IMU Calibration

[Explorocam]

Yes, you are correct that there are 2 sets of numbers. One is the Gyro and the other acceleration (accelerometers). The gyro measures the speed of the rotation and the accelerometers measure the position.

I just tried it. Look at the acc when sitting flat on the table, the X and Y axis read zero and the Z is -1 (that is, it is measuring 1 g as a result of gravity). Rotate the craft 90 degrees and hold it. Z goes to 0 and the axis about which you rotate will go to 1 (or -1 depending on which way you rotate it) because gravity is now pulling straight down on that axis (instead of Z.)

So if Z = 1 (or in this case -1) and X, Y read zero, then the system is level. If X or Y is non-zero, then the system (our copter in this case) is not level. Give it a try.

As to the drifting problem of Explorocam... Watching the video of your hover.... it looks pretty good to me. I would just dial in a couple of clicks of trim after takeoff. If you feel that is not an acceptable solution, perhaps check your CG? Are your props and motors balanced?

CG is dead on. I check before each flight, and no matter what I do to the calibration or what angle I calibrate at it continues to drift to the right. I'm skeptical about adding trim, because it may be interpreted as a slow roll. Anyways, shouldn't the adding the envelope under the right side have made a difference and stopped the right drift? I used to use the Open Pilot CC board and we could calibrate the Accel Bias, same thing as IMU and we could really dial the sucker in.

 

[FlyingMachinesGuy]

Try some static tests and you can see for yourself.

If the craft is not moving, the gyro numbers should all be zero regardless of the angle that it sits on the table. This is because the gyros measure rotational movement speed, not position. Turn it sideways, or at a 45 degree angle or upside down. If it is not moving, then they should be zero (or very close). If you rotate the craft, the gyro numbers reflect how fast you rotate it, not how far.

Accelerometers measure force along a linear axis. I doubt that a piece of paper will show much difference but if you put a book or something under the craft, you will see that the gyros are still at zero, and the accs reflect the tilt in the changes i mentioned earlier.

I'm not sure I understand why not to dial in a few clicks of trim... I mean... that is what they are there for. I have seen people get rid of far more than that with trim. It should have no effect on GPS or RTH as they are controlled by the NAZA and not the radio.

 

[kloner]

It's the wrong way to deal with any flight controller...... you make it see level then everything you have in your radio is set to zero, nuetral. Naza will misbehave on ya if you try to make up rules with her. IMU cali makes it hover straight no matter how crooked the install, that's it's function. it's easy to get bubbles under foam tape and get her crooked

i started out doing what your suggesting till it would fight it each time it booted up. drift this, that. then i started flying and drifting isn't much an issue. It happens for about 2 seconds when i take my eyes off the craft and onto a monitor.

 

[Explorocam]

Try some static tests and you can see for yourself.

If the craft is not moving, the gyro numbers should all be zero regardless of the angle that it sits on the table. This is because the gyros measure rotational movement speed, not position. Turn it sideways, or at a 45 degree angle or upside down. If it is not moving, then they should be zero (or very close). If you rotate the craft, the gyro numbers reflect how fast you rotate it, not how far.

Accelerometers measure force along a linear axis. I doubt that a piece of paper will show much difference but if you put a book or something under the craft, you will see that the gyros are still at zero, and the accs reflect the tilt in the changes i mentioned earlier.

I'm not sure I understand why not to dial in a few clicks of trim... I mean... that is what they are there for. I have seen people get rid of far more than that with trim. It should have no effect on GPS or RTH as they are controlled by the NAZA and not the radio.

OK, what you say makes perfect sense. However I have to ask, why would DJI include IMU calibration if we are supposed to add trim to correct for drift, shouldn't the IMU correct the drift? One of the Email I got from DJI said to return to the dealer for replacement if the drift continues. I uploaded the last two vids for them to see to make sure I'm not doing something wrong.

If trim is added to the Open Pilot it will roll. We were instructed to not use trim for this very matter. I know Naza and Open Pilot are two different systems and therefore I'm not certain. But I really do appreciate your help, and I'll give it a try. Will I have to add the trim each and every time or just once?

 

[FlyingMachinesGuy]

I agree with you that you want to get everything as straight and balanced as possible, but from the video it just doesn't look that bad. If the aircraft drifts to the right, what you have is a constant slow roll to the right. Adding a couple of clicks to the left just gets rid of that roll. If things are way off, adding trim is not the way to go. Besides... just try it. Take off, hover. When it drifts, stop the drift and add a couple of clicks of left. Fly around a little and bring it back to a hover. Dial in a little more or less. I am not suggesting radical changes, but slight adjustment are exactly what the trims are for. And if you are not happy with any trim adjustments, just trim it back.

Like I said, try it yourself. Plug in your NAZA and open the calibration window. Look at the Acc numbers and then rotate the craft 90 degrees and hold it there and look at the numbers. Rotate it upside down and look. If you hold it at any 90 degree angle (that is, nose down, tail down, right down or left down) then the accs will change to reflect the new attitude.

Then rotate it and try to rotate it 90 degrees in one second while looking at the gyro numbers. If you can continue to rotate it at that same speed, then the numbers will remain around 90. If you rotate it faster, then the numbers will go up (because it is in degrees per second.)

Empirical testing bears this out, but I must admit that this is one of those cases where I would like to hear from someone from DJI.

 

[Explorocam]

@FlyingMachinesGuy,

I'll try the Trim or Sub Trim idea, I'll also let you guys know what DJI says. I agree it's not that bad, I'm just puzzled that the IMU calibration is not stopping, adjusting or reversing the drift. It seems as though the calibration makes no effect at all. Perhaps I could also try calibrating while rolling it towards the left. I'd really like to figgure the Naza out as I am planning on getting the WKM in the next few months.

 

[FlyingMachinesGuy]

This is another thing that I would like to hear from DJI about. From what I can tell, the calibration window is just a monitor of the sensors, and only calibrates when you click the button. I have worked with a number of DJI systems and a few others, and if your calibration was off by a large margin, flying might be difficult if not impossible. A slight drift is not a major calibration problem.

@Explorocam... as for the trims, I usually approach it this way. A well built and balanced aircraft will fly with just minor adjustment and require little if any more afterwards. I would suggest to trim it in flight and don't worry about the sub-trims at first. If it takes more than about 10-15 clicks of trim, then I would begin to question other problems. If it takes a few clicks and it is stable, then that might be all that is ever needed. Then... just fly it. Fly a few packs and see what it does. If it requires a lot of re-trimming each flight, then again I might look at other issues. I have half a dozen DJI systems, all of which have required very minor trim adjustments and only have required re-trimming when I make changes to the structure.

 

[kloner]

Just remembered what playing with trim did,,, it was almost a year ago i was trying to figure these out. What makes atti mode fly flat and level destroys manual and vice versa. I've always left the ground in manual and that didn't work for me. Atti has such a disconnected feeling i don't like lifting off in that so it mattered to me....

 

[Explorocam]

Just remembered what playing with trim did,,, it was almost a year ago i was trying to figure these out. What makes atti mode fly flat and level destroys manual and vice versa. I've always left the ground in manual and that didn't work for me. Atti has such a disconnected feeling i don't like lifting off in that so it mattered to me....

Bingo! I feel the same way, I like to take off and land in manual, sometimes I even like to fly in Manual so I can make dives, therefore adding trim would make manual a pain in the butt. How did you fix it? If I remember calibration worked for you.
I really wish I could figure out why calibration is not working for me. Hope DJI will have an answer.

 

[FlyingMachinesGuy]

Kloner makes a very good point. Different modes may respond differently to trim adjustment. I might also say that our flying preferences play a part also.

I have a buddy that flys like his a$$ is on fire all the time. He almost always flys in manual mode. His copters are setup to be very quick to respond (twitchy to me). Whereas I fly about half of my flights in Atti mode and never switch modes and I like mine to be more stable. My copters have landing gear hanging down, his don't. I want to shoot stable pictures and video and he wants to see how many times he can flip it before it hits the ground. He and I have different expectations as to what is a "stable" hover.

Explorocam... after re-reading the thread and doing a little research and looking at your numbers, I have to wonder if your calibration is in fact working. What flight modes do you fly in? And do you fly take-off to landing in that mode or do you switch between modes in flight? Does it drift in GPS hold mode? Or only in Atti/manual? Or all modes? It just sounds to me like if your calibration was not working at all, the aircraft would not fly well, if at all.

 

[Explorocam]

@FlyingMachineGuy,

I fly in all modes. I take off in manual and then switch to Atti for flying, then at times I'll Loiter by hitting GPS. The craft does not drift in Manual if I leave it level, and it works great in GPS hold, (holds perfect). Sometimes I let it fly back in return to home and land on it's own if I'm too high or too far to trust my skill, (it also flies great in autopilot/return to home; in fact it lands within a half meter from where it launched){Perfect in my opinion}. The only time I get the drift is in Attitude.

Here is what DJI Innovations said in response to my Email and videos:

Hi, William,
In Atti mode, it would drift slowly, but obviously yours is too much.
You can either contact dealer for replacement, or

raise the right side of IMU by adding a little sticker under it, not your frame and do not calibrate IMU. See if Atti fly better.

That's what DJI said.

 

[drdrums]

@Explorocam - interesting thread, my machine responds the same way! Definite drift to the right in atti mode; if I steady it in a hover and let the cyclic go neutral, it will start drifting right and pick up speed unless I give it left cyclic. To get it to sit still, I need to keep in a constant left cyclic.

Manual mode, though, there's no drift, flies like a stable heli. (I don't have the gps module.) I don't want to trim it for atti, because that will result in a constant roll left in manual. I can hover it almost hands-off in manual.

I have tried numerous times to calibrate the IMU, both with and without a level, raising the offending side. No difference, always flies the same. No CG problems.

If try shimming the naza box up (so its always at a slight angle relative to the frame), will this fix it? Will it screw up manual mode?

Also, I see a pattern here - everyone's machine seems to drift right (if it drifts)! No one appears to have left drift. Are people always in atti mode when getting the drift? Any in manual? Maybe there's a quirk in the naza design?

 

[DesJardins]

Is your compass on the right?

Did you recalibrate that compass after the IMU calibration?

 

[Explorocam]

@drdrums,

Good reply. Yes this very aggravating for me to. DJI's advice was to get a replacement from the dealer. The Dealer will not send me a new one until I send them the old one because they don't get their Nazas from DJI but rather a distributer. DJI has tried to be helpful while RC Flight Store in Vancouver has been little to no help at all. I can't send the Naza in because I need it right now and finances are tight enough that buying a new unit with the hope of the dealer/distributer refunding me for the drifting unit is not an option either.

So, it would seem that the IMU calibration isn't working and if you are like me, this problem never really occurred until you did the latest update with the calibration. Therefore, I'm suspecting it's a problem with the calibration update. DJI's other advice to this problem other than sending it back to my useless dealer was to (as you said) place a small shim under the left side of the unit and keep trying until it hovers level. Kind of an analog approach to a digital problem. This should not screw up manual because manual is like "axis lock" wherever you let the stick go that's the orientation the Naza will sit. If I'm not mistaken, Manual mode will not auto level, that's why you can invert the it.

@DesJardins,
Quite sure it's not the compass, two reasons: 1) drdrums doesn't have the compass on his unit, Naza only. 2) I have the compass but know that its calibrated because I don't get any drift or toilet-bowl effect in GPS mode. When I let the sticks go it sits there beautifully stable and I can fly in GPS mode to.

Frustrating on two fronts; the issue with the Naza's drift and the issue of the dealer treating me like a punk. Will have to deal with another dealer when I switch to a different dealer, cause it will never be RC flight Store again.

 

[kloner]

it is smart to buy from dealers listed on dji's website

 

[DesJardins]

I just like tossing out as much info as possible... Because these issues comin up are many times the person not knowing about the IMU calibration or the fact that when you change one thing you should re-run the assistant and make sure all settings are re-written even if you don't think it's needed.

I am curious as to why you need a Naza that's not working properly?
I'd have it out the door that day, never deal with that "dealer" again a my dealer sent me a replacement item right away before I sent my faulty item back.


Hobby-drones.com was amazing...

 

[Explorocam]

So if I follow your logic to it's logical conclusion you are saying I'm not smart. However, I think it was more of a case of lack of knowledge pertaining to the subject matter. Further, there are no register dealers in Canada. Everyone in Canada has to go through a distributer.

P.S. I'm not hurt or angry, I'm just busting your balls. :triumphant:

 

[kloner]

It's not that anybody isn't smart, just un educated to what the difference is between there dealer levels. Almost all of em get it from distributors and any service is straight out of pocket. there other dealers just swap stuff out no questions asked and they deal with getting it back and getting credit. Theres 2 levels. i'd still buy from somebody on that list if i was on the moon and the moon didn't have one in my planet,,,,, usa, china, take your pick

there is always a dealer out there that gets it and is doing the warranty part themselves outta pocket, but you'd have to ask around to find em...... personally i've never tried dealing with there customer service, but i picture it like align or any other chineese company, a whole different outlook on business practices and just a different moral value on the whole situation. anybody down a week over something working right is being neglected

 

[DesJardins]

So if I follow your logic to it's logical conclusion you are saying I'm not smart. However, I think it was more of a case of lack of knowledge pertaining to the subject matter. Further, there are no register dealers in Canada. Everyone in Canada has to go through a distributer.

Is this directed at me?
:-I

 

[Explorocam]

Is this directed at me?
:-I

No no, I was just busting Kloner's balls for fun. No biggy.