|May 11, 2010, 12:13 PM|
Today I did some vibration testing on my flybarless FireFox "PhuFox" helicopter using the PhuBar and ViewPort to look at the gyro data in real time.
On the graph below, the blue line is the roll rate after smoothing with the digital low-pass filter. The red line is the integrated roll angle. Green is the control signal going to servo #1. The helicopter was sitting on a soft block of egg-crate foam rubber for this test.
On the left side you see no vibration in the signals because the motor is off, but you can see that I moved the aileron stick back and forth to move servo 1.
On the right side of the graph are the same signals except that I throttled the motor up to flight speed. The effect of vibration on the signals is small but visible. The magnitude of this effect on the servo output, when measured, amounted to about 1% of the total servo range of motion. My subjective experience in flying both helicopters is that a moderate level of vibration has no noticeable impact on performance of the PhuBar, thanks to the low-pass filtering.
The change in level of the servo signal you see at t-6 resulted from pausing ViewPort while I throttled up, then I restarted ViewPort. At the higher throttle setting, the pitch curve on my radio sent a higher pitch setting to the receiver and caused all three servos to move to a slightly different position due to the CCPM mixing in the PhuBar.
There are four things that help reject vibration:
1) Mounting the enclosure to the frame with thick foam tape
2) Mounting of the gyro breakout board onto the main circuit board involves one connector at the outer edge that allows a certain amount of up and down movement. The center of the gyro board is then secured to the top of the clock crystal (the oblong metal can in the middle of the main board) by thick foam tape.
3) The IDG500 gyro board itself has a hardware low-pass filter on it that has a cutoff frequency of about 2100 Hz
4) The digital low-pass filter implemented in the software has a cutoff frequency of about 47Hz. I got this filter code from Bill Premerlani's published MatrixNav code that he uses for R/C airplane navigation. Before adding this filter there was an audible jittering from the servos even if the unit was dead still, just from noise in the signal.
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