A working engineer's take on "Feedforward Vs Feedback Control" from instrumentationtools.com
Why Feedback Alone Loses the Drum Level Fight
The original article summarizes the textbook trade-off accurately. Feedback PID reacts only after the controlled variable moves. Feedforward acts on the measured disturbance before the process feels it. However, feedforward demands an accurate process model and reliable disturbance measurement. The article uses a boiler drum as its example, and that example deserves deeper treatment. A boiler drum holds little liquid capacity. A sudden steam demand from a downstream unit swells or shrinks the apparent level. Feedback-only control then drives feedwater the wrong way during the transient. I have watched single-element drum level loops oscillate for hours after a turbine load ramp. The controller was tuned fine. The strategy was simply wrong for the service.
Building Three-Element Control in Yokogawa CENTUM VP
Three-element control combines feedforward and feedback cleanly. Steam flow provides the feedforward, drum level trims through feedback, and feedwater flow closes an inner cascade loop. Here is how I build it in Yokogawa CENTUM VP.
- Step 1: Create the inner PID block for feedwater flow. Use the PID function block in AUTO as the cascade slave. Tune it fast and tight, because it is your workhorse.
- Step 2: Add the drum level PID as the master controller. Set its output range in engineering units that match the steam flow signal.
- Step 3: Wire steam flow through a lead-lag block into the summer. The lead time compensates feedwater valve lag against the boiler's swell dynamics.
- Step 4: Configure dynamic compensation so that feedwater demand tracks steam demand during ramps. Start with lead and lag within thirty seconds of each other, then refine on trend data.
- Step 5: Add a feedwater characteristic station so operators can run the loop manually during startup. Configure bumpless transfer back to cascade before commissioning.
Second, use the function block detail builder to verify block execution order. A steam flow signal computed after the PID block runs adds one scan of delay. Therefore, sequence the arithmetic blocks first in the controller's task schedule. Moreover, document every gain in the summer block. Undocumented feedforward gains are the number one cause of mystery trips years later. (Check out our Yokogawa Analog Input and Digital Output Modules for reliable I/O handling).
How Emerson Ovation Teams Approach the Same Problem
Emerson Ovation implements the same strategy with its control builder. Ovation PID controllers support embedded feedforward inputs natively, so you avoid a separate summer block. First, attach the steam flow signal directly to the feedforward input of the drum level controller. Second, scale the feedforward gain to match the feedwater-to-steam mass balance, including blowdown. However, the same lead-lag discipline applies. Ovation lets you place the dynamic compensation on the feedforward path, and you should use it. Moreover, Ovation's point group security lets you lock the feedforward gain behind engineering access. I recommend doing exactly that after tuning, because operators occasionally zero the gain to "calm the loop" and destroy the load tracking. (For your Ovation upgrades, see the Ovation HART Analog Input Module and Analog Output Modules).
Fieldbus Details That Make or Break the Strategy
The flow signals feeding this strategy often arrive over FOUNDATION Fieldbus or HART. Fieldbus transmitters publish their process variable on a scheduled macrocycle, typically 250 milliseconds to one second. That update rate is fast enough for steam flow feedforward. However, confirm the control module location. If the PID runs in the DCS while the transmitter executes in the field device, link latency matters. Keep the inner feedwater flow loop fast regardless of protocol. Moreover, check transmitter damping settings before blaming the control strategy. A two-second damping filter in a steam flow transmitter adds phase lag that no lead-lag block can fully recover. Therefore, audit damping values during commissioning. Finally, trend both the feedforward contribution and the feedback trim contribution separately. When the trim does all the work, your feedforward gain or model is wrong.
Conclusion & Action Advice
Neither feedforward nor feedback wins alone. Feedforward rejects measured disturbances early, while feedback trim covers model error and the unmeasured rest. First, build the inner feedwater cascade loop and tune it tight. Second, add steam flow feedforward with honest lead-lag compensation. Moreover, separate the feedforward and feedback contributions in your trends so you can see who is working. Therefore, test the strategy with a real load ramp before handover, not just a setpoint step. Finally, lock the feedforward gains behind engineering access and document them for the next engineer.