minimal control energy using lqr

Hello everyone,

I am using lrq function with a desire to compute minimal control energy of a specific linear dynamic system, setting parameters Q and N as zero.

lqr function prototype: [K,S,e] = LQR(A,B,Q,R,N)

A and B are created using rss function, just for test. However, in some cases, lqr cannot be executed correctly with the error message quoted below,

The "lqr" command failed to stabilize the plant or find an optimal feedback gain. To remedy this problem: 1. Make sure that all unstable poles of A are controllable through B (use MINREAL to check) 2. Modify the weights Q and R to make [Q N;N' R] positive definite (use EIG to check positivity).

Has anybody encountered this problem? Could anybody give me some hints? Thanks in advance.

best wishes