The process under consideration is the simple cylindrical liquid receiver shown in Figure $1.$ Liquid flows

in at the mass flow rate Fi and is permitted to flow out at a possible different mass flow flow rate Fo$.$ The

tank$$s cross sectional area is considered uniform, with a constant value A$.$ The inlet flow rate is determined

by some upstream process; control valve is available at the outlet pipe; a pressure level sensor is available

for providing liquid level measurement; flow measurement devices are available for monitoring both inflow

and outflow. It is desired to design a control system that will maintain a constant liquid level in the tank.

$1.$ Identify the key controlled, manipulated and disturbance variables for this process.