How not to lose weight - Gottwald’s new verifiable weighing system for harbour cranes

 

Gottwald took up this challenge in co-operation with Germany-based Schenck Process GmbH, a global market leader for solutions in measuring and process technologies in industrial weighing. The goal was to develop a verifiable discontinuous totalizing automatic weighing system. The basic requirements for an officially approved mobile harbour crane weighing system were:

The basic principle behind the weighing system that was developed to meet these requirements is the measurement of the supporting forces of the rope pulleys at the boom tip. A similar approach was taken in the past in connection with double boom luffing cranes. However, with those cranes, the technical implementation was far easier because the boom tip was always at the same angle, which meant that the rope forces and, as a result, the bearing forces were always in the same direction.

 

If the crane has a single boom, this makes it far more difficult because the effective directions of the forces in the ropes differ depending on the boom angle, which, in turn, depends on the current working radius. This meant that a completely new, unique system had to be developed and implemented — and this is the biaxial measuring system.
The biaxial measuring system enables the supporting forces of the rope pulleys to be measured at the boom tip in two perpendicular directions by means of special load elements and sensors fitted to the boom tip. Combined with sophisticated corrective mathematic algorithms, the system takes into account and compensates any influences affecting the weighing process and the results, such as rope deadweight, boom inclination, vibration of the entire crane as well as in the hoisting drive train, which are caused by lifting heavy loads and rope pulley friction.
The new weighing system has its own dedicated weighing terminal installed in the crane cab. This terminal is equipped with a display and the memory prescribed for a calibrated system which automatically records the result of every weighing action and can be read out later to enable the weighing documents to be printed.
All the weighing results are shown on the display and stored in the tamper-proof calibrated memory. A printer can also be installed which will continuously print every weighing result as a printed form. The actual weighing process is, to a large extent, automatic, once the system has been activated, so that almost no interaction is required between the operator and the system.
THE WEIGHING PROCEDURE
In each working cycle of the crane, two measurements are usually required: one with the full grab and the second with the empty grab. This ensures that any material remaining in the grab after it has opened is detected and excluded from the amount for that move. This ensures that only the amounts actually transferred are recorded and invoiced.
In order to minimize vibrations in the mechanical system and achieve valid weighing results as quickly as possible, the weighing procedure takes place during a hoisting phase at a reduced, but constant speed. During this phase the system constantly evaluates the signals provided by the various sensors and calculates a load figure. However, this figure still is subject to dynamic oscillations.The system monitors this load figure and looks for a sufficiently long phase in which the signal does not exceed a pre-defined tolerance range. Once this phase has been detected, the figure found will be accepted as the valid result of this weighing cycle.
One of the greatest challenges is, of course, to find the right compromise between weighing accuracy and process speed. As always, accuracy and speed contradict each other by nature, which means that highly accurate weighing must result in a trade-off with handling capacity. But thanks to the weighing system configuration and the sophisticated evaluation algorithms, the time required to obtain a valid result has been minimized and this optimizes handling rates.
Once the system has registered the correct weight, the crane driver continues as usual with full performance to move the bulk material to its destination, where the weight of the empty grab is determined in the same way. If the type of bulk material is such that there is little risk of caking in the grab, it is also possible to reduce the frequency at which the empty grab is weighed.
Gottwald’s weighing system has been designed to ensure that, irrespective of the dynamic behaviour of the crane, the system must not, under any circumstances, return incorrect weighing results. Even under the toughest working conditions, accuracy comes first.
OFFICIAL APPROVAL AS VERIFIABLE WEIGHING SYSTEM To receive ‘verifiable’ status, the new Gottwald mobile harbour crane weighing system had to pass an extensive and demanding official verification procedure. The system had to be checked and tested at length by the German national metrology institute, the Physikalisch-Technische Bundesanstalt (PTB), before being awarded EU type approval. As well as the comprehensive theoretical tests carried out by
the PTB, which concentrated at that time on compliance with weighing accuracy demands in all operating states of the crane and tamper-proof security, practical tests were carried out in December 2010 using a crane equipped with the new system. These practical tests started, as with all automatic weighing systems, with static tests, which involved weighing predefined test weights. This ensures that, when the crane is in commercial use, irrespective of the situation, at any boom angle, load height and slewing angle, the same load always produces the same, correct weighing result. Finally, the inspectors checked that, in dynamic operation, the limit values for that operating method are adhered to.
These static and dynamic tests correspond exactly to those mandatory tests required when each individual installation is tested on-site before it goes into commercial operation.
As a result of the positive testing results achieved here in the theoretical and practical tests, the PTB issued a European ‘Type- Examination Certificate’ which allows the system to be used as a verifiable ‘discontinuous totalizing automatic weighing instrument’ in all the EU States and any other country that recognizes EU regulations.
STATIC VERIFICATION IN ACCORDANCE WITH OIML R 76-1 (NON-AUTOMATIC WEIGHING INSTRUMENT) The first part of the practical verification involved static tests with special, defined high-precision blocks weighing up to
50 tonnes in total. According to OIML, for ‘Non-automatic weighing instruments’ of accuracy class III, the permissible error during initial verification is 0.5 x the verification weighing system interval. In this case the interval is 100kg so the static weighing results had to be within a tolerance of 50kg. The maximum
deviation observed, however, was only 20kg so the Gottwald crane-mounted weighing system was well within the limit.
DYNAMIC VERIFICATION IN ACCORDANCE WITH OIML R 107-1 (DISCONTINUOUS TOTALIZING AUTOMATIC WEIGHING INSTRUMENT) In the second verification phase, dynamic tests with bulk material were required. For ‘Discontinuous totalizing automatic weighing instruments’ of accuracy class 0.5, the OIML rules define a maximum permissible error during initial verification of 0.25% of the total weighed amount of the bulk material in the test. So, as the test involved a total handling volume of 278.4 tonnes of bulk material, the permissible error was 696kg. Since the actual deviation was only 40kg for the prototype system, the Gottwald crane weighing system was well within the permissible error range.