Problem Statement

The University of Texas at Austin Nuclear and Applied Robotics Group utilizes a robot that senses alpha radiation in contaminated areas. The alpha radiation sensors are attached to a linkage attached onto the robot, which is an unmanned ground vehicle (UGV). In order to optimize the rate of which an area can be scanned, the sensors move in a sweeping manner by a four bar linkage, powered by a single stepper motor.

The UGV with the current linkage and sensors attached.

However, this linkage does not perform as expected, due to some engineering flaws in the design and assembly done by a previous team. The joints of the links were poorly assembled by the shoulder bolts and nuts, which caused significant friction in motion, and the gears were glued together to the driving shaft. 

Moreover, the coupler point of the linkage moves in a trajectory that is very close to a circular arc. This is troublesome because the mapping of the 'swept' surface area becomes a convoluted optimization problem. As the robot moves and the linkage 'sweeps', some of the area will be repeatedly 'swept' while some are barely swept, with respect to the robot's velocity. 

The current design trajectory. The dotted line shows the sensor motion (the circles in cyan are the sensors).

Furthermore, the current linkage complicates the docking procedure of the robot. To dock, the robot has to first find the charge station with its 'eyes' (sensors on the front of the robot), and then once found it travels close to the station, and rotates before backing into the station. Because the trajectory of the current linkage resemble closely to a 90° arc, the sensor has to be continuously adjusted throughout the docking procedure in order to stay out of the back of the robot. Other contains for the design include that the whole linkage assembly had to be secure from the top platform of the robot and there was some limit space in this platform because there were other sensors and communication devices. Also all of the linkage movement must not interfere with the sonar sensor in the front of the robot. This sensor has a range of about 200° so the sensor must not reach the area on the side of the robot close to the front. 

Approximate field of view of the sonar.

Our task was then to make a prototype that works with the given contains, and a design that improves the sweeping coverage of linkage. As a bonus, we could come up with a design that also facilitates the docking procedure.