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Automated Continuous Process Control Part 8

Tham khảo tài liệu 'automated continuous process control part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | c06.qxd 7 3 2003 8 27 PM Page 130 130 BLOCK DIAGRAMS AND STABILITY Figure 6-1.5 Block diagram with valve added. Controller TT Valve Heat exchanger Sensor transmitter Figure 6-1.6 Block diagram showing control loop. Sensor transmitter Figure 6-1.7 Block diagram showing control loop and disturbances. c06.qxd 7 3 2003 8 27 PM Page 131 BLOCK DIAGRAMS 131 Sensor transmitter Figure 6-1.8 Block diagram showing control loop and disturbances. transfer function that describes how Pu affects T. All three transfer functions as well as G1 describe the heat exchanger. A detail analysis of the process shows that when Pu changes it first affects the steam flow F and then affects T. So the following question develops How do we present these effects in the block diagram Figure 6-1.8 shows these effects. G5 is the transfer function that describes how Pu affects F. The figure shows that Pu first affects F and then F affects T. Comparing Figs. 6-1.7 and 6-1.8 we see that G4 G5G1. Thus we can draw block diagrams in different ways as long as they make physical sense. Look at Fig. 6-1.9 can we draw the block diagram that way Yes or no Why Before proceeding to another example it is important to show some simplifications of the block diagram just drawn. Remember that the transfer functions are in terms of Laplace transforms. The reason for using these transforms is that we can work with them using algebra instead of using differential equations. Thus we can use the rules of algebra with the block diagrams. The three diagrams shown in Fig. 6-1.10 can be developed starting from Fig. 6-1.7. In the figure Gm GVG1H transfer function that describes how the controller output m affects the transmitter output c GD1 G2H transfer function that describes how the inlet temperature Ti affects the transmitter output c GD2 G3H transfer function that describes how the process flow FP affects the transmitter output c GD3 G4H transfer function that describes how the upstream pressure from the valve Pu affects