Patent Publication Number: US-10767667-B2

Title: Electronically controlled valve, hydraulic pump, and hydraulic pump system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims foreign priority benefits under U.S.C. § 119 to Chinese Patent Application No. 201611030563.0 filed on Nov. 16, 2016, the content of which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to hydraulic technology, especially relates to an electronically controlled valve, a hydraulic pump with the electronically controlled valve, and a hydraulic pump system with switchable control functions. 
     BACKGROUND ART 
     A hydraulic pump is a power source in a hydraulic system, it converts mechanical energy from a driving motor or an engine into hydraulic energy for the hydraulic system&#39;s use. Different hydraulic systems or one hydraulic system in different working conditions has different requirements for pressure source, this requires that the hydraulic pump should have different control types to meet such requirements. 
     Control types for current hydraulic pumps are implemented mostly by using traditional mechanically controlled valves. For these mechanically controlled valves, a specific control function is implemented by a specific mechanical structure, and the combination of multiple functions is based on simple physical addition of single function. These mechanically controlled valves are complicated in structure and require a great variety of parts, which increases complexity of the assembly line and may cause errors easily. On the other hand, development period for these mechanically controlled valves is quite long, which results in higher investment and higher product cost. Furthermore, set values for each function of these mechanically controlled valves must be adjusted manually on a test stand, this is quite inflexible. 
     With the development of information technology and network technology, more and more hydraulic systems require seamless integration of hydraulic pumps to achieve digitalized and intelligent control for improving working efficiency of the hydraulic system, the traditional mechanically controlled valves cannot meet such requirement. 
     SUMMARY 
     An objective of the present invention is to provide an electronically controlled valve, a hydraulic pump based on an electronically controlled valve, and a hydraulic pump system with switchable control functions for at least partially solving at least one aspect of the aforementioned problems and mitigating or at least partially eliminating defects and deficiencies exist in the prior art. 
     To achieve the aforementioned objective, according to a first aspect of the present invention, an electronically controlled valve for a variable displacement pump is provided. The electronically controlled valve comprises: a control valve housing; a spool mounted displace-ably inside the control valve housing; and a spool control component. The spool control component works in at least three current levels to enable the spool to shift among at least three correspondent working positions: when the spool control component operates in an intermediate current I M , the spool works in a middle position enabling the displacement of the variable displacement pump to keep constant; and when the spool control component operates in one of a high current I H  higher than the intermediate current I M  and a low current I L  lower than the intermediate current I M , the spool works in a working position enabling the displacement of the variable displacement pump to keep increasing or decreasing. 
     According to an embodiment of the present invention, the electronically controlled valve is a digital valve, and the intermediate current I M , the high current I H  and the low current I L  are respectively discrete current values. 
     According to an embodiment of the present invention, the high current I H  of the electronically controlled valve is a current value within a continuous range higher than the intermediate current I M ; and the low current I L  is a current value within a continuous range lower than the intermediate current I M . 
     According to an embodiment of the present invention, the spool control component comprises: an electrical actuator and an adjusting spring. The electrical actuator and the adjusting spring are provided oppositely at two ends of the control valve housing and act on the spool in opposite direction. The electrical actuator applies different forces to the spool according to the current levels to move the spool to a correspondent working position. 
     According to an embodiment of the present invention, a predetermined spring force of the adjusting spring can be changed to adjust the value of the intermediate current I M  for the spool. 
     According to an embodiment of the present invention, the electronically controlled valve is arranged in a symmetrical structure, and positions of the electrical actuator and the adjustment spring at the two ends of the control valve housing are interchangeable. 
     According to an embodiment of the present invention, the control valve housing comprises: an inlet P which is in fluid communication with a pump outlet of the variable displacement pump; a work port A which is in fluid communication with a servo-mechanism for adjusting the displacement of the variable displacement pump; and an outlet T which is in fluid communication with a pump housing of the variable displacement pump. When the spool control component operates in the intermediate current I M , the electronically controlled valve works in the middle position, and the inlet P, the work port A and the outlet T are uncommunicated with each other, thereby enabling the displacement of the variable displacement pump to keep constant. When the spool control component operates in one current level of the high current I H  and the low current I L , the spool is displaced to enable fluid communication of the work port A and the outlet T to make the displacement of the variable displacement pump keep increasing. When the spool control component operates in the other current level of the high current I H  and the low current I L , the spool is displaced to enable fluid communication of the inlet P and the work port A to make the displacement of the variable displacement pump keep decreasing. 
     In addition, according to another aspect of the present application, a hydraulic pump based on the electronically controlled valve is provided. The hydraulic pump comprises: a variable displacement pump having a swash plate; an outlet piston chamber which is in constant communication with a pump outlet of the variable displacement pump, wherein, an outlet piston which is connected to an end of the swash plate is movably provided inside the outlet piston chamber; a servo piston chamber, wherein, a servo piston which is connected to the other end of the swash plate is movably provided inside the servo piston chamber; and the aforementioned electronically controlled valve, wherein, the electronically controlled valve is respectively in fluid communication with the pump outlet of the variable displacement pump, a pump housing, and the servo piston chamber through three ports on the control valve housing. The servo piston and the outlet piston act jointly on the swash plate to adjust an angle of the swash plate for changing the displacement of the variable displacement pump. 
     According to an embodiment of the present invention, the three ports of the electronically controlled valve respectively are: an inlet P which is in fluid communication with the pump outlet of the variable displacement pump; a work port A which is in fluid communication with the servo piston chamber; and an outlet T which is in fluid communication with the pump housing of the variable displacement pump. When the spool control component operates in the intermediate current I M , the electronically controlled valve works in the middle position, and the inlet P, the work port A and the outlet T are uncommunicated with each other, thereby enabling the displacement of the variable displacement pump to keep constant. When the spool control component operates in one current level of the high current I H  and the low current I L , the spool is displaced to enable fluid communication of the work port A and the outlet T to make the displacement of the variable displacement pump keep increasing. When the spool control component operates in the other current level of the high current I H  and the low current I L , the spool is displaced to enable fluid communication of the inlet P and the work port A to make the displacement of the variable displacement pump keep decreasing. 
     According to an embodiment of the present invention, the hydraulic pump further comprises a hydraulic control safety valve which is connected between the pump outlet and the servo piston chamber, the hydraulic control safety valve is configured to be opened when pressure at the pump outlet exceeds a predetermined value to enable a fluid to flow through the hydraulic control safety valve to enter into the servo piston chamber, thereby decreasing the displacement of the variable displacement pump, and closed when the pressure at the pump outlet does not exceed the predetermined value. 
     According to an embodiment of the present invention, the hydraulic control safety valve comprises: a safety valve housing; a hydraulic control spool, wherein, the hydraulic control spool is displace-ably mounted inside the safety valve housing; a hydraulic path, wherein, the hydraulic path is in fluid communication with the pump outlet, and enable the pressure of the pump outlet to act on the hydraulic control spool; and a set spring, wherein the set spring acts on the hydraulic control spool in a direction opposite to the action direction of the hydraulic path, and sets the predetermined value. 
     In addition, according to still another aspect of the present invention, a hydraulic pump system is provided. The hydraulic pump system comprises: the aforementioned hydraulic pump; at least one sensor which is connected to the hydraulic pump; and a controller which has at least one input end connected to the sensor and an output end connected to an electrical actuator of the electronically controlled valve of the hydraulic pump to perform control. 
     According to an embodiment of the present invention, the at least one sensor comprises at least one sensor selected from a group of the following sensors: an angle sensor which is used to detect an angle of the swash plate of the hydraulic pump; a first pressure sensor which is used to detect pump outlet pressure of the hydraulic pump; a speed sensor which is used to detect a rotation speed of the hydraulic pump; and a second pressure sensor which is used to detect load pressure. 
     According to an embodiment of the present invention, the output of the at least one sensor can be used for different control functions, and the at least one sensor and the controller are combined to form at least one of the following control configurations to perform at least one control function of the hydraulic pump: an electric proportional displacement control configuration which comprises the angle sensor and the controller, wherein, the controller calculates the displacement of the hydraulic pump based on an angle signal sensed by the angle sensor and control the electronically controlled valve to change the displacement of the hydraulic pump until a required displacement is reached; a pressure compensation control configuration which comprises the first pressure sensor and the controller, wherein the controller compares pump outlet pressure of the hydraulic pump detected by the first pressure sensor with a predetermined maximum working pressure, and controls the electronically controlled valve to change the displacement of the hydraulic pump to the minimum and keep the state when the pump outlet pressure of the hydraulic pump reaches to the predetermined maximum working pressure, and change the displacement of the hydraulic pump to the maximum and keep the state when the pump outlet pressure of the hydraulic pump is less than the predetermined maximum working pressure; a constant power control configuration which comprises the angle sensor, the speed sensor, the first pressure sensor and the controller, wherein, the controller calculates an input power of the pump based on the pump outlet pressure, the angle of the swash plate, the rotation speed and work efficiency of the hydraulic pump, and controls the electronically controlled valve to change the displacement of the hydraulic pump to maintain the input power of the hydraulic pump at a set value; and a load sensing control configuration which comprises the first pressure sensor, the second pressure sensor and the controller, wherein, the controller monitors the pressure values from the first pressure sensor and the second pressure sensor, and compares the delta value between the pressure values with a predetermined load sensing set value, in case the delta value is not equal to the load sensing set value, the controller controls the electronically controlled valve to change the displacement of the hydraulic pump until the delta value is equal to the load sensing set value. 
     The beneficial technique effects of the present invention include: 
     First, multiple control functions of different types of hydraulic pumps can be implemented via one single electronically controlled valve. Secondly, set parameters of control functions of hydraulic pumps can be changed conveniently, so that flexibility of hydraulic pump systems can be improved prominently and energy saving of hydraulic pump systems can be achieved, thereby improving efficiency of the overall application systems where the hydraulic pump systems are applied. Third, the control of the hydraulic pumps become more intelligent, and the integration of the hydraulic pumps with the overall application systems becomes very easy. Moreover, configurations of all control functions and priority levels of the control functions can be defined according to actual application requirements of customers. Furthermore, hydraulic pumps that exist in the market currently can be conveniently upgraded according to the present invention. Finally, the hydraulic pump systems are more compact because the peripheral control elements and sensors can be selected and detachably installed into/on the hydraulic pump systems, thus the hydraulic pump systems can be installed into different overall application systems easily. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the present invention are described with reference to the drawings, where reference numbers in the drawings represent correspondent components. The brief description of the drawings is as follows: 
         FIG. 1  is a schematic view of a hydraulic pump comprising an electronically controlled valve according to an embodiment of the present invention. 
         FIG. 2  is a schematic view of a hydraulic pump comprising an electronically controlled valve according to another embodiment of the present invention, wherein, a hydraulic control safety valve is included. 
         FIG. 3  is a schematic view of a hydraulic pump system comprising the hydraulic pump shown in  FIG. 1 ; 
         FIG. 4  is a schematic view of a hydraulic pump system comprising the hydraulic pump shown in  FIG. 2 ; 
         FIG. 5 a    is a schematic view of the hydraulic pump system as shown in  FIG. 3  in an electric proportional displacement control mode; 
         FIG. 5 b    is a schematic view of the hydraulic pump system as shown in  FIG. 4  in an electric proportional displacement control mode; 
         FIG. 6 a    is a schematic view of the hydraulic pump system as shown in  FIG. 3  in a pressure compensation control mode; 
         FIG. 6 b    is a schematic view of the hydraulic pump system as shown in  FIG. 4  in a pressure compensation control mode; 
         FIG. 7 a    is a schematic view of the hydraulic pump system as shown in  FIG. 3  in a constant power control mode; 
         FIG. 7 b    is a schematic view of the hydraulic pump system as shown in  FIG. 4  in a constant power control mode; 
         FIG. 8 a    is a schematic view of the hydraulic pump system as shown in  FIG. 3  in a load sensing control mode; 
         FIG. 8 b    is a schematic view of the hydraulic pump system as shown in  FIG. 4  in a load sensing control mode. 
     
    
    
     DETAILED DESCRIPTION 
     Technical solution of the present invention is explained in further detail below by way of embodiments in conjunction with  FIGS. 1-8   b . In this description, identical or similar reference numbers and letters indicate identical or similar components. The following description of embodiments of the present invention with reference to the drawings is intended to explain the general inventive concept of the present invention, and should not be interpreted as a limitation of the present invention. 
     Drawings are used to describe the contents of the present invention. Size and shape of components in the drawings do not reflect actual proportions of components in a hydraulic pump and a system comprising the hydraulic pump. 
     According to the general concept of the present invention, an electronically controlled valve is provided. The electronically controlled valve comprises: a control valve housing, a spool, an electrical actuator and an adjusting spring. The control valve housing comprising a P port, an A port and a T port. The P port is in communication with a pump outlet of a variable displacement pump via a first path. The A port is in communication with a servo piston chamber via a second path. The T port is in communication with a pump housing via a third path. The spool is mounted displace-ably inside the control valve housing. The electrical actuator is connected to the spool at one end of the control valve housing and the adjusting spring is provided at the other end of the control valve housing, thus the adjusting spring and the electrical actuator act on the spool oppositely. The spool works in three positions. When the spool works in a middle position, the P port, the A port and the T port are uncommunicated from each other; when the spool works in a servo pressure-decreasing position, the spool is in a position that enables communication between the A port and the T port; when the spool works in a servo pressure-increasing position, the spool is in a position that enables communication between the P port and the A port. The electrical actuator works in three current levels to enable the spool to shift among the three working positions. When the electrical actuator works in an intermediate current I M , the spool is in the middle position; when the electrical actuator works in a current level different from the intermediate current I M , the spool is moved to the servo pressure-decreasing position or the servo pressure-increasing position in the control valve housing. This current level which is different from the intermediate current I M  may be a high current I H  higher than the intermediate current I M  or a low current I L  lower than the intermediate current I M . 
     As an exemplary embodiment, the electronically controlled valve is a three-position three-way electronically controlled valve with one end provided with an electrical actuator and one end provided with an adjusting spring, and the electrical actuator and the adjusting spring are interchangeable to implement positive control or negative control. 
     As an exemplary embodiment, the electronically controlled valve is a digital valve, and the intermediate current I M , the high current I H  and the low current I L  are respectively discrete current values. 
     As an exemplary embodiment, the electrical actuator comprises, but is not limited to, a solenoid, a proportional solenoid, a relief valve, an electric proportional relief valve. 
       FIG. 1  is a schematic view of a hydraulic pump comprising an electronically controlled valve according to an embodiment of the present invention. As shown in  FIG. 1 , the hydraulic pump  1  comprises: a variable displacement pump  11  which is driven by a driving shaft  12 , an electronically controlled valve  20 , a servo piston chamber  13  and an outlet piston chamber  14 . The variable displacement pump  11  is, for example, an axial piston pump having a swash plate  133 . The angle of the swash plate  133  is adjusted by joint action of a servo piston  131  and an outlet piston which are connected respectively to two ends of the swash plate  133 . The electronically controlled valve  20  is, for example, a three-position three-way digital valve with its spool in a middle position (shown in  FIG. 1 ). The servo piston chamber  13  is provided with the servo piston  131  and a first spring  132  inside. The outlet piston chamber  14  comprises the outlet piston and a second spring. 
     In addition, the hydraulic pump  1  may further comprise a constant displacement pump  10 . The constant displacement pump  10  and the variable displacement pump  11 , for example, are driven by the same driving shaft  12  and arranged in series connection. (for example, as shown in  FIG. 1 , the constant displacement pump  10  is located in an upstream of the variable displacement pump  11 ), thereby substantially forming a pump group. 
     The electronically controlled valve  20  is, for example, a digital valve, which comprises a spool  201 , a control valve housing  202 , a solenoid actuator  203  and an adjusting spring  204 . The spool  201  is mounted displace-ably inside the control valve housing  202 . The control valve housing  202  of the electronically controlled valve  20  comprises a P port, an A port and a T port. The P port is in communication with a pump outlet  112  of the variable displacement pump  11  via a first path  15 . The A port is in communication with the servo piston chamber  13  via a second path  16 . The T port is in communication with a pump housing  18  via a third path  17 . 
     As shown in  FIG. 1 , the electronically controlled valve  20  is a three-position three-way valve, and works in at least three different current levels. 
     When the solenoid actuator  203  works in the high current I H , it generates an electromagnetic force which is greater than a spring force of the adjusting spring  204 , thereby enabling the spool  201  to move to a servo pressure-decreasing position, that is, a left position shown in  FIG. 1  (a position close to the solenoid actuator  203 ). In this case, the A port is in communication with the T port, and the pressure in the servo piston chamber  13  reduces. As the outlet piston chamber  14  is in constant communication with the pump outlet  112 , the outlet piston drives the swash plate  133  to rotate under the action of the high pressure of the pump outlet  112  of the variable displacement pump  11 , and the tilt angle of the swash plate  133  increases. The servo piston is driven by the swash plate  133  to move in an opposite direction, and the first spring  132  of the servo piston chamber  13  ensures constant contact between the servo piston  131  and the swash plate  133 . In this case, the displacement of the variable displacement pump  11  keeps increasing. 
     Moreover, as shown in  FIG. 1 , when the solenoid actuator  203  works in the low current I L , it generates an electromagnetic force which is smaller than a spring force of the adjusting spring  204 , as a result, the spool  201  moves to a servo pressure-increasing position, that is, a right position shown in  FIG. 1  (a position close to the adjusting spring  204 ). In this case, the P port is in communication with the A port, and the servo piston chamber  13  is in communication with the pump outlet  112 . The servo piston  131  drives the swash plate  133  to rotate under the action of the high pressure of the pump outlet  112  of the variable displacement pump  11 , and the tilt angle of the swash plate  133  decreases. The outlet piston is driven by the swash plate  133  to move in an opposite direction, and the second spring of the outlet piston chamber  14  ensures constant contact between the outlet piston and the swash plate  133 . In this case, the displacement of the variable displacement pump  11  keeps decreasing. 
     Based on the aforementioned principle, when the solenoid actuator  203  works in a high current level to enable the displacement of the variable displacement pump  11  to increase, the electronically controlled valve  20  is conducting positive control. In contrast, when the solenoid actuator  203  works in a high current level to enable the displacement of the variable displacement pump  11  to decrease, the electronically controlled valve  20  is conducting negative control. As the electronically controlled valve  20  can be designed into a symmetrical structure, the adjusting spring  204  and the solenoid actuator  203  respectively at two ends of the electronically controlled valve  20  can be simply exchanged to obtain a positive control function or a negative control function. Furthermore, a predetermined spring force of the adjusting spring  204  can be changed to adjust the value of the intermediate current I M  for the spool  201 . 
       FIG. 2  is a schematic view of a hydraulic pump  1 ′ comprising an electronically controlled valve  20  according to another embodiment of the present invention. The hydraulic pump  1 ′ further comprises a hydraulic control safety valve  30 . The hydraulic control safety valve  30  is used to provide safety protection for the hydraulic pump  1 ′ shown in  FIG. 1 . Specifically, the hydraulic control safety valve  30  is a two-position two-way valve which comprises a hydraulic control spool  301 , a safety valve housing  302 , a hydraulic path  303  and a set spring  304 . When a hydraulic force generated by the pump outlet pressure of the variable displacement pump  11  acting on the hydraulic control spool  301  is greater than a set force of the set spring  304 , the hydraulic control spool  301  works in a communicating position (left position as shown in  FIG. 2 ). In this case, a high pressure fluid from the pump outlet  112  of the variable displacement pump  11  is in communication with the servo piston chamber  13 , and the servo piston  13  de-strokes the variable displacement pump  11  to the minimum displacement under the action of the high pressure fluid. As there is no orifice between the servo piston chamber  13  and the hydraulic control safety valve  30 , the variable displacement pump  11  can have a rapid response. The hydraulic control safety valve  30  acts as a safety protection device, it can be optionally included in the following described hydraulic pump systems comprising the electronically controlled valve  20 . Details description of the hydraulic control safety valve  30  will be omitted for these hydraulic pump systems. 
     When each of the hydraulic pumps in  FIG. 1  or  FIG. 2  is equipped with a combination of controller(s) and sensor(s), a hydraulic pump system can be formed for implementing one or more control functions. In an actual application, a sensor is chosen according to a control function to be implemented, and multiple control functions can be implemented via the selected sensors. The sensor(s) can be selected to be detachably mounted in and connected to the hydraulic pump system for implementing certain control function(s). Alternatively, various sensors can be mounted in the hydraulic pump system in advance, and the implementation of a certain control function is realized by turning on or off sensor(s). The control functions comprise, but are not limited to, electric proportional displacement control, constant power control, pressure compensation control and load sensing control. 
     The aforementioned hydraulic pump system with various sensors mounted in advance will be described in detail hereafter, wherein, the implementation of a certain control function is realized by turning on or off sensor(s); and wherein, the electronically controlled valve comprised in this system conducts positive control in all following examples. 
     Specifically, as shown in  FIG. 3 , the hydraulic pump  1  shown in  FIG. 1  is installed with a controller  31  and several sensors. The sensors comprise, but are not limited to, an angle sensor  32 , a first pressure sensor  33 , a speed sensor  34  and a second pressure sensor  35 . The controller  31  has at least one input end connected to a sensor and an output end connected to the solenoid actuator  203  of the electronically controlled valve  20  for controlling the solenoid actuator  203 . The angle sensor  32  is used to detect a swashplate angle. The first pressure sensor  33  is used to detect pump outlet pressure. The speed sensor  34  is used to detect a rotation speed of the hydraulic pump  1 . The second pressure sensor  35  is used to detect load pressure. 
     The hydraulic pump system shown in  FIG. 3  with multiple control functions will be described in detail hereafter. Wherein the implementation of a certain control function is realized by turning on or off sensor(s). 
     I. Electric Proportional Displacement Control 
       FIG. 5 a    is a schematic view of the hydraulic pump system according to the embodiment of the present invention shown in  FIG. 3  in an electric proportional displacement control mode, wherein, the first pressure sensor  33 , the speed sensor  34  and the second pressure sensor  35  in the hydraulic pump system shown in  FIG. 3  are turned off. Of course, the hydraulic pump system shown in  FIG. 5 a    may also be obtained by mounting the controller  31  and the angle sensor  32  to the hydraulic pump  1  shown in  FIG. 1 . 
     In the hydraulic pump system shown in  FIG. 5 a   , the electronically controlled valve  20  works with the controller  31  and the angle sensor  32  to implement electric proportional displacement control. 
     Specifically, when the hydraulic pump system needs to increase displacement, the controller  31  provides a high current I H  to the solenoid actuator  203  to make the electronically controlled valve  20  work in the servo pressure-decreasing position, wherein, the A port and the T port are in fluid communication to enable communication between the servo piston chamber  13  and the pump housing  18 , so that the displacement of the hydraulic pump  1  increases. During the process, the controller  31  monitors output of the angle sensor  32 . When the displacement of the hydraulic pump  1  increases to meet the requirement of the system, the controller  31  provides an intermediate current I M  to the solenoid actuator  203  to make the electronically controlled valve  20  work in the middle position, so that the hydraulic pump  1  keeps working at current displacement. Similarly, when the hydraulic pump system needs to decrease displacement, the controller  31  provides a low current I L  to the solenoid actuator  203  to make the electronically controlled valve  20  work in the servo pressure-increasing position, wherein, the angle sensor  32  is used to monitor the swashplate angle when the displacement of the hydraulic pump decreases. When the required displacement is reached, the intermediate current I M  is provided to the solenoid actuator  203  to make the electronically controlled valve  20  work in the middle position, so that the hydraulic pump  1  works stably at current displacement. 
     II. Pressure Compensation Control 
       FIG. 6 a    is a schematic view of the hydraulic pump system according to the embodiment of the present invention shown in  FIG. 3  in a pressure compensation control mode, wherein, the angle sensor  32 , the speed sensor  34 , and the second pressure sensor  35  in the hydraulic pump system shown in  FIG. 3  are turned off. Of course, the hydraulic pump system shown in  FIG. 6 a    may also be obtained by mounting the controller  31  and the first pressure sensor  33  to the hydraulic pump  1  shown in  FIG. 1 . 
     In the hydraulic pump system shown in  FIG. 6 a   , the electronically controlled valve  20  works with the controller  31  and the first pressure sensor  33  to implement pressure compensation control. 
     Specifically, when the hydraulic pump system works, the controller  31  detects and monitors pump outlet pressure of hydraulic pump  1  via the first pressure sensor  33 . When the pump outlet pressure reaches to a predetermined maximum working pressure, the controller  31  provides the low current I L  to the solenoid actuator  203  to make the electronically controlled valve  20  work in the servo pressure-increasing position. After the displacement of the hydraulic pump  1  decreases to the minimum level, the intermediate current I M  is provided to the solenoid actuator  203  to keep the hydraulic pump  1  working stably at the minimum displacement. In case that the external load decreases and the pump outlet pressure decreases to a level lower than the predetermined maximum working pressure, the controller  31  provides the high current I H  to the solenoid actuator  203  to increase the displacement of the hydraulic pump  1 . When the displacement of the hydraulic pump  1  reaches to the maximum level, the intermediate current I M  is provided to the solenoid actuator  203  to keep the hydraulic pump  1  working stably at the maximum displacement. 
     A pressure compensation set value which is used as a pressure comparison reference value may be set as different value for different application. 
     III. Constant Power (Torque) Control 
       FIG. 7 a    is a schematic view of the hydraulic pump system according to the embodiment of the present invention shown in  FIG. 3  in a constant power control mode, wherein, the second pressure sensor  35  in the hydraulic pump system shown in  FIG. 3  is turned off. Of course, the hydraulic pump system shown in  FIG. 7 a    may also be obtained by mounting the controller  31 , the angle sensor  32 , the speed sensor  34  and the first pressure sensor  33  to the hydraulic pump  1  shown in  FIG. 1 . 
     In the hydraulic pump system shown in  FIG. 7 a   , the electronically controlled valve  20  works with the controller  31 , the angle sensor  32 , the speed sensor  34  and the first pressure sensor  33  to implement constant power (torque) control. 
     Specifically, when the hydraulic pump system works, the controller  31  monitors working pressure of the hydraulic pump  1  via the first pressure sensor  33 , the swashplate angle via the angle sensor  32  and the pump rotation speed via the speed sensor  34 , and then calculates a current input power of the hydraulic pump with consideration of the work efficiency of the hydraulic pump. When the input power of hydraulic pump  1  reaches to a set value, if working pressure of the hydraulic pump  1  needs to increase according to a system load, the controller  31  provides the low current I L  to the solenoid actuator  203  to decrease the displacement of the hydraulic pump  1  to ensure that the input power of the hydraulic pump  1  is kept at the set value. If the system load decreases, the controller  31  provides the high current I H  to the solenoid actuator  203  to increase the displacement of the hydraulic pump  1  to a level for maintaining the input power of the hydraulic pump  1  at the set value, or to the maximum level. 
     A constant power set value which is used as a power comparison reference value may be set as different value for different application. 
     IV. Load Sensing Control 
       FIG. 8 a    is a schematic view of the hydraulic pump system according to the embodiment of the present invention shown in  FIG. 3  in a load sensing control mode, wherein, the angle sensor  32  and the speed sensor  34  in the hydraulic pump system shown in  FIG. 3  are turned off. Of course, the hydraulic pump system shown in  FIG. 8 a    may also be obtained by mounting the controller  31 , the first pressure sensor  33  and the second pressure sensor  35  to the hydraulic pump  1  shown in  FIG. 1 . 
     In the hydraulic pump system shown in  FIG. 8 a   , the electronically controlled valve  20  works with the controller  31 , the first pressure sensor  33  and the second pressure sensor  35  to implement load sensing control. 
     Specifically, when the hydraulic pump system works, the first pressure sensor  33  monitors the pump outlet pressure, and the second pressure sensor  35  monitors load sensing feedback pressure. The controller  31  monitors and compares pressure values from the two pressure sensors. When the pump outlet pressure is not equal to a sum of the load sensing feedback pressure and a load sensing set value, the controller  31  provides one of the high current I H  and the low current I L  to the solenoid actuator  203  to change the displacement of the hydraulic pump  1  until the pump outlet pressure is equal to the sum of the feedback pressure and the load sensing set value, at this time, the controller  31  provides the intermediate current I M  to the solenoid actuator  203  to keep the hydraulic pump  1  working stably in current state. 
     A load sensing set value which is used as a comparison reference value may be set to different value for different ideal load condition. 
     Similarly, based on the aforementioned embodiments, other embodiments may be implemented with changes and variations. 
       FIG. 4  is a schematic view of a hydraulic pump system comprising the hydraulic pump shown in  FIG. 2 , wherein the hydraulic control safety valve  30  is included.  FIG. 5 b    shows the hydraulic pump system of  FIG. 4  in an electric proportional displacement control mode;  FIG. 6 b    shows the hydraulic pump system of  FIG. 4  in a pressure compensation control mode;  FIG. 7 b    shows the hydraulic pump system of  FIG. 4  in a constant power control mode;  FIG. 8 b    shows the hydraulic pump system of  FIG. 4  in a load sensing control mode. 
     In addition, according to the aforementioned embodiments of the present invention, it should be understood that any technical solution implementing a combination of any two or more of the aforementioned control functions via integration of required sensors also falls within the protection scope of the present invention. 
     It should be understood that the position terms such as “up”, “down”, “left” and “right” in the description of the present invention are used for explaining the position relationship shown in the drawings. These position terms should not be construed as limitation to the protection scope of the present invention. 
     The embodiments of the present invention are described in a progressive manner, and each embodiment focuses on differences from the other embodiments. The same or similar parts of the embodiments are referable for each other. 
     The description of the aforementioned embodiments is used to help understanding the present invention rather than to limit the scope of the present invention. 
     While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.