In the slides of the following link is expressed the concept I am trying to understand : slides

I am studying control systems. I have seen that if I have a zero in the RHP, by using decoupling I can move the zero from one input/output channel to another, so we can choose where it create less problems.

I have that this concept is not really clear to me. With a decoupler I can transform the transfer matrix into a diagonal matrix, so that the first input influences just the first output and the same for the other channels, but what does it means that I can move the RHP zero?

For example, consider the transfer matrix:

\$G(s)=\begin{pmatrix} \frac{2}{s+1} & \frac{3}{s+2}\\ \frac{1}{s+1} & \frac{1}{s+1} \end{pmatrix}\$

So, I am considering a system with two inputs and two outputs.

I use a decoupler of the type:

\$\begin{pmatrix} -1 & 1\\ 1 & \frac{-2(s+2)}{3(s+1)} \end{pmatrix}\$

then the transfer matrix after applying the decoupler will be:

\$\begin{pmatrix} \frac{s-1}{s^{2}+3s+2} & 0\\ 0 & \frac{0.3s^{2}-0.3}{s^{3}+3s^{2}+3s+1} \end{pmatrix}\$

It can be seen that the original transfer matrix has a zero in the right half plane, so with positive real part, at \$s=1\$ since \$det(G(s))=0\$ for \$s=1\$ .If I apply the decoupler I have that in both channels I have a zero at \$s=1\$ , so with positive real part, which imposes limitations in the achivable bandwidth in the frequency response.

So, what does it means that with the decoupling it is possible to move the unstable zeros from one channel to the other?

  • \$\begingroup\$ I really don't think you mean MIMO in the context that you have linked it. Please review your usage of the term MIMO because it applies to radio and communications systems and not control systems. I notice that you also used MIMO incorrectly on some of your other questions too. \$\endgroup\$
    – Andy aka
    Commented Feb 12, 2020 at 9:24
  • \$\begingroup\$ Thank you for making me notice this. I have corrected the link. \$\endgroup\$
    – J.D.
    Commented Feb 12, 2020 at 9:29
  • \$\begingroup\$ Could you please state your problem more clearly. What does your RHP zero mean exactly, which transfer function does have it? Please explain your specific problem and what are you trying to achieve so we can help you more on point. \$\endgroup\$ Commented Feb 12, 2020 at 9:41
  • \$\begingroup\$ I think the problem you have is that nobody I know would relate the term MIMO to control systems and, the usage of the expansion (multiple input multiple output) is also either meaningless, misleading or trivial in the context of Electrical engineering. I advise you to un-clutter those questions you have raised that use these terms. Having "a zero in the RHP in a MIMO system" is just the same as "having a RHP zero". \$\endgroup\$
    – Andy aka
    Commented Feb 12, 2020 at 9:46
  • 2
    \$\begingroup\$ @Andyaka SISO and MIMO are standard terms in control engineering. \$\endgroup\$
    – user110971
    Commented Feb 12, 2020 at 11:35

1 Answer 1


When you decouple a MIMO system you turn it into a series of SISO systems. So you transform your system from a single 2 inputs 2 outputs system into two independent SISO systems by having a diagonal matrix. Moving the unstable zero to one of the inputs you can control which output is affected. The result is that only one of the outputs remains decoupled since you transform the diagonal matrix into a triangular matrix.

Using this method you can move the drawbacks around the system. Maybe one of the outputs moves slowly. Hence you don’t need good performance on said output. However you need a fast response on the other output.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.