Electric linear-actuator pumping system

Abstract

A pumping system includes at least three electrically driven pumping units. Each pumping unit includes a linear actuator driven by at least one electric motor and driving a shaft. Pistons on each end of the shaft move within fixed cylinders to draw fluid from an inlet and expel the fluid to an outlet. Each pumping unit draws fluid at one end while expelling an equal quantity of fluid at the opposite end. The shaft velocity is controlled such that a total flow of the pumping system is constant, eliminating flow ripple.

What is claimed is: 
     
         1 . A pumping system comprising:
 a fluid inlet;   a fluid outlet; and   three or more pumping units, each pumping unit having
 a threaded shaft fixed to a piston such that axial movement of the piston in a first direction draws fluid from the inlet and axial movement of the piston in a second direction expels fluid to the outlet, 
 an axially fixed nut engaging the threaded shaft such that rotation of the nut results in proportional axial movement of the shaft, and 
 one or more electric motors drivably connected to the nut. 
   
     
     
         2 . The pumping system of  claim 1  wherein:
 the piston is fixed to one end of the shaft; and 
 a second piston is affixed to an opposite end of the shaft such that axial movement of the second piston in the first direction expels fluid to the outlet and axial movement of the second piston in the second direction draws fluid from the inlet. 
 
     
     
         3 . The pumping system of  claim 1  wherein each of the one or more electric motors drivably connected to the nut comprises:
 a ring gear fixed to the nut; 
 a plurality of rotors, each fixed to a pinion gear engaging the ring gear, and 
 a plurality of stators, each configured to establish a magnetic field to exert torque on a respective rotor in response to an electric current. 
 
     
     
         4 . The pumping system of  claim 1  further comprising a controller programmed to command the pumping units to produce a repeating trapezoidal flow rate comprising increasing stages, steady stages, and decreasing stages, wherein pumping units in an increasing stage offset pumping units in a decreasing stage such that a total flow rate is constant. 
     
     
         5 . The pumping system of  claim 4  wherein the controller comprises a plurality of local controllers each controlling one of the three or more pumping units. 
     
     
         6 . The pumping system of  claim 5  wherein one of the plurality of local controllers is designated as a master controller and coordinates the remaining local controllers. 
     
     
         7 . A pumping system comprising:
 a fluid inlet;   a fluid outlet; and   three or more pumping units, each pumping unit having
 a threaded shaft fixed to a piston such that axial movement of the piston in a first direction draws fluid from the inlet and axial movement of the piston in a second direction expels fluid to the outlet, 
 an axially fixed nut having internal threads, 
 a plurality of threaded rollers engaging the threads of the nut and the threaded shaft such that rotation of the nut results in proportional axial movement of the shaft, 
 a ring gear fixed to the nut, 
 a plurality of rotors, each fixed to a pinion gear engaging the ring gear, and 
 a plurality of stators, each configured to establish a magnetic field to exert torque on a respective rotor in response to an electric current. 
   
     
     
         8 . The pumping system of  claim 7  wherein:
 the piston is fixed to one end of the shaft; and 
 a second piston is affixed to an opposite end of the shaft such that axial movement of the second piston in the first direction expels fluid to the outlet and axial movement of the second piston in the second direction draws fluid from the inlet. 
 
     
     
         9 . The pumping system of  claim 7  further comprising a controller programmed to command the pumping units to produce a repeating trapezoidal flow rate comprising increasing stages, steady stages, and decreasing stages, wherein pumping units in an increasing stage offset pumping units in a decreasing stage such that a total flow rate is constant. 
     
     
         10 . The pumping system of  claim 9  wherein the controller comprises a plurality of local controllers each controlling one of the three or more pumping units. 
     
     
         11 . The pumping system of  claim 10  wherein one of the plurality of local controllers is designated as a master controller and coordinates the remaining local controllers. 
     
     
         12 . A method of controlling a group of at least three linear actuator pumping units, the method comprising:
 commanding a shaft of each of the at least three pumping units to repeatedly:
 travel in a first direction at a first speed for a first interval, 
 gradually change a shaft speed until the shaft is moving in a second direction at a second speed equal in magnitude to the first speed, 
 travel at the second speed for second interval equal in duration to the first interval, and then 
 gradually change the shaft speed until the shaft is moving at the first fixed speed; and 
   offsetting the speeds of the shafts of the at least three pumping units such that a total pump flow rate of the at least three pumping units is constant.   
     
     
         13 . The method of  claim 12  wherein a number of units currently changing shaft speed from the first speed to the second shaft speed is maintained equal to a number of units currently changing shaft speed from the second speed to the first speed. 
     
     
         14 . The method of  claim 12  wherein the speeds of the shafts are offset such that at any point in time, exactly one shaft is changing from the first speed to the second speed and exactly one shaft is changing from the second speed to the first speed.