How to Use Timers in Ladder Logic Programming?

Hello friends! I hope you are doing very well, today we have a very crucial topic which is “timers”. Yes! Exactly like what comes to your mind. For running equipment i.e. motor at a specific time and/or for some amount of time we need timers. Timers are used even before PLC in classic or relay logic conventional control. However, there is a big difference between capabilities and limitations between using physical timers in classic or old fashion relay logic and using software timers in PLC. By completing this article you will be able to know what are timers and their types and applications. In addition, we are going to show off how to use timers in ladder logic programming with examples.

What are timers used for in industrial applications?

Well! This is a very good question to start and its answer can be taken as a motive to learn timers comprehensively. Timers are used to turn on actuators i.e. motors for a specific amount of time or turn off them after a specific amount of time. They are used for scheduling tasks on and off based on the process sequences.

How do timers work?

There are many types of timers based on the functions. However, all timers are working in the same principle that they are preset to a specific amount of time. Then when their coils are energized, the timers’ contacts are changing over to the opposite of their initial states i.e. from ON to OFF or from OFF to ON based on the delay time and the timer type. Therefore, we can imagine the main components of timers are the coil and contacts as shown in figure 1 [1]. It shows a relay timer for Telemecanique type showing the setting for the time delays, coil, and the contacts.

Figure 1: physical relay timer

Advantages of Timers in PLC over relay timers

The relay timers are limited in time setting i.e. for 24 hours and a specific amount of physical NO and NC contacts. While timers in PLC are logical software components with the unlimited setting of time which makes them very flexible with logic requirements. In addition, using physical relay timers costs us more space and wiring. While we can use soft timers in ladder logic with no limitations. Types of timers as we are going to describe in detail later, are limited to basic functions in the relay timers while all types of timers can be implemented in PLC using ladder logic programming.

Timers types

All timers have a coil and contacts and the latter will be changed over their states by energizing their coil based on the preset time and the type of timer. In the following subsection the most common timer types are introduced:

ON-DELAY timer

This type of timer is used to postpone the start of running. Figure 2 depicts the timing diagram of the On-Delay timer. As shown in this timer delayed the output from the input by the preset delay time. The figure shows that, after the input gets started, the time counts up until the preset amount of time elapsed, then the output starts as long as the input is ON. However, in the second pulse of the input, we can notice the output was not started because there was no chance to reach the amount of delay time that is preset in designing the timer.

Figure 2: The timing diagram of the On delay timer [2]

OFF-DELAY timer

In this type of timer, we aim at delaying the shutdown of the output. Figure 3 depicts the timing diagram of the OFF delay timer in which, the output starts at the time input starts and the output lasts after a specific time delay after the input gets off. You can notice that, in the second pulse of the input, the output did not shut down after the second pulse of the input due to the incoming of another input pulse before the delay time reached the preset value.

Figure 3: The timing diagram of the Off delay timer [2]

Pulse timer

This timer type energizes the output in each rising edge of the input for a fixed amount of time. Figure 4 shows the timing diagram of a pulse-type timer. It shows the output comes active each time it finds a rising edge of the input and keeps active for a fixed amount of time which is preset for the timer.

Figure 4: Timing diagram of, the pulse timer [2]

 

Accumulative timer

In an on-delay timer, if the input does not stay on for the preset time delay period there would not be a chance for the output to be energized. So there should be another modified timer to accumulate the period each time the input is ON and activate the output when reaching the preset value. This timer is called an accumulative timer and its timing diagram is depicted in figure 5. In this example, assume the preset values of 12 s, the first input pulse shown in blue resumes for 4 seconds and it goes on for another 8 seconds. The time is accumulated from the first and the second pulse until reached the preset value which is 12 seconds. The output gets ON as long as the preset time has reached and the input is ON.

Figure 5: on-delay accumulative timer

Timers in PLC and ladder logic program

Timers of different types can be used in PLC with great flexibility. Now let’s learn and practice timers with our simulator. But before we get to start we just need to introduce ourselves to the timer blocks in the ladder logic program.

Generate ON delay timer instruction

Figure 6 shows the instruction for generating an ON Delay timer. As you can see the instruction has two inputs and two outputs. The first input “IN” is the input contact of type Boolean. This input contact triggers the timer and initiates it to start counting the time. The second operand is the “PT” by which we set the required time delay. Moving to the outputs, the first output is “Q” this is the main output of the timer and it turns to true when the timer reaches the preset time delay. The second output is the “ET” which reports the time elapsed so far since the timer started to count time. The “PT” and “ET” are of time data type format and can be in milliseconds, seconds, minutes, hours, days et cetera.

Figure 6: Generate ON delay instruction

ON-Delay timer example ladder program

Figure 7 shows a very simple example program that uses an ON Delay timer for delaying the running of a motor. The ladder code is shown on the right part of figure 7. It shows that one input contact connected to address “I0.0” and its tag named “trigger_timer” is connected to the timer input. And the preset value is set to be one minute “T#1m” which means one minute. Notice the format is very simple. You type T denoting the time format and then “#” followed by the amount of time and at last the unit of time which could be “MS” for milliseconds, “S” for seconds, “m” for minutes, “d” for days … et cetera. Moving to the outputs, the timer output “Q” is connected to the motor meaning that the motor will be running after the timer counts complete one minute. The other output is “ET” which tells the time elapsed so far since the timer set to start counting time. Let’s go to the simulator window on the left side. We added all inputs and outputs to control the inputs and show the output as well. You can notice that, by setting the input “trigger_timer” to true, the timer starts counting the time as you can read in the “ET” variable that shows the time spent so far “T#29S_335MS” meaning 29 seconds and 335 milliseconds have been elapsed since input set to true. Since the PT time is not reached, the output is still false as shown in blue color.

Figure 7: ON-Delay timer sample program with a simulator

Figure 8 shows the moment when the timer reached the PT value which is one minute in our example. You can notice on the left window, the ET value becomes one minute which is equal to the PT value. Therefore, the timer output turned to true and the motor is switched on consequently. To sum up, this example shows how the output has become ON after one minute delay of the input.

Figure 8: ON-Delay timer energized the output after one minute delay

OFF delay timer example ladder program

After we have shown how we can delay the start of a motor by using the On Delay timer. Here we are going to show how to delay the shutdown. Figure 9 shows an example of a very simple ladder program that uses an OFF delay timer to delay the shutdown of an output. As you can see on the left the input is ON and the output starts running with the input at the same time. Let’s try to set the input OFF as in figure 10, you can notice the output is still ON and the time counter is incrementing until it reaches the PT value which is set to one minute in our example. Figure 11 reports the moment at which the time counter reached the PT value which is one minute. At that moment the output or the motor is shut down. In a conclusion, the Off delay timer is our tool to delay the shutdown of output or terminate a process.

Figure 9: OFF delay timer example

Figure 10: The input is off while input is still run

Figure 11: the output shutdown delayed one minute

Pulse timer example

In this example, we are going to start and stop the motor in a pulsative operation mode thanks to the pulse timer type. That means the input to the timer will start the output to run for a specific amount of time that is present in the PT value regardless of receiving any other input pulses. In figure 12, the pulse timer example is shown. The very simple ladder logic program is displayed on the left window and the simulation is shown on the left window. You can notice that, once the timer received a high state of the input, it energized the output and started counting the time. However, when the input went OFF the output keep running ON for the designated while represented in PT value which is one minute for this example as shown in figure 13. Let’s test the condition of having another input signal as shown in figure 14. When the input goes again high which the last pulse does not complete, the output continues running and the timer does not reset its timing count. However, when the pulse time is reached as in figure 15, the output is shut down and the timer now is ready to start another pulse by noticing a rising edge of the input.

Figure 12: pulse timer example

Figure 13: the output continue running for its pulse period

Figure 14: another pulse does not reset the timer until complete one pulse period

Figure 15: output shutdown after completing pulse period

Accumulative timer ladder example

In this ladder example, we are going to show you how to continue accumulating the time until reaching the PS value at which the timer energizes its output as shown in figure 16. The example shows a very simple ladder code that has two inputs connected in parallel to OR logic. So any of these two inputs will trigger the counter to count up the time. On the left window, all inputs and outputs including timer operand and outputs are listed in simulating table to show the values while the program is executing.

Figure 16: an accumulative timer ladder example program

Let us start triggering the timer by one of the inputs as shown in figure 17. As you can notice the timer starts counting the time as shown in the ET value on the left window showing the simulation. What if we set the inputs OFF? In the ON Delay timer, the time will be reset. But in this timer, it does not and instead, it waits for another input signal to accumulatively increment the time till reaching the PT value as shown in figure 18. Let’s verify the concept by setting one of the inputs ON as shown in figure 19, the timer accumulates the timing counter until it reached the PT value which is set to one minute in this example. At that moment the output is energized as shown in figure 20. And finally, figure 21 shows how the timer is reset by switching on a reset button. One good practical example for using this type of timer is the maintenance schedule. For example, we can schedule the maintenance to be conducted after one year or after a specific amount of time or number of operating hours. So the timer will keep accumulating the time of operation regardless of the downtime and raise a flag to notify it is the time for performing maintenance.

Figure 17: starting the timer by enabling one of the inputs

Figure 18: setting the inputs off does not reset the timer

Figure 19: timer accumulates the time

Figure 20: the output goes ON after reaching the PT value

Figure 21: timer reset its timing count

What next

I would like to thank you so much for following up with our tutorial that far. So far you are familiar with timers types and how to utilize the suitable one for your task based on the logic you want to perform. Next tutorial we are going to go through counters showing their types and functionalities and for what reasons we need counters how we can use them appropriately.

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