Question

3. The following plot shows two possible ways to design a feedback control loop to obtain a desired rate of liquid flow. Assume that in both Systems I and II, the flow transmitter is direct-acting (i.e., the output increases as the actual flow rate increases) However, the control valve in System I is "air-to- open," meaning that an increasing pressure signal from the controller will open the valve more, thus increasing the flow rate. On the other hand, the control valve in SystemII is "air-to-close." The dynamics for both of the valves are negligible. (a) For each of these valves, what is the sign of its gain, Kv? (b) Which controller must be direct-acting? reverse-acting? (c) What sign should the controller gain have for each case? FC FT Liquid System FC FT Liquid System II

Answer 1