# Quasi-Passive Resistive Exosuit for Space Activities: Proof of Concept
## 2 Materials and Methods
### Proof of Concept
#### 2.2.3 Soft Sensors Technology  
As a lower limb assistive device targeted at applications in locomotion and mobility, it is important that the XoSoft exosuit can identify different events during the cyclic motion. Although the ultimate application aims to generate resistance for limbs in the zero-gravity of a space craft or low gravity planetary missions, this system will initially be tested on earth, thus the device will rely on contact with the ground. To achieve this, the XoSoft exosuit has a sensorized insole with pressure sensors in strategic places: (i) a rear sensor placed approximately under the heel, (ii) middle sensors placed between the heel and the toes, and (iii) a front sensor placed at the tip of the toes. These sensors are commercial Force Sensing Resistors, which measure the contact force transmitted through their surface. They are integrated into a custom made silicon insole that can be used in any shoe either on or off-planet. To capture the gait events, only a binary on/off signal is required and this can  
Figure 3: Control flow of the finite state machine (FSM), in which the events (heel strike, flat foot, front foot, toe off, positive speed inflection and negative speed inflection) determining the state changes are reported. be extracted with a simple threshold operation. The insole sensors provide the following signals to be used for gait segmentation: heel pressure signal (HS), outside insole pressure signal (OS), inside insole pressure signal (IS), toe pressure signal (TS). The measurement of the real-time derivative of knee angular displacement (KAD) for both knees, allows the system to segment the gait cycle identifying six different events as mentioned above: HS, FI, FrF, TO, PSI and NSI. Using this sensory data and sequencing, the specific control signals can be generated to provide the required resistance and actuator actions during the cycle. **Soft sensors**: The prototype used for this proof of concept integrates sensorized soft modules to measure angular displacement at the knees using soft capacitive strain sensors [91]. A three-electrode configuration incorporating micro-coaxial cables was used to shield the sensors from proximity effects and avoid parasitic capacitances. Conductive textiles and non-conductive silicone elastomers were used, because of their excellent mechanical properties in terms of stretchability, flexibility and compliance. They also bond effectively in the multilayer three-electrode structure, while providing conformability to the garment in which they are integrated. The electrodes are made from Electrolycra (Mindsets Online, Middlesex University, London, UK) a stretchable conductive textile made from silver plated nylon and elastic fibres. Ecoflex(r) silicone elastomer (Ecoflex 00-10, Smooth-On Inc., Macunige, PA, USA) was selected for the dielectric layers of the shielded capacitor. To provide data on the knee bending movements, three strain sensors were embedded in a commercial elastic knee brace (shown in Figure 4A,B [91]). They are rectangular in shape, with the longest side parallel to the sagittal axis, since the largest strain occurs in the sagittal axis direction. More specifically, one sensor is positioned over the patella while the other two sensors are placed on either side of the kneecap. Each sensor has an active sensing area of 162 mm\({}^{2}\). Since these capacitive elements are also sensitive to applied pressure, the integration of three sensors allows the simultaneous discrimination between strain (due to the bending of the knee) and pressure (due to accidental contact of one sensor with the surroundings). During usage the kneepad is worn in contact with the skin, under the exosuit, to directly measure the strain induced by the bending of the knee. A typical raw output during normal walking is shown in Figure 4C. **IMU-based Strategy**: To estimate the knee angle and velocity, it is also possible to use a combination of two IMUs placed on each shank and thigh respectively as demonstrated in [24; 92].