tidu271

Theory of Operation
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Thermocouple manufacturers usually provide a lookup table that offer excellent accuracy for linearization
of a specific type of thermocouple. The granularity on these lookup tables is also very
precise—approximately 1°C for each lookup value. To save microcontroller memory and development
time, an interpolation technique applied to these values can be used. An example of this method when
converting from voltage to temperature with eight lookup table entries is shown in Figure 13.
To perform a linear interpolation using a lookup table, first compare the measured thermocouple voltage to
values given in the lookup table, until the lookup table value exceeds the measured value being
converted. Then, use Equation 4 to convert the measured thermocouple voltage to temperature, where V LT
is the voltage lookup table array and T LT is the temperature lookup table array. This operation involves four
additions, one multiplication, and one division step, respectively. This operation can be done easily on
most 16- and 32-bit microcontrollers. Converting from temperature to voltage is the same, except that the
lookup tables and the temperature variables are reversed, as shown in Equation 4.
Figure 13. V-to-T Conversion Block Diagram for Thermocouple
VIN
VLT
n 1
LT LT LT
VLTn VLT
n 1
T T n 1 T n T n 1
TIN
TLT
n 1
LT LT LT
TLTn TLT
n 1
V V n 1 V n V n 1
(4)
14 Temperature Sensor Interface Module for Programmable Logic Controllers TIDU271–May 2014
(PLC)
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