tailieunhanh - Process Control Systems Episode 2

Tham khảo tài liệu 'process control systems episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Dynamic Elements in the Control Loop J 23 This observation is of particular significance for two reasons 1. Load variations are normally introduced by turning the valve in the outflow line thus changing k. 2. In most processes including this one k would not be a constant even if the load were fixed because the relationship between input and output is not linear. In a real liquid-level process fo c Vh where C is the flow coefficient of the valve opening. Then h Vh _ 1 ío c k Consequently k c x h so even if C remains fixed k still varies with level. Again fortunately this does not affect the dynamic gain. The time constant of such a process is not a constant but varies with k. But this is of little consequence because the dynamic gain is constant. The ratio V F must be recognized as the determining factor. It will appear again and again in different processes with different forms of variables but it is the fundamental time constant of any flowing system. Its units are those of time. For example gal gal min minutes. The phase angle between input and output of a first-order lag is the negative of ỘD in the vector diagram of Fig. . As To approaches zero ỘD approaches 90 and therefore the true phase lag approaches 90 . In the steady state however the vertical vector is zero hence the phase angle is zero. The phase of a first-order lag is mathematically described as 2irV F 0 tan-1 Substituting for V Fk Ộ1 tan-1 2tt To Since the phase can never exceed the first-order lag cannot oscillate under proportional control. This was also true of the integrating process. Therefore we can make a general statement that a singlecapacity process can be controlled without oscillation at zero proportional band. This means that the valve will be driven fully open or fully closed on an infinitesimal error so that the loop is operating at top speed all the time. Since the proportional band is zero no offset can develop. A single-capacity process must therefore be categorized as the