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| Error evaluation with load measurement on travelling crane using dynamometric axles | Go back to the FAQ | |
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| It is possible to measure the load lifted by a travelling crane by using an axle mounted at the balancing pulley (mountings in x/2) or at its fixed point (mountings in x/1).
The aim of this document is to analyze where the different measurement errors come from and to define their importance.
1. Errors due to the measurement system (dynamometric and electronic axle)
1.1. Errors due to dynamometric axles
The errors specific to dynamometric axles are either mechanical or thermal.
The mechanical errors (non-linearity, hysteresis and non-repeatability) mainly depend on dynamometric axle dimensions or the quality of the supports.
For standard SENSY axles set in tolerated h6 bores, the error is about 0.5% of the nominal load of the bridge.
It can be put back to around 0.2% on request, on the condition that the dynamometric axle is profiled.
On a special axle whose length/diameter ratio is too small, this error can go up to about 5%.
The zero drifts and sensitivity according to the temperature are usually unimportant because they don't go over 0.1% for 10°C.
1.2. Errors due to management electronics
The errors due to management electronics are usually unimportant because they don't go over 0.1%.
2. Errors due to hoisting device
2.1. Errors due to the height of the cable
Higher the load smaller is the part of cable supported by the dynamometric axle.
The importance of this error depends on the ratio between the cable weight and the nominal load hoisted.
The only way to cancel this error is to always do the measurements at the same height.
2.2. Errors due to frictions in the pulley blocks.
The consequence of frictions in the various pulleys is that the apparent load on the dynamometric axle seems lighter when the load is being hoisted and heavier when the load is being lowered.
It is possible to lessen the influence of these frictions by always taking the measurement after the same hoisting or lowering action.
2.3. Errors due to dynamic phenomenon’s
The hoisted loads rarely stay still and the resulting acceleration is added to that of gravity.
This influence can be lessened by treating the signal of the dynamometric axle with an adequate filtering of the electronics and by waiting for the load to be still.
Finally, we can say that the global measurement error is of around 1 to 2% in the applications where the dimensions of the dynamometric axle are coherent and where the hoisting device is in good condition.
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