Hi

I am new to acados and would like to set up MPC problems in Matlab with constraints, reference, weights, expressions etc that change over time (*time dependent, time varying, dynamic*).

To explore the possibilities, I first tried to add some changes to the minimal_example_ocp with the pendulum_on_cart_model that was given at github as an example to acados.

I already saw that after setting up the solver, it should be possible to change all those things for specific time steps (â€śstagesâ€ť) with:

```
ocp.set('constr_lbx', lbx_i, i)
```

So this would change the lower bound for x to the values in lb_x_i for stage i.

And similar one could do this for other bounds, references, expressions.

In the minimal_example_ocp code, there was already this line:

```
ocp.set('constr_lbx', x0, 0)
```

This line works fine. In general, if the stage is i=0, setting the lower or upper bound for x for only this stage works (at least it is runnable like this, I did not check if it in this case has an effect).

If using another stage, e.g.

```
ubx_6=[0.5; inf; inf; inf];
ocp.set('constr_ubx', ubx_6, 6);
```

this error message appears:

```
Error using ocp_set
ocp_set: error setting constr_ubx, wrong dimension, got 4, need 0
Error in acados_ocp/set (line 260)
ocp_set(obj.cost_ext_fun_type, obj.cost_ext_fun_type_e, obj.C_ocp, obj.C_ocp_ext_fun, field, value, stage);
Error in minimal_example_ocp (line 129)
ocp.set('constr_ubx', x_bound_step_6, 6);
```

I do not understand why this happens. Just to try, I also ran:

```
ocp.set('constr_ubx', [], ubx_6, 6);
```

to achieve the 0-dimension. Then the code is actually runnable, but I do not see the expected change in the result. To me it seems like nothing has changed by adding this constraint to stage 6.

Another thing I already tried was to change the reference y_ref:

```
ocp.set('cost_y_ref', [0;1;0;0], 10);
```

There, I got the error message:

```
Error using ocp_set
ocp_set: error setting cost_y_ref, wrong dimension, got 4, need 5
Error in acados_ocp/set (line 260)
ocp_set(obj.cost_ext_fun_type, obj.cost_ext_fun_type_e, obj.C_ocp, obj.C_ocp_ext_fun, field, value, stage);
Error in minimal_example_ocp (line 142)
ocp.set('cost_y_ref', [0;1;0;0], 10);
```

So also here, I just tried to satisfy the dimensions demanded by changing it to:

```
ocp.set('cost_y_ref', [0;1;0;0;0], 10);
```

In this case, it again was runnable and even gave the expected result. So this seems to work, I would just like to know why changing the dimensions was necessary and what the additional number exactly means.

The lines written above are the only thing I changed in the public available example.

I added them right after the line

```
ocp.set('constr_lbx', x0, 0) .
```

So my questions are:

- First, for this might already solve everything: Is there maybe a documentation I overlooked, which lists the things that can be adjusted for specific stages and how to do this for each of them?
- How do I achieve to change the lower and upper bound of x for a specific ctage i?
- Why does the error message with the dimensions appear?
- For the y_ref case, why is the additional value required and what does it mean?
- Or maybe is there other possibilities to achieve different constraints etc for different stages?

I hope my problem is understandable.

Thank you very much for your help!

Greetings

Sofie