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Problems analogue signal
5

Problems analogue signal

Problems analogue signal

(OP)
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.

RE: Problems analogue signal

I got next to nothing from your diagram. It isn't specific enough.

From your text description, it sounds like the Teleflo Diva is a four-wire device and the PLC is expecting a signal from a two wire device. That can be corrected with a signal isolator.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Problems analogue signal

(OP)
thanks, i did the drawing in paint in 2 minutes, but believe me the original isnt much different. Problem is that according to the diagram it only needs two wires and if it is 4 it should have been drawn as such. I am attaching a photo as we speak.

I think the second picture tries to say its 4 wire, however when we generate mA with a Fluke to their terminals it seems to work, so i dont kno what difference ther can be between the Fluke and this Teleflo DIVA of which a find no info on the web btw.

RE: Problems analogue signal

Ok the second diagram clears it up. The Teleflo is a two wire device. This means that it controls the current, but does not supply it. You need to add a power supply in series with the current loop in order for it to function. The change is simple, look at the top diagram of your second image. Connect the loop like that and it should work. The positive side of you analog input should connect to the 24 VDC terminal of the Teleflo. The negative side of the analog input should connect to the analog output of the Teleflo. Hope this helps.

RE: Problems analogue signal

(OP)
thanks for your help. However If i set my Analogue output in my PLC to Active it the supplies the 24Vdc in the current loop, why would I need an external power source?

RE: Problems analogue signal

(OP)
According to the top image thats the way to connect if i want the signal as Active (i.e power from Teleflo) and the bottom pic as passive (i.e. power from card). What you are saying is to make my card active and connect it as the top diagram? but then I would have two 24 Vdc connected together right?

RE: Problems analogue signal

Who makes the analog card and what is the part number?

RE: Problems analogue signal

(OP)
Its called METSO from Finland, I will have to look it up. What exactly would you like to see from the specs? We have many passive instruments connected and so far no problems, even signals from frequency converters which are active are also ok.

RE: Problems analogue signal

I want to read the instruction manual so I can create a drawing showing how to wire your analog signal. The previous posts have described how to wire up the signal, but you are still having problems so I thought I would make a drawing.

RE: Problems analogue signal

(OP)
Ok, find the requested manual attached. The card we are using is AI8C D201134.
Correct me if I am wrong but this is how I understand this.
If I want the 24 Vdc supply to come from the TELEFLO I configure my card as Passive and connect to terminals 14 and 12 on the Teleflo, On the contrary if I want the 24 Vdc supply to come from my card I configure my card as Active and connect to terminals 13 and 12 on the Teleflo.

RE: Problems analogue signal

(OP)
Amptramp, I have done as you say however the problem is that setting both my card and their system to passive there is o Vdc in the loop. If I set my card to passive and connect to their terminals 12 and 13 I am connecting to potential free contacts. If I set my card to passive according to the diagrama I should connect to terminals 14 and 12 but it still no go!

thanks for your help

RE: Problems analogue signal

(OP)
I just uninstalled acad, can you attach it as pdf, thanks

RE: Problems analogue signal

flinana:

I noticed I posted the ACAD drawing file (.dwg) instead of the .pdf file. Please find the .pdf file attached below.

I chose the new wiring configuration based on the observation the Teleflora analog ouput is a "sinking" output. However, even if the new configuration works, I would not be comfortable with this wiring arrangement. The Metso analog input card is a single-ended card. On a single-ended input card, all channels share a common connection. The wiring diagram I posted basically sets the analog output from the Teleflora on "top" of the channel's 249 ohm resistor. I do not know what kind of interaction this configuration would have with other channels on the input card.

Do you have a circuit diagram for the Teleflora analog output (similar to the diagram for the analog input found in the instruction manual you posted)? This information would be helpful.

Finally, the easiest way to get the loop working, as xnuke suggested, is to utilize a signal isolator.

RE: Problems analogue signal

(OP)
Cheers amptamp, I have tried this new configuration too and nothing, just changing the polarity. I will try using the isolator although I believe my colleagues have tried too, I will try it again anyway.

RE: Problems analogue signal

(OP)
and no unfortunately I have no more info from Teleflo.

RE: Problems analogue signal

Cheers flinana:

I would really like to find a solution to your problem. Is there any way to obtain a circuit diagram for the Teleflora analog output?

RE: Problems analogue signal

flinana:

Ok. If you get the loop working, please post the solution.

Also, sorry for the misspelling of Teleflo.

RE: Problems analogue signal

(OP)
The strangest thing is that when i use a multimeter and connect the lead to the +24Vdc from the Teleflo and the other lead to the GND i do read the correct mA!!

RE: Problems analogue signal

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.

RE: Problems analogue signal

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?
 

RE: Problems analogue signal

(OP)
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



 

RE: Problems analogue signal

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.

RE: Problems analogue signal

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

RE: Problems analogue signal

What do you have to lose?

RE: Problems analogue signal

Shall we vote about it?

I vote YES  smile

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Problems analogue signal

(OP)
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!!!!

RE: Problems analogue signal

(OP)
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!

RE: Problems analogue signal

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

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Problems analogue signal

(OP)
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

RE: Problems analogue signal

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

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Problems analogue signal

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

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Problems analogue signal

(OP)
this is fun although complicated due to the HTF used.

RE: Problems analogue signal

Molten salt? Or something newer and better?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Problems analogue signal

(OP)
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)

RE: Problems analogue signal

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?

RE: Problems analogue signal

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.

RE: Problems analogue signal

(OP)
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.

RE: Problems analogue signal

flinana:

Are you saying you got the Teleflow output working? If so, what did you do to get the output working?

RE: Problems analogue signal

My guess is the teleflo transmitter output is referenced to something other than Negative.
It's quite common to find that especially in odd-ball transmitters, they don't bother to isolate the output from ground.
You could try this little test:
   Put a 250 Ohm load on the Teleflo (active) and measure the voltage from ground to each terminal with the Teleflo at zero percent.
   Make the same voltage measurement with your PLC as passive with your calibrator at zero percent, I think you will find the voltages to ground are different.
Please report back.
If you already have it working, let us know what you did.

Regards
Roy
  

RE: Problems analogue signal

Yes, just looking at one of the pics you posted the Teleflo output is referenced to +24
Your PLC input is referenced to ground
So assuming the Teleflo Power supply is grounded you will get 24/250 = 96 mA as soon as you connect Teleflo + to PLC +

Unless you can isolate the Teleflo power supply from ground a signal isolator is the only way forward

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