Inspection of the International Space Station and other manmade objects in space is difficult because of the microgravity environment. Robots are a promising approach to accomplish these inspection tasks and later repairs, but must be able to maneuver across the surfaces. Because there is no gravity, the robot is at high risk of floating away, necessitating grippers that can adhere to the surface and resist the forces and torques of inspecting and moving on the structure.
An inchworm style mobile robot uses opposing gecko adhesive pads to stick to the surface, enabling mobility and inspection. The robot can fit in a 1.5-in. (≈3.8-cm) gap, and can climb on multiple surface types including glass, metal, composites, and blankets. Further, the robot works in any gravitational orientation (vertical, inverted, etc.), as well as in a zero-gravity field.
The development of the robot consisted of three major stages. First, a mechanism for consistently actuating and controlling the gecko pads (i.e., turning the adhesive forces ON and OFF) was designed. Second, the body section of the robot, responsible for giving the robot the capability of traveling linearly along a plane, turning along a plane, and plane-to-plane transitions (i.e., from wall to ceiling), was designed. The last stage was configuring the robot with sensors, and writing software to control the robot both autonomously and remotely.
The gecko pads were thoroughly tested in previous studies, and results identify that the gecko pads need to slide a distance of 40 to 60 microns along the surface being climbed to ensure the 60- to 80-micron-tall fabricated wedges (hairs) come into intimate contact with that surface to allow the generation of the intermolecular van der Waals forces that create the adhesion. Therefore, maintaining a coplanar alignment between the sets of gecko pads, as well as a coincident parallelism with the surface being climbed, is critical in accessing the full potential of the van der Waals intermolecular adhesion forces. This consideration, along with the fact that the gecko pad’s adhesion is actuated and controlled by the application or removal of a shear force, led to a design consisting of a linear guide rail and a pair of ball-bearing carriages actuated via a linear actuator. This design ensures coplanar alignment, and minimizes torsion and moments acting on the gecko pads. The guide rail constrains the motion of the gecko pads to be theoretically one-dimensional, therefore converting the force from the linear actuator to only act in shearing the gecko pads.
The robot is designed with a total of six motors: two linear actuators used to generate the shear force required to turn the gecko adhesives ON and OFF, two servo motors used to lift each of the two gecko modules, one servo to turn the gecko modules relative to one another, and one brushed DC motor used to power the rack and pinion that extends and contracts the robots gecko modules relative to each other.