Hello,

I am trying to implement a MPC controller on a quadrotor using acados. I have 5 inputs:

1 - The total thrust of the drone: T

2 - The quatrenion, which is made out of 4 elements to output a certain orientation for the drone: qw, qx, qy, qz.

I am using the python interface. So, the inputs are defined as follows:

```
T = MX.sym("T")
qw = MX.sym("qw")
qx = MX.sym("qx")
qy = MX.sym("qy")
qz = MX.sym("qz")
u = vertcat(T, qw, qx, qy, qz)
```

However, I have a problem. I need the quaternion [qw, qx, qy, qz] to be of unit norm:

`sqrt(qw**2 + qx**2 + qy**2 + qz**2) = 1`

Is there a way to inform the solver of this constraint?

I know that I can add lower and upper bounds on each element. And, I did that for the thrust input T.

However, I donâ€™t know how to make the quaternion of unit norm within the solver.

It would be of great if someone can help.

Thank you