The machining accuracy is an important index to evaluate the machine tool characteristics, and error measurement and compensation are effective methods to improve the machining accuracy with low cost. Error measurement is a prerequisite and basis for error compensation, so how to quickly and accurately measure the machine error is particularly important. To quickly and accurately detect the geometric error of multi-axis NC machine tool, a new method with laser tracker based on the sequential multi-lateration measurement principle is proposed in the paper. The laser tracker is used to measure the same motion trajectory of the linear axis and rotary axis of machine tool at different base stations. Based on the GPS principle, the space coordinates of each measuring point can be determined by large amount of measured data. Then according to the error model of the linear axis and rotary axis, each error can be separated. In the paper, the principles of sequential multi-lateration measurement are analyzed in depth. By establishing the mathematical model of sequential multi-lateration measurement for the linear axis and rotary axis, the algorithms for linear axis measurement and rotary axis measurement are derived and proved feasible by simulations, respectively. Meanwhile, the error separation algorithm is also deduced. The results of experiments show that this method can achieve quick and accurate detection of errors in multi-axis NC machine tool.

No abstract

The paper investigates the determinants of geographical distribution of international currencies in global financial market transactions. We implement a gravity model, in which international currency distribution depends on the characteristics of the source and destination countries. We find that the source country's currency is more likely to be used in the financial market transactions of the destination country if the bilateral trade and capital flows are large or the destination country's economy is the larger of the two. We also find that the level of development of the destination country's financial market and whether the two countries use a common language are important determinants of the currency distribution. In addition, our model suggests that, to be a true international currency, the renminbi should be used more extensively in the financial markets of the US and UK.

Error compensation technology as an economical and effective method for improving the machining accuracy of the machine tools has been widely applied in the manufacturing. How to quickly and accurately obtain machine tool error is an important prerequisite for achieving the above task. A laser tracker is adopted the multi-station and time-sharing measurement principle to achieve quick and accurate identification of the geometric errors of the machine tool. In this article, the error of single-station measurement of laser tracker is analyzed first, and then the principle and algorithm of multi-station and time-sharing measurement are given. Based on the kinematic model of the machine tool, the mapping relation equations between the space motion error and individual geometric error of the machine tool are established, and two different identification algorithms are derived, respectively. Meanwhile, the identification accuracy of two algorithms and the impact of different measurement areas and squareness errors on two algorithms are compared and analyzed in depth. The experiment results indicate that higher identification accuracy can be obtained by the second algorithm, and individual geometric error of the machine tool can be quickly and accurately separated by this algorithm.

No abstract