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Old Jul 15, 2012, 09:48 AM
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Originally Posted by bcollydas View Post
OK Blade then help me out here.
This is a quick diagram of my setup.
The bird is at work and I don't have it in front of me.
According to the diagram, what are the position numbers from the IMu and the GPS?

Right now I have my GPS set at -3cm for the Z axis.

Cheers
Bill
For Z value, just measure the distance from your estimated Cg to the GPS and and the IMU centre, these will be inputted as a negative values.
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Old Jul 15, 2012, 09:50 AM
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Originally Posted by bcollydas View Post
do you have a picture of the craft?
there u go
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Old Jul 15, 2012, 09:53 AM
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Originally Posted by Bengit View Post
Refer to pages 11,12 and 13 of the Manual 2.9. Everything refers to the C of G, its makes sense....If you have never compensated for Cg (large) changes I would suggest that you have been very lucky. The M will compensate for small changes.

I always try to get my Cg at aircraft centre (X and Y), this makes life easy when taking measurements, the only problem comes with estimating the Z value...this is the one that in most affected by increasing battery weight/adding gimbals and cameras.

The IMU and GPS want/need to know where the copter Cg is, NOT the CoC. Cheers
Go take your WKM multi fly it, land and then hang a lipo or some other weight off the end of one arm. You will quickly learn that the Gyros and Accelerometers do not care about where weight is, which is CoG. They will make the necessary compensations to get the IMU backt to zero point. CoG and Center of craft are most often similar and I believe that DJI tried to write a manual for beginners that don't understand the data that gyros and accs. need to do the job. So in an attempt to follow the K.I.S.S. method they actual caused you guys to start chasing XYZ rabbits ever time you add an ounce to the multi. This isn't my first rodeo.

I'm not the one having flight performance issues. My rig has flown excellent from day one. I have added FPV gear, changed props, changed cameras, and numerous other devices added. I have never readjusted the the XYZ and only gain adjustments. So as you've said.... "I've gotten lucky" many times.
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Old Jul 15, 2012, 09:54 AM
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Thank you Bengit, I will try that tomorrow.
So, the IMU will be -10cm and the GPS -13cm. Right?


Cheers
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Old Jul 15, 2012, 09:58 AM
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Originally Posted by epedemi View Post
there u go
epedemi,
pls check your z-values for your IMU and correct me if I am wrong but you have a positive value for your IMU which place it below the CG.
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Old Jul 15, 2012, 09:59 AM
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Originally Posted by tritan View Post
Correct me if I am wrong but do you have the IMU 4cm below your CG?
Looks like it, I would suggest that in most configs (IMU between Plates or on upper Plate), a positive value is not possible,..especially if a gimbal/cam is fitted ........UNLESS, LIPOs are fitted VERY high/above the IMU.
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Old Jul 15, 2012, 10:00 AM
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when i first got it, i just hung everything on, until i got a central CoG, as far as i can tell, are you not just measuring the GPS distance from the Centre? after all, the GPS only needs to know where it is in relation to the centre of the craft?
the GPS does not need to know anything in relation to CoG ?

eg if the GPS is 5cm back and you have 0 as a value, then you calibrate it, the GPS will think that where it is is the Centre of the craft? but in reality the craft is 5cm farther forward?

or am i being dumb? lol
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Old Jul 15, 2012, 10:04 AM
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Originally Posted by nicnaimless View Post
Go take your WKM multi fly it, land and then hang a lipo or some other weight off the end of one arm. You will quickly learn that the Gyros and Accelerometers do not care about where weight is, which is CoG. They will make the necessary compensations to get the IMU backt to zero point. CoG and Center of craft are most often similar and I believe that DJI tried to write a manual for beginners that don't understand the data that gyros and accs. need to do the job. So in an attempt to follow the K.I.S.S. method they actual caused you guys to start chasing XYZ rabbits ever time you add an ounce to the multi. This isn't my first rodeo.

I'm not the one having flight performance issues. My rig has flown excellent from day one. I have added FPV gear, changed props, changed cameras, and numerous other devices added. I have never readjusted the the XYZ and only gain adjustments. So as you've said.... "I've gotten lucky" many times.
Ok...whatever, just trying to explain what is in the manual (think you asked the question originally), happy flying.
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Old Jul 15, 2012, 10:06 AM
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Proof that X Y Z do not need to be changed based on distribution of weight ( CoG )
@3:30 we see the multi take a heavy load on one arm and no adjustments made. Gyros and Acc don't measure weight, only changes in space postition. CoG is often center of craft but not always.
DJI WooKong-M On Quadcopter Test collocetion (6 min 50 sec)
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Old Jul 15, 2012, 10:07 AM
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The best place, to have as little force as possible to change a multi in the air, is to have the CoG in the same plane/height as the propellers. For a quad that is a little bit higher than the frames and on a Y6 the same height as the center of the arms.

EDIT: And I am talking about the z- value here

This guy has grasped this and know what I am talking about - look at his multi:
http://www.rcgroups.com/forums/showthread.php?t=1599250
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Old Jul 15, 2012, 10:10 AM
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with CoG, you add your placement of stuff to gain a central CoG, eg, the craft sits level, so if your IMU is in a central postion, the only value would need altering would be possibly Z.

But surely if your placement grants you a level sitting craft, from varying positions, then the CoG is still central? therefore it is still heading back to Centre of craft?

Also, what do people think? would you advise to have the GPS as central to the craft as possible?
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Old Jul 15, 2012, 10:11 AM
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Originally Posted by Bengit View Post
Ok...whatever, just trying to explain what is in the manual (think you asked the question originally), happy flying.
It's been happy. I'm just trying to help. I've never had problems. Been at this for several years now. If you keep changing your X Y Z when the IMU is not actually being moved you are going to beging to have lots of bad wrong data coming from the Gyros and Acc.

I really think DJI made a big mistake when they wrote CoG in the manual in an attempt to keep thing simple and understandable. Lots have been led astray.
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Old Jul 15, 2012, 10:12 AM
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Originally Posted by bcollydas View Post
OK Blade then help me out here.
This is a quick diagram of my setup.
The bird is at work and I don't have it in front of me.
According to the diagram, what are the position numbers from the IMu and the GPS?

Right now I have my GPS set at -3cm for the Z axis.

Cheers
Bill

Hi Bill
since you pointing out the Diagram your C.O.G is between battery or ( middle of the battery ) then IMU starts from there and the GPS value would go higher( negative) and IMU value go lower( positive) in Dji assistant screen - this is the way i understood
Values are inputed incorrectly, it's still fly no problem IMU will work twice harder to balance the C.O.G also you will noticed during a take off and landing it may not straight up or straight down ( chicken dance)

Shiu
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Old Jul 15, 2012, 10:14 AM
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Originally Posted by Davebbb View Post
when i first got it, i just hung everything on, until i got a central CoG, as far as i can tell, are you not just measuring the GPS distance from the Centre? after all, the GPS only needs to know where it is in relation to the centre of the craft?
the GPS does not need to know anything in relation to CoG ?

eg if the GPS is 5cm back and you have 0 as a value, then you calibrate it, the GPS will think that where it is is the Centre of the craft? but in reality the craft is 5cm farther forward?

or am i being dumb? lol
All x,y and z references for the GPS and IMU are to the Cg of the craft (as stated in the manual). Anyone with RC experience (as you have done) would normally try and assemble the craft so that the Cg is at centre...hence by default, measuring the IMU/GPS to craft centre would be ok. As stated early, the only problem comes with the Z that can sometimes be hard to estimate when items are slung inder the craft. Just trying to help, A/C Weight and Cg is my job. Regards
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Old Jul 15, 2012, 10:16 AM
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Originally Posted by nicnaimless View Post
It's been happy. I'm just trying to help. I've never had problems. Been at this for several years now. If you keep changing your X Y Z when the IMU is not actually being moved you are going to beging to have lots of bad wrong data coming from the Gyros and Acc.

I really think DJI made a big mistake when they wrote CoG in the manual in an attempt to keep thing simple and understandable. Lots have been led astray.
Perhaps this is why so many are having issues with flight ?
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