1、Public Lessons Learned Entry: 6196 Lesson Info: Lesson Number: 6196 Lesson Date: 2011-7-26 Submitting Organization: JPL Submitted by: David Oberhettinger Subject: Shaker Self-Check Unexpectedly Exceeded the Dynamic Test Limit Abstract: Shaker system control software generates a self check immediatel
2、y prior to a test at a signal level that is normally considered to be of insignificant strength compared to test levels. During a dynamic test of the Juno High Gain Antenna, however, the self check excited a response that exceeded the level from the subsequent test. In planning for dynamic testing,
3、ensure that the self check signal will be set lower than any test level. Description of Driving Event: Electrodynamic shaker systems are used in spacecraft test facilities to simulate the vibration, shock, and other dynamic loads that spaceflight hardware will encounter during launch and mission ope
4、rations. Because large shaker tables are capable of producing dynamic impulses at levels that can damage or destroy the flight hardware, the test equipment is set and checked carefully prior to the test. Actions taken to confirm that the shaker is calibrated and operating properly include: The sched
5、uled test is first performed on a simulated test article that has a total mass approximating that of the actual test article. This allows the test conductor to confirm that there are no equipment defects, such as table stiction (Reference (1) due to inadequate lubrication, and that the intended ener
6、gy level is actually delivered to the mass simulator. Whenever the shaker is operated, the shaker controller software automatically initiates a self-check as a standard procedure. The self-check is intended to provide a strong enough signal to produce a reliable system check, but the self-check sign
7、al level is normally considered to be of insignificant strength compared to test levels. In July 2010, an open-loop sine burst test was conducted at the NASA/Caltech Jet Propulsion Laboratory (JPL) to exercise the three flight interface points on the High Gain Antenna (HGA) for the Juno spacecraft (
8、Reference (2). Prior to running any tests on the flight hardware, the shaker (Figure 1) was checked out by running a sine sweep test and a sine burst test using a simple mass simulator for the antenna. The sine burst profiles from this simulation indicated that the shaker would generate a proper tes
9、t. However, the self check step of the simulation was not discussed and its input levels to the flight hardware were not examined because the self check level is not considered to be a test parameter. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-F
10、igure 1. A typical test configuration for the JPL large shaker, this photo depicts Mars Exploration Rover under test) The flight hardware HGA was installed on the shaker. Prior to conducting the sine burst test at various levels, the engineering team ran a low level sine sweep survey test that confi
11、rmed that the antenna responses matched the predictions and provided a measure of confidence in the health of the antenna and test configuration. When the first sine burst test was run, however, it was found that the self check that had immediately preceded the test had excited HGA responses beyond
12、levels expected from the sine-burst test itself. Whereas the highest response expected on the HGA during the sine bust test was 33g, the self check generated 67g. Subsequently, a detailed inspection of bond joints found a repairable 3-inch crack in the HGA (Figure 2) that may have been caused by the
13、 aberrant self check. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Figure 2. Red arrow on the HGA points to the damage site JPL typically sets shaker self check levels to be relatively high for sine burst testing (as compared to sine sweep or rand
14、om vibration testing) because of the need in open-loop testing to generate a strong signal that will provide a reliable self check. Sine burst testing involves subjecting each orthogonal axis of the test item to five-to-ten cycles of a sine wave whose peak is equivalent to the qualification load lev
15、el. Since the test is intended to impart a quasi-static load to the test item, the test frequency must be below the fundamental resonant frequency of the test item. Such a “strong” self check signal is not ordinarily considered hazardous because it is still rather low in amplitude compared to a typi
16、cal test. However, sine burst is intended as a non-resonant test (all input well below the test article first resonance), whereas the self check is a 5 second random burst from 10 to 400 Hz and excites all the significant modes for most test articles. Following the Juno HGA incident, JPL changed its
17、 test procedures to require the test operator to verify that the input loads due to the self check levels are set lower than the lowest level associated with the hardware test. References: 1. “High Energy Spectroscopic Imager Test Mishap,” NASA Lesson Learned No. 0903, NASA Engineering Network, May
18、10, 2000. 2. “Juno High Gain Antenna Sine Burst Test - System Self Check High Levels,” JPL Problem/Failure Report No. 28952, JPL Problem Reporting System, July 14, 2010. Lesson(s) Learned: The control software for electrodynamic shaker test equipment generates a self check signal immediately prior t
19、o a test; the strength of this signal may exceed that generated by the actual test, posing a risk to the test article. Recommendation(s): In planning for dynamic testingparticularly for sine burst testingensure that the self check signal will be set lower than any test level by analyzing the input l
20、evel to the test article. Evidence of Recurrence Control Effectiveness: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-JPL has referenced this lesson learned as additional rationale and guidance supporting Paragraph 6.12.5.2 (“Engineering Practices:
21、 Protection and Security of Flight Hardware”) in the Jet Propulsion Laboratory standard “Flight Project Practices, Rev. 7,” JPL DocID 58032, September 30, 2008. Documents Related to Lesson: N/A Mission Directorate(s): Science Exploration Systems Additional Key Phrase(s): Additional Categories.Test &
22、 Verification Additional Categories.Spacecraft Additional Categories.Payloads Additional Categories.Hardware Additional Categories.Ground Equipment Additional Categories.Flight Equipment Safety and Mission Assurance.Advanced planning of safety systems Engineering Design (Phase C/D).Spacecraft and Sp
23、acecraft Instruments Additional Categories.Test Article Additional Categories.Test Facility Additional Info: Project: Juno Approval Info: Approval Date: 2011-10-14 Approval Name: mbell Approval Organization: HQ Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
