Tuning of PID controller for level/flow control loop

What is a PID Controller?

The term PID stands for proportional integral derivative and it is one kind of device used to control different process variables like pressure, flow, temperature, and speed in industrial applications. In this controller, a control loop feedback device is used to regulate all the process variables.

This type of control is used to drive a system in the direction of an objective location otherwise level. It is almost everywhere for temperature control and used in scientific processes, automation & myriad chemical. In this controller, closed-loop feedback is used to maintain the real output from a method like close to the objective otherwise output at the fixe point if possible. In this article, the PID controller design with control modes used in them like P, I & D are discussed.

PID Controller Block Diagram:

A closed-loop system like a PID controller includes a feedback control system. This system evaluates the feedback variable using a fixed point to generate an error signal. Based on that, it alters the system output. This procedure will continue till the error reaches Zero otherwise the value of the feedback variable becomes equivalent to a fixed point.

This controller provides good results as compared with the ON/OFF type controller. In the ON/OFF type controller, simply two conditions are obtainable to manage the system. Once the process value is lower than the fixed point, then it will turn ON. Similarly, it will turn OFF once the value is higher than a fixed value. The output is not stable in this kind of controller and it will swing frequently in the region of the fixed point. However, this controller is more steady & accurate as compared to the ON/OFF type controller.

What is a level control loop?

From a control theory point of view, level loops are, or behave likely, integrating process and will have a pole at the origin on the pole-zero map. The response of these systems to a stepwise input would be an ever-increasing output and there is no steady state to which the process will evolve. For this reason, there are important aspects to be considered when applying the tuning recipe to an integrating process as flows: -

• The test needs special attention as the tank can easily overflow or dry out if the output and input flows are not correctly paced. In that sense, the test would apply one step up followed by one step down, so to stabilize the system.

• Put the loop in AUTO and change the PID parameters to the new set. It is important to move gradually in the direction of the revised parameters, so to avoid abrupt changes in the response and perform some additional testing to check the level response and de-tune the controller if needed. 

The tuning rules will normally result in a tight response for the control loop. That would be desirable for some of the levels, but it will counteract the whole purpose of using a surge tank. Therefore, suitable methods should be used here as, for example, the Zak Friedman’s averaging level rule, which is based on the material balance of the process. The following steps can be followed: -

• Define the maximum “common” disturbance FD in the inlet flow. That could be done by observing the plant historic data and finding the larges disturbance during normal operation. As alternative, a 20% of the outlet flow can be used.

• Put the loop in AUTO and change the PID parameters to the new set gradually. Check the engineering units to have the correct parameters for the control algorithm. Different sets of LD can be used to adjust the controller behaviour if needed.

These two procedures can lead to a set of parameters for both types of level controllers, strict control, and smooth level control. Finding the right set, though, will be challenging and time consuming, especially when some iteration is needed.

Finding the best set of parameters

Tuning level loops can be a demanding activity specially because of its integrating characteristic. A poor tuning set can easily lead to oscillations and dealing with them will be more complicated than for self-regulating loops. Besides, the de-tuning of the parameters found from the conventional rules cannot guarantee that the level constraints are met.

The PID tuning software helps you to find the best set of parameters for each level loop controller. Strict or smooth control, or a combination of both, can be achieved easily by performance and robustness specifications.

Choose a PID tuning software for level loops — you get: 

• Fewer interventions during the tuning

• The best responses for both strict and smooth level control

• No oscillations

• More robust tuning

• Better process stability