The safe and stable operation of the power grid is paramount for modern society. Within this network, oil-immersed transformers are critical components for voltage conversion and power transmission. Periodic internal inspection is essential to diagnose potential faults such as insulation aging, oil degradation, or winding deformations, which can lead to catastrophic failures. Traditional manual inspection is a complex, time-consuming, and hazardous process, requiring technicians to enter confined spaces filled with insulating oil. To address these challenges, the development of an autonomous inspection platform is highly desirable. This article presents the comprehensive structural design and analysis of a bionic robot specifically engineered for fault detection within oil-immersed transformers.
The internal environment of a transformer presents unique constraints that directly inform the design of the inspection bionic robot. The primary structure consists of a steel tank, a laminated iron core, and copper windings. The inspection pathways are the narrow gaps between these windings and between the windings and the tank wall. This environment necessitates a bionic robot with a compact, streamlined form factor, high maneuverability for navigation in tight spaces, and complete pressure sealing to operate while fully submerged in insulating oil. Furthermore, the robot must carry vision systems for fault identification and possess sufficient propulsion and depth control for full three-dimensional mobility.
The overall design philosophy is derived from biomimetics, observing the efficient locomotion of aquatic creatures. The proposed bionic robot features a hybrid propulsion system combining a two-degree-of-freedom (2-DoF) pectoral fin mechanism at its front and a dual-tail fin mechanism at its rear. A dedicated buoyancy control unit is integrated at the robot’s center of mass for precise depth adjustment. This multi-modal design allows the bionic robot to execute complex maneuvers including forward/backward motion, vertical ascent/descent, and turning on the spot, which are crucial for thorough inspection in cluttered environments.
Mechanical Architecture of the Bionic Robot
The mechanical design is segmented into three primary subsystems: the pressure hull and sealing system, the biomimetic propulsion system, and the buoyancy control system. Each is designed to meet the stringent requirements of the operational environment.
Pressure Hull and Sealing System
Inspired by the hydrodynamic profiles of fish, the outer shell of the bionic robot is designed for low drag and minimal flow disturbance. To facilitate assembly, maintenance, and internal component layout, the hull is constructed in three modular sections: a nose cone, a central body, and a tail section. The nose cone houses a transparent, sealed camera dome. The central and tail sections form the main pressure vessel containing all electronics, batteries, and actuators. The hull is fabricated from ABS plastic, chosen for its excellent impact resistance, dimensional stability, ease of machining, and adequate strength for the expected pressure differentials.
The sealing between the central and tail hull sections is critical for waterproof integrity. A groove-based O-ring sealing mechanism is employed. An O-ring sits in a precisely machined groove on the central hull face. When the tail section is bolted on, its mating surface compresses the O-ring, creating a tight seal. The contact pressure \( P_m \) on the sealing interface must always exceed the external hydrostatic pressure \( P \) to prevent leakage. The design ensures this condition is met even at maximum operational depth. The O-ring material is Nitrile Rubber (NBR), compatible with transformer oil, with dimensions selected per standard specifications (e.g., inner diameter \( d_1 = 119 \, \text{mm} \), cross-section \( d_2 = 1.9 \, \text{mm} \)).
Biomimetic Propulsion System Design
To achieve full 3D mobility (surge, sway, heave, and yaw), a hybrid propulsion strategy is adopted. The primary thrust for forward motion is generated by the dual-tail fins. Each tail fin is driven by an independent waterproof servo motor, mounted at a fixed dihedral angle (e.g., \( 30^\circ \)) relative to the horizontal plane. By oscillating these fins in phase, a strong forward thrust is produced. Differential control of the fins’ oscillation amplitude or center phase enables tight turning maneuvers.
The 2-DoF pectoral fin mechanism provides auxiliary propulsion and, more importantly, enables fine attitude control and low-speed maneuvering. Each pectoral fin is actuated by two servos arranged in a serial kinematic chain. The first servo (flapping servo) controls the up-and-down stroke of the fin, while the second servo (feathering servo), mounted on the output of the first, controls the angle of attack (feathering) of the fin blade. The complex motion of the fin is a synthesis of these two simple oscillations. The forces generated by the pectoral fins can be analyzed by considering the hydrodynamic pressure on the fin blades. For a fin oriented with specific flapping and feathering angles, the resultant force can be resolved into components in the robot’s body-fixed coordinate frame.
Let \( \alpha_R, \alpha_L \) be the angles between the fin normal vector and the robot’s XZ-plane for the right and left fin, respectively. Let \( \beta_R, \beta_L \) be the corresponding angles between the fin normal projection and the X-axis in the XY-plane. The hydrodynamic force vectors \( \mathbf{F_R} \) and \( \mathbf{F_L} \) acting on the fins can be projected to generate forces along the robot’s X (surge), Y (sway), and Z (heave) axes. The combined force vector \( \mathbf{F} = [F_x, F_y, F_z]^T \) on the robot body from the pectoral fins is given by the transformation:
$$ \begin{bmatrix} F_x \\ F_y \\ F_z \end{bmatrix} = \begin{bmatrix} \mathbf{F_R} & \mathbf{F_L} \end{bmatrix} \cdot \begin{bmatrix} \cos \beta_R \cos \beta_{Rt} & \sin \alpha_R & \sin \beta_R \\ \cos \beta_L \cos \beta_{Lt} & \sin \alpha_L & \sin \beta_L \end{bmatrix}^T $$
By independently modulating \( \alpha_{R,L} \) and \( \beta_{R,L} \), the bionic robot can generate controlled forces in various directions, enabling hovering, lateral translation, and pitch/roll adjustments.
| Component | Degrees of Freedom | Primary Function | Actuation Method |
|---|---|---|---|
| Dual Tail Fins | 1 per fin (Oscillation) | Primary forward thrust, Yaw control | Independent Servo Motors |
| Pectoral Fins (Pair) | 2 per fin (Flap & Feather) | Attitude control, Low-speed maneuver, Heave/Sway force | Serial Servo Actuators |
Buoyancy Control System Design
Active depth control is achieved through a piston-based buoyancy adjustment unit. This system changes the robot’s net buoyancy by altering its displaced volume of oil. The mechanism consists of a waterproof cylinder, a piston, a lead screw, a gear-reduced DC motor, and limit sensors. By driving the motor, the lead screw translates the piston within the cylinder, thereby changing the volume of oil displaced by the robot’s hull. This directly modulates the buoyant force \( F_b \):
$$ F_b = \rho_{\text{oil}} \cdot g \cdot V_{\text{displaced}} $$
where \( \rho_{\text{oil}} \) is the density of transformer oil, \( g \) is gravity, and \( V_{\text{displaced}} \) is the total volume displaced by the robot.
The system is designed to handle the pressure at maximum operational depth. The forces on the piston at depth \( h \) include the hydrostatic pressure force \( F_{\text{oil}} = \rho_{\text{oil}} g h A_{\text{piston}} \) and the force due to internal pressure differential \( F_{\Delta p} = \Delta p \cdot A_{\text{piston}} \). The motor and lead screw must provide a thrust \( F_{\text{thrust}} \) greater than the sum \( F_{\text{resistance}} = F_{\text{oil}} + F_{\Delta p} \) to move the piston. Hall-effect sensors at the cylinder ends provide positional limits to prevent overstroke and damage.
| Parameter | Symbol | Value | Unit |
|---|---|---|---|
| Piston Diameter | \( d_p \) | 13.2 | mm |
| Piston Area | \( A_p \) | 136.8 | mm² |
| Cylinder Stroke | \( L \) | 70 | mm |
| Max Displaced Volume | \( \Delta V \) | 9.58 | cm³ |
| Max Buoyancy Force Change | \( \Delta F_b \) | ~0.83 | N |
| Max Operational Depth | \( h_{max} \) | 10 | m |
| Oil Density | \( \rho_{oil} \) | 880 | kg/m³ |
| Hydrostatic Force at \( h_{max} \) | \( F_{oil} \) | ~12.0 | N |
| Pressure Differential Force (1 atm) | \( F_{\Delta p} \) | ~13.9 | N |
| Total Resistive Force | \( F_{resist} \) | ~25.9 | N |
| Actuator Max Thrust | \( F_{thrust} \) | 140 | N |
| Safety Factor | \( F_{thrust} / F_{resist} \) | ~5.4 | – |
Dynamic Analysis and Fluid Simulation
To validate the hydrodynamic design and predict the performance of the bionic robot, computational fluid dynamics (CFD) simulations were conducted using ANSYS Fluent. The primary goals were to assess the streamlined hull’s drag characteristics and to analyze the thrust generation of the biomimetic propulsors.
Hull Drag Analysis
A simplified 3D model of the robot hull (without appendages) was imported into the simulation environment. After meshing with refined boundary layers, a steady-state flow analysis was performed at a typical cruising speed. The resulting velocity contour plots around the hull showed smooth, attached flow with minimal wake turbulence, confirming the low-drag characteristics of the biomimetic shape. This is essential for efficient propulsion and stable navigation inside the transformer.
Tail Fin Propulsion Simulation
The thrust generation of the dual-tail fin system was investigated using transient CFD with dynamic meshing. User-Defined Functions (UDFs) were employed to prescribe the sinusoidal oscillatory motion of the tail fins: \( \theta(t) = \theta_{max} \sin(2 \pi f t) \), where \( \theta_{max} = 45^\circ \) and \( f = 2 \, \text{Hz} \). The simulation revealed the complex vortex interaction between the two fins. As the fins oscillate, leading-edge vortices are shed and interact, creating a coupled jet flow that enhances overall thrust compared to a single fin. The instantaneous thrust force \( F_T(t) \) was calculated by integrating pressure and shear stress on the fin surfaces. The results showed a periodic thrust profile with peak thrust occurring when the fins were near their maximum velocity during the stroke reversal.
A comparative study was performed to evaluate the efficiency of different fin geometries: the implemented biomimetic shape, a simple rectangular fin, and a diamond-shaped fin. The time-averaged thrust \( \bar{F_T} \) and propulsive efficiency were compared. The biomimetic fin shape, inspired by caudal fin morphologies, demonstrated superior performance, generating higher thrust per unit of input power, which validates the biomimetic design approach for the bionic robot‘s propulsor.
| Fin Geometry | Time-Averaged Thrust \( \bar{F_T} \) (mN) | Peak Thrust \( F_{T_{max}} \) (mN) | Relative Efficiency Index* |
|---|---|---|---|
| Biomimetic Shape | 152.3 | 410.5 | 1.00 |
| Rectangular Fin | 138.7 | 375.2 | 0.91 |
| Diamond Fin | 121.6 | 332.8 | 0.80 |
*Efficiency Index normalized to the performance of the Biomimetic Shape.
Dynamic Modeling
The equations of motion for the 6-DoF bionic robot can be derived using a Newton-Euler or Lagrangian approach. The robot’s motion in the fluid is governed by rigid-body dynamics and hydrodynamic forces. A simplified planar (surge-heave) dynamic model can be expressed as:
$$ (m + m_{a,x}) \ddot{x} = F_{T,x} + F_{P,x} – D_x(\dot{x}) $$
$$ (m + m_{a,z}) \ddot{z} = F_{T,z} + F_{P,z} + F_b – mg – D_z(\dot{z}) $$
$$ I_{yy} \ddot{\theta} = M_{T} + M_{P} $$
Where:
- \( m \) is the robot mass, and \( m_{a,x}, m_{a,z} \) are added mass coefficients in surge and heave.
- \( F_{T,x}, F_{T,z} \) are thrust forces from fins in x and z directions.
- \( F_{P,x}, F_{P,z}, M_P \) are control forces and moment from the pectoral fins.
- \( F_b \) is the buoyancy force from the control system.
- \( D_x, D_z \) are drag forces, typically modeled as quadratic: \( D(\dot{u}) = \frac{1}{2} C_d \rho A \dot{u} |\dot{u}| \).
- \( I_{yy} \) is the moment of inertia in pitch, and \( M_T \) is the pitching moment from thrusters.
This model forms the basis for designing control algorithms for autonomous navigation of the bionic robot within the transformer.
Conclusion and Future Work
This article has presented a detailed structural design and analysis of a biomimetic robot for inspecting oil-immersed transformers. The design addresses the key challenges of the confined, fluid-filled environment through a compact, streamlined hull with a robust sealing system. A hybrid propulsion strategy employing a 2-DoF pectoral fin pair and a dual-tail fin system was developed to grant the bionic robot high maneuverability for navigating narrow gaps. An active piston-based buoyancy control unit was designed and verified to provide reliable depth regulation against the hydrostatic pressures encountered inside a transformer.
CFD simulations validated the low-drag hull form and demonstrated the effective thrust generation of the biomimetic tail fins, showing their superiority over simpler geometric shapes. A dynamic model was outlined to describe the robot’s motion, providing a foundation for future control system development. The integration of these subsystems results in a versatile bionic robot platform capable of performing detailed internal inspections, thereby enhancing grid reliability and safety while reducing downtime and human risk associated with traditional methods.
Future work will focus on the detailed fabrication and integration of a full prototype, comprehensive hydrodynamic testing in a transformer oil analog, and the development of robust localization and autonomous navigation algorithms to guide the bionic robot through the complex internal geometry of a power transformer.