End effector with force controlling mechanism

Abstract

A precision soft-touch end effector has a mounting plate attached to a robot arm. The plate supports a stepper motor. The output shaft of the stepper motor is connected through a spring to an elongated finger that slides in a central longitudinal slot of the plate and supports a first wafer gripping post, while on the end opposite to the first wafer gripping post the mounting plate pivotally supports two L-shaped fingers with a second and third wafer gripping posts on their respective ends. The mounting plate in combination with the first sliding finger and two pivotal fingers forms the end effector of the robot arm which is thin enough for insertion into a wafer-holding slot of a wafer cassette. The end effector is equipped with a mapping sensor for detecting the presence or absence of the preceding wafer, wafer position sensors for determining positions of the wafer with respect to the end effector, and force sensors for controlling the wafer gripping force. Several embodiments relate to different arrangements of gripping rollers and mechanisms for control of the gripping force and speed of gripping required for gripping the wafer with a soft and reliable touch.

1 . A precision soft-touch gripping end effector for flat objects, said precision soft-touch end effector having a longitudinal axis, each of said flat objects having a plane and a peripheral edge, said precision soft-touch end effector comprising: 
 a mounting plate;    a linear precision drive mechanism supported by said mounting plate;    at least a first moveable gripping post located on said longitudinal axis, a second moveable gripping post located on one side relative to said longitudinal axis, and a third moveable gripping post located on the other side relative to said longitudinal axis, said first moveable gripping post, said second moveable gripping post, and said third moveable gripping post forming a plane of at least three gripping posts, embracing each of said flat objects, and being moveable simultaneously in the inward/outward direction with respect to said peripheral edge when each of said flat objects is placed into said end effector; and    a gripping force control means for controlling the gripping force with which said gripping posts grip said peripheral edge by controlling operation of said linear precision drive mechanism.    
   
   
       2 . The precision soft-touch end effector of  claim 1 , wherein said first moveable gripping post is located on a side of said flat object diametrically opposite to said gripping force control means.  
   
   
       3 . The precision soft-touch end effector of  claim 2 , further comprising a first linking member having means for supporting said first moveable gripping post, a second linking member having means for pivotally supporting said second moveable gripping post, and a third linking member having means for pivotally supporting said third moveable gripping post, said second linking member and said third linking member being pivotally connected to said first linking member, said first linking member, said second linking member, and said third linking member being connected to said linear precision drive mechanism via said gripping force control means.  
   
   
       4 . The precision soft-touch end effector of  claim 3 , wherein said linear precision drive mechanism comprises a stepper motor, and wherein said second gripping post comprises at least one first gripping roller, and said third gripping post comprises at least one second gripping roller.  
   
   
       5 . The precision soft-touch end effector of  claim 4 , wherein said means for supporting said first moveable gripping post comprises a first arm connected to said stepper motor through said gripping force control means; said means for pivotally supporting said at least one first roller comprises a first arm pivotally attached to said second linking member; said means for pivotally supporting said at least one second roller comprises a second arm pivotally attached to said first linking member; said gripping force control means comprising main spring means for simultaneously urging said first moveable gripping post, said at least one first gripping roller and said at least one second gripping roller to said peripheral edge of said flat object.  
   
   
       6 . The precision soft-touch end effector of  claim 4 , wherein said stepper motor has an actuating member; said gripping force control means comprises a sliding member moveable in the direction of said longitudinal axis, main spring means located between said actuating member and said sliding member, and a means for controlling compression of said main spring means in terms of said gripping force.  
   
   
       7 . The precision soft-touch end effector of  claim 6 , wherein said main spring means is a coil spring, said means for controlling compression of said main spring is a pressure sensor connected to said stepper motor for stopping operation of said stepper motor when said coil spring is compressed with a predetermined force.  
   
   
       8 . The precision soft-touch end effector of  claim 5 , wherein said stepper motor has an actuating member; said gripping force control means comprises a sliding member moveable in the direction of said longitudinal axis, said main spring means being located between said actuating member and said sliding member, and a means for controlling compression of said main spring means in terms of said gripping force.  
   
   
       9 . The precision soft-touch end effector of  claim 8 , wherein said main spring means is a coil spring, said means for controlling compression of said main spring is a pressure sensor connected to said stepper motor for stopping operation of said stepper motor when said coil spring is compressed with a predetermined force.  
   
   
       10 . The precision soft-touch end effector of  claim 6 , wherein said main spring means is a coil spring, said means for controlling compression of said main spring is a limit switch connected to said stepper motor for stopping operation of said stepper motor when said coil spring is compressed with a predetermined force.  
   
   
       11 . The precision soft-touch end effector of  claim 5 , wherein said gripping force control means further comprises: a first leaf spring that additionally urges said at least one first roller towards said peripheral edge; a second leaf spring that additionally urges said at least one second roller towards said peripheral edge; a first sensor for measuring compression of said first leaf spring; and a second sensor for measuring compression of said second leaf spring; said first sensor and said second sensor being connected to said stepper motor for stopping operation of said stepper motor when either of said first leaf spring or said second leaf spring is compressed to a predetermined gripping force.  
   
   
       12 . The precision soft-touch end effector of  claim 1 , further comprising position sensor means for determining position of said at least three gripping posts substantially with respect to said plane of said flat object and sensor means for determining that said plane of said at least three gripping posts deviates from said plane of said flat object.  
   
   
       13 . The precision soft-touch end effector of  claim 12 , wherein said position sensor comprises at least one mapping sensor for determining that the position of destination of said flat object is free for unloading said flat object and at least one through-beam sensor for determining position of said peripheral edge of said flat object.  
   
   
       14 . The precision soft-touch end effector of  claim 4 , further comprising position sensor means for determining position of said at least three gripping posts substantially with respect to said plane of said flat object and sensor means for determining that said plane of said at least three gripping posts deviates from said plane of said flat object.  
   
   
       15 . The precision soft-touch end effector of  claim 14 , wherein said position sensor comprises at least one mapping sensor for determining that the position of destination of said flat object is free for unloading said flat object and at least one through-beam sensor for determining position of said peripheral edge of said flat object.  
   
   
       16 . The precision soft-touch end effector of  claim 7 , further comprising position sensor means for determining position of said at least three gripping posts substantially with respect to said plane of said flat object and sensor means for determining that said plane of said at least three gripping posts deviates from said plane of said flat object.  
   
   
       17 . The precision soft-touch end effector of  claim 16 , wherein said position sensor comprises at least one mapping sensor for determining that the position of destination of said flat object is free for unloading said flat object and at least one through-beam sensor for determining position of said peripheral edge of said flat object.  
   
   
       18 . A method for gripping and handling precision flat objects with soft touch comprising the steps of: 
 providing a precision soft-touch end effector comprising a linear precision drive mechanism, at least three gripping posts laying substantially in a plane with at least two of said three gripping posts being moveable by means of said linear precision drive mechanism with respect to an imaginary or real circular flat object having a circular peripheral edge, said at least three gripping posts embracing said circular peripheral edge from opposite sides, said flat object having a plane; gripping force control means for controlling the gripping force with which said gripping posts grip said peripheral edge by controlling operation of said linear precision drive mechanism, first sensor means for determining position of said plane of said at least three gripping posts substantially with respect to said plane of said flat object, and second sensor means for determining that said plane of said at least three deviates from said plane of said flat object;    checking a position of said three gripping posts with the use of said first sensor means;    checking a position of said three gripping posts with the use of said second sensor means;    initiating said linear precision drive mechanism for moving said at least two gripping posts toward said edge of said flat object;    dividing the path of movement of said at least two gripping posts towards said edge at least into a first stage which is an initial stage of said movement and a second stage which is a final stage of said movement; and    controlling operation of said linear precision drive mechanism via said gripping force control means so that in said first stage said at least two gripping posts move with a speed higher than at said second stage.