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Zeroing sensor 4-20 mA SM1234

Created by: Heberto_hbo at: 3/20/2012 10:48 PM (11 Replies)

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3/20/2012 10:48 PM	Rate (0)
Heberto_hbo Posts: 16 Rating: (2)	Good day, I'm trying to make a routine using TIA V11 SP2, S7-1200 CPU1214C DC/DC/RLY, WINCC Advanced V11 SP2, acquiring a value from an Analog Sensor 4-20 mA, I already scale the input for this sensor using the block that I found in the forum link, but my main concern right now is that all sensors 4-20 mA have a zero drift for 4 mA, I mean, if I have a 0 to 15Kpsi sensor, my 0 value for 4 mA could be between -100 to 100 Psi, so I would like to make a Zero sensor routine that allow me to set the inicial value to zero in the HMI application. also, I would like to know how can I get negative values while I have the values below 4 mA for example... from 0 to 15Kpsi using a 4-20 mA sensor, if I have a sensor disconnected, broken cable etc (0 mA) that will be -3750 Psi that must be shown in the HMI, that indicate me a sensor disconnected, with the block that I'm using for scale a 4-20 mA sensor I cannot get these values.. only 0 Psi even with the sensor disconnected. this block is in the S7-1200 product support it has the library for download it and it can be used also to check a broken wire, . Any suggestion about this will be appreciated. Thanks in advanced, Best Regards
Heberto_hbo Posts: 16 Rating: (2)
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3/21/2012 7:38 AM	Rate (0)
Marcjan Platinum Member Joined: 4/22/2010 Last visit: 1/15/2025 Posts: 5697 Rating: (715)	Hello, Please give us the link to that block, so we know what you are talking about. Regards, Marcjan
	Problem solved? Please let us know! Please no technical questions by PM, use the forum sections for your questions.
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3/22/2012 11:09 PM	Rate (0)
Heberto_hbo Posts: 16 Rating: (2)	Hi, Thanks for replying, I'm taking the blocks from the following link: https://support.automation.siemens.com/WW/llisapi.dll?func=cslib.csinfo&lang=en&objid=42087579&caller=view How can you use analog 0-20 mA signal modules and signal boards with 4-20 mA signals?. Thanks, Best Regards.
Heberto_hbo Posts: 16 Rating: (2)
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3/23/2012 7:23 AM	Rate (0)
Marcjan Platinum Member Joined: 4/22/2010 Last visit: 1/15/2025 Posts: 5697 Rating: (715)	Hello, Have you seen this thread? https://www.automation.siemens.com/BR/tf/WW/en/Posts/67810#top Regards, Marcjan
	Problem solved? Please let us know! Please no technical questions by PM, use the forum sections for your questions.
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3/23/2012 2:19 PM	Rate (0)
Heberto_hbo Posts: 16 Rating: (2)	Good day, Actually I already review it, but what I want to do is a routine to perform a Zero sensorfrom theHMI to the PLC, that's all I would like to do, I already acquire data from the sensors but the offset for 4 mA is not always Zero, it is between -100 and 100 Psi, for project requirements, operator need to Zero this value to start from 0 Psi and keep monitoring the pressure.....any suggestion about this will be appreciated even in SCL or FB, Ladder etc. Thanks, Best regards.
Heberto_hbo Posts: 16 Rating: (2)
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3/23/2012 2:59 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 1/24/2025 Posts: 15558 Rating: (2468)	Heberto_hbo Good day, Actually I already review it, but what I want to do is a routine to perform a Zero sensorfrom theHMI to the PLC, that's all I would like to do, I already acquire data from the sensors but the offset for 4 mA is not always Zero, it is between -100 and 100 Psi, for project requirements, operator need to Zero this value to start from 0 Psi and keep monitoring the pressure.....any suggestion about this will be appreciated even in SCL or FB, Ladder etc. Thanks, Best regards. In reading your request, I'm not clear exactly what the trouble seems to be. The analog input module reads 0-20mA regardless of the 'zero' point. What you are talking about it rescaling your input based on new information about a specific point along the input curve, in this case, 0 psi. Does -100psi = 4 mA and +100psi = 20mA (approximately)? If so, you are simply changing the scaling by offsetting a point near the middle. If the amount of change is acceptably small, you can simply compute the offset from 0 and offset all other readings by that amount. It depends on the linearity and all of your device over the range you use it. The more of the range you use, and the more accurate you need it to be, the more you will need to rescale rather than simply offset. If it were me, I'd likely scale the input counts to 0.0-20mA (for -27648 to 27648 counts)as that value is consistent regardless of the operator. Then I'd set a 4-20mA range within that as my input range equated to -100 to +100 with 0psi set to be the output value when the input is precisely 12.0mA. Quite a few NORM/SCALE combinations, but it becomes easier to troubleshoot. Once you have it the way you like, you may remove some of the intermediate NORM/SCALE blocks and just go from counts to psi (if you need the speed/memory).
	science guy
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3/23/2012 5:27 PM	Rate (0)
Heberto_hbo Posts: 16 Rating: (2)	Huggy, Good day, Thanks for repying, The problem actually is the values of "Zero" for the application, I mean, I already did the scale of the signal from 4 to 20 mA 0 to 15Kpsi, but the sensor sends for 4 mA signal 3.8 mA and that's when my problems comes, because the process need an exact Zero value (could be -3 to +3 Psioscillating between Zero) in the HMI for Graphic and logging.. so basically I need to Zero any deviation from the sensor only in the 4 mA value....the operator with no pressure in the system will see an Offset from the Zero value...and he will have the option to perform a Zero sensor... setting this offset of -100 or 100 Psi to Zero in the HMI and graphic/logging values... I'm recording a Pressure value for test purpose in a pressurizing system (Lab application) so an offset in the zero between -100 and 100 Psi for Zero is not allowed, a difference in the Zero of +/- 3 Psi is the actual recommended. Let me know if it is not clear yet so I can attach some pictures and give more information. Thanks and best regards.
Heberto_hbo Posts: 16 Rating: (2)
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3/23/2012 7:21 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 1/24/2025 Posts: 15558 Rating: (2468)	Ok, I think I understand better. The range 4-20mA = 0-15,000psi. The 4mA is +/- 0.xmA. What we do not know now is whether the 15,000psi varies with the 0psi value, so you either offset or rescale. Have you done tests to determine if the 15kpsi also varies by the same amount, or does it need rescaled? If your tests conclude an offset is appropriate, then you just add or subtract a value from the output. However, I think what you are saying is that your tests concluded it also needs to be rescaled. To rescale based on a single point is not good either, at least in the lab I work. If you are having them rezero, have you done testing to validate values throughout the range to make the best change (offset or rescale)? If the 15kpsi is absolute and is always precisely 20mA, then it is simple. However, that would need to be a very special pressure sensor interface to guarantee that. So, what is it exactly you need help with? Logic to rescale, or to offset? It sounds like you have the scaling part handled quite well though. It is quite ok to use the 0-20mA signal with 3.8mA to 19.8mA instead of 4-20mA. Just reNORM and then reSCALE when the operator sets a new zero. You will need to have saved the full scale range value equivalent to 27648 counts to do that. Be sure to set that tag as 'retain'.
	science guy
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3/27/2012 5:42 PM	Rate (0)
Heberto_hbo Posts: 16 Rating: (2)	Good day, Basically logic for Offset, I was trying to do some logic that works, but it should work for every sensor connected, I mean, if I change the sensor, there will be another offset so, should basically work for every sensor.. I was reviewing some logic, verifying the actual reading of the sensor and comparing with zero.. if the value was negative... it will add the same value when the operator hit the zero button in the HMI.. if the value is positive.. the system will substract the same value when the operator hit the Zero sensor button in the HMI....another thing is that when a sensor is disconnected there will be a value reading in the HMI of -3750 for a pressure sensor of 4-20 mA, 0-15000 Psi....with 4 as Zero...basically I'm working on the solution like I told you before.. but I would like some suggestion if somebody has done a similar logic..... Let me hear your suggestions.. Thanks and best regards..
Heberto_hbo Posts: 16 Rating: (2)
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9/8/2014 10:01 PM	Rate (0)
Devs Posts: 2 Rating: (0)	Hello Heberto, Did you get this issue resolved? I am working on a similar logic to offset the 4mA and 20mA values whenever a new sensor is installed. Please advise on how you resolved this issue. Thank you.
Devs Posts: 2 Rating: (0)
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