What is the correct control mode for Zubax Myxa ESCs via FMU PX4 Autopilot with Cyphal

[Hiep (Hiep Truong Minh)]

Hi, i’m buidling UAV using FMU (Pixhawk V5X) - PX4 Autopilot and 4 Zubax Myxa ESCs for controlling motor (Flycat i-rotor 5010-750kv) via Cyphal. Which motor control mode should I use?
In my case:
FMU PX4 Autopilot ( current latest firmware of PX4 Autopiot used) : 2 main outputs to control :

uavcan.pub.udral.esc.0.id: 111
uavcan.pub.udral.readiness.0.id: 112

first 4 index of udral.esc.0 will give output to control for 4 ESCs (min:0 - max:1) below is sreenshot from Yukon:

1. When i used ratiometric torque ( setpoint_rat_torque ) => but It can accelerate motors to the maximum velocity even at very low setpoint values.( from FMU output), it can take off and fly up vertically and i can not control its velocity to land.

uavcan.sub.setpoint_rat_torque.id:        111
uavcan.sub.readiness.id:                  112

video flying with setpoint_rat_torque:

2. When i used ratiometric voltage ( setpoint_rat_voltage) => with this mode, i can control the velocity by Throttle Sitck (throttle stick increased => velocity increased and vice versa) from Remote Control (Flysky i6s) but it makes a grinding sounds when rotors ramping And Velocity ouputs are not asynchronous => UAV can lift up a little bit and fall down.

uavcan.sub.setpoint_rat_voltage.id:        111
uavcan.sub.readiness.id:                  112
mns.ratiometric_to_absolute_mul: 20 # battery: 20V

3. When i used velocity control mode => setpoint_velocity not working, setpoint_rat_velocity_u9 can work but not like what i expected ( Rotos ramping did not follow Throttle Sitck, and still grinding sounds )

uavcan.sub.setpoint_rat_velocity_u9.id:        111
uavcan.sub.readiness.id:                  112
mns.ratiometric_to_absolute_mul: 5000

And for each mode above, i have tried to change also other parameters to make my UAV flying as what i expected but still can not!
I researched alot and it looks like : velocity control mode is high recommeded But what i tried is above result.
Please help.

[pavel-kirienko]

Apologies for the delayed response.

@Hiep

When i used ratiometric torque ( setpoint_rat_torque ) => but It can accelerate motors to the maximum velocity even at very low setpoint values.( from FMU output), it can take off and fly up vertically and i can not control its velocity to land.

Sounds like the maximum current is set too high, or the value of mns.ratiometric_setpoint_min is non-default and high. Regardless, the torque control mode is poorly suited for multicopters because most (all?) flight controllers attempt to control the propeller thrust rather than torque; the torque is proportional to the derivative of thrust as you can see from a standard propeller equation (more on this in the reference manual).

@Hiep

When i used ratiometric voltage ( setpoint_rat_voltage) => with this mode, i can control the velocity by Throttle Sitck (throttle stick increased => velocity increased and vice versa) from Remote Control (Flysky i6s) but it makes a grinding sounds when rotors ramping And Velocity ouputs are not asynchronous => UAV can lift up a little bit and fall down.

The grinding sound indicates that the controller loses the synchronization, which could be caused by rapid transients. Consider reducing the motor.voltage_ramp parameter if you are keen on using the voltage control mode. It is not recommended, however, as explained in the reference manual; normally you should prefer the velocity control mode.

@Hiep

When i used velocity control mode => setpoint_velocity not working, setpoint_rat_velocity_u9 can work but not like what i expected ( Rotos ramping did not follow Throttle Sitck, and still grinding sounds )

As far as I know, PX4 cannot publish setpoints as uint9. What you’re seeing is caused by a type mismatch because you make the ESC subscribe to a float16[] topic using uint9[]; the ESC reads misformatted inputs and behaves erratically.

Ideally the PX4 firmware should be extended to support publication of velocity setpoints as float16[] instead of just ratiometric setpoints. Alternatively, we could add a new float16[] subscription to Telega similar to setpoint_rat_torque and setpoint_rat_voltage that accepts ratiometric velocity; that would be setpoint_rat_velocity.