offboard::set_position_velocity_ned

[ucas-zihaowang]

Hello,

In the funtion Offboard::Result Offboard::set_position_velocity_ned( PositionNedYaw position_ned_yaw, VelocityNedYaw velocity_ned_yaw);

What does VelocityNedYaw mean? Average velocity in each direction ? Actual flying, I set different VelocityNedYaw, the fly speed is always too fast.

Thanks.

[julianoes]

It's actually a bit confusing. I believe what the speed sets is the "feed-forward" term for the control. @Jaeyoung-Lim or @MaEtUgR maybe you can explain it better.

[Jaeyoung-Lim]

@happyzihao Could you elaborate on what do you man by too fast? Is it moving faster than the desired setpoint velocity?

[julianoes]

@happyzihao you can limit the speed using the param MPC_XY_CRUISE.

The velocity that you add to the MAVSDK interface is a control feed-forward.

[julianoes]

Note to self: we should describe that better in the API docs.

[PeymanAmirii]

Thanks for your answers. In control, feed-forward usually used for disturbances or outer effects. How do you use velocity as feedforward? For example the current point is [0, 0, 0] and the desired one is [10, 0, 0]. If we use set_position_velocity_ned(PositionNedYaw(10, 0, 0, 0),VelocityNedYaw(1, 2, 0, 0)), what happens?

[julianoes]

Without getting into the details of controls, what are you trying to do?

[PeymanAmirii]

I want the quadcopter to fly from its current point A to point B. I want to know what happens if vecotors AB and target velocity have different directions and we use the command set_position_velocity_ned(PositionNedYaw(B_north, B_east, B_down, 0),VelocityNedYaw(target_velocity_north, arget_velocity_east, arget_velocity_down, 0)).

[julianoes]

You want to fly from A to B, ok. And then you lost me.

[PeymanAmirii]

Thanks for your answer. What I am trying to say is that we can send target velocity and target position using the command set_position_velocity_ned. Conisder that a drone is at point A and we set point B as the target point and also we consider a target velocity vector for the dorne. If we use the command set_position_velocity_ned to send the target velocity and pont but the vector from point A to B has different direction with target velocity, then what happens? (I think that target velocity and vector from A to B should always have the same direction, but how the command handles the case if they don't have the same direction)

[julianoes]

Ok, so I think the PX4 controller will go in a straight line between A and B. However, if you, for instance know that there is a strong wind coming from one side you could command the feedforward against that wind and therefore the controller would be able to resist the wind better (at least initially) because it doesn't need to be perturbed by the wind first to find out that it needs to correct for it, but instead you can fight it earlier.

If your feedforward is not against wind but just wants to fly somewhere else, let's say to fly a round trajectory, then you will essentially fight the position controller, and you'll end up with something in between, based on your tuning and how strong your feedforward is.

Does that make sense?

[PeymanAmirii]

Thanks. So I think when we send both the target velocity and target position, the output is a sum of the outputs of the separate position controller and velocity controller. Therefore, they can conflict with each other.

[julianoes]

Right.