Tri-axial Motion Sensing with Mechanomagnetic Effect for Human-Machine Interface

Abstract

Haptic sensing has been critical to human-machine interaction for wearable robotics, where interaction force sensing in the three-dimensional (3D) space play a key role in stimulating environmental proprioception, predicting human-motion intent and modulating robotic fine-motions. While normal pressure sensing has been widely explored in the uniaxial way, it is still a challenging task to capture shear forces along the tangential directions where typical capacitive or resistive sensing is difficult to implement. Moreover, integration of uniaxial force sensing modules in one unit would produce a bulky system that is impractical for wearable applications. Herein, this paper proposes a tri-axial motion sensing method based on mechanomagnetic effect, where both normal and shear forces can be captured through the magnetic field monitoring in the 3D space. A soft magnetic film was designed and fabricated to induce compliant deformations under tri-axial loads, and the flexible deformations can be captured through the magnetic flux changes via the hall-effect. Both simulation and experimental results are provided to justify the sensor performance and validate its potential applications to human-machine interaction.

Appendix

1.1 Fabrication of Magnetic Film

The flexible magnetic film was fabricated with mixture of polydimethylsiloxane (PDMS; SYLGARD 184, Dow Corning) and NdFeB powders. The mass ratio of PDMS and NdFeB is 1:5. The mixture was cast into a thin film of the thickness about 0.3 mm using a knife coater. After curing for 30 min at 100 ℃, the film was manually cut into a circular shape with the diameter of 4 mm and was automatically magnetized to saturation in the magnetization equipment (AMH-500 Hysteresisgraph, Laboratorio Elettrofisico).

1.2 Fabrication of Elastic Layer and Sensor Assembling

The elastic layer of the proposed sensor was made from PDMS (SYLGARD 184, Dow Corning) with the mass ratio of curing agents being 10:1. The air bubbles in the elastomer mixture was removed by the vacuum pump. Then the mixture was cast into a mold and cured for 30 min at 150 ℃. After the magnetized film was cured, it was put into the groove on the upper surface of the elastomer, then the groove was filled with PDMS mixture to obtain the elastomer assembly that was cured for 5 min at 150 ℃ in an oven. Finally, the elastomer was bonded to the flexible circuit board with a silicone adhesive.