PID tuning improves process efficiency

Tuning a PID controller correctly can bring the setpoint closer to the constraint with reduced variability.

12/11/2013


Figure 1: In a closed-loop system, processes can be optimized by tuning the controller to reduce the error between the PV and the SV. Courtesy: Yokogawa Corp. of AmericaProcess industry plants must optimize regulatory and advanced control to maximize profitability while maintaining safe operation. Regulatory control stabilization is key to achieving these goals. Stabilization can often be improved through closer evaluation of a plant’s regulatory control loops. 

Most of these loops are operated by a proportional-integral-derivative (PID) controller. By better understanding how to tune these loops, plant personnel can improve quality and efficiency while ensuring plant safety. Moreover, achieving regulatory control stabilization forms the foundation for advanced process control implementation, which can further optimize operations. 

In addition to maintaining safe operation, a stable regulatory control system can increase profitability by reducing emissions and energy consumption, and increasing the life span of equipment. By automating troublesome control loops, the need for manual operations is reduced, saving labor hours and increasing product consistency. 

This article offers an introduction on how to achieve a stable, well-tuned regulatory control system, with emphasis on improved PID control. 

Control loop basics

In a typical control loop, a parameter, such as temperature or pressure, must be controlled. This parameter is called the process variable (PV). A sensor usually measures the PV, and this measured variable is provided as feedback to the controller in a closed-loop system. 

The desired value for the PV, such as 40 F in the case of a temperature control system, is called the setpoint variable (SV). If the PV is only 30 F, for example, the controller will react to adjust its output to increase the temperature. 

A constraint is the limit at which the process can be performed safely and efficiency. For example, the heat at 42 F will destroy the compounds in a liquid during a certain process. Therefore, the process must remain below this constraint. However, a process operating at 36 F is so far below the constraint that quality is lost and/or the process becomes inefficient. Thus, the goal is to control the PV and keep it as close to the SV as possible with minimal variability. When variability is minimized, the SV can be moved closer to the constraint, improving operations. 

PID controllers

The PID controller is the most commonly used controller type in process plants, with more than 95% of the control loops in a typical plant under PID control. The familiar PID controller can be a great tool for improving quality, energy efficiency, and production. 

PID controllers offer good performance in a variety of operating conditions and they can be operated in a simple, direct manner. They can be stand-alone or embedded, and they can be used for one or for many control loops. They can also be combined with sequential logic and advanced control functions to form complex automation systems.

The PID algorithm consists of three basic elements: proportional, integral, and derivative. The setting for each of these three elements is varied to achieve an optimal response: to maintain the PV as close to the SV as possible with minimal variability. Each element is linked to a certain task that has a specific effect on the control loop. 

A PID controller receives the measured PV data from a sensor, and then determines the difference (error) between the measured PV and the desired SV. It then computes the desired controller output or manipulated variable (MV) based on the error. The MV in turn acts on the final control element (for example, a steam valve), which in turn affects the process to change the PV (see Figure 1). 

There are many techniques for calculating the tuning constants for PID controllers. Most are based on developing a mathematical model for the combined final control element, process, and measurement. 

The model for describing the controller is simply the PID Algorithm. If a model can be determined for the process (final control element, process, and measurement), then a mathematical model for the entire loop is in place. At that point, tuning the controller becomes a matter of matching the controller tuning constants (P, I, and D) to the parameters of the process model, and optimizing the three PID constants to produce a desired response. 

In the real world, the controller output is usually not the only signal that affects the system. There may be elements within the environment that cause deviations, such as ambient noise. These elements are called disturbances. While disturbances must be factored in when controlling the process, they are also intentionally introduced on a temporary basis as an initial step to tune the controller. 

Tuning a PID controller

The process of tuning a PID controller involves five steps:

  1. Introducing a disturbance into the control loop
  2. Fitting the resulting response to a mathematical model
  3. Using tuning correlations to calculate controller parameters
  4. Implementing the new P, I, and D parameters
  5. Documenting the results. 

 

 

 

 

The first step in tuning the PID controller involves inserting a disturbance that creates a new controller output (CO) into the loop. This is called a bump test. The disturbance introduced into the loop must be large enough to force a clear PV response, and the response must be large enough to distinguish it from any noise in the system. Unmeasured disturbances can corrupt the PV data, so larger CO bumps are better (see Figure 2). 

Self-regulating process vs. integrating process

Figure 2: Data from the bump test should show a dynamic cause and effect from the change in the controller output. In this example, a CO bump causes the PV to move significantly. Courtesy: Yokogawa Corp. of AmericaAny number of model types is possible for describing a process. But before choosing a model, one must first determine if the process is self-regulating or integrating (non-self-regulating). This determination is essential in tuning a controller because the different processes require different mathematical models. 

After a change in the CO is introduced, a self-regulating process will eventually find a steady state as long as the CO and disturbance variable remain steady, as the process has an internal method of regulating the PV that balances the change in the CO. For example, in a flow loop, if a valve is opened an additional 10%, the flow will increase to a new value and will stay at that value until the valve is moved again. Most processes are self-regulating. 

However, in an integrating process, the PV will keep changing unless the CO is returned to its original position. With an integrating process, the PV will continue to rise or fall in a linear fashion in relationship to the CO change. This means an integrating process can be difficult to control. An example of an integrating process is changing the liquid level in a tank. If more flow is introduced into the tank, the level will keep rising; if more flow is let out, the level will continue decreasing. 

Tuning control loops for self-regulating processes tends to be easier than tuning integrating processes. Negative consequences can happen very easily and quickly if an integrating loop is tuned incorrectly. The inability to achieve equilibrium after a change in the CO is introduced in an integrating process could lead to problems, for example, a liquid overflowing or running dry, which could cause equipment (pump) damage, environmental damage, or significant safety issues. 

Self-regulating processes require both proportional and integral (and occasionally derivative) control modes for good performance. Despite their nomenclature, integrating processes rely much less on the integral control mode. Different tuning rules must be applied because the process models are different. 


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