You are here : Control System Design  Index  Book Contents  Appendix B  Section B.6 B. SmithMcMillan FormsB.6 Matrix Fraction Descriptions (MFD)A model structure that is related to the SmithMcMillan form is that of a matrix fraction description (MFD). There are two types, namely a right matrix fraction description (RMFD) and a left matrix fraction description (LMFD). We recall that a matrix and its SmithMcMillan form are equivalent matrices. Thus, there exist two unimodular matrices, and , such that
This implies that if is an proper transferfunction matrix, then there exist a matrix and an matrix , such as
where and are, for example, given by
We next define the following two matrices:
where and are matrices. Hence, can be written as
Combining (B.6.2) and (B.6.6), we can write
where
Equations (B.6.7) and (B.6.8) define what is known as a right matrix fraction description (RMFD) . It can be shown that is always columnequivalent to a column proper matrix . (See definition B.7). This implies that the degree of the pole polynomial is equal to the sum of the degrees of the columns of . We also observe that the RMFD is not unique, because, for any nonsingular matrix , we can write as
where is said to be a right common factor. When the only right common factors of and are unimodular matrices, then, from definition definition B.7, we have that and are right coprime. In this case, we say that the RMFD is irreducible. It is easy to see that when a RMFD is irreducible, then
Remark B.1 A left matrix fraction description (LMFD) can be built similarly, with a different grouping of the matrices in (B.6.7). Namely,
where
The left and right matrix descriptions have been initially derived starting from the SmithMcMillan form. Hence, the factors are polynomial matrices. However, it is immediate to see that they provide a more general description. In particular, , , and are generally matrices with rational entries. One possible way to obtain this type of representation is to divide the two polynomial matrices forming the original MFD by the same (stable) polynomial. An example summarizing the above concepts is considered next.
Example 2.2 Consider a MIMO system having the transfer function
Solution
