How are PLC Timers Used in Timed Switch Operations?

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⌚Timestamps:
00:00 - Intro
01:20 - Importance of timers
03:04 - Structure of timers
04:42 - Types of timers
06:52 - Best practices for timer usage
08:00 - Advanced timer functions
08:50 - Conclusion

Timers are one of the most often-used functions in PLC programming, and for good reason.

Timers allow the programmer to delay taking action for a period of time, to allow actions to proceed only for a set period of time, or to allow actions to continue for a total amount of time, even if the time is accumulated over several distinct periods.

These actions make advanced alarming strategies possible and allow the transfer of operation from a primary pump to a secondary pump after the primary pump has been in service for a set period of time.

Timers are pre-defined functions. They are added to a PLC program as an instruction block, with parameters such as Enable, Accumulated Time, and Timer Reset built into the timer function.

While there are several libraries of timers available, we will concentrate on IEC timers in this video, since most current PLC programming tools use this set of timers.

In a PLC program, timers let us know how much more time is left in the current operational step, and they indicate how long to wait when a valve is told to open to evaluate if the valve did open.

In complex systems with multiple actuators, sensors, and processes, timing functions help synchronize the actions of different components.

Timers are critical in many industries such as manufacturing assembly lines, chemical processing, batch operations such as paint manufacture, and others.

Timers all have a memory address. Depending on the PLC model, this address can be a file number and element, a simple timer number, such as T12, or a numeric address.

The “Time Base” is the smallest unit of time tracked by the timer, such as 1.0 sec, 0.1 sec, or 0.01 seconds.

The Preset is the timer setpoint.

The Accumulated value is the elapsed time.

All of these parameters are built into the timer function. We just need to provide the correct timer type, add the correct values to support our application, and then let the timer do its thing!

▶️ On-Delay Timer: When the logic upstream of the timer on the same rung is true, the timer is enabled. The timer times until the preset is reached, and the done bit is set. If the timer input is no longer enabled, the timer resets. Once set, the done bit stays true until the rung goes false.

▶️ Off-Delay Timer: An off-delay timer acts somewhat in the reverse of an on-delay timer. Once the time rung goes true, the done bit is set immediately. When the timer input goes false, the timer begins to time and after the preset is reached, the done bit turns off.

This type of timer is useful for delaying an action until a certain time period has elapsed.

▶️ Retentive Timer: This type of timer is like the on-delay timer, except the timer does not reset when the timer input goes false. The accumulated time is preserved, and when the timer is enabled again, it continues to accumulate time until the preset time is reached. At that point, the done bit is set.

With a retentive timer, a reset instruction must be executed to reset the timer. A typical use of retentive timers is to track run time on a piece of equipment so that the optimal time for maintenance can be determined.

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Видео How are PLC Timers Used in Timed Switch Operations? канала RealPars