Design of a flexible optical force sensor for normal and shear force measurement in haptics

Abstract

In recent decades, human robot interaction as well as human computer touch interfaces such as touch screens have become ubiquitous in our daily lives. Numerous works have been done on tactile pressure sensing for human-robot interaction through various transduction mechanisms. However, majority of them focus on measuring one- or two-dimensional forces. Therefore, the current thesis aims to develop a flexible, thin, low cost tri-axial sensor based on optical intensity modulation. The sensor works on the principle of coupling loss of light between multi-mode waveguides and a touch tactor, when the tactor is deformed. Change in contact area of the tactor with the waveguide results in the guided modes of light to couple outside, decreasing the intensity. The structural configuration of the sensor array has been studied by the finite element method. The waveguides are fabricated from transparent and flexible polymers that enable a simple, low cost, robust and easy to scale system. The designed sensor is able to successfully measure normal force from 0 - 10N (0-1315kPa) and shear force from 0 -3 N (0-395 kPa) with a signal to noise ratio of 31.66 dB. Having an offset error of only 0.42 % for 200 cycles, the sensor showed excellent durability. Being flexible, the sensor could conform to curved surfaces with a bending radius as small as 14 mm. The sensor can be packaged into a lower form factor with a thickness of 500 μm by changing the mechanical properties of the material used. The results show potential of the proposed sensor to measure tri-axial contact forces during various human-robot interaction tasks.