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⌚Timestamps:
00:00 - Intro
00:40 - Types of PLC timers
00:49 - PLC timer values
02:30 - PLC timer examples
02:53 - On-delay timer
05:24 - Off-delay timer
07:15 - Retentive on-delay timer
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In this video, we will be talking about PLC timers. We will explain what PLC timers are, how they work, the different types of timers, and give some examples of how they are used.
PLC timers are internal PLC instructions that can be used to delay input and output signals in the PLC program.
These timers operate like relay timers but you cannot hold a PLC timer in your hand and they do not need to be connected to wires to operate.
There are three main types of PLC timers:
- the on-delay timer,
- the off-delay timer,
- the retentive on-delay timer.
All of these PLC timers have a timer file name, a preset value, an accumulated value, a timer enabled bit, a timer timing bit, and a timer done bit.
- The timer file name is a PLC tag. This tag is where you can find the timer’s preset and accumulated values and the timer enabled, timing, and done bits.
Some PLCs create the timer file name for you and other PLCs have you create the timer file name.
For all of my timer examples, I will use the timer file name Timer1. I will also use a selector switch to enable and disable the timers.
- The timer preset value is the number that shows the delay length of the timer. This number can be changed by just typing it in or you can have the PLC write a number into this value. This number does not change during the timer’s normal operation.
- The timer accumulated value is the number that shows how long the timer has been timing. The timer accumulated value does not function the same on all of these timers.
- The timer enabled bit will be on when the timer is enabled.
- The timer timing bit will be on when the timer is timing.
- The timer done bit does not function the same on all of these timers.
I will be using Studio 5000, which is the PLC programming software for Allen-Bradley ControlLogix PLCs.
But the things you will learn here in these examples can be applied to any other PLC software. Because the overall timer functions are pretty much the same for any other PLC brand.
Let’s start by placing a new on-delay timer instruction on a PLC rung with a selector switch input.
Type the timer file name Timer1.
Next, enter the timer preset so that it equals ten seconds.
Not all PLC timers use the same time base, some have time bases that cannot be changed while others will give you multiple time base options.
I will discuss what happens to the PLC on-delay timer, off-delay timer, and retentive on-delay timer when the selector switch gets turned on and off.
Setting up an off-delay timer is the same as an on-delay timer.
Retentive on-delay timers are a lot like on-delay timers. The only major difference with retentive on-delay timers is how the timer accumulated value acts when you turn the selector switch on and off and that it needs a reset bit.
Timer applications:
1) On-delay timer
On-delay timers are used a lot in PLC programs and in many different ways. For example, an on-delay timer can be used on a conveyor’s start button.
The timer could be set up so that the start button would need to be pushed and held for five seconds before the conveyor would start.
Another on-delay timer could also be set up to sound a conveyor start-up warning horn and light.
2) Off-delay timer
Off-delay timers are used when you need something to stay running longer than everything else. For example, cooling fans and pumps might continue to run after a machine pauses from doing work.
3) Retentive on-delay timer
Retentive on-delay timers can be used to track the runtime of equipment.
They can be set up to trigger an indicator light to let someone know the equipment has run long enough and requires maintenance to inspect and clean it.
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