SIMATIC TDC, FM458, T400

Minimum cycle time of FM 458-1DP

Thread Starter: Pandiani   Started: 7/8/2009 10:52 PM   Replies: 4

  7/8/2009, 10:52 PM

Pandiani Joined 10/31/2005 Last visit: 12/9/2012 Posts: 238 Rating:  (20)	(0)  Hello, I wonder what is minimum cycle time that is set when using FM 458-1 DP to fast closed loop control. I have  found data in the manual that FM 458-1DP supports defining basic cycle of even 100 us, but that typical cycle time (T0) is defined as 1ms for any real application. What is your experience? Thanks
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  7/9/2009, 10:49 AM

KSN Joined 8/25/2006 Last visit: 5/19/2013 Posts: 898 Rating:  (85)

(0)  Hello, You are right, that in most common applications the basic cycle of 1 ms will be sufficient. But I think that such cycle time resolution in the FM came from Simadyn-D system, where it was possible to control the thyristor power section using e.g. ITDC-module. May be in such applications it was necessary to have tasks with the same cycle time as HW interrupts (3.3ms or 2.7ms due to line voltage frequency). Also I think there may be some cases where it is necessary to control extremly fast analog signals using such cycle tasks as 100us.

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  7/9/2009, 1:25 PM

miami Joined 8/31/2005 Last visit: 5/16/2013 Posts: 978 Rating:  (121)	(0)  Yes, you can use the 100µs, for one or two (simple) close loop control(s). If you want to do more close loop controls or complexe closed loop controls, you will have to inrease the task time. mfG / Best Regards Dirk
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  7/9/2009, 1:59 PM

mumbles Joined 9/27/2005 Last visit: 4/4/2013 Posts: 160 Rating:  (16)	(0)  If in doubt that your sampling time is too quick have a look online with the PSL (processor utilisation) block.  This can quickly tell you if you are running your tasks too quickly or if you have too many blocks located in the faster tasks. Hope this helps
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  7/9/2009, 7:21 PM

Pandiani Joined 10/31/2005 Last visit: 12/9/2012 Posts: 238 Rating:  (20)	(0)  Well, most transmitters have electrical damping constant (built in of about 100 ms for SITRANS P DS III= like explained here. I doubt there are tranmistters that can perforemed much better. For such application cycle of about 2-3 ms is more than sufficient. I really wonder where 100 or 200 us is needed (what sensors or transmitters are needed). Thanks
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