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Adding real-value with 0,1

Created by: Csar1985 at: 12/14/2011 11:56 AM (11 Replies)

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12/14/2011 11:56 AM	Rate (0)
Csar1985 Posts: 9 Rating: (0)	Dear all, We're having a small when programming an totalizer. Every pulse we get from the flow-meter we add 0.1 to a real-Value Now when the number gets bigger it sometimes doesn't add 0,1 but 0,09. I've checked the sequence and came across this We start with 19000 NExt time he adds 0,09 is at 19001,2 the we get 19001,29 (+1,29) next 19003,88 (+2,59) next 19006,47 (+2,59) next 19008,96 (+2,49) Does anyone know how this is possible? I've checked this: https://support.automation.siemens.com/WW/llisapi.dll?func=cslib.csinfo&objId=22172161&load=treecontent&lang=en&siteid=cseus&aktprim=0&objaction=csview&extranet=standard&viewreg=WW but did not found an answer to this problem. Tnx in advance
Csar1985 Posts: 9 Rating: (0)
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12/14/2011 12:07 PM	Rate (0)
Darius Platinum Member Joined: 9/28/2005 Last visit: 5/2/2023 Posts: 2957 Rating: (242)	Hello, Indeed, your numbers aren't so big to cause the problem. Very strange issue. I also would like to see feedback from other users about this issue. In the mean time maybe you could post part of your code where you calculate the totalizer ? One tip from me. I use REAL values only for represent analog sensors in engineering units format, but for totalizers allways use DINT. Its the same size, but allows to count much much more and still have required precision.
	Best regards, Darius
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12/14/2011 2:42 PM	Rate (1)
miami Platinum Member Joined: 8/31/2005 Last visit: 4/24/2023 Posts: 2194 Rating: (320)	Hello, when you start with 19 000 and you add 0.1 you have 6 digits. Between 6 and 7 (in decimal system) digits the floating numbers beginn to be rounded. Without having calculated, I think it is possible you have this effect. Have a look on the definition of REEL: You have 23bit mantissa and 8bit for the exponent and a signum bit (all in dual system), this is in decimal system a matissa with 6 or 7 digits. On a S7-300/400you should use DINT, here you have no rounding. Just start with 190000 (for 19 000) and add 1(for 0.1) each time. DINT can go up to 2 147 483 648 (which is 214 748 364.8 for you) If you have a S7-1200 you should use LREAL (long real with 52bit matissa) instead of REAL. Attachment REAL.pdf (310 Downloads)

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This contribution was helpful to 2 thankful Users Darius Marcjan

12/15/2011 12:15 PM	Rate (0)
hlapak Regular Member Joined: 11/25/2011 Last visit: 6/11/2022 Posts: 16 Rating: (1)	Hi Csar1985, could you please post a screenshot how you make the sum? best Regards,

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12/15/2011 1:06 PM	Rate (0)
Kris1976 Posts: 5 Rating: (1)	We do a totaliser with 2 counters (reals). First one just counts up to a certain limit. Ones this value exceeds this limit, we substract the limit from this first counter and add it to the second counter value. Like this, the first counter value always stays small enough so there is no rounding error and the second value ensures that we can have big totalised values. Its a bit more complicated to work with, but we are able to make a totaliser with reals
Kris1976 Posts: 5 Rating: (1)
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12/15/2011 1:37 PM	Rate (0)
Marko Bursic Platinum Member Joined: 9/8/2009 Last visit: 3/15/2023 Posts: 1410 Rating: (149)	you can use oscat library to implement quasi LREAL variable: TYPE REAL2 : STRUCT R1 : REAL; RX : REAL; END_STRUCT END_TYPE FUNCTION R2_ADD : REAL2 TITLE = 'R2_ADD' // //R2_add adds a real to a double real which extends the accuracy of a real to twice as many digits // //uses: REAL2 [UDT11] // VERSION : '1.0' AUTHOR : hugo NAME : ADD FAMILY : R2 VAR_INPUT X : Real2; Y : REAL; END_VAR VAR temp: REAL; tR2_ADD: REAL2; END_VAR BEGIN temp := X.RX; tR2_ADD.RX := Y + X.R1 + X.RX; tR2_ADD.R1 := temp - tR2_ADD.RX + Y + X.R1; R2_ADD := tR2_ADD; (* revision history hm 20.3.2008 rev 1.0 original version *) END_FUNCTION Look in OSCAT lib manual, I think you can use a part of this UDT (Rx or R1) as "most significant" so you can rapresent a totalizer with only one real number (for example on panel), but you have double precision when accumalting numbers. However is not recomended to use real numbers for totalizer, except if you have fragment (non integer) parts to totalize
	Last edited by: Marko Bursic at: 12/15/2011 1:50 PM
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11/1/2017 10:42 AM	Rate (0)
LuisPim Member Joined: 9/11/2015 Last visit: 3/27/2023 Posts: 6 Rating: (2)	Siemens REAL precision fault: Same problem on a 1518 fw2.1, and its not that I am reaching the last significant number. Problem starts right from 0.0. Try to run on each cycle: fRealNumber := fRealNumber + 0.1; You will get numbers like 28.70007 100.399 149.6999 229.8048 1768.015 and so on. Same problem with LReal, seems like last bit of the number is random.

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11/1/2017 11:08 AM	Rate (0)
CJ4 Posts: 103 Rating: (36)	LuisPim Siemens REAL precision fault: It is time you make a study of how floating point numbers (REAL) are represented by digital computers. Wikipedia's description should help but this calculator makes it more visual: http://www.exploringbinary.com/floating-point-converter/ Perhaps it is best for you to use 64-bit integer with a whole number divider. Attachment Decimal to Floating-Point Converter - Exploring Binary.png (131 Downloads)
CJ4 Posts: 103 Rating: (36)
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This contribution was helpful to 2 thankful Users LuisPim SMIT SHAH

11/2/2017 10:31 AM	Rate (0)
Piotr.M Platinum Expert Joined: 6/24/2013 Last visit: 5/25/2023 Posts: 1505 Rating: (388)	Hi You may need to use Long Real (64-bit Real) for Totalizer and covert result to Real just for processing/display. Library can be found on this forum. 1. convert value to be added to Long Real 2. Add to Totalizer (using Long Real Add function) 3. convert Totalizer to Real for rest of your program (to a different memory location so Long Real value is not overwritten during conversion)

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2/15/2020 10:21 AM	Rate (0)
shankar78 Experienced Member Joined: 1/28/2010 Last visit: 5/22/2023 Posts: 62 Rating: (0)	Hello experts, With reference to below query and discussion, Shall we consider it the software issue in Siemens PLC? I have also faced same issue for totalizer but instead of adding 0.1 in real , 1 added in DINT , converted DINT to real and then divide by 10.0. But in some cases if we need to calculate deltaP it creates issue If P1= 10.0 & P2=0.9 then P1-P2=10.0-10.9= 0.9 but sometimes it gives result as 1.0. Shankar LuisPim Siemens REAL precision fault: Same problem on a 1518 fw2.1, and its not that I am reaching the last significant number. Problem starts right from 0.0. Try to run on each cycle: fRealNumber := fRealNumber + 0.1; You will get numbers like 28.70007 100.399 149.6999 229.8048 1768.015 and so on. Same problem with LReal, seems like last bit of the number is random.

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