Tham khảo tài liệu 'automated continuous process control part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 7 3 2003 7 55 PM Page 190 190 MULTIVARIABLE PROCESS CONTROL If there is an odd number of positive Kjs the value of mil will be positive and furthermore its numerical value will be between 0 and 1. Positive interaction is the most common type of interaction in multivariable control systems. In these systems each loop helps the other. To understand what we mean by this consider the blending control system shown in Fig. and its block diagram shown in Fig. . For this process the gains of the control valves are positive since both valves are fail-closed. The gains K11 K21 and K12 are also positive while the gain K22 is negative. The flow controller GC1 is reverse acting while the analyzer controller GC2 is direct acting. Assume now that the set point to the flow controller decreases the flow controller in turn decreases its output to close the valve to satisfy its new set point. This will cause the outputs from valve Figure Control system for blending process. Figure Block diagram showing how signals and variables move. 7 3 2003 7 55 PM Page 191 INTERACTION AND STABILITY 191 GV1 and process G11 to decrease. Because K21 is positive the output from G21 also decreases resulting in lowering the analysis x. When this happens the analysis controller GC2 also decreases its output. This causes the outputs from GV2 and G12 to decrease and the output from G22 to increase. Figure shows the arrows that indicate the directions that each output moves. The figure clearly shows that the outputs from G11 and G12 both decrease. This is what we mean by both loops help each other. When there are an even number of positive values of Ki-s or an equal number of positive and negative values of Kij s the value of m-s will either be less than 0 or greater than 1. In either case there will be some Pij with negative values in the same row and column. The interaction in this case is said to be a negative interaction. It is important to realize that for a
