Self-Tuning Controllers Auto-Select P, I, D Values

09/01/1997


Tuning a PID controller is conceptually simple--observe the behavior of the controlled process and fine tune the controller's proportional (P), integral (I), and derivative (D) parameters until the closed-loop system performs as desired. However, PID tuning is often more of an art than a science. The best choice of tuning parameters depends upon a variety of factors including the dynamic behavior of the controlled process, the controller's objectives, and the operator's understanding of the tuning procedures.

Self-tuning PID controllers simplify matters by executing the necessary tuning procedures automatically. Most observe the process' reaction to a disturbance and set their tuning parameters accordingly. However, no two go about accomplishing those tasks in the same way.

'Heuristic' self-tuners, for example, attempt to duplicate the decision-making process of an experienced operator. They adjust their tuning parameters according to a series of expert tuning rules such as 'IF the controller overreacts to an abrupt disturbance THEN lower the derivative parameter.'

Model-based approach
A more common approach to automatic parameter selection, however, involves a mathematical 'model' of the process--an equation that relates the present value of the process output to a history of previous outputs and previous inputs applied by the controller. If the model is accurate, the controller can predict the future effect of its present efforts and tune itself accordingly.

For example, a process that reacts sluggishly to a step input can be modeled with an equation that gives the current output as a weighted sum of the most recent output and the most recent input. A self-tuner can choose the weights in that sum to fit the model to the observed process behavior. With the model in hand, the self-tuner can go on to determine how much proportional, integral, and derivative action the process can tolerate. In the case of a sluggish process, the model will show that the controller is free to apply aggressive control efforts. The self-tuner will then set the P, I, and D parameters to relatively high values.

Variations on the theme
Exactly how high or low the tuning parameters should be set depends on the performance objectives specified by the operator. If, for example, the settling time is to be limited to some maximum value, the required tuning parameters can be determined by analyzing the time constant and the deadtime of the process model. On the other hand, if excessive overshoot is the operator's principal concern, the controller can be configured to select tuning parameters that will limit the rate of change of the process variable.

Self-tuning controllers also differ in their data collection techniques. Some apply a series of artificial disturbances to the process in order to observe how it behaves. Others make do with data collected during normal loop operations. The latter approach limits the waste and inconvenience caused by intentionally disturbing the process, but generally produces much less useful information about the process' behavior.

Which of these many variations is appropriate for a given application of self-tuning control is up to the operator. A single universally applicable technique has yet to be developed.





No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
Control Engineering Leaders Under 40 identifies and gives recognition to young engineers who...
Learn more about methods used to ensure that the integration between the safety system and the process control...
Adding industrial toughness and reliability to Ethernet eGuide
Technological advances like multiple-in-multiple-out (MIMO) transmitting and receiving
Virtualization advice: 4 ways splitting servers can help manufacturing; Efficient motion controls; Fill the brain drain; Learn from the HART Plant of the Year
Two sides to process safety: Combining human and technical factors in your program; Preparing HMI graphics for migrations; Mechatronics and safety; Engineers' Choice Awards
Detecting security breaches: Forensic invenstigations depend on knowing your networks inside and out; Wireless workers; Opening robotic control; Product exclusive: Robust encoders
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
News and comments from Control Engineering process industries editor, Peter Welander.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
Anthony Baker is a fictitious aggregation of experts from Callisto Integration, providing manufacturing consulting and systems integration.
Integrator Guide

Integrator Guide

Search the online Automation Integrator Guide
 

Create New Listing

Visit the System Integrators page to view past winners of Control Engineering's System Integrator of the Year Award and learn how to enter the competition. You will also find more information on system integrators and Control System Integrators Association.

Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Control Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.