Loop tuning software: Self-adjusting platform follows process changes

-- Control Engineering, 4/2/2008

Does your process control application respond well to derivative and zero-shaping strategies in  PID control, but your process changes over time so that it is not possible to get the full benefit from these features? Microstar Laboratories says it has a solution.

Some processes change over time, making it difficult to apply the PID approach and its extensions without continual tuning. A good strategy at one condition degrades into one that tends to oscillate, overshoot, or respond slowly once things change. Microstar Laboratories says its data acquisition processor (DAP) boards for PC-based applications can be the building blocks for a better approach. Use these off-the-shelf components to build a self-tuning system for hard-to-control processes. Every DAP board runs a real-time operating system, DAPL, that you control from PC software. A new DAPL command—Pidzst, a self-tuning PID control algorithm with zero-shaping—keeps your system in full control during the self-tuning cycles, never interfering with feedback processing.

This is aimed especially at systems that need frequent loop-tuning adjustments. These can be due to command-level changes, to avoid poorly damped oscillations, and to maintain a good response time as conditions change. Ordinary derivative gain can become less effective or in some cases aggravate a tendency to oscillate when a control loop is poorly tuned. This system applies a zero shaping gain on the command input, which the company says can reduce tracking errors but can also slow response if this adjustment becomes unnecessary.

Every DAP board includes a processor running a real-time operating system, DAPL, that you configure and monitor from PC software. You specify the real-time behavior of the DAP board by downloading commands for DAPL to execute at runtime. The new Pidzst processing command is best suited for applications in which natural disturbances are relatively rare and not sufficient to provide adequate loop tuning information. Pidzst provides a self-scheduling, low-level injection test plus analysis processing that runs in real time and does not interfere with control loop operation.

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 ms on every channel, without ever missing an update.

Microstar offers all the supporting documentation and demonstration software online for you to review, plus the company offers evaluation hardware at no charge.

—Edited by Peter Welander, process industries editor, peter.welander@reedbusiness.com,
Process & Advanced Control Monthly
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