Sooner or later, advanced process control, or APC, comes up when the topic of process optimization is raised. If the goal of a production plant is to optimize efficiency and throughput, then such a discussion should certainly take all options into consideration – especially those that are deemed "advanced." That word on its own suggests a level of performance that is closer to optimal. After all, it’s advanced, right?
The PID fulfills the basic regulatory control needs of most production facilities around the world. It’s worth noting that PID control provides the foundation for many advanced solutions. Indeed, APC and other supervisory control solutions typically sit on top of a plant’s PID-based regulatory control layer. For APC to do its job and to make optimization possible, the underlying PIDs must first do their job.
In any discussion of process optimization and the use of APC, it's good to keep things in perspective. In that debate about PID vs. APC, here are some thoughts to consider:
First things first
The PID remains the dominant control technology used in industry. Practitioners from across process industries would generally agree that the PID is well-suited for maintaining safe, reliable control of most production processes. They might also acknowledge that most plant staff fail to tune their PID control loops with sufficient regularity. That lack of attention degrades a controller’s performance as the corresponding process dynamics change over time. Because an APC solution ultimately relies on a plant’s PIDs, it is subject to the same challenge of changing dynamics.
Rather than implement an advanced control solution, it can often be more cost-effective to invest in optimizing a plant’s existing regulatory controllers. Let’s get first things first here. The simple act of tuning PIDs has been shown to provide significant financial benefits in terms of increased quality and throughput and decreased waste and energy consumption. The gains can come very close to the value made possible with APC with no additional investment required.
Costs and benefits
It should be no surprise that APC solutions are more complex to implement and maintain than their more-basic counterpart, the PID. Advanced model-predictive control (MPC) solutions use multiple empirical models of a given process’s dynamics along with sophisticated algorithms to proactively drive the process towards its optimal operating condition. APC’s modeling function enables adjustment of multiple dependent variables such as set point, valve position, etc. – a function that can require significant knowledge during both initial application configuration and ongoing application maintenance.
Because the two technologies are subject to the same changing process dynamics, both require regular attention to function optimally. Whereas the knowledge needed to tune PID controllers is easily obtained, getting the knowledge and skills to maintain an APC solution is comparatively less simple. More often than not, maintaining an APC implementation involves a site visit from the solution vendor. And as the saying goes, “There’s no such thing as a free lunch.” Keeping both costs and benefits in perspective is always worthwhile.
The case of APC
The PID controller operates as a feedback technology, which means it is entirely reactive in nature. Specifically, the PID takes a measured process variable as input and adjusts the controller output based on the calculated error. A PID can be expected to make numerous adjustments in response to error as the full impact of a disturbance is gradually realized. Unlike the PID, an APC solution utilizes advanced modeling to predict the size and timing of process disturbances and to proactively initiate corrections that mute, if they don't fully negate, their impact.
Although the PID has proved to be reliable and cost-effective, it has its limitations, and those often make the case for utilizing APC. Indeed, some processes are so sensitive to disturbances that the predictive and autonomous capabilities of APC make all the difference. What’s more, the performance of such processes is usually under the constant watch of trained staff who can coordinate adjustments with the APC vendor as needed. With certain processes, the financial benefits of doing these can far outweigh the cost of the service call.
As with most process control technologies, there are usually pros and cons to take into account. Experience has shown that many control-related challenges can be addressed with a plant’s existing PID controllers. The PID’s low cost of configuration and maintenance along with its flexible design are among its greatest virtues. But it’s important to acknowledge the PID’s limits. As a proof point: The market for APC technologies continues to thrive!
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