434  

Figure 5 .  Noise floor showing measurements filtered using low bandpass of 10 Hz  

Calibration  of EFT Setup s 

BATC developed several procedures to calibrate its EFT setups as a system. These include h armonic  stinger tests, f requency response function tests , and speaker tests.  EFT setups are first analyzed to   identify their modal characteristics  with and without the mechanism intended for measurement.  To ensure  a good correlation between analysis and tests, the spring rates of the load cells themselves are characterized and are accurately modeled. Once the model of the setup is properly correlated with test  data, and its predicted response is validated through combinations of sine sweeps and tap testing, an  integrated model of the correlated setup and the unit under test is evaluated to ensure adequate stiffness  of the setup. Again, there should be no gains that would affect measurements  in the expected jitter  frequency range. A t this stage, the FMP is ready to proceed for  calibration as a unit.    The calibration of the FMP entails three parts. First, the setup  is shown to have the necessary sensitivity  to accurately make the necessary measurements. T he noise floor is identified and compared to the  program requirements which are either specified or derived  from higher level requirements . At a  minimum, the noise floor is required to be one order of magnitude below the minimum measured  force/torque in the six degrees of freedom.    Secon d, the forces measured by the FMP are validated using two separate and independently calibrated  data acquisition systems and three loading methods . This is performed to isolate and quantify the source  and magnitude of potential random measurement errors  and identify the load cells/channels that may  be  erroneous . Harmonic stinger test s are used to excite the setup at certain frequencies . A calibrated load  cell is placed at the interface between the stinger and the EFT setup to measure the input force. The input   force is also measured using the setup as the sum of the contributions of all load cells  through its  transformation matrix relating the individual loads to the resultant forces and moments  of the FMP . The  input load is applied separately along the di fferent axes of the setup and is required to match the  measured output load. The Spectral Dynamics data acquisition system at BATC’s Vibration Lab is often  used as the first independent mean for measurement . A second portable data acquisition system is used  to ensure concordance with the reference input load. The fixity of the FMP  setup is evaluated  to ensure  that the most adequate and practical configuration among clamped, simply supported and free-free that   delivers the most separation between the modes of the setup and the jitter forces frequencies is used.  Finally, Speaker tests  are used as a source for input loads that can be p laced at different locations of the  setup and different heights  above the interface plate of the FMP . The calibration consists in applying  known forces using a speaker precisely located and oriented on the setup and measur ing the reaction  forces and moments imparted on the surface of the plate.  The speaker is attached to a fixture through an  encapsulated l oad cell . The speaker is actuated using a form generator to deliver  loads of varying  magnitudes at particular input frequencies . Tests are required to show that the input load from the