If there were no unmeasured disturbances, feedback control would not be necessary. Process engineers and operators could home in on the best PID output and just leave it at this value. In fact many process engineers are much more comfortable with setting a stream flow per a process flow diagram...
Much of the differences in approaches to controller algorithms and tuning can be traced back to assumptions made about the type and importance of disturbances. Each method has merits based on the disturbance frequency, location, and time lag.
Slow oscillations can be difficult to recognize especially when the period is beyond the typical time frame of the trend chart or there are intervening disturbances or recycle. Slow oscillations can be more detrimental to product quality because the large period means the amplitude is less attenuated by intervening volumes.
A unified approach to PID Control has been found that enables a common and simplified method for setting PID tuning parameters. Key features can be used to eliminate the need for retuning to deal with different dynamics and objectives.
The process variable has slow decaying oscillations. Control theory text books indicate decreasing the PID gain should make the loop more stable. You decrease the PID gain. The oscillation gets worse. You decrease the gain again. The amplitude and the period get bigger. You repeatedly decrease the PID gain.
We are aware that too high of a PID gain can cause excessive oscillations and even instability. The ultimate gain for processes with no steady state on PID horizon is usually much higher than our comfort level.
Many of the most important process variables, such as vessel and column composition, pressure and temperature, do not reach a steady state in the time frame of PID action. Batch composition, pH and temperature and, of course, level have no steady state.
The PID is by far the most prevalent controller in the process industry. Here we step back for a view of the basics of the proportional, integral, and derivative modes. These PID controller modes have distinct advantages and disadvantages and consequences if one mode dominates.
Older Distributed Control Systems (DCS) and analog controllers tended to have different tuning setting units and methods of implementing integral and derivative action. A lack of understanding of the difference between the old and new PID features and tuning settings can lead to poor and even unstable control when migrating...