Synthesis Error Modelling And Compensation For Heavy Duty Machine Tools That Considerate Environmental Temperature

Internal and external heat source of machine tools lead to large thermal deformation, which is the main source that affects the machining accuracy. As for heavy duty machine tools under non-temperature control environment, the proportion of thermal error caused by environmental temperature change are higher, the thermal deformation has hysteresis effects due to environmental temperature, and the hysteresis length fluctuates with seasonal weather, and the deformation response amplitude is of nonlinear relation with temperature change. The environmental temperature problem is the main obstacle for the modelling and compensation of synthesis error including geometric error and thermal error. This paper focuses on the modelling and compensation strategy of heavy duty machine tool located on non-temperature control workshop. The main research contents in this paper include the following aspects.At first, with regard to the problem of slow delay thermal deformation arose by environmental temperature fluctuations of heavy duty machine tool located at no temperature control workshop, based on a lot of experiments and tests, consideration with Fourier series decomposition, Newton’s second law of thermodynamics, and the Principle of lumped heat capacity, an analytical form model was brought out for thermal error modelling and took the place of measured temperature, the analytical predicted model was with time, frequency and phase information, the model contained current temperature and history temperature, and could take the place of measured discrete temperature.Then the difference of machine tool thermal error caused by environmental temperature and the inner heat source had been discussed. The thermal error response caused by external heat source is always lags from the environmental temperature, there is no environmental temperature measuring point where the temperature is approximate linear with the thermal error. While for the thermal error caused by internal heat source, the optimized measuring point in machine tool body could be found where the temperature is approximate linear with the thermal error. So a spindle thermal error model that considerate the environmental temperature was built. The model was composed with two parts, one part is the thermal error model that only caused by environmental temperature, and the other parts is the thermal error that excluded environmental temperature but only considerate the heat source caused by the rotated of spindle. This modelling method was proved to be more appropriate for the heavy duty machine tools.Thirdly, the geometrical and thermal error from moving axes has more serious influence to the machining accuracy for large machine tools, which is more difficult to be measured and identified with respect to ordinary sized machine tools. The characteristic of moving axes synthesis error for heavy duty machine tools had been discussed, and a rapid measuring and identifying method for the geometrical and thermal error was brought out. In combination with laser vector method, a Doppler laser interferometer was put into use to identify geometric error includes positioning error, straightness error and perpendicularity errors under different temperature. Combining with the thermal response model of the environment temperature and the thermal error model of spindle unit, a comprehensive error model of time, space, temperature was established. And the integrated error model parameter identification and prediction effect validation was put forward.Fourthly, the way to integrate the comprehensive error model into numerical control system was discussed. Based on open fieldbus, the integrate error compensation model was embed into the HNC818-B CNC system, at the same time, the model was also integrated into Siemens840D numerical control system with PLC programming. The compensation results was verified with experiment under three large floor type boring and milling machine tools, two gantry type milling machine tools equipped with Siemens840D numerical control system and one vertical machining center equipped with HNC818-B CNC system. The experiment result showed that the synthesis error compensation is of adaptable with different working conditions. Under different environmental temperature, changing moving axes location and variable spindle rotate speed, at least60%synthesis error could be reduced. And the warm-up time was reduced from several hours to less than30minutes, the machining accuracy and the efficiency were both enhanced.At last, this paper discussed the strategy to guarantee the stability and reliability of synthesis error compensation system in application. For the potential temperature sensor system failure, a statistical hypothesis test method was put forward with information entropy, with which method the information correlation coefficient of sensor data was obtained at first, then the information correlation coefficient between the sample and population was compared on real-time to judge whether there is a failure with temperature sensor. Several least squares support vector machines (LS-SVM) was built among the variable of population, once failure occurred in certain tunnel, the calculated’normal value’could take the place of the failure tunnel data, by which ways the failure tunnel could still work until the continuous machining process completed, and the machining failure could be avoid.