Variability Appearance and Disappearance

The post, Variability Appearance and Disappearance originally appeared on the Control Talk blog.

Particularly confusing is variability that seems to come out of nowhere and then disappear. There are not presently good tools to track down the sources because it turns out most of them are self-inflicted involving automation system deficiencies, dynamics and transfer of variability. You need to understand the possible causes to be able to identify and correct the problem. Here we provide fundamental knowledge needed on the major sources of these particularly confusing sources of variability.

One of the most prevalent and confusing problems are oscillations that break out and disappear in cascade control loops and loops manipulating large valves and variable frequency drives (VFD). The key thing to look for is if these oscillations only start for large changes in controller output. If you have a slow secondary loop, valve or VFD, for small changes in controller output over a period of several controller executions for small setpoint changes or small disturbances, the secondary loop, valve or VFD can keep up with the requested changes. For large changes, oscillations break out. The amplitude and settling time increase as the degree of mismatch between requested rate of change and the rate of change capability of the secondary loop, valve or VFD. The best solution is of course to make the capabilities of what is being manipulated faster. You can make a secondary loop faster by decreasing the secondary loop’s dead time and lag times (faster sensors, filters, damping, and update rates) and making the secondary loop tuning faster.  You can make the control valve faster by a higher gain and no integral action in positioner, putting a volume booster on the positioner output(s) with booster bypass valve slightly open to provide booster stability and increasing the size of air supply lines and if necessary, the actuator air connections. You can make VFDs faster by making sure there is no speed rate limiting in the drive setup, keeping fast speed control with VFD in the equipment room (not putting it into a much slower control system controller) and increasing the motor rating and size as needed. If the problem persists, turning on external-reset feedback with fast accurate readback of the process variable of the manipulated secondary loop, actual position of valve and speed of VFD can stop the oscillations.

Another confusing trigger for oscillations is a low production rate. The process gain and dead time both increase at low production rates causing oscillations as explained in the Control Talk Blog “Hidden factor in Our Most Important Loops”.  Also, stiction is much greater as the valve operating point approaches the closed position due to higher friction from sealing and seating surfaces. Valve actuators may also be undersized for operating with the higher pressure drops near closure. Stiction oscillations size and persistence increase with valves designed to reduce leakage. Most valve suppliers do not want to do valve response testing below 20% output because it makes the valve dead band and resolution worse. The installed flow characteristic of linear trim distorts to quick opening for a valve drop to system pressure drop ratio at maximum flow is less than 0.25. The amplification of oscillations from backlash and stiction and the instability from the steep slope (high valve gain) from the quick opening installed characteristic cause the oscillations to be larger. Even more insidious is the not commonly recognized reality that a VFD has an installed flow characteristic that becomes quick opening if the static head to system pressure drop ratio is greater than 0.25 triggering the same sort of problems. Signal characterization can help linearize the loop but you still need adaptation of the controller tuning settings for the increase in the process gain from the hidden factor and the increase in dead time from transportation delays and you are still stuck with stiction. Besides the increase in the VFD gain multiplying effect on the open loop gain, there is an amplification of oscillations from the 0.35% resolution limit of traditional of VFD I/O card and the dead band introduced in VFD setup in a misguided attempt to reduce reaction to noise. Then there are oscillations from erratic signals and noise from measurement rangeability problems discussed in last month’s Control Talk Blog “Lowdown on Turndown”.  Low production rates can also cause operation near the split range point and crisscrossing of the split range point causing consequential persistent oscillations from the severe nonlinearities and discontinuities besides greater stiction. Again external-reset feedback can help but the better solution is control strategies and configurations to eliminate the unnecessary crossings of the split range point as discussed in the Control Talk Column “Ways to improve split range control”. 

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