Active damping control for bending oscillations of a forklift mast using flatness based techniques

Forklift trucks, such as the STILL FM-X ensure fast transport of heavy loads for logistics. The mast carriage of the FM-X can be shifted to increase the reachability of loads within a warehouse. Modern forklift trucks are constructed in a lightweight mode to increase their maximum operation velocities, load capacities, and to save costs. Hence, the mast has a limited stiffness and will be subject to oscillations of deflection. They are induced by the shifting of the mast carriage. This paper deals with the active damping of such bending oscillations. A 2-DOF control is applied. A differentially flat output of the system is calculated by using the inverse of the controllability matrix. Based on the flat output a model inversion is applied to the linear model for feedforward control. Due to costs and computational efforts the feedback loop is designed as a scalar proportional controller to stabilize the mast around the reference trajectory. The proposed control approach allows for damping of the bending oscillations and asymptotically stabilizes the system around the reference trajectory. Measurement results from a STILL FM-X 14 validate the improvement by the control.