Control Notes

Since my company, OptiControls Inc, specializes in the optimization of process control systems, I thought it would be appropriate to begin my blog with a non-technical description of the problem it solves for customers, and how it is done.

What is automatic control?

Cruise Control

One of the best-known domestic examples of automatic controls would be the cruise control of a motor car.  The cruise control keeps the car’s speed constant, despite road gradient and wind direction.  When the road runs uphill or downhill, the cruise control automatically changes the accelerator position to keep the car’s speed constant.

Similarly, industrial processes have automatic control systems for keeping them under control and maintaining all process conditions close to their specified operating levels.  For example, at a power station, the water level in the boiler, steam temperature, and steam pressure (as well as many other items) are kept in check by the automatic control system. Complex process plants can have hundreds or even thousands of individual temperatures, flows, levels, pressures and other conditions that are controlled simultaneously.

Simple Flow Control Loop

At the core of an automatic control system are individual controllers – each controlling one aspect of the process.  Each controller monitors a specific process condition via feedback from a sensor and compares it to the desired value (set point).  The controller tries to correct any difference between the measurement and set point by changing its output to the process, which changes the position of a final control element (like a valve) and drives the process back towards the set point.  This loop consisting of the measurement, controller, final control element, and process, is called the control loop.

So what’s the challenge with this?

Three-Loop Controller

Industrial controls need to be properly tuned to do a good job of regulating all the process conditions. Improperly tuned controls can cause unsafe process conditions, poor product quality, unnecessary plant shut-downs, longer start-up times and higher operating and maintenance costs.

An example of where humans act like controllers is when we regulate the water temperature while taking a shower.  If the water is too cold, we open the hot water tap a bit and when the water is too hot we close it a bit.  And we all know from experience how important it is to turn the tap the right amount and at the right speed. If we turn it too much or too fast we will get burnt or chilled, if we turn it too little or too slow we will be uncomfortable for a longer period.

A controller has adjustable settings that govern the magnitude and rate of the changes it will make to the process.  The magnitude and rate of the controller’s output changes should be optimized for the dynamics of the process it is controlling.  If the controller reacts too fast the process will overshoot its set point. If the controller reacts too slow it will take too long to get to set point. Getting the right tuning settings for some complex processes can be quite challenging.

How well are industrial controls performing?

Unfortunately, poorly functioning controls are very common in industry.  Various studies have shown that up to 30% of controllers do not function in automatic control mode at all, while another 30% of control loops function quite poorly in automatic control mode.

In many cases the problems exist because the personnel who originally installed the control system were not well skilled at optimizing the controls.  The controllers were tuned very roughly and only well enough to get the process up and running, frequently leaving much room for improvement. In addition to this bad start, process dynamics often change during operation, and the integrity of control equipment deteriorates over time, which further reduces the effectiveness of the controls.

How should automatic controls be optimized?

You should always do control loop optimization in a systematic way, working closely with the process operator and process engineer. Before a controller is tuned, the purpose of the control loop and the control objective are established. Then the design of the control loop is reviewed and diagnostic tests are run to ensure proper performance of the measuring device and final control element.  Assuming no problems are found, the controller is tuned to work in harmony with the dynamics of the process it is controlling, and to meet the overall control objective of the loop.

The dynamic behavior of the process is determined by analyzing data from a simple process response test.  Appropriate controller settings are calculated using tuning formulas or a computer program. Finally, the new settings are entered into the controller and one or more response tests are done to ensure the process is being controlled properly and that the control objective is met.  Ideally, the controller’s performance will be monitored periodically for a few days after tuning and under different process conditions to verify improved operation.

Stay tuned!
Jacques Smuts – Author of the book Process Control for Practitioners