Hello,
BoBo asked a good question about the location of the gps and that made me look a bit deeper into the literature that Dji have produced. It would seem their might be a few misleading statements, I refer to the location of the gps on the M600 top deck. They say there is a virtual centre of gravity 26mm below the "centre frame" what exactly is the centre frame and do they mean the top and bottom layers of the frame or an imaginary place in between. 26mm is a precise dimension but the centre frame is not???
The Z axis for the gps is based on the COG of the craft, 26mm below the centre frame, then when you look on the Assistant 2 software, the location of the gps in the Z axis is given as minus 230 for the factory fitted gps antenna. If the cog is the start point for all Z measurements one might assume its a positive number but DJI call it a negative one.
Will any of the admins jump in with an explanation of the thinking behind the confusing numbers?

Ricci

Hi,
Finishing my ops manual (needed very badly, dealing with all the required paperwork to finally fly), but I still need to find out where the centre of gravity of my M600 Pro (+ronin mx+panasonic GH4) exactly is.
I know I'm not the first to ask but haven't seen many accurate answers so far.

Thanks a lot

Hi,
This is going to be a bitch to get exactly so give me some time to work a way to get it.

Ricci

ricci2 Posted at 2017-3-8 11:00
Hi,
This is going to be a bitch to get exactly so give me some time to work a way to get it.

Take your time. Already glad somebody answered

cheers

Kover Posted at 2017-3-8 13:22
Take your time. Already glad somebody answered

cheers

Hi,
I will need you to weigh the craft with all the bits on and then after that comes the string and maths.

ricci

ricci2 Posted at 2017-3-8 16:04
Hi,
I will need you to weigh the craft with all the bits on and then after that comes the string and maths.

Ok, I will do that this weekend.
You an engineer by any chance?

Thanks.

Kover Posted at 2017-3-9 03:57
Ok, I will do that this weekend.
You an engineer by any chance?

Hi,
yes, I design F1 cars in my time and fix and diagnose things the rest of the time for my sins.

ricci

Kover Posted at 2017-3-9 03:57
Ok, I will do that this weekend.
You an engineer by any chance?

Hello Kover,
For your paper work to be complete, does it ask for the centre of mass, or centre of gravity, they are different, not by much but different enough to make a difference.

ricci

ricci2 Posted at 2017-3-9 16:18
Hello Kover,
For your paper work to be complete, does it ask for the centre of mass, or centre of gravity, they are different, not by much but different enough to make a difference.

Hi Ricci,

Here are some numbers :
Weight of RPAS : 9,147 kg (6 batteries of 598 g included)
Weight of the Ronin MX : 2,15 kg
Weight of transmitting device attached to Ronin MX : 46 g
Weight of camera body : 480 g
Weight of lens 14 mm : 55 g
Weight of lens 42 mm : 130 g
Weight of lens 45-175mm : 210 g

They are asking me for the mass and the centre of gravity.

Much obliged,
Kover

Kover Posted at 2017-3-11 06:27
Hi Ricci,

Here are some numbers :

Hello Kover,
Thank you for the the weights. I have posted a corrected COG for the M600 and I hope when I get the drawings to do the same for your M600p. what lens do you want to run with for the figures?

thanks

Ricci

ricci2 Posted at 2017-3-11 14:48
Hello Kover,
Thank you for the the weights. I have posted a corrected COG for the M600 and I hope when I get the drawings to do the same for your M600p. what lens do you want to run with for the figures?

Hello,

I hope to use all lenses but for the figures let's say the 42 mm.

thanks,
Kover

Kover Posted at 2017-3-12 00:01
Hello,

I hope to use all lenses but for the figures let's say the 42 mm.

Hello Kover,
I have been waiting for DJI to send me the cad drawings for the pro but because the weights are similar to the M600 you might be able to use the info I posted for the M600 based on the supplied cad drawings.
I will keep you updated when I get the drawings.

ricci

How to measure and calculate CoG.

To measure the CoG in the Z axis you can do it by putting the craft on the side and measure the weight at the points where it touches the ground.  
On the pictures it would be at the landing gear (station a) and at one rotor (station b). See attached pictures and drawing.

To make the calculation a reference plane (0) is created. The distance from ”0” to ”a” cold be anything but to make it simpel it could be set to 1 meter if you are measuring in meters or it cold be 100 cm if you are measuring in cm.

Now you take a scale and place under the landiggear and make note of the weight or mass if you will. Next you put the scale under the rotor and make note of the weight/mass at this point.

It is importent that the craft is close 90 degrees on its side or you will not measure/calculate the correct massmoment hence the correct COG.

One more thing you have to measure is the distance from ”a” to ”b” which is basically the height from the ground to the rotor when the craft is placed on level ground or at least when all the rotors have the same distance to the reference floor.

Now you have all the numbers to put into the last equation to calculate the CoG from the ”0” reference plane.
To get the CoG from station ”a” which is the foot of the landinggear you just subtract the number you used as reference value from ”0” to ”a”, from the the number you calculated in the equation.  

(Moments can be added together and Weights can be added together but Arm lenghts cannot.)

NB: On the pictures the flightbatteries and battery for the Ronin-mx are not attached to the craft, which should be the case if you want to calculate the ready for flight CoG.

      The scale on the picture is only for demonstration because it is too small to handle the actual mass of the craft. so I don´t have any actual numbers to put into the equation!

ricci2 Posted at 2017-3-25 05:58
Hello Kover,
I have been waiting for DJI to send me the cad drawings for the pro but because the weights are similar to the M600 you might be able to use the info I posted for the M600 based on the supplied cad drawings.
I will keep you updated when I get the drawings.

Ok, thanks a lot

nil Posted at 2017-3-26 07:25
How to measure and calculate CoG.

To measure the CoG in the Z axis you can do it by putting the craft on the side and measure the weight at the points where it touches the ground.  

Thanks Nil, I'm sure this will help lots of people.
It's a pitty DJI isn't more helpful. Being the builder of this machine, I'm pretty sure they have all the necessary figures available.

Cheers

Nil, great method.  I've been thinking about this over the weekend...scary..as I am unqualified to do so. What is the point of inputting the Z COG since that will change after the landing gear retracts?  After adding a custom gimbal to ours  we left the Z at  -230 and noticed no irregularities. Alternatively, if one is more concerned with positioning wouldn't one be more concerned with the measurement from the base of the GNSS antennae/receiver....and considering that the M600 does not carry a survey grade receiver then the error of the Z will be more than the the total height of the airframe anyhow? Any insight is appreciated.

Beard Posted at 2017-4-3 10:42
Nil, great method.  I've been thinking about this over the weekend...scary..as I am unqualified to do so. What is the point of inputting the Z COG since that will change after the landing gear retracts?  After adding a custom gimbal to ours  we left the Z at  -230 and noticed no irregularities. Alternatively, if one is more concerned with positioning wouldn't one be more concerned with the measurement from the base of the GNSS antennae/receiver....and considering that the M600 does not carry a survey grade receiver then the error of the Z will be more than the the total height of the airframe anyhow? Any insight is appreciated.

Hi Beard,
Theoretical maths and real world maths often part company when extraneous factors influence the result unless all the varying anomalies are all taken into consideration as we know by trying to predict the weather. Kover, initially asked for the theoretical COG for some bureaucratic paperwork so he could fly in his country, why knowing the Cog of a craft that has a mass of 9.6kg has to do with things is beyond me, its probably the paper work for a full sized aircraft schedules. sorry off topic, you make a good point which is why I calculated using the supplied CAD drawing but alas it was only for the M600 but as a paperwork exercise it would be enough to give Kover the number to tick a box on the forms and it was a DJI supplied drawing.
The Cog of a dynamic object not on a fixed orbit and influenced by exterior vector forces like wind, pitch and yaw, will have very little chance of being calculated to any accuracy, however; with DJI’s altitude control mechanisms, stability is very good so maybe when the craft is geostationary it might have an almost static Cog,  then Gravity gets involved so altitude is a factor, the plight continues infinitum.
Nil was pointing out a practical way to find a static Z, coordinate to meet Kovers needs I believe and it will not relate to the X,Y,Z, coordinates for the IMU, because if they did every craft will be different due to the payloads. The Imu and flight controllers as far as I can calculate needs the centre of inertia for the craft as do the gps antennas, accelerometers because they all need a point in space to reference themselves too so they can calculate how much correction to apply to maintain an equilibrium around that point which is why the 230mm works. It puts the Centre of inertia around 26mm below the top plate of the M600 and the Cog 54mm below the bottom plate (No camera or gimbal).
Lastly I have experimented by moving the IMU and gps X and Y, position to see what effect it had apposed to the Z, position. Results: the Z, axis had no visual effect when moved 100mm down, the X and Y, showed visual signs of drifting when altered 100mm out. You can draw your own conclusions; however keep in mind this was a visual conclusion.
Sorry if I have confused matters.

ricci

ricci2 Posted at 2017-4-3 16:29
Hi Beard,
Theoretical maths and real world maths often part company when extraneous factors influence the result unless all the varying anomalies are all taken into consideration as we know by trying to predict the weather. Kover, initially asked for the theoretical COG for some bureaucratic paperwork so he could fly in his country, why knowing the Cog of a craft that has a mass of 9.6kg has to do with things is beyond me, its probably the paper work for a full sized aircraft schedules. sorry off topic, you make a good point which is why I calculated using the supplied CAD drawing but alas it was only for the M600 but as a paperwork exercise it would be enough to give Kover the number to tick a box on the forms and it was a DJI supplied drawing.
The Cog of a dynamic object not on a fixed orbit and influenced by exterior vector forces like wind, pitch and yaw, will have very little chance of being calculated to any accuracy, however; with DJI’s altitude control mechanisms, stability is very good so maybe when the craft is geostationary it might have an almost static Cog,  then Gravity gets involved so altitude is a factor, the plight continues infinitum.

No confusion at all. Fantastic reply. Thanks for sharing the results of your experiment!!

Beard