Thermocouple signal of the nonlinear processing and improved method

Abstract: the thermocouple temperature measurement accuracy is determined by many factors, in addition to the thermocouple itself in accuracy, installation method, thermocouple compensation wire, mainly determined by the thermocouple secondary collection device on the precision. This paper introduces the working principle of the thermocouple, on the basis of analyses the second collection element three key factors affected the accuracy of the thermocouple signal acquisition, including thermocouple signal acquisition circuit analysis, thermocouple signal nonlinear processing and cold junction compensation method, optimized based on the above three key factors, can greatly improve the thermocouple signal acquisition precision, thus improve the thermocouple temperature measurement precision.

3 thermocouple signal of the nonlinear processing

3. Nonlinear processing

1 thermocouple signal thermometer table collection of thermocouple thermoelectric potential must go through the cold end compensation correction, can obtain the reference temperature 0 ℃ cases thermocouple thermoelectric power, the correction formula is

type: E ( T，0 ℃） For reference in the measured medium temperature 0 ℃ when the actual temperature T ℃ corresponding thermoelectric potential, mV; E ( T, t0) For the measured thermoelectric potential under constant temperature t0, mV; E ( t0，0 ℃） For reference when the temperature 0 ℃ temperature t0 thermocouple thermoelectric potential, mV.

after correction, the actual thermoelectric potential, according to the thermocouple indexing table, can be found to correspond to the actual temperature of the measured medium [ 3] 。 Look-up table method is suitable for the unit is easy to store large amounts of data, calculation speed of the computer system, but it is difficult to apply to the data storage space is limited and CPU speed slower thermometer table and other embedded systems, if the embedded system using the look-up table method, it would take a large storage space, and look-up table time is longer, can't satisfy the measurement requirement of real-time.

as a result of the thermocouple thermoelectric potential and a high nonlinear relation with the temperature signal, can not be expressed as a linear function relation of the two, if not make linearization processing, directly according to the thermoelectric potential thermocouple indexing table lookup, find the workload big, not suitable for the embedded thermocouple acquisition system, so you need to the thermoelectric potential and temperature signal of thermocouple linearization processing, reduce the computing effort.

starting from the nonlinear characteristics of the thermocouple signal of thermocouple temperature signal of the approximate linearization by subsection slope method, this method is suitable for embedded system, and high accuracy, less computation, do not take up larger storage space.

3。 2 optimization method of nonlinear processing

thermocouple nonlinear signal processing using piecewise approximation method of slope. Piecewise approximation method of slope is the thermocouple in a certain range of temperature

degrees of difference and the linear transformation of the electric potential difference value in this range as

the area between the slope correction, plus the area between the initial value, get the non linear correction formula for

the actual temperature value = slope value & times; （ Linear transformation value - Interval period of voltage initial value) Initial value (+ interval period of temperature 3)

type linear transformation in the value of after cold junction compensation of thermocouple thermoelectric potential, such as type ( 2) The E ( T，0 ℃） Value.

in order to improve the thermocouple temperature measurement accuracy, piecewise interval do, the more the thermocouple calculated value and actual value error is smaller, the higher precision, but the piecewise interval will increase the run time calculation, concrete block number needs based on the actual needs of thermocouple acquisition precision and frequency of CPU speed.

K dividing thermocouple, for example, assume that the thermocouple 0 ℃ ~ 1372 ℃ was divided into 17 segments, each segment of initial temperature, initial temperature, electric potential, the temperature corresponding potential, as well as the slope of the segment. Concrete block as shown in table 1. Calculation formula is:

type: Tabs for absolute temperature of hot end of thermocouple, ℃; Eabs for reference when the temperature 0 ℃ hot end of thermocouple thermoelectric potential absolutely, & mu; V； EAi for reference when the temperature 0 ℃ degrees between the period of the beginning of thermoelectric potential, & mu; V； Ki of the segment between the slope in the region, & mu; V/℃； Tqi of the period between initial temperature in the region, ℃.

assumption by type ( 2) Realize the cold junction compensation of K dividing the thermocouple thermoelectric potential for 41859 & mu; V, obviously the value on the 12th piecewise interval, the initial temperature is 950 ℃, the slope 39 & mu; V / ℃, the area between the period of starting thermoelectric potential for 39314 & mu; V, according to the type ( 4) Can get the actual temperature value for the Tabs = ( 41859 - 39314). / 39 + 950 = 1015. 3 ℃。 Look-up table available, 41859 & mu; V is the corresponding temperature is 1015 ℃. Look-up table values differ with piecewise slope calculated value 0. 3 ℃, achieve the goal of the nonlinear. Apparently piecewise approximation slope method can be implemented through software programming and easy to implement on the embedded system