CTEK Automation Control Application APN007 - Manual (Page 30)

Application Note – Automation Control Application
APN007 25 July, 2014
25
Min/Max Range and Sensor Calibration:
Range refers to the range or precision of the analog to digital (A/D) conversion. Ctek 1216 I/O
modules provide 12 bit conversion resulting in a theoretical 4096 discrete values, 0 - 4095. Ctek
I/O modules start with a range offset of 7 making the theoretical available range 7 - 4095. Once
the sensor is calibrated the actual available range may be somewhat smaller than the theoretical
due to sensor characteristics and minor variations in the reference voltage.
Calibrating an Analog sensor - The Min and Max range fields can be used to
automatically calibrate the sensor. To perform calibration the sensor must be powered up
and properly configured within the Automation Control application. Once the sensor is
configured the application automatically records the extremes (high/low) values for the
sensor. Some sensors have a built in calibration feature. In that case select the minimum
(low) level calibration on the sensor and leave it in that position for at least 10 seconds.
Repeat this process for the high or maximum value, again waiting at least 10 seconds.
If the sensor does not have an internal calibration feature there are two possible
approaches to calibration. First, consult the sensor vendor's literature. The vendor will
certainly define the Min and Max units that the sensor is defined for, for instance -30 to
+70 degrees. Also, it is common for the specifications to indicate a minimum and
maximum output of the sensor at a specified reference voltage. Those values can be
used to trim the Min and Max range inputs for the sensor. As an example, if a sensor is
specified to output 0.25 volts on the low end of the scale for a reference voltage of 5 volts
that would equate to 5% (0.25 / 5.00) of the total range which would make the Min Range
equal to 205 (4095 * 0.05) and the Max Range equal to 3890.
An internal calibration feature is also available that analyzes the actual reference values
available on the module. To use this feature the input pin is connected to the I/O modules
ground for 10 seconds followed by connecting the input pin to the sensors reference
voltage for 10 seconds. Once the input pin has been exposed to the extremes for 10
seconds each simply enter an asterisk (*) character in the MIN and Max range fields and
press submit. The asterisk characters will be replaced with the Min and Max range values
sensed during the calibration process.