Development of a flexible high-friction pressure sensor with structural colors for soft gripper fingers

Abstract

For energy-efficient and lightweight sensor applications, structural colors were utilized to sense the pressure applied to the surface. However, conventional structural color sensors have been usually fabricated with nanoscale features focusing on high pressure sensitivity with small load limits. In this study, a flexible surface with high frictional force and wide pressure-sensing capability was fabricated by ultra-precision machining for soft gripper applications. Friction and grip detection are very important for gripping performance and sensors for soft grippers are required to cope with loads of over 1 kg. Here, microscale three-dimensional trapezoidal patterns were fabricated and transferred to a polymer to increase the contact area for higher friction and improve structural color expression. The effects of pattern geometry on structural color changes due to the varying loads were analyzed using the hue, saturation, and brightness (HSB) model. The hues of the patterned surfaces decreased with loads. Clear color changes were apparent as the pattern width and depth varied; patterns with blue colors (hue of ~240) at no-load status sensed pressure most effectively. Based on the experimental results, a sensor was designed to have a clear color change in terms of pressure and then was applied to soft gripper fingers. Significant color changes were evident as the pressure changed during gripping even under large loads of up to 1200 g; the high friction (compared to that of a plain surface) also improved gripping performances. The sensor provides intuitive information detectable by the naked eye, and there is no need for any power source, wire, or data acquisition system. This research can contribute to the development of multi-functional, lightweight energy-efficient systems.