Control Talk Blog

Deadband and Resolution Limit Cycle Causes and Fixes

Deadband and resolution cause sustained equal amplitude oscillations called limit cycles that cannot be eliminated by controller tuning. A resolution limit causes limit cycles if there is integrating action at just one point in the control loop either in the process (e.g. level), positioner, or the process controller (e.g. PID).

Where Do Process Dynamics Come From?

Do you lie awake at night wondering what the source of process dynamics is? Do you wonder why temperature and composition controllers tend to oscillate at low production rates and low levels? Are you perplexed why some controllers need a lambda factor of 2 and others need a lambda factor...

Checklist for Loop Analysis by Trend Charts

Humans have an incredible capability to see and analyze patterns. The setup of the data historian and trend chart can either hide or enable the recognition of essential patterns. The check list can help you identify the root cause of poor loop performance and a potential solution.

Dead Time Dominance Does Not Have to Be Deadly

Severely dead-time-dominant loops are particularly challenging because a control loop cannot see and start to correct for an unmeasured disturbance until after one dead time. Complete correction takes at least two dead times. Also, such loops are more susceptible to noise, since there is not a major process time constant...

The ABCs of Controller Tuning

All of the major tuning methods end up with the same expressions for PID gain, reset time, and rate time when the tuning objective is maximum disturbance rejection. Differences come down to tweaking of the a, b, and c coefficients.

What are the Alternatives to Reduce Noise?

Nearly all loops have noise. Whether you see the noise is a matter of amplitude, frequency, resolution, and data compression. Signal filters and damping can reduce noise but the penalty is a measurement lag and deterioration in the ability to reject disturbances.

Future PV Values are the Future

Control loops have to deal with disturbances and setpoint changes, yet operator displays give a static picture. The simple identification of the loop deadtime and calculation of the process variable (PV) rate of change can be used to provide intelligent trends and future trajectories that not only help the operator...

Checklist for Best PID Performance

There is an incredible offering of PID features and options enabling maximum disturbance rejection and setpoint response but also the coordination of loops and unit operation optimization. While the full aspects of the PID capability are book worthy, this checklist and an excerpt from an ISA Automation Week 2012 paper...

Checklist for Best pH Measurement Performance

The composition measurement with by far the greatest sensitivity and rangeability is pH.  Consider that pH routinely detects changes at 7 pH to the 9th decimal place and for a 0-14 pH scale covers 14 orders of magnitude of hydrogen ion concentration.  New glass and reference designs have dramatically reduced...

Checklist for Best Inline Flowmeter Performance

Nearly all process inputs are flows. The measurement of flow is important for process analysis, metrics, and modeling, reducing variability of process inputs, enabling feedforward and ratio control, and isolating valve nonlinearities by secondary flow loops. Inline flow meters offer the best accuracy and rangeability.