Abstract
Introduction
Vibration control strategies have received much attention in many engineering fields, such as civil, marine, and aerospace. 1 Especially in spacecrafts, payloads such as antennas, optical mirrors, and scientific equipment easily fail in the vibration environment.2–5 Active control systems can give good performance in mitigating oscillations. However, in some circumstances, the active control methods are limited by its possible instability and its power requirement. 6 Therefore, passive vibration control and active–passive integrated vibration control as proven methods are more widely used in space industry than active control.6,7 Usually, linear vibration absorbers were applied as passive means of control to prevent excessive vibration energy transmission. 8 However, they are designed to suppress the oscillation vibration according to a specific excitation frequency. 9 Linear dynamic vibration absorbers are ineffective and cause the counter effect in other frequencies.
Nonlinear designs are proposed for attenuating broad frequency vibration energy transmitting to the primary system by introducing nonlinear elements.9,10 As an essentially nonlinear local attachment, the nonlinear energy sink (NES) is a powerful vibration absorber without obviously changing the natural frequencies of systems.11–15 Vakakis et al. 16 discovered the important phenomenon of targeted energy transfer, in which the vibrational energy from a linear system is directly transferred to a passive NES in an irreversible manner. Based on this mechanism, the NES has been widely designed to suppress the undesirable vibrations.17–19 NES attached to continuum primary linear structures, such as rod, 20 beams,21–23 drill-string, 24 plate, 25 elastic strings, 26 and hollow rotor systems 27 is considered in literatures. According to the NES applications, the NES has been designed to various types, like nonsmooth 28 or nonpolynomial. 29 In particular, new NES designs based on the rotating NES mass, 30 highly asymmetric NES, 31 modified NES considering negative linear and nonlinear stiffness components, 32 a two-degree-of-freedom (2-DOF) NES device, 33 and a asymmetric magnet-based NES 34 have been introduced to provide better vibration reduction performance. A mixed multiple scale/harmonic balance method is used to obtain the equations describing the slow- and fast-flow dynamics of NES-controlled systems. 35 Two different quasi-periodic response regimes in strongly nonlinear absorbers have been revealed. 36 The response regimes of oscillators coupled to an NES excited by the narrow band stochastic force have been researched theoretically and numerically. 37 A transformation is proposed to obtain the decay envelope formulas, which can be applied to identify the NES damping.38,39 The reduction of galloping vibrations for an aeroelastic system through an NES has also been investigated. 40
In terms of performance indicator of the NES, the displacement response in time domain or the energy dissipation/transition in fixed frequency was often used in the above literatures. Important frequency domain behavior existed in nonlinear systems is usually ignored. The power flow analysis approach can better reflect the nonlinear control performance in the frequency domain, as it combines both forces and velocities as well as their relative phase angle. 41 This method has been largely developed to study linear vibration control systems.42–47 Recently, the technique is receiving growing attention in nonlinear dynamical systems. Royston and Singh 48 applied vibration power transmission as a performance indicator in nonlinear mounting systems. Xing and Price 49 proposed a generalized method for addressing many vibration control problems in engineering. Xiong et al. 50 examined the power flow characteristics of the integrated system consisting of an equipment, a nonlinear isolator, and a flexible ship excited by sinusoidal wave. Xiong and Cao 51 analyzed the vibration energy dissipation mechanism of a 2-DOF system with irrational nonlinearity. Yang and co-workers52–56 developed power flow analysis methods to investigate the nonlinear dynamic systems, which include the power flow characteristics of the Duffing oscillator and the 2-DOF systems for vibration isolations and absorptions. However, the NES is completely different from nonlinear absorbers in the above research. The power flow investigated is the spatial-dependent variable, not the time-dependent variable.54,55 So the properties of the power flow in the NES and the satellite are studied. It is hoped that more energy will flow into the NES. To the best of the authors’ knowledge, power flow characteristics of the NES have not been studied yet.
In the present study, a power flow behavior of the NES is investigated to obtain good designs and applications of the NES. A 2-DOF whole-spacecraft structure with the NES attached is adopted. The harmonic balance method and numerical integrations are used to solve the dynamic governing equations. The instantaneous and time-averaged input and dissipated and absorbed powers of the coupling system are formulated. Power absorption ratio is introduced to indicate the NES performance and is compared with the kinetic energy of the structure. Moreover, the effects of the varying NES parameters on the vibration mitigation performance of the NES are revealed. Finally, conclusions for improving the performance of the NES are provided.
Mathematical modeling
2-DOF structure with NES
A scaled model of the spacecraft studied here is shown in Figure 1. The 2-DOF system with a specific set of parameters is an equivalent model of the scaled model whole-spacecraft.
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With the parameters, a well-designed NES reduces effectively the vibration of the whole-spacecraft. As shown in Figure 2, the 2-DOF system is excited by a harmonic displacement

The scaled spacecraft structure.

The 2-DOF primary structure with an NES attached.
The dynamic governing equations of the whole-spacecraft vibration reduction system are obtained as
In equation (1),
Analytical approximations and numerical integrations are adopted in this study in investigating the dynamic performance of the NES in terms of power flow. The harmonic balance method is applied to obtain the relationship between power flow variables and system parameters. Meanwhile, numerical integrations of equation (1) based on the fourth-order Runge–Kutta approach are used to examine the analytical results. Both experimental data and numerical simulation via the finite element revealed that the whole-spacecraft vibrates periodically. 55 Thus, only the periodic responses are sought in the following.
Analytical approximations and numerical results
The harmonic balance method
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can be used to obtain the first-order approximations of the dynamic responses of the nonlinear system representing periodic solutions. Thus, the displacement responses of the integrated whole-spacecraft vibration reduction system may be assumed as
Substituting these approximation terms in equation (1) and equating the coefficients of the related harmonic terms, we obtain
Thus, the relationship between the responses and the system parameters is provided by equation (4). Based on the Newton–Raphson method, an algorithm is used to solve the abovementioned nonlinear algebraic equations.
Numerical integrations based on the fourth-order Runge–Kutta method are conducted to verify the above approximation results. The variables are defined as
Using equation (5), we observe that equation (1) is transformed into a set of six first-order differential equations
The numerical dynamic response solutions of the nonlinear system can be readily obtained from equation (6) by the fourth-order Runge–Kutta method.
The primary structure parameters are chosen as

Verification of the harmonic balance method formulations (
Power flow analysis
The basic principles of vibration power generation, dissipation, and absorption in the dynamic coupled systems are revealed. The instantaneous input power into the integrated system is the product of the excitation velocity with the corresponding force, that is
For the coupled system, vibration energy is dissipated by the damping in the primary structure and the NES. Thus, the total instantaneous dissipated power is
The instantaneous power absorbed by the NES equals the power dissipated by the damping in the NES and can thus be expressed as
The time-averaged absorbed power over a cycle is
Here we introduce vibration power absorption ratio to evaluate the vibration reduction performance of NES. Both the satellite and the NES can absorb vibration energy from the vibration generator. The more energy the NES absorbs, the less energy the satellite is flowed. So power absorption ratio may be defined as the following formulation
Clearly, a large power absorption ratio means a good performance of NES. In comparison, the kinetic energy of a structure is often used as a measure to examine the NES control effectiveness.
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The maximum kinetic energy
Results and discussion
In this section, the variations of power flow variables in the frequency domain are obtained and are shown in the following figures. The vertical coordinate of
Figures 4 and 5 examine the influence of varying NES mass

Time-averaged (a) input power and (b) absorbed power of systems with varying mass

Kinetic energy (a) of the oscillator and power absorption ratio (b) with varying mass
Figure 5(a) and (b) presents the variations of the power absorption ratio and kinetic energy of the oscillator in the frequency domain, respectively. Figure 5(b) shows that the power absorption ratio of NES is good at all frequencies over the 160 Hz frequency band with the exception of an antiresonance region near 28 Hz. The drop off of the power absorption ratio is acceptable since the NES is antiresonant at this frequency, which can be seen in Figure 3(c). Figure 5 demonstrates that the increase in the NES mass enhances the vibration absorption performance of the NES at all frequencies by increasing power absorption ratio and reducing kinetic energy of the oscillator. However, a relatively light NES is required in engineering design. The NES mass can only increase in a certain range.
Figures 6 and 7 consider the effects of varying NES viscous damping coefficients

Time-averaged (a) input power and (b) absorbed power of systems with varying damping coefficient

Kinetic energy (a) of the oscillator and power absorption ratio (b) with varying damping coefficients
Figure 7 plots the variations of power absorption ratio and kinetic energy of the oscillator against the excitation frequency. The figure shows that the power absorption ratio of NES is high at all frequencies except at an antiresonance region. This result is also caused by the antiresonance of the NES response amplitude
Figures 8 and 9 investigate the effects of varying cubic nonlinear stiffness

Time-averaged (a) input power and (b) absorbed power of systems with varying nonlinear stiffness

Kinetic energy (a) of the oscillator and power absorption ratio (b) with varying nonlinear stiffness
Figure 9 presents the variations of power absorption ratio
Conclusions
The power flow analysis approach is for the first time proposed for quantitatively evaluating the dynamic performance of an NES in the frequency domain. A system of 2-DOF whole-spacecraft structure with the NES attached is adopted. Analytical approximation and numerical integration are applied for solving the dynamic equations of the whole-spacecraft vibration reduction system. The effects of the varying NES parameters on the vibration absorption performance of the NES are revealed. The investigation yields the following conclusions: (1) the NES can produce quite high power absorption ratio over a broad frequency spectrum except at an antiresonance region near 28 Hz, (2) the harmonic balance method is verified by numerical integration to solve this mathematical model, (3) the NES vibration reduction performance becomes better at all frequencies as the NES mass increases, (4) the increase in cubic nonlinear stiffness of the NES significantly enhances its performance only around the first resonance frequency of system, and (5) the effects of NES viscous damping on the performance in the high-frequency range are different to that in the low-frequency range.
