Constr_lbx wrong dimension error in MATLAB

User Questions
1

Hi.

I am getting a Wrong Dimension error when I try to set constr_lbx and constr_ubx of the acados_ocp solver object.
Within the acados_ocp_model object I have set Jbx to a single row matrix with one of the columns being 1 and the rest being zero. According to the Problem definition this should give me a state constraint on just that single state controlled by constr_lbx and constr_ubx.
I am succesfully able to set that single bound value for the acados_ocp_model object, i.e:

nbx = 1;
Jbx = zeros(nbx,nx);
Jbx(1,2) = 1;
ocp_model.set('constr_Jbx', Jbx);
ocp_model.set('constr_lbx', -10);
ocp_model.set('constr_ubx', 10);

But when I try to adjust the bounds dynamically during operation by setting the constraints of the acados_ocp object it fails:
ocp.set('constr_lbx', -10);
However the following works:
ocp.set('constr_lbx', model.x0);

So it seems like Jbx isn’t applied on the resulting solver object?
This aligns with the examples where constr_lbx is used to specify the initial condition. But if it is supposed to work with Jbx being specified, how would you specify the initial conditions?

Best regards
Thomas Jespersen

2

Hi Thomas,

% change values for specific shooting node using:
%   ocp.set('field', value, optional: stage_index)
ocp.set('constr_lbx', x0, 0)

The intial condition is implemented as bounds on x on the initial stage 0.
I think if you update your bounds stagewise for stage indices 1,.., N-1 it should work fine.

However, this

 ocp.set('constr_lbx', model.x0);

should indeed throw an error if nbx != nx, since it tries to set this value for lbx for all stages.
I would check ocp.model_struct.dim_nbx and ocp.model_struct.dim_nx to verify they are set correctly.

Cheers,
Jonathan

1 Like
3

Thanks, that does clarify some of it.
However if I take the minimal_ocp_example.m script and add the following:

nbx = 1;
Jbx = zeros(nbx,nx);
Jbx(1,1) = 1;
ocp_model.set('constr_Jbx', Jbx);
ocp_model.set('constr_lbx', -1)
ocp_model.set('constr_ubx', 1)

This correctly puts a bound on the position state of the pendulum.
However if I want to change this bound dynamically during the loop using ocp.set('constr_lbx', -0.5, 0) then I get the following error:

Error using ocp_set
ocp_set: error setting constr_lbx, wrong dimension, got 1, need 4

Error in acados_ocp/set (line 196)
ocp_set(obj.ext_fun_type, obj.C_ocp, obj.C_ocp_ext_fun, field, value, stage);

Error in minimal_example_ocp (line 131)
ocp.set(‘constr_lbx’, -0.5, 0)

However no error is thrown if I change the bound with the full state vector (a 4 element vector in this case):

% change values for specific shooting node using:
%   ocp.set('field', value, optional: stage_index)
ocp.set('constr_lbx', x0, 0)

Regards Thomas

4

Hi Thomas,

I agree the following is not super intuitive.

The intial condition is implemented as bounds on x on the initial stage 0.

So let me try to elaborate a bit more:
Internally lbx, ubx, Jbx/idxbx at stage 0 are used to implement the initial condition.
Since this defines the x_0 fully for an optimal control problem, the state constraints on the horizons are not enforced at the initial stage by default.
This is done by this line:

ocp_model.set('constr_x0', x0);

A more flexible way is to set the bounds at the initial stage is setting Jbx_0, lbx_0, ubx_0, e.g.

ocp_model.set('constr_Jbx_0', eye(nx));
ocp_model.set('constr_lbx_0', x0);
ocp_model.set('constr_ubx_0', x0);

The constraints on x, defined through lbx, ubx, Jbx are only enforced at the intermediate stages 1,...,N-1.

Regarding the example:
After the modification you suggested, i.e. the following:

% constraints
ocp_model.set('constr_type', 'bgh');
U_max = 80;
ocp_model.set('constr_lbu', -U_max); % lower bound on u
ocp_model.set('constr_ubu', U_max);  % upper bound on u
ocp_model.set('constr_Jbu', 1);

nbx = 1;
Jbx = zeros(nbx,nx);
Jbx(1,1) = 1;
ocp_model.set('constr_Jbx', Jbx);
ocp_model.set('constr_lbx', -1)
ocp_model.set('constr_ubx', 1)

ocp_model.set('constr_x0', x0);
% ... see ocp_model.model_struct to see what other fields can be set

this works to update the corresponding lower bounds on x.

for i = 1:N-1
    ocp.set('constr_lbx', -0.5, i)
end

I hope this clarifies it.

Best,
Jonathan

5

Indeed, that explains a lot. Thank you.
Unfortunately I still observe some oddities. If I apply the Jbx, lbx and ubx as mentioned above (with the limits of +/- 1) before I run the solver and then just before calling the solver I apply:

ocp.set('constr_lbx', -0.5, 1)

Then it seems like the lower bound constraints for the other stages are disabled, since the cart position diverges towards the end and clearly exceeds the original constraint of -1.
image

Although if I do:

ocp.set('constr_lbx', -0.5, 1)
ocp.set('constr_ubx', 0.5, 1)

the result is different and controlled.
image

So if you don’t mind me asking:

  1. When you use ocp_model.set('constr_Jbx', Jbx);, ocp_model.set('constr_lbx', -1) and ocp_model.set('constr_ubx', 1) how many stages is this applied to?
  2. What does ocp.set('constr_lbx', -1) effectively do compared to ocp_model.set('constr_lbx', -1) - so for both cases no stage is specified. I know from the interface code that ocp_model.set('constr_lbx', -1) just sets the according model_struct variable, but how does this propagate into setting the actual ocp compared to setting it with ocp.set?
  3. By setting ocp_model.set('constr_x0', x0); does this mean that Jbx is set to identity for stage 0 and the lbx and ubx is set to x0? So this is the same as the 3 lines you wrote with e.g. constr_Jbx_0?
  4. Can Jbx be defined differently and independently for each stage?

Thanks once again.

6

Hi Thomas,

Unfortunately, I can not reproduce the plots you posted.
Maybe you can upload your example somewhere.

I will try to answer the other questions though:

Stages 1 to N-1, stage 0 is done via constr_x0 or Jbx_0, lbx_0, ubx_0 as mentioned above.
For the terminal stage there are Jbx_e, lbx_e, ubx_e

When creating the ocp object, the formulation is taken from ocp_model. Interacting with ocp_model after creating ocp, does not influence the solver and should not be done.

Exactly.

That’s not supported by the Matlab interface.
However you could just define Jbx to select the component you want to constraint at some point at set trivial bounds at the non relevant stages.

Cheers