# A Simple Foot Plantar Pressure Measurement Platform System Using Force-Sensing Resistors
## 2 Materials and Methods  
Measuring instrument is designed to consist of 30 FSR 402 sensors. Sensors are made by Interlink Electronics with a diameter of 12.7 mm, a thickness of 0.46 mm, a range of style sensitivity of 100 g-10 kg, and a range of pressure sensitivity of 1.5-150 psi [12]. Prior to the experiment, each sensor was calibrated by applying an initial testing load (0-6000 g with increasing interval of 200 g) in the active area of the sensor. The determination of the calibration load limit of 6000 g is based on the results of previous studies using the same sensor which shows the load in the largest heel area for flat insole is only 3.35 kg/cm\({}^{2}\) (43 g with 12.7 mm active area diameter) [8]. The characteristics of the sensor behavior response is presented in Figure 1[13]. From the validation results obtained, the relationship V (Volt) and L (kg) in the form of polynomial Equation (1) is as follows:  
The red solid line in Figure 1 indicates the polynomial fit of the calibration measurement graph between voltage (volt) and load (kg). The polynomial fit equation is expressed below:  
\[\text{L}=927.7757\text{ V}^{3}\text{ - }1643.867\text{ V}^{2}+1083.49\text{ V - }31.02378 \tag{1}\]The dimensions of the \(40\times 40\times 6\) cm tool are made of a 3 mm thick steel plate frame and platform is made of 10 mm thick multiplex. On the measuring platform there is a display of foot to guide the subject while standing on it, as presented in Figure 2a. The sensors are attached right underneath the display area of the foot, 15 sensors on the soles of the left and right foot. To obtain a fully covered load distribution on the foot, the measurement are divided into four areas i.e., heel area or rear foot (heel or rear 31% of foot length (FL)), middle (arch or mid foot, 58% FL), front without the radius of the foot (85% FL), and the radius of the foot (100% FL) [14].  
Sensor placement position is presented in Figure 2b.Three sensors are attached on the rear foot (sensor #1-#3) where the sensor 1 is placed in the center of heel (CH) [15].Other four sensors are attached on mid foot (sensor #4-#7) and the remaining seven sensors are placed in the front area without the radius of the foot (sensor #8-#14), and one sensor is located on the thumb toe (sensor #15). The designed tool is used to scan the weight of the subject soles with a shoe size of 42 (FL = 25.9 cm). However, the coordinate placement of the sensors, as seen in Table 1, is still valid in the subjects with the shoe sizes 40 and 41.  
Figure 3 shows the hardware and software of the built-in foot static load gauge system. Each FSR sensor is connected to one 2.7 k\(\Omega\) resistor. The sensor output voltage is read by the Arduino MEGA 2560 microcontroller using a 15 pin analog input bit [16]. Then, the voltage is sent to the DAQ LabVIEW software via a USB serial to be converted into loads using Equation (1). To process and display data on a computer screen according to the wishes of the software interface with C# or C Sharp language. The use of C# language allows intertwined communication with software in LabVIEW.  
Figure 1: Relationship between voltage (V) and load (kg) of FSR 402 sensor.  
Figure 2: Simple foot static load measuring device: (**a**) measurement platform, (**b**) the location of the sensor placement.