ArduSub Horizontal+Vertical Thruster Coupling

[HanJIK]

Thank you
When the gain is 100%, when 4 horizontal motors are operated and then 2 vertical motors are operated, the PWM of motors 3 and 4 drops. Why is the PWM value dropping?

[EliotBR (Eliot)]

@HanJIK

When the gain is 100%, when 4 horizontal motors are operated and then 2 vertical motors are operated, the PWM of motors 3 and 4 drops.

Was that confirmed by checking SERVO_OUTPUT_RAW, or did the motors just slow down while the PWM stayed the same?

If the output PWM is actually lower for those two motors then that will be related to your flight mode, joystick command, and/or vehicle frame. If the speed is lower despite the same servo output values I expect it would be caused by insufficient power for running all 6 thrusters at high speed at the same time.

[HanJIK]

Was that confirmed by checking SERVO_OUTPUT_RAW , or did the motors just slow down while the PWM stayed the same?

YES, I checked with SERVO_OUTPUT_RAW.

Flight mode was manual mode.
The vehicle frame was selected as shown in the figure below.

I don’t know what the joystick command means.

[EliotBR (Eliot)]

@HanJIK

YES, I checked with SERVO_OUTPUT_RAW.

Ok, I’m not sure why that would be the case.

The vectored frame has equal factor magnitudes for motors 1, 2, 3, and 4, so I would expect them to operate with the same output magnitudes while in manual mode. I’ve raised this internally in case one of our software engineers has a better idea.

@HanJIK

I don’t know what the joystick command means.

I meant the way you’re commanding the vehicle to move (e.g. how you’re pushing the joystick axes, or the MAVLink messages you’re sending if you’re doing your tests with code).

[EliotBR (Eliot)]

@EliotBR

The vectored frame has equal factor magnitudes for motors 1, 2, 3, and 4, so I would expect them to operate with the same output magnitudes while in manual mode. I’ve raised this internally in case one of our software engineers has a better idea.

Following up on this,

@williangalvani

Forward and vertical motion are coupled by default. The MOT_FV_CPLNG_K parameter should disable it, but I’ve never tried it.

See the following code sections:

ArduPilot/ardupilot/blob/master/libraries/AP_Motors/AP_Motors6DOF.cpp#L461-L464

ArduPilot/ardupilot/blob/master/libraries/AP_Motors/AP_Motors6DOF.cpp#L85-L91

For some additional context,

@rjehangir

This was my doing. When first flying the vectored ROV, we found that there was interference between the vertical thrusters and rear thrusters that causes pitching in forward motion. This significantly decreased that, but does reduce overall thrust.

[HanJIK]

thanks for reply
Does the pwm of motors 3 and 4 decrease and then stop when 6 motors are operated at the same time due to the problem of the horizontal and vertical setting values?

[EliotBR (Eliot)]

@HanJIK

Does the pwm of motors 3 and 4 decrease and then stop when 6 motors are operated at the same time due to the problem of the horizontal and vertical setting values?

I’m not sure what you mean by this. The rear thrusters have their PWM reduced to avoid the vehicle pitching forwards too much when going forwards and down, as explained above. It’s possible to turn off that effect by setting the pitch coupling parameter to zero, but it may make the vehicle harder or less intuitive to control.

[HanJIK]

@EliotBR

I’m not sure what you mean by this. The rear thrusters have their PWM reduced to avoid the vehicle pitching forwards too much when going forwards and down, as explained above. It’s possible to turn off that effect by setting the pitch coupling parameter to zero, but it may make the vehicle harder or less intuitive to control.

Thank you~~