Static Calibration of a Force sensor based on Fiber Bragg Technology

The tactile sensor, developed at the “Università Campus Bio-Medico di Roma”, has a rectangular structure of acrylic resin, where 9 fiber optic sensors, based on FBG technology, are placed; in fact there are 3 rows of optical fibers, in order to have a 3 x 3 sensors array (on each row there are 3 FBGs), as it’s possible to see in figure 3.1.

Polydimethylsiloxane (PDMS) layer is dripped over the fibers and covers all the entire structure, in order to have a flexible surface and to shield the fiber optics. It was chosen this particular silicone rubber for its physical and mechanical properties [2]:

* usability over a wide range of temperature (at least -100 °C ÷ 100 °C);
* the shear modulus G ( 100 kPa ÷ 3 MPa) ;
* small temperature variations of the physical constants (except for the thermal expansivity, α≈ 20 ⋅10 -5 K -1 );
* low chemical reactivity (except at extremes of pH).

However between the three sensors lines, two grooves are made in in order to avoid cross-talk, as it’s possible to see in the previous figure.

Each FBG sensor, long 10 mm, is placed in a security basis between two solid acrylic resin. Moreover, in figure it’s possible to see several protruding cylinders on external surface: they essentially make easier the assembly with other rectangular sides which have the same hole. This elements are used in order not to escape PDMS while this silicone rubber is falling over the acrylic resin structure; then when this operation is finished and when the rubber is become rugged, the external wall can be removed and the protruding cylinder may be trimmed. It also important to remember that the nine fiber optic sensor have different wavelength, in particular three wavelengths for each row, as it’s showed in figure 3.3.