DJI s900 with 3DR Pixhawk - initial parameters and maiden flight

Hi again,

has been a while since my last blog entry. I already had some succesful flights with my s900 and the Pixhawk. It is a good and reliable team and I'm quite pleased with the performance. Nevertheless I want to go on with my build log to ease the entry steps for other pilots thinking about this combination. 

In this part of the build log I want to give you some hints on setting up some of the more important Pixhawk parameters before you do the first flights.

When you leveled the accelerometers and learned the RC controls in the initial setup, you might have tried to arm the copter already. I had an issue with a missalligned compass, where Pixhawk statet a “Compasses inconsistent“ failure. That means acceleration sensor's forward direction (together with internal compass) and external compass forward direction are off with more than 45 degrees misalignment.
This issue was a bit odd to solve for me. I tried recalibrations of Pixhawk, multiple  alignments of GPS sensor with different combinations of compass direction parameter, but I couldn't solve the problem for two evenings. Maybe I made some other mistakes and I can't tell what did the trick in the end but eventually everything worked out and s900 is performing perfectly. RTL, all flight modes and autonomous functions are working flawless. I find it a bit strange and even scary to fly and have not really figured out why it is working. But I assume there was something like a “Layer-9 issue“ (human failure) with not rebooting Pixhawk after the first correct configuration of compass and flight controller. So my final parameter settings with Pixhawk mounted in backward facing direction and compass mounted in forward facing direction looks like this:

 

So now with everything correctly configured for arming the copter you will most likely face the next special behaviour of s900 and Pixhawk combination. When you arm the copter the motors may not spin at all. The reason is that the DJI ESCs integrated at the end of the arms below the motor mounts are expecting a certain minimal PWM value for start rotating the motors. If you push up your throttle after arming (Be Careful!)  the motors will come to life nonetheless, but I have used to the behaviour that motors start spinning after arming. There are two parameters you will have to tweak for this. The first one is the “Mot_Spin_Armed“ parameter. This was added to APMs/Pixhawks by the developers in recent SW versions. Of course you can also leave that turned off if you like your motors stand still after arming, but find that a security issue as you won't be able to tell if copter is armed or not. OK there is a LED you can check, but this can be failing or covered by obstacles or cables. Six ESCs and motors failing at once seems a bit more improbable to me.
The second step to get your motors spinning when armed is raising the lower threshold of minimal PWM output at ESC connectors of the Pixhawk. The parameter for this is called “Thr_Min“. You will have to play a bit with the values until all motors kick in reliable after arming asthe ESCs seem to differ a bit for low PWM threshold. Even on one and the same copter. Maybe due to some part deviations of some resistors or capacitors on the ESC's input filters. I haven't investigated further. So after all my mot_spin_arm and thr_min parameters look like this:

If your motors all start spinning now after arming then CONGRATULATIONS! Now you are basically ready to start a first flight attempt if you haven't added any further equipment else as battery, Pixhawk, power supplies (UBECs) and receiver. It seems that without any gimbal or other heavy equipment the s900 flies quiet reliable on the Pixhawk's stock PID values.
(Please be aware that these initial values might change over time and in different SW versions!)

I have recorded some footage of the maiden flight for you all with stock PID parameters. You can tell from the video that these are OK for some first circles and testing. But take it easy and don't crash your toy on the first steps (BTW: have I told you already that I'm not responsible for you crashing your equipment when you are following my hints? HA! Now you read it and I'm not to blame for anything anymore ;) )

On last tip for pilots using the pre-3.3 software version for your Pixhawk:
The automatic gear functionality for retractable landing gear is to be implemented in 3.3 and ongoing versions. At least this is my up-to-date knowledge.
I am running version 3.2 when writing this. Call me a scaredy cat, but I am not willing to trying Beta-SW on a machine meant to carry 2000€ of photo equipment and weighing itself in at nearly the same amount of money ;)
So back to adding retractable landing gears to the pre 3.3 SW Pixhawk:
The workaround is, using one of the gimbal controls in the “additional gear" menu of APMPlaner2 (or whatever configuration software you are using). 

Select the according RC channel you want to use (in my case, Channel 8 of my RC transmitter) and select the correct output on the Pixhawks backside output pin bar. Counting of Pixhawks additional outputs starts with number 'RC9' on the right edge of housing (directly viewing on pin bar ). More info on this can be found here: http://copter.ardupilot.com/wiki/landing-gear/
When you have done that, it should look like this:

So that is it for today.

Now you  should have a DJI s900 combined with a Pixhawk that can at least fly and raise its legs. Isn't that a nice starting point for going on to a solid machine for carrying some weight?! In my next post I will show you all my setup for adding an XCAM A10 GoPro gimbal with 360° rotation ability.

Until then: wish you save flying, a nice Christmas and a happy new year if I can't make it to write the next post.
All the best!

Poor man's styrofoam cutter

Hi everybody. Still working on my DJI s900 and the next post will be posted soon hopefully, but for the time being I have another little project to share.

Two months ago I bouhgt a small Blade Nano QX. An eighteen grams leight and 14cm long & wide Quadcopter. Partly for praticing in my flat and partly for another small project next year, where I will  give a two hours evening lecture about Multicopters in german "Volkshochschule".
(Volkshochschule is some kind of public adult education center in Germany.)
But back to the Nano QX. It is a nice little quad an I wanted to try to use it for some special surprise on a company party. and this surprise is where I needed a styrofoam cutter for. You will see at the end of the post what it is for. :)

So let's start with the poor man's hot wire cutter.
You will need the following:

• Some thin copper wire
• Two bigger screws or nails
• Something to hold the screws/nails (I used my small vise)
• Your solder iron

Additionally I used some more wire and two 1.5 liter water bottles for tensioning the hot copper wire which you will see on the pictures.

So how is the setup for the "hot-wire-styrofoam-cutter":

• clamp the two screws into the vise
• wrap the copper wire around the first screw
• wrap it two to three times around the tip of your solder iron
• wrap the open end around the second screw

What you will get from this is a setup that looks hopefully something like this:

 As you can see on the picture I have wrapped some more wire around the neck of the 1.5 liter bottles and used these bottles as weights to tension the hot copper wire.
Here is another overview of this simple setup:

And a detailed view of the tip of my cheap solder iron:

Only thing you need to do now is heat up your iron as much as you can (I set mine to 450 degree Celcius). Your copper wire will, of course, get hot. And it will have several temperature regions with which you will need to experiment at cutting your styrofoam. 

Cutting will then look something like this:

The cutted styrofoam has some really decent edges and is not frayed in any way (falling appart into many small bubbles of styrofoam; like when you try to cut with a knife). Sometimes there a small and thin plastic fibres sticking on the cutted foam edges, but you can just pull them of.

So what can you do now with this setup?
I glued some printed paper on a 10mm styrofoam board from the local hardware store, cut it with my poor man's styrofoam cutter and the result looks like this:

Quite some nice and clean edgeds!
Stick this onto a Nano QX (or the quadcopter of your choice ;) ) and start to fly:

 

I think this looks really nice in flight and has a really nice effect when there is a small Smart vehicle flying around your flat ;)

OK, the readers with a sharp eye will write now that this is no paper/styrofoam-sandwich on this QX. And yes this is just the two printed paper pages glued together in that video, but only paper was to soft and sluggish during flight and lead to crashing the Nano QX over and over again.
I will post the final video of the flight with the paper/styrofoam snadwich here when it is finished. But until then I wish you mich fun with cutting styrofoam. :)

All the best!
Sebastian

Making Servo connectors

Making your own servo connectors

If there is one thing that I hated learning, it must be making servo connectors. Everything is small and fidgety and not very easy to assemble, i was a shear pain to begin with. Today it is one of the things i enjoy doing best. It has given me the ability to shorten or lengthen cables on my builds keeping the builds neat and tidy. It takes making quite a few cables to get it right, at times even cables that i thought were done properly, ended up being redone because of loose connections. Thats why i use a bit of solder on the wire. 28 gauge wire is relatively thin so using a dab of solder enhances the chance of a better connection. Anyway here comes a quick guide on how i make my header pin connectors and what you need to purchase to make them.

things you need
Terminal connectors female or male
28 gauge Silcone Wire
Crimping tool
Housing 1-6 slots remember to purchase the required amout of slots needed.
Wire cutters these are great use them for practically everthing, very cheap too.

Getting the stuff you need.

 

Making the servo connector 

DIY H4 home made frame

See the DIY frame in action, no tweeking done on the FC, flown by one of my students who is a complete novice and has no experience with quadcopters.

You might also want to read this article on the CC3D

Well was a bit bored so I decided to make a home made frame and see how it faired compared to buying a Ebay frame. So I went to the local hardware store and purchased a piece of 15mm x 15mm square wood measuring 2 meters in length. The length was a bit overkill, but could not find a shorter length, so i guess the rest will be used to repair the arms, when it eventually breaks. I also purchased a small square plastic box that is used by electricians to mount and combine electrical wires

 

Bought this cheap mounting plate

 

Measured and drilled four holes for the mounting plate used four plastic bolts and fastened the mounting plate with plastic nuts.

 

Mounted 4 plastic bolts at the top of the mount and tighten with a thin plastic screw. The screw will act as a stand-off.

 

Slotted the power distribution board on the mounting plate

 

used 4 plastic stand-offs to fasten the board and drilled four holes at the bottom for the power cables to the ESC´s.

 

Soldered the main power cables to the PDB,There is a small switch under the board which enables the power to be switched on/off. The board worked on its maiden flight but suddenly cut off and i could not reinitiate any 12 volts, i did not test why, but simply resolved the issue by resoldering and connecting the main power cables to one of the + and - from the ESC's, bypassing the on/off switch and making the 12 volts live at all times. This PDB is not sold with a manual so the Internet and common sense was the way i had to go by setting it up.

 

Soldered a JST XH connector to the LED pins, this will deliver 12 volts to the LED light strip that I will mount at a later stage.

 

I then soldered 4 positive and 4 negative wires using each corner and fed the wires through the holes in the housing. These wires will be later hooked up to four ESC's on the quad.

 

The ESC's power cables are now done, was a bit of a hassle as the soldering plates on the PDB are small and close together, make sure to check that your cables are not shorting.

 

Powering the CC3D can be done in two ways, I chose to use a UBEC, I soldered the + and - to the second LED pins/pads. The UBEC will convert the 12v from the PDB to 5v for the CC3D. The second method of powering the CC3D is by using the BEC on the ESC and plugging it directly to one of the motor pins on the CC3D.

 Method 1. using the UBEC, The + and - wire leads from the UBEC

 

 Plug it into motor channel 1, remember to get the polarity right + is the middle rail, - is the bottom rail closest to the edge of the board and the top rail is the signal cable to the ESC.

 

Method 2. Using the BEC from the ESC. Simply power the CC3D using the BEC of the ESC. It plugs into the one of motor channel, giving the CC3D the required 5v.

 

Mounted the CC3D and tightened it down with 4 plastic bolts.

Cut two sleeves and fed the signal cables to the CC3D.

 

 

Mount the cable for the reciever port, this will later connect to the radio reciever.

The cable for the main port, if you are going to be using other units like GPS, telemetry and so on. My setup is not using additional units, so this cable was removed.

  

Solder a female 3.5 mm banana plug on each of the power cables for the ESC's and cover it with shink tube  

 

Drill four holes in each corner of the housing, this will be used to mount the housing on the frame.
At this point you now have a almost ready to fly housing that can be removed on the fly and be used on another unit without too much hassle. Stage 1 done.

Building the frame.

 Cut two pieces of wood mine were 41.5 cm, try to keep the edges as straight as possible.

 

Drill two holes on each shaft, making sure that there is equal space on both sides. The more centered the housing is the better the center of gravity.

 

Using 25mm bolts mount the housing on the frame.

 

Tighten the screws firmly. 

 

To stop the frame from flexing too much, you will need to cut out and mount an adequate plate, using some thin light weight hobby wood. I used som Carbon that i had left over from a hobbyking H4. I cut the frame and drilled the necessary holes and tied it all together. if you don´t have an old frame, find something that is light weight. The length of the plate can be much shorter. Make sure to check the flex on the frame.

 

The top side. 

 

Both bottom flex plates mounted

 

Cut two more pieces of wood, They will make the arms.

 

Again measure an drill two holes, making sure that the arm is bang in the middle. You are going to need a top plate to stabilize the arm.

 

Mount the second arm in the same manor as the first. 

 

Make a flex plate for the top. 

 

Mount four motor mounts and a motor, one on each arm.

 

Connect each ESC to one of the power outlets from the housing, and feed the signal cable from the ESC up though the sleeve in to the housing 

 

Solder a header pin on to the signal cables or you can make your own header pins if you have the required tools

 

Mount the four signal wires from the motors accordingly

 

Mount the cables from the reciever port to the reciever.

 

Had to cut the lid of the housing as the wires stuck to far up. Anyways, not entirely done, the cabling needs to be done properly, for now the unit is ready for testing.

HP SimonK 30A ESC Brushless Speed Controller BEC 2A for F450 F550 Multicopter

Well these two ESC´s just arrived in through the door, thought i would give you a quick review.
The specifications for this ESC are as follows:

Highest efficiency 100% N-FET design.
No low voltage cutoff.
No over temp cutoff.
Super high refresh rate, no buffering of the input signal, resulting in more than 490Hz response rate.
16KHz motor frequency, giving fastest response of the motor, and quietest operation as well (no 8KHz squeal).
Super simple, foolproof operation! Nothing to program other than the throttle range.

ESC Specs:
Cont. current: 30A
Max. current: 35A
Input Voltage: 2-4S lipo / 4-12 cell Ni-MH/Ni-Cd
BEC: 5V 2A (Linear mode）
Size: 55x26x8mm (length 145mm including power cable)

Weight: ~22g

 

Very neatly made.

 

It has a printed manual with safety precautions and product specifications and a wiring diagram, which is actually a + in my book, many of these cheap ebay ESC´s are often sent without.

 

The ESC output has three 3,5mm female banana plugs, so you will at least need to have the male version to finish off your wiring. 

 

I hooked Both ESC´s to my home made test bench and they both fired up. 

 

This is where i needed some expert advice so my Co-editor Sebastian helped me do a test of a Turnigy multistar 20Amp esc and compare it to the 30Amp ebay esc. This is not a very precise test but it gives an indication of how effecient it it. Using an Iphone program AKlite. You need both motors running at the Same RPM, place your phone on the table next to your setup, use the same battery and motor to do the test. In theory the battery should be fully charged in both tests. Then you rev up each motor, to the same meter reading on the graph of the AKlite, this will indicate that the motors are spinning at roughly the same RPM. Now you can compare both readings from the Watt meter. In this case the Ebay ESC ran the same amperage as the more expensive Turnigy, but drew slightly less watts at low RPM. Need to put a prop on and test again under load to see if it is still more efficient. Under load the Ebay esc was even more efficient, but again, this is a very rough test and not very accurate.