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1. A cryogenic torsion balance using a liquid-cryogen free, ultra-low vibration cryostat

   https://doi.org/10.1063/5.0089933  

   Fleischer, S. M.; Ross, M. P.; Venkateswara, K.; Hagedorn, C. A.; Shaw, E. A.; Swanson, E.; Heckel, B. R.; Gundlach, J. H. (June 2022, Review of Scientific Instruments)

   We describe a liquid-cryogen free cryostat with ultra-low vibration levels, which allows for continuous operation of a torsion balance at cryogenic temperatures. The apparatus uses a commercially available two-stage pulse-tube cooler and passive vibration isolation. The torsion balance exhibits torque noise levels lower than room temperature thermal noise by a factor of about four in the frequency range of 3–10 mHz, limited by residual seismic motion and by radiative heating of the pendulum body. In addition to lowering thermal noise below room-temperature limits, the low-temperature environment enables novel torsion balance experiments. Currently, the maximum duration of a continuous measurement run is limited by accumulation of cryogenic surface contamination on the optical elements inside the cryostat. 

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2. Capturing modeling pathways using the Modeling Assessment for Physics Laboratory Experiments

   https://doi.org/10.1119/perc.2020.pr.fox  

   Fox, Michael F.; Pollard, Benjamin; Ríos, Laura; Lewandowski, H. J. (September 2020, 2020 Physics Education Research Conference Proceedings)

   null (Ed.)

   A choose-your-own-adventure online assessment has been developed to measure the process of modeling undertaken by students when asked to measure the Earth's gravitational constant, g, using a simple pendulum. This activity forms part of the Modeling Assessment for Physics Laboratory Experiments (MAPLE), which is being developed to assess upper-division students' proficiency in modeling. The role of the pendulum activity is to serve as a pre-test assessment with apparatus that students are likely to be familiar. Using an initial sample of student data from a development phase of the assessment, we show that the pendulum activity is able to discriminate between a range of student processes that are relevant to understanding student engagement with modeling as a scientific tool. 

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3. A New Vibrational Control System in Nature: Flapping Flight

   Taha, H; Kiani, M (January 2019, AIAA SciTech)

   Vibrational control is an open loop stabilization technique via the application of highamplitude, high-frequency oscillatory inputs. The averaging theory has been the standard technique for designing vibrational control systems. However, it stipulates too high oscillation frequency that may not be practically feasible. Therefore, although vibrational control is very robust and elegant (stabilization without feedback), it is rarely used in practical applications. The only well-known example is the Kapitza pendulum; an inverted pendulum shose pivot is subject to vertical oscillation. the unstable equilibrium of the inverted pendulum gains asymptotic stability due to the high-frequency oscillation of the pivot. In this paper, we provide a new vibrational control system from Nature; flapping flight dynamics. Flapping flight is a rich dynamical system as a representative model will typically be nonlinear, time-varying, multi-body, multi-time-scale dynamical system. Over the last two decades, using direct averaging, there has been consensus in the flapping flight dynamics community that insects are unstable at the hovering equilibrium due to the lack of pitch stiffness. In this work, we perform higher-order averaging of the time-periodic dynamics of flapping flight to show a vibrational control mechanism due to the oscillation of the driving aerodynamic forces. We also experimentally demonstrate such a phenomenon on a flapping apparatus that has two degrees of freedom: forward translation and pitching motion. It is found that the time-periodic dynamics of the flapping micro-air-vehicle is naturally (without feedback) stabilized beyond a certain threshold. Moreover, if the averaged aerodynamic thrust force is produced by a propeller revolving at a constant speed while maintaining the wings stationary at their mean positions, no stabilization is observed. Hence, it is concluded that the observed stabilization in the flapping system at high frequencies is due to the oscillation of the driving aerodynamic force and, as such, flapping flight indeed enjoys vibrational stabilization. 

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4. The Response of an Inerter-Based Dynamic Vibration Absorber With a Parametrically Excited Centrifugal Pendulum

   https://doi.org/10.1115/1.4053789  

   Gupta, Aakash; Tai, Wei-Che (August 2022, Journal of Vibration and Acoustics)

   Abstract The inerter has been integrated into various vibration mitigation devices, whose mass amplification effect could enhance the suppression capabilities of these devices. In the current study, the inerter is integrated with a pendulum vibration absorber, referred to as inerter pendulum vibration absorber (IPVA). To demonstrate its efficacy, the IPVA is integrated with a linear, harmonically forced oscillator seeking vibration mitigation. A theoretical investigation is conducted to understand the nonlinear response of the IPVA. It is shown that the IPVA operates based on a nonlinear energy transfer phenomenon wherein the energy of the linear oscillator transfers to the pendulum vibration absorber as a result of parametric resonance of the pendulum. The parametric instability is predicted by the harmonic balance method along with the Floquet theory. A perturbation analysis shows that a pitchfork bifurcation and period doubling bifurcation are necessary and sufficient conditions for the parametric resonance to occur. An arc-length continuation scheme is used to predict the boundary of parametric instability in the parameter space and verify the perturbation analysis. The effects of various system parameters on the parametric instability are examined. Finally, the IPVA is compared with a linear benchmark and an autoparametric vibration absorber and shows more efficacious vibration suppression. 

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5. A modified Michelson interferometer to measure sub-milliradian changes in angle

   https://doi.org/10.1063/5.0100720  

   LeDesma, C_K; Ross, M_P; Daly, B_E; Hoyle, C_D; Mola, M_M (August 2022, AIP Advances)

   Modern short-range gravity experiments that seek to test the Newtonian inverse-square law or weak equivalence principle of general relativity typically involve measuring the minute variations in the twist angle of a torsion pendulum. Motivated by various theoretical arguments, recent efforts largely focus on measurements with test mass separations in the sub-millimeter regime. To measure the twist, many experiments employ an optical autocollimator with a noise performance of ∼300 nrad/Hz in the 0.1–10 mHz band, enabling a measurement uncertainty of a few nanoradians in a typical integration time. We investigated an alternative method for measuring a small twist angle through the construction of a modified Michelson interferometer. The main modification is the introduction of two additional arms that allow for improved angular alignment. A series of detectors and LabView software routines were developed to determine the orientation of a mirror attached to a sinusoidally driven rotation stage that oscillated with an amplitude of 0.35 mrad and a period of 200 s. In these measurements, the resolution of the interferometer is 8.1 μrad per fringe, while its dynamic range spanned 0.962 mrad. We compare the performance of this interferometric optical system to existing autocollimator-based methods, discussing its implementation, possible advantages, and future potential, as well as disadvantages and limitations. 

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