Loop control: Self-tuning device minimizes process disruptions


In any process plant, there is usually a PID loop or two that needs more attention than others. Self-tuning controllers can often help that, but they frequently introduce their own peculiarities to a process. Microstar Laboratories has designed a self-tuning process that uses its standard hardware components and a software package that the company promises can adjust on the fly with minimal process disturbance.

( Read an article on self-tuning control strategies here and watch for a feature article: Fundamentals of Self-Tuning Control in the July issue of Control Engineering .)

Self-tuning controllers typically find out how a process works by manipulating their control efforts and seeing how the process variable responds. Different controllers use different approaches and some are more disruptive than others. Microstar’s data acquisition processor (DAP) board runs a real-time operating system (DAPL) that allows you to specify the behavior of the DAP and use a processing command, called a PIZST, to create a testing sequence.

The gain adjustments use Microstar’s iterative feedback technique. This approach reportedly estimates a gradient direction for loop tuning improvements using measurements rather than pure mathematical analysis. It does not determine specific gains to apply but produces incremental adjustments that improve existing gains.

Every DAP board runs DAPL, and can execute the PIZST command. An application based on a single board using a basic PID algorithm and optimized for multiple channel operation can be updated at time intervals as short as 0.05 milliseconds on every channel, without ever missing an update. DAPstudio, a Windows application you can use to control DAPL on any DAP board is available for download from Microstar’s Website, and you can download the PIZST command to test in a proof-of-concept experiment for your own application.

Detailed supporting documentation is available online , along with demonstration software for testing and evaluation. Moreover, you can order hardware for evaluation at no charge.

—Edited by Peter Welander, process industries editor, PWelander@cfemedia.com , Control Engineering Weekly News

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