Calibration of dynamic thermocouple during friction and cutting with laser interferometry

Abstract

The main tendency of nowadays is wide spreading of an adaptive control systems in different production processes and machines, which used temperature as feedback parameter. In spite of temperature transducers development, dynamic (natural or tool-work) thermocouple is still the most convenient method. However its relationship between temperature and thermo-emf is difficult to determinate due complex physical phenomena in contact. Existing calibration methods have significant disadvantages that limit their accuracy. The purpose of the study is development and experiment of the novel dynamic thermocouple calibration method. The method involves simultaneously measurement of thermo-emf and temperature of dynamic thermocouple by laser interferometry during the cutting process. The thermo-emf measurement circuit consist of mercury slipring, IC amplifier and analog to digital converter. Implemented temperature measurement method involves recording of the thermocouple element thermal expansion fields by laser interferometry and calculation temperatures employing the coefficient of thermal expansion (CTE) of the element material. Due to use experimental conditions close to real in contrast to conventional methods, developed method has higher accuracy. Comparing to infrared thermometry, the involved for temperature measurement method, due to the use of light in the visible region of the spectrum, has a larger spatial resolution and a lower achievable field of view. More over it is less sensitive to the thin-films interference in oxidized object surfaces due to higher reflection coefficient of the films for visible light that eliminates the false shift of the measured temperature. And since the method involves CTE to calculate the temperature, which, unlike the emissivity coefficient in infrared thermometry, does not depend on surface quality changing and can be measured with high accuracy by modern dilatometry, the developed method has high reliability. The study experiments the method efficiency by orthogonal cutting of steel with cemented tungsten carbide tool at different cutting speed. The first stage experiment obtained the temperatures of the tool-workpiece contact immediately after process interruption and corresponding thermo-emf. The results can help in the properties study of dynamic thermocouples used for temperature measurement in tribology and machining, including in an automatic control systems.