The circuit will continuously reset causing all of its switches to go gratata tatata tatata. There is a bogus effect of self latching. This instantaneous change cannot be adapted in electro-mechanical equipment, causing both timers to repeatedly switch on/off. The moment at which coil of T2 charges the status of N.C of its switch contact rapidly flips from N.O to again N.C: Please let me explain that such circuit can never function: ![]() There is a one-timer variant that is just as simple to understand, and has the advantage of being a little shorter: The two-timer variant of the Flasher pattern shown above isn’t the only way to do this. Typically this higher speed task would be driven by a timer interrupt which preempts the main logic. You may be able to create a “high speed task” that runs more often than the main program. If you need your timers to be more accurate than this, then you’ll have to dig into the documentation of your PLC. If the program scan time is 10 ms, then the Flashing Light coil’s period will range from 1000 ms to 1020 ms (which is much more reasonable). That’s because each timer is going to vary from 500 to 750 ms. If your PLC has a 250 ms scan time then the period of the Flashing Light coil in the above example will vary anywhere from 1000 ms to 1500 ms. This is important because it affects the repeatability and accuracy of your timers. Newer PLCs have typical scan times in the tens of milliseconds, and PC-based PLCs like Beckhoff’s TwinCAT might have scan times in the sub-millisecond range. Older PLCs with slow processors and long ladder logic programs can have scan times in the hundreds of milliseconds. Program scan times can vary widely in PLCs. That means it acts like a one-shot or a pulse. Timer 2’s output contact only stays on for one program scan. If you want the Flashing Light coil to be on initially, change the Timer 1 contact in the third rung from normally open (N.O.) to normally closed (N.C.). ![]() ![]() it starts off for 500 ms and then turns on for 500 ms). Some things to note: the Flashing Light turns on during the second half of the cycle (i.e. Now both timers are off, so on the next program scan Timer 1 will start timing again and the cycle repeats. On the next scan, this causes the rung input condition to Timer 1 to turn off, which immediately resets Timer 1, which means Timer 1’s output contact also turns off, resetting Timer 2 on the same scan and also turning off the Flashing Light coil. After another 500 ms, Timer 2 is done timing and its output contact turns on. When this happens, Timer 2 starts timing, and the Flashing Light coil turns on as well. After 500 ms, Timer 1 is done timing and its output contact (sometimes called “done bit”) turns on. Here’s how it works: initially both timers are off, and their elapsed time values are set to zero.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |