Skip to Content

Register Login

Industry Online Support

Technical Forum

Navigation

0-20mA to 4-20mA

Created by: Mishal11 at: 7/3/2014 5:49 PM (28 Replies)

Rating (3)

Thanks 5

Result pages:: |; 1 | 2 | 3; |

29 Entries

Entries per page: 10 | 20 | all

7/4/2014 7:12 PM	Rate (0)
Mishal11 Posts: 69 Rating: (0)	So if i understand this right, a 0-20mA module can't be used with 4-20mA and the oposite is true. ?
Mishal11 Posts: 69 Rating: (0)
Suggestion To thank Quote Answer

7/4/2014 9:34 PM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 11/18/2024 Posts: 12296 Rating: (2691)	Hello Mishal; Not directly on thechannel of your analog input module. You could uesome intermediary electronic circuit, a converter. See here, for example: https://www.google.ca/search?q=convert+0+20ma+to+4+20ma&tbm=isch&tbo=u&source=univ&sa=X&ei=5wC3U6yhMpCGqgbghILQAg&ved=0CEIQsAQ&biw=1600&bih=754 Or follow the suggestions already provided. Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 7/4/2014 11:31 PM
Suggestion To thank Quote Answer

7/4/2014 11:13 PM	Rate (0)
William_B Posts: 5225 Rating: (1196)	Ste39 Mishal11 Hi daniel So this means that it is not possible to scale it in any way ? am i tied to the 0-20ma on the inputs and outputs ? Hello Mishal11, Havea look here, if you can help: How can you standardize and destandardize analog values with STEP 7 V5.5? Mishal11 So if i understand this right, a 0-20mA module can't be used with 4-20mA and the oposite is true. ? Hi there. If you have an analog card that can read 0..20mA, you can use this to read your 4..20mA signal. Please look again at the note "Ste39" left for you. What happens is that the measured values between 0..4mA you "throw away". You need to do some software scaling so that what you measure only use the analog reading values between 4mA and 20mA. The downside is that you loose 20% of your range.. but maybe that is okay for you. So the question is - how accurate do you need to measure? Forgive my ignorance.. have you ever worked with analog readings before? Do you know how to handle the analog value? If you are unsure.. about rescaling the analog value to give engineering units, please let us know. Good luck. w
William_B Posts: 5225 Rating: (1196)
Suggestion To thank Quote Answer
This contribution was helpful to 1 thankful Users Ste39

7/5/2014 12:41 AM	Rate (0)
Mishal11 Posts: 69 Rating: (0)	Hi William I have limited experience with analog signals. Regarding my case i am using a 0-10 bar sensor 4-20 mA that goes to the input of the module, and as for the output, it goes to a VFD that also runs on 4-20mA, i have tried many different ways to try to scale or offset the 4mA with no success.
Mishal11 Posts: 69 Rating: (0)	Last edited by: Mishal11 at: 7/5/2014 12:43 AM Last edited by: Mishal11 at: 7/5/2014 12:42 AM
Suggestion To thank Quote Answer

7/5/2014 9:55 AM	Rate (0)
William_B Posts: 5225 Rating: (1196)	Mishal11 Hi William I have limited experience with analog signals. Regarding my case i am using a 0-10 bar sensor 4-20 mA that goes to the input of the module, and as for the output, it goes to a VFD that also runs on 4-20mA, i have tried many different ways to try to scale or offset the 4mA with no success. Kind greetings. It is difficult for me to "see" what you have done already. Maybe you already on your way to resolve your own problem. It would be helpful if you could show what you have done.. * connections sketch * hardware configuration * coding [Practical Advice] Allow me to start at Chapter 0. Please do these quick experiements to get a "feel" for the analog values. I suggest you first work with the analog output. Have the signal go through a resistor first. If it is a 500ohm (0.25W) resistor you can read a voltage peaking at 10V. Whatever resistor you take.. make sure the resistor Watts rating can handle the power flowing through. Remember.. Volts = ohms x Amps Watt = Volts x Amps = Volts x Volts / ohm = Amps x Amps x ohms Then experiment with the analog output word (PQWxxx) through the status chart / watch table. For this you need to have no program. Then take volt measurements with your meter. Change the PQW value between 0 and 27648 (explaination to follow). See the effect. Now write down the Volt value measured at 27648. Now calculate the 20% value. In my case with 10V it will be 2V. Manipulate the PQW value until you get that 20%. Write that down. Next step is to connect the analog output to the analog input. Again manipulate the PQW value and this time read from the analog input word (PIWxxx) what the effect is. You know what the 100% value and 20% value (more or less) should be. [Coding] For some background information I suggest you do some reading. From this forum.. [LINK] From the FAQ.. [LINK] The analog format that Siemens uses for unipolar analog signals, will have the internal signal (PIW / PQW) change from 0 and 27648. In your case you are going to use only a portion of this, because you want to measure from 4mA and your card handles from 0mA. So you will have the internal signal change from 20% of 27648... 5530 up to 27648. If you use library functions FC105 and FC106, you might run into a bit of trouble. Remember that these functions assume you are using the full range. I suggest you do some maths and project what the physical values (Bar / Hz) should have been if you could use 0mA.. it is going to be negative values. Does this help? Tell us with what you still struggle with and give us some more information about the questions I asked at the beginning. Greetings. w
William_B Posts: 5225 Rating: (1196)
Suggestion To thank Quote Answer

7/7/2014 5:10 AM	Rate (0)
C00lb0! Advanced Member Joined: 10/15/2009 Last visit: 2/9/2021 Posts: 20 Rating: (0)	Hi Mishal11, Use limits and Scale fc105 block for scaling. hope you get good results.

Suggestion To thank Quote Answer

7/7/2014 11:46 AM	Rate (1)
eisenpferdreiter Posts: 1058 Rating: (93)	Hi Mishal11, the answer to your question why there are no configurable settings with your module has been given detailed already by dchartier. Nothing more to say about it - its a cheap module and a compromise with low resolution. Can you use your 4 to 20 mA Transmitter/Actuators with 0 to 20 mA inputs/outputs? Yes, you can - but you have to be aware of the fact, that you will loose 20% accuracy. If this is ok with your application, you can do so as intended. To help you for your decision: the module has 8 bit resolution, that means 256 possible states. If you drop 20% you will reduce that to 200 states. As the module uses the Msb (most significant bit) as sign bit the 256 states are acatually +/- 128 and the 200 states are +/- 100 bit. In your reply to William B. you state that you have inputs 0 to 10 bar. If you use 0 to 20 mA you will have a resolution of 10bar/128 states, which means your resolution is0,078125 bar per bit, so roughly 0,08 bar resolution. If you use 4 to 20 mA at 0 to 20 mA input, you would have a resolution of 10 bar/100 states, which is about 0,1 bar resolution. Actually this could still be good for many applications, but of course you will have to take into account for your decision. You can check other inputs/outputs with same calculation. Just as William B. stated already, you cannt use library functions FC105 and FC106, as these are designed for full range, but you only have 8 bit resolution. For scaling inputs with 0 to 20 mA signal on 0 to 20 mA input, you will have to do following calculation: Engineering Value= (((MAE - MAS) / effective states) * RAW_VALUE) + MAS MAE...........................measuring area end - real MAS...........................measuring area start - real effective states..........with your module 128 - real RAW_VALUE............digitized input value from module - byte You see that you need to convert RAW_VALUE before you can do the calculation. You can do so by using a MOVE block, where you move RAW_VALUE to a real variable. If you use 4 to 20 mA signal on 0 to 20 mA input, calculation would be: Engineering Value = (((MAE - MAS) / effective states) * (RAW_VALUE - 20%FS)) + MAS MAE...........................measuring area end - real MAS...........................measuring area start - real effective states...........with your module 102.4 - real RAW_VALUE.............digitized input value from module - byte 20%FS.......................20% full states, in your case 25.6 - real So in your example with 0 to 10 bar on 4 to 20 mA with 0 to 20 mA input: half of measuring range is 5 bar, which will be converted to 12 mA (middle between 4 mA and 20 mA). Engineering value = (((10 - 0) / 102.4) * (76.8 - 25.6)) + 0 bar = 5 bar the RAW_VALUE will be (128digit / 20 mA) * 12 mA = 76.8 digit For calculating outputsyou take same equation, but calculate RAW_VALUE instead of Engineering Value, as Engineering Value would be given in that case. I hope I could help you along. Best regards
eisenpferdreiter Posts: 1058 Rating: (93)
Suggestion To thank Quote Answer
This contribution was helpful to 1 thankful Users William_B

7/8/2014 5:26 PM	Rate (1)
Piotr.M Platinum Member Joined: 6/24/2013 Last visit: 11/18/2024 Posts: 1652 Rating: (426)	Hi You can scale it in software by changing values sent to analog output to force it change from 4 not from 0mA Blocks which scale (AI to engineering) and unscale (engineering to AQ) attached as zipped source file. Attachment AIAQ_4_20mA.zip (629 Downloads)

Suggestion To thank Quote Answer

7/13/2014 4:46 PM	Rate (0)
Mishal11 Posts: 69 Rating: (0)	Thanks i do get the point now, thanks to you guys I appreciate your support. In my case i have already ordered an other module that can handle 0-20ma / 4-20ma, which i think would run better.
Mishal11 Posts: 69 Rating: (0)
Suggestion To thank Quote Answer

7/13/2014 4:48 PM	Rate (0)
Mishal11 Posts: 69 Rating: (0)	Piotr.M Hi You can scale it in software by changing values sent to analog output to force it change from 4 not from 0mA Blocks which scale (AI to engineering) and unscale (engineering to AQ) attached as zipped source file. Dear Piots.M Thanks for the replay, how do open the AWL file you sent ?
Mishal11 Posts: 69 Rating: (0)
Suggestion To thank Quote Answer

|

Share this page:

Share this page on...

Follow us on