Question:
Why is on-off control not suitable for continuous process control?
Options:
A) Minimizes dead time
B) Causes continuous cycling
C) Eliminates steady error
D) Provides linear output
View Answer
Correct Answer: B) Causes continuous cycling
Detailed Explanation
On-off (bang-bang) control operates in only two states:
- Fully ON
- Fully OFF
When the process variable crosses the setpoint:
- Output switches abruptly
- The process overshoots
- Output switches again
This results in continuous cycling around the setpoint rather than smooth regulation.
For continuous processes (temperature, pressure, flow):
- Smooth modulation is required
- Actuators should not continuously switch
- Stable steady control is preferred
PID control provides gradual, proportional adjustment instead of abrupt switching.
Practical Issues with On-Off Control
- Oscillations around SP
- Equipment wear (valves, relays)
- Poor accuracy
- Large temperature or pressure swings
Why Other Options Are Incorrect
A) Minimizes dead time → On-off does not affect process dead time.
C) Eliminates steady error → It oscillates around SP, not eliminate error smoothly.
D) Provides linear output → On-off output is binary, not linear.