The design and application of a nonlinear series compliance actuator for use in robotic arms

The work presented here addresses the problem of providing robotic manipulators with improved force control and force sensing capabilities. A nonlinear compliant element for use in cable driven robotic manipulators was designed and built. The compliant element provides a nonlinear (exponential) force displacement behavior. This device is similar to the human hand in that it provides a large dynamic range (defined as the ratio between the largest exertable force and the smallest exertable force), near constant dynamic resolution, early contact detection and changing stiffness of the element. Additionally, the compliant element was designed with ease of manufacturability and assembly in mind and sensitivity for off the shelf components. The effects of placing these compliant elements in line with a cable transmission system were investigated. System performance as a function of element preload was studied. Dynamic response for an open loop linear system equivalent was obtained and used to suggest controller designs. Using these suggested controller designs, the dynamic response of the nonlinear closed loop (position) system was observed. A set of design requirements was developed and a compact design for the compliant element is presented. An experimental force displacement curve for the element was obtained and the position tracking performance of the actuator was evaluated. Next, the compliant element was incorporated into the Compliant Arm Design for Digging (CADD), a 4 degree of freedom manipulator designed in collaboration with Andrew Curtis to examine the performance of the compliant elements in a 'real world' setting. The design presented in this work was incorporated into 2 degrees of freedom of the manipulator and the compliant element designed by Curtis was included in the remaining 2 degrees of freedom. Thesis supervisor: J. Kenneth Salisbury Title: Principal Research Scientist 3