Date of Award
Spring 2023
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Mechanical Engineering
First Advisor
Schimmels, Joseph
Second Advisor
Murray, Allison
Third Advisor
Voglewede, Philip
Abstract
Traditional robotic manipulators are designed for accurate absolute positioning and require a highly structured environment to perform simple operations. They are unable to compensate for small discrepancies in the relative positioning of the robot end-effector with respect to its environment. A task-appropriate compliance allows a robotic manipulator to compensate for small discrepancies in the position or orientation of an object when the object is constrained. A better means of realizing task-appropriate structured compliant behavior is needed to allow a robot to perform dexterous manipulation.In this thesis, three agonist-antagonist variable stiffness actuators (VSAs) are used to independently control the joint stiffnesses and positions in a 3-DOF finger. The VSAs allow the finger to achieve a large range of controllable 2-dimensional linear elastic behavior at the fingertip. The joint design and actuation strategy enable the independent control of the angular position and stiffness of each joint, without coupling. Each VSA employs two motors in opposition adjusting the length of a cable running through a series of pulleys with a spring-loaded lever. A quasi-static model was developed to evaluate the elastic performance of the design. A set of geometric parameters were optimized to produce the desired quadratic force-deflection behavior for one half of the VSA so that, when the two halves act in opposition, a linear torque-angular deflection relationship is obtained at the joint. A prototype was assembled and tested to verify the performance of the VSAs and the controllable endpoint compliance. The optimized half-VSA mechanism produced approximately quadratic tension-deflection behavior in the tendons. While the experimental relationships did not closely match the analytical results and differed between joints, the nearly quadratic behavior was achieved in all joints, allowing the antagonistic variable stiffness actuators to generate linear variable stiffness. The controllable linear behavior at the joints was used to adjust the particle planar (2D) compliance at the fingertip. The shape and magnitude of the experimentally obtained fingertip compliance ellipses matched the analytical results well for most configurations.