Singular Q matrix in LQR design leads to problems?

[Christian Wolf]

Hello,

I wanted to design a LQ controller. In general the matrix R must be pos
definite and I think Q pos semi-def. Thus it should be possible to use a
singular matrix Q which should lead to a pole at 0 in the closed loop.
(Do not ask why I want to do that.)

However if I try to do this I get an error like this:

error: are: 5: the computed dimension of the solution does not equal N
error: called from:
error:   /usr/share/octave/packages/control-2.6.2/care.m at line 162,
column 12
error:   /usr/share/octave/packages/control-2.6.2/lqr.m at line 95,
column 12
error:   /home/.../sim/1/controller.m at line 25, column 3

Can you explain this to me?

Thanks a lot
C. Wolf