Due to inertia, during horizontal movement of the hoist travelling trolley, loads suspended from the cables are accelerated or respectively decelerated in relation to the hoist travelling trolley in a delayed fashion. These acceleration or deceleration processes lead to a horizontal deviation of the load receiving element in relation to the position of the hoist traveling trolley. This deviation creates an undesirable oscillation of the loads attached to the supporting cables.
One of the constant tasks of a crane operator, therefore, is to counteract these oscillatory movements. A practised and attentive crane operator will achieve this through skillful countersteering during the transport movement. If, however, the operator is unpractised or unattentive, the transportation operations and handling times may be considerably extended. In the worst case, there will be a higher risk of collisions and accidents.
There are known oscillation damping devices by CePLuS in Magdeburg which use high-performance cameras with microprocessors for measuring a horizontal deviation of the load receiving element. These high-performance cameras are mounted to a hoist travelling trolley and measure the movements of the loads so they can adapt the velocity of the hoist travelling trolley while traversing in order to prevent undesirable oscillation of the loads from occurring.
Reflectors are attached to the load receiving element in order to measure the deviation of the load receiving element. The camera mounted on the hoist travelling trolley is directed downwards, i.e. in the direction of the load receiving element, and determines the position of the reflector relative to the hoist travelling trolley. The deviation of the load receiving element is computed from this position data for the reflector.
A drawback of the CeSAR system by CePLuS has been that the time intervals for determining the deviation are too large for realtime dynamic control, and further, the resolution with regard to the accuracy of measurement of the camera measurement system is insufficient to meet the demands of the realtime dynamic control. In addition to this detrimental system data, the overall size of the CeSAR oscillation damping system has proved to be disadvantageous, since the reflectors which must be attached to the load receiving element have unfavourable dimensions.
The patent is based on the idea that if two cables are strung from the trolley to the front and back corners of the spreader, and the crane moves forward, the motion will tend to shorten the front cable and lengthen the back cable.
To enable this effect, two cable length sensors are advantageously placed on the hoist traveling trolley in such a way that the two cables are intersecting. One of the cable length sensors is placed in the front of the hoist traveling trolley and the other is placed in the rear of the hoist travelling trolley. The anchorage point of the respective cables is extended in a diagonal fashion from the respective front part to the respective rear part and from the hoist travelling trolley to the load receiving element.
By this method of guying and the cable length measurement, the horizontal deviation of the load receiving element is exactly determined by using simple trigonometric relationships stored in an algorithm in a computer unit.
The calculation method involves the following steps:
• Measurement of a first diagonal distance between the rear part of the hoist travelling trolley and a front part of the load receiving element and simultaneous measurement of a second diagonal distance between a front part of the hoist travelling trolley and a rear part of the load receiving element;
• Transmittal of the two measured values to an electronic data processing means;
• Insertion of the two measured values into a predetermined algorithm stored in a computer unit connected to the electronic data processing means;
• Determination of an initial value which is equivalent to the horizontal deviation of the load receiving element in relation to the hoist travelling trolley.
As the angle of deviation is preferably required for further calculations of the hoist travelling trolley/load receiving element, the angle of deviation stretched between the verticals and the supporting cables is determined in a second mathematical step, which likewise involves using simple trigonometric relationships. The angle of deviation can then be used as an input variable for the subsequent calculations of the motion system of the travelling trolley/load receiving element.
