Research On CNC Machine Tool Error Measurement, Modeling And Real Time Compensation Based On Ethernet Distributed Control System

Error compensation technology is one of the most crucial methods to improve themachining accuracy of computer numerical control (CNC) machine tools, with which threeresearch directions related are: error measurement and identification, error modelingmethodology, and error compensation implementation method. This thesis is sponsored byChinese National Science and Technology Key Special Project, the National ScienceFoundation Project, and the Specialized Research Fund for the Doctoral Program of HigherEducation. Some experiments were carried out on a five-axis machining center and twothree-axis vertical machining centers, in order to conduct the research on efficient errormeasurement, accurate error modeling, and real-time error compensation. In addition, areal-time error compensation system based on Ethernet distributed control method isdeveloped to compensate for multiple machine tools simultaneously in order to improvethe machine performance under different temperature conditions.The main contents of this thesis are described as follows:(1) According to the structural kinematic features of different kinds of five-axis machinetools, a universal kinematic model is established based on the homogenous transformationmatrix method so as to describe the kinematic chain transfer from the tool coordinatesystem to the workpiece coordinate system. Furthermore, a universal volumetric errormodel can be obtained based on the analysis of error components possibly induced by themain machine links and the using of rigid motion theory. Then the volumetric errorcompensation values can be calculated within the machine working volume. In order tomeet the requirement of real-time compensation, a data processing strategy, in whichdifferent error components are classified and modeled with different methodologiesaccording their regularity of distribution, is proposed to simplify the volumetric errormodel aiming at increasing the calculation speed.(2) An error measurement and identification method based on the double ball-bar (DBB) instrument is proposed according to the moving characteristic and error distribution of therotary axis. Through the MATLAB simulation experiment, different error’s influence onthe DBB measuring patterns can be obtained. A real DBB measuring experiment wasconducted on a five-axis machine tool, the results of which showed that the proposedmeasuring method is able to measure five error components simultaneously with highefficiency and accuracy. Moreover, this measuring method could be carried out underdifferent working conditions to obtain the error variations with temperature. A thermalerror modeling method based on the natural exponential model is developed to predict andcompensate for the thermal errors of the rotary axis.(3) Either on three-axis or five-axis machine tools, translational axes are the mostimportant links, of which the error components are the crucial sources of machineinaccuracy. The laser interferometer can be employed to measure the positioning error andthe straightness error of translational axes. The measurement results show that thepositioning error is obviously affected by the temperature field, especially by the variationsof the ambient temperature and screw nut temperature. Based on the analysis of positioningerrors under different temperature conditions, a natural exponential model is established todescribe the non-linear relationship between the thermal term in the positioning error andthe temperature variables. Compared with a multiple regression analysis model, theproposed modeling method performs better in terms of prediction accuracy and robustness.As to the straightness error, it has relatively small value compared with the positioningerror. In addition, it is almost independent from the temperature variations. Therefore, itcan be regarded as the pure geometric error and modeled by a polynomial with the positioncoordinate.(4) Much heat is produced during the high-speed rotation of the spindle, so the thermalerror caused by the thermal deformation is one of the major sources of machine inaccuracy.This thesis combines the advantages of both grey model (GM) and artificial neural networkin terms of data processing to propose a novel error prediction model, namely grey neuralnetwork (GNN). According to the structure of the prediction model, three types of GNNare introduced and analyzed in this thesis: serial grey neural network (SGNN), parallelgrey neural network (PGNN), and inlaid grey neural network (IGNN). In SGNN, somegrey models with different length of data sequence firstly play an important role inpreprocessing the original thermal errors and temperature data, establishing a first-orderdifferential equation. Then, the neural network receives the predicted thermal errors fromdifferent grey models to make the nonlinear optimization by adjusting the weight and threshold of the network neurons based on the back propagation (BP) training algorithm.After these two steps, a complete error model is established to predict the final thermalerror of the machine tool spindle. In PGNN, one GM and one BP network are utilized topredict the thermal error, respectively. Then an effective combination algorithm combinesthe results of GM with BP to output the final data as the thermal error compensation value.In IGNN, the topological structure of the BP network is optimized by adding a grey layerbefore the input layer and a white layer after the output layer, in order to reduce therandomness of the original data and enhance the robustness and the fault-tolerant ability.Compared with the traditional GM and BP network, these three types of GNN prove betterin terms of prediction accuracy, calculation convenience and robustness. What’s more, theyrequire less to the original data. Thus, the new proposed models are properly applied todifferent working conditions to compensate for the spindle thermal error of machine tools.(5) A real-time error compensation system based on Ethernet distributed control methodis developed to compensate for multiple machine tools simultaneously, in which theexternal machine zero point shift (EMZPS) function is utilized as the implementationinterface between the compensation system and the CNC machine tools. From the overallarchitecture, the compensation system is composed of two main parts: the PC controlcenter and the programmable machine controller (PMC). The former one has fastcomputation speed and large data storage, so it is responsible for calculating the volumetricerror compensation values of different machine tools. The latter one is directly connectedwith CNC system, so it is responsible for the real-time data processing parts, i.e. inquiringthe machine position coordinate and looking up the corresponding compensation value inthe data table stored in PMC. The proposed Ethernet distributed error compensation systemcan make full use of the data computation characteristic of both PC and PMC. It is not onlycapable for the complex mathematical model calculation, but also meets the requirement ofreal-time error compensation. In addition, the PC control center and the machine tool PMCis connected through the Ethernet cable under the Fanuc Open CNC API Specifications(FOCAS) protocol, which is featured by high data transmission speed, strong reliability,easy hardware connection, and convenient module expansion.(6) Two kinds of error compensation experiment were conducted by utilizing the errormeasuring method, modeling methodology, and error compensation implementationtechnique proposed in this thesis. The first kind of experiment was conducted on afive-axis machining center with a rotary table. Firstly, a laser interferometer and a DBBinstrument were employed to conduct the error measurement on translational and rotary axes. Then, error component models were established under different temperatureconditions. The error compensation value applied to each feed axis could be obtainedthrough the volumetric error model, which includes the main error components of themachine. At last, the error compensation was implemented through the developedcompensation system. Experimental results showed that the volumetric accuracy wasimproved significantly after error compensation. The second kind of experiment wasconducted on multiple CNC machine tools. Error models for each machine tool should beobtained firstly, and then different machine tools could be compensated through theEthernet distributed control system. Experimental results showed that the proposed errorcompensation system could be utilized to improve the accuracy of multiple machine tools.It has much higher compensation efficiency compared with the traditional errorcompensation methods. Both the above experiments proved that the proposed errormeasuring method, error-modeling methodology, and the Ethernet distributed errorcompensation system were of significant use to improve the machine tool performanceunder different temperature conditions. This compensation system can work stability;therefore, it has high practical value and significance for popularization.