Abstract:
A variable displacement pump control system comprising a variable displacement pump having a movable element for controlling pump displacement, a hydraulic motor for moving the movable element, and a transducer for producing an electric signal corresponding to the actual position of the movable element. A comparator compares the electrical signal from the transducer and an electrical signal corresponding to the desired displacement of the pump and produces an error signal. A control operates in response to the error signal to meter fluid flow from the pump output to the hydraulic motor. The control comprises a relief valve, a compensator valve and a servo valve. The servo valve is preferably a single-stage valve that includes damping orifices to provide stability.

Description:
This invention relates to variable displacement pumps and particularly to variable displacement pump control systems. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     In the use of variable displacement hydraulic pumps, it is desirable to vary the displacement of pump in response to a control in order to obtain maximum efficiency. It has heretofore been suggested that an electro hydraulic control system be provided. Typical such systems are shown in U.S. Pat. No. 4,139,987 wherein the system senses the load and varies the displacement. 
     Most prior art servo controls for controlling pump displacement through yoke positioning use a two-stage servo valve with a flapper nozzle arrangement as typified in U.S. Pat. No. 4,139,987. Such arrangements are costly to manufacture. Other two-stage servo valves used in this type application use a two spool arrangement but the small pilot spool stage has a low tolerance to contamination due to the small size. 
     Since single-stage servo valves are inherently more contamination tolerant and less costly than two-stage servo valves, they have been considered for use with the yoke-positioning controls of hydraulic pumps. However, such single-stage servo valves and pump combinations up to now have been limited to these applications which require relatively low flow rates and low speed response times, for example 500 milliseconds. In such applications requiring higher flow rates and faster response times, for example 70-100 milliseconds, single-stage servo valves become unstable due to the large flow forces action on the spool of the single-stage valve. For this reason the two-stage servo valve and pump combination predominate where higher flow rates and faster response times are required. 
     Among the objects of the present invention are to provide an electro hydraulic variable displacement pump control system which functions in response to actual movement of a movable element in the pump which controls displacement of the pump and functions to quickly and efficiently control the pump displacement. 
     In accordance with the invention, the variable displacement pump control system comprises a variable displacement pump having a movable element for controlling pump displacement, a hydraulic motor for moving the movable element, and a control module comprising a transducer for producing an electric signal corresponding to the actual position of the movable element, means for producing an electrical signal corresponding to the desired pump displacement, means for comparing the electrical signal from the transducer and the electrical signal corresponding to the desired displacement of the pump and producing an error signal, and a control operable in response to the error signal to meter fluid flow from the pump output to the hydraulic motor. The control comprises a relief valve, a compensator valve and a servo valve. The servo valve is preferably a single-stage valve that includes damping orifices to provide stability. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of the variable displacement control system embodying the invention. 
     FIG. 2 is a longitudinal sectional view of a solenoid controlled servo element used in the system. 
     FIG. 3 is a fragmentary sectional view on an enlarged scale of the servo element shown in FIG. 2. 
     FIG. 4 is a block diagram showing the manner in which the electronic controller can be made to control pump displacement in response to differential pressure, engine torque or engine speed. 
    
    
     DESCRIPTION 
     Referring to FIG. 1, the variable displacement control system embodying the invention comprises a variable displacement pump 10 having a movable element 11 for controlling pump displacement, and a control module 15 comprising a position transducer 12 operable to produce an electrical signal corresponding to the actual position of the movable element, a comparator 13 for comparing the electrical signal from the transducer 12 and a command electrical signal corresponding to the desired position of the movable element and producing an error signal, and an electronic modulator 14 for modulating the error signal. 
     The pump 10 is preferably of the yoke controlled type wherein the element 11 is a yoke movable by a yoke actuating cylinder 16 against the action of a yoke return cylinder 17. The yoke return cylinder 17 urges the yoke to a position corresponding to full displacement of the pump. 
     The control module 15 is operable below a predetermined output pressure of the pump 10. The control module 15 operates by metering fluid flow from the pump 10 output to or from the yoke actuator cylinder 16 in the pump 10 in response to an error signal. The error signal is generated by the comparator element 13 which sums the position signal from the yoke position transducer 12 and a command signal representing the desired yoke position, i.e. subtracts the signal representing the actual yoke position from the input signal representing the desired yoke position. The error signal is transmitted to the electronic modulator 14 which converts the error signal to a pulse train signal having a pulse width proportional to the magnitude of the error signal. 
     The control module 15 further comprises a control in the form of a maximum pressure relief element 18, a compensator element 19, and a servo element 20. 
     The relief element 18 is adjustable for setting a preselected maximum system pressure and opens the compensator element 19 to a reservoir tank upon sensing the preselected maximum pressure. 
     The compensator element 19 is a three-way valve having a compensator spool positioned between a pilot chamber 20a at one end of the compensator element, and a spring chamber 21 at the opposite end of the compensator element. A spring member 22 acting on the spool member is positioned in the spring chamber. The spring chamber 21 serves as an accumulator volume 21a and has the spring member 22 acting on the spool member positioned therein. A passage 23 in the spool member connects the pilot chamber with the spring chamber and is formed with a restriction or orifice 24. The pilot chamber 20 is connected to the output of the pump by lines 25, 26 for applying pump output pressure against the spool member. The spring chamber 21 is connected to the input of the relief element 18. 
     The spring rate of the spring member 22 is selected to oppose movement of the compensator spool by the pump output pressure until the output pressure exceeds the pressure setting of the relief element 18. The combination of the orifice 24 and the accumulator volume of the spring chamber 21 serve to dampen momentary excessive rates of pressure rise that may occur in the pump output and prevent premature actuation of the relief element. With the foregoing construction of the compensator element 19, the compensator spool remains stationary up to the pressure setting of the relief valve element 18. 
     The compensator element 19 further includes first, second and third ports 27, 28, 29. The first port 27 connects the compensator element 19 to the head end 16a of the yoke actuator cylinder 16 through line 30; the second port 28 connects the compensator element 19 to the service port 38 of the servo element 20 through line 31; and the third port 29 connects the compensator element 19 to the output of the pump 10 through line 32. With the spool member in its spring-held position, the first and second ports 27, 28 are in communication with each other and communication is established between service port 38 of servo element 20 and the head end 16a of the actuator cylinder 16. With the spool member shifted by pressure at the maximum pressure setting from the pump output, communication between the servo output port and the head end 16a of the actuator cylinder 16 is interrupted and communication between the third port 29 and the head end 16a of the actuator cylinder is established. 
     The servo element 20 serves as a three-way valve having a pressure port 33 connected to the pump output through lines 25, 34, a tank port 35 connected to the reservoir tank by lines 36, 37, and as previously mentioned, a service port 38 connected to the head end 16a of the actuator cylinder 16 through the second port 28 of the compensator element 19. Referring to FIG. 2 the servo element also includes a servo spool 40 having a service port land 41 which serves to cut off fluid flow between the service port 38 and both pressure port 33 and the tank port 35 depending on the position of the servo spool 40. The servo spool 40 is positioned between a pair of proportional solenoids 42, 43 and also between a pair of opposed servo spring members 42a, 43a positioned in spring chambers 42b, 43b. The spring members act to center the servo spool in a neutral position in the absence of energization of either of the solenoids. In the neutral position of the servo spool 40, the service land 41 blocks flow from either the pressure or tank ports to or from the service port 38. However, the service land 41 is formed with an underlap 44, i.e. the width of the land is slightly less than the opening of the service port, and in the neutral position the underlap 44 with respect to the service port forms a restricted passage or orifice between the service port 38 and the pressure and tank ports 33, 35. In the neutral position these orifices serve to vent the head end 16a of the actuator cylinder 16 to the reservoir tank thereby insuring positioning of the yoke to its full stroke position. 
     The servo spool 40 is formed with an axial passage 45 which is intersected by a first radial passage 46 which is at all times in communication with the tank port. The axial passage 45 connects the opposed spring chambers with the tank port through a pair of restrictions or orifices 47 formed at each end of the spool 40 in the inserts 40a and a pair of second radial passages 48 each of which is in communication with each spring chamber 42b, 43b in the neutral position of the servo spool. As the servo spool 40 shifts in either direction, the spool moves the fluid in the spring chamber toward which the spool is moving through the orifice 47, the axial passage 45 of the spool 40 and the orifice 47 in the far side. These orifices 47 restrict the flow from one spring chamber to the other to dampen the movement of the servo spool through the restricted displacement of fluid from one of the other of the spring chambers 42b, 43b thereby improving the response of the servo spool 40 to the energization of one or the other of the solenoids. (FIG. 3) 
     As previously mentioned, the electronic modulator 13 generates a pulse train signal having a pulse width proportional to the magnitude of the error signal. The electronic modulator 13 directs the generated signal to one of the other of the solenoids 42, 43 of the servo element depending on the sense of the error signal thereby energizing the appropriate solenoid, the width of the generated signal determining the magnitude of movement of the servo spool 40. Movement of the servo spool 40 serves to meter fluid flow to or from the head end 16a of the yoke actuator cylinder 16 through the compensator element 19. Movement of the servo spool 40 to the left, as shown in FIG. 1, directs pump output fluid flow to the cylinder 16 to destroke the yoke 11, i.e. reduce pump displacement, and movement to the right directs fluid flow from the head end 16a of cylinder 16 to the tank reservoir thereby allowing the yoke return cylinder 17 of the pump to onstroke the yoke 11 to increase pump displacement. In the event of the attainment of maximum pressure the compensator element 19 is actuated and it disconnects or overrides the servo element 20 from control of the pump yoke 11. 
     The manner in which the system can be utilized for controlling pressure, torque or speed is shown in FIG. 4. The pressure, torque, or speed is sensed using conventional transducers, not shown, and the signal generated by the transducer is fed to the electronic controller and compared with the desired pressure, torque, or speed signal and the difference between the sensed and desired signal generate another signal which is fed to the control module for on-stroking or de-stroking the pump.