Question 12 CEL01 - Chief Engineer - Limited

The Correct Answer is C **Explanation for Option C (Correct):** The derivative mode (D) of a Proportional-Integral-Derivative (PID) controller is designed to respond to the speed at which the process variable is moving away from the setpoint (i.e., the rate of change of the error). Mathematically, the derivative term calculates the derivative of the error signal with respect to time ($K_d \frac{de}{dt}$). If the error is changing quickly, the derivative control action will be large, providing a quick, anticipatory response to sudden disturbances or rapid process changes. This anticipation helps to dampen oscillations and improve the stability of the system. Therefore, the control action is directly proportional to the rate at which the error is changing. **Why the other options are incorrect:** * **A) It is a control mode that produces a control action that is proportional to the gain.** * This is too vague. While gain ($K_p$, $K_i$, $K_d$) is a proportionality constant used in all modes, the output of the derivative mode is specifically proportional to the rate of change of the error, not just a general relationship to gain. The gain itself is merely a tuning parameter. * **B) It is a control mode that produces a control action that is proportional to the accumulation of error over time.** * This describes the **Integral mode (I)**. The integral mode accumulates (sums up) the error over time, addressing steady-state offset and driving the error to zero. * **D) It is a control mode that produces a control action that is proportional to the error.** * This describes the **Proportional mode (P)**. The proportional mode produces an output that is directly proportional to the instantaneous magnitude of the current error.