Tham khảo tài liệu 'essentials of process control phần 6', ngoại ngữ, ngữ pháp tiếng anh phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 304 PART TWO Laplace-Domain Dynamics and Control Rearranging gives yset r2 So Eq. gives the closedloop characteristic equation of this series cascade system. A little additional rearrangement leads to a completely equivalent form V GịG2GC Gcị yset 2 1 GịGcị 1 G2G J2 2 An alternative and equivalent closedloop characteristic equation is 1 G GC1 1 G2Gcz 0 The roots of this equation dictate the dynamics of the series cascade system. Note that both of the openloop transfer functions are involved as well as both of the controllers. Equation is a little more convenient to use than Eq. because we can make conventional root locus plots varying the gain of the Gc2 controller after the parameters of the GC controller have been specified. The tuning procedure for a cascade control system is to tune the secondary controller first and then tune the primary controller with the secondary controller on automatic. As for the types of controller used we often use a proportional controller in the secondary loop. Since it has only one tuning parameter it is easy to tune. There ị is no need for integral action in the secondary controller because we don t care if there is offset in this loop. If we use a PI primary controller the offset in the primary i loop will be eliminated which is our control objective. EXAMPLE . Consider the process with a series cascade control system sketched in I Fig. . A typical example is a secondary loop in which the flow rate of condensate from a flooded reboiler is the manipulated variable M the secondary variable is the flow I rate of steam to the reboiler and the primary variable is the temperature in a distillation I column. We assume that the secondary controller Gif I and the primary controller Gcĩ j are both proportional only. Ì G Cl AI Gc2 K2 I In this example I c. 1 G. 1 5 l s 1 2 5s 1 Conventionalcontrol. First we look at a conventional single proportional controller Kc I that manipulates M to control lAh The .
