In modern power transmission line maintenance, fixed-wing UAVs have become indispensable tools for long-range inspection missions. As the operational range and payload requirements increase, these fixed-wing UAVs grow in size and complexity, demanding rigorous pre-flight calibration procedures. In particular, the inertial measurement unit (IMU) and magnetic compass must be calibrated precisely before each flight. Traditional manual calibration requires multiple operators to lift and tilt the heavy fixed-wing UAV through pitch and roll angles of at least ±10° and a full 360° rotation, posing significant safety risks and potential damage to the UAV. To address these challenges, we designed a dedicated pre-flight inspection platform that automates the calibration process. This paper presents the platform’s architecture, mechanical design, control system, and field validation results.
1. Introduction
The application of fixed-wing UAVs in power transmission line inspection began with line-stringing operations and has evolved to include thermal imaging, LiDAR scanning, and vibration monitoring. With the integration of advanced sensors, the mass of fixed-wing UAVs such as the CW-30 has increased to over 17 kg. Pre-flight calibration of the IMU and magnetometer requires precise orientation changes: pitch ±10°, roll ±10°, and yaw 360°. Manual handling of such heavy fixed-wing UAVs is not only labor-intensive but also hazardous; accidental drops can cause injuries or UAV damage. Therefore, we developed a motorized platform that can automatically execute these calibration maneuvers, ensuring consistency and safety.
2. Requirements Analysis
Field investigations were conducted to define the platform’s technical specifications, as summarized in Table 1.
| Parameter | Value |
|---|---|
| Compatible fixed-wing UAV models | CW-30 / CW-20 |
| Maximum payload capacity (kg) | ≥ 34 |
| Pitch angle range (°) | ±10 |
| Roll angle range (°) | ±10 |
| Yaw rotation range (°) | 360 (unlimited) |
The platform must accommodate the UAV’s geometry and center of gravity (CG). A simplified static model of the fixed-wing UAV was constructed from four main components: fuselage, wings, booms, and tail. The CG projection lies approximately at the intersection of the fuselage and wing leading edge. This point must coincide with the rotation axis of the platform to maintain stability during tilting.
3. Overall Design
The proposed platform consists of four major modules: the rotary table module, the dual-axis tilt module, the support module, and the electrical control module. Figure 1 (inserted later) shows a conceptual illustration of the platform with a fixed-wing UAV mounted.
3.1 Rotary Table Module
The rotary table module provides the 360° yaw rotation required for magnetometer calibration. The fixed-wing UAV is secured to the top plate using a combination of flexible cushions and stainless steel ring clamps. The cushions conform to the UAV’s glass-fiber-reinforced plastic (GFRP) top cover, while the ring clamps grip the carbon-fiber booms (60 mm diameter) which offer high load capacity. A slewing bearing with integrated gear teeth is mounted underneath the table. This bearing features IP67 sealing and hardened alloy steel raceways, ensuring durability in dusty or rainy field environments. The slewing ring is bolted directly to a flange on the tilt module to minimize height and weight.
3.2 Dual-Axis Tilt Module
The dual-axis tilt module is responsible for IMU calibration by tilting the platform ±10° in pitch and roll. We employ a decoupled Hooke’s joint (universal joint) design, separating the two orthogonal axes (A-axis for pitch, B-axis for roll) to allow independent actuation. Two electric linear actuators (pushrods) control each axis. The pushrods are selected for their compact size, self-locking capability, and precise stroke control. Figure 2 (not shown) illustrates the kinematic arrangement.
Kinematic analysis was performed using UG software for eight extreme loading cases (Table 2). The minimum clearance between moving parts was 2.3 mm (Case 5 and Case 6 at the U-shaped clamp), satisfying ISO 10218-1 safety requirements.
| Case | A-axis angle (°) | B-axis angle (°) | Critical clearance location |
|---|---|---|---|
| 1 | +10 | 0 | A-axis clearance |
| 2 | -10 | 0 | A-axis clearance |
| 3 | 0 | +10 | B-axis clearance |
| 4 | 0 | -10 | B-axis clearance |
| 5 | +10 | +10 | U-shaped clamp clearance |
| 6 | -10 | -10 | U-shaped clamp clearance |
| 7 | +10 | -10 | U-shaped clamp clearance |
| 8 | -10 | +10 | U-shaped clamp clearance |
Strength verification of key components was carried out. The long shaft (A-axis) is modeled as a simply supported beam with a concentrated load at mid-span. For a load of 0.6 kN and support spacing of 400 mm, the bending moment is:
$$M = \frac{F \times L}{2} = \frac{600 \, \text{N} \times 0.4 \, \text{m}}{2} = 120 \, \text{N·m}$$
The section modulus of the 40×40 aluminum profile (LD30, 6063-T5) is W = 2600 mm³. Hence the bending stress is:
$$\sigma = \frac{M}{W} = \frac{120{,}000 \, \text{N·mm}}{2600 \, \text{mm}^3} = 46.15 \, \text{MPa}$$
With a safety factor of 3, this remains below the allowable stress [σ] = 58.3 MPa. The U-shaped clamp (made of AISI 304 stainless steel) was analyzed via finite element method under a 588 N load. The maximum von Mises stress was 45 MPa, well below the yield strength of 205 MPa.
3.3 Support Module
The support module consists of a central column (40×40 aluminum profile) and a cross-shaped base. The column is pinned to the A-axis crossbeam of the Hooke’s joint at its top and fixed to the base at its bottom. Because the joint transmits only in-plane bending moments, the column is in pure axial compression. Buckling analysis is performed:
The slenderness ratio λ is calculated as:
$$\lambda = \frac{\mu l}{i} = \frac{2 \times 400}{12.2} = 65.6$$
Where i = √(I/A) = √(52000/332) = 12.5 mm. The Euler critical stress is:
$$\sigma_{cr} = \frac{\pi^2 E}{\lambda^2} = \frac{\pi^2 \times 60{,}000}{65.6^2} \approx 137.5 \, \text{MPa}$$
The actual compressive stress under 60 kg load:
$$\sigma = \frac{F}{A} = \frac{588 \, \text{N}}{332 \, \text{mm}^2} = 1.77 \, \text{MPa}$$
This is far below the critical stress, ensuring safety.
3.4 Electrical Control Module
The control module converts operator commands into motor signals that drive the two linear actuators. To enhance safety during field operation, a wireless remote control using 433 MHz RF modules is adopted. The system controls two relays, each driving one pushrod for extension/retraction. The 433 MHz band offers low power consumption and good penetration, suitable for outdoor environments. The controller also provides emergency stop functionality.
4. Application and Validation
After fabrication, the platform underwent rigorous testing in accordance with DL/T 875-2016. First, all extreme tilt positions were verified: no collisions or interference were observed. The platform moved smoothly without jamming. Repeatability tests over three consecutive cycles showed a maximum positioning error of ±0.5° for both pitch and roll axes. The yaw rotation achieved full 360° with consistent angular accuracy.
Load testing was then performed using a CW-30 fixed-wing UAV (mass 17 ± 0.2 kg). The UAV was mounted on the platform and subjected to ±10° pitch and roll cycles, followed by a continuous 360° yaw rotation. The platform operated smoothly, and the UAV remained securely fastened throughout. The IMU and magnetometer calibration were successfully completed, as confirmed by the flight controller’s status messages.
Field application was carried out during a routine inspection of a 750 kV transmission line. The platform preformed pre-flight checks on the fixed-wing UAV, including IMU and compass calibration. The entire process was completed within 5 minutes without manual lifting. The platform’s stable operation and accurate orientation adjustments significantly improved pre-flight efficiency and eliminated safety risks.
5. Conclusion
We have designed and validated a pre-flight inspection platform specifically for fixed-wing UAVs used in power transmission line inspection. The platform automates the critical calibration of IMU and magnetometer by providing independent pitch, roll, and yaw motions. The decoupled Hooke’s joint driven by electric linear actuators ensures precise ±10° tilting, while the slewing bearing enables full 360° rotation. Field tests demonstrate that the platform operates smoothly, safely, and with high repeatability. By replacing manual handling with remote-controlled automation, the platform reduces labor requirements and eliminates the risk of dropping the heavy fixed-wing UAV. Future improvements may include integrating angle sensors and closed-loop control for even higher accuracy and full autonomous calibration sequencing.