12 bit unipolar resolution

Hi all,

Does a 12-bit resolution PLC module have a sufficient accuracy to carry out a standard PIT/FIT processes? I'm looking at a module cut sheet, and it lists 50 Hz, 60 Hz, 250 Hz and 500 Hz specs for this feature . Are these sampling frequencies? I've dealt with AB analog modules in the past that were top of the line 24-bit resolution, but I think that level of accuracy is not always needed.

Any insight into this would be appreciated.

Thanks!

Too many TLA's. What's PIT? What's FIT?

danw said:

Too many TLA's. What's PIT? What's FIT?

Click to expand...

I was afraid to ask.

Can you provide the complete catalog # of your analog module.

12-bit resolution is one part in 4096.

For a 0 - 10 volt analog signal that means the smallest detectable increment is 2.44 millivolts (10 volts divided by 4096).

For a 4 - 20 MA analog signal the smallest detectable increment is 3.9 microamps (16 mA span divided by 4096).

In more practical engineering units, for a pressure sensor with a span of 0 - 500 PSI a 12-bit analog input can detect pressure changes no smaller than 0.122 PSI (500 divided by 4096).

That's best possible resolution. Noise on the analog signal wiring may make that resolution difficult to achieve in your environment.

Until recently, we still had a lot of 12 bit stuff on our SCADA. Had to do a lot of work-around like splitting number into 2 words, etc...

Hi all,

I've attached the cut sheet. PIT=pressure instrument transmitter.
We've got a 0-100 PSI range on our PIT. It looks like 12-bit resolution should be OK. For VFD speed control, I have to look into it, 12-bit may not be the best resolution, as we got a span of roughly 0-3600 rpm. Will have to provide an output in frequency 0-60 Hz to have a better control.

Thanks for feedback!

Regards,
EE

You should also check the specs on your transmitters. No point in using 16-bit analog input modules if the transmitter's analog output is 12-bit. The same reasoning applies to the VFD's analog input compared to the PLC's analog output.

The resolution of the analog input determines the smallest change in the PV that the PLC will see. If the PLC makes no change in the output when there is no error PV=SP and the smallest possible change to the output when the PV=SP+/-1 then the PV is always going to vary by 3 units of input resolution, assuming the smallest possible change in output doesn't result in PV overshooting SP.


is 100psi/2^12*3=.07 PSI oscillation acceptable to your process?
is 3600rpm/2^12*3=2.63 rpm oscillation acceptable to your process?


Using 0-60 Hz instead of 0-3600 RPM won't make any difference unless you are running in to the limit of floating point precision on your platform.