Vibration Analysis Pro

The extended vibration analysis is called by a buttin in the model status dialog:

Vibration levels are important, don't ignore them!

General about vibrations

Vibrations are always an important issue. Helicopters do have vibration always, it's normal and can never be avoided completely.
However, VBar needs informations from sensitive sensors about the rotational rates as well as the acceleration of the model.
VBar uses highly refined algorithms to differentiate between the wanted signals and the noise from vibrations.

In general, the algorithm is not the limiting factor, but the hardware sensor itself. On very high vibration load some ugly physical effects appears. So it's always a good idea to make sure to have as little vibration as possible.

Vibration Rectification - gradual influence
This term names an effect where on vibration influence, the sensor starts to show more and more offset from zero. The effect is caused by minimal nonlinearities of the sensor output. Unfortunately this effect cannot be removed by software, since it is not possible to distinguish between the actual motion and the vibration effect.

Vibration rectification appears always, and gets gradually worse on higher vibration levels. It never occurs out of nowhere.

Sensing Limit - influence beyond a treshold
All sensors, but in particular the accelerometers do have a measurement limit. If the limit is reached and exceeded, the output
value is no longer showing the actual activity. This as well can lead to offsets, which cannot be removed by software. Reaching or exceeding the limit usually is not a problem for the gyros, but it is for the accelerometers. So this usually only affects rescue.

Sensor Blinding - heavy immediate influence
If vibrations get extreme, the sensor can reach it's mechanical limit, and output almost completely random data. This is stressing the sensor element in a way that it may fail, or get stuck at one end of it's physical travel. The level of vibration where this happens is extremely high, and there will be at least lots of "Extreme Vibration Level" entries in the log file. Often it goes together with mechanical destruction of parts of the helicopter or the electronics just because of the vibration.

Direction of vibration
Vibrations can occur on all 3 gyro axis and all 3 lateral axis. Usually they do appear more or less on all axis mixed together, since the unit cannot be mounted 100 % rigid. In this case a lateral vibration will lead to a slight tilt, which is seen on the gyro axis as well.

Another important parameter is the frequency. On helicopters, all rotating parts can create vibrations in the frequency of the rotation. This gives a good first hint to the source of vibration. The analyzer is working with a frequency of 1 kHz, which allows to detect vibrations up to 500 Hz which is 30.000 1/min. This is sufficient to even detect motor vibrations. However, i.e. gears may create even higher frequencies which may not be detectable, but still can have the described effects.

Functions of the extended Vibration Analyzer

Gyro Single Axis FFT
The known frequency spectrum view displays only the aileron axis of the system. This is known to be the most critical. This tool
now allows to use any other sensor as source of the spectrum analysis. This is done by the "Axis to Use for FFT". (FFT means Fast Fourier Transform which is the algorithms name). It allows to show one axis a time. The viblevel value is just a hint with no unit. The red line is the limit for the gyros. The other sensors may have different scales.

Vibration Log File / View
This tool allows to write a logfile during testing or during flight. The logfile stores the "viblevel" as value without unit, and additionally all vibration values of the highrange accelerometers in "g". 1 g is 9.81m/s2. The measurement range of the highrange sensor is +- 200g.

With "Show Graph", the logged data can be viewed. The logfile is started in the first moment the checkbox is ticked, and stopped if unticked. The file is continued if activated again. Maximal one file per flight is created. It only can be viewed during the model is still connected.

Detailed Accelerometer View
This sections shows the accelerometer readings of all sensors. You can activate the display with the "Activate" button. This in turn deactivates the FFT display.

The lowrange sensor is a the precise one, with a measurement range of 16g. The highrange sensor is able to read up to 200g, but it is not that precise because of the high range. Both readings shall roughly show the same value. The highrange one is displayed slower, so it's better readable.

Noise is what the highrange sensor sees as unwanted signal. A few´s are quite normal, here this is just normal measurement noise. The bars on the right display all values as bars. The first two bars show the actual value from about -3g to +3g. First is lowrange, second is highrange. The third bar is the vibration in g´s.

The gray triangle is shown with a warning sign, if the range of the lowrange sensor is completely exhausted. This is not necassary the limit of the system, but it shows what the lowrange one is capable of, and when the highrange sensor has to take over.


Interpretation of Graphs.

Example of Vibration Logs:

Logo 200 with very low vibrations. The maximum is sol low, that only a few discrete steps are visible. Resolution is 1g.

Graph of a LOGO 700 with 3 different Headspeeds

Each graph is scaled individually, so to get a comparable reading, the maximum value on the right has to be checked to find out which scale the curve has.