Learning PID loop tuning from an expert

Hands-on experience helps accelerate the PID learning curve.

12/03/2014


Figure 1: Small-capacity pumps are common in chemical plants, where they are used to control the liquid level inside storage tanks as well as a variety of other applications that require constant pressure throughout the chemical process. Courtesy: YaskawaSoon after graduating from college, I got a job at a chemical plant as an electrical and instrumentation technician. Being a young guy with little experience, my supervisor paired me up with a grizzled veteran named Tim. Of the many things I learned from Tim, one of the most valuable was a simple process for tuning a proportional, integral, and derivative (PID) control loop. I learned about basic control systems and how they worked as part of my education. Unfortunately, all of my experience with PID loops had been in textbooks. I never had the firsthand experience of tuning a PID control loop in the field.

My first chance at tuning a PID loop came during a trip with Tim to the ethylene production area on the west side of the chemical plant. An operator in the ethylene control room had placed a trouble call for a 500 gal chemical tank that was not maintaining an accurate level (see Figure 1). Tim reminded me that PID control loops can be found in a variety of applications that require constant control of liquid level, pressure, flow, temperature, or tension, to name a few. In this case, the operator was trying to keep a constant level in a 500 gal chemical additive tank that was a part of a chemical line process, but it was not working properly (see Figure 2). 

First things first

The first thing Tim taught me about PID control systems was the basic components. "The first thing you need is a setpoint signal," he said. In our situation, the setpoint signal was a 0-10 Vdc signal from a potentiometer in the control room that the operator used to set the level he wanted to maintain in the tank. "The second thing you need in your system is a feedback signal," said Tim. The feedback device in this case was a liquid level transducer that provided a 4-20 mA signal based on the level of the liquid in the tank. Tim explained that the last item required in the system is the actual PID controller.

Figure 2: This control system diagram is typical of a chemical plant chemical line process containing an additive tank. Courtesy: Yaskawa America Inc.

Although it has been quite a few years since I graduated from college, at that time, the controller was a self-standing module that received the setpoint and feedback signals and performed the PID number crunching. It provided a 4-20 mA output signal that controlled a valve that fed the tank. It also had a built-in small strip chart recorder that showed the liquid level over long periods of time. You can still purchase stand-alone PID control modules today, but the software can also be found in VFDs, and most commonly in PLCs and building control systems (see Figure 3). PID control loop software can be found inside of the PLC that runs an entire control room and provides a sophisticated graphical look of the entire control system on a variety of monitors and control desks. Even though today's PID graphics look much better than that old strip chart recorder, the PID control method used today is basically the same. 

The proof is in the testing

Figure 3: The output signal of a PID controller can be used to control a valve or can be fed to a VFD that controls the speed of a pump motor to control process flow or tank level. Courtesy: Yaskawa America Inc.When we got to the job site, the first thing we did was test the liquid level sensor, which provided feedback to the controller. After a few minutes, we determined the sensor was in good working order because it provided 4 mA at low level and 20 mA at high level. The next thing we checked was the setpoint signal. Tim stood in the control room and adjusted the potentiometer from minimum to maximum while I measured the signal where it connected to the PID module. The potentiometer was functioning properly and measured 0 Vdc at minimum and 10 Vdc at maximum. The last thing we checked was the valve itself. We connected a small variable milliamp supply that provided 4-20 mA to the valve and watched it open and close without a problem. So the culprit in our nonfunctioning PID control system appeared to be the PID module itself. A quick trip to the parts depot provided us with an identical replacement module and we headed back to the ethylene control room. I figured we would swap the module; adjust the proportional, integral, and derivative selector switches to the same settings as the old one; do a quick test of the system; and be back at the shop in time for lunch.

The PID module we used had three selector switches on the side: one for the proportional gain setting, one for the integral time setting, and one for the derivative time setting. In most cases, we would have set the selector switches on the new module to the identical settings of the old module and called it a day. But Tim thought we could improve on the performance of the control loop and saw a chance to teach me some of the finer points of PID loop tuning. 


<< First < Previous Page 1 Page 2 Next > Last >>

The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
Pipe fabrication and IIoT; 2017 Product of the Year finalists
The future of electrical safety; Four keys to RPM success; Picking the right weld fume option
A new approach to the Skills Gap; Community colleges may hold the key for manufacturing; 2017 Engineering Leaders Under 40
Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
The cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Power system design for high-performance buildings; mitigating arc flash hazards
VFDs improving motion control applications; Powering automation and IIoT wirelessly; Connecting the dots
Natural gas engines; New applications for fuel cells; Large engines become more efficient; Extending boiler life

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me