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Sep 27, 2013, 01:38 AM
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The microquad honeymoon is over

It's every bit as crash prone & fragile as expected. Every day of flying requires a bag of propellers. The propeller mounts tend to break off.

Not sure exactly how to repair the motor mounts. Glue isn't very robust. You can't replace the motor mounts without replacing the entire frame, which takes 1 week to rebuild. They may eventually need plywood doublers. Discovered the propellers can sometimes be bent back into place, after a mild crash.

Microquad Crashes (1 min 1 sec)

A sudden tendency to flip over in normal flight appeared. A motor+ESC was replaced but found not to be the problem. Increasing the I2C speed to support the MS5611 was not the problem. The ESC's were not malfunctioning. The turboPWM wasn't too fast. The autopilot wasn't overflowing or byte wrapping.

The problem was tracked down to the attitude control feedback. The PID limits have to be extremely low, compared to brushed motors. It took recording the PID values & PWM values on the ground to notice at what offset a flip would occur.

Since the microquad was too small to mount on a test stand, the only way was to fly as low as possible, wiggling the cyclic to try to get it to flip, & accepting propeller damage.

The px4flow seemed to do a lousy job separating angular motion from horizontal motion. It uses a lowpass filter to get the gyro centers. It reads only 1 noisy gyro value for each frame. Then it has a complicated set of rules for subtracting angular movement.

The mane requirement is the angular motion has to be above a threshold. Then the angular motion has to be in the same direction as the pixel motion. Then the angular motion clamped below the detected pixel motion. Some of the limits & constants are configurable, using mavlink packets.

There was a failed attempt to remove angular motion outside the px4flow, using the mane gyros. That caused crazy oscillations. So the calibration in the px4flow would take some doing to improve upon.

Some autopilot flights below 1m were failures. It just flew away without detecting position.

The PX4flow has presented a case of far sightedness. It's most accurate at 1m altitude. Below 1m altitude, it has a very hard time detecting position. Above 1m, the sonar falls over. The focus can be adjusted for lower altitude, but it has to fly in a narrow altitude band. That's the price of a very high magnification, high brightness lens which can detect surface features from far away.

A 2nd camera would have benefited the low altitudes, but there wasn't enough planning or time to discover the PX4flow limits & design a 2 camera optical flow solution. Planning ahead & expecting the worst are skills you need in the struggle of life.
Last edited by Jack Crossfire; Sep 27, 2013 at 03:28 AM.
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