1. Nature of the Control Action: In linear controllers, the output is proportional to the error signal in a continuous manner. However, an ON-OFF controller's output is a discontinuous function of the error. Because the output "jumps" abruptly between states rather than following a straight-line relationship with the input error, it is fundamentally a
non-linear controller.
2. Performance Characteristics:
• Oscillation: Because it cannot provide partial output, the controlled variable constantly oscillates around the setpoint.
• Simplicity: Contrary to one of the options, these are exceptionally simple and inexpensive to design and implement.
• Deadband: Most ON-OFF controllers incorporate a differential or "deadband" to prevent rapid cycling (chattering) of the control element.
3. Why other options are incorrect:
• Linear controller: Incorrect, as the output is discontinuous.
• Proportional band is 100: This terminology refers to proportional (P) controllers, where a proportional band of 100% means the gain ($K_p$) is 1.
• Complex to design: Incorrect; they are the least complex of all control modes.