# Robotized Mobile Platform for Non-Destructive Inspection of Aircraft Structures
## 2 Structural Design and Control System  
### Assumptions and Initial Calculations  
The MP was designed taking into account some assumptions related to the non-destructive testing systems as well as motion requirements and adhesion to the tested surfaces. Analyzing various solutions discussed in the previous section, the following assumptions were made:  
* The classical propeller drive or electric ducted fan (EDF) was assumed to be used for the generation of a payload by creating a negative pressure. This approach allows for appropriate adhesion of the MP for various curvatures of the tested surfaces. These types of driver ensure high efficiency and low weight in relation to the power output.
* The body in the form of a shell structure is to be made of glass-epoxy laminate. This design allows for a compact structure, and the use of a glass-epoxy laminate allows for low weight and high durability.
* The MP should be equipped with a four-wheel chassis, which usually ensures vehicle stability and good traction.
* The minimum speed was assumed considering the speed to scanning with the selected testing system: Olympus OmniScan MX with eddy current array (ECA) testing technique, which is typically used for inspection of aircraft. Measurements are to be made using the SAB-067-005-032 low-frequency eddy current array probe, for which the scan speed is 0.1 m/s. The MP should mimic the movement of the ECA probe during inspection by following the predefined trajectories.  
* The speed during idle travel of MP is assumed to be not less than 0.5 m/s.
* The assumption for wheels were the following: diameter of 70-100 mm, tread made of a material with a high friction coefficient (rubber, polyurethane or silicone) close to 1. Higher friction coefficient will allow to obtain a greater lifting capacity of the entire MP with the same propeller drive thrust.
* Wheel drive is to be implemented with DC motors with integrated gear to minimize the mass of the MP.
* The weight of the whole MP is assumed to be up to 2 kg, including ECA probes and cabling, which was estimated after a preliminary review of available components, propeller drive performance, and price.
* DC power supply with a voltage in the range of 12-48 V is provided via converter from the power grid, which is governed by the selected ECA system, where the probe is connected to the defectoscope by wire.  
Taking into account the assumed weight of MP of 2 kg and the diameter of the wheels of 100 mm, the torque of the motors driving the wheels was determined to be 0.98 Nm with a rotational speed of 95 rpm under the assumed linear speed of 0.5 m/s. The next step was to determine the minimum thrust that the propeller drive should produce so that the MP can move on surfaces inclined at arbitrary angles in the range of 0-180\({}^{\circ}\). Considering the assumed mass of the MP and three values of the static friction coefficient of 0.25, 0.5, and 0.75, the thrust force was determined and presented graphically in Figure 1 The maximal values of a thrust force and the related angles of inclination for the considered values of a friction coefficient are given in Table 1.  
The use of wheels with a tread made of a material with a high friction coefficient allows the use of a propeller drive with lower thrust and lower power consumption. This allows the use of smaller and lighter components, e.g., regulator, battery or power cables.  
Having these assumptions and the results of the initial calculations, the development of the control system and construction of mechanical parts of MP were performed.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Friction Coefficient, –** & **Angle of Inclination, \({}^{\circ}\)** & **Thrust Force, N** \\ \hline
0.25 & 104 & 80.9 \\
0.5 & 116.6 & 43.8 \\
0.75 & 126.9 & 32.7 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Maximal values of a thrust force for the considered friction coefficients.  
Figure 1: Relations between thrust force and angle of inclination for the considered friction coefficients.