In daily process control operations, periodic oscillations act like an “invisible killer” to equipment health. Even attenuating oscillations subtly accelerate valve wear and other issues. Accurately identifying the type of oscillation, quickly pinpointing its source, and implementing effective solutions are key to improving automation levels and operational stability. This article delves into the identification patterns of three common types of oscillations and their respective treatment strategies, employing a systematic approach to solve control issues and turn automation systems into stabilizers, rather than sources of oscillation.
What Are the Types of Oscillations and Their Features?
In-phase Oscillations
Characteristics: The process variable (PV) and controller output (OP) rise and fall simultaneously, with symmetrical curves.
Cause: External strong disturbances or excessive proportional action.
Diagnosis and Solution:
Reduce the proportional gain by one-third. If the oscillation worsens, an external disturbance is likely, requiring identification and removal of the disturbance source.
If the oscillation disappears, it confirms that the proportional action was too strong. Maintain the adjusted parameters.
Out-of-phase Oscillations
Characteristics: The peaks of PV correspond to the “zero-change” point of OP, creating an inverse relationship.
Cause: Overly strong integral action.
Diagnosis and Solution:
For self-regulating processes: Set the integral time to the oscillation period or divide by three. This typically eliminates the oscillation.
For integrating processes: The integral time must be significantly increased.
Non-smooth Oscillations
Characteristics: PV shows a square wave, while OP appears sawtooth-like.
Cause: Non-linearity in the control valve (e.g., sticking, viscosity).
Solution: Repairing or improving the valve is the only effective solution (e.g., lubrication, tightening packing, stem calibration, adjusting the positioner, or replacing the valve). Adjusting PID parameters is generally ineffective.
Key Insight: If oscillation phase characteristics are unclear, prioritize reducing proportional action.
In-Phase Oscillation Deep Dive:
When PV and OP peaks significantly deviate from setpoint values, precise measures must be taken based on the behavior in manual mode:
Manual Oscillation Disappears:
Cause: Proportional action is too strong, or the loop is coupled with another.
Solution: Reducing proportional gain by one-third typically resolves the oscillation. If coupling is the issue, reducing the gain of any related loop also resolves the issue. Afterward, fine-tune the integral time or consider advanced control strategies (e.g., feedforward, cascade).
Manual Oscillation Intensifies:
Cause: External periodic disturbances, often from upstream oscillations.
Solution: Primary strategy: Locate and adjust the interfering loop (follow the “upstream to downstream” principle). Backup strategy: If disturbance sources cannot be eliminated, significantly increase the proportional action to suppress the disturbance.
Manual Oscillation Remains Unchanged:
Cause: Measurement noise or weak control action.
Solution: Increase measurement filtering and substantially enhance the proportional action (e.g., increase proportional gain by three times or more). Observe the oscillation behavior. If the amplitude does not increase significantly, continue strengthening the control.
Systematic Process for Eliminating Oscillations
Define the Problem: Establish the goal of “zero amplitude oscillations”. Periodic oscillations indicate that there is a recurring disturbance in the system. Even with a high control rate, these oscillations should not be ignored.
Parameter Tuning:
Out-of-phase oscillations: Significantly reduce integral action.
Non-smooth oscillations: Repair the valve or actuator—adjusting parameters won’t be effective.
In-phase Oscillation Root Cause Analysis:
Check in Manual Mode: Do not rush into parameter adjustments.
Oscillation Persists/Increases: The issue is likely external. Look for upstream or external oscillation sources or periodic process disturbances. If the disturbance cannot be eliminated, enhance the control action.
Oscillation Disappears: The loop is the source, or there is coupling. Reducing the proportional action (either in the affected loop or a coupled loop) by one-third should resolve the issue. Prioritize lowering the performance of lower-priority loops.
Conclusion: Oscillations serve as a benchmark for automation performance. A low control rate indicates significant room for improvement, while a high control rate with frequent interventions suggests missing critical control loops. High control rates with minimal intervention but regular oscillations highlight hidden control problems. Therefore, eliminating all periodic oscillations is essential to ensuring the stable operation of the system and unlocking the full potential of automation. Even attenuated oscillations or occasional manual interventions indicate that further system optimization is possible. Moving beyond single-loop performance, utilizing valve positioning freedoms, and implementing multivariable optimization should be the direction for continuous improvement in process control.