Tripody: An Overconstrained 3-SPR-like Parallel Robot for High-Reach Construction Tasks

2026-07-28

AI Summary

This work addresses the lack of lightweight, high-stiffness robotic systems capable of millimeter-level precision for complex indoor elevated tasks by proposing Tripody, a wheeled three-degree-of-freedom parallel robot. Innovatively replacing the spherical joints in a 3-SPR architecture with universal joints introduces controllable overconstraints, while distributed elastic deformation accommodates kinematic incompatibilities—thereby significantly enhancing torsional stiffness without compromising translational accuracy. The system integrates custom linear actuators, SE(3) state estimation, forward kinematics modeling, and task-space control, and supports modular end-effectors. Weighing 33 kg with a working height of 1.7–3.4 m and a payload capacity of 32 kg, Tripody achieves closed-loop positioning errors below 0.6 mm. Compared to its spherical-joint counterpart, it exhibits up to a 454% increase in torsional stiffness and demonstrates a maximum relative hole-position error of only 4.5 mm in open-loop drilling tasks.

Abstract

Many ceiling construction tasks still rely on heavy serial manipulators that are difficult to deploy in cluttered interiors, motivating lightweight, field-ready alternatives that reach ceiling height while maintaining millimeter-level accuracy and the stiffness demanded by overhead tool loads. We introduce Tripody, a wheeled 3-DoF parallel robot for high-reach tasks that replaces the base spherical joints of a classical 3-SPR (3 legs; S: base spherical joint; P: actuated prismatic joint; R: end-effector revolute joint) morphology with universal joints, intentionally overconstraining the mechanism; small, distributed elastic deflections absorb the resulting incompatibilities, preserving predominantly translational motion. The 33kg system extends from 1.7m to 3.4m in height, supports a continuous 32kg payload, and offers a modular end-effector interface for ceiling operations. We detail the mechanical design - including custom linear actuators and a kinematic-compatibility analysis - and a control stack for accurate positioning that combines SE(3) state estimation, forward kinematics, and task-space control. In experiments, Tripody exhibits similar in-plane stiffness to a spherical-base variant but substantially higher torsional stiffness - an increase of 67% at 1.7m, 196% at 2.6m, and 454% at 3.4m - while maintaining negligible cross-axis coupling. Closed-loop positioning with a total station converges below 0.6mm across the entire workspace; pure model extrapolation achieves a 95th-percentile error of 2.7mm (max 3.6mm). Finally, we demonstrate task-level ceiling-drilling feasibility in an open-loop study by drilling a 15-hole pattern with 4.5mm maximum relative hole-position error after rigid alignment. These results support overconstrained, compliance-absorbing 3-SPR-like architectures as a practical path to lightweight, high- reach, millimeter-accurate construction robots.

Problem

• high-reach construction
• parallel robot
• overconstrained mechanism
• millimeter-level accuracy
• stiffness

Innovation

• overconstrained parallel robot
• 3-SPR architecture
• torsional stiffness enhancement
• compliance-absorbing mechanism
• high-reach construction robotics