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S7-1200 Timers Inaccurate

Created by: dangerous at: 9/29/2009 8:58 AM (7 Replies)

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9/29/2009 8:58 AM	Rate (0)
dangerous Posts: 35 Rating: (1)	I almost do not want to ask this question, it could be embarrassing! I'm trying to log data over a specific time period, within that time I want to make sure that I get the correct number of updates from my remote device. i.e. if I update every second over 2000s then I want to make sure that I get 2000 updates +/- 1 update. To test the accuracy of the timers I created a cyclic 1s timer using TON and I generate a positive pulse to add 1 to a register - thats all. I check the updates against my stop watch and find that I am missing about 10% of updates after only 200sec - I don't know if this is a scan issue or what but I have written similar software inS7-300/400's and its always workedwell. I need more accuracy than this, can any suggest an alternative method? Attachment s7-1200 timer issue.JPG (1112 Downloads)
dangerous Posts: 35 Rating: (1)
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9/29/2009 11:01 AM	Rate (0)
Mr D Posts: 21 Rating: (4)	To achieve higher accuracy with timed operations you should maybe look at using the cyclic interrupt OB with a specified interrupt scan time. You can then guarantee that you execute a piece of code at specific time without the need for timers....................
Mr D Posts: 21 Rating: (4)
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9/29/2009 5:22 PM	Rate (0)
Denilson Pegaia Platinum Expert Joined: 9/23/2005 Last visit: 1/17/2025 Posts: 4394 Rating: (1463)	Hi, I don't know yet why, but I measure some acurace problems in other simple test: 1 - run a TON with very large setpoint. 2 - read the actual elpased time 3 - subtract: actual elpased time - last cycle elapsed time = delta time between two call 4 - sum: delta time between two call + total time = total time 5 - compare total time and the direct reading ofelapsed time from TON There is a difference of 2s per hour. This difference don't occours if you use the same algoritm but with RD_SYS_T Other alternative for cycle OBs is to read the time clock and compare it. However it takes some processing time.
	Denilson Pegaia Siemens Industry Sector Technical Support
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9/29/2009 7:21 PM	Rate (0)
dangerous Posts: 35 Rating: (1)	Hi Pegaia, Thanks for getting back to me but I think that my problem was related to the operation of the .Q timer output - I was using it to increment a counter at one second intervals and this proved to be inaccurate. I've never encountered this problem before its a bit worrying to be honest. Could the PLCs scan time offset the .Q event?
dangerous Posts: 35 Rating: (1)
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9/30/2009 6:53 AM	Rate (0)
fritz Diamond Member Joined: 10/7/2005 Last visit: 1/17/2025 Posts: 3030 Rating: (1058)	Hello dangerous I haven't "played" with S7-1200's yet (and promised myself I won't until this "toy" PLC supports STL, but that's another story and we are drifting away from the subject) but whatever "general" Timer related accuracy issues may exists, they will be compounded by the way you are using the Timer. Put simple, you are usinga self triggering Timer which leads tothe following chain of events: Scan X: One second haselapsed -> TON Timer OutputON Scan X+1: Call Timer with a ZERO RLO at it "IN" input -> TON Timer OutputOFF (and add one to your Double Integer counter, no need for the P_TRIG instruction by the way, your Timer output is only ON for one scan). Scan X+2: Start TON Timer again. What this means is that there will beTWO "surplus"cylces which will make every TON second count equate to "1 second + two scan cycle times" (which is "normal" for self triggering timers). I hope this helps.
	Last edited by: fritz at: 9/30/2009 12:27 PM Corrected last statement as there are actual two "empty" scans Cheers fritz The advice and opinion given in this thread is that of the author and does not necessarily reflect the views of I S Systems Pty Limited. To find out how I S Systems can help you with your automation needs please visit www.issystems.com.au.
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9/30/2009 12:20 PM	Rate (0)
dangerous Posts: 35 Rating: (1)	Fritz, thanks for your comments. I understand your explanation, when I first created this little bit of software I had the increment function imediately after the timer (and no P_TRIG function). Using this method I got no updates at all making me think that the .Q bit was not set until the end of the PLC scan - hence the rung reversal. I am going to try using a cyclic OB and see if this is more accurate. Thanks for all your help
dangerous Posts: 35 Rating: (1)
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9/30/2009 1:14 PM	Rate (0)
fritz Diamond Member Joined: 10/7/2005 Last visit: 1/17/2025 Posts: 3030 Rating: (1058)	Hello again dangerous I would think too that doing the increment right after the TON should have worked, but as said, I haven't used the 1200's yet so who knows why it didn't work. Anyway, I just editedmy previous post as I realised that you will actually have two"empty" PLC scans which need to be added to the1 second time. Either way, cyclic OB usage will (should hopefully) give you a more accurate result, it will certainly elimate the "empty scans" issue with the self triggering TON. Since youare busy testing, I suggest you also try a test using the CPU clock memory byte(simple rising edge count on the 1Hz bit will do the job). Assuming theCPU clock memory byte is based on the internal real time clock you should get the accuracy of the real time clock (+/- 60 sec per month as per technical spec). Food for thought anyhow and good luck.
	Cheers fritz The advice and opinion given in this thread is that of the author and does not necessarily reflect the views of I S Systems Pty Limited. To find out how I S Systems can help you with your automation needs please visit www.issystems.com.au.
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2/25/2010 12:31 PM	Rate (0)
Denilson Pegaia Platinum Expert Joined: 9/23/2005 Last visit: 1/17/2025 Posts: 4394 Rating: (1463)	Hi, I know that it doesn't make much sence, but try to not use the values of the DI in your logic. Else save the Timer outputto a coil (to a M, for example) and use this aux variable in the other points.
	Denilson Pegaia Siemens Industry Sector Technical Support
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