Thermal Error Components Research Articles

Time-varying positioning error modeling and compensation for ball screw systems based on simulation and experimental analysis

Thermal expansion of ball screw systems affects the machining accuracy of machine tools significantly. This paper intends to provide a comprehensive error compensation method for the time-varying positioning error of machine tools. To confirm the thermal deformation mechanism of ball screw systems, experiments have been designed to study the thermal behaviors of a ball screw system under varying temperature conditions. An exponential algorithm is proposed to predict the temperature variation pattern of the ball screw based on finite element analysis, and the actual thermal boundary conditions of the ball screw system are exactly defined according to the proposed algorithm and the experimental results. Then, a comprehensive compensation model is established based on the decomposition of the initial geometric error and thermal error components. Finally, a real-time error compensation system is developed for machine tools based on the function of external machine original coordinate shift and fast Ethernet data interaction, and satisfactory results have been achieved for the compensation experiments on a machining center.

A Comprehensive Approach for Thermal Error Model Optimization for ANN-Based Real-Time Error Compensation in CNC Machine Tools

Thermally induced errors play a critical role in the control of machining accuracy. They can account for as much as 70% of dimensional errors in produced parts. Since thermal errors cannot totally be eliminated at the design phase, errors compensation appears to be the most economical solution. Accurate and efficient modeling of the thermally induced errors is an essential part of the error compensation process. This paper presents a comprehensive approach for thermal error modeling optimization. The proposed optimization method is based on multiple temperature measurements, Taguchi’s orthogonal arrays, various statistical tools and artificial neural networks to provide cost effective selection of appropriate temperature variables and modeling conditions as well as to achieve robust and accurate thermal error models. The proposed approach can be effectively and advantageously used for real-time thermal error compensation since it presents the benefit of straightforward application, reduced modeling time and uncertainty. The experimental results on a CNC turning center confirm the feasibility and efficiency of the proposed optimization method and show that the resultant model can accurately predict the time-variant thermal error components under various operating conditions.

Precision Enhancement of <i>Ucp710</i> 5-Axis Machine Tool by Real Time Thermal Error Compensation

The thermal error model of the ucp710 5-axis machine tool was acquired by the homogeneous coordinate transformation, and includes 17 thermal error components. The thermal behavior of the ucp710 machine tool has been tested and the temperature fields were recorded by the compensation control and temperature sensing systems. The developed compensation system has been applied to the ucp710 5-axis machine tool, and the accuracy has been improved about 1 times after compensation, demonstrating a very high potential for the error compensation of CNC machine tools.

Real Time Thermal Error Modeling and Compensation of 5-Axis NC Grinding Machine Tool

The thermal error model of the 5-axis grinding machine tool was acquired by the homogeneous coordinate transformation, including 17 thermal error components. The thermal volumetric error real time compensation model was built by using the multiple regression analysis. The thermal error compensation control system and the temperature sensing system were developed and used as real-time compensation for the 5-axis grinding machine tool.

Thermal behavior of a machine tool equipped with linear motors

Development of a feed drive system with high speed and accuracy has been a major issue in the machine tool industry. Linear motors can be used as efficient tool to achieve the high speed and accuracy. However, a high speed feed drive system with linear motors, in turn, can generate heat problems. Also, frictional heat is produced at the ball or roller bearing of LM block when driven at high speed. It can affect the thermal deformation of the linear scale as well as that of the machine tool structure. In this paper, important heat sources and resulting thermal errors in a machine tool equipped with linear motors were investigated when it was operated at high speed. The thermal deformation characteristics were identified through measuring the thermal error caused from thermal deformation of the linear scale and the machine tool structure. The dominant thermal error components were identified from the thermal error analysis using finite element method. It was shown that the proposed analysis scheme is efficient in identifying the dominant thermal error components and its magnitudes such as the thermal expansion and movement of the linear scale, thermal deformation of the machine tool slide.

An application of real-time error compensation on an NC twin-spindle lathe

A unique form of the error synthesis model, which combines both the geometric and thermal errors of the NC twin-spindle lathe, is given by the homogeneous coordinate transformation method. Estimating the variation of the slopes of the error profiles enables the separation of the geometric and thermal errors. A set of strategies is presented for modeling different types of thermal error components according to their properties. The temperature field of the machine is monitored on-line by 16 thermal sensors. Based on measured temperatures and the coordinates of the slides, pre-established empirical models estimate the thermal error components in real-time. The kinematic model then synthesizes the estimated thermal and geometric error components to determine the planar error under the current machine conditions. Based on the determined planar error, the computer sends out the compensation signals to the machine’s controller. The controller cyclically picks up the compensation signals approximately every 10 ms and corrects the planar error by shifting the reference origin of the slides. In the compensation cutting test, the effectiveness of this method was verified, the size variations of the workpieces could be reduced from 60 to 14 μm, and the taper could be reduced from 50 to 15 μm/cm.

Thermal error measurement and modelling techniques for the five-degree-of-freedom spindle drifts in computer numerically controlled machine tools

Thermally induced errors have been significant factors affecting machine tool accuracy. In this paper, the spindle thermal error is examined, where thermal error components with five degrees of freedom (DOFs) are considered. An effective measurement system has been devised for five-DOF thermal errors, consisting of gap sensors and thermocouples around the microcomputer-interfaced environment. Several thermal error modelling techniques are also implemented for the thermal error prediction: multiple linear regression, neural network, system identification methods, etc. The performance of the thermal error modelling techniques is evaluated and compared, giving the system identification method as the optimum model having the least deviation. The developed system for the thermal error measurement and modelling has been practically applied to a computer numerically controlled machining centre, and the spindle thermal errors are effectively compensated using the microcomputer-machine tool interfaced networks. The machine tool accuracy has been improved about four to five times typically.

Thermal Error Measurement and Modeling Techniques for the 5 Degree of Freedom(DOF) Spindle Unit Drifts in CNC Machine Tools

Thermally induced errors have been significant factors affecting the machine tool accuracy. In this paper, the spindle thermal error has been focused, where the 5 degree of freedom thermal error components are considered. An effective measurement system has been devised for the 5 DOF thermal errors, consisting of gap sensors and thermocouples around the micro-computer interfaced environment. Several thermal error modeling techniques are also implemented for the thermal error prediction: multiple linear regression, neural network and system identification methods, etc. The performance of the thermal error modeling techniques is evaluated and compared, giving the system identification method as the optimum model having the least deviation. The developed system for the thermal error measurement and modeling was practically applied to a CNC machining center, and the spindle thermal errors were effectively compensated around the micro computer-machine tool interfaced networks. The machine tool accuracy was improved about 4-5 times typically.

A comprehensive error compensation system for correcting geometric, thermal, and cutting force-induced errors

A comprehensive error compensation system has been developed to correct geometric, thermal, and cutting force-induced errors on a turning centre. The basic approach to error compensation is proposed in this paper. The implementation of error compensation control and of hardware configuration of the system are also presented. A total of 11 geometric and thermal error components and 10 cutting force-induced error components can be compensated for using this system. Performance evaluations have been carried out using actual cutting tests. Experimental results show that the diameter accuracy of the part has been improved more than 5 times and taper accuracy of the part has been improved about 5 times.

Thermal error modelling for real-time error compensation

A modelling strategy for the prediction of both the scalar and the position-dependent thermal error components is presented. Two types of empirical modelling method based on the multiple regression analysis (MRA) and the artificial neural network (ANN) have been proposed for the real-time prediction of thermal errors with multiple temperature measurements. Both approaches have a systematic and computerised algorithm to search automatically for the nonlinear and interaction terms between different temperature variables. The experimental results on a machining centre show that both the MRA and the ANN can accurately predict the time-variant thermal error components under different spindle speeds and temperature fields. The accuracy of a horizontal machining centre can be improved through experiment by a factor of ten and the errors of a cut aluminium workpiece owing to thermal distortion have been reduced from 92.4 µm to 7.2 µm in the lateral direction. The depth difference due to the spindle thermal growth has been reduced from 196 µm to 8 µm.

Real-time Compensation for Time-variant Volumetric Errors on a Machining Center

An error compensation system has been developed to enhance the time-variant volumetric accuracy of a 3-axis machining center by correcting the existing machine errors through sensing, metrology, and computer control techniques. A general methodology has been developed to synthesize both the geometric and thermal errors of machines into a time-variant volumetric error model. Instead of the well-known 21 geometric error components, 32 machine linkage errors are formulated as a 4D error field including the space domain and the time domain. Different types of models are proposed for different kinds of thermal error components. A compensation controller based on an IBM/PC has been linked with a CNC controller to compensate for machine errors in real time. This scheme has been implemented on a horizontal machining center and has been shown, using metrology instruments, to improve the machine accuracy by an order of magnitude. A cut workpiece inspected using a coordinate measuring machine (CMM) has also shown that dimension errors have been reduced from 92.4 μm to 18.9 μm in a dimension of 404 × 310 mm2 and the depth difference of milled surfaces has been reduced from 196 μm to 8 μm.

An Investigation Into The Relative Accuracy OfBall-screws And Linear Encoders Over A BroadRange Of Application Configurations And UsageConditions

It has been found that ball-screw expansion is the single largest thermal error source within a broad range of CNC machine tool configurations and over a wide range of running conditions when rotary encoder position measurement is used. This paper describes thermally related axis positioning errors exhibited by ballscrew with rotary encoder and linear encoder feedback systems when used within CNC machine tools. Indicative data resulting from many thermal tests has been used to support the analyses. The paper details the thermal error components that make up the total thermal error in both types of positioning system. It is known that the position of the measuring system on an axis can affect the accuracy achieved at the component through the Abbe error. It is accepted that machining often occurs in uncontrolled environments and that thermal errors from external sources can combine to cause additional thermal errors on the work-piece. The study proposes that an axis using a ball-screw and rotary encoder for position measurement exhibits offset, scale and reversal errors that change quickly according to the thermal state of the ball-screw. Linear encoder measurement systems exhibit the same error categories, but generally with smaller maximum values, and slower rates of change. The thermal errors exhibited by ball-screws are primarily due to an inability to decouple the measuring system from the heat in the ball-screw and mounting system that sets the position of the tool relative to the work-piece. Transactions on Engineering Sciences vol 23, © 1999 WIT Press, www.witpress.com, ISSN 1743-3533