## 4 Designing Sensors for Soft Robots
### Emerging Deformable Optical-Waveguide Based Sensing Technologies  
An optical fibre combines a core and a cladding, made of materials with different refractive indices. The core material has a considerably higher refractive index than the cladding and because of this a light ray incident on the cross-section of the core, within a certain range of incident angles, gets internally reflected and transmitted along the length of the fibre. Commercial optical fibres used in communications have very smooth surfaces and display negligible light loss over large distances. However, if the surface of the core-cladding interface becomes rough or the refractive index difference between the core and the cladding decreases, light loss in the fibre increases. It has also been observed that when the waveguide is bent or the cross-section of the core is changed due to an external force, light loss increases at the core-cladding interface. With sufficiently lossy optical fibres, it is possible to create optical fibres at centimetre scale that can be used to detect fibre bending by monitoring the change in the intensity of light transmitted through the fibre. Moreover, the core and cladding can also be fabricated from soft silicones with different refractive indices to make light wave-guide-based sensors that are stretchable and capable of detecting both fibre bending and externally applied forces [39; 40]. Many optical fibre sensors based on inherent light loss have been developed to monitor deformations in soft structures. [PERSON] et al. developed stretchable optical waveguides using two different silicone elastomers as core and cladding [39]. These waveguides can be stretched up to 85% strain and can detect stretch, bending and external forces, and have been utilised for proprioception and exteroception in soft robotic fingers. [PERSON] et al. have embedded stretchable optical waveguides into an underwater gripper to achieve bend sensing in an underwater robot, demonstrating the suitability of the technique in extreme conditions--in this case high hydrostatic pressure [40]. In contrast to sensors based on different core and cladding materials, [PERSON] et al. developed gold-coated optical channels in which the light reflected off the gold coating was transmitted internally [41]. These optical waveguides were able to predict linear strain and indentation forces from the transmission losses. One of the issues associated with optical waveguide-based soft sensors is their sensitivity to multiple stimuli (force, stretch and bending), and the difficulty in interpretating the data they provide. A sensor that was sensitive to bending and insensitive to pressure would enable us to measure true bending deformation and would therefore be useful for proprioception of soft manipulator environment interaction. This prompted us to research sensors that were based on flexible optical fibres that are insensitive to pressure. We developed a fibre-optic cable sensor that can be made from off-the-shelf materials using a simple, and inexpensive method of fabrication [42]. The pressure-insensitive bend sensor was fabricated using the commercially available Super EskaTMSimplex Cable SH1001-1.0. These cables are composed of a thin optic fibre with a transparent core made of Polymethyl-Methyl acrylate (PMMA) and is cladded in a layer of fluorinated polymer. Through mechanical abrasion, the optic fibre was made lossy such that the light loss due to bending in a centimetre scale cable is substantial enough to allow characterisation of the bending curvature. The abraded fibre was moulded inside a silicone structure for structural stability (Figure 12A). The fibre-based sensor was integrated into a pneumatically actuated soft finger (based on PneuNets design--see Figure 12B [42]. As the finger was actuated at different pressures, it gradually deformed from pose A to pose D and the optical light transmission losses in the sensor waveguide increased leading to a decrease in sensor output values (Figure 12C).