Research Article Design and experiment of a passive damping device for the multi-panel solar array Advances in Mechanical Engineering 2017, Vol. 9(2) 1–10 Ó The Author(s) 2017 DOI: 10.1177/1687814016687965 journals.sagepub.com/home/ade Yongfang Kong and Hai Huang Abstract With the space technology development, large flexible space deployable structures have been used widely. Studying on the vibration control for large flexible space deployable structures becomes very important. In this study, a novel passive vibration damping device is developed for the multi-panel sun-orientated deployable solar array. Its upper strut contains a viscous damper while the lower strut is rigid. The device is lockable and located near the solar array root hinge to increase the structural damping without reducing the fundamental frequency. This design will not influence the original functions of the solar array, such as folding, deploying, and sun tracking. The corresponding finite element models are established, and the properties of the damping device are investigated by modal analysis and transient response analysis. The damping mechanism design for a certain type of solar array is presented. The associated modal tests based on a solar array test sample verify the effectiveness of the device. Conclusions are drawn to help define design guidance for future damping device implementations. Keywords Deployable solar arrays, vibration control, viscous damper, finite element analysis, mechanical design, modal test Date received: 8 July 2016; accepted: 13 December 2016 Academic Editor: Fakher Chaari Introduction With the space technology development, large flexible space deployable structures, such as large-scale solar arrays, antennas, and space manipulators, have been used widely. These components have structural features like large scales, high flexibility, low stiffness, and low damping. Therefore, large vibration is easily induced when the flexible components are disturbed, which may seriously affect the system performances. However, many on-orbit interference factors are inevitable, such as the disturbance generated by the spacecraft attitude adjustment, orbit transfer, space debris impact, and thermal shock. In order to improve the life of large flexible space deployable structures and the system’s running accuracy, the structural vibration must be effectively controlled. On vibration control for large flexible space deployable structures, various ways have been explored. These ways can be divided into two different categories, passive and active. One passive method is to transfer the host structural vibration energy. An adaptive passive damping system using tuned mass dampers (TMDs) was developed for the low-order modes of spacecraft large solar arrays.1 Another common passive method is to increase the structural damping. For example, a damper consisting of a titanium flexure and viscoelastic damping material was designed for the rigid Hubble Space Telescope (HST) solar array, SA-3. It was mounted in the base of the mast to achieve the School of Astronautics, Beihang University, Beijing, P.R. China Corresponding author: Hai Huang, School of Astronautics, Beihang University, 37 Xueyuan Road, Haidian District, Beijing 100191, P.R. China. Email: hhuang@buaa.edu.cn Creative Commons CC-BY: This article is distributed under the terms of the Creative Commons Attribution 3.0 License (http://www.creativecommons.org/licenses/by/3.0/) which permits any use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.sagepub.com/en-us/nam/ open-access-at-sage). 2 improvement in the overall structural damping level and suppress structural vibration induced by the control system.2 A mast damping system was developed for the Shuttle Radar Topography Mission’s 60-m-long deployable mast which served to deploy an outboard antenna.3 The mast damping elements were located at the canister attachment truss aft bipod and the vertical strut to control the mast’s first two orthogonal bending modes and first torsional mode, respectively.4 The other passive way is to improve the structural stiffness. This is normally implemented by a new design, such as a novel design of the composite structural lattice frame for the spacecraft solar arrays,5 and a tape spring hinge designed by an optimization method for the deployment of solar arrays.6 For active vibration control, considerable modes have also been studied, like positive position feedback,7 autonomous decentralized intelligent control,8 and proportional-derivative (PD) control.9 Actually, active vibration control methods can achieve excellent results. However, all of these active methods require additional energy devices and have difficulties in the controller design. The methods for improving part of the structure have higher requirements on the design with only limit increase in the stiffness. On the contrary, increasing the structural damping with passive methods, especially only attaching a damping system to the host structure, is easier to implement. Additionally, it has the advantages of high reliability and good economic benefits. Moreover, A Rittweger et al.10 researched four different damping principles for the corresponding damper device predesign. These principles were based on the following physical effects: hydrodynamic concept, hydraulic concept, coulomb friction, and viscoelastic bushings. For good accuracy and high stability, dampers designed according to the first concept, especially viscous dampers, are adopted most in engineering projects. Thus, passive damping method based on the viscous damper is advisable for space structural vibration control. Today, the multi-panel sun-orientated deployable solar array is commonly utilized to provide spacecraft power generation. Most deployable solar arrays employ rigid honeycomb panels. In order to satisfy the requirements of spacecraft such as cost, mass, and power growth capability, honeycomb panels become larger and are even replaced by tensioned flexible blanket systems for some missions.11 Large solar arrays probably easily bring low-frequency and longtime bending modal responses, threatening the safety of the spacecraft structures. Hence, vibration damping for large solar arrays is necessary. The HST vibration suppression is a successful example of passive control applications.2 However, its damper was in a series damping mode and had a trade-off between the damper stiffness and damping performance. The final damper made the SA-3 Advances in Mechanical Engineering fundamental bending frequency decreased, which had a negative impact on the HST jitter performance. In this study, a novel passive vibration damping device is developed for the multi-panel sun-orientated deployable solar array. Its upper strut contains a viscous damper while the lower strut is rigid. This device has the following features. First, this is a fluid viscous damper with no stiffness. Second, the oblique strut damper is attached to the solar array root hinge in a parallel damping mode, increasing the structural damping without reducing the fundamental frequency. Third, the damping device can be folded at launch, deployed along with the solar array deployment and then be locked. Furthermore, the device will not influence the sun-orientated function. The rest of the article is organized as follows: in section ‘‘Solar array damping device,’’ the proposed damping device is introduced. In section ‘‘Finite element modeling and analysis,’’ the corresponding finite element models are established and the properties of the damping device are investigated. Section ‘‘Damping mechanism design’’ provides the damping mechanism design. Experimental results are presented in section ‘‘Experiments.’’ And the last section draws the conclusions. Solar array damping device The multi-panel solar array on-orbit works in the deployed state, which belongs to the typical cantilever beam pattern. Generally, its first mode is the bending vibration mode and the corresponding frequency is much lower than other vibration modes, so the bending vibration is most likely to be aroused. Adding an oblique member near the solar array root hinge to improve the structural stiffness or damping was considered as the vibration suppression method. Early in the project design phase, the corresponding finite element models were created. The models were employed to analyze the control effect of different methods. Simulation results indicated that the improvement in structural stiffness and vibration suppression was poor by only adding a rigid rod to the root. Practically, most modal strain of the deployed solar array’s first vibration mode is concentrated at the root. Therefore, attaching damping element to the root can well damp the structure. Moreover, the linear viscous damping approach was selected based on good accuracy, high stability, simple design concept, and amenability to existing analysis tools.4 Considering the viscous damper has no stiffness, a parallel damping concept was employed to develop the solar array damping device. The purpose of such damping device design is to increase the structural damping without reducing the fundamental frequency. As mentioned before, for the general multi-panel solar Kong and Huang 3 (a) C (b) Lockable A hinge A D C B D B Figure 2. (a) Stowed state and (b) developed state. 1 x ðt Þ d = ln , n xðt + nTd Þ Figure 1. Schematic of the solar array damping device. array, the out-of-plane bending vibration is its main vibration mode. Consequently, we developed a damping device to especially achieve a substantial increase for the modal damping ratio in the first bending mode. Figure 1 presents a schematic of the proposed solar array damping device. The upper strut (1) is a viscous damper while the lower strut (2) is a rigid rod. They are connected by a hinge which can be locked after deployment to improve the reliability of the mechanism. A beam (3) attached to the yoke is used to connect with the rigid rod. The viscous damper is supported by a vertical beam (4) added between the solar array drive assembly (SADA) and the root hinge flange. With the lockable hinge, the upper and the lower strut can be folded at launch (see Figure 2(a)) and combined into one strut after deployment (see Figure 2(b)). The device is installed between the root hinge flange and the yoke, which will not affect the sun-orientated rotation. Thus, this design will not influence the original system functions. To assess the damping performance in vibration suppression for the deployed solar array and optimize its design, the analytical models are needed. The passive damping performance is usually evaluated by the system equivalent modal damping ratios. For an underdamped linear system, one of its modal damping ratios can be drawn by analyzing the free vibration response in that modal frequency. The free vibration displacement response of the system under a single mode can be expressed as xðtÞ = Aejvn t sinðvd t + u0 Þ ð1Þ where A and u0 are parameters defined by the initial conditions, j is the modal damping ratio, vn is the undamped natural frequency of the system, and pffiffiffiffiffiffiffiffiffiffiffiffi ffi vd = 1 j2 vn is the damped natural frequency. For j 1, the equivalent modal damping ratio can be analyzed by the free decay method j’ d 2p ð2Þ where d is the logarithmic decrement and n is the number of selected amplitude interval period. Therefore, transient response analysis and modal test, which can obtain free vibration response, are utilized to analyze the equivalent modal damping ratios in the finite element models and the testing system, respectively. Finite element modeling and analysis Modeling and analysis for the deployed original solar array and the solar array with damping device have been performed using MSC/PATRAN. The solar array studied here contains a yoke, three solar panels, a root hinge, and three pairs of hinges between the plates. In modeling, the plate is located in the XY plane of the global coordinate frame, and the long side is along the X axis. The beam element has been employed to model the yoke, while the laminate shell element has been applied to model the solar panel. And the bush element has been employed to model the hinge, while the linear viscous damper element has been used to model the damping device. The damping law is F = CV, where F is the damping force, C is the damping coefficient, and V is the relative velocity between the ends of the damper. The solar panel is honeycomb sandwich structure with length 3 width 3 thickness of 2258 mm 3 1826 mm 3 30 mm for every piece. Both the upper and the lower panel have three layers with thickness of 0.3 mm. They are made of M40 carbon fiber/epoxy composite. The core is regular hexagon aluminum honeycomb, with wall thickness of t = 0.04 mm and side length of l = 4 mm. It can be equivalent to orthotropic continuous structure as the following formulas12 4 t 3 2 t E11 = E22 = pffiffiffi Es , E33 = pffiffiffi Es , 3 l 3l pffiffiffi 3 3g t G12 = Es 2 l pffiffiffi ð3Þ 3g t g t Gs , G31 = pffiffiffi Gs , G23 = 2 l 3l 8 t r = pffiffiffi rs , m12 = ms 3 3l 4 Advances in Mechanical Engineering Table 1. The performance parameters of honeycomb core equivalent material. Parameters Value E11 (MPa) E22 (MPa) E33 (MPa) G12 (MPa) G23 (MPa) G31 (MPa) r (kg/m3) m 0.165584 0.165584 827.9 0.031047 77.94 116.9 42.158 0.33 Figure 3. The finite element model of the deployed original solar array. Table 2. The equivalent six-direction stiffness of hinges. Directions Root hinge Hinge between the plates X (N/m) Y (N/m) Z (N/m) RX (N/m/rad) RY (N/m/rad) RZ (N/m/rad) 3,240,000 266,000 357,000 1680 5558 58,600 7,428,000 993,600 517,200 1104 792 54,840 where g is generally chosen between 0.4 and 0.6 in practice. Here, it is chosen to be 0.5. Es, Gs, rs, and ms are Young’s modulus, shear modulus, density, and Poisson ratio for the core wall material, respectively. These parameters are assigned as Es = 71.7 GPa, Gs = 26.95 GPa, rs = 2740 kg/m3, and ms = 0.33. After calculating, the honeycomb core equivalent material parameters needed to define three-dimensional orthotropic material in MSC/PATRAN, are shown in Table 1. Every hinge is modeled as bush element with length of 80 mm. Their equivalent six-direction stiffness is listed in Table 2. Figure 3 illustrates the finite element model of the deployed original solar array. Based on it, a linear viscous damper element and a steel solid beam element are attached to the solar array root hinge, as shown in Figure 4. They are used to model the parallel damping device. Figure 4. The finite element model of the solar array with damping device. Table 3. First four order frequencies of the original solar arrays (Hz). Order Frequency 1 2 3 4 0.17278 0.59229 1.0897 2.5252 For the solar array with damper, all degrees of freedom of its left end points are also fixed to make modal analysis. No matter what value the damping coefficient of the damper is, its frequencies are nearly unchanged. The first order frequency remains to be f1 = 0.17278 Hz. This verifies that the passive parallel damping method does not reduce the fundamental frequency. Transient response analysis Modal analysis The original solar array modal analysis is carried out in cantilever state with all degrees of freedom of the root hinge end points fixed. Its first four order frequencies are listed in Table 3. The first four modal shapes of the original solar array are presented in Figure 5. The results indicate that the first mode of the general multi-panel solar array is out-of-plane bending mode. According to the modal analysis results, a vertical sinusoidal excitation whose frequency is close to the first order modal frequency is exerted at the middle point of the model free end for 5 s. The force is given by f ðtÞ = sinð0:4ptÞ ð4Þ The direct method is used to do transient response analysis. The original overall structural damping ratio is set to 0.5% based on empirical knowledge. Many Kong and Huang 5 Figure 5. Modal shapes of the original solar array: (a) first, (b) second, (c) third, and (d) fourth. Figure 6. The displacement response curves of the solar array with damper. Figure 7. Result with the model structural damping ratio of 1%. researches also assume a damping ratio value around 0.5% of critical in the baseline model, such as HST solar array damper research.2 For the solar array with damping device, the displacement response curves of the same point under different damping coefficients are shown in Figure 6. At the first 5 s, the response is forced vibration response. After that, the result becomes free vibration response. It can be seen that the amplitude decay rates are not the same. At first, the decay rate is accelerated with the increase in the damping coefficient. When the damping coefficient increases to a certain value, the decay rate reaches the fastest. Then, as the damping coefficient becomes larger again, the decay rate will slow down. Thus, there is an optimum damping coefficient. As shown in Figure 6, the amplitude attenuates the most quickly at the coefficient of 3e5 N s/m. After analysis, the optimum damping coefficient of the damper element in such size for this solar array is about 3e5 N s/m. According to equation (2), the solar array modal damping ratio in the first bending vibration mode is calculated to be about 2.1% with damper of the optimum damping coefficient. In this situation, more than four times the original is achieved, which is a substantial increase. Dynamic response analysis for structural damping variation In order to further understand the effect of the original overall structural damping ratio on this passive vibration control, the original damping ratio value is changed in the transient response analysis. Other parameter values and the motivate settings still remain the same as the above. The damping coefficient is selected to be 3e5 N s/m. Figure 7 shows the transient response result with the model structural damping ratio of 1%. It can be calculated that the modal damping ratio in the first 6 Advances in Mechanical Engineering Figure 9. The finite element model of the certain type of solar array with damper. Figure 8. The final configuration design of the solar array damping mechanism. bending vibration mode is equivalent to 2.5%, in this case, 2.5 times the original. Combined with the result above, it is clear that when the original structural damping ratio is relatively small, after attaching the parallel damping, the damping ratio is improved more markedly compared to the bigger one. Note that when the solar array is deploying, the angular velocity of the yoke becomes faster and faster, and until about fully deployed it comes up to 30 deg/s, which means that the damper may be pulled at a velocity of 0.1 m/s. However, the damper velocity at work is on the order of 1e–4 m/s, which is quite low. At low speed, the equivalent linear damping coefficient should meet the value of 3e5 N s/m to achieve an excellent vibration damping effect. If the coefficient keeps the same, then the damping force at high speed will be too large to damage the structure. Hence, the damper was designed as nonlinear. The damping law is as follows13 F = CV N Damping mechanism design This parallel damping passive control design is applied to a certain type of satellite solar array. In order to minimize the changes in the original solar array yoke and root hinge, simultaneously considering the requirement of eliminating interference between the rods, the final configuration design is shown in Figure 8. The anticipated stowed state is that the lower strut can be folded within the yoke. During the deployment, the viscous damper should be pulled out a few millimeters to ensure the solar array hinges can be locked. However, this distance does not require too long. Here, it was set to about 5 mm. Viscous damper design According to the above requirements and the dimensional characteristics of the actual deployable solar array, the mechanism dimensions were determined. The viscous damper dimensions were also identified. Figure 9 illustrates the corresponding finite element model of this deployed solar array with damper established in MSC/PATRAN. The optimum damping coefficient of the additional damper element under this size was analyzed to be about 3e5 N s/m. When the original overall structural damping ratio is 0.5%, after attaching such damper, the modal damping ratio in the first bending vibration mode is equivalent to about 5%, 10 times the original. ð5Þ where N is an exponent. It is determined by the damper internal structure and usually designed to range from 0.3 to 1 for structural dampers. The double-rod viscous damper was selected. And its cylinder was filled with silicone fluid. The dimensions of the final produced damper are shown in Figure 10. Viscous damper performance test was conducted. The experimental data on the damper velocity and damping force are shown in Table 4. Fitting the experimental data in equation (5), the damper damping force–velocity relation is calculated to be F = 5:52 3 V 0:484 ð6Þ where the damping force unit is kN and the velocity unit is m/s. The damping force–velocity fitting curve is shown in Figure 11. From the experimental data, it can be analyzed that when the velocity is 0.1 m/s, the damping force is less than 2000 N, which ensures the structural safety. On the other hand, at working velocity from 1e–4 to 6e–4 m/s, its equivalent linear damping coefficient is between 5e5 and 2e5 N s/m. The corresponding transient response analyses were done after changing the damping coefficient in the finite element model to be 5e5 and 2e5 N s/m, respectively. The analytical results in Figure 12 verify that this damper enables the model equivalent modal damping ratio to be more than 4.5%, nine times Kong and Huang 7 Figure 10. The dimensions of the final produced damper (mm). Table 4. Velocity and damping force of the produced damper. Velocity (m/s) Damping force (kN) Velocity (m/s) Damping force (kN) 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0008 0.001 0.047 0.067 0.087 0.0995 0.115 0.1275 0.149 0.1695 0.002 0.003 0.004 0.02 0.04 0.06 0.08 0.1 0.2445 0.302 0.347 0.93 1.26 1.44 1.6 1.74 Figure 11. The velocity–damping force fitting curve. the original, which is also substantial. Thus, the actual viscous damper meets the expected requirements. The lower strut and locking mechanism design During the launch phase, the upper strut and the lower strut should be folded. Consequently, the lower strut was designed into two separate rods which were linked via a screw and a connecting bar. Then, the viscous damper could be folded between the two rods. After deployment, the two struts should be combined into one strut, becoming an axial force element. Hence, the Figure 12. Results of the solar array with damper under different damping coefficients. hinge connecting the two struts was designed to be lockable. A common lockable spring-pin design concept was implemented. The spherical rod end bearing of the piston rod was replaced by a self-designed connector having a pin-hole (see Figure 13(a)). It has a M8 thread with the length of 24 mm, which can be screwed into the piston rod. Except the vicinity of the central hole and the pin-hole, the connector is hollowed out to reduce weight. There is one pin on each side of the lower strut to improve the locking reliability. The compression spring for locking is placed in the locking cover and pressed by the adjusting bolt. The design result of the 8 Advances in Mechanical Engineering Figure 15. Schematic diagram of the experimental setup. Figure 13. (a) The self-designed connector and (b) the lower strut. Figure 14. The locking process: (a) stowed configuration (unlocked) and (b) locked state. lower strut is shown in Figure 13(b), where the two separate parts are symmetrical. The material for the lower strut, the connecting bar, and the locking covers is aluminum alloy to make additional weight as light as possible. However, the stiffness in the locking portion should be ensured, so the material for the self-designed connector and pins is steel. Furthermore, a steel sleeve for each pin is embedded in the lower strut. The locking process is as follows. When the solar array is stowed, the pins on both sides are pressed against the surface of the connector, as shown in Figure 14(a). With the deployment of the solar array, the pins slide along the surface. When the angle between the damper and the lower strut reaches 180°, the two pins are pressed into the pin-hole by the springs at the same time, successfully locked as shown in Figure 14(b). Experiments The modal parameters of a linear vibration system, including the system modal frequencies and damping ratios, can be obtained by modal test. Therefore, the associated modal tests of a solar array test sample have been performed to verify the effectiveness of the damping device. The main concerned mode is the solar array’s first bending vibration mode, so the method of exciting the first vibration mode is applied for the tests. Figure 15 illustrates the schematic diagram of the experimental setup for the solar array sample attached with damping device. In the experiments, one end of the sample was bolted to a rigid fixture. The excitation and measurement point was in the middle of the free end. At this point, the first bending mode has the largest amplitude. The solar array’s free vibration at the first mode was easily obtained by impacting the excitation point or giving the solar array initial displacement manually in the direction perpendicular to the plate. And the displacement signal at the same point was measured by a laser vibrometer. Take the natural logarithm of both sides of equation (1) lnjxðtÞj = jvn t + ln½Ajsinðvd t + u0 Þj ð7Þ where vd can be obtained from the collected displacement data using the fast Fourier transform (FFT) algorithm. When the second part takes the same value, the slope is equal to 2jvn, which is the attenuation coefficient. The first bending modal frequency and the modal damping ratio can be calculated from the frequency response and the attenuation coefficient. A hammer test results of the original sample and the sample with damping device are presented in Figures 16 and 17, respectively. Compared to Figure 16(a), Figure 17(a) illustrates that with damper, the vibration displacement response amplitude decays more rapidly. For the original sample, the damped natural frequency is analyzed to be about 2.594 Hz and the attenuation coefficient is 20.037275, so its modal damping ratio is about 0.23%. For the sample with damping device, the damped natural frequency is increased to about 3.319 Hz. Figure 17(c) shows that the attenuation coefficient has two distinct values, one is 20.25347, while the other is 20.050972. Therefore, at the initial vibration stage, its modal damping ratio is equivalent to 1.22% and then the damping ratio is reduced to 0.24%. The reason for such phenomenon may be that with large vibration amplitude, the damper has a great damping effect at the initial stage, while the vibration decays to a certain Kong and Huang 9 Figure 16. Hammer test results of the original sample. Figure 17. Hammer test results of the sample with damping device. Table 5. Modal test results. The original sample Hammer test 1 Hammer test 2 Initial displacement excitation test The sample with damping device The first bending modal frequency (Hz) Modal damping ratio j The first bending modal frequency (Hz) Modal damping ratio j Initial stage Later stage 2.594 2.594 2.594 0.23% 0.23% 0.26% 3.319 3.319 3.319 1.22% 1.28% 1.32% 0.24% 0.25% 0.26% extent, the damper cannot be pulled due to its static friction force, and almost has no damping effect. Other hammer tests and initial displacement excitation experiments were also implemented. The test data were analyzed and their results were quite similar. Table 5 lists all modal test results. The experiments reveal that the modal damping ratio in the first bending vibration mode of the sample with passive damping device can achieve more than five times the original when the damper is effective. While the vibration is quite small, the damper turns to playing a supporting role to suppress vibration. Conclusion A novel passive vibration damping device for the multipanel solar array is presented. The device is lockable and located near the solar array root hinge to increase 10 the modal damping ratio in the first bending vibration mode without reducing the fundamental frequency. Moreover, it ensures that the solar array maintains the original functions, such as folding, deploying, and sun tracking. There is a viscous damper at the upper strut to provide passive damping. Analytical and experimental results validate the effectiveness of this parallel damping method. With the finite element models, the properties of the damping device are investigated. The findings demonstrate that the damping coefficient has an optimum value and this passive vibration control improves the modal damping ratio more markedly when the original structural damping ratio is relatively small. The passive damping device implemented for a certain type of satellite solar array was produced. Simulation results verify that the actual viscous damper can meet the requirements and increase the modal damping ratio substantially. The associated experiments based on a solar array test sample reveal that the passive damping device has a great damping effect at large vibration. While the vibration is quite small, the damper almost has no damping effect; however, it can play a supporting role to suppress vibration. For future damping device applications, the viscous damper design may be further improved, especially minimizing its static friction force. Additionally, the upcoming experiments based on the real solar array can help to obtain more results. Acknowledgements The work was carried out at the Key Laboratory of Spacecraft Design Optimization and Dynamic Simulation Technologies, Ministry of Education, China, under contract with Beijing Institute of Control Engineering. Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research work was supported by the National Natural Science Foundation of China (Grant No. 11672016), which the authors gratefully acknowledge. Advances in Mechanical Engineering References 1. Kienholz DA, Pendleton SC, Richards KE, et al. Demonstration of solar array vibration suppression. In: Proceedings of SPIE’s conference on smart structures and materials. Orlando, FL, 13 February 1994, vol. 2193, pp.59–72. SPIE (the international society for optics and photonics). 2. Anandakrishnan SM, Connor CT, Lee S, et al. Hubble space telescope solar array damper for improving control system stability. 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