PLC analogue input resolution and signal scaling/selection???? 1

[hizdadd]

hi everyone
pretty new at this PLC stuff so really need some help.
i have a pressure transducer that has a range of 0-5bars, and a siemens 314ifm PLC with integrated analogue inputs(12 bits)
the problem being that i need a high resolution, and the only pressures i am interested in is between 4-5bars, the rest i do not need.
now my pressure transducer gives me a current of 20mA at 5 bars and 4 mA at 0bars,and 16mA at 4 bars. i need only the 4-5bar pressure(16-20mA) to be spread over the 12 bits to get a higher resolution.
is this possible and if yes please tell me how?the range between0-4 bars is of no use to me
can i do this without changing my transducer. maybe by varying the range in the plc

 

[danw2]

The PLC's A/D spans the 4-20mA range, so it has only so many bits to look at the 20% of the 4-20mA range of interest to you. You can probably figure out how to scale only the range of interest to you, but that won't increase the resolution.

FYI, dumb transmitters have only a specified, single range. The only adjustment is a trim adjustment, usually approx ±1% of span.

"Smart" transmitters's 4-20mA outputs can be ranged throughout the input scale, so that the 4-20mA output, in your case, would be 4-5 bar; 4mA = 4 bar, 20mA = 5 bar.

The HART protocol is typically used to re-range the 4-20mA output on smart transmitters with a handheld communicator or PC software and a HART modem.

All the major instrumnet companies have smart transmitters, but they cost multiples of their dumb counterparts.

What you want to do is routinely done with smart transmitters.

 

[hizdadd]

thanks danw2,
so is there anything in the data sheets that will let me know of whether the transducer is dumb or smart?

 

[danw2]

Rules of thumb:

If it looks something like this, the range can be configured

If it looks like this, it probably is not range-configurable

Look in the spec sheet for "HART".

Siemens uses terminology like,
"Spans (infinitely adjustable) over nominal measuring range and max. permissible test pressure"

"The parameterization is performed using control keys or via
HART communication"

 

[hizdadd]

thank you for all your help,
i have looked up some smart transducers which are of reasonable cost($400-$1000) heres a link to one of them:

http://www.ueonline.com/bulletins/tx200-b.pdf

just another question
so basically this is what i'm doing, designing a leak detection system(by measuring the pressure drop), so basically ill be charging a test piece to a pressure of 5 Bars and sealing it, and then measure the pressure drop, which i calculate not to be more than a few hundred pascals.
so heres my question, i now have a smart pressure transducer which is capable of measuring btw 0-5 bars, i have programmed it so that it gives me an output of 4mA at 4 bars, and 20mA at 5 bars. now while the piece is charging my transducer will be exposed to the 0-4 bar range, my question being while the transducer is exposed to the 0-4 bar range what will its output be, or will there be no output, will this range inflict any damage on my transducer?
you might ask why the 0-4Bar range isn't important , its because it has nothing to do with the part testing, and data aquisition.

 

[danw2]

1) The TX200A with the zero/span pots will not cover the 4.0-5.0 bar range, because the zero adjustment is limited to 0.5% adjustability.

A call to the distributor revealed that the HART model is supposed to be rangeable 4.0-5.0 bar.

2) Assuming the HART model, when the pressure is 0-4 bar (gauge), the output of the transmitter will be 4.0mA. The 4mA will only increase when the applied pressure is above 4.0 bar.

3) A 0-4 bar applied input pressure will not inflict damage on the transducer.

4) Expect to see a pressure change due the charge gas temperature change over time. Once you've applied pressure sealed it, the chamber sealed pressure will vary with temperature. So depending on what tolerance you can accept for pressure deviations, you need to be sure that ambient temperature shifts that cause pressure changes are not interpreted as leaks.

5) Configuring a HART device requires a HART communicator or software and modem, which costs 4x-10x what the transmitter costs. Be sure to buy it from a distributor who will configure the range for you. The factory might pre-configure the range, but the spec sheet doesn't say that.

 

[LunaApplication]

Hi everyone,

I just finished the test between SCADA software and HART devices (FISHER 4320 and HART Output module) through Smart Wireless Gateway.

From the gateway webpage, we can read all the status of the device, and I used Modbus Mapping to communicate with our SCADA software.

I attached the screen shot that I got from the test, maybe you will be step forward with your HART devices.


Luna Li
Application Engineer

http://www.icpdas-usa.com

 

[LunaApplication]

It should be here:

http://files.engineering.com/getfil...47a4-846f-b75f6899dc2f&file=Hart&Kingview.png

This the SCADA software I used for testing:

http://www.icpdas-usa.com/king_view.php

Luna Li
AE
www.icpdas-usa.com