What is the VibrationTest Tool?

The tool evaluates the vibration generated by the motors using the MK build-in sensors, so no additional hardware is needed.

The VibrationTest works with 1.2 and 1.3 FC boards but unfortunately does not work in combination with ME boards without a hardware patch.

The tool can be used to balance props, balance motor/prop assemblies, experimenting with different motor-mounts and so on. As the tool only uses the sensors on the MK FlightControl board, no extra hardware is required.

As the tool can follow the vibrations caused by the prop, it is possible to deduce the motor RPM at a given speed value. This allows checking if a ESC/motor assembly performs as expected.

For Aerial Photography, it would perhaps be possible to attach a second FC board to a camera to visualize the vibrations the camera is exposed to. However, this is under investigation if this produces usable results.

Why is it important to reduce vibrations?

The FlightControl sensors pickup the vibrations and this deteriorates performance; the new FC ME comes with vibration dampers in order to reduce the vibrations transferred to the FC.

Reducing vibration is very important, especially for Aerial Photography or Video. Vibrations result in blurry footage. Some claim that high frequency vibrations can be picked up by the Optical Image Stabilization lens assembly which is incorporated in almost all recent cameras. The OIS is not designed for these vibrations and makes thing worse, even when you switch the OIS feature off in the camera menu.

Propellers can be balanced the traditional way. However it was discovered that also the motors need balancing. One way to accomplish this is let a small unbalance in the prop compensate the motor unbalance, so the orientation of the prop is important (see this post). Looking at the vibraions of a running motor allows balancing the prop/motor assembly as a whole.

How does it work?

The standard FlightControl program samples all sensors periodically and provides a command to read the latest sample from all sensors. The MKTool uses this command to generate graphs of the analog values. In a first approach the VibrationTest used this standard command to monitor vibrations. The problem is that this mechanism only allows to sample the values 30 times a second. The actual shape of the signals cannot be determined and one can only hope to catch the peak values. In order to get reliable results the values need to be monitored for a long time (a minute for example).

This new approach is based on dedicated FlightControl software providing a new command. The new command instructs the FC to sample one of the sensors as fast as possible and store the info in its internal memory. Afterwards the recorded data can be read to analysis. This allows to grab a reliable signal in a very short time. This approach can capture about 11000 samples a second, 500 times more than the MKTool can.

This is an example of the signal measured by the Roll-ACC sensor:

Current Project status

Version 1.2 is available

At the time of writing, the tool was tested in combination with:

• PC/Windows XP and Vista
• FC 1.1, FC 1.2 and FC 1.3

• The Tool does not seem to work well in combination with FC ME without hardware modification.

• Opening the virtual serial port of a Bluetooth dongle might fail. I suppose this is the same issue as the most recent MKTool has, will be solved in the next version.

Theoretically the tool should also run on MAC and Linux.

Features/fixes in the pipeline

• Tool to help measuring RPM

• Measurement parameters in different dialog -> more room for results

• Configurable sampling-rate -> sampling over a longer time is needed is some cases

• Allow to connect two FC boards for measurement of camera vibrations
• Integrated backup and restore of flight settings
• Integrated installing and restore of FC code

MK Forum thread for discussions
<frederic AT rc-flight DOT be>

How do I install the tool?

The tool is written in programming language called Python.

The most logic way to run the tool is to install support for Python on your PC (if you do not have it already) and the Python libraries the tool depends on. Python is also available for Linux and MAC; the tool should also run on these platforms, but I have never tried it.

If you do not feel like installing Python, a version that is compiled to a windows executable is provided.

So, you have two options.

Option 1: Install Python and required libraries

For a Windows machine I propose you install:

• ActivePython for Windows Press the big "ActivePython Download Now" button.

• PySerial (support for serial port access) Select "pyserial-2.5-rc1.win32.exe" on that page.

• wxPython wxWidgets for Python Select the the win32-unicode version for Python 2.6

• NumPy, the fundamental package needed for scientific computing with Python

Once you have Python support on your PC, unzip VibrationTest_1_2.zip to a directory on your PC.

Option 2: Get the compiled version

unzip VibrationTestExe_1_2.zip to a directory on your PC

How do I use the tool?

Prepare your MK

Secure your MK

I use luggage straps to secure my MK to the table while it is still able to vibrate:

For MKs with bigger motors I have found that it is increasingly important to make sure that the straps do not vibrate too much and stay out of the propwach as much as possible Using some soft materials to isolate the straps from the frame and to minimize the strap vibrations improve the quality of the measurements, see picture below.

Later in this page it will be explained how the quality of the measurements can be checked.

Installing the FlightControl software

The VibrationTest tool only works in combination with a special version of the FlightControl software. It must be flashed in the FC board before using the test and you need to reinstall your regular software version afterwards. This just takes a few minutes. Switching the software is done the usual way, using the MKTool.

You will find the required hex file in the same directory with the VibrationTest tool.

The FC code for the VibrationTest is based 0.74d but is not fit for flying! Before flying, you will need to reinstall your initial FC code. When the version you use for flying is 0.74d, your settings should be preserved. In combination with other versions, it might not be the case. It is a good idea to backup yout settings to PC (via the MKTool) before installing the VibrationTest FC code.

After insatlling the FC code for the VibrationTest, the LC-Display in the MKTool will look like this:

It is normal that a there is a warning for a missing BL-Ctrl. When you have 4 motors for example, it should complain that it did not find motor 5.

Starting the VibrationTest tool

In the directory where you unzipped the tool, dubbleclick VibrationTestGui.py (in case of the Python version) or VibrationTestGui.exe

Using the VibrationTest tool

Checking the reliability/accuracy of the results

Before you start, it is good to check the reliability of the test and setup.

You should alway start with a sweep and look at different sensors. If the vibrations is rather constant over motor-speed, the combination might be well balanced or you might not be measuring what you are hoping to measure. In this case, sticking some tape to the prop should should unbalance it and you should see a difference.
You can have a look at the recorded signals, the filtered signals and perhaps the spectrum. The picture below shows an example of (raw) a signal of good quality:

The signal below is regular but goes slowly up and down. This is not a big problem as this will be filtered out later; this is still good:

The signal in the next picture is of low quality. Propably other components than the motor are vibrating:

If the signals seem ok, you can repeat the sweep multiple times, using one of the sensors. Each time you should get similar results. Results will never be 100% the same.

Another test you should do, is let the tool measure multiple times at the same speed; for example "10*100,10*200". The measurement at the same speed should report the about the same amount of vibrations.

The figure below shows the result of such test with a "Roxxy 2824-34 / EPP1045 / 3s / Standard ESC" combination. All but the green line are from the Nick sensor. The black line are vibrations measured at speeds 130 and 185. The other lines are sweeps. This does not look too bad.

When the results are disappointing, one should experiment with the way the MK is attached. One could fix the MK more firmly to the desk or on the contrary allow more movement. One should also and make sure nothing else but the MK can vibrate.

It is also possible that the filter parameters might not be optimal for your setup. In the figure below I have almost disabled the filter and switched to "pp" measurement:

This is another example of such a test (Turnigy 2836-750 / APC 12x3.8 / 4s / TowerPro ESC):

Examples

Balancing example: Roxxy 2824-34 / EPP1045 / 3s / Standard ESC

Effect of prop position, Roxxy 2824-34 / EPP1045 3s / Standard ESC

The importance of the position of the propeller relative to the motor was first reported by RCGroups member "Old Man Mike", see this post. My tool also shows this effect. However, I observed that sometimes rotating the prop did not produce significant variations...

I discovered that when I first balance the prop as good as I can using the VibrationTest, rotating the prop produced very little difference.

I come to the conclusion that I need to deliberately unbalance the prop a bit in order to find the "sweet spot". I also observed that when I unbalance the prop by sticking some tape on one side of the prop the vibrations peak while they go trough a minimum when I stick some tape to the other side. I think this supports the theory that the slightly unbalanced prop can compensate for unbalances in the motor.



On the X-asis the rotation of the prop in steps of 60 degrees. The red line is with a prop that I balanced before (needs some tape opposite to the prop size-markers). The green line shows vibrations when I attach an additional strip of tape (so unbalancing it). The blue line shows what happens when I attach tape to the other side (where the markers are).

Balancing example: Turnigy 2836-750 / APC 12x3.8 / 4s / TowerPro ESC

The red line is the initial balancing. The prop seemed best balanced with no additional tape.
The green line are the measurement while rotating the prop (steps of 45 degrees)
The blue line is the re-balancing, due to the repositioning of the prop the results are better. The first 3 measurements showed that no tape produced the best results. In the last 3 measurements of the blue line, I experimented with smaller strips and this produced the best result.

Influence of ME ACC capacitors

This example shows why the VibrationTest does not work in combination with FlightControl ME boards. ME boards have bigger capacitors on the ACC lines and these suppress the signal we are interested in. 1.3 boards have 0.1 uF capacitors while ME has 1 uF capacitors.

To be sure that the issue was due to these capacitors I replaced one of the capacitors and performed some measurements. The first picture below shows the raw signal without capacitor (0uF). The next picture shows the signal with a 0.1uF capacitor, so this corresponds with the capacitor used on 1.3 boards. For the last picture a 1uF capacitor (so the normal value for an ME board) was installed. As you can see the signal is completely suppressed...




The pictures below show the spectra of the same signals. As you can see, the 0.1uF cap already filters a lot but leaves the vibration signal visible.

Changelog

Older versions

• 1.0

VibrationTest_1_0.zip
VibrationTestExe_1_0.zip

Links

VibrationTest related

• VibrationTest Project in Subversion

• MK Forum thread for discussions

• Mail FredericG: <frederic AT rc-flight DOT be>

Related Threads

• Thread: Vibrationsdämpfung der Antriebsmotoren

• Thread: Tilt servo play => Vibrations

• Thread: Equilibrage des Hélices RC (French)

• Video: Is the Optical Image Stabilization of the FX35 causing problems?

• Thread: Multikopter (mikrokopter, UAVP, x-ufo-3rd, etc) AP pictures

Related Projects

• Old Man Mike "Test Fixture Evaluates Motors/Props/ESCs for Quadcopter Performance"

Used Libraries and Tools

• Python

• wxPython

• wxGlade

• NumPy

• py2exe

Technical references

• Sampling

• Fourier Analysis

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