Design of a compliant load cell with adjustable stiffness

Manipulation at the sub-micron scale often requires force-sensing capabilities of milli-to nanonewton forces. This article presents a novel design of a compliant load cell with mechanically adjustable stiffness. The system enables adapting force sensitivity to the requirements of a specific application. The principle of the stiffness adjustment is based on a preloaded spring, that stores the potential energy used to compensate the effort needed to deflect the compliant structure of the load cell. Unlike Micro-Electro-Mechanical Systems (MEMS), the new mechanism can be fabricated at the centimeter-scale. This reduces the fragility of the system and facilitates interchange of endeffectors. A main advantage of this solution is the possibility to use one common force sensing device for diverse applications at various scales, such as in biotechnology, semiconductor nanoprobing or microassembly. We describe the analytical model of the load cell and use it to simulate the performance of the stiffness adjustment mechanism. The analytical results are then validated by finite element method (FEM) and experiments performed on a large-scale stainless-steel prototype. Empirical results show that the overall stiffness can be tuned to nearzero and beyond, resulting in a bistable mode. The presented model brings freedom for designing the sensitivity adjustment, and the experimental part shows the ability to reduce the stiffness of the prototype by approximately 200-fold, achieving a force sensing resolution of 0.41 mu N