[PROBLEM] No output from pressure sensor

Shafiqson · Apr 19, 2012

Hi all,

I'm quite new to PLCs and instrumentation, but somehow I happen to be in responsibility of the instrumentation of a pipe system, involving some water tanks, pumps and different sensors.

I have this pressure sensor I am trying to connect to the analog module of the PLC, but I'm unable to get a decent reading of the output signal from the sensor, even with an amperemeter!

The technical specifications are as follows:

PLC: Mitsubishi FX3U w/ analog module FX2N-8AD
Sensor: DMP 457 (BD Sensors) 0-10 bar @ 4-20 mA

The sensor is connected as thus:

24V+ -> Vs+ [Sensor] Vs- -> I+ [PLC] COM -> 24V-
GND on the sensor is connected to the same ground as the analog module.

Could someone help out??

icky812 · Apr 19, 2012

Section 6 of the analog user manual shows that there is a bit more to the wiring than that.

KalleOlsen · Apr 19, 2012

Hi and welcome to the forum!

If it is a 2-wire sensor, you must be sure to have a 24VDC driving source in the loop. Some AI-cards has this option built-in (I don't know about Mitsub).

Kalle

nonuke · Apr 19, 2012

Like Kalle mentioned, since it is a 2 wire sensor, and the analog input channel is passive, you have to power the loop with an external 24VDC.
From the +24VDC of the Power Supply, go to the + of the sensor and from the sensor -, go to the + of the analog input channel and connect the - from the analog input channel to the - of the Power Supply.

nonuke · Apr 19, 2012

You can also do the wiring according to the analog input manual. You will get the same results.
In any case you can see that you must power the current loop with 24VDC.

danw · Apr 19, 2012

It sounds like you already have a 24Vdc power supply, from your description of the wiring:

24V+ -> Vs+ [Sensor] Vs- -> I+ [PLC] COM -> 24V-

I think you might be missing the shorting link between Vx+ and Ix+ (where x is channel number) as mentioned by the *3 note

Shafiqson · Apr 19, 2012

Thanks for the replies everyone!

Things I forgot to mention:

i) I have an external 24V supply powering the sensor.

ii) The V+ and I+ terminals are shorted, as indicated by the manual.

I've successfully connected a temperature sensor (RTD) using the same supply and identical wiring, being able to read currents correctly.

KalleOlsen · Apr 19, 2012

- Is your mA-meter fuse OK?
- If you measure over the AI card terminals, do you still not have any current?
- Is it a 4 or 2 wire sensor?
If it is the latter, try to switch the 24V supply polarity.

Kalle

Saffa · Apr 19, 2012

Let's first confirm that your sensor is working properly! Connect it up (via a small fuse) directly to your 24V power supply: 24V+ to sensor positive, sensor negative to multimeter positive, multimeter negative to power supply 0V.

With your meter on mA setting and probes in correct holes, you should see 4.0mA or so with your pressure sensor sitting at atmospheric pressure. Of course it's nice to know your meter is working, do you have another functional sensor you could use to test your meter and its fuse? I've blown mine way too many times when going to do a hasty voltage reading and not noticing my probes still in the mA slot...

Shafiqson · Apr 19, 2012

KalleOlsen said:
- Is your mA-meter fuse OK?
- If you measure over the AI card terminals, do you still not have any current?
- Is it a 4 or 2 wire sensor?
If it is the latter, try to switch the 24V supply polarity.

Kalle

i) mA-meter is fully functional
ii) Haven't measured over the card terminals
iii) 2 wire sensor, will try to swtich polarity tomorrow

Saffa said:
Let's first confirm that your sensor is working properly! Connect it up (via a small fuse) directly to your 24V power supply: 24V+ to sensor positive, sensor negative to multimeter positive, multimeter negative to power supply 0V.

With your meter on mA setting and probes in correct holes, you should see 4.0mA or so with your pressure sensor sitting at atmospheric pressure. Of course it's nice to know your meter is working, do you have another functional sensor you could use to test your meter and its fuse? I've blown mine way too many times when going to do a hasty voltage reading and not noticing my probes still in the mA slot...

As I am mentioning to Kalle, the meter is working perfectly fine.

It seems I should clarify, or mention, one thing. The meter and PLC both DO show a current, 4 mA, occasionally jumping to 4,1 mA or even 4,2. But that is the most I am able to get out of the sensor. I am expecting to see something in the lines of 5,6 mA..

leitmotif · Apr 19, 2012

Saffa said:
Let's first confirm that your sensor is working properly! Connect it up (via a small fuse) directly to your 24V power supply: 24V+ to sensor positive, sensor negative to multimeter positive, multimeter negative to power supply 0V.

With your meter on mA setting and probes in correct holes, you should see 4.0mA or so with your pressure sensor sitting at atmospheric pressure. Of course it's nice to know your meter is working, do you have another functional sensor you could use to test your meter and its fuse? I've blown mine way too many times when going to do a hasty voltage reading and not noticing my probes still in the mA slot...

Take sensor and put on bench
Connect a source of pressure to it and a known pressure gage
Put pressure on it
Check response.

From what you have said so far youo have no idea of what is happening.

Dan Bentler

Saffa · Apr 20, 2012

leitmotif said:
Take sensor and put on bench
Connect a source of pressure to it and a known pressure gage
Put pressure on it
Check response.

From what you have said so far youo have no idea of what is happening.

Dan Bentler

Shafiqson said:
It seems I should clarify, or mention, one thing. The meter and PLC both DO show a current, 4 mA, occasionally jumping to 4,1 mA or even 4,2. But that is the most I am able to get out of the sensor. I am expecting to see something in the lines of 5,6 mA..

Yes, it would have been handy to know that it was reading 4.0mA to start - both on your meter and in your PLC. Everyone just wasted a lot of time checking your PLC's I/O wiring etc.

Dan's approach would be the best next step now.

This is really not a difficult task even with only a basic understanding of how 4-20mA works and the right tool, i.e. a loop calibrator.

nonuke · Apr 20, 2012

Shafiqson said:
Thanks for the replies everyone!

Things I forgot to mention:

i) I have an external 24V supply powering the sensor.

ii) The V+ and I+ terminals are shorted, as indicated by the manual.

I've successfully connected a temperature sensor (RTD) using the same supply and identical wiring, being able to read currents correctly.

Since you are powering the sensor with an external supply, you should try to connect the sensor to I+ only, and remove the jumper to V+

Shafiqson · Apr 20, 2012

leitmotif said:
Take sensor and put on bench
Connect a source of pressure to it and a known pressure gage
Put pressure on it
Check response.

From what you have said so far youo have no idea of what is happening.

Dan Bentler

I connected to it a pressure source of approx. 5 bar. But I am still unable to get a reading from the sensor, apart from the 4 mAs.. Maybe I am forgetting something basic? The sensor is completely new, having rested on the shelf since 2010, when it was purchased. I tried another sensor, but it yielded the same result.

The datasheet: http://www.bdsensors.com/file/305_ba_dmu_schiff_e_19_05_09.pdf

Saffa · May 1, 2012

Did you ever get this problem sorted? I didn't notice it pop up in my emails again...

If you had 5 bar on it, and you're still only getting 4mA... on two different sensors... then it must be something incredibly simple. sure the valve from the compressor to your test rig / manifold was open?

[PROBLEM] No output from pressure sensor

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