Grip as Needed, Glide on Demand: Ultrasonic Lubrication for Robotic Locomotion

TL;DR

Friction is traditionally treated as a fixed property in robotic interfaces, but this paper shows ultrasonic lubrication can actively modulate friction to enable locomotion. It presents two friction-control modules (cylindrical and flat) and demonstrates their integration into inchworm- and ovipositor-inspired systems, achieving bidirectional movement with efficiencies around 94% and 93%. Friction reduction is demonstrated across dry and wet, rigid and soft, and even ex-vivo tissue interfaces, indicating broad applicability. The work suggests ultrasonic lubrication can simplify mechanical design and expand locomotion capabilities in diverse environments, with potential impact from medical devices to cluttered-environment robots.

Abstract

Friction is the essential mediator of terrestrial locomotion, yet in robotic systems it is almost always treated as a passive property fixed by surface materials and conditions. Here, we introduce ultrasonic lubrication as a method to actively control friction in robotic locomotion. By exciting resonant structures at ultrasonic frequencies, contact interfaces can dynamically switch between "grip" and "slip" states, enabling locomotion. We developed two friction control modules, a cylindrical design for lumen-like environments and a flat-plate design for external surfaces, and integrated them into bio-inspired systems modeled after inchworm and wasp ovipositor locomotion. Both systems achieved bidirectional locomotion with nearly perfect locomotion efficiencies that exceeded 90%. Friction characterization experiments further demonstrated substantial friction reduction across various surfaces, including rigid, soft, granular, and biological tissue interfaces, under dry and wet conditions, and on surfaces with different levels of roughness, confirming the broad applicability of ultrasonic lubrication to locomotion tasks. These findings establish ultrasonic lubrication as a viable active friction control mechanism for robotic locomotion, with the potential to reduce design complexity and improve efficiency of robotic locomotion systems.

Figures (5)

• Figure 1: Design and integration of cylindrical and flat friction control modules for bio-inspired locomotion. (a) Cylindrical module: a $10~\mathrm{mm}$ ring-shaped resonator with four bonded piezoelectric plates, supported at nodal lines by an internal frame to minimize interference with oscillation. Finite element analysis confirmed operation in the second flexural mode at $22.7~\mathrm{kHz}$. (b) Flat module: a $32 \times 10 \times 1~\mathrm{mm}$ slider resonator with piezoelectric plates bonded at antinodes and supported via a ball-joint groove at nodal lines. Simulation results predicted the third flexural mode at $21.2~\mathrm{kHz}$. (c) Inchworm-inspired locomotion system: two cylindrical modules connected by a push–pull cable-sheath mechanism and actuated by a double-acting cylinder, enabling bidirectional motion through selective activation of each module. (d) Ovipositor-inspired locomotion system: two flat modules, one fixed and one mounted to a linear actuator, enabling bidirectional motion through selective activation of the movable module during the motion cycle.
• Figure 2: Experimental setup for the friction characterization experiments. A miniature load cell mounted along the sliding axis measured friction forces via a bracket guided on a horizontal stage, while the slider was mounted to a vertical linear guide for motorized horizontal motion and free vertical displacement. A 100 g weight ($\approx$1 N) provided the normal load. Two protocols were tested: (i) sliding with and without continuous vibration at 280 V to assess sustained lubrication, and (ii) sliding with the excitation voltage ramped from 0 to 280 V to evaluate dynamic friction modulation. Tests were performed on dry and wet PLA, sandpapers of different grit sizes, granular soil, and ex-vivo porcine intestinal tissue.
• Figure 3: Vibration characterization of cylindrical and flat modules. (a) Frequency response showing resonance peaks at $22.9~\mathrm{kHz}$ for the cylindrical module and $21.4~\mathrm{kHz}$ for the flat module. (b) Vibration amplitude as a function of applied voltage, exhibiting linear growth up to the $\sim4~\mu\mathrm{m}$ measurement limit of the . Both modules achieved amplitudes exceeding $2~\mu\mathrm{m}$ at ultrasonic frequencies ($\geq20~\mathrm{kHz}$), meeting design requirements for ultrasonic lubrication.
• Figure 4: Time-lapse sequence of the inchworm locomotion system traversing a rigid PLA track. By selectively activating ultrasonic lubrication at each friction control module, forward propulsion is achieved following the inchworm motion cycle. Images are shown at one-second intervals. Refer to supplementary videos 1 & 2 for the demonstration of the use cases presented in this paper including the inchworm locomotion case.
• Figure 5: Friction modulation experiments across a range of surfaces. (a) Measured friction forces under forward and backward sliding show that the vibrating slider effectively reduces friction on PLA (dry and wet), soil, sandpaper, and biological tissue (colon). (b) Voltage modulation of coefficient of friction (top row) and corresponding friction reduction percentage (bottom row). On PLA, reduction is consistent with air squeeze-film formation and exhibits nonlinear saturation. Sandpaper surfaces show higher reduction for finer grit (240) compared to coarser grit (150), due to enhanced squeeze-film effects in smaller asperities. Soil produced relatively low reduction owing to its unstable granular contact. On wet PLA with olive oil, viscous adhesion increased friction but ultrasonic vibration still enabled reduction via liquid squeeze-film formation. On colon tissue, viscoelasticity damped vibration energy, limiting reduction. (c) Summary of friction reduction for different substrates during forward–backward sliding (left) and voltage modulation (right) experiments. On pig colon tissue, the slider achieved friction reduction of approximately $37\%$ and $27\%$, respectively, indicating potential for enhancing the locomotion efficiency of sliding robotic medical devices.