| At present,the rapid development of manufacturing industry benefits from the support of precision machining manufacturing technology.Precision displacement measurement technology and devices,as key components to control machining quality and feedback position information,have been widely used and attached more and more importance.Especially for high-end CNC machine tools,which are the basis of manufacturing equipment,absolute position sensor,as the key component of the fully closed-loop control system,plays a decisive role in the machining accuracy of the machine tools under the condition of linear motor as servo control drive.Grating is the best choice at present to meet the requirement of not only high precision but also absolute position sensor with large range for machine tools.However,due to the technology blockade imposed by foreign grating manufacturers on our country and relatively backward domestic manufacturing technology,we are forced to rely on imports for a long time.Facing this situation,in order to promote the progress of China's manufacturing industry,independent innovation and development of other absolute position sensors will become an inevitable trend.The author's laboratory has been devoted to the research of time-grating measuring technology and devices for a long time.With the theory of space-time conversion,time-grating displacement sensor emerged.Previous studies have confirmed that time-grating do not require high manufacturing accuracy and can achieve large range measurements.Considering that the absolute displacement time grating of traditional structure can't play a full role in machine tools with long measuring travel due to the restriction of cable,a time-grating displacement sensor which can achieve large range measurement while maintaining high accuracy positioning is proposed in this paper.It only has cable connections on fixed parts to enhance its applicability in machine tools.The following work has been done around the design of the sensor;1)Firstly,the principle of multi-electric field coupling based on time-grating is proposed.Four orthogonal signals from the first-stage sensor with M measuring periods are taken as the excitation signals of the second-stage sensor with N measuring periods to form a sensor A with M+N measuring periods.The combination of the two sensors increases the number of periods and realizes the reflective structure design of the single-sided outgoing line and the increase of resolution;2)Designed an absolute displacement sensor structure based on multi-field coupling.Once again applying the principle of multi-field coupling,the four orthogonal signals from the first-stage sensor are combined as excitation signals of the third-stage sensor with N-1measuring period to form a sensor B with M+N-1 measuring period.Sensors A and B form a differential structure,which realizes absolute positioning in a manner similar to vernier caliper measurement;3)The mathematical model of measurement error of the first-stage sensor before multi-field coupling and the sensor B after multi-field coupling is established.The measurement errors of sensors before and after coupling due to different signal amplitudes,non-orthogonal phase and nonlinear electric field are also analyzed.When the signal amplitude is unequal and the phase is not orthogonal,the error of the first-stage sensor changes from the primary error and the secondary error to the multiple harmonic error related to the period of the combined sensor under the multi-electric field coupling.Under the influence of nonlinear electric field,the error of the first-stage sensor is the fourth error,and it is still the fourth error after the multi-electric field coupling;4)In order to verify the periodic error characteristics of the sensor,a sensor prototype with parameters M=136 and N=5 is made by PCB process,the circuit for signal acquisition and processing is designed,the excitation signal generator and other equipment are selected,and a convenient and reliable experimental platform for displacement measurement is established;5)Test and structure optimization.The measurement error of the first-stage sensor and its correlation with the error of sensor B after multi-electric field coupling have been verified experimentally.The experimental analysis shows that the measurement error of sensor B coupled by multi-electric field is the sum of the errors of each combination stage sensor in the same measuring range,and mainly represents fourth error.After the error mechanism of multi-electric field coupling is mastered,the structure of the sensor is optimized by increasing the number of periods of the second-stage sensor and the third-stage sensor.The experimental results of the sensor prototype optimized to N=M show that within 200 mm measuring range,the positioning accuracy reaches ± 200 nm and the resolution is raised to half of the original one.In summary,based on the principle of multi electric field coupling,this paper proposes a kind of high-precision absolute time grating displacement sensor whose precision and resolution are amplified compared with the same size time grating.The time-grating structure based on the coupling of multiple electric fields is reflective,whichrealizes the single side outgoing line.Compared with the traditional opposite type structure sensor,it greatly expands its practicability and reliability under complex working conditions.This major research result not only shows the development potential of time-grating,but also lays a solid foundation for the subsequent industrialization of time-grating displacement sensor,and makes a significant contribution to the development of precision displacement measurement technology in China and even the whole manufacturing industry. |
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