Self-Mixing Laser Interferometry: In Search of an Ambient Noise-Resilient Alternative to Acoustic Sensing

TL;DR

Self-mixing interferometry (SMI) enables noncontact vibrometry with fringes that occur for each displacement of $\\lambda/2$, enabling tactile sensing for extrinsic contact. The authors build a robotic fingertip incorporating an SMI sensor and a microphone and evaluate performance in time- and frequency-domain tasks using an Audio Spectrogram Transformer for spectral classification. They find SMI is robust to broadband ambient noise in time-domain tests and often outperforms acoustic sensing under targeted disturbances; motor noise remains a challenge and higher readout bandwidth is proposed as a future improvement. The results support SMI as a practical ambient-noise-resilient tactile sensing modality and provide design assets for further development.

Abstract

Self-mixing interferometry (SMI) has been lauded for its sensitivity in detecting microvibrations, while requiring no physical contact with its target. Microvibrations, i.e., sounds, have recently been used as a salient indicator of extrinsic contact in robotic manipulation. In previous work, we presented a robotic fingertip using SMI for extrinsic contact sensing as an ambient-noise-resilient alternative to acoustic sensing. Here, we extend the validation experiments to the frequency domain. We find that for broadband ambient noise, SMI still outperforms acoustic sensing, but the difference is less pronounced than in time-domain analyses. For targeted noise disturbances, analogous to multiple robots simultaneously collecting data for the same task, SMI is still the clear winner. Lastly, we show how motor noise affects SMI sensing more so than acoustic sensing, and that a higher SMI readout frequency is important for future work. Design and data files are available at https://github.com/RemkoPr/icra2025-SMI-tactile-sensing.

Figures (7)

• Figure 1: Self-mixing interferometry. Every time the target displacement D changes by $\lambda$/2, a discontinuity or fringe appears in the SMI signal.
• Figure 2: SMI readout circuit.
• Figure 3: Mechanical design of the fingertips.
• Figure 4: Measuring extrinsic contact of objects falling into a hard plastic cup. The laser shows increased isolation from ambient noise.
• Figure 5: Setup for the cup shaking experiment. Either three 14 mm M6 bolts or a chunk of playdough are placed in the cup while the robot rotates from +60° to -60°.