Further Enhancements

After the exciting flight last week, I had to repair the Quadcopter as one of the arms broke in a landing.  So I decide to make a few enhancements:

1. Put in a new APM controller and GPS to fix the Loiter issue.  This did not turn out well as the new controller was defective but the GPS seemed to improve things.  The new GPS is Hobbyking brand, old one was a "no-name" with some mods to make the cable work.
2. Improve power feed to the controller and receiver by using a UBEC (battery eliminator) instead of the power feed from the ESCs.  This is recommended due to the interference from the ESC and I do think  it improved things.
3. Get rid of extra wires in the controller wiring harness.  Done.
4. Install new arm.  Did this, I learned from the dearly departed Flying Flowerpot quadcopter that it is almost impossible to properly fix a quad's arm and they are reasonably cheap.
5. Performed a test flight and analyzed logs.  Seemed to improve things with few glitches on barometer and GPS and more stable flight.  Testing will continue.

Putting Pixhawk aside

Micro APM mounted on standoffs on plywood base

Wider view

After many days of debugging Pixhawk problems with ESC calibration, channel mapping, poor stability.. I put it aside and went back to an APM 2.7 unit for the SK450 dead cat.  The pixhawk seems like a nice unit, but the factory support was mediocre, the warranty is 3 months, and it seems pretty finnicky about ESCs, powering, and such.  Online support from forums is usually helpful but not always and not in this case.

Removing the Pixhawk was reasonably easy but the APM was a pain as it is a micro unit and is not nicely packaged.  I mounted it on standoffs as shown and replaced the GPS with a known good unit as well, let's see what happens.

New Drone is Down, Debugging Now

The new Deadcat drone no longer works.  I cannot figure out why and have spent about 6 hours trying everything.  It seems to be related to the Pixhawk flight control unit interacting with the ESC (speed controllers).  Below is a summary of the problem that I sent to 3DR, the designers and manufacturers of the Pixhawk.  I will let you know what happens.

--------------------------------------------------------------------------------------------------------------------------

I might have a bad Pixhawk autopilot.  I bought it in December and it worked fine for about a month.

Here is my config:

SK450 frame

4 x Turnigy plush 30A ESC (red power lead is disconnected - Pixhawk is connected to signal and ground only)

4 x Turnigy Multistar 2216-800kv motors

Pixhawk

3DR GPS module

3DR battery sensing module

Spektrum DX6i Transmitter

Orange R615X receiver

Various turnigy 2200mah batteries

Mission Planner on a PC for ground station

It worked like a charm before, reference this Youtube video (excuse the lousy quality).  Deadcat SK450 Test Flight 1

				Deadcat SK450 Test Flight 1
				View on youtu.be	Preview by Yahoo

No crashes, I was still test flying in the basement as it is 20 degrees outside with snow.  I was upgrading it as the R615X receiver has limited range.  So I  tried a new receiver, the Orange R800X, and never really got it to work, then I went back and did the following:

- reloaded arducopter 3.2.1 by first loading the plane software, then reloading the quadcopter software, also pressed the reset button on the pixhawk

- recalibrated all my ESC s manually one by one

- did many full setups: compass, accelerometer, radio, .. calibrations

- checked a lot of the parameters and adjusted them, then set them back if there was no improvement

- verified the radio was working by connecting servos to the receiver and also by doing the manual one by one calibration of the ESCs

- verified the pixhawk was seeing the radio inputs, and was putting out the correct outputs, such as increasing throttle on all 4 output channels when the radio throttle was advanced after arming

Here are the problems that I am seeing:

1. The USB connection fails 50% or more of the time.  I have to power cycle the pixhawk and/or plug and unplug the USB cable.  I tried two different USB ports on my laptop, no difference.  This is pretty consistent for the past 5 days.  It worked fine before.

2. The ESCs continuously complain that the initial throttle input coming from the Pixhawk is not low enough.  Here is a phenomenon I saw today regularly:

1. I calibrate the ESC by connecting it directly to the throttle output of the receiver, and go through the calibration procedure.  The ESC responds with OK tones.  
2. I try the motor still connected to the receiver, and it revs up and down with the throttle input.
3. I move the ESC input over to the Pixhawk, arm the Pixhawk, and then the motor responds properly after arming.  Note that I did not power down the pixhawk or receiver.
4. I power down the quad (radio, pixhawk etc.), and start it back up.  I get normal startup tones from pixhawk and the ESCs complain that they have no throttle input (this is normal as the safety switch is off)
5. I turn on safety switch, arm the pixhawk and get OK tone, but the ESC complains that the throttle input is not low enough again.  Motors will not function.
6. WEIRD!
7. I also tried calibrating one ESC through the pixhawk instead of connecting to receiver (step 1).  The ESC seems to calibrate successfully.  I execute step 3 successfully.  I get the same problem at step 4.

3. Parameters do not seem to be reliably stored in the Pixhawk.  It keeps resetting the battery failsafe settings and sometimes when I try to write a parameter change via mission planner I get an error "Bad Number", which is not very useful.

4. Now, I cannot get the Pixhawk to connect to either receiver, the previously used and flown R615X or the new R800X.  The radio calibration screen shows no activity, but the receiver is definitely connected to the transmitter as I test it with a servo or connect it to an ESC and can throttle the motor up and down.

I am stumped.  If I had my trusty Tektronix oscilloscope from EE school, I would check the signal on the pixhawk ouputs but alas, I don't have one.

Deadcat SK450 Test Flight

Test Flight Successful

Finished the basic build of the quad and was able to get a quick successful test flight in the basement after about 5 hours of messing around with the quad, my radio, and Mission Planner.

Finally worked out all the little issues and had a successful test flight:

1. The Deadcat needed to be balanced as it was naturally nose heavy.  Moving the battery allowed me to balance the quad.
2. I played around with ESC calibration and radio settings for hours because the props kept spinning when I armed the quad.  My old APM equipped quad did not do this.  Well it turns out that the default for Mission Planner with Pixhawk is to make the blades spin slowly when armed.  I did manage to program the ESCs to play tunes when they power on, but that is not much use.
3. Turning on the throttle failsafe is a waste of time as it is triggered as soon as you arm the quad, causing the arm function to fail.
4. Use loctite on all bolts, the one place that where it was skipped (landing skids), a bolt came loose.
5. Need to make up a cheat sheet with the meaning of all Pixhawk and ESC sounds and lights.  It is hard to understand failures otherwise.
6. Had to reverse all the RC channels except throttle.
7. It seems smoother than the last quad as the props are balanced but it is more twitchy on the controls, so will have to do some more tuning.

Progress

The quadcopter is coming together now that I have the proper parts for the Dead Cat configuration.  The arms are all attached although it was hard to get the screws to align, the new GPS is mounted on a foldable mast, and the ESC wiring is done (not shown).  I forgot to mention previously that I added carbon fiber motor mounts (about $8) as the fiberglass original parts are not very sturdy and break in a crash.

I am now waiting for a PPM adapter to arrive.  The Pixhawk only supports PPM input from the receiver (single wire interface for all channels) and the Spektrum receiver does not have this feature, so a conversion board is needed (about $8).  I am also waiting for a new camera and gimbal.

Dead Cat Config

Note the GPS in the center left of the photo

Carbon Fiber motor mount

Evaluation of Homebuilt Quadcopter Components

I put together a table with my evaluation of the components I used to build the quadcopter and the rough cost of the quadcopter: $320.

Part	Source	Rating (1=best)	Approx cost	Comments
Hobbyking SK450 Glass Fiber Quadcopter Frame 450mm	Hobbyking	1	$20.00	Well documented, rugged
TURNIGY Plush 25amp Speed Controller	Hobbyking	1	$13.00	Well documented, works well, does not overheat, used 4
Turnigy Multistar 2216-800Kv 14Pole Multi-Rotor Outrunner	Hobbyking	1	$16.00	Works well, used 4 units
Hobby King Quadcopter Power Distribution Board	Hobbyking	2	$5.00	Works well but hard to mount
Extended Landing Skid Set for SK450 Quadcopter Frame	Hobbyking	2	$11.00	Works but a bit too flexible
Slow Fly Electric Prop 1045R SF	Hobbyking	3	$3.00	Works, inexpensive, but prone to breakage
TURNIGY BESC Programming Card	Hobbyking	1	$5.00	Well documented, works well
Turnigy 2200mAh 3S 20C Lipo Pack	Hobbyking	1	$10.00	Good power delivery, good value
OrangeRx R615X DSM2/DSMX Compatible 6Ch 2.4GHz Receiver w/CPPM	Hobbyking	1	$10.00	Works well, good value, compatible with Spektrum DX4i Transmitter
HobbyKing HKPilot Mega Mini Combo Flight Controller GPS and Power Module	Hobbyking	4	$120.00	Flimsy USB, poor documentation
5" (inch) Travel Micro USB Cable, Twin Pack Cables	Amazon	1	$10.00	Great way to protect USB on APM
Hobbypower Top Pin APM2.6 External Compass APM Flight Controller w/ UBLOX NEO-6M GPS	Amazon	2	$70.00	Works great, great value, no documentation
Tenergy 1-4 Cells Li-PO/Li-Fe Balance Airsoft Battery Charger	Amazon	1	$20.00	works great, great value
2 of NEEWER® 3.5mm Gold Plated Bullet Banana Plug Connector for RC Battery 10-Pairs	Amazon	1	$7.00	Works well, used 2 packs
Dowels, screws, glue, velcro, flower pot	Home Depot	1	$25.00	Cheaper than hobby store
Total cost of built unit (not including broken Hobbyking Controller)			$319.00

Building and Flying Quadcopter Part 11

Quadcopter Drone ready for tethered test flight

Making progress, I found the link to set up the motors, speed controls, etc and got the wiring for the ESCs and motors set up properly.  Then I mounted the propellers and spent a while trying to figure out how to arm the drone, which I found at this link on the wiki.

I also needed a way to test-fly the drone, given my propensity to crash quadcopters.  I came up with a system using an old piece of plywood, with holes to allow rope to be attached to the drone landing gear.  The rope is wrapped around some tall screws on the plywood to allow adjustment.  This is shown in the photo above and actually worked out well.

Then our first test flight shown above.  The drone worked, although the controls need adjustments, particularly the throttle.  You can see me testing throttles, forward motion, yaws left and right, and left and right movement - all the basic movements.  So now it is back to the wiki and Mission Planner to find out how to tune the controls.

Building Quadcopter Part 6

Photo 1: ESC (speed control) and motor mounting

Photo 2: More detail on the ESC and motor

Continuing with the build, I attached bullet connectors to the Turnigy 25A Plush speed controls (ESC) using the soldering technique mentioned in the previous blog entry.  The motor leads were shortened to 2 cm to make the wiring tidy and to reduce latency.  I then used heat shrink to insulate the bullet connectors.  The Turnigy Multistar 2216 motors were also mounted on the frame.  The speed controls are connected to the motors under the frame arms and the connections to the power distribution panel are also underneath.  This is shown in Photo 1 and 2 above.