AME Unit 11 Q1

A discontinuous or on-off control system is a type of controller that has only two states: on or off. The output signal is switched between these two states depending on whether a measured process variable is above or below a preset setpoint.

How it Works:

• Setpoint: This is the desired value for the process variable.
• Process Variable: This is the actual measured value of the controlled quantity.
• Controller: Compares the process variable to the setpoint.
• Output: Either fully on or fully off, depending on the comparison.

Example:

• Thermostat: A common example is a thermostat. If the room temperature falls below the setpoint, the heater turns on. When the temperature reaches the setpoint, the heater turns off.

Characteristics:

• Simple: Easy to understand and implement.
• Economical: Often less expensive than more complex control systems.
• Oscillations: Tends to produce oscillations around the setpoint due to the abrupt on/off switching.
• Limited Accuracy: Not suitable for applications requiring precise control.   1. Process Control: A Comprehensive Guide to Implementation and Understanding in Industrial Systems – Wevolver www.wevolver.com

Applications:

• Simple heating and cooling systems: Homes, small commercial buildings.
• Pump control: Starting and stopping pumps based on level or pressure.
• Lighting control: Turning lights on and off based on light levels.

Image:

Opens in a new windowx-engineer.org onoff control system

In summary, on-off control is a basic control strategy that can be effective for simple systems but may not be suitable for applications requiring precise control or high performance.  

1. On-off Vs PID Temperature Controller: Differences You Need To Know – GoSwitchgear

A discontinuous or on-off control system generates a bang-bang response. This means the output switches abruptly between two states: fully on or fully off.  

1. On/off Control (Bang-bang Control) | Closed-loop Control Systems | Textbook

2. Bang–bang control – Wikipedia

Key characteristics of the response:

• Oscillations: The system tends to oscillate around the setpoint due to the abrupt changes in the output.
• Over-shoot and under-shoot: The process variable often overshoots or undershoots the setpoint before stabilizing.
• Steady-state error: There’s often a steady-state error, meaning the process variable doesn’t settle exactly at the setpoint.

Visual representation:

Opens in a new windowwww.researchgate.net bangbang control response

Factors affecting the response:

Hysteresis: Adding hysteresis (a delay in the controller’s response) can also help dampen oscillations.

System dynamics: The characteristics of the system being controlled (e.g., mass, inertia, time constants) significantly influence the response.

Dead band: Introducing a dead band (a range of values around the setpoint where no action is taken) can reduce oscillations but may increase steady-state error.

While on-off control systems are simple and often sufficient for basic applications, their inherent limitations in accuracy and speed can be addressed through several techniques:

Improving Accuracy:

• Deadband: Introducing a deadband, a range of values around the setpoint where no action is taken, can reduce the frequency of switching and thus reduce oscillations.   1. Deadband – Wikipedia en.wikipedia.org
• Hysteresis: Implementing hysteresis, where the on and off setpoints are different, can also help to dampen oscillations and improve accuracy.
• Proportional Band: While not strictly an on-off control, a proportional band can be introduced to modulate the output based on the error between the setpoint and the process variable, improving accuracy.

Improving Speed of Response:

• System Optimization: Improving the dynamics of the system being controlled can directly affect how quickly it responds to changes. This might involve reducing mass, inertia, or time constants.
• Faster Actuators: Using faster actuators can also improve response time. However, this often comes with increased cost and energy consumption.

Limitations and Considerations:

• Trade-offs: Improving accuracy or speed often comes at the expense of the other. For example, increasing the deadband can reduce oscillations but may also increase steady-state error.
• Complexity: Introducing more sophisticated techniques like proportional bands or hysteresis can increase the complexity of the system.
• Alternative Control Strategies: For applications demanding high accuracy and speed, more advanced control strategies like PID (Proportional-Integral-Derivative) control might be necessary.

In conclusion, while on-off control systems are simple and cost-effective, their performance limitations can be partially addressed through techniques like deadband, hysteresis, and system optimization. However, for demanding applications, more advanced control strategies should be considered.