Siemens gets active on vibration7 December 2010
Siemens Drive Technologies Division has developed a new active system for reducing the vibration on a container crane.
Dr Elmar Schäfers, manager of mechatronics research and development at Siemens Drive Technologies, said: “Higher, faster, better: for crane builders, this means installing their container bridges in a minimum of space, but still catering to a higher clearance, while at the same time ensuring faster throughput in freight handling.”
At a presentation in Erlangen, Germany, Joachim Zoll, manager for cranes at Siemens explained the background to the new system. Container cranes are usually used for loading and unloading container ships. The containers are transported from a ship by a container crane, hoisted to the shore by a trolley where it is set aside, ready for further transport. As the trolley holding the crane moves, a vibration is generated in the container crane, with the direction of travel of the crane trolley dictating the location of the vibration.
The amplitude of the vibration can reach up to a metre in the direction the crane trolley is travelling in, although at times of high wind, the distance could be increased further. As modern harbour cranes are also lighter and taller constructions than many older versions, they must also require less quay space to allow transport vehicles to drop the freight.
Crane structures may require stiffening, if the vibration has to be reduced, but it is often not enough to just use thicker metal sheets. If a stiffer crane structure is required, the crane span cross-sectional area and overhead clearance may also require modifications on the container crane, requirements which can be difficult to execute on such cranes.
To reduce vibration of the cranes, passive mass dampers are often used; a weight of up to 60USt may be suspended on a pendulum device. The vibration frequency is set by the length of the pendulum, and energy is removed from the vibrating mass through a damper which reduces the vibration of the crane.
However, passive mass dampers of this kind are costly to integrate in the structure of the crane, which only results in a limited damping effect with considerable materials needed.
Firms must ensure tolerance values are complied to during lifting and lowering container applications; deviations of only a few centimetres can usually be tolerated when picking up or setting down a container on a ship or on land, but when the container bridge is vibrating too much, it is impossible for the crane driver to continue with any precision.
To counter this, the dynamic response must then be reduced in order to keep the level of vibration at a minimum. This is a time-consuming activity while transferring containers, and due to the additional costs required, it is less desirable. Siemens' system aims to reduce the issues met during such applications.
Schäfers continues: “With the power drive damper Siemens provides an opportunity to fully compensate for the increased vibration susceptibility under these conditions. This opens up new perspectives in terms of speed and throughput of freight transfer using container cranes.”
Zoll added, “Siemens wants to launch active vibration damping with linear motors on the market by the middle of next year in cooperation with a crane builder.”
The firms’ active vibration damper uses a linear motor with ‘intelligent’ motion which is mounted on the girder with a lighter than conventional weight attached. Vibrations are detected by a sensor, and algorithms use the generated values to calculate the movements of the linear motor required to neutralise the natural oscillation of the girder.
The ‘intelligent’ acceleration and braking of the linear motor means the forces are directed into the girder. Therefore, vibrations are more quickly and effectively reduced, when compared to utilising conventional processes. The firm says that the power drive damper can be integrated with existing container cranes, generally without the requirement for structural modifications.