Preview-Based Relative-Motion Control of an Insertion Tool for Neural-Thread Placement in Pulsating Tissue

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Article Summary

Robotic neural-thread placement requires regulating the insertion-tool tip relative to tissue that moves with cardiac and respiratory pulsation. This paper develops a preview-based relative-motion controller that estimates latency-delayed periodic surface motion, predicts it over a short horizon, and uses offset-free model predictive control to regulate relative placement while limiting actuator effort and lateral relative velocity. In MuJoCo, the 1-DOF controller achieves 12.0\um\ free-space and 1.9\um\ contact RMS relative-placement error, versus 18.3/176.8\um\ for delayed-feedback impedance and 286.1/275.5\um\ for lab-frame PD, at the cost of higher peak contact force (3.43 versus 2.00~mN) since offset-free tracking drives the tip fully to the commanded depth rather than yielding against the tissue. In 3 DOF, coupled preview reduces contact lateral shear from 1.34 to 0.50~mm/s with 2.1\um\ lateral RMS error. A feasibility-restored octagonal shear formulation keeps the QP solvable under degraded sensing by adding a bounded shared slack: at 10\um\ RMS per-axis sensing noise, where a matched cost-only controller violates the 0.80~mm/s budget in all 10 seeds (mean/maximum 0.988/1.175~mm/s), the soft-octagon controller completes all 10 seeds with no fallback and no measured violation (0.653/0.712~mm/s), with its operating envelope characterized up to 15\um\ RMS. A two-vertex Lyapunov certificate for the controller's actual finite-horizon error-feedback gain holds over $-40\%/{+}50\%$ reflected-mass mismatch. The modeled tip is a rigid contact point, and the study is simulation-only: flexible-thread and carrier-needle mechanics, a validated transient-force constraint, biological damage thresholds, and hardware-realistic sensing and timing remain required before deployment.

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