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Multirotor  Vibration

Multicopters are very sensitive to vibrations. Their flight control unit relies on IMU sensors to determine flight attitude and stability. Other sensors carried as payload can also be affected . This basic study shows how to determine the source of vibration and how to design the frame to avoid it.
 
There are many sources of vibration such as;
  1. Unbalanced propellers
  2. Bent motor shafts
  3. Bad propeller adapters
  4. Bad motor alignments
  5. Bad bearings
  6. Structural resonance from Landing Gear, Booms and Sensor mounts.
The first 5 items can be identified and addressed using the Vortex Spectrum Analysis. More on this will be covered in future tutorials. In this tutorial, we will discuss structural resonance which is often overlooked when multirotors are designed. Structural resonance is a major contributor to vibration.
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Fortunately DynexHobby have developed a simple software application to calculate the critical resonant frequency. The goal is to design the multirotor to operate away from the resonant frequency. For example if your quadcopter loiters at 50Hz and happens to be your motor RPM during this flight mode, then your are likely to experience structural resonance. This will show up in aerial video footage as "jello." Recall that 50Hz = 50 x 60 = 3000RPM.

Here is an example. DynexHobby operates the trusty X525 quadcopter. It is observed that during rapid climb to altitude, video jello is experienced around 4000RPM. We tend to run small size propellers hence the high speeds. In this example we will examine the structural resonance of the four booms.

To access the Multirotor designer, click on the "quadcopter" symbol in the main menu and then click on "Design a multirotor."
By clicking on each button, we can define the geometry, mass and material properties of the X525.
Clicking on the "solve" button produces the final solution. In this case, the natural frequency of the boom is 71Hz or 4260RPM. So it appears that when flying around this motor speed, the X525 will experience structural resonance.
So how can we avoid this problem? There are a few solutions available to us.
  1. Select a different propeller size that will allow the quadcopter to operate at a different RPM. There are limitations such as reduced flight performance to consider.
  2. Change the length of the boom. This can have some impact.
  3. Change the boom material to carbon fiber. This has a substantial impact. Carbon fiber will shift the natural frequency of the boom into higher frequencies avoiding the problem.
  4. Change the mass where the motor is mounted. This can be achieved by shifting the ESC location.
Try it for yourself. Note that the application can also calculate resonant frequencies for other items such as landing gear. Any structure that can be represented as a cantilevered beam is appropriate for this type of analysis.
 
There are many products on the market that claim to dampen the vibration. Although this is partly true, the real cause for the vibration still exists. It is better to eliminate the root cause than cover it up. The chart below illustrates this affect. In an undamped structure, the vibration amplitude is high. When the vibration is damped the amplitude is reduced however it still exists.