More representative spacecraft random vibration testing

Abstract

The launch phase is the most demanding mechanical environment typical satellites experience. In order to verify that a payload or piece of equipment will survive the expected loads experienced during launch, it is subject to prescribed vibration environments. However, current vibration testing methods tend to overtest. This means the harshest environment a satellite and its equipment must survive is the testing, not the launch. Consequentially, design process compromises are made, moving the focus from surviving the launch to surviving the testing. Vibration testing involves shaking the test article in each of the three standard directions (X, Y and Z) according to the provided testing specifications. These specifications are based of the single launch environment which is split into three for practical reasons, but which leads to overtesting. One of the causes for equipment overtesting is that items are normally tested along its three orthogonal axes (i.e. X, Y and Z). However, the body axes of the equipment are not always in line with the structure it is attached to. Even if the body axes do align, the dynamics of the coupled system mean any vibration at the base of the larger structure is unlikely to be acting all on the same axis (or axes) at the interface between the satellite and equipment. Another key difference between the testing environment and launch environment is the direction of the vibrations. The launch vibration environment is a single 3D environment, while testing is usually comprised of three single axis vibrations tests. This thesis presents two alternative testing methods that separately, or together, can create a test campaign which better matches the environment the piece of equipment would see during launch. The first method, the Angle Optimisation Method, looks at testing the piece of equipment is mounted at an offset angle on to the shaker rather than the traditional three orthogonal mounting directions. The method optimises the testing angle for the piece of equipment such that testing responses are closer to those seen when the equipment is attached to the higher level assembly. This method focuses on covering the maximum Root Mean Square (RMS) values for each quantity (e.g. sum of interface forces, and acceleration at centre of mass) obtained from the coupled system tests - resulting in a test campaign of one to three separate tests, each with altered input directions. This results in RMS values much closer to the desired higher level testing values than the traditional testing. The second method, the Dual Input Method, looks at adding a secondary smaller vibration source at a specific location on the test item. The method finds the best location to attach the second vibration source that produces a more representative testing of the piece of equipment when compared to the higher level testing. It also determines what the input should be at this specific point. This method looks at improving the correlation of the Operational Deflecting Shapes (ODS) of the equipment when tested in isolation and when attached to the higher level assembly. Response Vector Assurance Criterion (RVAC) is used for the correlation of the ODS. Two case studies were undertaken to demonstrate the benefits of these methods. The first was a computational case study that both methods were applied to. In this case study the Angle Optimisation method was able to reduce the amount of over testing by up to 70% compared to the traditional testing method. While the Dual Input method was able to improve the correlation between the equipment and coupled system responses by nearly 50%. The second case study was an experimental application of the Angle Optimisation Method. This case study successfully showed that it was possible to implement this method as a physical test. A custom angled interface plate was manufactured to the specifications determined by the Angle Optimisation method. In addition to showing the successful implementation of this method, the over testing was reduced by roughly 50% when compared to the traditional method.