Thursday, March 5, 2015

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
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

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.

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