Getting a (Gecko) grip : surface conformation for dry adhesion assisted robotic grasping
Keywords: Engineering.
Mechanical Engineering.

Nature continues to stimulate engineering solutions for real world problems. Understanding how the gecko, for example, relies on Van der Waals forces to climb various surfaces led to the fabrication of materials that employ the same working principles. The field of studying and creating these bio-inspired dry adhesives has expanded over the past twenty years, with continuous advances in manufacturing capabilities and adhesive performance. However, while the material has previously been used in wall climbing and grasping demonstrations, increasing its controllability and active surface conformation have yet to be fully explored. This dissertation addresses the question of how to develop gecko adhesive controllability with intermittently-active surface confirmation, for utilization in real world robotic applications. This work begins with a study of direct indenting as a manufacturing technique for creating varying microgeometries. A machining method that was previously developed to create microgeometries in wax blocks is adapted and studied in the context of more durable materials such aluminum. Microgeometries with desired behaviors are achieved through empirical testing and parametric design studies. A large-strain elastic-plastic finite element analysis simulation was constructed to understand and ultimately predict the effects of different tool geometries and approach trajectories on the machined microgeometries achieved in metal. Tests using instrumented cutting tools agree with the predicted forces and shapes. The simulation makes it possible to determine whether a desired geometry will be achievable and to predict the required tool path. Building off this capability, the work explores three categories of surface conformation required for different adhesive applications: microscale, mesoscale, and multiscale. Each is discussed with research findings and explored solutions to achieve effective use of gecko adhesives in different scenarios. Microscale conformation and controllability are demonstrated with the development of an electrostatically actuated gecko adhesive clutch. Active conformation through electroadhesion is utilized as a triggering mechanism for toggling between engaged and disengaged adhesion states. The clutch takes advantage of the ability to create new microfeatures, including microwedges with different dimensions from those used in previous work. Combined with electrostatic actuation, the new features provide a level of controllability not previously obtained with directional gecko-inspired adhesives. The work takes advantage of the micromachining analysis to also create spatially-varying arrays of microfeatures. For additional controllability, nonstick prism microfeatures were additionally created to promote sliding of the adhesive when the clutch is not engaged. The adhesion-based clutch involves an integration of these effects to achieve electronically-controlled microscale conformation and adhesion. Use of the clutch is demonstrated as an emergency brake for an MRI compatible teleoperated robotic system and to maintain the grasp force in a low inertia robotic gripper when the motor is turned off. Mesoscale conformation is demonstrated with air-promoted contact in an augmented suction and adhesion tool for side-picking bulky and irregular objects. Suction grippers are a good solution for picking retail objects but are usually deployed in a top down configuration. Gecko adhesives have been used to lift irregular and deformable objects in the past, with thin film grippers that are also used in the top down approach. Other efforts of grasping using gecko adhesive have relied on internal forces normal to the grasp surface. When picking items in densely packed shelf spaces, the grasp surface is limited and often irregular. By synergizing suction gripper technology and thin film gecko-inspired adhesives, secure and successful side grasps of bulky objects can be achieved. A new mode of mesoscale conformation is introduced to ensure successful grasps with the gecko adhesives through a gentle stream of air. The air acts as a highly compliant cushion for a flexible thin film backing for the adhesive. The new gripper technology was integrated with Toyota Research Institute's grocery shopping robot, expanding its successful grasps of store items. Finally, multiscale conformation is discussed and demonstrated with a limited Degree of Freedom (DOF) robotic system using hybrid adhesion and passive alignment mechanisms. Previous work in developing hybrid electrostatic and dry adhesion gripping pads has paved the way for applying gecko adhesive grasps on microrough surfaces. With this capability, systems for bimanual handling of bulky, deformable, and irregular items can be developed. Overcoming the pose control limitations of a bipedal robot's end-effectors prompts a new end-effector design that integrates multiple length scales of conformation to achieve secure grasps. This includes an alignment mechanism, a compliant backing, and a hybrid adhesion pad integrated into a custom gripper for a bipedal robot. The new technology allows the robot to pick previously ungraspable items with <2N squeezing forces. In summary, this work presents manufacturing requirements of a directional dry adhesive material and implementation considerations for tackling and improving robotic task execution. It is meant to inform future design and use of gecko-inspired adhesives with active surface conformation as a tool to effectively solve real world challenges.