## 4 Designing Sensors for Soft Robots
### Optic Fibre Based Pose Sensor Design  
The optic fibre-based sensor described here measures the distance between a reflective surface and the tips of the optic fibres used for transmission and reception of light signals. Varying the fibre-to-tip distance as a function of the robot's bending motion alters the light intensity reflected back which in turn allows us to determine the robot's pose. In this particular implementation, we use three light sources (in this case light emitting diodes (LEDs)) throwing light via three optic fibres onto three reflective surfaces that move according to the amount of bending and the direction of bending experienced by the soft robot. The amount of reflected light (modulated by the robot motion) is subsequently collected by the three receiving fibres and read out by an optoelectronics circuit at the base of the receiving fibres. As any change in pose affects those distances, there will be concurrent changes in light intensity at these reflective surfaces. When compared to other optic-based sensing techniques (such as Fibre-Bragg Grating (FBG) methods), light intensity modulated sensing is a relatively low-cost option that mitigates various complexities relating to system set-up, signal acquisition and processing. Our curvature sensor system was developed using three sets of optical fibres and inextensible wires that allowed us to calculate the bending radius, the bending direction and the elongation of a soft manipulator section from the arc lengths of these wires--all of which were embedded along the longitudinal axis of a flexible, cylindrical module emulating the flexible portion of a soft robot arm. In our test rig, the flexible module itself was attached to a rigid section housing the optic fibres. To achieve this, three channels were introduced at 120\({}^{\circ}\) intervals across the cross section of each module, to house the required wires (in the flexible module) and transmitter/receiver optic fibres (in the rigid section), respectively, as per Figure 9. Within the flexible module, the wires are fixed to the far end and to small magnets at the other end where it connects to the rigid section. The magnets are moved when the flexible module bends in different directions. The surfaces of the magnets are highly reflective and thus used to reflect the light from the tip of the transmitting fibres back to the receiving fibres. Due to their magnetic properties, additional magnets can be integrated within the rigid section to ensure that the measuring wires are always kept taut. Bending in the flexible segment causes changes to the length of the wires within the three channels, with the effect that the magnets are drawn towards or moved away from the top of the flexible module. As stated, the distances between transmitter/receiver and magnet influence the amount of reflected light, effectively turning the receiver into a light intensity-modulation sensor. By measuring the length of all three wires, we can calculate, with some precision, the amount of bend, the direction of bend and the elongation of the flexible module. In terms of prototype production, we used three Keyence Digital Fibre sensors FS N10 to transmit and receive light via three pairs of optic fibres (transmission and reception). The flexible module was constructed from EcoFlexTM, a plastic resin commonly used in the fabrication of soft manipulators [17]. The flexible module length, pre-extension, was \(l=55\) mm and had a diameter of 25.5 mm. The channels in the flexible module are symmetrically positioned, each one \(d=7.75\) mm from the central axis, and with a radius of 3.5 mm, taking into account its inner lining. Made from the articulator sections of drinking straws, this lining helps lubricate the channel surface for the fibres within. The rigid lower section of the test platform (see Figure 9B) houses the ends of the optic fibres and the magnets within its three chambers, such that the magnets are able to move up and down.