mandag den 17. august 2015

DJI s900 with 3DR PixHawk - frame assembly hints

Hi everybody. Sebastian again here on Peter's multicolor blog.
It is time for my next blog entry. This time I want to let you take part in quite big project: building up a DJI spreading wings s900 helicopter for professional usage with a 3DR Pixhawk flight controller.
DJI is delivering no printed documentation with its s900. You have to download it from the web or you can have a look at one of the build videos. I will only give some tips here for the first part of my build log.

First: I was not very sure what has to be in the package prior to delivery. DJI is delivering more parts than you need for assembly. And this is not only true for some spare prop blades. So don't be to curious when you have some screws left over, when the frame is assembled. What was left after my build is:
  • Spare bumpers for motor mounts 
  • Spare pairs of prop blades with plastic washers, regular velcro tape and velcro battery straps

  • Spare battery connectors and rubber rings for landing gear

  • Spare red plastik fasteners for locking motor arms in place 

Second: even when DJI is delivering more than you need, the opposite is also true for things you should have at hand when you start the build. What I did find handy and useful is the following:
  • Loctite (blue one is sufficient)
  • Allan keys in several sizes
So on to the main tips for the assembly:
Assembly is quite straight forward. There is a high degree of preassembly in your delivered box. I.e. the mechanics and servos for the retractable landing gear are already mounted at the center frame plates.
Before you are too enthusiastic when starting the assembly and you directly trying to puzzle your way through sticking the landing gear into the frame's mounts, make sure to slide on the thick black rubber rings onto the main carbon gear beams. I missed that in the first excitement and now my gear has gone through one disassemble and reassemble more of the ones of most owners during the first build day.
For mounting the landing gear to the foldable landing gear mounts, you need to use the screws with the very flat heads. After mounting the gear, you will some screws left over as spare parts (see fotograph)

Also don't forget to add some loctite to all screws you mount to the gear and frame. DJI has prebalanced the propulsion systems to minimize vibrations from the start on, but safety first! And I read about some owners who had to sent their frames back to DJI for recalibration because of massive vibrations.
With the gear mounted to the frame, the next (and last) parts waiting for attention are the motor arms. Arms are numbers from M1 to M6. Starting from M1 with counter clockwise motor rotation and red color as the forward right arm. Next is the second red arm with clockwise prop rotation on the front left. And then following the black ones with alternating motor rotation (ccw, cw, ...) in ccw direction around the frame.
What was very important here at my frame: checking that the upstanding plates between the upper deck and lower you mount the arms to are aligned correctly (see photograph).

There plates are only mounted with one screw and therefore can pivot around their vertical axis. One plate at my frame was slightly out of correct alignment and therefore the thread of the arm mounting screw grabbed in a wrong way into the thread. I might have screwed up one of my threads if I hadn't checked again when the screw was only very hard to drive in.

If you have a bad feeling about something during any build, always double check what you are doing. It's always very upsetting when you damage your expensive equipment because of too hasty decisions or laziness for not a wanting to read some instructions again.
So with the arms mounted correctly, it is time to connect the power and control cables. The connectors for that are located under a flat covering dome on the lower frame plate. This cover is loosened with 4 screws on the bottom of the lower frame plate.

When you have removed the dome cover first, you can connect the power cables of the rotor arms. For that DJI added a special L-shaped tool to tighten the cable lugs into the power mounts. DJI gives the advice to only tighten the screws as far as aligning the edge of the screws to the edge of the mounting brackets. At my frame the clamping force reached with that screw tightness didn't feel very reassuring at some of the power bracktes. With some vibrations always left during regular copter operations, I am not very excited to find out about an easy loosening power connection for two or more of my motors during mid flight. I rather buy a new power bracket or even a new lower frame plate than loosing a complete frame with camera because of a weak power connection (See next picture at the copper brackets. Black screws not in line with edge of brackets).

Second to last step is connecting the esc control cables. That is straight forward as DJI uses poke yoke style (can not only be mounted in correct direction) servo connectors for the ESC  control cables.

Last advise is to double check the fitting of the power cables into the precut holes of the cover dome. There is a chance to damage the silicone isolations of the power cables if you remount the cover and don't double check the power cables fitting. The following picture shows one of the black ground power cables with is exactly positioned WRONG. It would be sqeuzed and pinched if not corrected before retightening the cover dome.

If you have finished mounting the copter's  arms there isn't really much left to do. Mounting the GPS holder (if you use the one delivered from DJI). Mounting the inner part of the upper carbon fibre plate again. And that's it. You have finished the basic build up of your s900 frame.

The next steps like mounting an Atto-Pilot 180A current sensor and building an antivibration mount for the pixhawk flight controller into the s900 will follow in some next posts.

Have fun flying!