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Minimum Impedance Manipulation

Jaydev Desai, Robert Howe


 
FEEDFORWARD MODELS 
+ 
LOW IMPEDANCE 
= 
GOOD POSITION CONTROL 
+ 
LOW CONTACT FORCE  
WITH UNCERTAINTY

I worked on the Whole Arm Manipulator (WAM) combining its minimal impedance and manipulation capabilities. The goal of this work  was to develop a good impedance model for the WAM which can be varied over a large range from being very stiff to very compliant like the human arm. We purchased the  DS1103 board manufactured by DSPACE, Germany. This board enabled us to do our control task at much higher speeds than conventional control boards. We also purchased an ATI Gamma F/T force-sensor that was placed at the end-effector of the WAM. On this force-sensor we attached a semi-hemispherical cap that was used to probe the environment. Since the contact forces are non-zero at points of contact, we used the force information to determine the exact contact point on the cap (Intrinsic tactile sensing: Bicchi, Salisbury and Brock). This information was used to determine the exact location of a point on the object in the inertial frame. After "scanning" the object in minimal impedance mode we could offline do a 3D reconstruction to obtain the approximate shape of the object. We could also gather knowledge about the object's compliance by varying the end-effector stiffness and "probing" the object. Combining the knowledge of the object compliance with its approximate shape, we could grasp the object with the appropriate grasping force and manipulate it in space with the WAM.

 

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