Motion Trajectory Measurement of NC Machine Tools Using Accelerometers
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Abstract:
NC controllers use different types of compensation systems to improve motion accuracy of feed drive systems against pitch error, friction, backlash, and elastic deformation. Compensators for static error, such as pitch and squareness errors, are tuned semiautomatically. However, for dynamic error such as quadrant glitches and vibration, parameter tuning takes too much time. In this study, motion trajectory measurement for parameter tuning using accelerometers has been proposed. In the methods, displacements of each axis can be obtained from measured accelerations along each axis. Although the obtained displacements include some errors, such as setting error, sensitivity error, and integral error in numerical integration, the errors can be compensated for based on the feedback positions measured simultaneously. To confirm the feasibility of the proposed methods, measurement tests using a grid encoder are carried out. Results of the measurements confirm that the circular trajectories and vibrations can be measured by the proposed method. Automatic parameter tuning method for the backlash compensator is also proposed.Keywords:
Machine tool
Rotary encoder
NC controllers use different types of compensation systems to improve motion accuracy of feed drive systems against pitch error, friction, backlash, and elastic deformation. Compensators for static error, such as pitch and squareness errors, are tuned semiautomatically. However, for dynamic error such as quadrant glitches and vibration, parameter tuning takes too much time. In this study, motion trajectory measurement for parameter tuning using accelerometers has been proposed. In the methods, displacements of each axis can be obtained from measured accelerations along each axis. Although the obtained displacements include some errors, such as setting error, sensitivity error, and integral error in numerical integration, the errors can be compensated for based on the feedback positions measured simultaneously. To confirm the feasibility of the proposed methods, measurement tests using a grid encoder are carried out. Results of the measurements confirm that the circular trajectories and vibrations can be measured by the proposed method. Automatic parameter tuning method for the backlash compensator is also proposed.
Machine tool
Rotary encoder
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Citations (9)
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The technological manipulation of the piezo-electric actuator could compensate for the errors of the machining precision during the process of machining which lead to an elevation and enhancement in overall precisions. This manipulation is a very convenient method to advance the precision for nations without the solid knowledge of the ultra-precision machining technology. There were 2 divisions of researches conducted to develop the UPCU for precision enhancement of the current lathe and compensation for the environmental errors as shown below; The first research was designed to measure and real-time correct any deviations in variety of areas to achieve a compensation system through more effective optical fiber laser encoder than the encoder resolution which was currently used in the existing lathe. The deviations for a real-time correction were composed of followings; the surrounding air temperature, the thermal deviations of the machining materials, the thermal deviations in spindles, and the overall thermal deviation occurred due to the machine structure. The second research was to develop the UPCU and to improve the machining precision through the ultra-precision positioning and the real-time operative error compensation. The ultimate goal was to improve the machining precision of the existing lathe through completing the 2 research tasks mentioned above.
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Cube (algebra)
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Thermal errors cause serious dimensional errors to a workpiece in precision machining. A feed drive system generates more heat through friction at contact areas, such as the Linear encoder and the guide, thereby causing thermal expansion which affects machining accuracy. Therefore, the thermal deformation of a Linear encoder is one of the most important objects to consider for high-accuracy machine tools. This paper analyzes the increase of the temperature and the thermal deformation of a Linear encoder feed drive system. During temperature variation is measured by using thermocouples , meanwhile, the thermal error of the guide is measured by a laser interferometer. A thermal error model is proposed in this study by using back propagation neural network (BPN). An experiment is carried out to verify the thermal error of the guide under different feed rates and environmental temperature.
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An instrument capable of measuring arbitrary dynamic NC tool paths in a three-dimensional space has been developed. This paper describes the estimation method for 23 error parameters concerning the link length errors, inclination of link axises and bearings. The estimated errors parameters have been applied to tool path trajectories which are calculated from rotary encoders output angles. The results show significant accuracy improvement. The fact that the measurement of a machine motion in 125 square tool path is possible with an error of 8μm error, show the validity of the proposed measurement device and that of the error parameters estimation method.
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The position errors in the axis direction of a vertical machine center have been measured by means of the VM101 linear encored measurement system. The character of the backlash error is discussed; results show that the backlash error has great influence on the position error. The position accuracy is enhanced after the backlash errors are compensated.
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Errors of the signals from Electric Encoder can cause inaccurate measurement results.Offset voltage error of SIN/COS and Mechanical offet angle error was analyzed.The compensatory algorithms were proposed to eliminate errors in measurement results.In the actual servo system tests and verifies the error compensation method's efficiency and reliability.
Error Analysis
Rotary encoder
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An instrument capable of measuring arbitrary dynamic NC tool paths in a three-dimensional space has been developed. In this paper, we describe the structure of the newly modified device and the estimation method for 11 error parameters concerning the link length errors and inclination of link axises. The estimated errors parameters have been applied to tool path trajectories which are calculated from rotary encoders output angles. The results show significant accuracy improvement. The fact that the measurement of a machine motion in rectangular tool path is possible with an accuracy of ±3 μm error and that the accuracy of circularity tests are much improved, show the validity of our proposed measurement device and that of the error parameters estimation method.
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