The amplitude of this vibration can be up to one meter depending on the construction type and size of the container crane. Through [strong] high wind this could be even further increased. To reduce this vibration, the crane legs for example can be strengthened. But it is often not sufficient to simply use thicker metal sheets. When constructing a stiffer crane structure, crane span cross-sectional area and overhead clearance must often also be modified on the container crane and this is for most container cranes difficult to realise. According to Uwe Pietryga, Head of the Electrical Department at Kocks, modern harbour cranes are also of a lighter construction than their predecessors. They are also taller. Furthermore, they must also not take up too much quay space, allowing space for transport vehicles to drop the freight. Another option to reduce vibration is the use of passive mass dampers. A weight of up to 60 tonnes is suspended on a pendulum device. The vibration frequency is set by the length of this pendulum and energy is removed from the vibrating mass via a damper which finally damps the disturbing vibration of the crane. A passive mass damper of this type is, however, extremely costly to integrate in the structure of the crane – the materials required are considerable and the damping effect is limited. Compliance with tolerance values is essential when picking up and setting down containers. Deviations of up to several centimeters can normally be tolerated when picking up or setting down a container on a ship or on land. When the container bridge is vibrating too much, it is not possible for the crane driver to load or unload with any precision. The dynamic response of the motion then has to be reduced considerably in order to keep the excitation of vibration to a minimum. This consumes a great deal of time when containers are being transferred and, due to the additional costs that it causes, it is undesirable.
New methods
Therefore Kocks was looking at new methods in the design and equipment to ensure foreseeable dynamic behaviour of the crane structure under consideration of the impact through the drive forces (dynamic response). So another option had to be found to counteract any vibration. The company turned to Siemens for mechatronic support for the development of a scalable crane simulation tool. By using computer simulation Kocks was able to verify the results of the computer simulation with its findings compiled on a real Kocks Boxer container crane. With the tool it was also able to create a “crane toolbox” for easy adaption and simulation of other container cranes, emulate the drive chain and implementation of external forces (i.e. wind). By using a FEM tool it was also able to create a computer model of the entire crane structure and predefined “sensor points” as virtual measurement points in the model. As a result the company designed a mass damper to counteract the vibration on a container crane. With the simulation tool it defined its optimum position, weight and damping settings for the mass damper. It was also able to predict the crane structure behaviour before the crane has been built!
Intelligent damping
Together with Siemens Drive Technologies Division , Siemens developed a power driven damper with reduced weights, better damping performance and a potential for a range of other applications. Siemens developed a technique for active damping by using a linear motor with “intelligent” motion. It is mounted on the girder and a weight is attached, but is many times lighter than for a conventional, passive vibration damper. A sensor detects the vibration of the girder and algorithms use these values to precisely calculate how the linear motor has to be moved to neutralise the natural oscillation of the girder. Through intelligent acceleration and braking of the linear motor, the forces are directed into the girder. The vibration is reduced much more quickly and effectively than when using conventional techniques. “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. 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,” explains Elmar Schaefers, Manager of Mechatronics R&D at Siemens Drive Technologies. The power drive damper can be integrated in existing container cranes generally without the need for structural modifications. According to Joachim Zoll, Manager for Cranes at Siemens Drive Technologies, 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.