Commit 54a1fa8b authored by Sam Calisch's avatar Sam Calisch
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<img src='img/package.jpg' height=300px>
### Noncontact, bipolar, nonlinear loadcells
### Bipolar, nonlinear loadcells
I've become interested in nonlinear load cells -- those whose transfer function changes slope in order to be very sensitive at small loads and less sensitive at larger loads (in order to maximize range). One way to think about this transfer function is as a soft transition into a protective stop.
Nonlinear load cells have been made by geometric design of flexures, as in this <a href="https://dspace.mit.edu/handle/1721.1/119789">thesis</a>. This is very cool, but seems it might be sensitive to contamination and the contact forces might exhibit some hysteresis. It also doesn't seem to lend itself to miniaturization.
Another approach is to use a flexure that is more linear but to measure displacement using a physical principle with a nonlinear transfer function. One example of such a principle is frustrated total internal reflection, as the reflectivity is a near exponential function of gap distance. <a href='https://link.springer.com/article/10.1140/epje/i2017-11542-4'>This paper</a> demonstrates the measurement of film thickness using this principle (and as a bonus includes some force measurement based on Hertzian contact between planar and spherical lenses!).
Another approach is to use a flexure that is more linear but to measure displacement using a physical principle with a nonlinear transfer function. One example of such a principle is frustrated total internal reflection, as the reflectivity is a roughly logarithmic function of gap distance. <a href='https://link.springer.com/article/10.1140/epje/i2017-11542-4'>This paper</a> demonstrates the measurement of film thickness using this principle (and as a bonus includes some force measurement based on Hertzian contact between planar and spherical lenses!).
I think a bipolar nonlinear load cell could be made based on frustrated reflection that was very sensitive around the origin, while having a very large maximum range. It could also be made extremely small and inexpensive (led, photodiode, a few miniature lenses, likely encapsulated in an oil).
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