Control of unstable processes with dead time by PID controllers

A novel control structure with PID-type controllers is proposed in this paper to deal with first-order unstable processes with dead time (UFOPDT). The PD controller in the inner loop is aimed to stabilize the unstable process while the purpose of the PID controller at the outer loop is to improve the performance of the overall system. Stability analysis is based upon the Nyquist stability criterion. And the tuning schemes arc the classic Ziegler-Nichols step response methods as well as the tuning rules for stable second-order systems with time delay. It is shown that the proposed method is able to control the UFOPDT process satisfactorily and has made significant improvements over the existing methods in terms of performance and simplicity.