GPS Mounting

[Kolb]

I am a bit confused with the mounting of the GPS unit on my XM8. Hoping that someone can help me out.

My IMU is placed right in the center (CG), so X Y Z settings are always 0. Correct?

The GPS however is located more to the front and right. And I have a bit of trouble understanding how to fill out the X Y Z figures.

X offset: 5cm (from CG to GPS boom)

Y offset: 11cm (from CG to GPS boom)

Z offset: 8cm
You measure from the CG (bottom of IMU) to the top of the GPS?

Is it that simple? I just want to be sure and double check with the experts before taking it into GPS flight

Cheers!

 

[ChrisViperM]

First step should be to find your real Center of Gravity.....this will be the reference point for your measurements of IMU and GPS. It should be in the center of your copter, but many times it is not the case. Don't measure from your IMU to GPS.....both measurements have to be taken from the Center of Gravity and be set in the Assistant software. COG -->IMU.......COG -->GPS Never do this: IMU --> GPS .... if your IMU is not sitting dead center on the Center of Gravity

Once you have found the real Center of Gravity, take your measurements from there - as for the IMU you measure to the center of the IMU. Everything moving along the green line in the Assistant software means negative figures.....to me there is no logic behind why I have to put a negative figure when the GPS is higher than the COG, but there is a lot of things not logical with DJI....just take it as a fact.





The second video is for the NAZA, but it will give you the right idea....:

Hope this helps


Chris

 

[Kolb]

Wow, thanks for that!

IMU:
X 0
y 0
Z -8

GPS:
X 5
Y 10
Z -14

 

[ChrisViperM]

Glad it helped....but you have your Center of Gravity 8 cm above the center of the frame....? (reffering to your IMU values on the Z-axis)

PS: Don't forget to find your Center of Gravity only if everything (LiPo's, Gimbal....) is already mounted to your copter !!!!


Chris

 

[Kolb]

You get me confused.

My CG is somewhere on my center plate. The IMU is located 8 cm (measured from CG to halfway IMU) above the CG. Hence the negative value.
CG was located with everything on it!

 

[ChrisViperM]

My CG is somewhere on my center plate.

That's good.

X-axis is forwards (+ value) or backwards (- value) ....looking from above at the copter
Y-axis is right (+ value) or left (- value)
Z-axis is down (+ value) or up (- value) ....looking from the side at the copter.

Let's say your IMU is mounted on the center plate, but 5 cm to the front and 3 cm to the left, your setting would look like this:

X = + 5
Y = - 3
Z = 0

If you mount the IMU at the same spot as your COG, your value would look like this

X = 0
Y = 0
Z = 0



The values you have posted :

IMU:
X 0
y 0
Z -8

.... would mean two things:

1) Your IMU is mounted 8 cm above the center plate....
2) Your COG is 8 cm below your center plate.....

I know it's confusing, but again: If your Center of Gravity is somewhere in the center of your center plate and your IMU is mounted somewhere on that center plate, you can't have a value of -8 on the Z-axis for your IMU settings....


Chris

 

[Kolb]

Hi Chris, this should be more clear. We are both right I guess

 

[ChrisViperM]

Cool.....seems clear now and sorted out. :tennis:

Just one question: By looking at your photo, it looks like the IMU is mounted on top of the Flight controller box....not so good. It should be away from other electonics as far as possible. Also make sure you have a good vibration dampening pad to mount the IMU.....the DJI IMU is very vunerable to vibrations....lot's of crashes due to that....that's why they had to come up with a vibration dampening unit for the DJI S800 / Wookong:

http://aerialmediapros.com/store/s8...s800-evo-vibration-dampning-absorber-kit.html

Chris

 

[Kolb]

The IMU is mounted on a separate mounting plate, you can't tell from the photo, it is an optical illusion
I just 3M doubled taped it to that specific IMU plate. They seem to contradict themselves because they explicitly tell you only to use the 3M tape to mount the IMU, especially to avoid vibrations.

I know about that special vibration dampening system that they came up with, but I am not flying a S800, but a Droidworx XM8, they are supposed to be more stable and vibration free... So I am hoping I'll be fine

Thanks for your help!

 

[ChrisViperM]

OK....then it's clear. Didn't want to sound like a teacher, I was just worried about your beautiful Droidworx......:tennis:

I wish you a lot of crash-free flights....



Chris

 

[Kolb]

Thanks Chris, hope to stay crash free as well!
I'll be traveling to your capital city in a few days, without the bird though

 

[ChrisViperM]

Enjoy your stay........ Weather should be great :tennis:



Chris

 

[saidquad]

I wonder if mounting GPS in a longer stick would be any good. I have mounted the GPS on 26cm CF stick (it is foldable) but I don’t dare to fly and test it. I just thought maybe it is good to have the GPS as high as possible (away from the rest of equipment). Any advice? Honestly I don’t understand why we should mount the GPS on a stick anyways.

 

[ChrisViperM]

The GPS is very vulnerable to Radiation Interference coming from all the electonics on the copter......putting it on a stick solves this problem most of the time. If the stick is too long, it will start to swing/vibrate and you have the next problem....GPS doesn't like to swing either....

That's how the MikroKopter guys are doing it: http://www.mikrokopter.de/ucwiki/en/GpsShield


Chris

 

[saidquad]

Thank you Chris. What height would you recommend for mounting GPS on a S800? I have heard that it should be higher than props. Is that so?

 

[ProPilotWannaBe]

Thank you Chris. What height would you recommend for mounting GPS on a S800? I have heard that it should be higher than props. Is that so?

I mounted mine on my S800 in the center on a 10cm stick. The main this is for the props not to interfere with the LOS signal, and get away from the noise of the other equipment as Chris said. To me, the 10cm accomplished both.

 

[saidquad]

I'll use the 10cm stick too. Thank you.