Hydraulic pump with speed and pressure compensation

A method of controlling a pump displacement of a variable displacement pump, the variable displacement pump having a yoke carrying a swashplate, the variable displacement pump further having a rotating group being driven by an electric motor to rotate about a drive shaft axis, the electric motor providing a variable rotational speed for the rotating group of the variable displacement pump, and the yoke being pivotally movable relative to the drive shaft axis, the method comprising the steps of:

• establishing a maximum displacement position for the yoke;
• determining whether the speed of the electric motor exceeds a predetermined speed;
• when the variable rotational speed of the electric motor exceeds the predetermined speed, actuating a speed compensator that includes a displacement limiting actuator that is adapted to pivot the yoke toward a minimum displacement position for establishing a new maximum displacement position that decreases a maximum permitted stroke length allowed by the swashplate, the new maximum displacement position being inversely proportional to the variable rotational speed of the electric motor;
• determining whether pump pressure at a pump outlet of the variable displacement pump exceeds a predetermined pressure;
• and
• when the pump pressure exceeds the predetermined pressure, actuating a pressure compensator that includes a pressure compensating actuator that is adapted to further pivot the yoke from the new maximum displacement position set by the displacement limiting actuator of the speed compensator toward the minimum or no displacement position so that the stroke length allowed by the swashplate is less than the maximum permitted stroke length.

A pump system comprising:

• a variable displacement pump including:
• a pump housing defining a pump inlet and a pump outlet;
• a drive shaft that extends through the pump housing, the drive shaft being rotatable about a drive shaft axis;
• a rotating group within the pump housing, the rotating group being coupled to drive shaft such that the rotating group rotates in concert with the drive shaft about the drive shaft axis, the rotating group including a piston block defining a plurality of cylinder bores positioned about the drive shaft axis, the rotating group also including pistons having piston bodies that reciprocate within the cylinder bores to pump hydraulic fluid from the pump inlet to the pump outlet, the pistons also including piston shoes;
• a yoke within the pump housing, the yoke carrying a swashplate, the piston shoes of the pistons of the rotating group being configured to slide along the swashplate as the rotating group rotates about the drive shaft axis, the yoke being pivotally movable relative to the drive shaft axis to adjust a swashplate angle of the swashplate relative to the drive shaft axis, the swashplate angle establishing a pump displacement of the variable displacement pump per rotation of the rotating group by establishing a stroke length of the pistons within the cylinder bores, the yoke being pivotally movable relative to the drive shaft axis between a maximum displacement position and a minimum or no displacement position, the pistons having a maximum stroke length when the yoke is in the maximum displacement position and a minimum stroke length stroke length when the yoke is in the minimum displacement position, the yoke being spring biased toward the maximum displacement position;
• a pressure compensator for adjusting the pump displacement based on a pump pressure at the pump outlet, the pressure compensator including a pressure compensating actuator for moving the yoke in a de-stroking direction from the maximum displacement position toward the minimum displacement position, the pressure compensator also including a pressure compensation line for applying hydraulic pressure to the pressure compensating actuator to move the yoke in the de-stroking direction, the pressure compensator also including a pressure compensator valve for opening and closing fluid communication between the pressure compensation line and the pump outlet, the pressure compensator valve being movable between a closed position in which fluid communication is blocked between the pump outlet and the pressure compensation line and an open position in which fluid communication is open between the pump outlet and the pressure compensation line, the pressure compensator valve being biased toward the closed position by a biasing force, the pressure compensator valve being configured to open when the pump pressure at the pump outlet exceeds the predetermined pressure;
• an electric motor for driving rotation of the drive shaft, the electric motor providing a variable rotational speed;
• a speed compensator including:
• an orifice;
• a fixed displacement pump coupled to the drive shaft such that the drive shaft drives rotation of the fixed displacement pump, the fixed displacement pump being configured to draw hydraulic fluid from an inlet side of the variable displacement pump and to pump the hydraulic fluid through the orifice;
• a speed compensation line adapted to receive hydraulic pressure from the pump outlet;
• a displacement limiting actuator connected to the speed compensation line and adapted to move the maximum displacement position of the yoke toward the minimum displacement position such that a new maximum displacement position is established and a maximum permitted stroke length allowed by the swashplate is decreased, and the pressure compensating actuator being adapted to destroke the yoke between the new maximum displacement position and the minimum displacement position;
• a speed compensation valve for opening and closing fluid communication between the pump outlet and the speed compensation line, the speed compensation valve being biased toward a closed position where fluid communication between the pump outlet and the speed compensation line is blocked, the speed compensation valve being configured to move to an open position to open fluid communication between the pump outlet and the speed compensation line when a differential pressure across the orifice exceeds a predetermined amount, the differential pressure across the orifice being proportional to the speed of the electric motor, and when the differential pressure across the orifice exceeds the predetermined amount, a pressure at an upstream side of the orifice acts on the speed compensation valve to move the speed compensation valve from the closed position to the open position.