PID Tuning Fundamentals: Good Control Should Be SIMPLE

After all these years the Proportional-Integral-Derivative (PID) controller remains the workhorse for process manufacturers. Still, after more than 50 years of use in an industry many practitioners continue to view the tuning of PID controllers as a “black art”. While the PID can be justified for maintaining its long-standing industry dominance, it’s long overdue for the PID and regulatory control, in general, to be viewed as something other than complex. We propose SIMPLE instead.

The PID serves as the bedrock of regulatory control and it provides the foundation on which supervisory control solutions are built. Since the PID is here to stay for the foreseeable future, it seems worth considering how to make the PID work for you and your plant’s needs. Here’s one view on how to keep things SIMPLE:

• Safety: When designing the control objective the associated safety considerations should be ranked higher than all other factors. Simply put: Safety trumps everything else…always. While the underlying motivation may be profit, safety should be the one thing that girds decision making on the plant floor.
• Impact: Knowing the interaction that exists between and among control loops is important information when designing and implementing a control strategy. That same insight is essential to executing an appropriate corrective action when performance issues arise.
• Management: If management desires a certain type of performance, then it is the job of engineering to either satisfy that performance objective or to explain in a logical manner why the goal is not achievable. Communication that’s based on a clear understanding of a process’ capabilities assures that all parties are on common ground.
• Profit: If a plant’s financial performance is dependent upon efficient control, then it’s essential for production staff to understand the critical role that control plays. Business considerations such as throughput and quality are directly linked to a plant’s control environment. Maintaining an awareness of the relationship between effective control and plant profitability is a key to success.
• Longevity: Most plant managers capitalize their investments in production equipment and other large assets. While one eye should surely be on current production levels the other should remain fixed on long-term success. As such production staff need to be mindful of trouble-makers that can have a short-term negative impact on performance and lead to long-term equipment failure.
• Equipment: Process equipment and components are expensive and their cost should be taken into consideration when formulating a PID control loop’s performance objective. While tuning a loop aggressively may achieve tighter Set Point tracking, it may also accelerate the associated valve’s mean time to failure.

Keeping control SIMPLE can assure that PID controllers are used to their fullest potential, enhancing safety and maximizing profitability. The next post will offer input on what type of data is best for tuning PID control loops.

Dennis Nash is President and CEO of Control Station, Inc., an award-winning supplier of process diagnostic and optimization solutions. For more information about Control Station, visit the company’s profile here on the Industrial Automation Exchange.