Research On Keytechniques Of Form-Free Measurement In Precision Engineering

Precision measurement technology is the basis of engineering science and the key ofmodern manufacturing technology. The form tolerance measurement technology is animportant research topic of precision measurement technology. Even it has been developingand evoluting, the form tolerance measurement technology follows the basic measurementmode, based on which no matter general or special targeted precision measuring instrumentshave to get the nominal geometric model and theoretical parameters before the measurementprocess. The instruments are set according to the nominal information, then geometricinformation is obtained, and data processing and error assessment are carried out accordingly.In scientific research and industrial practice, there are some key components withoutgiven nominal geometrical models and parameters before the measurement. It is a realproblem for precision engineering that how to accomplish the measurement process and errorassessment. For the traditional precision measurement mode, it is difficult to complete themeasurement process because there is not known nominal information. Through surveyingand mapping technologies some data are obtained with a low accuracy which cannot meet therequirements of the precision engineering. With reverse engineering, the digital modelreconstructed generally does not have clear structural features so that it is difficult tounderstand its mathematical meaning, and the reverse engineering does not process the errorevaluation.Based on the "small deviation hypothesis" for precision parts, the Form-freemeasurement mode is proposed, where the measuring instrument can process themeasurement for dimensions and form tolerances without knowing the geometrical form,parameters, and precision positioning. To realize the Form-free measurement, it is the base toobtain reliable geometric data. Key techniques to realize the Form-free measurement modeare researched. The main contents are as follows:(1) The "Form-free measurement mode" for precision engineering is described and itskey techniqes are analyzed. Several aspects of precision measurement, such as measurementand measurement process, measurand, nominal geometric model of measurand, units ofmeasurement, measurement methods and measurement of complex shape, and measurementuncertainty are analyzed systematically. The features of traditional precision measurementmode and reverse engineering are analyzed, and it is concluded that they can not measure the complex measurand without given nominal geometric model. The basic idea of Form-freemeasurement mode and its basic process are analyzed. The key techniques to obtaingeometric data to realize the Form-free measurement mode is analyzed. The methods of formrecognization and fast operators for Form-free measurement mode are introduced. And theprospects and advantages of the Form-free measurement mode are described.(2) The Abbe principle is revisited and a generalized Abbe principle is presented. Theclassical Abbe principle and its development are reviewed and it is pointed out that the focusof the Abbe principle is the first order errors caused by the angle motion of the carriage underdifferent structure configuration. The errors due to the non-rigid base and non-rigid carriageare analyzed, and the error orders caused by the different elements configurations areanalyzed. It is found that first order errors exist even the classical Abbe principle is compliedwith. Furthermore implied conditions of the Abbe principle are discovered, and a generalizedAbbe principle with "collinear five elements" is presented. The two-dimensional andthree-dimensional Abbe principle are analyzed, the structures complied with thetwo-dimensional and three-dimensional Abbe principles are discussed, and it's concluded thatit is not more complicated to satisfy to the three-dimensional Abbe principle than to satisfy tothe two-dimensional Abbe principle. And the way to decrease the Abbe errors during theprocess of design and fabricating of the complex measuring machine is analyzed.(3) A measuring machine featuring high precision, micro stepping, fast speed and largestroke is developed to realize the Form-free measurement mode. According to the basicrequirements of the Form-free measurement mode, the overall design principles aredetermined, the measuring machine structure is configured, and critical parts are designed andanalyzed. A closed aerostatic guideway with vacuum pre-load is invented, an H typetwo-dimensional coplanar stage is invented, an axis Z with pneumatic balance is designed,and a vibration isolation assembly is fabricated.(4) The motion control system of measuring machine, the control system of pneumaticbalance for axis Z, the control system of aerostatic vibration isolation system, and the probingsystem are built. A probing system is built, and the framework of the measurement software isprogrammed. Based on an open and programmable controller, the servo control system,including motors, drives, controllers and displacement measurement systems, are calculatedand chosen. The synchronous motion control of dual-motor drive for axis Y is realized. Thebalance to the gravity of axis Z and braking function when the power is off are accomplished.As a sample, a tactile probe is utilized to set up the connection to the control system, the method of capturing the trigger signal is determined, and the related parameters of thecontroller are set up. The framework of the measurement software is programmed with VisualC#.(5) An adaptive sampling method for the complex measurands without given nominalmodel and parameters is presented. Special points that reflect the trend of the measurand areobtained. The third order spline interpolation technique is adopted for the interpolation pointsof special points from the measurand, and the equal curvature expolation technique is utilizedto obtain the extrapolation points of existing sample points. The interpolation andextrapolation points are compared to determine the next probing start point. Numericalexperiments are carried out with MATLAB for an open simple curve and a closed complexcurve to verify the stability, accuracy, efficiency and reliability of the sampling method.(6) Experiments are carried out. The motion control system is commissioned to achievethe predefined motion. A comparative experiment on the novel and traditional aerostaticguideways is carried out. The straightness errors and positioning errors of every axis and thesquareness errors between them are measured with a laser interferometer. With the controlsystem a micro stepping of0.5μm is achieved. The vibration isolation system is adjusted andits performance is tested.