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| 9/8/2014 10:12 AM | ||||||||
Joined: 1/16/2012 Last visit: 9/8/2022 Posts: 72 Rating:
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Hi, Just recently I've been working with AI module and I was a bit confused. I'd like to clafiry some vague things about it for good :) It was a 16-bit analog input module and VEGABAR 82 (4..20 mA) sensor with the measuring range -1..+100 bar and smallest measuring range 0.025 bar. As I understand, its span is 100 or 101 bar, not sure about its settings, but its not that important. Let's say it's 100 bar. Span / smallest measuring range = 100 / 0.025 = 4000 measuring units 4..20 mA: 20 - 4 = 16 mA So, each sensor's unit: 16 / 4000 = 0.004 mA On the PLC side it's: 0..27648 = 27648 units 16 / 27648 = 0.00058 mA (you can find the same value in Siemens specs) My colleague told me to show just one decimal place in the HMI, because "the sensor is not accurate enough". I understand that sensor and PLC have two different resolutions and that sensor is not accurate enough, but I think that I could show 2 decimal places and that value would still be correct.
Is my theory correct? How can I calculate number of decimal points to represent? How do you calculate how many decimal places to represent for different analog sensors? Thanks for you replies. Regards, Giedrius K. |
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| 9/8/2014 4:28 PM | ||||||||
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Posts: 1058 Rating:
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Hello Giedrius_Kai, I guess digitalisatuion is not the problem in this case, it should be quite precise. But what accuracy is the sensor? You have a range of 100 bar. If the sensor is 1% precise, your error from sensor will already be 1 bar at scale end. Even if your sensor is a very precise one, I doubt it would be better than 0.1%. Thus showing 2 decimals would be feigning accurracy. regards |
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| 9/9/2014 7:51 AM | |
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Joined: 1/16/2012 Last visit: 9/8/2022 Posts: 72 Rating:
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I think its accuracy was 0.1 %. So, 1 bar is equal to 0.16 mA, because 16 mA (mA range) / 100 bar (bar range) and 0.1 bar is ±0.016 mA, because of the accuracy. Why is it feigned then? If it were a ±0.16 mA, I would agree with you. |
| 9/10/2014 1:59 PM | |
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Posts: 1058 Rating:
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Dont think in mA, think in engineering units: if the accuracy of your sensor is 0.1%, it means your maximum error coming from sensor is already 0.1 bar. To that you have to add some minor errors for converting to 4 to 20mA and converting mA to digital value (conversion error, linearity error).If you then show a pressure value with 2 digits, like 94.79 bar, the last digit is actually feigned, as your error is for sure more than 0.094 bar. The last digit could never give you any accuate information, as error is too big - thats why it would be feigned accuracy. With PLC you can never be more precise than the sensor used. regards |
| 9/11/2014 7:25 AM | |
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Joined: 1/16/2012 Last visit: 9/8/2022 Posts: 72 Rating:
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Thanks eisenpferdreiter for the explanation. |
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