The article presents an experimental methodology for evaluating fluid muscle stiffness using a controlled linear drive and a measuring chain based on a strain gauge force sensor. The measuring assembly consists of a strain gauge sensor, a signal converter, a programmable logic controller, and an electric actuator ensuring precise positioning. The measurements were performed at a vacuum level of 95% and at three extension lengths, i.e., muscle deflection from the base position (30, 60, and 90 mm), in order to test multiple load conditions. The results demonstrate a change in force response under vacuum loading and highlight the influence of granule rearrangement on stiffness variability at larger deformations. The proposed methodology ensures repeatability, diagnostic reliability, and suitability for characterizing soft robotics actuators, while also creating a robust experimental framework for the systematic evaluation of the stiffness characteristics of fluid muscles across a wide range of testing and loading conditions.
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