Modelling and Control of the Steering in an Articulated Forklift using Rapid Control Prototyping

Abstract: This report focuses on the steering of a Very Narrow Aisle (VNA) forklift, which undergoes frequent transitions between manual and wire guidance modes. Manual operation is employed outside narrow aisles, while wire guidance mode is utilized within them.VNA forklifts are commonly employed in environments where space optimization and productivity are of utmost importance. The forklift’s steering system operates hydraulically through the movement of two cylinders controlled by a proportional valve, which in turn is controlled by a current input. To implement the steering control, a Speedgoat target machine, a rapid control prototyping platform, is utilized. The control algorithm is developed in Simulink Real-Time and integrated with the forklift’s MCU through the Speedgoat target machine, connected via a CAN bus. The Speedgoat control strategy utilizes the distance from the wire (DFW) and heading angle(HA) to generate a pivot angle request. In contrast, the current control strategy aims to minimize DFW, HA, and the pivot angle using P controllers. For the Speedgoat control strategy, the pivot angle request is compared to the current pivot angle to produce a steering command. A PD controller is applied to the heading angle for rapid stabilization of steering changes, while a PI controller is used to ensure the actual pivot angle follows the desired pivot angle. To minimize the distance from the wire, a P controller with two different settings, depending on proximity to wire locking is employed. The control strategy also incorporates bumpless transfer techniques to ensure smooth transitions between manual and wire guidance modes by gradually adjusting the impact of wire guidance and manual steering. Anti-windup measures are taken to prevent integral wind-up effects, and various PID tuning methods are explored to determine the optimal controller parameters. A simulation model is developed to simulate the manual steering of the forklift. The manual steering implemented in Speedgoat exhibits smoother behavior compared to the current configuration, albeit with slightly longer time delays during start and stop events. When switching between manual and wire guidance modes, the Speedgoat configuration provides a smoother transition. This is attributed to the utilization of bumpless transfer techniques, which minimize abrupt valve switches and mitigate the undesired zig-zag motion. In wire guidance mode, the Speedgoat configuration generally produces smaller steering commands. However, when the forklift is in close proximity to wire locking, the proportional gain is increased, resulting in higher steering commands. This accelerates the reduction of DFW, leading to a shorter time until the forklift is locked onto the wire. Thus, the Speedgoat controller can compete with the current controller in terms of locking time while maintaining smoother behavior during wire acquisition. However, smaller steer-commands reduce the likelihood for the forklift to acquire the wire when it is approached more aggressively.