Thermal Deformation Error Modeling And Correcting Research For Articulated Arm Coordinate Measuring Machines

With the rapid development of manufacturing science and technology, the modern manufacturing industry is developing towards the directions of high- effective, high-precision, high-qualitative and high-intelligent, which brings higher and higher demands on precision measurement and process. The thermal deformation of precision instruments, precision machine tools and other devices is the key restraint for further improving the accuracy of precision measurement and process. Coordinate measuring machines have developed for about fifty years, and articulated arm coordinate measuring machines developed later. However, because of its advantages of high precision, flexible use, low demand for operational environment and portable, articulated arm coordinate measuring machines are being widely used and promoted, also highly demanded in China. These machines can work with temperature ranging from 10℃to 40℃. The measurement error caused by thermal deformation is the major one among all their error sources, so only correcting the thermal deformation error can realize the high precision measurement. Based on the measurement models of articulated arm coordinate measuring machines, multi-temperature error sources and error properties for the measuring machine have been studied first time, and the thermal deformation error correction model based on BP neural network and neural network integration have been built. The experiments show that the work can effectively reduce thermal deformation error of the measuring machine.Based on the measurement models and error models of articulated arm coordinate measuring machines, this paper deeply researched their thermal character, analyzed the influence of measurement errors resulted from the thermal deformation of joint components, arms and circular grating sensors on articulated arm coordinate measuring machine under the action of internal and external heat sources. Especially, for the mutual position change between moving and fixed scales of gratings resulted from joint component deformation, Fourier spectrum analysis method was applied to build the optical field output model involving fixed and moving gratings pose parameters, to introduce the mathematic formulas for output optical field with fixed and moving gratings unparallel mutually. All of which are simulated in Matlab. This paper also gave Moire fringe error values and their variation trend for different offset angle. The fuzzy group analysis method has been adopted to select the temperature measurement points for articulated arm coordinate measuring machines thermal deformation model building and compensation. The two points in which as the best temperature measuring points to join in the modeling have defined.Using neural network mathematical modeling of articulated arm coordinate measuring machines thermal deformation error was studied and analyzed. A BP neural network model with a single hidden layer was built, which inputs were the 3-D measurement data of measuring heads and temperature data at the two temperature measuring points on machine, and outputs were the changes of the space coordinate points compared to that at 20 degree. Considering the influence of measurement errors and temperature errors coming from machines'pose, the paper proposed the thermal deformation error correction model for decision-level data fusion on a basis of neural network integration. This model has two sub-networks, one based on space coordinate points is thermal deformation error neural network model using space point coordinates measured as input characteristic variables under single machine pose, and the other one based on the poses of measuring machines is to use the six angle values of selected limited space measuring points in different measuring poses as input characteristic variables. Fusing the two network data can improve the generalization ability of models.A data collection system for articulated arm coordinate measuring machines was developed successfully, which includes the circuits for data collection of six circle grating sensors and temperature sensors on two temperature measuring points and the circuit for communicating with upper monitor. Software compilation for lower computer was performed. The data collected by the system was used for modeling as the data sample.We also set up the temperature experiment system for articulated arm coordinate measuring machine. The experiment of temperature field distribution of the articulated arm coordinate measuring machine was done. Measuring a standard bar using the measuring machine, its length changed with ambient temperature and internal thermal, that shows the measuring machine thermal deformation error shall be corrected. The data samples needed in models were collected for a long time, and using these samples to exercise and simulate built models. The experimental results certified the effectiveness of correcting the articulated arm coordinate measuring machines'thermal deformation errors by these models set up, especially the fusion models in which the original errors can be reduced by half.