In the Roboship project at the research group RAM of University of Twente, a robot on a rails was developed to inspect ballast tanks of large cargo ships. Nowadays, inspection of these walls is human labour, while the conditions are severe (i.e. toxic fumes and slippery surfaces). The robot drives on a rail and inspects the wall visually with a camera and uses a sensor to measure the thickness of the ballast walls. The thickness is used to determine if a wall needs to be repaired to prevent further degradation of the ballast tank wall. The goal of my project was to design and build a 3 DOF (degrees of freedom) prototype end-effector for the inspection robot of the Roboship project.

Requirements of the end-effector

The desired end-effector should be upgraded from 2 to 3 degrees of freedom, while the weight and dimensions needed to be minimized. Next to that, the end-effector should be able to measure between a radius range of 100 mm (difference between min and max range) and should establish this in a time of less then 5 seconds with an accuracy of less than 10 mm.

Design of the end-effector

Since it was a prototype, all the materials needed to be 3D printed, hence an unique spindle and gear mechanism was designed to be able to rotate and translate the thickness sensor with two gearbox DC motors. The other degree of freedom was establish by rotating around the center point of the end-effector. The DC motors were actuated by custom made PCB’s with Arduino environment mimicking to be Dynamixels. A discrete PID controller was implemented to achieve the accuracy and settling time. A Graphical User Interface (GUI) was created in Matlab to control the end-effector and evaluate its behavior. The desired position of the end effector can be set in Cartesian coordinates from the GUI.

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