Consider the schematic diagram of a controlled blending process shown in Fig. 8.1. The control objective is to control the mass fraction of the exit stream, x, by adjusting inlet flow rate, w₂, using a feedback controller. The mass fractions of a key chemical component in the inlet streams, x₁and x₂, are constant, and mass flow rate w₁is a disturbance variable. The liquid volume V is constant. The composition sensor / transmitter (AT) and the current to pressure transducer (I/P) are both direct acting devices. What is the minimum amount of information you would need in order to decide whether the feedback controller, AC, should be reverse acting or direct acting?

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I/P < Electrical signal Pneumatic signal AC <--Set point x2 = 1 w2 X1 %3D wi Хт AT х Figure 8.1 Schematic diagram for a stirred-tank blending process and control system.