Learned a lot about the inertial referencing circuit yesterday and today. The 4th order filter used on the displacement part is to compensate for the phase change seen by the accelerometer.
Phase change of the accelerometer explained:
Even though the natural frequency of the accel is much higher than our analysis band, the phase change associated with the natural frequency is substantial enough at our analysis band that it must be accounted for. To do this, we referenced the accel against one with a much much higher natural frequency. This gave us a transfer function for the accel(FRF between circuit accel and reference accel), which you can fit a model to in Matlab using invfreqs. Paul then does some magic and creates a filter that will mimic this behavior for the displacement sensor.
Also, one of the parts of the circuit for the accel is to account for the laser sensors measurement delay. We had to switch one of the resistors in this section because the laser sensor we are using now is different then before.
Once all the circuitry was changed to account for the new sensors, I ran another RR test and got a good RR for the entire analysis band. Brad was pleased but wanted me to gain tune the circuit at 65Hz to see if we could get the high frequency part better. Gain tuning involves changing the variable resister to try and match the gains of the two velocities from the sensors. If done correctly, the gains will be identical and the only thing that will cause problems is the phase difference between the two signals.
The rest of the day I spent preparing the instrumentation to get shipped to White Sands for Leslie's test.
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