Compliant Mechanisms That Use Static Balancing to Achieve Dramatically Different States of Stiffness

Abstract Stiffness in compliant mechanisms can be dramatically altered and even eliminated entirely by using static balancing. This requires elastic energy to be inserted before operation, which is most often done with an additional device or preloading assembly. Adding such devices contrasts starkly with primary motivations for using compliant mechanisms, such as part count reduction, increased precision, and miniaturization. However, statically balanced compliant mechanisms with a fully monolithic architecture are scarce. In this article, we introduce two novel statically balanced compliant mechanisms with linear and rotary kinematics that do not require preloading assembly, enabling miniaturization. Static balance is achieved by the principle of opposing constant force and extended to a rotational device by using opposing constant torque mechanisms for the first time. A constant force mechanism based on existing work is used and inspired a novel constant torque mechanism. A single-piece device is obtained by monolithically integrating a bistable switch for preloading, which allows static balance to be turned on and off. The linear device reduces stiffness by 98.5% over 10 mm, has significantly reduced device complexity and has doubled relative range of motion from 3.3% to 6.6% compared to the state of the art. The rotary device reduces stiffness by 90.5% over 0.35 rad.