As we know there are gyro sensors on multicopters, and some of them even have an accelerometer. Multicopter that are equipped with gyros and accelerometers can achieve stabilization requiring little even no control from the pilot. They automatically stay horizontally levelled which is a great feature for a beginner because it makes the quadcopter much easier to fly. However before attempting to fly your quadcopter for the first time you should ensure you have a calibrated ACC sensor.

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It’s just good practise to verify your quadcopter’s accelerometer is properly calibrated before flights.

How to Calibrate ACC in Multiwii GUI

Place the board or quadcopter flat on a table, plug the board into the USB connection of your computer. Startup the MultiWii Config GUI, connect to the COM port and find the ACC sensor readings.

It should read close to 0 (within the range -10 to 10) for PITCH and ROLL, and close to either 256 or 512 in the Z axis. If it’s outside this range you should calibrate your ACC again, and find a more horizontally levellved surface to do this.

Click the CALIB_ACC button above the graph section and wait a few seconds. The ACC sensor readings should now be close to the values in the above paragraph, so the ACC PITCH and ROLL near zero, and Z near 256 or 512. The Z value of 256 or 512 will depend on what sensor and firmware version you are using, both values are valid.

How does accelerometer work?

I spent a few paragraph explaining how this works in a quadcopter in practice. However I will relate that to more software-wise here.

In Simple words the ACC can sense the orientation of the multicopter. It is required for the self-level flight feature (Angle and Horizon as explained here). The ACC sensor is also required in the altitude hold feature (along with Baro or sonar), estimates the velocity of the quadcopter in the vertical direction.

What’s after the Calibration? Trimming

The accelerometers need to calibrated at least once to let the sensor know what is level, if you calibrated on a slope it would take that as being level and you screen display would show as level. If you tried to fly in this condition you would be forever flighting to keep it level as we see it but it would always want to revert to its “learned” level, calibrated on a level surface and our level would be the same its “learned” level.

Nothing is perfect, neither your “level surface” nor your Accelerometer. After Calibration, it might still not be 100% “levelled”, so you need to manually trim it. It’s a process of trial and error. You should adjust it until you can take your hands off the stick, and it still stays in the air levelled.

Once we have a level we all agree on then you need to trim out the discrepancies that we had to trim out in gyro mode. Prop up one side of your quad and calibrate, you will see the pitch and roll indicators come to level, take the prop away and the roll will move to one side saying its not level, re-calibrate and all will be well.

Tx trim tells the Tx what the true pot center for the selected a/c is. settings are kept in Tx memory and dont change unless you move trim switches
Acc Trim tell the accelerometer what true level is for selected a/c. setting are lost if acc is recalibrated.