Process Control Systems & Automation Flashcards

What is feedforward control, and how does it differ from feedback control?

Answer: Feedforward measures a disturbance and applies corrective action before it affects the process variable; feedback corrects after the process variable has deviated

Feedforward control measures an incoming disturbance and compensates proactively before the disturbance affects the controlled variable, whereas feedback control only corrects after a deviation from setpoint has already occurred.

What is the Ziegler-Nichols closed-loop method used for in process control?

Answer: Determining initial PID tuning parameters by finding the ultimate gain and ultimate period of the loop

The Ziegler-Nichols closed-loop method determines PID tuning parameters by bringing the controller to its ultimate gain (where sustained oscillation occurs) and measuring the ultimate period, then applying tuning formulas.

What is 'integral windup' in a PID controller, and when is it most likely to occur?

Answer: Accumulation of integral error during a period when the controller output is saturated (at its maximum or minimum limit), leading to overshoot when the constraint is removed

Integral windup occurs when the integrator keeps accumulating error while the controller output is at its limit (saturated), causing a large overshoot once the limit is no longer active; anti-windup strategies limit this accumulation.

In a split-range control strategy, what is the typical application?

Answer: Dividing a single controller output signal (0–100%) to operate two final control elements in sequence — often one for heating and one for cooling

Split-range control uses one controller output to sequentially drive two (or more) control valves: for example, 0–50% output operates one valve (e.g., cooling) and 50–100% operates another (e.g., heating), providing two-directional process manipulation.

What is 'dead band' in a process control valve or controller, and what is its primary negative effect?

Answer: A range of input change to which the final control element does not respond, causing offset and limit cycling around the setpoint

Dead band is the range over which the controller output must change before the final control element responds; it causes the process variable to cycle around the setpoint in a limit cycle rather than settling precisely.

What is the purpose of a ratio controller in process automation?

Answer: Maintain the ratio of two process streams at a desired value by adjusting one stream proportionally to the other

A ratio controller maintains a fixed proportional relationship between two process flows (wild and controlled streams), ensuring the controlled stream tracks changes in the wild stream at the desired ratio.

Which ISA (Instrumentation, Systems, and Automation Society) standard defines the symbols and identification used on Process & Instrumentation Diagrams (P&IDs) for control loops?

Answer: ISA-5.1

ISA-5.1 (Instrumentation Symbols and Identification) is the standard that defines the graphical symbols, alphanumeric identification, and functional diagramming conventions used on P&IDs for instruments and control loops.