Soon 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.
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
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.