Intercepting Radio Channel Values

cbrviking · June 5, 2020, 5:33pm

I want to intercept the value sent from the radio through the PX4 Cube to a servo. I have successfully intercepted the output from an autopilot command but not one coming up the radio.

I have the mode for SERVO9 set to 57 (radio channel 7) and it drives the servo, but I can’t find in the software where I can intercept and read the commanded value.

Any help?

Jamie

vierfuffzig · June 6, 2020, 9:27am

@cbrviking can you be a bit more specific about what exactly you’re trying to achieve?

RCn input and SERVOn output are generally separated, so any RCn input can be mapped and scaled to any output function.

check https://github.com/ArduPilot/ardupilot/tree/master/libraries/RC_Channel for reference on RC input processing.

cbrviking · June 7, 2020, 8:45pm

More specifically, I want to use an unused RC channel from the ground to adjust a gain in-flight. I have the channel coming up on RC channel 7, going through the PX4/Cube and coming out channel 9 (On the AUX out bus). I want to grab the value on it’s way through (It won’t actually go to a servo) and use it to adjust my gain.

I have successfully grabbed values coming out of the autopilot for motor tilt (Mode 41) to do other things but I can’t detect mode 57 come through.

vierfuffzig · June 7, 2020, 10:21pm

depending on what exact gain you want to adjust you might want to look at transmitter based tuning: https://ardupilot.org/plane/docs/common-transmitter-tuning.html
and check if your desired parameter is among the existent selection:
https://ardupilot.org/plane/docs/parameters.html#tune-param-transmitter-tuning-parameter-or-set-of-parameters

maybe have a look at how RC_Channel input is used in AP_Tuning:

github.com

ArduPilot/ardupilot/blob/0528daecd85e9372f470f09fbfa05733a4f066b1/libraries/AP_Tuning/AP_Tuning.cpp#L183




if (selector_start_ms) {
    // no tuning while selector high
    return;
}


if (current_parm == 0) {
    return;
}


RC_Channel *chan = rc().channel(channel-1);
if (chan == nullptr) {
    return;
}
float chan_value = linear_interpolate(-1, 1, chan->get_radio_in(), channel_min, channel_max);
if (dt_ms > 500) {
    last_channel_value = chan_value;
}


// check for controller error
check_controller_error();

hope that helps.

cbrviking · June 8, 2020, 5:06pm

Ok, I’m making a little progress. I have the tuning setup to switch off channel 8 and take it’s value from channel 7 and I can see it switch but I’m having trouble figuring out where it reads the value from channel 7.

cbrviking · June 8, 2020, 7:59pm

Ok, I figured it out. Thanks for pointing me here!