While working with the folks at HeliFlight Center, we have discovered a potential issue with the Lemon Rx DSMX 8ch PPM receiver that can cause a "lockup" condition of the Pixhawk Autopilot.
A recent customer presented a multicopter "build" for troubleshooting incorporating the Pixhawk autopilot and a Lemon RC DSMX 8ch PPM receiver The Lemon Rx was connected via the RC port. During flight, the aircraft would become completely unresponsive to control inputs and eventually crash if certain control inputs were made to the video gimbal system.
With the aircraft in test on the bench, a servo channel used for an axis on the camera gimbal had its servo-limit set to the maximum travel value of 150%. The lockup was reproduced whenever this channel transitioned from 145% to 146%, and repeatably reproduced with both the Spektrum DX7 and DX9 transmitters. Using a DSO, the 145% and 146% PWM values were measured at the Lemon RC PPM output at 2.196 mS and 2.202mS respectively.
Further investigation revealed setting any channel PWM value equal to or greater than 146% (2.202 mS) would cause a complete lockup of the Pixhawk Autopilot, regardless of the number of channels configured/in use or the values of those remaining unused channels.
Additional testing was done to determine if the large PWM value (> 2.2 mS) was generated within the Spektrum transmitter or an error/artifact generated by the Lemon RC receiver.
Using the same DSO test setup, a channel was again configured for the 150% max value and measured to be 2.1 mS at the Spektrum DX9 trainer-port (PPM) output. Without disturbing this configuration, this EXACT channel was again measured at the Lemon RC receiver PPM output with a value of 2.23 mS.
Empirically, it appears the Lemon RC receiver adds some "offset" to the received PPM waveform timing that may cause individual PWM channel values to exceed 2.2 mS.
More importantly, should any individual channel PWM value exceeds 2.2 mS, the Pixhawk may become unresponsive with disastrous results.
With this said, any ideas here?
Steve
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