-------------------------------------------------------------------------------------- 

                      ROOT_LOCUS
                                      ___           
                      R(s)         + / _ \  E      _____   C
                              _____\/ \   \_______| G(s)|_________\ 
                                   /\ /_  /       |_____|   |     /
                                     \___/                  |      
                                      - ^    _____          |            
                                        |___| H(s)|_________|           
                                         B  |_____|              
                      
                      For stabilty 
                      G(s)*H(s)=(K*(s-z)*../((s-p)..)=>gain< 1 @ 180deg 


                                   IMAGINARY      
                                      ^
                                 _   /|\  How loop poles move 
                                 /|   |   as loop gain is increased
                                /     |  
                       <--X  X->|<-X  |____\ REAL
                                \     |    /
                                _\|   
                              
                      
                                         IMAGINARY      
                                            ^
                                       _   /|\    
                                       /|   |
                                      /     | 
                                      \     |
                       <--X  X->Z  X->|<-X  |____\ REAL
                                      /     |    /
                                      \
                                      _\|   
                                    
                      
                      
                                         IMAGINARY      
                                            ^
                                       _   /|\    
                                       /|   |
                                      /     | 
                                      \     |
                       <--X  X->Z  X->|<-X  |____\ REAL
                                      /     |    /
                                      \
                                      _\|