Pressure transducer issue w/MLX1100

DaveZR2

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Join Date
Feb 2010
Location
Boston
Posts
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I have a weird problem that I hope is indicative of something simple that someone here can help me with.

I'm using a 24v input, 0-10v output, Vac-45psi pressure transducer. I'm supplying + and - from my 24v power supply to the transducer. I'm taking the signal from the transducer and sending it to the IV1(+) terminal on my Micrologix 1100. The IA COM terminal is getting - from my 24v power supply.

Everything seems to work fine for hours at a time, until randomly the pressure reading will be full vacuum when it should be something else. Applying pressure to the transducer doesn't change anything. I checked the voltage from IV1(+) to my - terminal on the power supply...0V.

What's going on? Anything I should check? Why does cycling the power solve the problem?o_O

Thanks in advance,
Dave

What I've found is when I remove power to the control system and then power up again, the problem goes away. For reference the voltage between IV1(+) and - on the 24v power supply is around 2.4V at atmospheric pressure after powering up again.
 
I am using a raw value of 255 as my atmospheric pressure point.

If the value is >255 then the scale parameters are:
Input Min: 255
Input Max: 1023
Scaled Min: 0.0
Scaled Max: 45.0

If the value is <255 (vacuum) then the scale parameters are:
Input Min: 0
Input Max: 255
Scaled Min: -30.0
Scaled Max: 0.0

What do you mean by data specs?
 
bce123 said:
What is the voltage reading at this point
it would help to post your logic also
Click to expand...

The voltage is 0.0V when the error is occurring and 2.4V after powering up again (actual pressure is atmospheric in both cases)

I can post my logic but wouldn't you agree that the error is occurring independent of the logic?
 
DaveZR2 said:
I have a weird problem that I hope is indicative of something simple that someone here can help me with.

I'm using a 24v input, 0-10v output, Vac-45psi pressure transducer. I'm supplying + and - from my 24v power supply to the transducer. I'm taking the signal from the transducer and sending it to the IV1(+) terminal on my Micrologix 1100. The IA COM terminal is getting - from my 24v power supply.

Everything seems to work fine for hours at a time, until randomly the pressure reading will be full vacuum when it should be something else. Applying pressure to the transducer doesn't change anything. I checked the voltage from IV1(+) to my - terminal on the power supply...0V.

What's going on? Anything I should check? Why does cycling the power solve the problem?o_O

Thanks in advance,
Dave

What I've found is when I remove power to the control system and then power up again, the problem goes away. For reference the voltage between IV1(+) and - on the 24v power supply is around 2.4V at atmospheric pressure after powering up again.
Click to expand...

How often does this problem occur? If often, then I would remove the sensor from the system and bench test it with a DMM probing the output terminal. If the output looks abnormal, then the problem is isolated to the sensor or the supply.
 
DaveZR2 said:
The voltage is 0.0V when the error is occurring and 2.4V after powering up again (actual pressure is atmospheric in both cases)

I can post my logic but wouldn't you agree that the error is occurring independent of the logic?
Click to expand...

Sounds like it. I would still isolate the sensor from the system to bench test it just in case the problem is the analog input channel.
 
DaveZR2 said:
I am using a raw value of 255 as my atmospheric pressure point.

If the value is >255 then the scale parameters are:
Input Min: 255
Input Max: 1023
Scaled Min: 0.0
Scaled Max: 45.0

If the value is <255 (vacuum) then the scale parameters are:
Input Min: 0
Input Max: 255
Scaled Min: -30.0
Scaled Max: 0.0
Click to expand...

Aside from your transmitter problem, why not just scale the input once?

Input Min: 0
Input Max: 1023
Scaled Min: -30
Scaled Max: 45

And are you sure its "PSI" and not "inHG"?

Just curious.
 
Mickey said:
Aside from your transmitter problem, why not just scale the input once?

Input Min: 0
Input Max: 1023
Scaled Min: -30
Scaled Max: 45

And are you sure its "PSI" and not "inHG"?

Just curious.
Click to expand...

Scaling once for above atmospheric (psig) and once for below atmospheric (inHg).
 
I suspect an intermittently bad connection.

If you have 0 volts (when there is not a vacuum) across the analog input (+) to (-), then either

1) the power supply is bad or not making connection to the transmitter, or

2) the transmitter is not producing the correct output, or

3) there is an open circuit preventing the transmitter's voltage from reaching the AI or from powering the transmitter (same as 1).

I'd look for #3 first.
 

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