- Programmable Logic Controllers (PLCs) are the backbone of industrial automation, providing precise control over complex processes.
- One of the essential components of PLC programming is the use of timers, which facilitate time-based control operations.
- In this article, we explore the ON Delay Timer (TON) and OFF Delay Timer (TOF) in the context of PLC ladder logic, with a focus on simulations using Schneider Electric EcoStruxure Machine Expert Basic software.
- These timers are critical for implementing delays, measuring intervals, and ensuring synchronized operations in various industrial scenarios.
- By understanding the properties, inputs, and outputs of these timers, as well as their practical applications, we can enhance the reliability and efficiency of automated systems.
ON Delay Timer (TON)
Explanation
The ON Delay Timer (TON) is used to delay the activation of an output after the input signal has been turned on. It ensures that an action starts only after a specified time has passed since the input was activated.
Properties
- Enable (EN): Activates the timer when set to true.
- Preset Time (PT): The time duration the timer must count before activating the output.
- Done (DN): A status bit that becomes true when the accumulated time reaches the preset time.
Inputs:
- EN: Signal to start the timer.
- PT: The delay duration.
Outputs
- DN: Timer completion indicator.
Ladder Diagram – ON Delay Timer
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Process Scenario Example – ON Delay Timer (TON)
A conveyor belt system requires a delay before starting a secondary belt to ensure the first belt is clear.
- Input (I0.0): Start button.
- Output (Q0.1): Secondary conveyor belt motor.
- Preset Time (PT): 20 seconds.
Description
Ladder Diagram Example
Explanation
- When the start button (I0.0) is pressed, the TON timer (TM0) is enabled.
- The timer counts for the preset time of 20 seconds (PT).
- After 20 seconds, the DN(TM0.Q) bit is set, turning on the secondary conveyor belt motor (Q0.0).
Click here for Designing 2 out of 3 Voting Logic in Control Systems: A Step-by-Step PLC Ladder Diagram Tutorial with Video
OFF Delay Timer (TOF)
Explanation
The OFF Delay Timer (TOF) delays the deactivation of an output after the input signal has been turned off. It ensures that an action continues for a specified time even after the triggering input has been deactivated.
Properties
- Enable (EN): Activates the timer when set to true.
- Preset Time (PT): The time duration the timer must count down before deactivating the output.
- Done (DN): A status bit that becomes true when the accumulated time reaches the preset time after the input is deactivated.
Inputs
- EN: Signal to start the timer.
- PT: The delay duration.
Outputs
- DN: Timer completion indicator.
Ladder Diagram – Off Delay Timer
Process Scenario Example – OFF Delay Timer (TOF)
A cooling fan should continue to run for a certain time after a machine is turned off to ensure proper cooling.
- Input (I0.0): Machine operation signal.
- Output (Q0.0): Cooling fan.
- Preset Time (PT): 15 seconds.
Description
The cooling fan should continue running for 15 seconds after the machine stops to ensure it cools down properly.
Ladder Diagram Example
Explanation
- When the machine operation signal (I0.0) is active, the TOF timer (T0) is enabled.
- When the machine operation signal (I0.0) is turned off, the TOF timer starts counting down from 15 seconds (PT).
- The cooling fan (Q0.0) remains on until the TOF timer reaches 0, ensuring a 15-second run time after the machine stops.
- After 15 seconds, the timer done bit DN(TM0.Q) bit is OFF, turning off the cooling fan coil output (Q0.0).
Click here for Designing 2 out of 4 Voting Logic in Control Systems: A Step-by-Step PLC Ladder Diagram Tutorial with Video
Summary
- Timers are indispensable in PLC programming, enabling precise control and synchronization of industrial processes.
- The ON Delay Timer (TON) and OFF Delay Timer (TOF) each serve distinct functions: delaying the activation of outputs and extending the duration of output signals after input deactivation, respectively.
- Through simulations in Schneider Electric EcoStruxure Machine Expert Basic software, we can effectively visualize and test these timer functions in real-world scenarios.
- By mastering the use of these timers, engineers can design more robust and efficient automated systems, ensuring seamless operation and improved productivity in industrial environments.
- Understanding and applying these timer concepts is crucial for anyone involved in PLC programming and industrial automation.
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