## 2 Materials and Methods
### Acquisition System for Determining the Phase of Passing and Walking Characteristics
#### 2.1.2 Sensorial System  
The sensory system comprises eight resistive pressure sensors mounted on the back of the sole. Sensors are positioned according to the pressure zones of the sole, having a proper positioning for each sandal size, made with a template. (Figure 5). The optimal positions were established by studying the existing insole models in trade, by studies performed on a group of people (male/female, different sizes) using pressure plates (for orthopaedical use), and by experimental positioning on the prototype obtained. As a result, an eight-sensor configuration was chosen to cover the sole's essential pressure areas. Each sensor is mounted in mechanical support fixed by a screw on the sandal's sole. This bracket consists of a cylindrical housing that supports the sensor. It is attached to the sole using an eccentric piece. When the sensor cannot be mounted precisely in the desired position, the eccentric part can reposition the cylindrical housing in the prescribed position. A sliding cylinder ending in a spherical cap represents the moving part and the contact with the ground. By pressing the sandal on the ground, this cap performs a translational movement in the cylindrical housing by pressing the pressure sensor. Thus the sensory network measures the distribution of body weight. Figure 6 shows the conceptual photograph of the pressure sensor and an image of the sensor placed on the sole. Figure 5: (a) Distribution of pressure sensors on the sole; (b) one-legged stepping cycle. All the components were designed using Solidworks and ANSYS, simulating accurate materials and stresses corresponding to the load with a body weight of 120 kg (Figure 7). The aim is to use pressure (weight) sensors to determine the main points where the pressure exerted by each step is high. Given many sensors, an ATMEGA2560 controller was adopted because 8 sensors will be used for each sole, and the number of inputs held by the controller must be significant. An FSR sensor is made of a piezoresistive material that can modify its resistance depending on the weight it measures. These sensors are resistors that vary linearly, considering conductance vs. resistance under an applied weight. The proposed application's advantages include the operation with low energy consumption and the fact that the implementation, as a thin film, allows mounting in very narrow and tight places. The sensors are read using the electronic current-to-voltage converter circuit to obtain precise values of the measured weight. Such circuitry is required because the evolution of the signal provided by the sensor, depending on the weight, is recorded as current values (mA). The electronic circuit in Figure 8 takes the current as an input, providing a voltage value as output that can be read with the microcontroller. The MCP6004 operational amplifier is part of this design because it was necessary to obtain a linear signal compared to a classic voltage divider, and the current bias input is minimal. In addition, compared to a simple divider, this design keeps the same voltage applied to the FSR sensor even if other resistors or other sensors are connected in parallel. The cost of the data acquisition system (sensors, circuits, processing unit) is around 200 Euro. In addition, the theoretical principle of having a virtual ground between the terminals of the FSR sensor is applied. Finally, the formula determines the output voltage of the circuit:  
\[V_{OUT}=\frac{-V_{DRIVE}}{R_{FSR}}\times R_{F}\]  
where \(V_{OUT}\) represents the output voltage, \(-V_{DRIVE}\) represents the supply voltage, and \(R_{F}\) is the load resistance. Figure 6: Sensor mounted on the sole of the sandal—prototype 3D printer, ABS—positioning detail using eccentric, sensor mounting detail in the housing. Figure 7: (a) Sensor support components—Solidworks design—cylindrical housing, eccentric support mounting, sensor support base, pressure sensor, ground contact cylinder (finished with spherical part); (b) Sensor simulation details using ANSYS.