Analog Problem

BoxerBrats

Member
Join Date
Jan 2005
Location
Chico California
Posts
187
OK question, I have a pressure transducer for pressure range of 0 200 PSI. Puts out a 4 - 20 MA signal. I installed this yesterday and the lines are empty so there is 0 psi, but my PLC is saying there is 200 PSI. I took my fluke measures the MA signal it shows .004 MA.

We tested this before it left the shop using exact PLC and transducer worked perfect, though granted never tested with zero PSI.

We are using a AD PLC with a F0-4AD2DA-2 analog card.

Like i said this was tested here worked fine. I am wondering if the polarity of the wire had been reversed would this have caused the problem?

Any suggestion would be great. At this time I am basically brain dead we had put in over 24 hours last 2 days installing this, and the drive ect....
 
Hi,


I know nothing about AD PLCs but you say that 4-20mA equals 0-200 psi. Now you measure close to 0mA when you should have 4 mA. No matter what PLC you are using you should have 4mA. Check the wiring again. Is there really supply voltage to the sensor?

Then of course you should not have 200 psi at 0mA. Your program should generate an alarm for wire error.
 
Last edited:
BoxerBrats said:
but my PLC is saying there is 200 PSI.
Do you really mean 200 PSI ? or it's the equivelant binary counts. I am saying that just to give you a hint that thier may be an error in your scaling equation (assuming that you really mean 200 PSI)
 
Reversed polarity will certainly result in a zero signal. Try swapping the leads. Also make sure you have continuity to the device, and make sure the contractor isn't using wire nuts to splice the analog wiring.
 
u may check by giving a variable Analog Signal
try to get Values for 0 ma to 20 ma
check the wiring ... may be troble over there

Wiring is a single line or (sink / source) ?
pls provide drawaings.

Thanks
sandymax
 
The reason you are seeing 200psi is because the analog input is less than 4mA. I'm not sure if other PLCs do this, but the way you are scaline the analog value in AD's plc will result in a full scale value when the analog input is less than 4mA. It has to do with a division error. With the type of system you are using the pressure transmitter on, you should use a snubber on the pressure transmitter.
 
I doubt very much indeed if it is anything to do with a division error - it is most probably likely that the analogue input is configured to full scale on wire break .
First thing - put a simulator on the loop and see what you get out of the PLC - you know what 12mA should give you - then check the way the input is setup for broken wire . That leaves your transducer , you can prove that in a second with a 24V supply and your ammeter .
 
Well, since the analog card he is using does not have a full scale on wire break option, that’s not why he’s seeing full scale with less than 4mA.

His scale logic is -
LD Raw Analog Value (0 – 4095)
MUL Full Scale Value (0 – 200)
DIV Raw Analog Max Value (4095)
OUT Scaled Analog Value

I don’t remember what the Raw Analog Value is when the input is less than 4mA, but it’s not 0. Therefore we are back to the Division error.

Anyways, what you can do in your logic is to test the Raw Analog Value to make sure it’s in range. If Raw Analog Value is not greater than or equal to 0 and less than or equal to 4095 then LD K(Full Scale Value or Minimum Scale Value) and OUT to Scaled Analog Value. You would insert this logic after the scale logic. You could also turn on some alarm bit, Pressure Transmitter Error Alarm, and then use this alarm bit in your program to disable the pumps or whatever.

Since you are reading .004mA, first check for proper voltage in the loop, second simulate 4-20mA to the Analog Card (verify the card is working). My guess, a blown transmitter due to a surge in water pressure. Even if this isn’t the problem, make sure you have a snubber on the pressure transmitter, this will help prevent pressure surges from rupturing the diaphragm in the pressure transmitter.
 
Think again - if the device is stuffed and there is no current in the loop - why is the PLC displaying FSD ? Scaling is not a divide function is it ? it would fall over all the time wouldn't it ? how many times is the input to a scale 0 ? it would fault wouldn't it ? scale is a multiply by function if you want to make it smaller , you don't have to divide , you mulitply by a fraction - original post stands , do things in order rather the speculating - I don't know this PLC - though it does sound a bit basic (wait for the shouts over that) if you can't scale broken wire upcsale or down - but I don't have to - if there is no current in the loop , two things could be happening - One- scaled up on broken wire , two - card stuffed - either way , fault finding is five minutes .

Sherlock Holmes had the right idea - when you have eliminated what it can't be - what it is is what you are left with .
What you are saying is that if the loop is reading 4 mA or under (zero input) the thing will divide by zero and give FSD ? please !
 
Last edited:
We are using a AD PLC with a F0-4AD2DA-2 analog card.
I thought that was a voltage card...ie 0 to 5 or 0 10 volt.

The manual: http://web5.automationdirect.com/static/manuals/d0optionsm/ch7.pdf


f04ad.jpg


If so an full voltage is constant wouldnt it always show full scale?
 
Last edited:
Old Sherlock wins again - wouldn't make a lot of difference what current was applied to a voltage card , if anything flows it will read max volts ,maybe ? perhaps ?
Could it still be a divide by zero error Tark ? If it is a voltage card , might answer the question - also remember that many current cards are actually voltage cards with those little resistor things slung in - and if one of those gets cooked - as is the want with siemens s7 300 cards , then it tend to read full scale all the time - disconnect the devices , use a meter and find where the problem is .
 
One shouldn't be too surprised to find that when making a fruit salad with fish parts instead of fruit, the result is bound to be somewhat other than expected.

I'm curious... if this is indeed a 0-5V/10V module (and so it seems) how is it that your "test" with 4-20 showed that all was well?

Since you assumed this is a 4-20 mA module, doesn't your "test" include placing the meter in series and then, after reading the mA, calculating what the raw number should be... and then verifying that the PLC shows that number?
 
It doesn't matter , it is a divide by zero error .
The only possibility that crossed the floggles mind was a current card where the ballast resistor was cooked causing this , but back to basics - as you rightly said , what happened to the meter in this . don' have to get too technical , 4 , 12 ,16 ,20 is enough to make sure the thing is OK . I have had cooked siemens cards with cooked resistors that looked ok at low currents , but went cuckoo when things were wound up a bit because the resistors were only partially cooked .
Do you suppose you fruit salad tasted fishy because you used cooked resistor flavour fish ? , or because your fish were in fact fowl ?
If tin whistles are made out of tin , what are fog horns made out of ?
 
OK

first my mistake on part num F0-4AD2DA-1 It is 4-20 MA, and I know math is right this was all tested in shop with the same PLC, same program, and same pressure transducer.

I really think I have my polarity backwards. Would have probably caught it, just brain dead after to many hours
 
Fred Floggle said:
- I don't know this PLC -

What you are saying is that if the loop is reading 4 mA or under (zero input) the thing will divide by zero and give FSD ? please !

Well Fred, I do know this PLC. I did not say it was a division by zero error, I said it was a division error.

If you set-up the scaling as AD shows in their manual, and you apply less than 4mA to the analog input, it will go full scale. I know this happens, because I have seen it happen more than once. I had a pressure transmitter taken out by lightning, and the PLC showed full scale. I had an analog input that was bouncing between min and max scale when it should have been at min. Come to find out the transmitter was putting out 3.84mA instead of 4mA.

I guess the important point is that this is an unexpected result and people who use AD's products should be aware that this can happen. Depending on the application, this could be a big problem.
 

Similar Topics

Hi We're having issues with an FX1N-1DA-BD Analog output module. We're using an metallic contamination sensor MCS 1000 to detect particles. This...
Replies
1
Views
61
Hi, I tried today to get RD3A and WR3A instructions to work on the PLC and it didn't work, but it worked in simulation. I followed the clone...
Replies
18
Views
1,243
Hi, Sorry for my bad english, I'm from Poland. I've decided to learn basic PLC programming as hobby and to maybe get a better job. I have...
Replies
2
Views
1,300
Good evening, just a dump question, I’ve configured channel no 13 with scalling as attached, then shown not linear reading, even when i simulate...
Replies
6
Views
1,678
I have a SLC5/05 with a 1746-NI16V. I am trying to configure it for single ended inputs. Please notice in the screenshot the upper SCP is...
Replies
21
Views
5,454
Back
Top Bottom