Hydrostatic constant speed drive system

Abstract

A hydrostatic constant speed drive system comprises a variable pump (1), a variable motor (2) having an inlet connected to an output port of the pump by a first main oil line (L1) and an outlet connected to an input port of the pump by a second main oil line (L2), and a controller (20) configured to adjust the swash angles of the pump and the motor, wherein in a non-braking mode of operation, in a first motor speed range, the controller (20) controls the swash angle of the motor to remain maximum and adjusts the swash angle of the pump to achieve a change in the motor speed, in a second motor speed range, the controller (20) controls the swash angle of the pump to remain maximum and adjusts the swash angle of the motor to achieve a change in the motor speed, and in a braking mode of operation, hydrostatic braking is implemented by only controlling the swash angle reduction of the variable pump (1).

Description

Technical Field

[0001] This application relates to a hydrostatic constant speed drive system, and more particularly to a hydrostatic constant speed drive system that simultaneously employs both a proportional variable pump and a proportional variable motor as speed regulating elements. Background Technology

[0002] Hydrostatic drive systems are widely used in agricultural and forestry machinery, construction machinery, and rail vehicles. These systems are used for both driving and operating the equipment. Some construction machinery, such as rail grinding machines, milling machines, and graders, requires a stable speed during operation; otherwise, the quality of work cannot meet requirements. Traditionally, construction machinery controls speed by having the driver operate the throttle and brakes. For example, to maintain a constant speed, the throttle needs to be increased when going uphill, and the throttle needs to be decreased or the brakes applied when going downhill. This method has significant drawbacks. First, maintaining a constant speed requires the driver to constantly monitor the situation and manually operate the system, leading to high labor intensity and inherent instability and inaccuracies. Second, frequent and prolonged operation of the brake system can cause it to overheat, reduce braking capacity, and even lead to brake failure. Summary of the Invention

[0003] One objective of this application is to provide a hydrostatic constant speed drive system with constant speed control function, which can improve the quality of operation and reduce the labor intensity of the driver.

[0004] Therefore, this application provides a hydrostatic constant speed drive system in one aspect, comprising:

[0005] Variable displacement pump;

[0006] A variable displacement motor has its inlet connected to the output port of a variable displacement pump via a first main oil circuit, and its outlet connected to the input port of the variable displacement pump via a second main oil circuit. The variable displacement motor is equipped with a motor speed sensor.

[0007] The controller is configured to execute the non-braking and braking modes of the hydrostatic constant speed drive system based on the comparison between the received speed input signal and the actual motor speed detected by the motor speed sensor, so as to realize the constant speed drive operation of the hydrostatic constant speed drive system.

[0008] In the non-braking operating mode, within the first speed range of the variable motor, the controller maintains the swing angle of the variable motor at its maximum and increases the swing angle of the variable pump to increase the speed of the variable motor, or decreases the swing angle of the variable pump to decrease the speed of the variable motor. Within the second speed range of the variable motor, which is higher than the first speed range, the controller maintains the swing angle of the variable pump at its maximum and decreases the swing angle of the variable motor to increase the speed of the variable motor, or increases the swing angle of the variable motor to decrease the speed of the variable motor. The boundary point between the first speed range and the second speed range corresponds to the speed of the variable motor when both the variable pump and the variable motor are at their maximum swing angle.

[0009] In braking mode, the controller implements hydrostatic braking by reducing the swing angle of the variable pump, thereby reducing the speed of the variable motor.

[0010] In one embodiment, the controller includes a pump controller and a motor controller, the pump controller being configured to cooperate with the motor controller to implement the non-braking operating mode, and the pump controller being configured to implement the braking operating mode independently.

[0011] In one embodiment, the pump controller includes a pump drive controller, a hydrostatic brake controller, and an output switch. The pump drive controller is configured to generate a pump sway angle command in a non-braking operating mode, and the hydrostatic brake controller is configured to generate a pump sway angle command in a braking operating mode. The pump sway angle commands generated by the drive controller and the hydrostatic brake controller are sent to a variable pump via the output switch to control the sway angle of the variable pump.

[0012] In one embodiment, during operation of the hydrostatic constant speed drive system, one of the first and second main oil circuits is a pump output side oil circuit, and the other is a pump suction side oil circuit; furthermore, the hydrostatic constant speed drive system further includes:

[0013] A jumper circuit, connecting the first and second main oil circuits, and equipped with normally closed first and second brake switch valves; and

[0014] An overflow oil passage is connected to the intermediate part of the bridging oil passage located between the first and second brake switch valves, and a proportional overflow valve is arranged in the overflow oil passage;

[0015] In the braking operation mode, when the oil pressure on the pump suction side is higher than the opening pressure of the proportional relief valve, the proportional relief valve is opened due to the pressure, and the brake switch valve on the pump suction side of the first and second brake switch valves is opened, causing the oil circuit on the pump suction side to overflow, thus keeping the oil pressure on the pump suction side not higher than the opening pressure of the proportional relief valve.

[0016] In one embodiment, the overflow oil passage leads to the oil tank.

[0017] In one embodiment, the hydrostatic constant speed drive system further includes a return oil passage equipped with first and second check valves, and an overflow oil passage leading to the intermediate portion of the return oil passage located between the first and second check valves.

[0018] In one embodiment, the hydrostatic constant speed drive system further includes a first pressure sensor disposed in the first main oil circuit and a second pressure sensor disposed in the second main oil circuit; and

[0019] The controller further includes a drag torque controller, which is configured to:

[0020] The pump suction side oil circuit pressure and the pump output side oil circuit pressure are obtained by the first and second pressure sensors;

[0021] In the braking operating mode, an electrical signal corresponding to the opening pressure of the proportional relief valve is determined and sent to the proportional relief valve; and

[0022] In braking operation mode, when the oil pressure on the pump suction side is higher than the oil pressure on the pump output side, the brake switch valve closer to the pump suction side oil line of the first and second brake switch valves is opened, while the brake switch valve closer to the pump output side oil line is kept closed.

[0023] In one embodiment, in the braking operating mode, the electrical signal determined by the drag torque controller corresponding to the proportional relief valve opening pressure is related to the pump swing angle, and the proportional relief valve opening pressure value is set such that the hydrostatic braking torque generated by the hydrostatic constant speed drive system can maintain the constant speed of the hydrostatic constant speed drive system.

[0024] In one implementation, during braking operation, the drag torque controller calculates the pump suction side oil circuit pressure limit value based on the allowable drag torque of the variable pump's power source, the pump output side oil circuit pressure, and the current pump swing angle, and determines the electrical signal corresponding to the proportional relief valve opening pressure based on the pump suction side pressure limit value.

[0025] In one embodiment, the variable pump is an electro-proportional variable pump, and the variable motor is an electro-proportional variable motor.

[0026] The hydrostatic constant speed drive system according to this application has a fully constant speed control function and hydrostatic braking capability, requiring no human intervention and greatly reducing the driver's workload. Attached Figure Description

[0027] The foregoing and other aspects of this application will be more fully understood through the following detailed description with reference to the accompanying drawings, in which:

[0028] Figure 1 This is a schematic diagram of a hydrostatic constant speed drive system according to a feasible embodiment of the present application.

[0029] Figure 2 This is a graph showing the output torque and speed of the hydrostatic constant speed drive system.

[0030] Figure 3 This is a hydraulic circuit diagram of a hydrostatic constant speed drive system according to another feasible embodiment of this application. Detailed Implementation

[0031] This application generally relates to a hydrostatic constant speed drive system, which is particularly suitable for driving various agricultural and forestry machinery, engineering machinery, and rail vehicles.

[0032] According to a feasible embodiment of the present application, a hydrostatic constant speed drive system is... Figure 1 The diagram schematically illustrates that the hydrostatic constant speed drive system includes a closed-loop hydraulic drive circuit, which mainly includes: a piston-type closed-loop variable pump 1, driven by a power source (especially an engine or motor) not shown in the figure, and preferably an electro-proportional variable pump; and a piston-type variable motor 2, whose inlet is connected to the output port of pump 1 through a first main oil circuit L1, and whose outlet is connected to the input port of pump 1 through a second main oil circuit L2, thereby being driven by pump 1 through the first or second main oil circuits L1 and L2, and preferably an electro-proportional variable motor.

[0033] The output shaft of motor 2 is equipped with a motor speed sensor 3 to detect the speed of motor 2.

[0034] Motor 2 can be used to drive the wheels of a traveling device. For example, as shown in the figure, the output shaft of motor 2 drives wheel 5 through transfer case 4.

[0035] Alternatively, motor 2 can be used to drive both the wheels and working elements of the traveling equipment. Alternatively, motor 2 can be used solely to drive the working elements of various equipment. For ease of understanding, the following detailed description with reference to the accompanying drawings uses only the example of motor 2 driving the wheels of a vehicle.

[0036] The first and second main oil circuits L1 and L2 are equipped with first and second pressure sensors 6 and 7, respectively, to detect the hydraulic oil pressure in the first and second main oil circuits L1 and L2.

[0037] Furthermore, a bridging oil circuit L3 connects the first and second main oil circuits L1 and L2. One end of the bridging oil circuit L3 is connected to the first main oil circuit L1, and the other end is connected to the second main oil circuit L2. A first brake switch valve 8 is provided in the first side portion of the bridging oil circuit L3, and a second brake switch valve 9 is provided in the second side portion. They are normally closed and open when a signal is received.

[0038] An overflow oil passage L4 is connected to the middle section of the bridging oil passage L3 between the first and second side portions. A proportional overflow valve 10 is arranged in the overflow oil passage L4. The first end of the overflow oil passage L4 is connected to the middle section of the bridging oil passage L3, and the second end leads to the oil tank 11.

[0039] In addition, the hydrostatic constant speed drive system also includes a replenishing pump (not shown), which is connected to the first and second main oil circuits L1 and L2 and is used to replenish oil to the first and second main oil circuits L1 and L2.

[0040] Figure 1 The hydrostatic constant speed drive system is equipped with a corresponding controller 20, which is connected to the various components of the hydrostatic constant speed drive system described above, and is used to detect the status of the hydrostatic constant speed drive system and control the operation of the hydrostatic constant speed drive system.

[0041] The controller 20 includes a pump controller 21 and a motor controller 22. The pump controller 21 includes a pump drive controller 23, a hydrostatic brake controller 24, and an output switch 25. The drive controller 23 and the hydrostatic brake controller 24 output control signals to the output switch 25, and the output switch 25 sends control signals to the pump 1 to control the operation of the pump 1. The motor controller 22 sends control signals to the motor 2 to control the operation of the motor 2.

[0042] The controller 20 also includes an instruction generator 26, which receives the speed input signal 27 and generates speed instructions.

[0043] The controller 20 also includes a pump and motor control switch 28, a subtractor 29, and an actual vehicle speed feedback terminal 30. The actual vehicle speed feedback terminal 30 is used to receive the actual speed of the motor 2 detected by the motor speed sensor 3. The pump and motor control switch 28 receives the speed command from the command generator 26 and the actual speed from the actual vehicle speed feedback terminal 30 to determine whether the vehicle speed is controlled by the pump or the motor. The pump and motor control switch 28 outputs a status value corresponding to the determination result to the pump drive controller 21 (pump drive controller 23) and the motor controller 22.

[0044] Subtractor 29 subtracts the actual rotational speed from the actual vehicle speed feedback terminal 30 from the speed command from command generator 26, and outputs the result (difference) to pump drive controller 21 (pump drive controller 23 and hydrostatic brake controller 24) and motor controller 22.

[0045] The hydrostatic brake controller 24 also receives the hydraulic oil pressure in the first and second main oil circuits L1 and L2 detected by the first and second pressure sensors 6 and 7.

[0046] The controller 20 also includes a drag torque controller 31, which receives the hydraulic oil pressure in the first and second main oil circuits L1 and L2 detected by the first and second pressure sensors 6 and 7, and receives the pump control signal from the output switch 25. The drag torque controller 31 also controls the operation (pressure signal) of the first and second brake switch valves 8 and 9 (opening and closing) and the proportional relief valve 10.

[0047] The controller 20 controls the pump 1 and the motor 2 based on the speed input signal 27, thereby achieving the desired vehicle speed. The vehicle speed corresponds to the rotational speed of the motor 2. Figure 2 The curves in the diagram illustrate the relationship between the output torque and speed of the hydrostatic constant speed drive system. Figure 2 The horizontal axis represents the rotational speed of motor 2 (which also represents the vehicle speed), and the vertical axis represents the output torque of motor 2 (which also represents the vehicle's traction force). Figure 2 The zero-speed point W0 on the curve corresponds to the vehicle's stationary starting state. During the transition from zero-speed point W0 to the first operating point (maximum torque point) W1, the speed and torque of motor 2 increase synchronously, generally occurring during the vehicle's initial start-up phase. After reaching the first operating point W1, as the speed of motor 2 increases, the torque of motor 2 decreases, and the motor's operating point (represented by speed) experiences a second operating point W2, eventually reaching the third operating point W3. After reaching the third operating point W3, an increase in speed leads to a sharp drop in torque. Therefore, the area after the third operating point W3 is generally not suitable for use as the operating range of a hydrostatic constant-speed drive system.

[0048] According to this application, the point of maximum swing angle of pump 1 and motor 2 is defined as the second operating point W2.

[0049] In the non-braking state, when the hydrostatic constant speed drive system is in acceleration mode, the first speed range between the operating point W0 and the second operating point W2 is used as the pump-controlled vehicle speed stage. During this stage, the controller 20 controls the motor 2 to maintain its maximum swing angle to ensure the output of the largest possible torque to accelerate the vehicle, and controls the pump 1 to gradually increase its swing angle to increase the vehicle speed. The second speed range between the second operating point W2 and the third operating point W3 is used as the motor-controlled vehicle speed stage. During this stage, the controller 20 controls the pump 1 to maintain its maximum swing angle, and controls the motor 2 to gradually decrease its swing angle to increase the vehicle speed. Conversely, in the non-braking state, when the hydrostatic constant speed drive system is in deceleration mode, similarly, the second speed range is used as the motor-controlled vehicle speed stage. During this stage, the controller 20 controls the pump 1 to maintain its maximum swing angle, and controls the motor 2 to gradually increase its swing angle to decrease the vehicle speed; the first speed range is used as the pump-controlled vehicle speed stage. During this stage, the controller 20 controls the motor 2 to maintain its maximum swing angle, and controls the pump 1 to gradually decrease its swing angle to decrease the vehicle speed.

[0050] The controller 20 uses the second operating point W2 as the critical point for switching between pump control and motor control during acceleration. Below the second operating point W2, the vehicle speed is controlled by the pump; above the second operating point W2, the vehicle speed is controlled by the motor. Therefore, under these road conditions, the corresponding vehicle speed needs to be matched with the swing angle of pump 1 or motor 2, and closed-loop speed control is performed to maintain a constant vehicle speed.

[0051] This system can maintain a constant vehicle speed under various road conditions. When the vehicle is in normal forward driving, it is assumed that the first main oil circuit L1 is the high-pressure side, i.e., the pump output side, and the second main oil circuit L2 is the low-pressure side, i.e., the pump suction side.

[0052] The controller 20 controls the vehicle speed based on the speed input signal 27. The speed input signal 27 can be a current or voltage signal from a manually operated element, human-machine interface, etc., which is processed by the command generator 26 of the controller 20 to generate a speed command. This speed command and the speed feedback from the actual vehicle speed feedback terminal 30 are sent to the pump and motor control switch 28 to determine whether it is pump control or motor control. The pump and motor control switch 28 outputs a status value to the pump drive controller 23 and the motor controller 22. The pump drive controller 23 or the motor controller 22 compares the current vehicle speed command and the vehicle speed feedback to obtain the speed deviation and performs PID calculation. The pump drive controller 23 outputs a pump swing angle command to the pump output switch 25, and the pump output switch 25 outputs a pump swing angle command to pump 1; or the motor controller 22 outputs a motor swing angle command to motor 2.

[0053] As mentioned earlier, in the non-braking operating mode (normal acceleration or deceleration mode) of the hydrostatic constant speed drive system, between the operating point W0 and the second operating point W2, the controller 20 controls the motor 2 to maintain its maximum swing angle and controls the pump 1 to gradually increase its swing angle to increase vehicle speed, or controls the pump 1 to gradually decrease its swing angle to decrease vehicle speed. Between the second operating point W2 and the third operating point W3, the controller 20 controls the pump 1 to maintain its maximum swing angle and controls the motor 2 to gradually decrease its swing angle to increase vehicle speed, or controls the pump 1 to maintain its maximum swing angle and controls the motor 2 to gradually increase its swing angle to increase vehicle speed. This allows the vehicle speed to be maintained at a constant value.

[0054] On the other hand, in the braking mode of the hydrostatic constant speed drive system, between the working point W0 and the third working point W3, the vehicle speed can be reduced by controlling the reduction of the swing angle of pump 1 to apply hydrostatic braking to the vehicle.

[0055] The controller 20 controls the swing angle of pump 1 or motor 2 to increase the vehicle speed to the desired value. After the vehicle speed reaches the desired value, the controller can perform closed-loop control on the swing angle of pump 1 or motor 2 based on the rotational speed of motor 2 detected by motor speed sensor 3, so that the vehicle speed is kept constant at the desired value, thereby achieving constant speed control.

[0056] During constant speed driving, the controller 20 determines in real time whether the vehicle speed deviates from the expected value based on the comparison between the motor speed sensor 3 and the speed input signal 27. If it is in a non-braking state, the speed is adjusted by adjusting the swing angle of the pump or motor to keep the vehicle speed at a constant value. If it is in a braking state, the closed-loop hydraulic drive circuit is controlled to automatically perform hydrostatic braking, wherein the vehicle speed is reduced by controlling the swing angle of the pump 1 to keep the vehicle speed at a constant value.

[0057] During hydrostatic braking (e.g., when a vehicle is going downhill), as the flow rate of control pump 1 decreases, the vehicle exerts a reverse drag on motor 2. This results in the flow rate at the outlet of motor 2 exceeding the required flow rate at the inlet of pump 1, causing the outlet pressure of motor 2 to increase and the inlet pressure to decrease. The vehicle's own kinetic energy drives motor 2 to operate as a pump, and pump 1 operates as a motor, driving the power source. The vehicle's kinetic energy is converted into heat energy, which is absorbed by the power source and the hydrostatic constant speed drive system.

[0058] During hydrostatic braking, the first pressure value detected by the first pressure sensor 6 (representing the inlet pressure of the motor 2) and the second pressure value detected by the second pressure sensor 7 (representing the outlet pressure of the motor 2) are sent to the hydrostatic brake controller 24, which determines whether the vehicle is in a braking state based on the current first and second pressure values.

[0059] To address this, during forward driving, the hydrostatic brake controller 24 monitors and compares the pressure values ​​of pressure sensors 6 and 7 in real time. When the pressure value of sensor 6 is less than the pressure value of sensor 7, such as during downhill driving (when the system performs hydrostatic braking), the hydrostatic brake controller 24 determines that the vehicle is in a braking state. During reverse driving, when the pressure value of sensor 7 is less than the pressure value of sensor 6, the hydrostatic brake controller 24 determines that the vehicle is in a braking state.

[0060] When it is determined that the vehicle is in a braking state, the hydrostatic brake controller 24 outputs the pump swing angle control value to the pump output switch 25, and the pump output switch 25 outputs the pump swing angle command to the pump 1 to adjust (further reduce or maintain) the pump swing angle.

[0061] If the vehicle is going downhill at excessive speed, due to mechanical inertia, motor 2 will be dragged by the vehicle and rotate at high speed. The hydrostatic brake controller 24 will output a continuously decreasing pump swing angle signal, causing the outlet pressure of motor 2 to continuously increase to generate greater braking torque, thereby decelerating to a constant speed.

[0062] When a vehicle travels down a steep slope, if the component of the vehicle's weight exceeds the sum of all resistance forces, the hydrostatic constant speed drive system does not require driving force, and the vehicle speed will gradually increase. During hydrostatic braking, pump 1 enters motor mode and reverses the drag force source, applying drag torque to it. The maximum allowable drag torque of the power source is usually limited. If pump 1 applies a drag torque exceeding the maximum allowable drag torque, it may cause the power source to stall or even be damaged.

[0063] By increasing the opening pressure of the proportional relief valve 10 and setting the opening pressure of the proportional relief valve 10 in the drag torque controller 31, on the one hand, the opening pressure of the proportional relief valve 10 can be set so that the hydrostatic braking torque generated by the hydrostatic constant speed drive system can maintain the constant speed operation of the hydrostatic constant speed drive system (i.e., constant vehicle speed). On the other hand, protection of the power source can be provided during hydrostatic braking. This is described in detail below.

[0064] When the vehicle is in normal forward driving mode, it is assumed that the first main oil circuit L1 is the high-pressure side, i.e., the pump output side, and the second main oil circuit L2 is the low-pressure side, i.e., the pump suction side. The drag torque controller 31 determines the vehicle braking state based on the pump control signal issued by the output switch 25. Furthermore, the drag torque controller 31 monitors and compares the pressure values ​​of pressure sensors 6 and 7 in real time. When the value of sensor 6 is less than the value of sensor 7, the drag torque controller 31 outputs an open signal to the second brake switch valve 9 to open the second brake switch valve 9, while the first brake switch valve 8 remains closed. Simultaneously, the drag torque controller 31 calculates the pump suction side pressure limit value based on the allowable drag torque of the power source, the pump output side pressure, and the current pump swing angle. The drag torque controller 31 determines the electrical signal of the proportional relief valve 10 based on the pump suction side pressure limit value, and this electrical signal determines the opening pressure of the proportional relief valve 10. The drag torque controller 31 sends this electrical signal to the proportional relief valve 10. When the pump suction side pressure reaches this opening pressure, the proportional relief valve 10 opens, and a portion of the hydraulic oil in the second main oil circuit L2 flows into the oil tank 11 through the second side portion of the bridging oil circuit L3 (via the second brake switch valve 9) and the relief oil circuit L4 (via the proportional relief valve 10), thus limiting the pump suction side pressure to no higher than the pump suction side pressure limit value. By setting the opening pressure of the proportional relief valve 10, the hydrostatic constant speed drive system can generate a matching braking torque to maintain the vehicle's constant speed.

[0065] When the vehicle is in normal reverse driving mode, the second main oil circuit L2 is the high-pressure side, i.e., the pump output side, and the first main oil circuit L1 is the low-pressure side, i.e., the pump suction side. The drag torque controller 31 determines the vehicle braking state based on the pump control signal from the output switch 25. Furthermore, the drag torque controller 31 monitors and compares the pressure values ​​of pressure sensors 6 and 7 in real time; when the value of sensor 7 is less than the value of sensor 6, the drag torque controller 31 outputs an open signal to the first brake switch valve 8 to open the first brake switch valve 8, while the second brake switch valve 9 remains closed. Simultaneously, the drag torque controller 31 calculates the pump suction side pressure limit value. The drag torque controller 31 determines the electrical signal corresponding to the opening pressure of the proportional relief valve 10 based on the pump suction side pressure limit value. The drag torque controller 31 sends this electrical signal to the proportional relief valve 10. When the pump suction side pressure reaches this opening pressure, the proportional relief valve 10 opens, and a portion of the hydraulic oil in the first main oil circuit L1 flows into the oil tank 11 through the first side portion of the bridging oil circuit L3 (via the first brake switch valve 8) and the relief oil circuit L4 (via the proportional relief valve 10). This limits the pump suction side pressure to no higher than the pump suction side pressure limit value, thereby generating a matching braking torque and maintaining the vehicle at a constant speed.

[0066] When a vehicle is on a steep downhill slope, the hydrostatic constant speed drive system needs to provide matching braking torque to maintain a constant vehicle speed. If the proportional relief valve 10 is set to a constant opening pressure (relief pressure) based on the maximum drag torque and maximum pump sway angle, this pressure may not be sufficient to maintain the braking pressure required to maintain a constant speed, thus failing to achieve the same constant speed as the set speed. This hydrostatic constant speed drive system includes a drag torque controller 31, which solves this problem. Pressure feedback from the first pressure sensor 6 and the second pressure sensor 7, along with the pump sway angle value from the output switch 25, simultaneously enter the drag torque controller 31 to determine whether braking is in progress. If braking is in progress, and depending on the vehicle's direction of travel, the drag torque controller 31 outputs an open signal to the brake switch valve 17 or 18, and outputs a pressure limit value related to the current pump sway angle to the proportional relief valve 10, thereby generating matching braking torque to maintain the vehicle's constant speed.

[0067] According to another feasible embodiment of the present application, a hydrostatic constant speed drive system in Figure 3 The Chinese character represents a schematic representation. Figure 3 The implementation methods and Figure 1 The difference in the implementation method is that the second end of the overflow oil passage L4 is not connected to the oil tank, but to the return oil passage L5. One end of the return oil passage L5 is connected to the first main oil passage L1, and the other end is connected to the second main oil passage L2. A first check valve 32 is provided in the first side portion of the return oil passage L5, and a second check valve 33 is provided in the second side portion. The second end of the overflow oil passage L4 is connected to the middle portion of the return oil passage L5 located between the first and second side portions. The first and second check valves 32 and 33 are oriented in opposite directions and are oriented to allow hydraulic oil to flow from the middle portion of the return oil passage L5 to the first and second main oil passages L1 and L2, but not to allow hydraulic oil to flow from the first and second main oil passages L1 and L2 to the middle portion of the return oil passage L5.

[0068] for Figure 3 In the hydrostatic constant speed drive system shown, the towing torque controller 31 determines the opening pressure of the proportional relief valve 10. When the pump suction side pressure reaches the opening pressure of the proportional relief valve 10, the proportional relief valve 10 opens, and a portion of the hydraulic oil in the pump suction side oil circuit flows into the pump output side oil circuit through the portion of the bridging oil circuit L3 connected to the pump suction side (via the corresponding brake switch valve 9 or 8), the relief oil circuit L4 (via the proportional relief valve 10), and the portion of the return oil circuit L5 connected to the pump output side (via the corresponding check valve 33 or 32). Therefore, the pump suction side pressure is limited to not exceeding the permissible pump suction side pressure.

[0069] Figure 3 Other aspects of the hydrostatic constant speed drive system in China Figure 1 , Figure 2The hydrostatic constant speed drive system shown is the same, so it will not be described again here.

[0070] In summary, the hydrostatic constant speed drive system according to this application has a fully constant speed control function, requiring no human intervention and greatly reducing the driver's workload. Furthermore, it exhibits good speed control stability and high control precision.

[0071] Furthermore, it adapts to various road conditions; it improves the system's hydrostatic braking capability, maintaining constant speed even on steeper downhill slopes. Moreover, it fully utilizes the braking capacity of the power source (especially the engine or electric motor), minimizing heat generation in the hydraulic system.

[0072] In addition, the absence of service brakes avoids the risk of service brake system failure and improves driving safety.

[0073] While this application has been described herein with reference to specific exemplary embodiments, the scope of this application is not limited to the details shown. Various modifications may be made to these details without departing from the basic principles of this application.

Claims

1. A hydrostatic constant speed drive system, comprising: Variable pump (1); A variable displacement motor (2) has its inlet connected to the output port of a variable displacement pump (1) via a first main oil passage (L1), and its outlet connected to the input port of the variable displacement pump (1) via a second main oil passage (L2). The variable displacement motor (2) is equipped with a motor speed sensor (3); and The controller (20) is configured to execute the non-braking working mode and the braking working mode of the hydrostatic constant speed drive system based on the comparison result between the received speed input signal (27) and the actual motor speed detected by the motor speed sensor (3), so as to realize the constant speed drive operation of the hydrostatic constant speed drive system. In the non-braking operating mode, within the first speed range of the variable motor (2), the controller (20) controls the swing angle of the variable motor (2) to remain at its maximum, and increases the swing angle of the variable pump (1) to increase the speed of the variable motor (2), or decreases the swing angle of the variable pump (1) to decrease the speed of the variable motor (2); within the second speed range of the variable motor (2) which is higher than the first speed range, the controller (20) controls the swing angle of the variable pump (1) to remain at its maximum, and decreases the swing angle of the variable motor (2) to increase the speed of the variable motor (2), or increases the swing angle of the variable motor (2) to decrease the speed of the variable motor (2); the boundary point between the first speed range and the second speed range corresponds to the speed of the variable motor (2) when both the variable pump (1) and the variable motor (2) are at their maximum swing angle; In the braking mode, the controller (20) implements hydrostatic braking by controlling the reduction of the swing angle of the variable pump (1), thereby reducing the speed of the variable motor (2).

2. The hydrostatic constant speed drive system as described in claim 1, wherein, The controller (20) includes a pump controller (21) and a motor controller (22), the pump controller (21) being configured to cooperate with the motor controller (22) to implement the non-braking operating mode, and the pump controller (21) being configured to implement the braking operating mode independently.

3. The hydrostatic constant speed drive system as described in claim 2, wherein, The pump controller (21) includes a pump drive controller (23), a hydrostatic brake controller (24), and an output switch (25). The pump drive controller (23) is configured to generate a pump swing angle command in a non-braking operating mode, and the hydrostatic brake controller (24) is configured to generate a pump swing angle command in a braking operating mode. The pump swing angle commands generated by the drive controller (23) and the hydrostatic brake controller (24) are sent to the variable pump (1) through the output switch (25) to control the swing angle of the variable pump (1).

4. The hydrostatic constant speed drive system as described in claim 2 or 3, wherein, When the hydrostatic constant speed drive system is in operation, one of the first and second main oil circuits is the pump output side oil circuit and the other is the pump suction side oil circuit; The hydrostatic constant speed drive system also includes: A jumper circuit (L3) is connected between the first and second main oil circuits and is equipped with normally closed first and second brake switch valves; and An overflow oil passage (L4) is connected to the intermediate part of the bridging oil passage (L3) located between the first and second brake switch valves, and a proportional overflow valve (10) is arranged in the overflow oil passage (L4); In the braking operation mode, when the oil pressure on the pump suction side is higher than the opening pressure of the proportional relief valve (10), the proportional relief valve (10) is opened due to the pressure, and the brake switch valve located on the pump suction side of the first and second brake switch valves is opened, causing the oil circuit on the pump suction side to overflow, so that the oil pressure on the pump suction side is not higher than the opening pressure of the proportional relief valve (10).

5. The hydrostatic constant speed drive system as described in claim 4, wherein, The overflow oil passage (L4) leads to the oil tank (11).

6. The hydrostatic constant speed drive system as described in claim 4, wherein, The hydrostatic constant speed drive system also includes a return oil passage (L5), which is equipped with first and second check valves. The overflow oil passage (L4) leads to the middle part of the return oil passage (L5) located between the first and second check valves.

7. The hydrostatic constant speed drive system as described in any one of claims 4 to 6, wherein, The hydrostatic constant speed drive system further includes a first pressure sensor (6) installed in the first main oil circuit (L1) and a second pressure sensor (7) installed in the second main oil circuit (L2); and The controller (20) further includes a drag torque controller (31), the drag torque controller (31) being configured to: The pump suction side oil circuit pressure and the pump output side oil circuit pressure are obtained by the first and second pressure sensors; In the braking operating mode, an electrical signal corresponding to the opening pressure of the proportional relief valve (10) is determined and sent to the proportional relief valve (10); and In braking operation mode, when the oil pressure on the pump suction side is higher than the oil pressure on the pump output side, the brake switch valve closer to the pump suction side oil line of the first and second brake switch valves is opened, while the brake switch valve closer to the pump output side oil line is kept closed.

8. The hydrostatic constant speed drive system as described in claim 7, wherein, In the braking mode, the electrical signal determined by the drag torque controller (31) corresponding to the opening pressure of the proportional relief valve (10) is related to the pump swing angle, and the opening pressure value of the proportional relief valve (10) is set so that the hydrostatic braking torque generated by the hydrostatic constant speed drive system can maintain the constant speed of the hydrostatic constant speed drive system.

9. The hydrostatic constant speed drive system as described in claim 8, wherein, In the braking working mode, the drag torque controller (31) calculates the pump suction side oil circuit pressure limit value based on the allowable drag torque of the power source of the variable pump (1), the pump output side oil circuit pressure, and the current pump swing angle, and determines the electrical signal corresponding to the opening pressure of the proportional relief valve (10) based on the pump suction side pressure limit value.

10. The hydrostatic constant speed drive system according to any one of claims 1 to 9, wherein, The variable pump (1) is an electro-proportional variable pump, and the variable motor (2) is an electro-proportional variable motor.

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