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Mar 06, 2007, 11:43 PM
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Digital modulation flight test


So let's get to the point. After 2 weeks of development, 50,000 dead marines in Iraq, and 1 broken fingernail, finally flew with all the controls passing through digital modulated radio and custom firmware. The digital modulation flight was the most dangerous, difficult, time
consuming flight test in U.Know.Where. since 1940. Merely thinking of what the motor would do on that first test made you want to extricate
yourself from Earth.

So we tested and tested as much as possible with bare servos, without powering the motor, without the rotor, and finally with the rotor. It
was so heavily tested, the first flight was as smooth as can be for a copter with a 12oz payload.

Other than the weight issue, flying took some getting used to. The heavy payload, inability to see the LED's with the payload attached, and
clunkiness of the digital modulation firmware made the first flight percarious indeed, but as long as you kept the speed down, all the usual
maneuvers were eventually accessible. Still didn't dare fly over 30 ft away.

Miraculously, the firmware didn't crash at any time during the test flight.

Yes the custom firmware control is very clunkly. There are longer delays in propogation from stick to servo. The stick sampling rate is definitely slower even though the radio bandwidth is 40-50 commands/sec. The servo precision is probably identical to stock firmware but it's hard to tell because of the sampling rate. By the end
of the flight, it was flying roughly as well as it did under stock firmware.

Solving the RF emission + stick potentiometer crosstalk required relocating the antenna away from the electronics. That eliminated most
of the crosstalk but not all. If you touch the antenna, the motor will start. Fortunately, turning off the transmitter guarantees the motor won't start.

Really wanted a range test using a 50,000 candlepower halogen to indicate the autopilot enable state. It worked in the dumpy apartment
but never in the field no matter how many times we tried. The transmitter firmware was getting erased every time it was transported.
This phenomenum has happened with PIC's for the last 3 years and its cause is still unknown.

Modified the transmitter's bootloader to resist spurius erase commands and moved all the pulldowns to 1k to resist RF emissions. Theoretically, the RF interference was sending its bootloader into an erase loop. That solved the problem, but not in time for the halogen
test. Only 1 more flight test because it was time to head back to Silicon Valley.

The Gumstix is required to process the servo commands and was in full attendance for this flight. It takes several 32 bit multiplies and
divides to convert stick position to duty cycle. With its 30 sec initialize time, knowing when the gumstix is initialized is still an issue.

When fixed to the copter, the Gumstix entered reset every time we powered up the transmitter and every time we did extensive servo
commands. Switching the servo power off the Gumstix bus and onto the speed controller bus solved the problem for now. Seems the servo twitch on transmitter boot and any excessive servo movement may have glitched the Gumstix bus enough to trigger a reset.

Then, the ESC would not initialize. To initialize, it ended up requiring a minimum duty cycle. Really really wanted a minimum duty cycle of 0 for those first motor tests, but the ESC would not be
denied. Gave it the duty cycle for minimum servo deflection and it booted up harmlessly, FOR NOW.

Finally, the broken fingernail was not from an aircraft part but from the CPU fan on the battery charger. Need to unplug it before touching
the cell count switch instead of after. What country puts out this painful sack of protoplasm anyways?

For the range check, spooled up the motor. You see, the receiver is programmed to slowly spool down the motor if it doesn't recieve a valid packet in 1 second. This algorithm was tested and retested with servos.

After only 317ft, the custom firmware worked perfectly, gradually spinning down the motor as predicted. Sadly, the range with digital modulation was similar if not less than the range with stock modulation. It seemed to drop off suddenly, unlike stock modulation. It's hard to compare because the stock modulation just destroyed your model if you ever lost contact.

Incidentally, this business of measuring gap sizes to send data is called phase modulation by the pros. Every day, an improvised phase modulation circuit blows up 10 Americans who have no clue what phase modulation is.
Last edited by Jack Crossfire; Mar 07, 2007 at 12:09 AM.
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