A novel optical 3D force and displacement sensor – Towards instrumenting the PapillArray tactile sensor

Abstract

As robots move into unstructured environments, tactile sensing will become an essential capability of any robotic gripper attempting to engage in dexterous object manipulation. Sensing the frictional properties of the sensor-object interface (along with contact forces and torques) is essential for determining the minimum grip force required to securely grasp an object; however, most existing artificial tactile sensors are incapable of sensing the property of friction. In previous work, a design was presented for a grip security sensor – the PapillArray – which consists of an array of independent silicone pillars with different uncompressed heights, inspired by the papillae in the skin of the human finger pad. It was shown that tracking the relative deflections of the pillar tips can be used to detect incipient slip; and when incipient slip is detected, measuring the 3D force on the pillar provides an estimate of the coefficient of static friction. In a continuation of this work, here we present a method of instrumenting the movement and forces experienced by the pillars of the PapillArray. A novel pinhole camera design is implemented whereby deformation of the pillar results in both movement and change in area of a spot of light, projected onto a quadrant photodiode. The four quadrant photodiode signals are mapped to true 3D displacement and force using multivariate regression. The accuracy of the mapping’s position and force estimates are validated using both video-tracking of the pillar tip and measurements made with a 3D force sensor, respectively, when stimulating the pillar with a spiral XY displacement pattern at various Z compressions. The overall displacement estimation error (mean ± SD) in the X, Y and Z axes was 0.012 ± 0.077 mm, 0.041 ± 0.074 mm and 0.010 ± 0.023 mm, respectively, for a full-scale deflection of 10.0 mm in X and Y, and 3.25 mm in Z. The overall force estimate error (mean ± SD) was 0.002 ± 0.045 N, -0.007 ± 0.049 N and 0.073 ± 0.118 N, for full-scale force of approximately 4 N in X and Y and 11 N in Z, respectively. The sensitivity of the pillar to vibration (between 100 and 1000 Hz) was also tested, as this may help to detect incipient slip events. The pillar can detect vibration up to 1000 Hz for a vibration displacement amplitude as small as 0.35 μm. This technique can be used to instrument all pillars in the PapillArray sensor, thereby facilitating incipient slip detection and friction measurement. Such a sensor would be of particular benefit in a robotic gripping application where it is important to apply only the grip force required to prevent slip of the object of interest, both to reduce the energy usage of the gripping system and to avoid damaging the gripped object.