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Loadcell Stiffness and Resonant FrequencyRev. C
When selecting a loadcell, the user should keep in mind that with any loadcell, the stiffness and resonant frequency can effect signal accuracy. The possible effects of relative compliance of the loadcell to that of the loading source is especially likely to be a major error factor with low force loadcells. TAT loadcells are available in both compliant and stiff models. Conventional low force load cells, on the other hand are generally very compliant.
The force to be measured by the loadcell may originate from a very stiff or a very compliant source. The loadcell selection should take the source stiffness into account. If the source is very compliant, such as a soft spring, then even a fairly compliant and low resonant frequency load cell may be adequate. This is not, however, always the case and the signal frequency response requirements and its effects together with the digital signal sampling rate could still be an error contributor.
As a rule of thumb, the resonant frequency of the loadcell and the structure attached to it in combination must be more than twice the sampling frequency and even much higher than that to assure that no sampling error will occur. A much higher ratio would be lead to even more accuracy with any sampled signal.
If the source is fairly stiff (that is the force will change if the source is not constrained by the loadcell) then only a very stiff loadcell can accurately measure the force. In this case, the loadcell is acting as a physical constraint on the movement of the structure where the force is originating.
If, on the other hand again, the load source is fairly compliant, then a compliant loadcell may be advantageous if the mounting/attaching of the loadcell will unavoidably induce a relatively large strain into the load cell. The high compliance of the loadcell will reduce the chance of a damaging overload being forced into the loadcell. In general, such a high compliance will not allow high sampling rates to produce a highly accurate signal.
It is the combination of the total mass supported by the loadcell (its own mass and the mass of the load source that determine the resonant frequency. The resonant frequency, in turn, limits the sampling rate and how rapidly the load can change and still be measured to the required accuracy. Some models of TAT load cells are fairly low in mass with high stiffness. Those models will have a high resonant frequency, which will allow high sampling rates.
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