EQUIPMENT INTERFACE LOAD CHARACTERIZATION IN ACOUSTICS

Transcription

1 1 EQUIPMENT INTERFACE LOAD CHARACTERIZATION IN ACOUSTICS Nicolas Ludovic LARUE (1), Jean Marie LOME (2), Alice PRADINES (3) (1) Mechanical analysis and test engineer, EADS Astrium - 31 avenue des Cosmonautes, Toulouse cédex 4, France, nicolas.larue@astrium.eads.net (2) Head of mechanical R&D, EADS Astrium - 31 avenue des Cosmonautes, Toulouse cédex 4, France, jean-marie.lome@astrium.eads.net (3) Structures and mechanics engineer, CNES - 18, avenue Edouard Belin, Toulouse cédex 09, France, alice.pradines@cnes.fr ABSTRACT Today equipment units of spacecraft are specified in terms of interface random acceleration to simulate the mechanical environment during spacecraft acoustic tests. This specification leads to very high interface loads and internal accelerations, much higher than those seen during acoustic tests. In the frame of the MEFIFA2 R&T co funded by CNES, EADS Astrium achieved a test campaign on a typical equipment including random and acoustics runs. Those tests helped to determine the kind of solicitation in both cases in order to set up a more accurate qualification procedure. This study showed that the internal subsystems of the equipment respond to direct acoustic solicitation rather than to random vibration of the supporting panel transmitted through the interface. We therefore consider that for this equipment a direct acoustic qualification would have been more adapted. Keywords : Acoustics random solicitation interface direct acoustics 1 CONTEXT During launch, spacecraft must endure a severe mechanical environment induced by the launcher. One of the sizing loads is the acoustic load due to rocket engine noise and pressure fluctuation in the boundary layer. In the qualification process of spacecraft to mechanical environment, acoustic tests are conducted in a reverberant chamber with a diffuse sound field. This load on the spacecraft results in a mechanical solicitation on the various equipment units (batteries, electronics, ) mounted on the panels. These solicitations are of 4 types and can be simply modelled as on figure Fig. 1 : equipment unit solicitation Case 1 : direct acoustics on the mounting panel generates out of plane random vibration Case 2 : same thing but exciting higher order panel eigenmodes. Out of phase translation generating rotation Case 3: in plane excitation by random response of adjacent panels Case 4: direct acoustic excitation of the unit s eigenmodes In real life, those four solicitations are more or less coupled depending of the considered frequency band. In the design and integration process of the spacecraft, each equipment unit has its own qualification process under the responsibility of the subcontractor. Major subsystems whose mass exceeds 100 kilograms such as SEVIRI (the Second Generation Meteosat telescope) are qualified with an acoustic test. But smaller parts such as electronic boxes are qualified with a random vibration test on a shaker to simulate the solicitation they will endure during spacecraft level acoustic test. This means that the assumption is made, that the excitation of type 4 is negligible (or at least neglected) since it is not reproduced by this kind of test. It also means that the 2 4 European Conference on Spacecraft Structures, Materials & Mechanical Testing 2005 Noordwijk, The Netherlands, May 2005 (ESA SP-581, August 2005)

2 2 excitation of type 2 is not modelled and is supposed to be covered by the excitation of type 1 and 3 at test level. Those random tests are currently specified in terms of acceleration PSD (Power Spectrum Density) at the mounting interface of the unit, separately on the three axes for a rigid mounting on the shaker. The spectrum generally consists of a raising slope up to 100 Hz, one or several constant levels up to 1000 Hz, and a decreasing slope up to 2000 Hz (see on figure 2) matches the requirements of the PSS to be qualified by random tests. This unit has a typical configuration with an external box and wedge locked electronic cards. Figure 2 : typical qualification random spectrum The levels are determined either from the ESA s PSS (Procedures, Standards and Specifications) or from the experience on other spacecraft. The specified levels must cover the peaks of PSD, which will be obtained during the spacecraft level test. For that reason, the accelerations at the unit base during the qualification test are very often much higher than those measured during spacecraft acoustic tests at the interface between the unit and its mounting panel, especially in terms of RMS value (Root Mean Square). In the frame of the MEFIFA2 (Mesure des Efforts d InterFace en Acoustique, InterFace Loads Measure in Acoustics) R&T study co funded by CNES (French National Space Agency) and EADS Astrium, Astrium tried to better characterize the solicitations of a typical equipment unit in order to propose a more adapted specification method, which would avoid the current over specification. 2 TESTS DESCRIPTION To characterize the solicitations in random and acoustics, we have made random tests on a shaker for a typical equipment unit, and then we have mounted it on a typical panel for an acoustic test. The unit used is the MSTH (Thermal and Structural Model) of the EAIM (Inertial and Magnetic Actuator Electronics). This model is fully representative of flight models mounted on spacecraft like SPOT 4, SPOT5 or Helios. The dimensions are 500*300*300 mm and 33 kg. This unit is therefore part of the big units, but it Fig. 3 : exploded view of the EAIM - the electronic cards (not represented) are wedged between the internal plates 2.1 Random tests Random tests have been conducted with the qualification spectrum of the EAIM on two axes : orthogonal to the interface plane (Z axis) and in plane on the longitudinal axis (X axis), which corresponds to the out of plane axis of the cards. No test was made on the third axis since the modal behaviour presents no interest. 2.2 Acoustic tests The used assembly had to enable the simulation of several kinds of configurations that can be found on spacecraft layout : in the centre of a panel, close to a side in order to get various eigen frequencies and modal shapes of the loaded panel. This enables to separate the contributions of the global panel modes and the local unit modes. We therefore used a typical sandwich panel of 1m*1m with aluminium skins and honeycomb similar to those used for spacecraft panels. Massive bars on the four sides of the panel, representing clamped conditions, simulate the boundary conditions of spacecraft. Four configurations have been tested : EAIM centred on the panel EAIM close to a side of the panel EAIM close to a side, and a bar removed (free condition) EAIM alone, hanged in free-free conditions

3 3 In comparison to spacecraft tests, this assembly cannot simulate the solicitation of type 3, in the mounting plane. Fig. 4 : acoustic test assembly Fig.5 : acoustic assembly configurations 2.3 Instrumentation During the tests, we have measured the PSD of the interface loads with piezo electric cells. Only the out of plane loads were measured. We also measured the PSD of acceleration at the unit interface close to the load measure to well characterize the interface solicitation. We also measured the 3-axis acceleration PSD on the box at mid height to separate the influence of the box modes. We also measured the PSD of acceleration on four points located on electronic cards on the X (out of plane) direction, as well as two internal points close to the base. Those 6 internal measures enable to appreciate the local dynamical behaviour of the subsystems of the EAIM. 3 RESULTS EXPLOITATION 3.1 Random tests The random tests have enabled to better understand the modal behaviour of the EAIM MSTH. The first modes on the longitudinal (X) axis are local bending modes of the electronic cards at about 250 Hz. The box modes are at higher frequencies. The modes on the out of interface plane (Z) axis are box modes at frequencies between 500 and 1000 Hz. In terms of solicitation, we obtain an acceleration PSD consistent with the input spectrum. The interface loads measured at the four corners of the unit are plotted on fig. 6 and 7. On the X test (fig. 6) the local modes of the cards generate an important peak of force at 250 Hz, the box modes generate important loads beyond 500 Hz. On the Z test, the major peak is due to a global mode, we cannot see any influence of local modes. It is interesting to notice that the X-axis test generates the highest Z interface loads : the solicitation axis is not necessarily the most representative for the response in that axis. One must take into account the lever arms, which induce coupling between axes. The internal accelerations are consistent with the expected ones : some peaks on local modes and global modes. 3.2 Acoustic tests The analysis of the acceleration PSD on the four corners of the interface EAIM / panel shows that solicitation depends on the configuration. This corresponds to what was expected. Fig. 8 shows the acceleration in the centred configuration, and fig. 9 in the shifted configuration. The centred configuration solicitates the four corners with the same levels, whereas the shifted configuration yields much lower levels close to the panel side. This enables to analyse the influence of the base excitation on the internal response. Concerning the in plane acoustic solicitation, a comparison with the measured issued from the SPOT and Helios spacecraft acoustic tests shows that the levels obtained are similar in our study : the type 3 solicitation has little influence compared to the three others. 3.3 Comparison of random and acoustic tests Fig. 8 and 9 also show that the interface accelerations are very different from the random qualification specification, which is also plotted on the figures. The frequencies and levels do not correspond to the measures.

4 4 In terms of interface loads, fig. 10 shows the measures for the different tests and configuration for the same measurement point. The highest curves correspond to random tests, the three others to the three acoustic configurations on the panel. Beyond 100 Hz, the interface loads are lower in acoustics, whereas the accelerations are higher than the random specification (fig 8 and 9) : higher accelerations generate lower loads in acoustics. This is due to the flexibility of the interface which filters the loads in acoustics whereas in random the interface remains stiff on the whole frequency range. The difference in the solicitation (not in phase or not the same levels on all points) can also account for a part of the difference. In terms of internal accelerations, the results are shown on fig. 11. The plots represent the acceleration on the same point for the two random tests (highest plots) and for acoustic assembly tests. Like the interface loads, the internal accelerations are lower in acoustics than in random test. Beyond 250 Hz, one also notices that the internal acceleration does not depend on the assembly configuration. Yet the interface acceleration does depend on the configuration : the internal response is not generated by the random solicitation of the unit interface. The most probable hypothesis is that the electronic cards accelerations are mainly due to direct acoustic solicitation (type 4 on fig. 1). The test of the EAIM alone was decided after those considerations: the EAIM has been hanged alone in free-free conditions in the acoustic chamber. The internal accelerations measured on this test are similar to those measured when the EAIM was mounted on the panel, in terms of levels as well as in terms of frequencies. The conclusion is therefore that direct acoustics on the EAIM is the main contributor to the internal response of the equipment unit. 4 CONCLUSION Through this study, EADS Astrium has shown that for the EAIM MSTH, the qualification process through random tests does not enable to represent the solicitation endured during spacecraft acoustic tests. Random tests generate too low interface accelerations and too high interface loads. For internal behaviour, EADS Astrium has shown that in the case on the EAIM MSTH (typical of this mass range) the responses are generated by direct acoustics and do not depend on the assembly configuration (position on the panel, modal behaviour of the assembly). This R&T study, co-funded by CNES and EADS Astrium leads to consider another strategy for the qualification of such units. The qualification for the internal subsystems would be obtained by means of an acoustic test on the unit. The interface would then be qualified by a quasistatic test. The design data is the interface load, not the acceleration. And what is important is the RMS value rather than the PSD for the failure of the interface. The value of the qualification quasistatic load would be obtained either by acoustic analysis at spacecraft level or from experience.

5 5 Fig. 6 : interface load on the X random test Fig. 7 : interface load on the Z random test

6 6 Fig. 8 : interface acceleration on the centred configuration Fig. 9 : interface acceleration on the shifted configuration

7 7 Fig. 10 : interface load for random and acoustic tests for the same point Fig. 11 : internal accelerations

8 8 Fig.12 : internal acceleration equipment on the panel / equipment alone