Set Point Filters Do Not Compensate for Noise But They Can Improve A Control Loop’s Overall Performance
Names – like looks – can be deceiving. While jumbo shrimp are big relative to other shrimp there’s very little about them that could be considered gargantuan. So there should be no surprise to learn that in the realm of process control a Set Point Filter has nothing to do with filtering noise within a control loop’s data.
Two different types of noise filtering solutions were covered recently. Specifically, Internal Filters were highlighted as an effective means for compensating for Process Noise while External Filters – both PV Filter and CO Filter – were cited for addressing the effects of Signal Noise. Both are valuable tools in establishing effective control loop performance as they allow the controller to literally “block out the noise” and focus on regulating a process’ true dynamics. In the same sense a Set Point Filter can be applied to smooth a PID controller’s response and improve overall control loop performance. While a Set Point Filer doesn’t compensate for noise in the process data it does mitigate excessive variability associated with large changes to Set Point.
A few aspects of the Set Point Filter worth considering include the following:
Unintended Consequences
Sudden and large changes to Set Point can have negative unintended consequences particularly when a PID controller with a large controller Gain – and associated proportional action – is involved. Significant overshoot and oscillati
on can result as the controller reacts to a large change. The resulting variability can be counter-productive from a Set Point tracking perspective. As a general rule controllers are designed to mute excessive variability – not to accentuate it.
As a strategy the Set Point Filter limits the negative impact of large Set Point changes by metering it out in smaller increments.
Applying a Measured Approach
The Set Point Filter achieves the desired, aggregate change to Set Point but it does so by metering it out in smaller increments. Gradually enacting the change still allows an aggressively tuned controller to deliver the rapid response that’s desired. However it also allows the controller to avoid the overshoot that often occurs in response to large changes. If you’ve ever used a Set Point Ramp, then you’ve employed the concept of Set Point Filtering.
Filtering in the Real-World
Processes like some Temperature Control (e.g. furnaces) and Level Control are good examples where use of a Set Point Filter can prove advantageous. With these and many batch processes it’s common for large adjustments to be administered as they transition from one to another phase of production. The Set Point Filter accommodates the goal of effective disturbance rejection while muting the overshoot which can accompany large fluctuations in Set Point.
Industrial Options Abound
No additional hardware is needed to implement a Set Point Filter. Rather, it’s a strategy that a variety of modern control systems can accommodate. Many PLCs and PACs like ControlLogix from Rockwell Automation as well as distributed control systems such as Honeywell’s Experion PKS include advanced blocks for the purpose of Set Point Filtering. Simply consult your supplier for details.
Filtering as a whole is geared towards improving the PID’s abilities to regulate control and to maintain efficient loop performance. Whether using an Internal Filter or External Filter to address noise in the process variable signal or applying a Set Point Filter to guard against unnecessary volatility, these tools allow the PID controller to do its job more effectively.
