Question

Feed-forward control is used to:

• A. Eliminate feedback from the process
• B. Compensate for measurable disturbances before they affect the output
• C. Replace the need for a sensor
• D. Tune the PID controller automatically

Hint:

Think of feed-forward control as "preventive action" (measuring cause) and feedback control as "corrective action" (measuring effect).
Feed-forward control requires a process model and direct measurement of the disturbance.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question asks for the fundamental objective and operating principle of feed-forward control.
In industrial process control, feed-forward is an advanced control strategy that works in conjunction with feedback control to handle disturbances more effectively.

Step 2: Key Formula or Approach:
Feedback control is reactive: it measures the controlled variable (output), compares it to the setpoint, and takes corrective action only after an error has occurred.
Feed-forward control is proactive: it measures the disturbance directly, predicts its effect on the output, and takes corrective action before the disturbance can cause any deviation in the output.

Step 3: Detailed Explanation:
Let us analyze how feed-forward control operates:
1. A sensor measures a major process disturbance (such as feed flow rate or inlet temperature changes) before it enters the process equipment.
2. The feed-forward controller uses a mathematical model of the process to calculate the exact adjustment needed in the manipulated variable to cancel out the predicted effect of this disturbance.
3. The manipulated variable is adjusted immediately, compensating for the disturbance.
Because it acts preemptively, feed-forward control can theoretically achieve perfect control (zero deviation) if the process model is perfectly accurate.
However, because not all disturbances are measurable or perfectly modeled, feed-forward control cannot completely replace feedback control.
Instead, they are usually combined (feed-forward-feedback system) where feed-forward handles major measurable disturbances, and feedback handles unmeasured disturbances and modeling errors.

Step 4: Final Answer
Thus, feed-forward control is used to compensate for measurable disturbances before they affect the output, which corresponds to option (B).