B
Take for example a tank with an inlet valve and a 4-40mA actuator controlled by a Proportional only controller with a A 4-20mA level gage.
The flow out of the tank increases, the tank level begins to decrease due to the imbalance between inflow and outflow. While the tank level decreases the error as reported by the gage (SP – Tank Level) increases our proportional controller (PV - Inflow) increases the controller output proportional to this error. Consequently, the valve controlling the flow into the tank opens wider and more water flows into the tank. As the level continues to decrease, the valve continues to open until it gets to a point where the inflow matches the outflow. At this point the tank level remains constant, and so does the error (it has reached a steady-state ess). Then, because the error remains constant the P controller will keep its output constant and the controlled inlet valve will maintain its position. The system now remains ‘balanced’ with the tank level remaining below than its set point. This residual error is called Offset. Close the valve at any other rate than you opened it and shut off valve will respond to the constant Kc without being able to factor time and the tank will retain some level of the offset.
Is it that simple?
The flow out of the tank increases, the tank level begins to decrease due to the imbalance between inflow and outflow. While the tank level decreases the error as reported by the gage (SP – Tank Level) increases our proportional controller (PV - Inflow) increases the controller output proportional to this error. Consequently, the valve controlling the flow into the tank opens wider and more water flows into the tank. As the level continues to decrease, the valve continues to open until it gets to a point where the inflow matches the outflow. At this point the tank level remains constant, and so does the error (it has reached a steady-state ess). Then, because the error remains constant the P controller will keep its output constant and the controlled inlet valve will maintain its position. The system now remains ‘balanced’ with the tank level remaining below than its set point. This residual error is called Offset. Close the valve at any other rate than you opened it and shut off valve will respond to the constant Kc without being able to factor time and the tank will retain some level of the offset.
Is it that simple?