CPT Process Control & Automation 3 — Questions and Answers
Question 1: What is the primary purpose of a distributed control system (DCS) compared to a standalone controller?
- To control a single loop with greater accuracy
- To centralize all control in one processor for speed
- To distribute control across multiple processors while providing centralized monitoring (Correct answer)
- To eliminate the need for field instruments
Correct answer: To distribute control across multiple processors while providing centralized monitoring
A DCS distributes control functions across multiple controllers while allowing operators to monitor and manage the entire plant from centralized operator stations.
Question 2: In a programmable logic controller (PLC), what is a 'scan cycle'?
- The time required to calibrate all analog inputs
- The sequential process of reading inputs, executing logic, and updating outputs (Correct answer)
- The rate at which the HMI refreshes the display
- A diagnostic routine run during startup
Correct answer: The sequential process of reading inputs, executing logic, and updating outputs
A PLC scan cycle consists of reading all input states, executing the control program logic, and then writing the results to all outputs, repeated continuously.
Question 3: What does 'split-range control' involve?
- Using two setpoints on a single controller
- Sending a single controller output to two or more final control elements that operate in sequence (Correct answer)
- Dividing the process into two separate control loops
- Using half the proportional band for fine control
Correct answer: Sending a single controller output to two or more final control elements that operate in sequence
Split-range control sends one controller output signal to multiple valves that each operate over a different portion of the output range (e.g., 0–50% and 50–100%).
Question 4: Which term describes the change in process variable per unit change in controller output, expressed as a ratio?
- Process time constant
- Process gain (Correct answer)
- Dead time
- Reset rate
Correct answer: Process gain
Process gain is the ratio of the change in the controlled variable to the change in the manipulated variable, characterizing how sensitive the process is to control action.
Question 5: A thermocouple produces a signal based on which principle?
- Change in electrical resistance with temperature
- Voltage generated at the junction of two dissimilar metals (Correct answer)
- Expansion of a bimetallic strip
- Frequency shift of an oscillating crystal
Correct answer: Voltage generated at the junction of two dissimilar metals
Thermocouples operate on the Seebeck effect, where a voltage is produced at the junction of two dissimilar metals proportional to the temperature difference.
Question 6: What is 'dead band' in the context of a control system?
- The frequency range where the controller gain is zero
- A range of input values over which no output change occurs (Correct answer)
- The time required for the valve to travel from closed to open
- The minimum detectable signal from a sensor
Correct answer: A range of input values over which no output change occurs
Dead band is a range of the input (or error) signal within which the controller takes no corrective action, which can cause limit cycling or persistent offset.
Question 7: Which safety instrumented system (SIS) term describes the probability that a safety function will fail to operate on demand?
- Safety integrity level (SIL)
- Probability of failure on demand (PFD) (Correct answer)
- Mean time between failures (MTBF)
- Risk reduction factor (RRF)
Correct answer: Probability of failure on demand (PFD)
Probability of failure on demand (PFD) quantifies the likelihood that a safety system will not perform its intended protective function when a hazardous event occurs.
CPT Process Control & Automation 3
What is the primary purpose of a distributed control system (DCS) compared to a standalone controller?