Control Notes

A control loop tuner should be proficient in using a variety of tools to be effective in any tuning situation. Customers often ask me how I tune loops, and my answer is that I use several tools – depending on the situation. Here is an overview of the tools I frequently use when analyzing and optimizing control loops.

Process Historian

Indispensable for much more than tuning, the process historian is one of my most-used tools. I use it to check valve linearity, analyze process interactions, compare loop performance before and after tuning, design feedforward controllers and characterizers, and to analyze the step-response of a process for tuning the controller.

Some plants where I work have no OPC connection for retrieving real-time process data, or don’t allow installing data collection software to collect real-time process data. Then their process historian is my only way to access plant data. I often analyze the step response for tuning purposes using the historian’s user interface, but if it is easy enough to export data to Excel, I will go that route and analyze the data using tuning software on my laptop.

For fast-responding loops, I ask the system administrator to speed up the sampling rate, because the default sampling interval on most historians is 30 to 60 seconds, which is too slow for analyzing fast loops. A one-second sampling interval is required for flow and liquid pressure loops, five seconds for most other loops, while 30 to 60 seconds serve only the slowest loops.

Some control loops I work on have processes that take hours to respond. In more than half of these cases I can go back in history and find sufficiently large operator-induced step changes that I can use for analysis and tuning. That saves me from having to do step tests and wait hours for the process to respond. I always try to get at least three of these step changes, but I prefer to have more if the process models change from one step-test to the next. This saves me a lot of time on slow-responding processes because the complete response is already in the historian. This also minimizes the need for disturbing the process with additional step tests.

Process Historian

Excel

When I analyze step-test data directly on the historian, I use a pre-built Excel spreadsheet to simplify the data analysis and controller tuning calculations. I take down a few readings from the historian and enter them into the spreadsheet, and it calculates the process characteristics, and recommends tuning settings. It supports self-regulating and integrating process types, and has Cohen-Coon, Ziegler-Nichols, Lambda/IMC, Dead-Time, Surge-Tank, and Level-Averaging tuning rules. It also allows me to speed up or slow down the loop response by calculating different tuning settings, based on my tuning objective. Every thing I need for my tuning calculations!

Excel Tuning Calculator

Loop Simulation Software

Loop Explorer is a simulation and tuning software tool that I developed to give me insights into how a loop would respond to setpoint changes and disturbances. This is essential for obtaining optimal tuning settings for the loop’s control objective. The simulator is especially handy when I use the spreadsheet to analyze the step response, since the spreadsheet does not have its own simulator. I also use the Loop Explorer software in my training classes to demonstrate many concepts related to process characteristics, PID controllers, and controller tuning.

Loop Explorer Software

Tuning Software

Of course I also use commercial tuning software. I recommend that every plant who does tuning in-house invest in good tuning software and have it accessible in every control room. If I work at a plant that already has high-end tuning software installed, I use their software. Otherwise I use the tuning software I have on my laptop. High-end tuning software applications analyze process response and automatically identify process characteristics. They provide access to different tuning methods, and render simulations of loop response with the new tuning settings. They also have databases of controller types, so one doesn’t have to deal with manually converting tuning constants to suit a specific controller.

One very important point: Tuning software is just a tool and is no substitute for understanding process dynamics, PID controllers, and the tuning process. If you can’t tune control loops by manually determining process characteristics from step-response data, and applying an appropriate tuning rule to calculate tuning constants, you will likely not be successful with software either.

Operator Time Trends

When I do step testing, I mostly sit right next to the operator. Then we use his/her real-time trends for the control loop to monitor the response. When sitting next to the operator I can point to certain anomalies, and explain why I do certain tests. It is also a great time to get to know the operator, learn about the process he controls, and become familiar with the culture of the company.

P&ID and Operator Graphics

Before analyzing and tuning a control loop, I ask the operator to explain the process to me. He/she will often use their operator graphics to show me the streams into and out of the process, and the location of valves, pumps, heat exchangers etc. Process engineers will often give me a set of P&IDs that I refer to.

In several occasions I discovered that other interacting or subordinate loops have to be tuned first, or placed in manual, before I could attend to the loop of concern. I also find flow measurements, not being used for control, that I can trend for supplemental information on the control valve’s performance, or if there might be a need for implementing cascade control.

Operator Graphic

Pen, Paper, and Calculator

And don’t forget the traditional pen, paper and calculator. I find it handy and convenient to quickly draw a diagram on paper, take notes, or to quickly run through calculations. I would often transfer my written notes to electronic format for inclusion in my report after the day’s tuning, or while waiting for step-test results on a slow loop.

Hand Calculations

Process Walk-Down

Whenever possible, I go out to the plant with an experienced operator or engineer to take a look at the process, equipment, and physical location and condition of the control valves and instrumentation. One time I was dealing with a vastly oversized nitrogen injection control valve that was used to control pressure on a distillation column. The loop was completely unstable, regardless of any tuning settings we tried. We tried making 0.1% steps in controller output with the controller in manual mode. Stepping the controller output upwards from 1.5% to 2.4% the column pressure showed no response (no physical change in valve opening), but at 2.5% the pressure sharply decreased. When the operator and I went out to the valve and radioed back to the control room to repeat the test, we noticed that the valve position bumped by about 5% instead of the 0.1% change in controller output. We would never have known this if we did not go to the valve. After the faulty positioner was replaced we could stabilize the loop. (However, control was still poor because the valve was grossly oversized.)

Process Walkdown

Literature

I have several really good books on process control, instrumentation, control valves, processes, PID controllers, and tuning. Some of them are academically inclined, making them virtually useless for tuning controllers in real plants. But some others are much more practical in nature. The latter is obviously more suitable for practitioners. I track the sales of eight of these practical books on amazon.com and the top seller, Process Control for Practitioners, has sold more copies over the last two years than the next three books together. (Update: in 2014 it sold more than all the other practical books on controller tuning together.)

Summary

Even though I am a big proponent of tuning software, it is not the only tool available for analyzing and tuning control loops. It is important to consider the situation, and use the most appropriate tool or technique for analyzing and optimizing control loops – even if it comes down to doing manual calculations on a piece of paper.

Stay tuned,

Jacques Smuts

Principal Consultant at OptiControls and author of the book Process Control for Practitioners