IMU explanation --- how does a drone know it's level ??

[Jiggly-Wiggly]

Maybe this has been discussed in here before --- but can anyone give a simplistic yet thorough explanation for how a Phantom maintains level hovering?

IMU does this but how? I know the accelerometers must be at the core of it, but I'm not sure how an accelerometer knows it is level with respect to the horizon? And if the IMU is calibrated on a non-level surface, doesnt that mean the Phantom will maintain AGL height but slowly fly off in one particular direction in non-GPS mode?

 

[tevek]

This video explains why and how do an IMU calibration. My take is that the phantom needs to be set up and referenced on a dead level surface so that when in flight, it has an artificial understanding of what is level, the gimbal also assumes that position and the drone will fly straight and true. I recently had a pile of slanted flight and offset horizon issues coupled with an extended warm up time. Fixed with a recal of both IMU and gimbal.

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Credit to @msinger.

 

[Jiggly-Wiggly]

This video explains why and how do an IMU calibration. My take is that the phantom needs to be set up and referenced on a dead level surface so that when in flight, it has an artificial understanding of what is level, the gimbal also assumes that position and the drone will fly straight and true. I recently had a pile of slanted flight and offset horizon issues coupled with an extended warm up time. Fixed with a recal of both IMU and gimbal.

Guy in the video calls it "Internal Measurement Unit" --- not exactly inspiring confidence in a good explanation LOL

A video talking about calibrating an IMU on a level surface doesnt help me understand how it fundamentally works. That's like a video saying "A car travels down the road because the pistons are moving up and down". Not really helpful in knowing the basic electro-mechanics of what makes it do what it do.

 

[Nightjumper6mike]

Let's say you take a glass half full of water and place marks (sensors) on the glass at the LEVEL where the water touches. The sensors record this as a reference as to what LEVEL is. Basically the same as the artificial horizon instrument in an aircraft.
After the sensors have been registered, it doesn't matter what the water does as the sensors will know as water seeks its own level.

 

[Jiggly-Wiggly]

Let's say you take a glass half full of water and place marks (sensors) on the glass at the LEVEL where the water touches. The sensors record this as a reference as to what LEVEL is. Basically the same as the artificial horizon instrument in an aircraft.
After the sensors have been registered, it doesn't matter what the water does as the sensors will know as water seeks its own level.

OK cool --- but since there is no water inside a Phantom IMU --- how does it work to maintain level? I assume it must work on some tiny electronic device that can sense minute changes in gravity.

 

[Richard R]

The IMU has 6 sensors. The first three measure linear acceleration, the most significant is that due to gravity. These measure along the X, Y, and Z axes. When you calibrate the IMU, it looks at the signals coming for all 3 and assumes that those levels represent level. Once in the air, any variation in the ratios between the 3 are translated into angular tilt. The flight controller then moves the bird to bring the ratios back to the calibrated levels. The other three are angular rate sensors that are used to measure and control roll and yaw.

 

[tevek]

OK cool --- but since there is no water inside a Phantom IMU --- how does it work to maintain level? I assume it must work on some tiny electronic device that can sense minute changes in gravity.

It uses accelerometers that detect movement in three axis (x,y and z). When doing a cold calibration, the accelerometers will detect the position of the drone on a dead flat surface as being the reference point for when you take it out flying. This is separate from the barometer used for height calculation from HP, the compass used for the heading of the drone and the GPS used for its position in any given area.

 

[Jiggly-Wiggly]

The IMU has 6 sensors. The first three measure linear acceleration, the most significant is that due to gravity. These measure along the X, Y, and Z axes. When you calibrate the IMU, it looks at the signals coming for all 3 and assumes that those levels represent level. Once in the air, any variation in the ratios between the 3 are translated into angular tilt. The flight controller then moves the bird to bring the ratios back to the calibrated levels. The other three are angular rate sensors that are used to measure and control roll and yaw.

Ahh, now we're getting somewhere. It's perplexing though --- if the Phantom is pointed north and a wind gust comes along that is blowing west to east --- that would accelerate the Phantom to the east but the sensor would see that the same as a left roll and therefore increase RPM on the two left motors to compensate, thereby pushing the Phantom further east, right? But then the IMU would sense there is a right roll and therefore increase RPM on the two right motors to take it back to level. Very confusing stuff !!

 

[Monte55]

Isn't there a gyro in there somewhere.?

 

[Fester]

Go here, then look up flight controller basics. Explains basic flight controllers, IMUs, compasses, etc.
three-and-six-axis-gyro-stabilized-drones

 

[Jiggly-Wiggly]

Go here, then look up flight controller basics. Explains basic flight controllers, IMUs, compasses, etc.
three-and-six-axis-gyro-stabilized-drones

That is a really cool page for explaining the basics of how an IMU works --- thanks!

My nephew is 12 and starting to get into drones. I'm hoping I can explain the basic electronics to him so it doesnt seem confusing. I could pretty much describe everything to him except that **** IMU.