Active vibration damping for turntable ladders of fire trucks

The large heights that the ladder shall reach and the thus required ladder lengths render avoiding oscillations during operation solely by design and construction practically impossible. Such oscillations are induced by the ladder’s motion, or external influences like wind or sudden load changes at the cage. To actively dampen such oscillations and thereby to improve the ladder’s motion is the main objectives of the ongoing research partnership between our institute and Magirus, the market leader in the field of turntable ladders, that was established in 1998.

The oscillation damping only uses the hydraulic actuators that are already available, so that no additional actuators are required. Strain gauges are attached to the ladder close to the hub to measure strain in the longitudinal and lateral directions. Additionally, gyroscopes at the top end of the ladder measure angular velocities in three directions. The information gained from these sensors is combined to reconstruct the oscillation modes that are then actively damped. For ladders of up to 30 m working height, only the basic oscillation is significant, as higher-order modes are sufficiently attenuated by natural damping. With increasing ladder length, the natural damping of higher-order modes decreases, so that for ladders longer than 30 m, higher modes have to be considered for the control design. Additionally, especially for ladders with an articulated arm as the new Magirus M42L-AS shown above and featuring the single-extension design, coupled bending-torsional oscillations are excited during rotation.

Based on the respective model, a decentralized control concept is implemented, i.e. each axis is considered separately. For each axis, a 2 degree-of-freedom control concept is implemented. In each case, the oscillation modes whose natural frequencies are lower than the cut-off frequency of the actuators are considered for the controller design.