# Presentation: 504749

## Validation of Base-Driven Modal Survey Capability

**PI:  ****	****Scott Gordon/Code 542**
**Co-PI’s:  ****	****Brian Ross/Code 549**
**	****Dan Worth/Code 549**
**	****Dan Kaufman/Code 542**

## DDF Proposal - Overview

**Purpose:  To prove feasibility of Base-Driven Modal Survey as alternate technique to Fixed-Base Modal Survey for extracting modal parameters**
**Approach:**
- Perform standard modal survey/correlation on representative test article
- Perform base-driven modal survey on same test article
- Compare results/develop guidelines
**Rationale:**
- Correlated finite element models required to accurately predict flight loads
- Base-driven modal survey is a cost-effective means for extracting modal data necessary to correlate finite element models
- Currently fixed-base modal survey is only accepted way of extracting modal parameters (frequency, mode shapes, damping)
- Data does not exist to prove viability of base-driven modal survey

- Scott Gordon/Code 542

- October 24, 2003

## Base-Driven vs Fixed-Base Modal

**Need for Model Correlation**
- Improved flight loads predictions  More efficient structures/mechanical systems
- Requirement for  test verified models for Verification Coupled Loads Analysis
**Current Approach – Dedicated Fixed-Base Modal Survey Test**
- 1-2 weeks test effort (15 – 20 $K)
- Dedicated modal test facility
- Design/fab/analysis of mass mockups
- Pre-test analysis to ensure that test config = flight config
**Base-Driven Modal Survey Approach**
- Testing performed as part of standard vibration testing (sine/random)
- Modal extraction performed on shaker table – dedicated test facility not required
- Hardware tested in flight configuration, at flight input levels
**Applicabililty**
- Component/subsystems with few modes to be correlated
- Observatory level testing

- Scott Gordon/Code 542

- October 24, 2003

## DDF Test Article

**Cassini Infared Spectrometer (CIRS) Mass Mockup**
**Weight:  57 lbs**
**Dimensions:  30” x 30” x  6”**

- NASTRAN Finite Element Model

- CIRS Mockup on Slip Table

- Scott Gordon/Code 542

- October 24, 2003

## Test Instrumentation

- Scott Gordon/Code 542

- October 24, 2003

## Test vs Analysis Comparison

**The finite element model is considered test correlated if for significant modes (>10% modal effective mass) the following requirements are met:**
- Analytical frequencies are within 5% of the measured test frequencies
- Cross-orthogonality between the test mode shapes and the analytical predicted mode shapes show >0.90 on the diagonal and <0.10 on the off-diagonal
**Cross-Orthogonality is defined as**

- Scott Gordon/Code 542

- October 24, 2003

## Correlated CIRS FEM Using Traditional Modal Survey

- CIRS Mockup in Modal Facility

- Scott Gordon/Code 542

- October 24, 2003

## Vibration Test Runs

**Total Vibration Test Runs = 30 (10 per Axis)**

- Scott Gordon/Code 542

- October 24, 2003

## Modal Extraction Using Vibration Data

- Scott Gordon/Code 542

- October 24, 2003

## Cross-Orthogonality Results for 0.1g Sweep @ 4oct/min

- Mode not identified due to rocking of shaker head (See Page 12)

- Scott Gordon/Code 542

- October 24, 2003

## Comparison of Vibration Data with Correlated FEM

- Correlated FEM Mode Shape

- Vibration Data Mode Shape

- Scott Gordon/Code 542

- October 24, 2003

## Issues/Problems

**Only able to correlate first 8 modes of CIRS mockup using standard modal survey techniques because of limitations with test setup**
- Difficulty exciting rotational modes
- Insufficient time to troubleshoot problems.
**Rocking of shaker head during thrust axis vibration test resulted in poor quality data for the Z-axis vibration test**
- Need to examine other data sets to see if rocking is consistent across input types and levels
- Need to examine alternate means of processing data to improve quality
**Schedule constraints in getting access to modal facility and shaker facility limited time available to process data**
- All vibration runs complete and data archived
- Only 0.1g sweep @ 4 oct/min has been processed to date

- Scott Gordon/Code 542

- October 24, 2003

## Summary/Conclusion

**FEM model of the CIRS mockup created**
**CIRS mockup FEM correlated using standard Fixed-Base modal survey techniques**
**Entire Suite of planned vibration tests have been completed**
**Test data from the 0.1g @ 4oct/min test run has been processed and modal parameters successfully extracted**
**Excellent correlation for lateral (X & Y) modes.  Poor correlation for thrust (Z)**
**Two issues uncovered that must be explored further**
- Higher order modes not well correlated using either fixed-base or base-driven data
- Thrust axis (Z) rocking during vibration resulted in poor quality data
**Additional Work Planned (Under Internal AETD Funding)**
- Process all acquired test runs to determine how different vibration parameters effect quality of data
- Review vertical axis data across different runs to see if rocking behavior is consistent
- Process data using different input references to see if quality of data can be improved
_**DDF Investigation showed feasibility of using base-driven vibration data to extract modal parameters.**_

- Scott Gordon/Code 542

- October 24, 2003