaces; and a two-way damping valve attenuates vehicle body vibrations caused by uneven road surfaces. During suspension operation, the valve core opening of the two-way adjustable damping valve remains unchanged, meaning the suspension damping remains constant.

[0015] Semi-active suspension mode: Air springs, upper and lower accumulators buffer the impact of uneven road surfaces; a two-way damping valve attenuates body vibrations caused by uneven road surfaces. Unlike passive suspension mode, the ECU can adjust the valve core position of the two-way adjustable damping valve according to the vehicle's operating conditions, meaning the suspension damping is adjustable, thereby improving the adaptability of the suspension system to a certain extent. Compared to passive suspension mode, the improvement lies in the ECU's ability to issue control commands based on the vehicle's motion state, road surface, and load to adjust the valve core opening of the two-way adjustable damping valve, thus enhancing the suspension system's ability to adapt to different driving conditions.

[0016] Active suspension mode: Air springs, upper and lower accumulators buffer the impact of uneven road surfaces; a two-way damping valve attenuates body vibrations caused by uneven road surfaces, and the damping of the suspension system is adjustable. The ECU can issue control commands based on the vehicle's motion state, road surface, and load conditions to adjust the valve core opening of the two-way adjustable damping valve, thereby adjusting the damping of the suspension system. Compared to the semi-active suspension mode, this mode is improved by the ECU issuing control commands based on the vehicle's motion state, road surface, and load conditions to control a one-way motor to drive a one-way hydraulic pump to actively work on the hydraulic cylinder. By pumping hydraulic oil into the hydraulic cylinder, the wheels actively move vertically, thereby minimizing the vehicle's acceleration and wheel dynamic load. This mode further improves upon the semi-active suspension mode, as the ECU can actively control wheel movement based on the vehicle's operating conditions, reducing vehicle acceleration and wheel dynamic load, thereby maximizing vehicle ride comfort and safety.

[0017] Vehicle height adjustment mode: Onboard sensors measure the displacement of the front and rear axle suspensions and the vehicle's operating conditions in real time. The ECU calculates the optimal vehicle height based on the sensor information. This mode is used when there are significant changes in load, vehicle speed, road conditions, etc. Then, the ECU controls a one-way motor to drive an air pump to pump filtered and dried air into the air springs, and the vehicle height changes accordingly. The front axle displacement sensor and the rear axle displacement sensor then feed the real-time vehicle height back to the ECU, thus forming a closed-loop control until the vehicle reaches the optimal height. This mode is suitable for operating conditions where there are significant changes in load, vehicle speed, road conditions, etc.

[0018] Vehicle Body Attitude Adjustment Mode: To maintain vehicle body attitude during acceleration, braking, and cornering, the ECU controls a bidirectional adjustable damping valve and a one-way hydraulic pump to reduce the roll rate, roll angle, pitch rate, and pitch angle during acceleration, braking, and cornering. This reduces changes in vehicle body attitude under these conditions, improving ride comfort and driving safety. Onboard sensors measure the vehicle's operating conditions in real time and send the detected information to the ECU. The ECU calculates the optimal piston position in the hydraulic cylinder based on this information. Then, the ECU increases suspension damping by adjusting the valve core position of the bidirectional adjustable damping valve, reducing the pitch and roll rates. Simultaneously, the ECU controls a one-way motor to drive the one-way hydraulic pump, actively working on the hydraulic cylinder, further reducing the pitch and roll angles. Through integrated control of the bidirectional adjustable damping valve and the one-way motor, pitch rate, roll rate, pitch angle, and roll angle are all reduced. This mode is suitable for driving, braking, and cornering conditions.

[0019] Overload protection mode: When the relative speed between the vehicle body and the wheels is high, the suspension system will generate a large damping force, which may damage the hydraulic cylinder connecting parts and the vehicle body. An overflow valve is installed in parallel with the hydraulic cylinder. When the relative speed between the wheels and the vehicle body exceeds a certain value, the hydraulic pressure opens the overflow valve, directly connecting the upper and lower chambers of the hydraulic cylinder. This limits further increases in damping force, thus protecting the connection between the vehicle body and the suspension system.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] (1) The present invention designs the hydraulic system as an internal circulation type, eliminating the oil tank, filter and check valve. This not only simplifies the suspension structure and reduces the cost of the suspension, but also makes the suspension system almost maintenance-free after the first oil filling, which greatly reduces the cost of later maintenance. The suspension of the four wheels is exactly the same, which improves the versatility of the suspension parts.

[0022] (2) Compared to other active hydraulic suspension systems that use bidirectional motors and bidirectional hydraulic pumps as hydraulic drive mechanisms, this invention uses a combination of a unidirectional motor, a unidirectional hydraulic pump, and a two-position four-way directional valve to achieve the functions of a bidirectional motor and a bidirectional hydraulic pump. This not only expands the range of component choices but also further reduces the production cost of the suspension, because the price of unidirectional hydraulic pumps and unidirectional motors is much lower than that of bidirectional hydraulic pumps and bidirectional motors, and there are fewer types of bidirectional hydraulic pumps and bidirectional motors on the market, making it difficult to find suitable models. In addition, two-position four-way directional valves are not only low in cost but also come in a wide variety and are easy to purchase.

[0023] (3) This invention connects an overflow valve in parallel with a hydraulic cylinder. When the relative speed between the vehicle body and the wheels exceeds a threshold, the hydraulic oil in the lower hydraulic chamber can directly enter the upper hydraulic chamber through the overflow valve, thereby limiting the damping force within a certain range to prevent the connection between the vehicle body and the hydraulic cylinder from being damaged by excessive impact loads. Other active suspension systems avoid generating excessive damping force by setting a check valve on the piston of the hydraulic cylinder. Due to the small piston area, it is relatively difficult to arrange the check valve, and the maintenance is difficult when the check valve fails. It requires disassembling the entire suspension system, resulting in high maintenance costs. If the check valve needs to be replaced, the hydraulic cylinder piston also needs to be replaced. However, this invention can achieve the overload protection function with an overflow valve connected in parallel with the hydraulic cylinder. Moreover, when the overflow valve fails, maintenance is convenient, and even if the overflow valve needs to be replaced, the cost is extremely low.

[0024] (4) The response frequencies of the air spring system and the hydraulic system of the present invention do not overlap. Therefore, the same motor can be used to drive the unidirectional hydraulic pump and the air pump. This not only simplifies the suspension structure and improves the lightweight nature of the suspension system, but more importantly, it reduces the production cost of the suspension, thereby promoting the large-scale popularization of slow active hydraulic suspension systems. Finally, using one motor to drive both the hydraulic system and the air spring system simultaneously also reduces the control difficulty of the suspension system and facilitates the implementation of control strategies.

[0025] (5) In this invention, an upper accumulator and a lower accumulator are connected in series in the hydraulic pipeline. When the wheel vibrates at low amplitude and high frequency, the upper accumulator and the lower accumulator can work as spring elements. At this time, the hydraulic pump can stop working to reduce the energy consumption of the suspension system without reducing the ride comfort of the vehicle. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of a slow active hydraulic suspension system based on a directional valve according to the present invention.

[0027] Figure 2 This is a schematic diagram illustrating the overload prevention working principle of a slow active hydraulic suspension system based on a directional valve, as described in this invention.

[0028] Figure 3 A schematic diagram showing the vehicle body height when a vehicle using a slow active hydraulic suspension system based on a directional valve, as described in this invention, is traveling on a good road surface.

[0029] Figure 4 A schematic diagram showing the vehicle body height when a vehicle using a slow active hydraulic suspension system based on a directional valve, as described in this invention, is traveling on a bumpy road.

[0030] Figure 5 This is a schematic diagram illustrating the working principle of a slow active hydraulic suspension system based on a directional valve for lowering vehicle height, as described in this invention.

[0031] Figure 6 This is a schematic diagram illustrating the working principle of a slow active hydraulic suspension system based on a directional valve to increase vehicle height, as described in this invention.

[0032] In the picture:

[0033] 1-Two-way adjustable damping valve; 2-Upper accumulator; 3-Two-position four-way directional valve; 4-Air spring pressure reducing valve; 5-Air pump; 6-Air dryer; 7-Air filter; 8-First clutch; 9-One-way motor; 10-Second clutch; 11-One-way hydraulic pump; 12-ECU; 13-Lower accumulator; 14-Relief valve; 15-Wheel; 16-Road surface; 17-Hydraulic cylinder; 18-Air spring; 19-Body body; 20-Front axle displacement sensor; 21-Acceleration sensor; 22-Rear axle displacement sensor. Detailed Implementation

[0034] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments:

[0035] Example 1

[0036] like Figure 1 , Figure 2 , Figure 5 and Figure 6 As shown, a slow active hydraulic suspension system based on a directional valve includes a hydraulic system, an air spring system, on-board sensors, and an ECU.

[0037] The hydraulic system includes a bidirectional adjustable damping valve 1, an upper accumulator 2, a two-position four-way directional valve 3, a one-way hydraulic pump 11, a second clutch 10, a one-way motor 9, a lower accumulator 13, an overflow valve 14, and a hydraulic cylinder 17. The lower end of the hydraulic cylinder 17 is fixed on the swing arm of the wheel 15, and the piston rod of the hydraulic cylinder 17 is fixed on the lower side of the vehicle body 19. The upper and lower chambers of the hydraulic cylinder 17 are connected to the two ports of the two-position four-way directional valve 3. The bidirectional adjustable damping valve 1 and the upper accumulator 2 are installed between the upper chamber of the hydraulic cylinder 17 and the two-position four-way directional valve 3. The lower accumulator 13 is installed between the lower chamber of the hydraulic cylinder 17 and the two-position four-way directional valve 3. The two-position four-way solenoid valve 3 is connected to the one-way hydraulic pump 11 to form a hydraulic system circuit.

[0038] The air spring system includes an air spring 18, an air spring pressure reducing valve 4, an air pump 5, an air dryer 6, an air filter 7, a first clutch 8, and a one-way motor 9. The upper end of the air spring 18 is fixed to the lower side of the vehicle body 19, and the lower end is fixed to the upper side of the hydraulic cylinder 17, thus integrating the air spring and the hydraulic cylinder into one unit. The one-way motor 9 drives the air pump 5 through the first clutch 8, and the air pump 5 is connected to the air spring 18 through a pneumatic pipeline. The air inlet of the air pump 5 is sequentially connected to the air dryer 6 and the air filter 7. The driving device for both the hydraulic system and the air spring system is the same one-way motor 9.

[0039] The vehicle-mounted sensors include a front axle displacement sensor 20, an acceleration sensor 21, and a rear axle displacement sensor 22. The front axle displacement sensor 20 and the rear axle displacement sensor 22 are respectively installed on the upper side of the front and rear axles of the vehicle.

[0040] The ECU is also connected to the bidirectional adjustable damping valve 1, the two-position four-way reversing valve 3, the air spring pressure reducing valve 4, the one-way motor 9, the front axle displacement sensor 20, the acceleration sensor 21, and the rear axle displacement sensor 22 to control the various working modes of the slow active hydraulic suspension system.

[0041] Example 2

[0042] A control method for a slow active hydraulic suspension system based on a directional valve includes the following control modes:

[0043] (1) Passive suspension mode:

[0044] The motor response frequency of this invention is 0.5Hz to 5Hz. When the motor response frequency is exceeded, the invention enters passive suspension mode, the unidirectional motor stops working, and the opening of the bidirectional adjustable damping valve core is fixed. That is, the unidirectional motor cannot actively work on the hydraulic cylinder, and the suspension damping remains unchanged. The specific working principle is illustrated by taking a vehicle going over a speed bump as an example:

[0045] When a vehicle goes over a speed bump, the wheels move upwards first. At this time, the air springs are compressed and store energy, thus buffering the impact of the speed bump on the vehicle body. Simultaneously, the upward movement of the wheels causes the piston in the hydraulic cylinder to move downwards along the inner wall of the cylinder. The volume of the upper chamber of the hydraulic cylinder increases, reducing pressure, while the volume of the lower chamber decreases, increasing pressure. Hydraulic oil from the upper accumulator flows into the upper chamber of the hydraulic cylinder, and hydraulic oil from the lower chamber flows into the lower accumulator. At this point, the upper and lower accumulators act like two springs connected in series, further buffering the impact of the speed bump on the vehicle body. The hydraulic oil flowing from the upper accumulator into the upper chamber of the hydraulic cylinder passes through a damping valve, resulting in hydraulic loss, which rapidly attenuates the vibration. In summary, the air springs, upper accumulator, and lower accumulator buffer the impact of the speed bump on the vehicle body. The impact causes the vehicle body to vibrate, and the two-way damping valve quickly attenuates this vibration, thus achieving the buffering and vibration reduction function of the suspension system and improving vehicle ride comfort. This mode only achieves optimal performance under specific vehicle speeds and road conditions.

[0046] (2) Semi-active suspension mode:

[0047] Passive suspension only achieves optimal performance under specific vehicle speeds and road conditions, which negatively impacts vehicle ride comfort and handling stability. This invention adjusts suspension damping by controlling the valve core position of a bidirectional adjustable damping valve, thus enabling it to function as a semi-active suspension. The specific implementation process is as follows:

[0048] When a vehicle is traveling on a smooth road, to improve ride comfort, the suspension system damping should be reduced. The acceleration sensor sends the vehicle's motion status to the ECU, which then issues a control command to reduce suspension damping. This is achieved by adjusting the position of the two-way adjustable damping valve, thus placing the suspension in a low-damping state, which helps improve ride comfort. When a vehicle is traveling on a bumpy road, to improve ground clearance, the suspension system damping needs to be increased. The acceleration sensor sends the vehicle's motion status to the ECU, which then issues a control command to increase suspension damping. This is achieved by adjusting the position of the two-way adjustable damping valve, thus placing the suspension in a high-damping state, which also helps improve ground clearance. By setting different suspension damping values ​​for different driving conditions, the adaptability of the suspension system is improved to a certain extent.

[0049] (3) Active suspension mode:

[0050] Based on the semi-active suspension mode, this invention can also utilize a unidirectional motor to drive a unidirectional hydraulic pump to actively perform work on the hydraulic cylinder, thereby actively controlling the wheel movement and giving the invention the function of an active suspension. The specific implementation process is illustrated by taking a vehicle going over a speed bump as an example:

[0051] Vehicle sensors such as accelerometers and displacement sensors measure the vehicle's motion in real time. When the ECU detects that a wheel is moving upwards over a speed bump, it reduces the suspension system's damping by adjusting the valve core position of a bidirectional adjustable damping valve. Simultaneously, it controls a one-way motor to drive a one-way hydraulic pump, pumping hydraulic oil into the upper chamber of the hydraulic cylinder, thus actively moving the wheel upwards. When the wheel passes the highest point of the speed bump and begins to move downwards, the ECU issues a control command to control the one-way motor to drive the one-way hydraulic pump, pumping hydraulic oil into the lower chamber of the hydraulic cylinder, thus actively moving the wheel downwards. By actively controlling wheel movement, vehicle acceleration and wheel dynamic load can be significantly reduced. Vehicle acceleration is a primary indicator of ride comfort, and wheel dynamic load is a primary indicator of handling stability; therefore, the active suspension mode of this invention can maximize both vehicle ride comfort and handling stability.

[0052] (4) Vehicle height adjustment mode:

[0053] Vehicle height not only affects a vehicle's passability and the ease of entry and exit for the driver and passengers, but also significantly impacts ride comfort by influencing the natural frequency of the suspension system. Therefore, adjusting vehicle height is of paramount importance. The ECU calculates the optimal vehicle height based on signals from onboard sensors. When the vehicle needs to be raised, the ECU controls a one-way motor to drive an air pump via a first clutch. The air pump draws air, which has passed through an air filter and air dryer, into the air springs, increasing the vehicle height until the desired height is reached. When the vehicle needs to be lowered, the ECU controls the air spring depressurization valve to open, releasing air from the air springs into the atmosphere, causing the vehicle height to decrease until the desired height is achieved. During vehicle height adjustment, the front and rear axle displacement sensors continuously feed the measured vehicle height signals back to the ECU, forming a closed-loop control system and improving the accuracy of vehicle height control.

[0054] This mode is particularly important for buses and medium-to-heavy-duty trucks, because buses need to frequently get on and off, and adjustable vehicle height will greatly facilitate their operation; the sprung mass of medium-to-heavy-duty trucks can differ by several times between unloaded and fully loaded conditions. The natural frequency formula for the suspension system is:

[0055]

[0056] In the formula: K is the suspension stiffness, and M is the sprung mass;

[0057] As can be seen from the above formula, the natural frequency of the suspension of a medium or heavy-duty truck when it is unloaded and when it is fully loaded may differ by several times. This will deviate significantly from the vibration frequency that the human body is accustomed to, and the driver and passengers will feel very uncomfortable.

[0058] (5) Vehicle posture adjustment mode:

[0059] During acceleration, braking, and cornering, a vehicle experiences pitch and roll movements. This not only significantly impacts ride comfort but also negatively affects the psychological well-being of the driver and passengers. More importantly, the body roll angle directly affects vehicle handling stability, posing a serious threat to the lives of the driver and passengers. Therefore, it is essential to minimize changes in vehicle posture during acceleration, braking, and cornering.

[0060] The vehicle height adjustment mode can also adjust the vehicle's posture, but the response speed is too slow, making it unable to adjust the vehicle's posture in real time during acceleration, braking, and cornering. However, using a motor-driven one-way pump to work on the hydraulic cylinders can meet the response speed requirements for these operating conditions.

[0061] When the vehicle accelerates, brakes, or corners, the acceleration sensors send the vehicle's longitudinal and lateral accelerations to the ECU. The ECU calculates the optimal work done by the four-wheel suspension and then issues commands to adjust the vehicle's attitude. At this time, the damping of the suspension system is increased by adjusting the position of the two-way adjustable damping valve core. This increases the stiffness of the suspension system, thereby reducing the pitch and roll angular velocities of the vehicle under the aforementioned conditions. However, increasing the damping does not change the steady-state values ​​of the pitch and roll angles.

[0062] On the other hand, by controlling a unidirectional motor to drive a unidirectional hydraulic pump to pump hydraulic oil into the hydraulic cylinder, the steady-state values ​​of the pitch and roll angles can be changed.

[0063] In summary, by integrating the bidirectional adjustable damping valve with the unidirectional motor, the pitch angle, pitch rate, roll angle, and roll rate of the vehicle during acceleration, braking, and cornering can be reduced. On the one hand, this will greatly improve the ride comfort and handling stability of the vehicle, and on the other hand, it will also reduce the negative psychological impact of changes in vehicle posture on the driver and passengers.

Claims

1. A slow active hydraulic suspension system based on a directional control valve, characterized in that, The system includes a hydraulic system, an air spring system, and an ECU. The hydraulic system includes a bidirectional adjustable damping valve (1), an upper accumulator (2), a two-position four-way directional valve (3), a one-way hydraulic pump (11), a second clutch (10), a lower accumulator (13), and a hydraulic cylinder (17). The lower end of the hydraulic cylinder (17) is fixed to the swing arm of the wheel (15), and the upper end of the piston rod of the hydraulic cylinder (17) is fixed to the lower side of the vehicle body (19). The upper and lower chambers of the hydraulic cylinder are respectively connected to the two ports of the two-position four-way directional valve, allowing for bidirectional adjustment. The damping valve (1) and the upper accumulator (2) are installed between the upper chamber of the hydraulic cylinder (17) and the two-position four-way directional valve (3), and the lower accumulator (13) is installed between the lower chamber of the hydraulic cylinder (17) and the two-position four-way directional valve (3). The two-position four-way directional valve (3) is connected to the one-way hydraulic pump (11) to form a hydraulic system circuit. The air spring system includes an air spring (18), an air spring pressure reducing valve (4), an air pump (5), an air dryer (6), an air filter (7), and a first clutch (8). The air spring (18) is fixed at the lower side of the vehicle body and at the upper side of the hydraulic cylinder (17), so that the air spring (18) and the hydraulic cylinder (17) are integrated into one unit; the air pump (5) is driven by the first clutch (8), and the air pump (5) and the air spring (18) are connected by air pressure pipeline; the hydraulic system and the air spring system share the air spring system with the air spring system, and the air spring (9) is also driven by the one-way hydraulic pump (11) of the hydraulic system through the second clutch (10); the ECU is simultaneously connected to the bidirectional adjustable damping valve (1), the two-position four-way reversing valve (3), the air spring pressure reducing valve (4), the air spring (9), and the vehicle sensor to control the various working modes of the slow active hydraulic suspension system; the vehicle sensor includes the front axle displacement sensor (20), the acceleration sensor (21) and the rear axle displacement sensor (22), and the front axle displacement sensor (20) and the rear axle displacement sensor (22) are respectively installed on the upper side of the front and rear axles of the vehicle.

2. The slow active hydraulic suspension system based on a directional control valve as described in claim 1, characterized in that, An overflow valve (14) is provided between the upper accumulator (2) and the lower accumulator (13).

3. The slow active hydraulic suspension system based on a directional control valve as described in claim 1, characterized in that, The air pump (5) has its air inlet connected in sequence to the air dryer (6) and the air filter (7).

4. The control method for a slow active hydraulic suspension system based on a directional valve as described in claim 1, characterized in that, Includes the following control modes: Passive suspension mode: When the vibration frequency exceeds the set range, the system enters passive suspension mode, where the air springs, upper accumulator, and lower accumulator buffer the impact of uneven road surfaces; and the two-way adjustable damping valve attenuates the vehicle body vibration caused by uneven road surfaces. Semi-active suspension mode: The air springs, upper and lower accumulators buffer the impact of uneven road surfaces; the two-way adjustable damping valve dampes the body vibration caused by uneven road surfaces. Active suspension mode: The air springs, upper and lower accumulators buffer the impact of uneven road surfaces; the two-way adjustable damping valve attenuates the body vibration caused by uneven road surfaces; the ECU issues control commands based on the vehicle's motion state, road surface and load conditions to adjust the valve core opening of the two-way adjustable damping valve, thereby adjusting the damping of the suspension system. Vehicle height adjustment mode: On-board sensors measure the displacement of the front and rear axles and driving conditions in real time. The ECU calculates the optimal vehicle height based on the information fed back by the on-board sensors. Then, the ECU controls the one-way motor and air pump to make the vehicle height reach the ideal value. Vehicle attitude adjustment mode: On-board sensors measure the vehicle's operating conditions in real time, and then send the information detected by the sensors to the ECU. The ECU calculates the optimal position of the hydraulic cylinder piston based on the sensor information. Then, the ECU increases the damping of the suspension system by adjusting the valve core position of the bidirectional adjustable damping valve. On the other hand, the ECU controls the one-way motor to drive the one-way hydraulic pump to actively work on the hydraulic cylinder, reducing the vehicle's pitch and roll angles. Overload protection mode: When the relative speed between the wheel and the vehicle body exceeds a certain value, the hydraulic pressure will open the valve port of the relief valve, at which time the upper chamber and lower chamber of the hydraulic cylinder are directly connected.

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