newbie setup questions

schwett

Member
i picked up a (cheap, I think) s800, wkm, two li-po batteries, and futaba 8j transmitter to do some aerial photography. not video. i'll be using a custom mount for the camera setup which weight about 2.5lb. I don't need a video downlink, gimbals or servos, etc. i'm not too concerned yet about vibration as the shutter speeds will be very high, but i may look into dampening later. i put most of the kit together today and have some questions.
i had to replace the connectors on the batteries with xt60s which i found at another local shop (the place i bought the gear was out of stock.) to run two batteries, i assume i'll need a y-adapter and also another battery tray? without a second tray, it seems strange that the standard battery tray puts such a heavy object so far off center. is that really OK!??! i'm also not clear what the preferred way to power the PMU is. another splice from the batteries?
the screws which hold the mounting points for the two small struts on each leg of the gear are claimed to be M3.0x22 in the instructions. there's no way an M3.0 screw will fit in that hole.... is this a mistake in the instructions, or did i somehow mess it up? otherwise it all goes together just fine. it's the screw coming into the yellow dashed box from the right side in this illustration:
s800.PNG

finally, is it considered better to attach a camera mount to the two bars at the tops of the legs (which are directly connected to the arms by the locking connectors) or to attach it directly to the bottom plate of the center frame? if the latter, would people advise making new holes or using a couple of the 19 which already exist to hold the two parts of the center together? they're pretty small threads and i'm not sure about supporting a heavy-ish load that way.
thanks in advance for any help... i did a quick search on the forum but didn't find anything obviously relevant.
 

schwett

Member
got into the air today. realized the stupidity of my question about the power for the PMU. the big obvious cable coming from the base plate marked "PMU" would do it. i decided to fly with just one battery and used a bit of electrical tape in a few places on the skids where I still don't seem to have the right screws.1
the calibration sequence is a little confusing the way it's described in the manual. the axes they refer to them are rotated from what i'd have called them. rotating "around" the horizontal axis would mean to me that if you're holding the copter level facing forward, you'd be spinning it nose down, then back towards you, then around. not spinning it around from the "top" view, which seems to be the first step of the calibration sequence.
i tried flights on manual, attitude, and gps. first few hops seemed very unbalanced - the copter was tilted perhaps 20 degrees as soon as it was in the air and I had to bring it right back down. I adjusted the center of gravity with the assistant and it seems better. any tips on how to accurately measure the CG? I just used the edges of my fingers to balance the copter on the edges of the base plate, one in each axes, and then used the intersection of those two imaginary lines. the CG was slightly in front and to the left of the IMU. I would like to balance it better - i'm thinking a second battery tray and battery on the other side.
my takeoff and landing surface was slightly sloped, and the first couple landings the copter dumped over on it's side. the props seem no worse for the wear. they're surprisingly resilient.
in gps mode it doesn't really hover completely level and stable. should it? it tends to lean to one side or the other and very slowly rotate.
I did not try adjusting the gps/compass orientation or position.
here's a video illustrating my last test flight. on the last couple test flights I was able to land without tipping over! you can fairly clearly see right after takeoff the leaning and rotating behavior.

 
Last edited by a moderator:

schwett

Member
any reason not to use two of the DJI battery trays, one on each side, with two matching batteries connected for balance? it doesn't seem that two 5000mah batteries weigh much more or less than a single 10000mah battery.
 

Tahoe Ed

Active Member
The S800 with WKM can compensate for the unbalanced state of the battery. I am not sure that I saw much drift, you did not just let it settle into a GPS hover and then let it sit. That tells a lot about what is happening. However, have you rotated your GPS for the magnetic declination where you are flying? That can account for drifting, i.e., the Toilet Bowl effect, circular rotation around a point. The manual describes how to do this. You may want to take a look at it. http://www.dji.com/download/wookong-m-downloads/ You may want to reconsider not have a video down link. Being able to frame a shot and then trigger it from you Tx is a positive thing.
 

schwett

Member
The S800 with WKM can compensate for the unbalanced state of the battery. I am not sure that I saw much drift, you did not just let it settle into a GPS hover and then let it sit. That tells a lot about what is happening. However, have you rotated your GPS for the magnetic declination where you are flying? That can account for drifting, i.e., the Toilet Bowl effect, circular rotation around a point. The manual describes how to do this. You may want to take a look at it. http://www.dji.com/download/wookong-m-downloads/ You may want to reconsider not have a video down link. Being able to frame a shot and then trigger it from you Tx is a positive thing.

thanks for the reply ed!
it doesn't seems like it is fully compensating for the off-balance battery - it's tipped down to that side. if it was fully compensating would it be level, or just stable in position?

I did read the part in the WKM manual about the toilet bowl effect... but I didn't quite think that was what was happening. it seemed less consistent than that. is there a simple calculator to know how much to rotate the GPS/compass module? the manual doesn't specify, just a range of 10 to 30 degrees.
 

Tahoe Ed

Active Member
What gains are you using for Basic and ATTI? I would recommend that you use the gains as specified in the manual. That might help. If you are running default at 100 across the board that could be part of your issues. Go here: http://www.ngdc.noaa.gov/geomag-web/. I am in Las Vegas and I am 12 degrees of rotation clockwise.
 

schwett

Member
i doublechecked the gains and for some reason the settings hadn't stuck - they were all at 100.
i corrected that, replaced the slightly squishy tape mounting the imu with more rigid tape, and tightened a few other things up in the landing skid.
magnetic declination for san francisco is about 14 degrees clockwise. i wasn't sure which way that meant i should turn the GPS/compass, so i experimented a bit. still having some troubles:
it's still rotating in the first couple test flights. i think i *may* have gotten the compass oriented properly for the last flight, which starts at a minute in the video, but it seems to have become more unstable and started to flip over almost immediately. my intention was to just take off and let it hover in GPS mode every time, but it really wouldn't sit still.
i recalibrated it at the beginning of the session and then once in the middle to try and rule out bad calibration. as an aside, last time i had no problem getting into calibration mode, this time it took many tries and the LED was not quite responding every time to the mode toggle. otherwise controls seem very responsive.
 
Last edited by a moderator:

PeteDee

Mr take no prisoners!
Even at 100 the craft should hold, just be a bit squidgy.

I have never worried to much about MD, do your compass calibration and advanced IMU cal and you should be good to go.

Pete
 

schwett

Member
Even at 100 the craft should hold, just be a bit squidgy.

I have never worried to much about MD, do your compass calibration and advanced IMU cal and you should be good to go.

Pete

that's what I thought too. obviously something is not right.

it is far from being able to hold position and attitude in GPS mode. i didn't try atti or manual this last time. any suggestions!? it looks like the props are spinning at radically different speeds, but that may be an artifact of the video sampling.
 

schwett

Member
that's what I thought too. obviously something is not right.

it is far from being able to hold position and attitude in GPS mode. i didn't try atti or manual this last time. any suggestions!? it looks like the props are spinning at radically different speeds, but that may be an artifact of the video sampling.

i made some adjustments and thing are much improved. i mounted the second battery and a camera, adjusting the position of the two batteries to compensate for the camera being very slightly off center between the bipod legs. this put the CG right on the center in the x and y axes and about 5cm below the IMU in the z.

i think i had the IMU and GPS/compass positions way off; i failed to take into account the z height of the compass, and may have mixed up the other directions. i did it more carefully this time, recalibrated, and now she holds very, very steady in attitude mode.
still don't quite have the hang of the controls. it seems like it should be possible to rotate around while maintaining a fixed position, which so far i can only do in one direction.
i took a few shots - not with the final camera i plan to lift - but just to test. i don't think vibrations will be a problem at the shutter speeds i'm shooting at. focus may be a little harder to achieve remotely. these were manually focused prior to takeoff, which is obviously not ideal.
4087-aboveFortMason-2048.jpg


4160-fortMasonAerial-2048.jpg
 


schwett

Member
making some progress. properly balancing it and more carefully setting the imu and antennae positions may have been the trick. need some solution to constantly attaching and reattaching the compass. i epoxied both ends of the stalk to the fittings, and have been using tape to stick it on to the top plate. i'm thinking I might cut the stalk in half and epoxy some kind of screw and socket fastener to each half.

i took the height and distance limiters off after my last session (for a moment I bumped up against the 15M height limit and couldn't figure out why it wouldn't go higher at full throttle, lol)'GPS mode still seems dodgy, need to experiment with it in a much bigger field.


quick shot I took a few days ago.
4222-oldenburg-2048.jpg



it's a bit tougher this time of year since there actually people out and about when the sun rises and the light is good...


how sensitive is the "calibration?" I assume if i fly a few miles from the clear spot where calibration was done it's not going to matter.
 

Top