Motor-driven actuator with hydraulic force amplification

Abstract

The present invention provides an actuator ( 20 ) for selectively displacing an output member ( 33 ) against an opposing load (L) in response to the output ( 24 ) of a motor ( 23 ). The actuator includes a first transmission mechanism ( 50 ) for displacing the output member relative to the motor at a first nominal ratio with respect to the motor output; a second transmission mechanism ( 60 ) for displacing the output member relative to the motor at a nominal second ratio with respect to the motor output; a selector ( 34, 39 ) for coupling the motor output to the output member only through the first transmission mechanism ( 50 ) when the load is less than a predetermined value, and for coupling the motor output to the output member through the first and second transmission mechanisms ( 50, 60 ) when the load is greater than the predetermined value. The force exerted by the output member on the load is the sum of the forces transmitted through the first and second transmission mechanisms.

What is claimed is:  
     
         1 . An actuator for selectively displacing an output member against an opposing load in response to the output of a motor, comprising: 
 a first transmission mechanism for displacing said output member relative to said motor at a nominal first ratio with respect to said motor output;    a second transmission mechanism for displacing said output member relative to said motor at a nominal second ratio with respect to said motor output;    a selector for coupling said motor output to said output member only through said first transmission mechanism when said load is less than a predetermined value, and for coupling said motor output to said output member through said first and second transmission mechanisms when said load is greater than said predetermined value; and    wherein the force exerted by said output member on said load is the sum of the forces transmitted through said first and second transmission mechanisms.    
     
     
         2 . An actuator as set forth in  claim 1  wherein the force exerted by said first transmission mechanism on said output member is substantially limited to said predetermined value.  
     
     
         3 . An actuator as set forth in  claim 2  wherein said selector couples said motor output to said output member through said second transmission mechanism only when said load exceeds said predetermined value.  
     
     
         4 . An actuator as set forth in  claim 1  wherein the displacement range of said second transmission mechanism is substantially less than the displacement range of said output member, and wherein said second transmission mechanism is provided with a clutch capable of selectively connecting said second transmission mechanism to said output member at any point in said output member displacement range.  
     
     
         5 . An actuator as set forth in  claim 4  wherein said clutch is configured and arranged to automatically connect said second transmission mechanism to said output member whenever said load is greater than said predetermined value.  
     
     
         6 . An actuator as set forth in  claim 2  wherein said first transmission mechanism includes a spring preloaded to said predetermined value.  
     
     
         7 . An actuator as set forth in  claim 1  wherein said first transmission mechanism includes a ball-screw and nut.  
     
     
         8 . An actuator as set forth in  claim 1  wherein said second transmission mechanism includes a hydrostatic coupling.  
     
     
         9 . An actuator as set forth in  claim 1  wherein said motor is electrically powered.  
     
     
         10 . An actuator as set forth in  claim 1  wherein said motor output is a rotatable shaft.