Soft Multi-Directional Force Sensor for Underwater Robotic Application

Abstract

Tactile information is crucial for recognizing physical interactions, manipulation of an object, and motion planning for a robotic gripper; however, concurrent tactile technologies have certain limitations over directional force sensing. In particular, they are expensive, difficult to fabricate, and mostly unsuitable for underwater use. Here, we present a facile and cost-effective synthesis technique of a flexible multi-directional force sensing system, which is also favorable to be utilized in underwater environments. We made use of four flex sensors within a silicone-made hemispherical shell structure. Each sensor was placed 90° apart and aligned with the curve of the hemispherical shape. If the force is applied on the top of the hemisphere, all the flex sensors would bend uniformly and yield nearly identical readings. When force is applied from a different direction, a set of flex sensors would characterize distinctive output patterns to localize the point of contact as well as the direction and magnitude of the force. The deformation of the fabricated soft sensor due to applied force was simulated numerically and compared with the experimental results. The fabricated sensor was experimentally calibrated and tested for characterization including an underwater demonstration. This study would widen the scope of identification of multi-directional force sensing, especially for underwater soft robotic applications.

Waterproof Design of Soft Multi-Directional Force Sensor for Underwater Robotic Applications

“…However, by introducing the solid retaining ring to the updated sensor design the flex sensor's base remains fixed when the active sensing length is displaced due to the deformation of the shell. Therefore, the readings exhibit a repeatable trend, but opposite to what was reported previously [30], while there does exist variability in readings between sensors S 1 -S 4 in the case of this reporting, there is a clear and repeated trend that provides emphasis that a contact force is localized to a specific sensor. Although flex sensor outputs should have been precisely similar theoretically as hypothesized in the working principle of the sensor, they are different for individual cases (demonstrated in Figures 7-11).…”

“…The flex sensors S 1 and S 2 , on the opposite side of the array, increase in resistance as their curvature decreases. In the previous development of this sensor [30], the differential readings exhibited a completely opposite trend, as is consistent with the test results exhibited in Figure 9. To reiterate, this is likely due to the addition of a solid material retaining ring and entrapped fluid used to fill the internal pocket of the shell.…”

“…Figure 7 demonstrates the output data of the sensor when subject to normal force exerted at 0 • inclination. The result for this case is consistent with the output of the sensor due to normal force built in our previous study [30]. It should be understood that each flex sensor's differential reading is distinct from the others due to variable sensitivity and suitable positioning constraints during fabrication.…”

“…As shown in Figures 8 and 9 accordingly, when force at 30 • and 60 • to the vertical axis is applied directly onto a certain sensor, the corresponding differential reading of that flex sensor is minimum with respect to the remaining three flex sensors. In previous tests reported in [30], an increase in the flex sensor under indentation was visible. This was due to a complete soft construction of the silicone pad, which allowed for the flex sensor base to move more in line with its active sensing length.…”

“…Based upon the proven data set presented in this report, the sensor can be entitled to qualify for achieving the IP68 standard in terms of waterproofing. This is a significant improvement from the previous implementation of the multi-directional force sensor which could only achieve an IP67 level of protection against water [30]. The successful waterproofing test insinuates the sensor's prospects for underwater exploitation.…”