Question

The process and instrumentation diagram for a feedback control strategy to maintain the level (h) of a liquid by regulating a valve (V) is shown. The manipulating variable(s) is/are: 

• A. $F_1$ only
• B. $F_2$ only
• C. $h_m$ and $P_V$ only
• D. $h_{sp}$ and $P_V$ only

Hint:

In any control system, the manipulated variable is the one the controller actively changes to influence the controlled variable.

Answer 1

The Correct Option is A

In a feedback control loop, the manipulated variable is the variable that the controller adjusts in order to keep the controlled variable at its desired setpoint. Here, the controlled variable is the liquid level \(h\). The feedback loop measures \(h_m\), compares it with \(h_{sp}\), and the liquid level controller (LC) outputs a control signal that adjusts the valve position.

Step 1: Identify controlled and manipulated variables.
The liquid level in the tank is to be maintained. Thus, the manipulated variable should influence the inflow or outflow. In this system, the controller sends a signal to the valve \(V\), which controls the inlet flow rate \(F_1\). Therefore, the manipulated variable is \(F_1\).

Step 2: Check other options.
- \(F_2\) is the outlet flow, but it is not being controlled by the feedback loop. Hence Option (B) is incorrect.
- \(h_m\) is the measured value and cannot be manipulated. It is an input to the controller.
- \(P_V\) is a control signal to the valve but is not the process variable being manipulated; the physical quantity being changed is \(F_1\), not the valve pressure.
- \(h_{sp}\) is the setpoint, not a manipulated variable.

Step 3: Conclusion.
The only true manipulated variable in this feedback system is the inlet flow rate \(F_1\).