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Experimental evaluation of force frequency shifting for low-frequency vibration excitation

机译：低频振动激励的力频移实验评估

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摘要

A novel approach known as force frequency shifting (FFS) has been previously proposed for low-frequency (less that 1 Hz) vibration excitation. The technique requires a time-varying forcing function to be applied in a spatially varying fashion. The non-linear dynamic response produces excitation forces and moments at sum and difference frequencies of the force (i.e. f_z) and translational motion (i.e. f_x) frequencies. It is the difference frequency component (i.e. f_z - f_x) that produces the desired low-frequency excitation. The purpose of this paper is to examine the fundamental behaviour of the FFS concept in depth and explore the feasibility of the method for structural excitation of large-scale systems with low natural frequencies. First, a set of tests was performed on a laboratory prototype to quantify the character of the excitation output. The prototype system was instrumented at component interfaces allowing reaction forces to be measured and compared to analytical model results. The tests show that the model frequency predictions were very accurate. Force and moment amplitude predictions were nominally within 5% of the experimental values. However, some amplitude comparisons showed larger errors and can be attributed to a number of identified modelling and experimental issues. Next, a full-scale FFS prototype was used to perform vibration tests on a large test structure (inertial vibration isolation platform). The results from two tests were compared: (1) a traditional modal analysis Using an impact excitation and (2) an operating deflection shapes analysis using the FSS as the excitation. The comparison was excellent demonstrating the potential for the FFS technology to be used for modal testing of large structures.
机译：先前已经提出了一种称为力频移（FFS）的新颖方法，用于低频（小于1 Hz）振动激励。该技术要求时变强制函数以空间变化的方式应用。非线性动态响应会在力（即f_z）和平移运动（即f_x）频率的总和和差频率处产生激励力和力矩。差频分量（即f_z-f_x）产生所需的低频激励。本文的目的是深入研究FFS概念的基本行为，并探讨该方法用于低固有频率的大型系统结构激励的可行性。首先，在实验室原型上进行了一组测试，以量化激励输出的特征。在组件接口处对原型系统进行了检测，从而可以测量反作用力并将其与分析模型结果进行比较。测试表明，模型频率预测非常准确。力和力矩幅度的预测值名义上在实验值的5％以内。但是，某些幅度比较显示出较大的误差，并且可以归因于许多已确定的建模和实验问题。接下来，使用完整的FFS原型在大型测试结构（惯性振动隔离平台）上执行振动测试。比较了两个测试的结果：（1）使用冲击激励的传统模态分析和（2）使用FSS作为激励的运行挠度形状分析。比较非常好，证明了FFS技术可用于大型结构的模态测试的潜力。

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