2-DOF skew control of boom cranes including state estimation and reference trajectory generation

•A nonlinear model for the skew dynamics of a boom crane is presented.•A suitable skew control system is designed in two-degrees of freedom structure.•Appropriate state estimation and reference trajectory planning methods are designed.•Experimental results from a full-size harbor crane are provided.

In small and midsize harbors, boom cranes are used for multiple applications. These include bulk cargo handling and container transloading. For container handling, a spreader is attached to the crane hook. While grabbing a container with the spreader, both the position and the orientation of the spreader and the container must match. The spreader orientation is usually referred-to as skew angle. Other synonyms are yaw angle or spreader heading. The skew angle is controlled using a hook-mounted rotator motor. Since wind, impact, and uneven load distribution can cause skew vibrations, active skew control is desirable for facilitating crane operation, improving positioning accuracy, and increasing turnover. Different skewing device designs are used for different types of cranes. This contribution presents the skew dynamics on a boom crane along with an actuator model and a sensor configuration. Subsequently a two-degrees of freedom control concept (2-DOF) is derived which comprises a state observer for the skew dynamics, a reference trajectory generator, and a feedback control law. The control system is implemented on a Liebherr mobile harbor crane and its effectiveness is validated with multiple test drives.