Design of an In-Pipe Robot with Contact-Angle-Guided Kinematic Decoupling for Crosstalk-Suppressed Locomotion

Abstract

In-pipe inspection robots must traverse confined pipeline networks with elbows and three-dimensional fittings, requiring both reliable axial traction and rapid rolling reorientation for posture correction. In compact V-shaped platforms, these functions often rely on shared contacts or indirect actuation, which introduces strong kinematic coupling and makes performance sensitive to geometry and friction variations. This paper presents a V-shaped in-pipe robot with a joint-axis-and-wheel-separation layout that provides two physically independent actuation channels, with all-wheel-drive propulsion and motorized rolling reorientation while using only two motors. To make the decoupling mechanism explicit and designable, we formulate an actuation transmission matrix and identify the spherical-wheel contact angle as the key geometric variable governing the dominant roll-to-propulsion leakage and roll-channel efficiency. A geometric transmission analysis maps mounting parameters to the contact angle, leakage, and efficiency, yielding a structural guideline for suppressing crosstalk by driving the contact angle toward zero. A static stability model further provides a stability-domain map for selecting torsion-spring stiffness under friction uncertainty to ensure vertical-pipe stability with a margin. Experiments validate the decoupling effect, where during high-dynamic rolling in a vertical pipe, the propulsion torque remains nearly invariant. On a multi-material testbed including out-of-plane double elbows, the robot achieved a 100% success rate in more than 10 independent round-trip trials.

Figures (15)

• Figure 1: Prototype of the proposed V-shaped in-pipe robot enabled by a joint-axis-and-wheel-separation architecture.
• Figure 2: Conceptual comparison of architectures: (a) Coupled motion vs. (b) Our decoupled motion.
• Figure 3: 3D CAD model of the proposed in-pipe robot. The system integrates an AWD propulsion train and a centrally actuated spherical wheel for rolling reorientation.
• Figure 4: Geometric constraints and end-view configuration of the V-shaped robot inside a circular pipe. (a) Pipe cross-section schematic defining the pipe diameter $D_p$, the omni-wheel pair spacing $W_o$, the wheel radii $R_o$ (omni wheel) and $R_s$ (spherical wheel), the pipe center $O$, and the nominal omni-wheel-to-wall geometric clearance $H_{wp}^{0}$ shown in the equivalent 2-D model. (b) End-view configuration used for the geometric constraint and contact-angle analysis: the core structural design parameters $(L_1, L_2, a, b, n)$ determine the derived configuration angles $(\alpha_1,\alpha_2,\theta_1,\theta_2)$ and the total V-opening angle $\theta=\theta_1+\theta_2$. The distance $H_s$ denotes the offset between the omni-wheel centerline and the spherical-wheel center along the pipe diameter direction (as indicated). At the spherical-wheel--pipe interface, $\alpha_0$ marks the angular location of the nominal (ideal) contact point on the pipe wall, while $\alpha$ denotes the actual contact angle used later to characterize transmission crosstalk.
• Figure 5: Unified end-view model and notation used in both the static stability analysis (Section III) and the actuation transmission analysis (Section IV). The model defines the wheel--pipe normal forces $F_{N0},F_{N1},F_{N2}$ at the spherical wheel (index 0) and the two omni-wheel pairs (indices 1 and 2), the corresponding friction forces $f_0,f_1,f_2$, the gravitational loads $G_k$ of the main modules (indexed in the figure), and the joint torque $M_J$ at the V-arm pivot. The inset illustrates the spherical-wheel contact and defines the contact angle $\alpha$ as the deviation between the direction of the actual contact force/velocity transmission and the ideal circumferential direction for pure rolling about the pipe axis; the rolling torque $\tau_r$ and the induced axial-leakage component $\tau_p$ are also illustrated.