Design of a Compliant Six-Wheeled Robot for In-Pipe Inspection with Geometry Constraints

This paper presents the design of SPIINE, a compliant six-wheeled robot for in-pipe inspection with geometry constraints. SPIINE, the Self-Propelled Interior Inspector for Non-destructive-testing (NDT) Examination, is designed to navigate 53 mm (2 in) inner-diameter pipes with tight 90° elbows, including elbows with a curvature radius as small as 76 mm (3 in), while transmitting power and high-resolution video over long tethers verified to 110 m. In straight steel pipe, SPIINE can generate more than eight times its weight in pull force to support towing of NDT sensor payloads. We also present analytical models for estimating robot stiffness and normal-force generation along with prototype hardware tests that evaluate model accuracy. Across the tested displacement range, the vertical reaction-force model achieved a root-mean-square error of 0.21 N; for two tested pipe configurations, pull-force estimates were within 7–13% of the measured values. Hardware experiments demonstrate that SPIINE can traverse a tight 90° elbow in forward and reverse at eight tested entry roll orientations spanning 360°, generate 16.5 N of pull force in straight steel pipe, and traverse more than 60 m of mixed horizontal and vertical pipe with low measured power consumption of approximately 2–3.5 W across representative test segments.