Two more degrees of freedom must be in- 

 cluded: Grasping and linear movement; the 

 former is an obvious motion, the latter is not. 

 If one desires to push an object straight 

 ahead, the elbow and shoulder must pivot at 

 the same angular rate if the forearm is to 

 remain in the same horizontal plane. AL- 

 VIN's "joints" and all others as well, can only 

 function one at a time, hence a linear exten- 

 sion of the wrist must be provided to push or 

 pull in the same plane as the forearm. Figure 

 10.37 graphically illustrates the problem and 

 its solution. 



In Barringer's door-closing example, the 

 arm went through a progressively diminish- 

 ing rate of speed. This is another desirable 

 feature in a manipulator: Variable speed 

 control. Most manipulators, however, oper- 

 ate at only one speed. 



A further characteristic of the human arm 

 is that it tells the "operator" what force to 

 apply. Such force "feedback" is nonexistent 

 in contemporary submersible arms, but it is 

 sometimes quite desirable. As an example, in 

 1966 ALUMINAUT was operating off the 

 coast of St. Croix, Virgin Islands. A biologist 

 on one particular dive was quite anxious to 

 collect (intact) one of the many sea urchins 

 inhabiting the bottom. ALUMINAUT's ma- 

 nipulator has no force feedback or variable 

 speed control. Hence, each attempt to pick up 

 one of the delicate animals resulted in noth- 

 ing more than a few fragments of spines or 

 exoskeleton. Much to the biologist's conster- 

 nation, the task was finally abandoned. 



Obviously there are other components and 

 requirements that must be present or satis- 

 fied if the manipulator is to do its work. 



LOCATOR MOTIONS 



'ELBOW 



Fig. 10.36 Manipulator with six degrees of freedom. [From Ref. {33)1 



521 



