Tricopter based on ArduCopter, yaw-issues and solutions

Hi,

I just build a Tri based on ArduCopter. I had some issues with yaw and did some changes in the code. In this post I want to share my experience and I would like the developers to take my changes into the official release, that it will be easier for others to build a Tricopter.

First of all, this is the result:

One of the most interesting mechanical parts, the Yaw-Servo:

Now my yaw-issues and solutions, starting from code release 2.1:

1) Yaw-Servo not in middle position on "power on"

2) Yaw-Servo reverse with Mavlink RC7_REV

1) Yaw-Servo not in middle position on "power on"

Issue: On "power on" the Yaw-Servo moved into mechanical limit (bad sound, high current consumtion and reduced lifetime for the servo). After a few seconds it returned to the middle position.

Solution:

In file "radio.pde" in function "output_min()":

Replace:

APM_RC.OutputCh(CH_7, g.rc_3.radio_min);

By this:
    #if FRAME_CONFIG ==   TRI_FRAME
   APM_RC.OutputCh(CH_7,   g.rc_4.radio_trim); // Yaw servo middle position
   #else
    APM_RC.OutputCh(CH_7,   g.rc_3.radio_min);
   #endif

2) Yaw-Servo reverse with Mavlink RC7_REV

Issue: Depending on how you mount your Yaw-Servo the stabilisation of yaw can run in the wron direction. Reversing yaw on the remote doesnt help. You need to reverse the servo or change it mechanically. To reverse the servo:

Solution:

In file "ArduCopter.pde" in function "fifty_hz_loop()":

Replace:
APM_RC.OutputCh(CH_7, g.rc_4.radio_out);

by:
APM_RC.OutputCh(CH_7, ( (-1 * (g.rc_4.radio_out - g.rc_4.radio_trim) ) + g.rc_4.radio_trim ) );

I got this change from this Link! Now you can reverse the yaw-servo by putting "-1" to the parameter "RC7_REV" via Mavlink.

Regards, Igor

Glen,

I moved all the motors code into the AP_Motors library in 2.5.4. So you need to go into AP_MotorsTri.cpp and change somewhere around line 129 to be this:

// also send out to tail command (we rely on any auto pilot to have updated the rc_yaw->radio_out to the correct value)
// note we do not save the radio_out to the motor_out array so it may not appear in the ch7out in the status screen of the mission planner
if( _rc_yaw->get_reverse() == true ) {
_rc->OutputCh(AP_MOTORS_CH_TRI_YAW, _rc_yaw->radio_trim - (_rc_yaw->radio_out - _rc_yaw->radio_trim));
}else{
_rc->OutputCh(AP_MOTORS_CH_TRI_YAW, _rc_yaw->radio_out);
}

I've actually checked this fix into trunk so it'll come out in the next official version and you won't need to manually upload using Arduino anymore.

By the way, I don't actually have a Tri so I'm interested to hear if this fix works ok for you.

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Comments

Glenn M October 28, 2013 at 7:11pm

Ken, have you tried setting the RC7_REV parameter?

I haven't used a tri for a while, but last time I did I needed to set that for it to work correctly.
Ken Colangelo October 28, 2013 at 7:03pm

Yup, loaded the firmware from the Mission Planner. Tried Stable and one of the Betas - went back to Stable.

When there are no props installed I see the problem as well but it's really slow. After throttling up the yaw servo will gradually creep off to one side. Outside, with props on, the yaw sservo swings quickly to one side before liftoff and then I get nothing but spinning.
The servo is connected to Output 7. I tried both the stock prop orientation and the optional one with one prop reversed. I've tried with and without telemetry, as well as with failsafe disabled and no GPS nor compass. I'll be posting a video of it here in a moment.
Developer

Randy October 28, 2013 at 6:31pm

I've never flown a tri but that doesn't sound right. Of course you've loaded the tricopter software from the mission planner? (I'm sure you wouldn't make that mistake!) I just don't see why the yaw channel would move with the throttle. The yaw servo is plugged into output 7 on the APM?

It's true that the tri-copter software is not as polished as the rest of the multicopters (although in fact it's only the motor mixing that is tricopter specific) although Leonard from the dev team is building a tri so we probably will get to improving it over the next few months.
Ken Colangelo October 28, 2013 at 2:26pm

My APM 2.6 3.01 yaw servo tilts on throttle up. As I throttle up it tilts in proportion to the amount of throttle given. It centers fine when idle, the ranges from the TX all make sense in the mission planner. I've calibrated and re-calibrated a dozen times. I've updated firmware a few time, re-calibrated my ESCs 3 times. This is as good as it gets. I'm using the standard FliteTest David Windestal configuration. I think it's possible that APM as it stands is hit-or-miss when it comes to tricopters.
Developer

Randy April 25, 2012 at 6:21am

Glen,

I think maybe the fix is working but you've got some other issues there. I've commented on that thread, I hope it helps.
Glenn M April 23, 2012 at 9:33pm

Hi Randy, that change of yours didn't work for me. Details of what happened are here: http://diydrones.com/forum/topics/tricopter-yaw-issues
Glenn M April 22, 2012 at 11:57pm

Thanks for that. I'll give it a go and report back.
Developer

Randy April 22, 2012 at 10:56pm

Glen,

     I moved all the motors code into the AP_Motors library in 2.5.4. So you need to go into AP_MotorsTri.cpp and change somewhere around line 129 to be this:

// also send out to tail command (we rely on any auto pilot to have updated the rc_yaw->radio_out to the correct value)
// note we do not save the radio_out to the motor_out array so it may not appear in the ch7out in the status screen of the mission planner
if( _rc_yaw->get_reverse() == true ) {
  _rc->OutputCh(AP_MOTORS_CH_TRI_YAW, _rc_yaw->radio_trim - (_rc_yaw->radio_out - _rc_yaw->radio_trim));
}else{
  _rc->OutputCh(AP_MOTORS_CH_TRI_YAW, _rc_yaw->radio_out);
}

     I've actually checked this fix into trunk so it'll come out in the next official version and you won't need to manually upload using Arduino anymore.

     By the way, I don't actually have a Tri so I'm interested to hear if this fix works ok for you.
Glenn M April 22, 2012 at 7:18pm

I'm just setting up a tri, and need to reverse my yaw direction. In 2.5.4 I can't see the following code in arducopter.pde. Where should this be changed, and is this the best way to do it?

Replace:
APM_RC.OutputCh(CH_7, g.rc_4.radio_out);

by:
APM_RC.OutputCh(CH_7, ( (-1 * (g.rc_4.radio_out - g.rc_4.radio_trim) ) + g.rc_4.radio_trim ) );
eric.stubkjaer April 20, 2012 at 10:15pm

Got my tri flying last night. When using the KK board I ended up tilting the 2 front motors (about 7 deg) so the craft would hover in one position LEVEL. Before this the frame tilted to one side to counter the side thrust of the tail rotor which had to tilt to prevent yaw (all props spin same direction). With this frame and the APM2 the craft flew first time with default settings. Starting to tune but flys well already. I needed to add a servo reverser to correct the yaw servo direction. Am waiting for this in Ground Control but may try the software change soon.

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