Enhancing Control System Performance with Gap Control Techniques

Gap control is a key aspect of regulatory process control and can be applied in various continuous control systems. Essentially, gap control allows you to use different controller gains for different parts of your control process. There are two main parameters that need to be configured for gap control:

1. Gap Range – This is the range around the controller's setpoint, defining the area where the gap gain should be applied. Outside this limit, the normal controller gain takes effect.
2. Gap Gain – A gap multiplier can be adjusted, which is then multiplied by the main controller gain. The resulting value becomes your gain within the gap range.

In this article, I will focus on three specific cases where gap control should be applied:

• Sluggish level control to ensure stable flow to downstream processes.
• Managing non-linearity.
• Handling noisy signals.

1. Sluggish Level Control: Ensuring Stable Flow

Understanding the Challenge

For this point, consider a situation where you have level control, and it's crucial to send a stable flow to the downstream process without the need for tight level control. In such cases, applying gap control allows for smaller corrective actions within the gap range and larger actions when the level exceeds the gap area.

Configuring Gap Control

When setting gap control parameters, several factors should be considered. First, define the gap area based on your measured volume. If the volume is smaller, the gap range should be larger. The reason for increasing the gap range with smaller volumes is that a larger range allows the output to remain stable for longer periods, giving the controller enough time to maintain the level within the desired range.

Responding to Changes

For example, imagine you have a small volume and a sudden increase in inlet feed due to upstream changes. With a small volume, the level will rise quickly. If the gap range isn’t sufficient to compensate for this change through output adjustments, the level will exceed the gap range, triggering the main controller gain. This could result in significant fluctuations in your output flow, which should ideally remain stable, and may cause the level to oscillate around the gap area.

Key Considerations

It’s also important to avoid setting the controller gain to zero within the gap range. Even within the gap, some proportional action is needed to bring the level back into the range and prevent it from exceeding the gap area. If you set the gain to zero, disturbances can push the level outside the gap, causing larger and more immediate changes in downstream flow. Instead, maintaining some corrective action within the gap allows for smoother control and minimizes fluctuations.

2. Managing Non-Linearity in Control Valves

Identifying Non-Linearity

For this scenario, consider situations where your control valve operates in the nonlinear phase of its characteristic curve. To address nonlinearity, certain soft implementations can be configured, which I will discuss in future articles. However, when the valve is expected to operate in the nonlinear region for an extended period, applying gap control can be an effective solution.

Applying Gap Control

Gap control is useful in these cases because, in a nonlinear region, even a small change in valve output can result in significant changes in the process variable. For example, if your valve has an equal percentage (EQ%) characteristic and is operating at high capacity, even a minor adjustment in the valve output can lead to significant changes in the process variable. The same principle applies at lower capacities with a quick-opening valve.

Setting the Gap Gain

Since process gain is higher in certain parts of the process, applying gap control helps keep the controller gain as low as possible within the gap area. If the process is self-regulating, you should set the gap gain to zero. However, if the process is integrating, you should assign a very small value for the gap gain, as mentioned earlier.

3. Handling Noisy Signals

Diagnosing the Noise Source

For the this scenario, gap control should be considered if you determine that the noise in the process variable is caused by the measurement itself, and it oscillates at approximately the same frequency. To test this, you should put your control loop in manual mode and observe for some time to see if the noise continues. This will help you determine whether the noise is due to measurement issues or aggressive tuning.

Implementing Gap Control

If you observe during the open-loop test that the noise remains, it indicates that the measurement is the source of the noise. In this case, applying gap control can reduce unnecessary valve actions, thereby extending the valve's lifetime and minimizing unwanted effects on downstream processes.

Once you have data from the open-loop test, calculate the standard deviation of your process variable around the average value. This calculation will help you determine the minimum gap range parameter. After defining the gap range, set a smaller gap gain, or zero if the process is self-regulating.

Conclusion

Gap control is a useful tool in regulatory process control, essential for managing various operational challenges. By properly configuring gap range and gain, you can enhance stability and efficiency, whether dealing with sluggish level control, valve non-linearity, or noisy signals. Applying gap control strategically improves process reliability, extends equipment life, and minimizes downstream impact. Understanding these principles enables smoother operations and better control system performance.