82 K. J. Pederson et al. Fig. 9.1 Z-axis test setup test engineers. Force-limiting techniques have continued to improve and a comprehensive guide can be found in the NASA Technical Handbook “Force Limited Vibration Testing” [4]. Additionally, force limits can be enacted in vibration analysis in situations where negative margins are unrealistic due to a fixed base constraint. A method and case study of this can be found in reference [5], which discusses how force limiting was used on the CoNNeCT SCAN Testbed. This chapter discusses the common pitfalls of improperly using load cells during force-limited vibration testing, including the effect of mounting configuration on load cell sensitivity, and the in situ calibration performed to remedy this. At NASA Glenn Research Center’s Structural Dynamics Lab (SDL), a recent test on the NEXT-C thruster exhibited some of the potential consequences of these pitfalls. This test is referenced throughout this chapter to provide a real-world example of these known challenges. The NEXT-C ion thruster is a commercialized version of NASA’s Evolutionary Xenon Thruster (NEXT-C) built by Aerojet Rocketdyne. This thruster will provide in-space propulsion technology demonstration of the Double Asteroid Redirection mission (DART). The DART mission is part of the Planetary Defense Program and aims to alter the trajectory of a candidate asteroid, proving that this technology is capable of redirecting a future asteroid off a potentially hazardous earth bound trajectory. The thruster went through environmental vibration testing at SDL in November 2019 and, as of the writing of this paper, is tabbed to launch in July 2021. The thruster is mounted to the spacecraft through three radially oriented joints. Figure 9.1 depicts this test setup. In situ calibration is required to account for the parallel load path introduced by the pre-load bolt, shown in yellow in Fig. 9.2. There are two common methods to perform in situ calibration of load cells, and both include imparting a known load into the test setup and making sure the load cells accurately measure that load. One model uses a modal hammer to impact the test article with a known force and the other utilizes a base-driven excitation. For both the modal hammer and base-driven excitation, frequency response functions (FRF) are analyzed. For the modal hammer technique, the FRF between the load cell force and hammer force should equal one at low frequency signifying that the measured force into the structure is being accurately captured by the load cells. For the base-driven technique, the FRF magnitude between the base acceleration and load cell force, which is 1 divided by the apparent mass, should be equal to 1 (total suspended mass) at low frequency. Depending on how far off the FRF is from one, the load cell’s sensitivity would then be adjusted. The NEXT-C testing was performed utilizing the in situ calibration because there was no identifiable hard point to impact the ion thruster. Also, for structural health monitoring purposes, the test plan included a low-level random test that would adequately excite the rigid body motion. The two main force-limited testing topics discussed in this chapter are (1) the unknowns associated with which mounting hardware should be included in the measured mass of the test article and (2) the handling of load cells that are not aligned with the test axis and updates to the sensitivity of summed channels.
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