Zhijing Zhang

Micro-assembly precise coaxial alignment methodology based on surface roughness and reflectiveness matching

Purpose - The purpose of this paper is to propose a novel method of coaxial optical precision alignment based on surface roughness and reflectiveness matching. Design/methodology/approach - The micro-assembly experiment system set-up was constructed according to the principle of the coaxial optical alignment. The coaxial optical alignment error is theoretically analyzed and calculated. When the prism orthogonal alignment mechanism produces the error of 0.001°, the theoretical deviation was less than 0.87µm and the actual experimental micro-assembly platform assembly accuracy exceeded 3µm. A peg-in-hole precise assembly of punching pin micro-assembly experiment was done in order to validate feasibility of this method. Findings - The results indicate that coaxial optical precision alignment could be used for the assembly of complex micro-heterogeneous system which is integrated by similar devices, such as 3D complex micro-structures, silicon micro-electro-mechanical system MEMS devices and non-silicon MEMS devices with flat structure. Originality/value - The paper provides certain methodological guidelines for MEMS for high precision automatic assembly of complex 3D micro-structures.

An innovative method of modeling plane geometric form errors for precision assembly

For precision mechanical systems, different distribution characteristics of geometric form errors usually lead to different assembly contact states, and in turn, different assembly errors are formed, due to the propagation and accumulation of geometric form errors. Statistically, a batch of part machined by the same precision process possesses the identical distribution characteristics of geometric form errors. It is of great importance to improve assembly accuracy and productivity if the statistic distribution characteristics of geometric form errors can be understood. To solve the problem, two methods of modeling geometric form errors for single parts based on linear combination of basis shapes and modeling statistic geometric form errors for a batch of parts machined by the same process on the basis of principal component analysis are proposed in this article. Besides that, evaluating indicators of model accuracy are also proposed. The results of the case study imply that the two methods can model geometric form errors of single parts and statistically model geometric form errors of a batch of parts, effectively and reliably.

Error sensitivity analysis of a microassembly system with coaxial alignment function

– The purpose of this paper is to analyze the sensitivity of the motion error parameters in microassembly process, thereby improving the assembly accuracy. The motion errors of the precision motion stages directly affect the final assembly quality after the machine visual alignment. , – This paper presents the error parameters of the in-house microassembly system with coaxial alignment function, builds the error transfer model by the multi-body system theory, analyzes the error sensitivity on the sensitive direction using the Sobol method, which was based on variance, and then gets the ones which made a great degree of influence. Before the sensitivity analyzing, parts of the error sources have been measured to obtain their distribution ranges. , – The results of the sensitivity analysis by the Sobol method, which was based on variance, are coincident with the theoretical analysis. Besides, the results provide a reference for the error compensation in control process, for the selection of the precision motion stages and for the installation index of the motion stages of the assembly system with coaxial alignment. , – This kind of error sensitivity analysis method is of great significance for improving the assembly accuracy after visual system positioning, and increasing efficiency from the initial motion stage selection to final error compensation for designers. It is suitable for general precision motion systems be of multi-degree of freedom, for the method of modeling, measuring and analyzing used in this paper are all universal and applicative.

State space model based theory of assembly variation propagation modeling

Based on state space, a variation propagation model of multi-station assembly of precision mechanical system was developed. Machining errors generally have great effects on the performance of mechanical system, especially precision mechanical system. In a multi-station assembly process, machining errors at each station accumulate and propagate to other parts, which will eventually degrade assembly accuracy and performance of the system. The differential motion vector was adopted to represent machining errors by the state vector in the model. Considering form errors, an innovative concept of mating coordinate system was proposed. This mating coordinate system, defined by two mating surfaces, can reflect real mating conditions more accurately. Detailed derivation for variation propagation modeling was presented. Finally, state equation and observation equation that can be used to guide measurement were obtained. In conclusion, the model we established can guide fault diagnosis and process design evaluation of precision mechanical system.

Modeling Method for Assembly Variation Propagation Taking Account of Form Error

The propagation of variations, such as fixture errors and datum errors resulting from assembly and machining processes, has been extensively studied. However, only a few studies that focus on form error propagation in assembly systems have been implemented. Machining errors, especially form errors, have great impact on assembly accuracy and accuracy stability of precision mechanical systems. With form errors being the research object, a method for calculating mating variation and specifying mating coordinate is proposed to improve the accuracy of the variation propagation model. Taking into account the form error of mating surfaces, the assembly variation propagation of a precision mechanical system is analyzed, and the brief derivation procedure of the variation propagation model is introduced afterwards. The variation propagation model involves a new concept of mating variation specified by the two mating surfaces. An innovative method, the difference surface search based method, is proposed to calculate the mating variation amongst the mating surfaces. The obtained mating variation is then utilized to specify the mating coordinate in the variation propagation model. Moreover, FEM is employed to simulate the contact state of the two mating surfaces to demonstrate effectiveness of the proposed method. Meanwhile, the mating variation and mating coordinate obtained are incorporated into the assembly variation propagation model, which is then verified by a following case study through a comparison between the calculated results and the experimental results. The comparing results indicate that the established model improves the prediction of assembly accuracy. The developed model enables the investigation of various fundamental issues in variation reduction, including variation analysis, process monitoring, accuracy prediction, and accuracy control.

Design and error analysis of reconfigurable turn-milling machine tool

Reconfigurable turn-milling machine tool is the cooperative product of reconfigurable machine tool technology and turn-miling process. With the quick reconfiguration of different modules, the machine tool can adjust to produce a part family in a lower cost and higher efficience. The method and process of designing turn-milling RMT is discussed in this paper. Using commercial off the shelf (COTS) method, some possible configurations of machine tool were developed. Some defenition about RMT workspace were given to guide the reconfiguration of the machine tools. Also, this research has applied screw theory to perform error analysis and error compensation of turn-milling reconfigurable machine tools.

An entropy-based method to evaluate plane form error for precision assembly

The minimum zone method is currently a traditional means for evaluating form error. However, it cannot reveal the effects of form error on the scale of assembly accuracy. To address this deficiency, however, considering the propagation of geometric error in assembly process, this article proposed an entropy-based evaluation method for plane form error for precision mechanical system, which includes primary evaluation, convex hull search, top plane establishment, local entropy analysis and obtaining comprehensive evaluation index. Experimental results demonstrate that the proposed method can evaluate the plane form distribution error, which suggests the assemblability of parts, with the same form errors accurately and stably.

Distance measurement for rotation center to measurement table using triangulation method

Large aspheric mirrors with diameter over 200 mm are widely used in space communication system, large-scale astronomical telescope and X-ray facilities. However the measurement of these mirrors with large slope is a long-term problem. Scanning deflectometry method using autocollimator with rotary stage was proposed for measuring mirrors with large slope. The repeatability of the method has been confirmed as tens of nanometers. However, the absolute accuracy of the method is not confirmed since the distance between the rotation center of the rotary stage and the mirror under measurement is difficult to measure, which strongly affects the measurement accuracy of scanning deflectometry method. In this paper, we have proposed a triangulation method with CMOS sensor for measuring the distance from the rotation center to the measurement table (marble surface). The error analysis shows that the measurement uncertainty of this measurement method is about 40 microns. The repeated experiments show that the measurement accuracy of the proposed method is 49.7 microns, which meets the requirements of large mirror measurement with tens of nanometers’ uncertainty. This method also provides a new solution for the position measurement of the rotation center of the large scale rotary device.

Construction of Knowledge Base Based on Ontology

In order to achieve the goals of high precision, high efficiency and low cost for the assembly of multi-species and variable volume of precision micro-parts, this paper proposes a kind of method for building the ontology-based intelligent assembly knowledge base. As the traditional data model is difficult to express some complex knowledge, this knowledge base is driven by the ontology model instead of the traditional data model to solve the problem. This paper completes the knowledge base using the protégé software to carry out the consistency test, using the semantic web rules description language SWRL to construct reasoning rules, using Jess reasoning engine to realize the knowledge reasoning. Hence, generating new knowledge to update the knowledge base.

Simulation study on the eccentricity of floater in liquid floated gyroscope

This paper studies the effect of the floater eccentricity in a single degree-of-freedom Liquid Floated Gyroscope LFG on the fluid motion and the forces applied to the floater. The study is done numerically by using Computation Fluid Dynamics CFD based on the theory of Spherical Couette Flow SCF. The normal pressure distribution, the shear stress distribution, the streamline diagrams, the force and the torque applied to the eccentric floater are reported. The results of numerical simulations imply that the floater eccentricity significantly influences the fluid motion and the forces applied to the floater.