Design and analysis of selectively compliant underactuated robotic hands [electronic resource]
Keywords: Mechanical Engineering.

A balance between complexity and functional capabilities has been explored since the first years of multi-fingered robotic hands. In an age where DC motors are the de facto standard for actuation in robotics, the problem of needing to operate in a human-sized world puts severe constraints and limits on actuator size and placement in hands. While many successful examples of fully-actuated designs exist, these designs generally reflect the trade-offs and sacrifices imposed by such constraints. In that light, underactuation, employing fewer actuators than degrees of freedom, has gained attention as a method to achieve many of the functional capabilities of fully-actuated hands with fewer constraints on actuators and transmissions. Underactuated hands also have distinct advantages over fully actuated hands, especially when used on mobile robots, due to their reduced weight and control complexity, and the potential for increased robustness. However there is typically a trade-off in terms of reduced controllability or manipulability when handling grasped objects. When designing underactuated hands, extra care must be taken during the design process to ensure that such hands will grasp a wide range of object sizes and shapes robustly, particularly when friction is low and uncertain. Despite these concerns, underactuated hands have become increasingly popular in robotic and prosthetic applications. Robotic hands are also a venue in which novel, secondary mechanisms are often found. Devices such as differentials, valves, clutches, and low-power, shape-changing actuators have been used to improve grasp robustness on a wider range of objects and allow users more grasping and manipulation options. However, the location and placement of secondary actuators has not been studied in a comprehensive way with respect to the types of actuation methods possible. This is due in part to the lack of general analytic tools which enable designers to rapidly investigate their designs prior to the prototyping stage. Additionally, much of the analysis in the field of robotic hands is done once basic design choices have already been made, making subsequent analyses specific only to a set of design parameters specific to those choices. The same point can be made regarding quality metrics, which suffer from fragmented utilization due to the many different emphases placed on different design requirements. The primary goal of this thesis is to provide a framework for the analysis and evaluation of underactuated robotic hands. The first chapter discusses both the broad motivations for studying robotic hands and the specific contributions of this thesis. The next chapter reviews relevant designs from literature, analyses that have accompanied them, uses of secondary devices in underactuated hands, and the progress that dynamics simulators have made towards representing reality. In the next chapters, the issues related to modeling abstract, generic hand designs is discussed, and a kinematic framework is introduced to derive the force relationships between actuator and grasped object for many mechanisms commonly encountered in underactuated hands. Chapter 6 discusses difficulties associated with solving static force equations, and several methods are introduced to accomplish this. The last of these options relies on three-dimensional rigid-body dynamic simulations to evaluate the performance of compliant, underactuated mechanisms which may encounter conditions such as coulomb friction in contact and and damping at the joints. In the next chapters, these force relationships are derived and discussed for specific hand designs in the context of a force-field representation, and several performance metrics are derived which measure a hand's ability both to acquire and retain objects. The benefits of secondary actuation mechanisms are then discussed with two specific examples. First is the SRI/Stanford/Meka hand, a tendon-driven, compliant, underactuated hand capable of locking individual joints. Second is a mechanism implemented on the Seabed Hand, which increases the range of graspable objects and allows users to selectively change grasp properties based on their specific control needs. Finally, the impacts of friction are discussed, and the trends from simulations are compared with experimental data. From these experiments the benefits of secondary mechanisms can be demonstrated in a frictional world as well.