## 3 Drone System
### SBUS Communication Protocol  
The SBUS (Serial Bus) protocol is a widely used digital communication standard in radio-controlled systems. It enables efficient and reliable transmission of up to 16 control channels over a single serial line [33; 34]. This reduces wiring complexity while ensuring high-speed and low-latency communication. In drone systems, SBUS typically operates over a 2.4 GHz wireless link between the remote controller and the onboard receiver. The receiver decodes the SBUS signal and forwards control commands such as throttle, pitch, roll, and yaw to the flight controller, which executes the user's intended maneuvers.  
In this system, a Futaba T10J transmitter [32] sends user commands to the onboard receiver. The receiver then forwards the raw SBUS signals to the microcontroller. The microcontroller intercepts these commands, dynamically adjusts them based on the measurements from the sensor module, and forwards the modified signals to the Pixhawk flight controller. The SBUS frame structure consists of 25 bytes: a start byte, 23 bytes encoding up to 16 channels (each with 11-bit resolution), a flag byte for fail-safe and frame-loss detection, and an end byte [33], as summarized in Table 3.  
In this research, four primary SBUS channels of throttle, pitch, roll, and yaw were used for manual drone control. The SBUS signal was extended beyond conventional usage to support autonomous collision avoidance. When an obstacle is detected, the system computes a virtual repulsive force and dynamically adjusts the pitch and roll signals to steer the drone away from the hazard. Since indoor inspections are conducted at low speeds in confined spaces, the SBUS signal range was intentionally constrained to promote safe and precise control. During these operations, SBUS channel values were maintained near the neutral midpoint to avoid abrupt maneuvers, as illustrated in Figure 12.  
In this study, SBUS values from the Futaba T10J transmitter ranged from 360 to 1673. Through a series of controlled experiments in a 4 m \(\times\) 6 m indoor test area, the drone was found to operate most safely and stably within the SBUS range of 687 to 1344, centered at 1016. This range supports fine motion control, minimizing abrupt directional changes or high-speed responses that could lead to collisions in cluttered indoor environments.  
The effectiveness of this configuration was validated using the experimental drone system shown in Figure 10. By leveraging the SBUS protocol and real-time sensor feedback, the system achieved both responsive control and enhanced collision avoidance, making it well-suited for inspection tasks in complex indoor spaces.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Byte** & **Data** & **Description** \\ \hline
0 & Start byte (0 \(\times\) 0 F) & Indicates the beginning of an SBUS frame \\ \hline
1–22 & Channel data (22 bytes) & Encodes 16 channels (11 bits each) \\ \hline
23 & Flag byte & Fail safe/frame lost \\ \hline
24 & End byte (0 \(\times\) 00) & Indicates end of the SBUS frame \\ \hline \hline \end{tabular} Each channel value is an 11-bit integer ranging from 360 to 1673. The midpoint value, 1016, typically corresponds to a neutral or stationary control input.  
\end{table}
Table 3: SBUS frame structure.  
Figure 12: SBUS channel signal range and corresponding drone speed behavior.