added a note to IS documentation about unbalanced fans

docs/input_shaper.md

@@ -15,7 +15,7 @@ When tuning Input Shaper, keep the following in mind:
   1. **Focus on the shape of the graphs, not the exact numbers**. There could be differences between ADXL boards or even printers, so there is no specific "target" value. This means that you shouldn't expect to get the same graphs between different printers, even if they are similar in term of brand, parts, size and assembly.
   1. Small differences between consecutive test runs are normal, as ADXL quality and sensitivity is quite variable between boards.
   1. Perform the tests when the machine is heat-soaked and close to printing conditions, as belt tension can change with temperature.
-  1. Avoid running the toolhead fans during the tests, as they introduce unnecessary noise to the graphs, making them harder to interpret. This means that even if you should heatsoak the printer, you should also refrain from activating the hotend heater during the test, as it will also trigger the hotend fan.
+  1. Avoid running the toolhead fans during the tests, as they introduce unnecessary noise to the graphs, making them harder to interpret. This means that even if you should heatsoak the printer, you should also refrain from activating the hotend heater during the test, as it will also trigger the hotend fan. However, as a bad fan can introduce some vibrations, feel free to use the test to diagnose an unbalanced fan as seen in the [Examples of Input Shaper graphs](#examples-of-input-shaper-graphs) section.
   1. Ensure the accuracy of your ADXL measurements by running a `MEASURE_AXES_NOISE` test and checking that the result is below 100 for all axes. If it's not, check your ADXL and wiring before continuing.
 
   > **Note**
@@ -107,6 +107,7 @@ In the following examples, the graphs are random graphs found online or sent to
 | **These two graphs are really bad**: there is a lot of noise all over the spectrum. Something is really wrong and you should check all moving parts and screws. You should also check the belt tension and proper geometry of the gantry (racking) | ![](./images/IS_docs/shaper_graphs/insane_accels.png) | ![](./images/IS_docs/shaper_graphs/insane_accels2.png) |
 | These two graphs have some **low frequency energy**. This usually means that there is some binding or grinding in the kinematics: something isn't moving freely. Check the belt alignment on the idlers, bearings, etc... | ![](./images/IS_docs/shaper_graphs/low_freq_bad.png) | ![](./images/IS_docs/shaper_graphs/low_freq_bad2.png) |
 | These two graphs show **the TAP wobble problem**: check that the TAP MGN rail has the correct preload for stiffness and that the magnets are correct N52. Also pay attention to the assembly to make sure that everything is properly tightened | ![](./images/IS_docs/shaper_graphs/TAP_125hz.png) | ![](./images/IS_docs/shaper_graphs/TAP_125hz_2.png) |
+| Here you can see **the effect of an unbalanced fan**: even if you should let the fan off during the final IS tuning, you can use this test to validate their correct behavior: an unbalanced fan usually add some very thin peak around 100-150Hz that disapear when the fan is off during the measurement | ![](./images/IS_docs/shaper_graphs/fan-on.png) | ![](./images/IS_docs/shaper_graphs/fan-off.png) |
 | The graph on the left shows **a CANbus problem** (problem solved on the right): although the general shape looks good, the graph is not smooth but spiky. There is also usually some low frequency energy. This happens when the bus speed is too low: set it to 1M to solve the problem | ![](./images/IS_docs/shaper_graphs/low_canbus.png) | ![](./images/IS_docs/shaper_graphs/low_canbus_solved.png) |
 
 </details>