Measurement is the prerequisite technology for assembling precise devices in order to guarantee the quality requirements. As a typical category of assembly, peg-in-hole is the most widely employed assembly method in industrial fields, including precision instruments and machinery. Some of the measurement problems and solutions of two major categories of the peg-in-hole assembly, i.e. clearance fits and interference fits, were discussed. The parts to be assembled are small in size. The precision clearance fits require mating surfaces to avoid collisions during assembly; for the precision interference fits, the mating surfaces should not be damaged or generating abrasive chips during press-fit. The hole-shaft alignment and attitude adjustment are the main task need to be carried out precisely in assembly. Furthermore, real-time monitoring and connection strength measurement is also the important for interference fit assembly. Therefore, the purpose of this paper is to achieve precise hole-shaft alignment and attitude adjustment of these two kinds of fits and to real-time monitor the assembly process, as well as connection strength assessment of the interference fit. Two assembly instruments were built for interference fits and clearance fits to achieve precise alignment, attitude adjustment, and real-time monitoring. In addition, an ultrasonic testing apparatus was built to evaluate the connection strength. After calibration, the precision press-fit instrument can achieve high assembly accuracy and demonstrated with experiments. The prediction results of connection strength are in good agreement with experimental results with a relative error less than 20%. Furthermore, the instrument for clearance fits was also designed and introduced.
Press-fit assembly is one of the traditional methods for assembly of interference fitting parts, but the assembly quality cannot be acquired directly from this method. At present, the press-fit curve is employed for quality estimation, and thickwalled cylinder theory (TCT) is used for standard press-fit curve prediction. However, the evaluation results cannot predict the stress concentration occurred on the mating surface, and most interference fitting parts fail due to this reason. This paper aims to explore ultrasound as a tool for non-destructive evaluation of contact stress, and then acquire the contact stress distribution eventually. Therefore, it is more convenient and intuitive to evaluate the assembly quality based on this method.
Mechanical failure is primarily caused by coaxiality errors of axis. Traditional methods (synthetic gauge testing methods, method of rotary axis, coordinate measuring machine, etc.) for detecting coaxiality have some disadvantages such as: low efficiency, sensitive to human factors and cannot be used to measure the axis that is far apart without connection. In this paper, based on the principle of laser collimation, a method for measuring coaxiality errors of non-connecting axis with long distance was proposed. By rotating the laser module and the four-quadrant photo detector module respectively, the center and radius of the circle that formed by the laser spot were used to calculate the angular deviation and the parallel deviation. Finally, a measurement system based on the precision instrument deflection was designed to verify the rationality of the method.
Automatic assembly is widely used in precision component fabrication, and the manually assembled task for a small batch of the precise component is replaced by automatic system gradually. One of the key issues is manipulating the workpiece reliably. With the quick updating of the products, the end effectors need to pick more parts with different dimensions and configurations. In this paper, an exchangeable end effector for laser gyroscope assembly is presented. The end effector integrated with adsorption and clamping is designed to suitable for different type of workpieces. The three-position two-way magnetic valve is used to switch the adsorption and clamping. Based on the dimension and shape characteristics of the parts, the adsorption and clamping position of the end-effector was calculated. The maximum weight of the part is 0.5N, thus vacuum degree required for adsorption is 0.078MPa and the reaction time is 0.653s. The clamp for this gas circuit can generate the clamping force of 10N, which meets the requirement of clamping. Finally, the assembly tests were carried out to prove the feasibility of this exchangeable end effort.
The size of internal air gap of dynamic pressure motor is an important index to determine its performance and running stability. In order to improve the precision and automation degree of the internal air gap measurement for dynamic pressure motor, an automatic measurement equipment was developed based on modular design concept. The equipment was mainly composed of clamping module, automatic forcing module, and displacement measurement module. During measurement, the stator was mounted by clamping module with a flexible support at two ends. Then an external force was loaded by forcing module, which was a 3-D electric precision motion platform integrating one triaxial force transducer. A relative displacement was generated because of the internal air gap. And the relative displacement increased until the rotor and stator touched each other. Thus the air gap was transformed into an external micro displacement. Finally, the displacement was measured by measurement module, a 2-D precision motion platform integrating double inductive probes, with relative measurement principle. Experimental results showed that the measurement accuracy was about 0.2μm.
On account of the assembling demand for the metal framework of frequency stabilizing component for laser gyro, a precision assembly system was designed and developed in this paper. The coaxiality of the metal framework and the grooved mirror is the key index to assembly. Meanwhile, the grooved mirror is fragile and the assembly force has to be monitored and controlled in real time. The installation structure of force sensor is designed and a force control unit is established. The contact force can be detected and feedback to control the movement of the linear stages to complete the displacement in the vertical direction for the operation of pick up and place parts for end effector. To reduce the time of image processing, the machine vision unit is based on high repeatability accuracy of the stage to assist image mosaic, thus the calculation workload for image mosaic is reduced, and the time of image mosaic is about 0.5s.
An automatic flatness detection system for micro rings is developed. It is made up of machine vision module, ring supporting module and control system. An industry CCD camera with the resolution of 1628×1236 pixel, a telecentric with magnification of two, and light sources are used to collect the vision information. A rotary stage with a polished silicon wafer is used to support the ring. The silicon wafer provides a mirror image and doubles the gap caused by unevenness of the ring. The control system comprise an industry computer and software written in LabVIEW Get Kernel and Convolute Function are selected to reduce noise and distortion, Laplacian Operator is used to sharp the image, and IMAQ Threshold function is used to separate the target object from the background. Based on this software, system repeating precision is 2.19 μm, less than one pixel. The designed detection system can easily identify the ring warpage larger than 5 μm, and if the warpage is less than 25 μm, it can be used in ring assembly and satisfied the final positionary and perpendicularity error requirement of the component.
In the process of miniature parts’ assembly, the structural features on the bottom or side of the parts often need to be aligned and positioned. The general assembly equipment integrated with one vertical downward machine vision system cannot satisfy the requirement. A precision automatic assembly equipment was developed with double machine vision systems integrated. In the system, a horizontal vision system is employed to measure the position of the feature structure at the parts’ side view, which cannot be seen with the vertical one. The position measured by horizontal camera is converted to the vertical vision system with the calibration information. By careful calibration, the parts’ alignment and positioning in the assembly process can be guaranteed. The developed assembly equipment has the characteristics of easy implementation, modularization and high cost performance. The handling of the miniature parts and assembly procedure were briefly introduced. The calibration procedure was given and the assembly error was analyzed for compensation.
Adhesive bonding is a popular assembly method for miniaturized component composed of parts with different materials. However, it also inevitably introduces assembly stresses in some stress-sensitive structures, which adversely influence the accuracy of the performance, as well as the long-term stability of the component. An on chip piezoresistive micro stress test circuit was designed and tested in this paper. Piezoresistors were fabricated by doping Boron to n-type silicon wafer. The sensitive structure on the micro part was a flexible beam which had a thickness of 50 μm. Silver epoxy adhesive and double epoxy adhesive were used in the assembly process and the stresses parallel and perpendicular to the flexible beam were calculated. The experimental results showed that silver epoxy adhesive introduced larger stress because the difference in thermal expansion coefficients between silver epoxy adhesive and silicon are larger than that of double epoxy adhesive. Aging tests were also carried out and 46 percent of stress on average was released in 100 days at room temperature. This preliminary work implied that the stress distribution of the sensitive structure at all stages of the assembly could be measured using on chip micro stress test circuit.
In the paper, a precision press-fit instrument for assembling small interference fitting parts is introduced, which includes pressing module and parts alignment module. The pressing module was used to clamp and position parts, and parts alignment module was used for the two parts’ alignment. Through analyzing press-fit control method, component alignment and adjustment strategy, and machine vision device calibration method, the instrument meets the pressing requirements of precision small components. Finite element method is used to predict the reasonable range of press-fit force, and pressing result of the instrument is tested by experiments.
In assembly of miniature devices, the position and orientation of the parts to be assembled should be guaranteed during or after assembly. In some cases, the relative position or orientation errors among the parts can not be measured from only one direction using visual method, because of visual occlusion or for the features of parts located in a three-dimensional way. An automatic assembly system for precise miniature devices is introduced. In the modular assembly system, two machine vision systems were employed for measurement of the three-dimensionally distributed assembly errors. High resolution CCD cameras and high position repeatability precision stages were integrated to realize high precision measurement in large work space. The two cameras worked in collaboration in measurement procedure to eliminate the influence of movement errors of the rotational or translational stages. A set of templates were designed for calibration of the vision systems and evaluation of the system’s measurement accuracy.
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