While PID Control with Filter Provides an Answer for Process Noise, It Raises Other Questions.
Sometimes controller design feels like a choice between the lesser of not two but three evils. P-Only Control is simple and provides a fast response, but the resulting Offset hinders tight control. PI Control addresses Offset but it leaves room for improvement. And then there’s PID Control which enhances a loop’s Set Point Tracking but typically at the expense of the associated Final Control Element (FCE). If not for the ubiquitous nature of noise in industrial applications, then PID Control would be the clear choice. But since there’s no escaping noise, what’s a practitioner to do?
As shared previously, filtering – both Internal Filtering and External Filtering – is a commonly applied approach for mitigating the negative effects of noise. Of those options Internal Filtering corrects for Process Noise by mathematically manipulating the process’ signal. It allows for a process’ true, underlying dynamics to be understood and controlled. However, the manipulation also results in a muted controller response which can be counter-productive when speed is needed.
The following processes are generally suitable for PID Control with Filter:
- Large Vessel Pressure Control
A key characteristic of such a process is that it is generally slow and often noisy. Given the slow speed of the process’ dynamics it is largely immune to any negative aspects of filtering. Specifically, use of a filter may result in a muted controller response. Fortunately the delay is typically marginal and it plays an insignificant role in the control of such a sluggish process. What’s more, the filter effectively dampens noise in the PV signal which calms the Controller Output and protects the FCE from excessive movement.
- Distillation Reflux Control
Distillation columns are generally quite large, reaching several stories into the sky. Like the previous example they are characteristically slow and noisy. During normal operation a portion of the distillate is captured in a reflux drum for further processing. Like other processes involving liquids and gases, levels of process noise are generally high. This justifies the use of a filter as a means of steadying output and protecting mechanical assets. As with the previous example, the dynamics of Distillation Reflux Control are slow. Unlike a fast acting process the introduction of a modest delay will have little effect on the unit’s overall control capabilities.
These and other similar processes are well suited for PID Control with Filter. The correction for Process Noise allows the controller to deliver superior performance without overworking the associated FCE. But while PID Control with Filter may solve for noise, it introduces a new and different challenge – delay – and is even more complex to design. Practitioners must consider this when selecting a form of the PID controller. It’s no longer a choice between the least of three evils – it’s now four!
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