Research On Measurement And Evaluation System Of The Screw Thread Via Machine Vision Technology

The external thread is an indispensable part of mechanical linkage and tighten, which is widely used in instruments and electromechanical equipments. Thread contact-measurment including comprehensive detection for thread eligibility and single-parameter measurement such as the pitch diameter, pitch, thread angle etc. Generally, comprehensive detection is implemented manually with the screw thread gauge, single-parameter measurement is implemented with three-needle method, double ball screw etc and the measured threads eligibility can be estimated manually by checking with the standard accoding to the thread category and size, so this method slowly, inefficiently and easy cause man-made errors.With the development of the computer technology, machine vision technique has come to be mature, which can be applied to thread measurement to improve the detection efficiency and to achieve the non-contact measurement. In this paper, a normal thread measuring and evaluating system is designed based on WINDOWS operating system and C++ Builder 6.0 software, so that the middle diameter, pitch and thread angle of Metric screw threads and Unified screw threads can be measured non-contactly. The main research contents are as follows:(1) A thread measurement platform was designed, which is a two-dimensional mechanical platform, dirved by stepper motor, and a motion control systems was designed to control the two-dimensional mechanical platform move, which the core is MCU. A mechanical platform displacement measurement module was designed, in which a linear is used as the platform displacement detection device, MCU and FPGA as the core of signal processing circuit, so that the signal of linear can be collected and processed and the communicated with computer can be implemented.(2) A thread image processing module was designed, so the measured thread image information can be obtained by using the technology of DirectShow, and some image processing was carried out for the thread image, such as pre-processing, filtering, edge extraction processing etc, so that the thread contour information can be obtained.(3) A external thread parameter measurement module was designed, based on the thread measuring displacement, which can be obtained by using the grating signal processing module, and thread profile information, which can be obtained by using the image processing module, so thread parameters, such as pitch diameter, tooth-type angle and pitch, can be measured. (4) Based on the Metric and U.S. threads standard, a thread evaluation module was designed, in this module, the range of the pitch dimeter can be calculated according to the calibration parameters of the measured thread, and then the thread eligibility was evaluated by comparing measured pitch-diameter with the standard value. A thread measurement and evaluation module was designed for three-wire measurement, so the wire diameter can be measured and provided according to the calibration parameters of thread and the measured thread eligibility can be evaluated by comparing the measured pitch diameter with standard value, in C++ Builder 6.0 environment.Thread measurement experiment was accomplished on Metric and U.S. threads, and the experimental results and possible factor of the errors was analysed. The experimental results show that the measurement and evaluation of the thread parameters are implemented efficiently and accurately by using the measuring method and the application system of this paper provided. Not only the correct evaluation result is provided, but also the tedious artificial thread standards lookup is avoided and the measurement efficiency is increased consequentially.