Direct drive electromechanical linear actuators

Abstract

Direct drive electromechanical rotary-to-linear actuators include one or more electric motors mounted in a housing. Each motor includes a stator and a rotor. The motor drives a planetary drive mechanism that includes an elongated central shaft having one or more helical threads on an external surface coupled to the rotor for conjoint rotation. A planetary nut having helical threads on an internal surface is disposed concentric to the shaft, and a plurality of planetary rollers are disposed concentrically between the shaft and the planetary nut. Each of the rollers has a helical thread on an external surface that is complementary to and in engagement with a thread of the shaft and a thread of the nut. Rotation of the rotor is converted with mechanical advantage into linear movement of the planetary nut. The actuators provide backlash-free operation, higher stiffnesses, slew rates and frequency responses, and better overall efficiency.

1 . A direct drive electromechanical rotary-to-linear actuator, comprising: 
 an elongated housing;    an electric motor, including a stator fixed in the housing and a rotor supported for rotation relative to the stator; and,    a planetary drive unit, comprising: 
 an elongated central shaft coupled to the rotor for conjoint rotation therewith and having one or more helical threads on an external surface thereof;  
 a planetary nut concentric to the shaft and having a plurality of helical threads on an internal surface thereof; and,  
 a plurality of planetary rollers disposed concentrically between the central shaft and the planetary nut, each having a helical thread on an external surface thereof that is complementary to and in engagement with a thread of the shaft and a thread of the nut.  
   
   
   
       2 . The actuator of  claim 1 , wherein the central shaft is integral to the rotor of the motor.  
   
   
       3 . The actuator of  claim 2 , wherein the shaft and rotor comprise a magnetically permeable material.  
   
   
       4 . The actuator of  claim 1 , wherein the stator comprises high-cobalt-content laminations and the rotor comprises neodymium-iron magnets.  
   
   
       5 . The actuator of  claim 1 , wherein the central shaft includes a hollow center, and further comprising a linear variable displacement transducer (“LVDT”) disposed in the hollow center.  
   
   
       6 . The actuator of  claim 5 , wherein the LVDT comprises: 
 an elongated annular coil set having an end fixed relative to a first end of the actuator; and,    an elongated rod of a magnetically permeable material slidably disposed in the annulus of the coil and having a first end fixed relative to a second end of the actuator.    
   
   
       7 . The actuator of  claim 1 , further comprising: 
 a plurality of axial grooves in the housing adjacent to the planetary nut; and,    a plurality of axial splines on the nut, each slidably disposed in a respective one of the grooves.    
   
   
       8 . The actuator of  claim 1 , wherein the rotor comprises an elongated annular shaft having the planetary nut integrally defined within the annulus thereof.  
   
   
       9 . The actuator of  claim 8 , further comprising: 
 an absolute-angular-position encoder rotatably coupled to the central shaft through a gear.    
   
   
       10 . A direct drive electromechanical rotary-to-linear actuator, comprising: 
 an elongated housing;    a first electric motor, including a first stator fixed in the housing and a first rotor supported for rotation relative to the first stator;    a second electric motor, including a second stator fixed in the housing and an elongated annular second rotor supported for rotation relative to the second stator;    a planetary drive unit, comprising: 
 an elongated central shaft coupled to the first rotor for conjoint rotation therewith and having one or more helical threads on an external surface thereof;  
 a planetary nut having a plurality of helical threads on an internal surface thereof and coupled to the annular second rotor within the annulus thereof for conjoint rotation therewith and for relative axial sliding therein; and,  
 a plurality of planetary rollers disposed concentrically between the central shaft and the planetary nut, each having a helical thread on an external surface thereof that is complementary to and in rolling with a thread of the shaft and a thread of the nut; and,  
   means for locking respective ones of the first and second rotors against rotation.    
   
   
       11 . The actuator of  claim 10 , wherein at least one of the motors comprises a pancake motor.  
   
   
       12 . The actuator of  claim 10 , wherein the rotor locking means comprises a double-armature, double-disc solenoid clutch brake.  
   
   
       13 . The actuator of  claim 10 , wherein the rotor locking means comprises: 
 a clutch plate mounted on the first rotor for conjoint rotation therewith and between the jaws of a caliper brake;    a plurality of rocking levers having rollers disposed at respective first ends thereof and arranged to move between a first orientation in which the rollers are engaged in respective slots in an end of the second rotor, and a second orientation in which the rollers are disengaged from the slots;    a plurality of first springs biasing the rocking levers into the first orientation;    a second spring biasing the jaws of the caliper brake together and against the clutch plate; and,    a spool moveable between a first position compressing the second spring and thereby relieving the bias of the second spring on the caliper brake jaws, and a second position urging the rocking levers against the bias of the first springs and into the second orientation thereof.    
   
   
       14 . The actuator of  claim 13 , further comprising: 
 means for releasably holding the spool in the first position; and,    means for selectably releasing the spool from the first position for movement to the second position.    
   
   
       15 . The actuator of  claim 14 , wherein the means for selectably releasing the spool comprises a solenoid.  
   
   
       16 . The actuator of  claim 15 , wherein the means for releasably holding the spool in the first position comprises a plurality of ball bearings disposed in apertures in the spool and held captive in an adjacent circumferential groove by an arm of the solenoid.  
   
   
       17 . A direct drive electromechanical rotary-to-linear actuator, comprising: 
 an elongated stator housing;    an electric motor, including a stator supported in the housing for axial movement relative thereto, and a rotor supported in the housing for conjoint axial movement with the stator and rotation relative thereto; and,    a planetary drive unit, comprising: 
 an elongated cylindrical spindle having a long axis and coupled to the rotor for conjoint rotation therewith;  
 a drum disposed concentric to the spindle and having a thin, cylindrical sidewall with a plurality of helical corrugations therein, the corrugations having a pitch of φ relative to the spindle axis; and,  
 a plurality of disc-like planetary rollers disposed in a radially symmetrical array about the spindle, each being mounted on the stator housing for rotation about an axis skewed at an angle φ relative to the spindle axis and having a circumferential surface in frictional engagement with the spindle and convolutions in the circumferential surface in engagement with a respective one of the corrugations of the sidewall of the drum.  
   
   
   
       18 . The actuator of  claim 17 , wherein the rollers are located in adjacent planes perpendicular to the spindle, each plane containing a radially symmetrical array of two or more rollers.  
   
   
       19 . The actuator of  claim 17 , wherein the circumference of each roller includes two convolutions of a pitch given by π D tan φ, where D is the diameter of the drum.  
   
   
       20 . The actuator of  claim 17 , wherein the wall of the drum is strained from a cylindrical shape to a trochoidal shape incorporating a plurality of longitudinal zones having a smaller radius alternating with a plurality of longitudinal zones having a larger radius.