Problems analogue signal 5

[flinana]

Hi,
I have the following problem. I am suppose to receive a 4-20 mA signal from a system called TELEFLO DIVA supplied by Air Liquide.
Problem arises that when i connect the two wires to our plc input card (configured as active) we read nothing. However wheni connect a 4-20 mA generator (as passive) and generate to the card it read ok. I belive the signal is given in parallel and not in series, could this be?

I attach the diagram that was given to me.

 

[djs]

I wonder if you are running into a voltage limitation problem with the Teleflow output. It looks like your input card has a resistance of 250 ohms. Most multimeters have a resistance of about 2.5 ohms or less when measuring ma.

If you can connect the Teleflow to the analog inoput card and take some voltage measurements at various points. For example measure the voltage across the output of the Teleflow, then again across the analog input card. The dirty secret here is that the analog input card is really measuring voltage, not current. A 250 ohm resistor is placed across the voltage inputs so that a 20 ma signal will produce 5 volts to the ADC.

 

[roydm]

You say I get the right mA between +24 and Ground so the flow device is 4 wire (4 wire means it's powered e.g. 120, 240 or 24 volts going directly to the device)
You said your PLC works OK when you simulate a Passive signal (thats a 2 wire input meaning the Fluke is just creating a load on the input, not suppling the power). In other words you are trying to supply the loop power from both places.
If you re-configure your PLC input as passive it should work
Sometimes though you get into a situation where the inputs are not compatible because either both are referenced at different levels.
If it doesn't work please do a couple of tests and report back
With the plc input configured as passive see if the input terminals are grounded (measure input terminal to ground with multimeter.
Measure voltage at each of the flowmeters terminals to ground.
Connect up the flowmeter to the input module and once again measure the voltage between each plc input terminal and ground.
Disconnect the positive wire at PLC terminal and connect a mA meter in series, do you get mA?
Disconnect the negative wire at the PLC what is the current in positive wire now?
BTW, by ground, I am refering to the chassis, not negative what are you calling Ground in first sentence of my reply?

 

[flinana]

hGND is Earth. I have tried the following previously.

Active output TELEFLO and passive PLC Input. FAILED
Passive output TELEFLO and Active PLC Input. FAILED
Passive output TELEFLO and Passive PLC Input. FAILED
Active output TELEFLO and Active PLC Input. Have not tried.

GND in TELEFLO is connected to earth
Negative terminal in PLC input also connected to same earth.

When I set the input card as Passive I set up the multimeter as Active, generate a signal and it is received.

When I set the input card as Active I set up the multimeter as Passive , generate a signal and it is received.

The input card works fine, we have many other instruments connected and never had a problem. I believe the problem is the way the TELEFLO generates the mA.

One thing I haven't mentioned is that when I set-up both sides as passive and connect the negative of the plc card to another negative of another card I do read mA but the the actual mA I should be reading, it reads a fixed value of mA.
This is becoming a headache, We have commisioned this solar plant with around 1500 signals and this one we cant get!!

I need to try the last two posts, reading voltages at different points.

Thanks

 

[djs]

Sounds like you might have a ground loop. If one side of the PLC card is tied to ground and the ground of the Teleflow is also grounded, then if the controlling element in the Teleflow is between the output and the +24 side, then you have a ground loop. The only solution is to use a signal isolator.

 

[flinana]

Ok, We have signal isolators we use as barriers from ATEX area. Should I try one of these?

 

[djs]

What do you have to lose?

 

[Skogsgurra]

Shall we vote about it?

I vote YES

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[flinana]

OOOOOKKKKK, Will do, as soon as the transformer feeding the TELEFLO is repaired. Someone had a great idea and connected a heater to the 220Vac control panel plug!!!!

 

[flinana]

btw Skogsgurra I see u are Swedish. I have to put up all day with Swedish commissioning engineers for our 50MW steam Siemens Turbines . I too worked for Siemens in germany a while back!

 

[Skogsgurra]

I actually do some special investigations for the Finspong team - they are all over the world. Where are you?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[flinana]

I have been in the Norrkoping office for FAT tests, most of the Siemens are from there of the Finnspong office. I am in southern Spain, building solar power plants

 

[Skogsgurra]

I may have been quite close to where you are. The 'Four Solaire' in Cerdagne. Same principle?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[flinana]

not sure what that is. This is where I am

http://www.solucar.es/corp/web/en/our_projects/solucar/index.html

 

[Skogsgurra]

That's impressive. The Cerdagne Tower is an experimental, French plant. It is in the Pyrenees.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[flinana]

this is fun although complicated due to the HTF used.

 

[Skogsgurra]

Molten salt? Or something newer and better?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[flinana]

HTF = Heat Thermal Fluid, such as dowterm. We have done a Molten salt plant already. HTF is complicated due to its property changes with temperature. We heat oil up to 398 degress with the parabolic mirrors and then onto a steam generator (BKA Aalborg)

 

[sinueus]

I have been thinking that the problem might be caused by the scan sequence of the PLC. If one of the instructions in the program is reacting at the wrong time, it could cause this kind of problem. Do you have a detailed scan sequence and a ladder diagram?

 

[roydm]

Personally I hate using signal isolators but sometimes you have to bite the bullet , I think its the only way in this case.
Contractors love those panel outlets for running their drills etc. I suggest you install a circuit breaker sized to protect the transformer.

 

[flinana]

ok, so after doing all tests you people suggested it so happens that the problem is a voltage limitation problem with the Teleflow output.
I have tried using a level transmitter connected in parallel with the process and works fine. So I have ordered on to install and that way I will always have the level indication even if the TELEFLO goes down. Thanks everyone for their inputs.