Understanding Sensor Readings

[Liamus27]

Hi.

I was wondering if any technical experts can post some helpful info about sensor readings. I had my questions about compass readings helpfully answered in another thread but I read on a preflight checklist:

Check IMU Sensors: IMU +/- 0.1

So I know about IMU calibration (this is not a thread about that) and I know how to access these readings but I want to know what they mean? Am I right in thinking these readings are similar to a spirt level and show the current level i.e flatness of the aircraft's position?

 

[With The Birds]

Spirit level is a good analogy..... The spirit level only works in one dimension (axis) per bubble though. The imu is more akin to a marble on a flat surface. The x and y axis readings are interpolated so tilt in any direction can be determined. It is also able to measure acceleration in any direction in space (has a Z axis also). It has a mechanical component, thats why you should always recalibrate after a hard landing or other impact to your phantom.

 

[Liamus27]

This is useful thank you. What is the impact (if any) in this circumstance (IMU all properly calibrated on level cool surface etc):

I place my aircraft on an uneven surface for takeoff - what will my IMU reading show preflight and will it affect flight performance? My question is if I have a reading outside IMU +/- 0.1 does this always indicate a problem or will it waver based on how the aircraft is sitting?

I'm not sure I have explained myself very well so feel free to ask more!

 

[RoyVa]

Here is info on IMU:
IMU( Inertial Measurement Unit = Accelerometer + gyroscope + compass)
Roll, Pitch and Yaw. Without almost any drift.
The IMU is an electronic sensor device that measures the velocity, orentation and gravational forces of the Quadcopter.
These measurements allow controlling electronics to calculate the required changes in the motor speeds.
It's a combination of 3 axis accelerometer and a 3 axis gyroscope and a 3 axis magnetometer (compass) for better Yaw control.
The accelerometer measures acceleration and also force and measured the downward gravity force. As it has 3 axis it can work out the orentation of the quad.
The gyroscope measures angular velocity, in other words rotational speed around the 3 Axis.
A magnetometer measures the direction and strength of the earths magnetic field. The sensor can then determine which way is north and south. The pole locations are then used as a reference with the Yaw angular velocity around the gyroscope to calculate a stable Yaw angle.
Hope that helps!

 

[Liamus27]

Here is info on IMU:
IMU( Inertial Measurement Unit = Accelerometer + gyroscope + compass)
Roll, Pitch and Yaw. Without almost any drift.
The IMU is an electronic sensor device that measures the velocity, orentation and gravational forces of the Quadcopter.
These measurements allow controlling electronics to calculate the required changes in the motor speeds.
It's a combination of 3 axis accelerometer and a 3 axis gyroscope and a 3 axis magnetometer (compass) for better Yaw control.
The accelerometer measures acceleration and also force and measured the downward gravity force. As it has 3 axis it can work out the orentation of the quad.
The gyroscope measures angular velocity, in other words rotational speed around the 3 Axis.
A magnetometer measures the direction and strength of the earths magnetic field. The sensor can then determine which way is north and south. The pole locations are then used as a reference with the Yaw angular velocity around the gyroscope to calculate a stable Yaw angle.
Hope that helps!

Hugely helpful thank you.

So, my question above with a well calibrated IMU taking off from an uneven surface will be normalised in air based on a 0 IMU rating?

 

[rockydog]

Taking off from a non level surface is not a problem -(other than the danger of the quad tipping over when you start the motors if you're too far off level).
Once you're airborne, it will immediately revert to level flight if your IMU is calibrated properly.

 

[Liamus27]

Taking off from a non level surface is not a problem -(other than the danger of the quad tipping over when you start the motors if you're too far off level).
Once you're airborne, it will immediately revert to level flight if your IMU is calibrated properly.

That was my understanding - thank you for confirming

 

[Liamus27]

So my next sensor reading question is about the following advice I read(not IMU related):

I would add checking the area before start with a magnetometer app. If the magnetic filed detector shows anything beyond the threshold from 45 to 55 nT I do not calibrate the compass or even start the engines.

Has anyone used a magnetomter to check the area they fly? Seems overkill but I'm here to learn.