Clip-brazing for the design and fabrication of micronewton-resolution millimeter-scale force sensors

Abstract

We present clip-brazing as a new method to construct millimeter-to-centimeter-scale metal structures with micron-scale precision that is fast, scalable and extremely low-cost. Small structures with demanding requirements on both dimensional tolerances and dynamic force responses to time-varying loads are difficult to fabricate and expensive to iteratively prototype. The technique introduced here enables precise placement of strong metallic brazed bonds to create 3D structures with micrometric accuracy, which in this work is exemplified through the design and construction of a broadband micronewton-resolution force-sensing device. The fabrication method uses tensioned clips made from a silver brazing alloy wire to accurately place and control the volume of the metal that joins and supports the pieces that compose the micro-structures. The use of clips also allows the simultaneous fusion of all the connections in the structure during a single heating sequence, reducing tolerance stack-up. To analyze the quality and strength of the brazes, we employed scanning electron microscopy (SEM) on cross-sections and tensile testing on dogbone-shaped sample pieces, respectively. After proper calibration, the functionality of the constructed micro-force-sensing system was analyzed and demonstrated using constant a priori known weights and the vertical periodic forces produced by an 83 mg flapping-wing microrobot (including a 3 mg attachment truss). The static tests, in combination with solid-mechanics analyses and simulations based on finite-element analysis (FEA), provide compelling evidence of the high accuracy and precision of the force sensing system for frequencies below 167.5 Hz. Furthermore, the shape characteristics and average values of the measured periodic signals are compared to computational fluid dynamics (CFD) simulations and validated for two sets of flapping angles across the frequency range from 55 to 100 Hz. These results validate the proposed approach as a viable tool for microrobotic design and fabrication.