3.0 Apparatus. 3.1 Excitation System

Transcription

1 3.0 Apparatus The individual hardware components required for the GVT (Ground Vibration Test) are broken into four categories: excitation system, test-structure system, measurement system, and data acquisition and analysis system. The following sections describe each of these categories based on the individual instruments listed in Table 1. Table 1. List of Instrumentation for Ground Vibration Test Equipment HP 3567A Dynamic Signal Analyzer MB Electronics 2120 Power Amplifier MB Electronics 35 lb. Electro-magnetic Shaker Flexure Fixture Suction Plate/Electric Pump Star-Lite Experimental Aircraft Rubber Innertubes Accelerometer-Cable Support Mount Flexure Rest Kistler 8694M1 Piezotron Multi-Axial Accelerometer PCB 208 A20 Piezoelectric Load Cell Tektronix 561A Oscilloscope Kikusui DSS 6520A Digital Storage Oscilloscope HP 3567A Dynamic Signal Analyzer I-DEAS (Integrated Design Engineering Analysis Software) 3.1 Excitation System The excitation system consists of a DSA (Dynamic Signal Analyzer), power amplifier, electro-magnetic shaker, flexure fixture, pump, and suction plate. This section presents information on the specifications and/or use of each of these components in the GVT HP 3567A Dynamic Signal Analyzer The HP 3567A is an expandable PC Spectrum/Network analyzer that characterizes signals in the frequency and time domains (Fig. 8). 24

2 Figure 8. HP 3657A Dynamic Signal Analyzer with Pentium Interface The DSA provides the source input signals to electronic equipment and also records and analyzes the output from any measurement devices. For the GVT excitation system, the HP 3567A is used as a random-burst generator to drive the electronic equipment. The DSA has the capability of analyzing data from 16 to 48 simultaneous input channels and uses a MS-DOS operating system and Microsoft Windows user interface [13]. The analyzer s user interface allows access to five separate analysis-mode screens. These separate screens include the following modes: Hardware Setup, Measurement, Create Math, Time Capture, and Hardcopy. For the GVT, only two of the modes have been required: Hardware Setup and Measurement. The Hardware Setup mode is used to set the measurement hardware parameters: Global, Source, DAC, Front End, Trigger, Engineering Unit, and Identity. These parameters are accessed through virtual buttons on the Hardware Setup screen and allow the user to set the source signal, frequency span, instrument-response range, calibration ratio, trigger, and many other aspects of the electronic signals to be used in the analysis. The Measurement mode is used to control measurements and display results from tests. The Measurement main screen contains menus (MeasType and MeasParam) that allow the user to set signal analysis features. Some of these features include the frequency resolution, signal windowing, signal averaging, and type of calculation. Some of the calculation features of interest in a GVT include the FR (frequency response) and power spectrum of a signal. 25

3 3.1.2 MB Electronics 2120 Power Amplifier The MB 2120 MB provides adjustable power amplification up to 125 V and has a frequency range of 5 Hz to 20,000 Hz (Fig. 9); the required input voltage for the amplifier is 1 V RMS. The amplifier consists of three stages: a differential amplifier-type pre-amp stage, a push-pull voltage-amplifier driver stage, and a push-pull parallel power-output stage [18]. Figure 9. Power Amplifier The power amplifier is used in the GVT to amplify the random burst signal produced by the DSA. The amplifier is designed to operate the electro-magnetic shaker used for the GVT Electro-magnetic Shaker The electro-magnetic shaker converts the voltage signal sent by the amplifier into a physical displacement (Fig. 10). The displacement amplitude produced by the shaker is governed by the amplitude of the signal sent from the amplifier. The GVT is configured to drive the shaker without air cooling so the maximum current the shaker can utilize from the amplifier is 0.22 ma dc. Two shakers, a 50 lb. shaker and a 35 lb. shaker, are available for use in testing, but the 35 lb. shaker Figure lb. Electro-Magnetic Shaker provides the best FR data for the accelerometer/load-cell pair used in the Star-Lite GVT Flexure Fixture The flexure fixture is an axial component that transfers axial loads from the shaker to the structure (Fig. 11). The flexure fixture is made up of two stingers and a flexure rod. The stingers have a small cross section that is stiff in the axial direction but bends easily. The stinger allows the transmission of axial loads but bends when subjected to transverse forces or moments. The flexure is a stiff member that keeps the flexure fixture from buckling due to loads. For the preliminary GVT, a 1.5 ft. and a 3 ft. flexure are being tested. Figure 11. Flexure Fixture 26

4 3.1.5 Suction Plate and Electric Pump The suction plate and pump are used to attach the flexure fixture to the aircraft in a nondestructive manner. The pump forms a vacuum between the suction plate and surface of the aircraft. An o-ring and silicone grease form a seal between the plate and wing surface (Fig. 12). Figure 12. Pump, Suction Plate with O-ring, and Non-Destructive Mounting to Aircraft 3.2 Test-Structure System The test-structure system consists of the Star-Lite aircraft, the innertube constraints, accelerometer-cable support mount, and the flexure rest. This section presents information on the use of these components in the GVT Star-Lite Experimental Aircraft 3MAAP is performing the GVT on a Star-Lite aircraft located in room 202 of Woolrich Laboratories at the University of Texas at Austin. The Star-Lite is a single-seat, home-built experimental aircraft designed by Mark Brown of San Antonio, TX. The Star-Lite has a wing span of 21.5 ft., a length of 16.4 ft., and a height of 4 ft (Fig. 13). The empty weight of the aircraft is 275 lbs. Figure 13. Typical Star-Lite Aircraft. 27

5 3.2.2 Rubber Innertubes Rubber innertubes are placed under the landing gear in order to simulate free-support constraints of the Star-Lite (Fig. 14). The tubes provide a soft-support system to isolate the rigid-body frequencies well below the flexible modes. Figure 14. Innertube Supports Under Tires Approximate Free-Support Accelerometer-Cable Support Mount The accelerometer-cable support mount, built by 3MAAP in the machine shop of Woolrich Laboratories, provides a support relief for the weight of the accelerometer cable. The mount also provides an elevated stand to support the cable above and away from the Star- Lite (Fig. 15). Figure 15. Accelerometer-Cable Support Mount Flexure Rest The weight of the flexure fixture causes the thin cross-section of the stingers to easily bend. The flexure rest provides a method of holding the flexure fixture in a vertical position when 28

6 the pump and suction plate are not forming a vacuum against the surface of the wing (Fig. 16). Figure 16. Flexure Rest Supporting Flexure in Vertical Position 3.3 Measurement System The measurement system consists of the multi-axial accelerometer and the load cell that are used to measure acceleration and force in the GVT Kistler 8694M1 Piezotron Multi-Axial Accelerometer The Kistler 8694M1 is a quartz-based acceleration transducer that measures acceleration in a tri-axial coordinate system (Fig. 17). The accelerometer consists of three basic elements: the transducer body, the quartz sensing element, and a seismic mass. During testing, the sensing element is impacted by the seismic mass. The impact produces an Figure 17. Accelerometer electric charge proportional to the acceleration of the mass. The accelerometer is attached to a surface with wax [19] PCB 208 A20 Piezoelectric Load Cell The PCB 208 A20 is a quartz-based force transducer (Fig. 18). The load cell consists of two thin quartz disks that generate an electrical charge when subjected to a loading [20]. The charge is proportional to the force applied to the Aircraft. Figure 18. Load Cell 29

7 3.4 Data Acquisition and Analysis System The data acquisition and analysis system consists of four components: two oscilloscopes used in the preliminary test setup to monitor data quality, the DSA used to calculate the frequency responses of the data, and the I-DEAS software for calculating modal parameters Tektronix 561A Oscilloscope The Tektronix oscilloscope is a 4-channel oscilloscope that was used to monitor the four data channels (force, x acceleration, y acceleration, and z acceleration) simultaneously (Fig. 19). The 4- channel oscilloscope is no longer in use due to its role in ground-loop effects. Figure Channel Oscilloscope Kikusui DSS 6520A Digital Storage Oscilloscope The Kikusui oscilloscope is a 2-channel oscilloscope currently being used to monitor realtime output signals from the load cell and accelerometer in real time (Fig. 20). Figure Channel Oscilloscope HP 3567A Dynamic Signal Analyzer The DSA not only provides the power source to drive the excitation system, as described in section 3.1.1, but also supplies the 4 ma current necessary to power the load cell and multi-axial accelerometer. In addition, the DSA records the input force registered by the load cell and the acceleration measured by the accelerometer in order to convert the timedomain signal into the frequency domain through a FFT (Fast Fourier Transform). The data recorded by the DSA can be saved to a file for use in post process analysis. 30

8 3.4.4 I-DEAS (Integrated Design Engineering Analysis Software) I-DEAS is an integrated package of mechanical-engineering software tools developed by SDRC (Structural Dynamics Research Corporation). The software is installed in the Unix Workstation, Oberon, in the ASE/EM Learning Resource Center of Woolrich Laboratories under a licensing agreement between SDRC and the Aerospace Engineering Department (Fig. 21). Figure 21. I-DEAS Star-Lite Model on the Oberon Workstation The I-DEAS software is used in the post process analysis of FR data obtained from a GVT. The I-DEAS analysis extracts the modal parameters from GVT data and also provides a modal animation of the structure. 31