Force-Feedback Micro-Actuation
Distal voice-coil micro-actuator with integrated 6-axis F/T sensor provides closed-loop insertion depth control to ±50 µm and skin-tension monitoring at the needle interface.
The end-effector houses a voice-coil linear actuator (VCA) optimized for low-mass reciprocation of the needle holder. The moving carriage (8.4 g) rides on cross-roller guides with sub-micron straightness, driven by a current-mode amplifier at 8 kHz update rate. A 6-axis silicon strain-gauge F/T sensor sits between the actuator base and the needle cartridge interface, providing simultaneous force and torque feedback.
Insertion depth is controlled in a cascaded loop: an outer position loop (1 kHz) commands a force-saturated inner velocity loop, with a feed-forward Z-impedance term derived from a Kelvin–Voigt scalp tissue model (k ≈ 1.8 N/mm, c ≈ 0.04 N·s/mm). When the scalp-tension load cell crosses 0.6 N preload threshold, the controller transitions from velocity to force-tracking mode for the final 1.2 mm of stroke.
Modeled Fz vs. depth; first cyan band marks skin pierce, second marks graft release transition.
| Parameter | Value | Unit | Tolerance / Note |
|---|---|---|---|
| Actuator type | Voice-coil (VCA) | Custom Lorentz coil | |
| Carriage mass | 8.4 | g | |
| Stroke length | 15 | mm | |
| Peak force | 6.0 | N | |
| Continuous force | 2.4 | N | |
| Position resolution | 0.5 | µm | Linear optical scale |
| F/T sensor axes | 6 | Fx, Fy, Fz, Tx, Ty, Tz | |
| Force resolution (Fz) | 5 | mN | |
| F/T sample rate | 8 | kHz | |
| Insertion depth tolerance | ±50 | µm | |
| Closed-loop bandwidth | 350 | Hz | |
| Skin-tension threshold | 0.6 | N | Triggers force-mode handoff |