Application report by TU Munich, KRP-Mechatec TU Munich, Chair of Lightweight Construction (www.llb.mw.tum.de), Prof. Dr.-Ing. H. Baier KRP-Mechatec Engineering GbR (www.krp-mechatec.de), Dipl. Ing. C. Zauner
The Chair of Lightweight Design (LLB) at the Technical University of Munich is involved in numerous research projects in addition to its general teaching activities. Clients are mostly companies from the aerospace industry (e.g. EADS, KRP-Mechatec), which want to use the knowledge of the TU, its flexibility and the possibilities of its research facilities.
KRP-Mechatec is a strong partner when it comes to testing and developing mechatronic components. Through close contacts with the aerospace industry, it is technically up to date and can implement many measurement and development projects through close cooperation with the research institutes.
1. the measurement task
The company EADS-Astrium commissioned KRP-Mechatec to test a mechatronic satellite component, a so-called "Low Shock Release Unit" (LSRU), which the customer had developed and built for use on satellites. The LLB's equipment was used to carry out the tests. KRP-Mechatec in cooperation with the chair were asked to assess the suitability and quality of the component in a detailed report on the basis of reliable measurement data and to incorporate suggestions for improvement. The basis of the order was a specification sheet in which the qualification tests were specified.
The component to be tested is a novel device for holding down and releasing folded solar panels of a satellite. Up to five folded panels, each measuring 3.9m x 2.1m, are pressed together and held down on the satellite by several Low Shock Release Units during launch. When unfolded, the two solar generators each measure up to 23m x 2.1m.
When the solar generator is released, a spindle pulls out of an extremely low-friction, rotating nut integrated into the LSRU. The actual deployment of the solar panels then occurs via a drive and cable pulls at the joints connecting the individual panels.
Until now, the release of such panels has mostly been done pyrotechnically, but this involves large shocks to the satellite. The brief, very high mechanical vibrations caused by the explosion can cause defects in circuit boards, sensors and instruments, among other things, and ultimately lead to the failure of an entire satellite system. With unit prices of up to €200 million for a satellite, this is a high risk.
In total, three qualification tests with LTT transient recorders were performed at LSRU.
I. Measurement: Electrical measurement
Four LSRUs were to be tested, each having two electromagnetic actuators. This test demonstrated the electrical characteristics of the electromagnetic triggering mechanism. The accuracy of the used triggering pulse of 21V or 35V at 40ms duration, as well as the disturbance pulse of 21V or 35 V at 10µs duration and an edge rise of 150ns was verified. Likewise, the current consumption and, if necessary, the mutual induction of the magnets were measured and evaluated. The measurement was done with current clamps and recording with the LTT at up to 20MHz sampling rate.
II. measurement: sinus / random vibration test
Three release units were mounted on a large electrodynamic shaker with 80kN maximum force. In principle, this "shaker" works like a loudspeaker that excites a vibrating table with test samples.
The test consists of a sinusoidal and a random load test. A resonance search is performed before and after each load test to detect any damage.
The resonance search was performed from 5 to 2000Hz with a small amplitude of 0.5g. In the sinusoidal loading test, excitation is performed from 5 to 100Hz with an amplitude of maximum 18 g, and the random loading test was performed from 5 to 2000Hz with a loading of 16 grms. All tests were run in three excitation directions, giving a total number of 15 tests.
The tests recorded data from 18 accelerometers, a "pilot" signal from the shaker table, and two strain gauges. The sampling rate was set to 20kHz per channel (for a total of 21 active channels).
The vibration test showed that the mechanical loads during launch of a launcher are endured and that no unit falsely triggers.
III Measurement: Shock measurement and evaluation
The purpose of the measurement was to determine the mechanical shock generated when the LSRU is released. The release unit was mounted on a suspended aluminum sandwich plate. Four sensors (three-axis accelerometers with +/- 500 g) were used to measure the accelerations caused by the release unit. A total of 12 channels were used at a sampling rate of 400 kHz per channel.
For the final presentation of the results, a "shock response spectrum" was created, which also required the high sampling rate. It could be shown that the LSRU is characterized by a very low shock generation and thus other satellite components are only slightly loaded.
2. why measurement technology from LTT
The order placed demands on the measurement technology that could not be met with existing equipment. A solution was sought that would allow synchronous measurement on 21 channels in parallel and a sampling rate of up to 20 MHz per channel. Data on currents and voltages had to be measured. Since the control of the actuators has an edge rise of 150 ns, fast measurements were necessary. Likewise, the calculation of the "shock response spectrum" requires a high time resolution of the measured acceleration.
After careful examination of possible solutions, the chair decided to purchase a measurement system from LTT GmbH (http://www.tasler.de) consisting of two cascaded transient recorders with a total of 24 channels. The system will also be used in future projects, e.g. for the design of active and passive damping systems, as well as for acoustic measurements.
The software LTTview allows the storage of the measurement data in all common file formats. Since the chair uses the software FAMOS for further processing and analysis of the measurement data, all data from LTTview were saved in Famos format. Helpful for processing the data was the ability to use LTTview to easily zoom in and crop a spectrum of interest from a very large set of data online and then export it directly to online Famos.
For certain measurements, KRP-Mechatec cooperated with the Max Planck Institute Garching. On this occasion, measurement data were recorded in parallel with the measurement systems of the MPI using the LTT transient recorders. The comparison showed that the LTT transient recorders provided extremely accurate measured values. Considering the price difference, such a good result could not be expected.
The client for the test was highly satisfied with the results presented by KRP-Mechatec and the Chair of Lightweight Construction. The measurement data obtained with the LTT transient recorders fully met the wishes and requirements.
The KRP-Mechatec engineers also praised LTT for its work with the transient recorders. Close contact with the manufacturer LTT ensured smooth operation and rapid problem solving.
Numerous experiences in using the systems were documented and taken into account in further developments. In addition to the proven precision of the systems and the quality of the measurement data, the test managers praised the intuitively usable user interface of LTTview.