PID control of unstable processes with time delay: A comparative study

In this paper, several linear PID control methods for unstable processes with time delays, which have recently been reported in the literature, are studied in terms of a comprehensive set of control performance specifications. Their applicability and achievable performance are indicated with regard to different cases of normalized time delay for ease of user's choice of the methods. By analyzing these methods' pros and cons, a modified method which combines their respective strengths using simple linear time-variant and nonlinear control strategies is obtained and demonstrated with performance enhancement.