Noob help with Analog O/Ps

[Yendor33x]

Hi All,
I don't have a ton of experience with PLCs yet, so I appreciate any feedback I can get. What I am doing is using an Omega FMA1816A mass flow sensor reading scfh units. Let's say our spec is 2scfh @70psi for leak checking purposes. Assume anything greater than that value is a fail condition = bad part. The omega meter has an option for 0-5VDC or 4-20mA output signal.

I've done some reading on them, and I cannot determine which is the better path to go? As close as we are to the control pane - I can't see voltage drop being a huge issue. At the same time, which O/p type you get from the omega meter determines which kind of analog card for the PLC you need, correct?

For reference we will most likely be going with AB Compact Logix PLC; we have them in use everywhere in the plant.
Thanks!

 

[jetwax]

for analog signal transmition, industry standard is to use 4-20 mA.
not as susceiptable to noise, and cable length isn't really a concern.

Does the instrument your using have an adjustable setpoint to trigger the output?

Maybe you want to see how bad the part is?

in short, 4-20mA.

 

[Yendor33x]

The below is straight out of the product manual. It does not look like there is a setpoint you can set to trigger. How would I manage the current signal in the PLC to make my own setpoint? I don't think we really care about how "bad" the part is; rather if greater than setpoint 2scfh of flow = FAIL


Initiate a controlled gas flow after warm up. The flow signal output can be viewed
either on the LCD display, remote panel meter, digital multimeter, or other display
device.
If an LCD display has been supplied with the FMA1700A/1800A, the observed
reading is in direct engineering units (0 to 100% indication is optional).
Analog output flow signals of 0 to 5 VDC and 4 to 20 mA are attained at the appropriate pins of the 9-pin “D” connector (see Figure 2.a) on the side of the
FMA1700A/1800A transducer.
Meter signal output is linearly proportional to the mass molecular flow rate of the
gas being metered. By default calibration is done against 0 to 5 VDC output signal. If 4-20 mA output signal is used for flow indication on the FMA1700A/1800A,
which was calibrated against 0 to 5 VDC, the total uncertainty of the reading may
be in the range of +2.5% of full scale. Optional calibration for 4-20 mA output signal is available upon request at time of order. The full scale range and gas for
which your meter has been calibrated are shown on the flow transducers front
label.

 

[drbitboy]

Hmm, the thread title is Analog O/Ps; I interpret O/P as an OutPut; but the task seems to involve reading a Analog signal in the PLC, and the convention is usually to express the direction of the signal relative to the PLC, not the sensor/transducer/instrument, so that is an InPut (I/P) to the PLC.


I assume "controlled gas flow" means maintaining a constant pressure inside an enclosure, so you can measure the enclosure leak rate at that pressure.


Also, since the process is pass-fail, it doesn't really need a PLC per se, just something that lights red or green when the voltage or current signal is above or below, respectively, a target level. That could be done with analog parts.


Still, it might be easier with the PLC, which would provide flexibility (e.g. working in engineering units, once the mass flowmeter is calibrated, for easy adjustment if the target rate changes), and probably make it easier to troubleshoot in the future.

 

[Yendor33x]

drbitboy,

You are right. I should have titled the thread as PLC Analog I/P. I was thinking more along lines of the flowmeter having an output to the plc, which in turn is an analog input for the PLC. Thanks for that. Can you see how green I am?

More information I should have included in original post:
What we are trying to accomplish is leak testing components in an automatic situation. We will use PLC as this is a smaller piece of a larger automatic machining cell (leak testing after assembly in this case).

What needs to be accomplished/major goal: flow meter reads flow 24/7 every cycle. If there is a leak measured above the threshold value, the process needs to stop/fail.

Currently the automated process was done by an outside vendor using pressure decay method -- our new corporate lab spec wants us to read flow instead. So, here I am.

How does the PLC read an analog input to achieve said setpoint/threshold value in order to trigger an alarm & process stop?

 

[drbitboy]

How does the PLC read an analog input to achieve said setpoint/threshold value in order to trigger an alarm & process stop?

                              high_flow_alarm
---[GEQ                   ]---------( )-----------
   [SourceA  measured_flow]
   [SourceB            2.0]

Of course there would also be

• An analog input card to convert the flowmeter 4-20ma analog signal to a binary digital value.
• A scaling instruction as well to convert the analog input card's binary digital value to SCFH
  • This step may be merged into the card configuraion
• Some indication of the state of [high_flow_alarm] (light, HMI, whatever),
• Maybe some logic to control the test setup,

but there's the beef: if the measured flow is above 2.0scfh, then the high_flow_alarm bit would be 1 (fail test) otherwise it would be 0 (pass test).

 

[Yendor33x]

Interesting.. it doesn't look too bad from here.



I'll have to read into scaling instructions that you mentioned.
The model of flowmeter I picked allows for either 0-10vdc or 4-20mA signal & comes preset from manufacturer to read & display scfh units.

I'll have to check on being able to convert sensor input signal to scfh in plc

 

[drbitboy]

I'll have to read into scaling instructions that you mentioned.
The model of flowmeter I picked allows for either 0-10vdc or 4-20mA signal & comes preset from manufacturer to read & display scfh units.

I'll have to check on being able to convert sensor input signal to scfh in plc

https://www.plctalk.net/qanda/showthread.php?t=126614