Control System Design & Architecture Flashcards

Which document type formally defines the measurement ranges, alarm setpoints, and control modes for every instrument in a plant?

Answer: Instrument Index / Instrument Data Sheet

Instrument data sheets capture all engineering specifications for each tagged instrument, including range, calibration, alarm limits, and materials of construction.

In control system design, what is 'control loop interaction' (coupling) and why is it a concern?

Answer: The unwanted influence one control loop has on another's controlled variable

Loop interaction occurs in multi-variable processes where adjusting one manipulated variable also affects another loop's controlled variable, complicating tuning and stability.

A safety instrumented system (SIS) is designed to achieve SIL 2. Approximately what probability of failure on demand (PFD) range does SIL 2 require?

Answer: ≥10⁻³ to <10⁻²

SIL 2 corresponds to a PFD range of 10⁻³ to 10⁻² (average probability of failure on demand between 1 in 1,000 and 1 in 100) per IEC 61508/61511.

What is the difference between 'fail-safe' and 'fail-secure' in control system design?

Answer: Fail-safe moves to a safe process state; fail-secure maintains the last known state

Fail-safe drives the system to a predetermined safe condition (e.g., valve opens or closes), while fail-secure prioritizes maintaining the secured/locked state to prevent unauthorized access or process upset.

In a feedforward control strategy, the feedforward signal is derived from:

Answer: A measured disturbance variable that is expected to affect the process

Feedforward control measures a disturbance before it affects the controlled variable and applies a corrective action predictively, rather than waiting for error to develop.

Which network topology provides the highest redundancy for a plant-wide control network but requires the most cable?

Answer: Mesh topology

A fully meshed topology connects every node to every other node, providing multiple redundant paths but requiring significantly more cable than star or ring topologies.

A batch recipe calls for adding ingredient A only after the reactor temperature has reached 80°C. This sequential condition is best implemented using which design element?

Answer: Interlocking logic or sequential function chart (SFC) step transition

Sequential function charts (SFCs) or interlock logic define transitions between steps, ensuring ingredient A addition only begins when the temperature condition is satisfied.