PID (proportional, integral, derivative) control has existed for decades and there are many tools available to help implement PID to tune process control systems, thereby making it generally easy to deploy in most situations. Higher order analysis and techniques, however, are often needed for more complex control loops, products or systems where other factors must be considered, such as upstream and downstream operations and energy consumption. Safety and regulatory issues must also be considered, as these control loops and associated equipment do not exist in a vacuum.
ECS leverages PID control in systems nearly every day. Sometimes higher order tuning is needed when a notable event occurs and a problem is obvious, but sometimes it is the result of detailed analysis and a realization that something small but significant may be awry. Depending on the process or product, a very small adjustment can have huge financial impact.
That’s why it’s imperative to be familiar with all parts of the controlled system before attempting any remediation. The three rules I live by when performing this analysis are to know the system, then to know the system, and finally to know the system. We review related documentation and the physical process itself. Our primary focus is the area controlled by the PID system, but we also consider upstream and downstream operations as our loop may have a dramatic impact upon adjacent and even seemingly unrelated areas. Therefore, it’s critical to know all the physical items involved in the control loop, including utilities, capacities, devices, connections, sizing and other physical properties, as well as controller information and peripheral items like filters that may have been applied in control and tuning.
We collect copious amounts of statistical information before any remediation is attempted. This helps to ensure a complete understanding of what is occurring today and it provides a basis to compare eventual changes. Once actual testing starts, our first activity is to perform a bump test, which is done to establish exactly what kind of loop we are working with. Loops fall into three primary categories, with the majority being self-regulating loops, followed by integrating process loops and runaway loops. Establishing the type of loop sets the stage for the use of specific procedures and tools for effective tuning.
The next entry in this series will detail some of the characteristics of each type of loop and real-world techniques used in tuning. We will also cover what an atypical control loop is and how to approach them.
Rafael Zapata, lead engineer at ECS Solutions, a certified member of the Control System Integrators Association (CSIA). For more information about ECS Solutions, visit its profile on the CSIA Industrial Automation Exchange.
