Control Modes - ON / OFF Control

Summary

This video explains the fundamentals of measurement and control, focusing on ON/OFF control and its applications. It describes how the controller output toggles between ON and OFF based on the process variable's relation to the set point. The ON/OFF control mode is ideal for systems where fluctuations are acceptable, such as liquid level control and home heating systems. The video also touches on proportional control, highlighting its constant relationship between input and output, setting the stage for further exploration of control modes.

Takeaways

• 😀 ON/OFF control is a type of two-position control where the controller output switches between two fixed conditions based on the process variable and setpoint.
• 😀 The ON/OFF control mode is typically used in systems where fluctuation and cyclic control are permissible, such as large capacity systems.
• 😀 The controller is ON when the process variable is below the setpoint and switches to OFF when the process variable exceeds the setpoint.
• 😀 In ON/OFF control, the controller’s output drives the manipulated variable (e.g., valve) to an open or closed position, regulating the process.
• 😀 When the process variable falls below the setpoint, the controller switches to ON, opening the valve, and when it exceeds the setpoint, the controller switches to OFF, closing the valve.
• 😀 The fluctuation around the setpoint is due to the differential gap, meaning the process variable oscillates between the two positions before reaching equilibrium.
• 😀 The ON/OFF control mode is effective for systems where precise control is not required, and where a constant output is acceptable with periodic fluctuations.
• 😀 Common applications for ON/OFF control include liquid level control, home heating systems, and domestic water heaters.
• 😀 The ON/OFF control mode operates based on the error between the process variable and the setpoint, with no proportional or integral action involved.
• 😀 Proportional control (P) is the next topic in control modes, which involves a constant relationship between the input and output of the controller, differing from ON/OFF control.

Q & A

What is ON/OFF control in control systems?

ON/OFF control, also known as Two Position Control, is a type of control where the controller switches its output between two fixed conditions: ON when the process variable is below the set point, and OFF when the process variable exceeds the set point.

How does the ON/OFF control mode work in terms of process variable and set point?

In ON/OFF control mode, the controller monitors the difference between the process variable (current value) and the set point (desired value). The controller is ON if the process variable is below the set point, and switches OFF if the process variable exceeds the set point.

What happens when the ON/OFF controller switches from ON to OFF?

When the controller switches from ON to OFF, the valve closes, stopping the flow or changing the manipulated variable to its minimum position. This occurs when the process variable reaches or exceeds the set point.

Why does the process variable fluctuate around the set point in ON/OFF control?

The process variable fluctuates around the set point because of the controller's switching action. The controller only operates in the ON position until the set point is reached, then switches OFF. This creates a cycle of increasing and decreasing the process variable within a differential gap.

What is the differential gap in ON/OFF control?

The differential gap is the range between the set point and the point where the controller switches between ON and OFF. This gap prevents constant switching and allows the process variable to fluctuate within a defined range.

Can ON/OFF control be used in all applications?

ON/OFF control is typically used in applications where some fluctuation and cyclic control are permissible, such as large capacity systems. It is commonly used for liquid level control and systems like home heating or domestic water heaters.

What are some examples of applications that use ON/OFF control?

Examples of applications using ON/OFF control include liquid level control in tanks, home heating systems, and domestic water heaters.

How does the controller handle situations where the process variable drops below the set point?

When the process variable drops below the set point, the controller switches to the ON position, causing the manipulated variable (like a valve) to open and gradually increase the process variable toward the set point.

What is the primary characteristic of proportional control?

Proportional control (P) is characterized by a constant relationship between its input and output, where the controller output is proportional to the error between the set point and the process variable.

How does ON/OFF control differ from proportional control?

ON/OFF control operates in a binary manner, switching between two fixed conditions (ON and OFF), while proportional control adjusts the controller output continuously based on the magnitude of the error, offering smoother control.