Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 501)

Appendix F
Optional Analog Input Software Calibration
F–2
Calculating the Software Calibration
Use the following equation to perform the software calibration:
Scaled Value = (input value x slope) + offset
Slope = (scaled max. – scaled min.) / (input max. – input min.)
Offset = Scaled min. – (input min. x slope)
Calibration Procedure
1. Heat up / cool down your MicroLogix 1000 system to the temperature in
which it will normally be operating.
2. Determine the scaled high and low values you wish to use in your
application. In this example, scaled high value (which corresponds to 20
mA) is 31207 and scaled low value (which corresponds to 4 mA) is 6241.
3. Using an analog calibration source connected to the analog input channel
or your system’s input device placed at the 4 mA position, capture the low
value by setting and then resetting the CAL_LO_ENABLE bit. Ensure
that your low value lies within the conversion range of your analog input.
4. Using an analog calibration source connected to the analog input channel
or your system’s input device placed at the 20 mA position, capture the
high value by setting and then resetting the CAL_HI_ENABLE bit.
Ensure that your high value lies within the conversion range of your
analog input.
5. Set and then reset the CALIBRATE bit. This causes the MicroLogix to
calculate the slope and offset values used to perform the error correction
to the analog input.
The analog channel is now calibrated to
±
5 LSB at the calibration
temperature.
The recommended calibration period is once every 6 months. If an
application has a wide range of operating temperatures, a software
calibration should be performed every 3 to 4 months.
Example Ladder Diagram
The following ladder diagram uses three internal bits to perform the
calibration procedure. CAL_LO_ENABLE causes the ladder to capture the
4 mA calibration value and CAL_HI_ENABLE causes the ladder to capture
the 20 mA calibration value. CALIBRATE causes the ladder diagram to
scale the high and low values to the nominal values, which provides the slope
and offset values used to calibrate the analog input channel.