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Old January 23rd, 2004, 12:11 PM   #1
Sceptre
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Pressure Transducer Input

Good day,

I have a pressure transducer that i'm trying to hook to an analog input on my PLC. When I wire up the transducer and measure the voltage output with a meter i get 0.5v (0.5v = 0 PSI). When I hook the wires to the double-ended input, my reading immediately raises to 0.7v when checking it with a meter and when I view the variables in the software, i am getting values fluxuating between 0 and 4095 (12bit).

is there anything i might be doing wrong? or is there something I have to do when hooking up analog inputs?

any information would be greatly appreciated.
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Old January 23rd, 2004, 12:49 PM   #2
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Whole lotta shakin goin on!

Quote:
is there something I have to do when hooking up analog inputs?
Well the obvious answer to this part of your question is yes, of course there is. First and foremost you must know what kind of output your transducer has. Nice thing that there are only two types, those being voltage or current. If you have a current output, you will need to input that to your analog as current or the module will not interpret it correctly. I presume that you are using 1771 analog module with a PLC-5 of some description. If you are indeed using a transducer with a voltage output, then you still must tell the module what you are inputting to get a correct reading. Most common voltage outputs are 1-5 volts or 0-10v or something like that. Common current values are 4-20 ma or 0-10 ma etc. If I were you, I would find out exactly what I have from my transducer over the range I expected to measure and then tell the module how you want that interpreted. Also, on some modules you can select a current input mode by flipping a dip switch, others require the application of a loop resistor across the input terminals of the module to convert the loop current to a voltage drop that the module can deal with. Check out some things and let us know what you find and if you need more help, come back with updated info and we will go from there.
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Old January 23rd, 2004, 02:24 PM   #3
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i'm not as dumb as i look

i have a 0-5v transducer and 0-10v inputs. when i measure the transducer with a voltmeter i get 0.5v (which is correct) when i hook it to the PLC i get 0.7v. Its picking up 200mv from somewhere and I can't figure out where. It seems to be a ground loop but I can't seem to isolate it. The PLC has all optically isolated grounds and I am exciting the transducer from the 24vdc along with the PLC (same positive and negative connections).

i even pick up the 200mv when power is removed from the plc and the transducer is just screwed to the terminals.
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Old January 23rd, 2004, 02:42 PM   #4
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Ooooopppppsssss! No need to get testy!

I did not mean to be sarcastic nor condescending in my other post. However, some basic things get overlooked by some of us and I have done those very things so you would be in good company if you had. Now, on to your problem. With the new data to work with, I can not tell you exactly what might be your problem, however I can suggest checking your ground in this equipment setup. If you are not running shielded cable to the transducer, you need to. If you are, you need to make sure the shield is only grounded on one end and that can be in the panel where your PLC is along with all your other stuff. Never connect the shield to ground on both end of a run. It provides a ground loop and can really make things act strangely. Other than that, I do not have any suggestions for you.

Quote:
i even pick up the 200mv when power is removed from the plc d the transducer is just screwed to the terminals.
This is what makes me suspect a grounding problem. Electrical noise can account for the couple of hundred milivolts you are seeing and far worse. Let us know how you fare.
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Old January 24th, 2004, 08:54 PM   #5
ushidayo
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just a few thoughts

I don't know the answer to your problem, but I have a few suggestions on things to check. I assume the analogue input is wired correctly and the analogue card has been configured.

Does this change in voltage also occur when the transducer is connected to another input channel on the same analogue card? If not there might be a problem on the input channel.

Also, have you tried replacing the transducer with another voltage source (e.g. from a voltage calibrator) and checking the result?

Another thing worth checking is the wiring within the control panel. If the positive and negative wires for a control circuit are not kept close together, a loop antenna can be formed which will pick up noise. Are the positive and negative wires twisted together? Twisted pairs for analogue circuits are a common approach to reducing noise problems within control panels.
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Old January 25th, 2004, 06:29 PM   #6
panic mode
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Dear Sceptre,

Why don't you make it simple for everybody and post some
usable data like:
- transducer make and part number
- transducer configuration
- PLC type and analog card configuration
- connection diagram you are trying to use
- what multimeter and range you are using for tests
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Old January 26th, 2004, 12:23 PM   #7
Sceptre
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Quote:
Originally posted by panic mode
Dear Sceptre,

Why don't you make it simple for everybody and post some
usable data like:
- transducer make and part number
- transducer configuration
- PLC type and analog card configuration
- connection diagram you are trying to use
- what multimeter and range you are using for tests

my transducer is an Omega PX303-200G5V (0.5-5.5v output 200PSI). I have a Moeller PLC (XC-CPU101-128k) and the analog card is XIOC-8AI-U1. It is configured and when I hook a 5v input to it (variable power supply), it works fine. I am using a omega multimeter on 2v scale.

I don't have a wiring diagram right now, but its a very simple circuit. 24vdc and ground wired into power on analog card, power and ground of transducer connected to same power source. Signal from transducer goes to + input and ground is tied to negative input on analog card.

i hope this helps and someone can give me a little direction. I've tried separate power supplies for PLC and transducer, as well as various other configurations, nothing has worked for me so far.
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Old January 26th, 2004, 11:36 PM   #8
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I don't have a clue what the problem is, but...

In your most recent post you mention using ground for 0 volt common. I assume that the 0 volt from the 24V DC psu is grounded. Are the O volt commons that are connected to the Analogue card, transducer and analogue input negative terminal using wires that are connected directly to the 24V psu? or are they connected to ground?

When it comes to common mode noise it can be very difficult to predict the path that it will take through "ground". Just the other day I was about to start commissioning a motor control panel, and found 1 amp travelling through the panel earth even though the main breaker was off (haven't a clue where it was coming from, and certainly not the time to look).

Anyway, back to the point. Are the 0 volt common and panel ground separated in the panel, except for 1 ground connection after the power supply unit?
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Old January 27th, 2004, 06:39 AM   #9
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Pressure transducer?

To go back a few steps, have you looked at the o/p from the transducer using a digital meter? Have you monitored this o/p while simulating prssure increase on a test rig? My feelings are to isolate the problem, the above suggestions can be carried out in the calibration room. How long is the cable run from transducer to PLC? Does it run on tray or in trunking / ducts alongside AC supplies?
Jim.
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Old January 27th, 2004, 09:10 AM   #10
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At this point I don't have a panel or panel ground. I just have +24vdc and the negative connection for the voltage. This is powering my PLC and my transducer. This is wired up on a bench with absolutely nothing else hooked to it. I have tried wiring up the transducer to separate power supplies with separate negatives, as well as one supply with one negative point. My power/negative are twisted pairs for both my power supply to PLC as well as my power wires to transducer.

The transducer works fine as long as its not hooked to the PLC. it even works fine when interfaced to a data acquisition card in my desktop PC. Most wires are about 2 feet right now. The transducer wiring being about 3 feet.

I am completely confused right now....I have talked to Moeller and Omega and both say this should work fine. Also, I have tried it on another moeller PLC and it works fine. Which led me to believe that it with a problem with the PLC...but when I hook up a variable voltage supply in place of the transducer, it works fine.
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Old January 27th, 2004, 04:41 PM   #11
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Transducer problem

I can understand your confusion. This is the kind of problem (in my experience) which is usually corrected by using the most unlikely solution. Is it possible that the Transducer is picking up high frequency interference generated by the nearby CPU? In this case you could try passing the signal pair in a double loop through a couple of small carbon rings, much as you would do to suppress radio interference, and / or accross a R-C network. I'm not suggesting any of these will work, but it's a long shot and worth a try.
Jim.
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Old January 29th, 2004, 01:00 PM   #12
Sceptre
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just thought i'd let you guys know

i hooked up a current output transducer last night with a resistor to convert to voltage and it worked fine. so i went home last night...slept on it and came back completely confused this morning

i thought about op-amps with 1:1 gain ratio....realized that its just a filter...put my transducer into a op-amp (gain of 1), hooked the output and reference to the PLC and now it works like a charm

thanks for all the input
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