Detecting External Forces on an Autonomous Lawnmowing Robot with Inertial, Wheel Speed and Wheel Motor Current Measurements

Abstract: An autonomous lawn mowing robot moves around randomly within an area enclosed by a magnetic wire and makes decision based on sensor information. To ensure human and animal safety it is essential that the robotic lawn mower can detect and stop if, for instance, it is being lifted by a human. This thesis takes a look at how on-board sensors could be used to detect a few critical events, here called fault cases. Data such as acceleration, angular velocity and motor currents are recorded and then used to develop three methods for detection briefly de-scribed below. The Odometry method uses constraints on valid movement of the robotic lawnmower and a fault case is detected if estimated velocity in global coordinates violates these constraints. The pitch angle relationship estimates the relation between electrical currents needed to drive the robotic lawn mower at a certain speed in certain pitch angle. When the electrical currents corresponding to a certain pitch angle according to the relation deviates from measured currents a fault case would be detected. The frequency method is based on the idea that disturbances on signals caused by uneven ground should decrease when the robotic lawn mower is lifted or held. The method would then detect this damping of disturbances by examining frequency content. The best method is the pitch angle relationship while the other two proposed methods have potential but would need higher sampling frequencies and additional signals to fully perform satisfactorily. With additional information such as position of the robotic lawn mower the estimation of the global velocities could be significantly improved which in turn would improve the odometry method and serve as a complement to the current pitch angle relation. The frequency methods would also be valid if the sampling frequencies were much higher, some-thing that might not be as cost efficient as needed to make the method profitable.