## 4 Simulation Results Using PI, PID, and ISTSMC Strategy  
Figure 5 shows the performance of the PI controller in a MATLAB simulation when tracking the desired value of motor current \(I_{ref}\), which varies from 0A to 0.21A in pulsating form. The \(I_{m}\) in Figure 5 shows the measured value of the motor current, with a minor deviation from \(I_{ref}\). Figure 5 also shows the pitch (deg) of the VTOL system along the horizontal axis based on the PI controller. The input to the VTOL system is \(I_{m}\), which varies from 0 A to 0.21 A; this leads to the operation of the VTOL propeller assembly and initiates vertical uplift to attain the equilibrium position at \(0^{\circ}\). In addition, the VTOL assembly faces fluctuations when varying the pitch (deg) from \(-5^{\circ}\) to \(5^{\circ}\). The tracking error in steady-state  
Figure 4: Closed-loop system for VTOL system based on ISTSMC. conditions shows the reduced performance of the PI controller simulated in MATLAB, as shown in Figure 5.  
Figure 6 shows the results achieved in MATLAB based on the cascaded loop controller for the VTOL system. It shows the pitch in (deg) of the aluminum arm along the horizontal axis tracking the \(\theta_{ref}\) of the pulse generator signal with a period of 2.8 s, having a magnitude of pitch of two degrees along the horizontal axis and a pulse width of 50%. The tracking error is observed between \(\theta_{ref}\) and \(\theta_{m}\). Similarly, Figure 6 also shows the \(I_{m}\) measured value of the motor accurately tracking the \(I_{ref}\) based on the PI controller in a current control loop for the actuator model. Figure 7 shows the comparative analysis between the cascaded loop controller based on PI as an inner loop tracking \(I_{m}\) to \(I_{ref}\), PID at the outer loop tracking \(\theta_{m}\) to \(\theta_{ref}\), and the nonlinear control strategy using ISTSMC. In addition, Figure 7A,B illustrate the pitch of the VTOL system tracking the pulse generator signal \(\theta_{ref}\), as discussed in Figure 6, with \(I_{m}\) showing the motor current used to operate the actuator of the VTOL system, respectively. A significant tracking error of nearly 40% of the final value is observed in Figure 7A for the PID controller. To reduce the magnitude of the tracking error, the ISTSMC algorithm is applied, with the results illustrated in Figure 7C,D demonstrating the pitch (deg) of the VTOL system and the motor current of the actuator model, respectively. It is shown in Figure 7C that the magnitude of the tracking error is reduced to less than 1%. A tracking delay is also observed, indicating a delay in the response of the VTOL system due to its inertia opposing the uplift provided by the propeller to the platform. As the motor current increases, the VTOL platform's pitch is also increased to track the reference value \(\theta_{ref}\) as depicted in Figure 7D. Figure 7C confirms that the magnitude of the tracking error \(\mid\!\!\theta_{ref}-\theta_{m}\!\!\mid\) decreases, leading to a significant decrease in \(I_{m}\), shown in Figure 7D, compared with Figure 7B, for the cascaded loop controller. Besides this, the controller input for ISTSMC, shown in Figure 7D, is more uniform and continuous compared to the cascaded loop controller shown in Figure 7B, where rapid fluctuations can be observed that lead to reduced reliability and cause mechanical losses. Figure 5: Pitch of the VTOL system and motor current of actuator model based on PI controller. Experimental Results Based on PI and Cascaded Controller  
Figure 8 shows the experimental setup based on the QNET VTOL system. The 24 V DC energizes the motor with 11,000 rpm connected to a solid aluminum arm. The circuit board of the QNET VTOL consists of a PWM voltage-controlled power amplifier that generates a peak current of 2 A and a continuous current of 0.5 A.