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SM 1231 AI4 4-20mA issue

Created by: spokiir at: 4/24/2012 5:47 PM (6 Replies)

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4/24/2012 5:47 PM	Rate (0)
spokiir Posts: 13 Rating: (0)	Hello, I have SM 1231 AI4 card, with 2 analog 4-20mA signals, one from a pressure sensor, other for purity. I measure 19.25mA current from purity sensor with multimeter, but when I connect the wires with AI card, the value changes from 18 to 19 and is fluctuating between this two values. The value from pressure sensor is stable. wiring is as attached. The purity sensor is power supplied from different DC supply than the AI card. Please advice what I have wrong. plc: CPU 1212C AC/DC/Rly // 6ES7 212-1BD30-0XB0 AI: SM 1231 AI4 // 6ES7 231-4HD30-0XB0 thanks stano Attachment scan0001.jpg (926 Downloads)
spokiir Posts: 13 Rating: (0)
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4/24/2012 6:46 PM	Rate (0)
0_Einstein Grandmaster-Expert Joined: 7/7/2010 Last visit: 12/31/2025 Posts: 16342 Rating: (2521)	spokiir Hello, I have SM 1231 AI4 card, with 2 analog 4-20mA signals, one from a pressure sensor, other for purity. I measure 19.25mA current from purity sensor with multimeter, but when I connect the wires with AI card, the value changes from 18 to 19 and is fluctuating between this two values. The value from pressure sensor is stable. wiring is as attached. The purity sensor is power supplied from different DC supply than the AI card. Please advice what I have wrong. plc: CPU 1212C AC/DC/Rly // 6ES7 212-1BD30-0XB0 AI: SM 1231 AI4 // 6ES7 231-4HD30-0XB0 thanks stano In your NORM / SCALE blocks, the output datatype of the SCALE that you observe toggling between 18 & 19, what datatype is the tag? It should be real. The inputs are configured in pairs. Is the wiring matched with the PLC module configuration as well?
	science guy
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4/25/2012 10:22 AM	Rate (0)
spokiir Posts: 13 Rating: (0)	18-19mA, measured by multimeter. the word value is around 25000, but not stable at all. I have checked the manual for sm1231, but didn't find the solution any ideas?
spokiir Posts: 13 Rating: (0)
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4/25/2012 2:53 PM	Rate (0)
0_Einstein Grandmaster-Expert Joined: 7/7/2010 Last visit: 12/31/2025 Posts: 16342 Rating: (2521)	spokiir 18-19mA, measured by multimeter. the word value is around 25000, but not stable at all. I have checked the manual for sm1231, but didn't find the solution any ideas? How do you measure 19.25mA with a multimeter? Just curious. As to the problem, ~25000 counts is about right for 18-19mA. 27648 = 20mA. How much does the count value change if you normalize and scale it? Use the NORM function to scale the signal from counts to 0.0-1.0. Then use the SCALE function to scale the Normalized 0-1.0 to user units, or pressure (kPa/psi/etc.). If the user units are only changing a fraction of a kPa, that may be correct. You may need more filtering or better cable shielding or better signal common/grounding to reduce the actual bounce. Might need to relocate sensor feedback or power cable to reduce noise even further. We recently had a problem where shielded high quality VFD cable was simply too close to low signals that didn't have adequate shielding. Currently rewiring to correct. Yes, I know 4-20mA is supposed to be more immune to noise, but it does require some help to ensure everything is properly bonded unless your sensor provides excellent ground to signal isolation. I will not say the AI module has excellent ground to signal isolation as it does not seem to in my experience.
	science guy
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4/25/2012 3:29 PM	Rate (0)
Gorp Posts: 89 Rating: (16)	spokiir The purity sensor is power supplied from different DC supply than the AI card. Please advice what I have wrong. thanks stano Is 0V of the sensor supplyfor the purity sensor connected to the 0V (M) of thePLC power supply? I don't know the input circuit of the analog input but since it is not isolated I think it needs a return path for the (very small) current that flows into the module.
Gorp Posts: 89 Rating: (16)
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2/24/2014 5:10 PM	Rate (0)
GrasserF Posts: 340 Rating: (6)	new question is splittet to If 20mA is 27684, what's 4mA should be in NORM, 0 or 5336? 6ES7 231-4HD32-0XB0.
GrasserF Posts: 340 Rating: (6)
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3/26/2018 10:22 AM	Rate (0)
Jen_Moderator Posts: 8171 Rating: (174)	New question published by tvr126 is split to a separate thread with the subject connect a loop powered transmitter that outputs 4-20 mA. Best regards Jen_Moderator
Jen_Moderator Posts: 8171 Rating: (174)
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