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Additive manufacturing is more and more used in optics to produce opto-mechanical components as well as light transmission mediums, either for prototype evaluation or for functional part generation. It was previously shown that optical systems can benefit from the geometrical accuracy of the printed parts. Intrinsic defects such as surface roughness or volume birefringence can also be exploited for optical component design. We here present such use of particular properties of an additive manufacturing process based on photopolymerization. The final goal of the work is the design of a force sensor for collaborative robotics. More precisely, the aim is to design an optical force sensor to control the contact force between a human body and a magnetic source controlled by a robot for medical purpose. Optical sensors are known to have major interests in harsh environments where classical electrical sensors cannot be used due to, like here, electromagnetic compatibility issues. Two 3D-printed designs of optical force sensors are compared. The first one, conceptually developed in a previous work, is using polarization modulation due to force-induced birefringence to modify optical transmission in a sensor based on a monolithic original geometry. For such a case, additive manufacturing appears as a powerful production technique as the 3D part must be transparent and at the same time obtained with an accurate complex geometry. The second design is based on the volume scattering properties of printed transparent parts. For the first time to our knowledge, we show that the optical system made out of a beam expander and a cylindrical lens, necessary to achieve an optical line, can be replaced by a simple prismatic 3D-printed element. Using the Polyjet technology developed by Stratasys Ltd, a line can simply be obtained using the 1D volume light scattering inside the printed medium. The variation of line properties is then related to the mechanical strain induced by the force to be measured. In other words, the optical properties we rely on are linked to the bulk liquid material, its photopolymerization during printing and finally the impact of mechanical stress on the printed component. The sensitive element in the force sensor can be seen as a metamaterial with properties which depend on its micrometric structuration. The micro-structuration size is not related to the standard minimum feature size as claimed by the manufacturer but to the additive manufacturing process itself. In our case, a Stratasys Connex 350 printer has been used with an acrylate transparent material. Opto-mechanical properties such as birefringence, surface roughness, elasto-optic coefficients have been measured. The ability to generate an optical line using natural 1D volume light scattering in a printed parallelepiped with polished surfaces is experimentally demonstrated. As potential application, the parallelepiped is used to replace a cylindrical lens in an amplitude modulation force sensor. The sensor response is measured. Thus, additive manufacturing appears to be a promising technique to achieve optical components and to integrate optical sensors in future 3D-printed mechatronic systems.
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