Research On External Thread Parameters Measreument Based On The Universal Tool Microscope And Charge-coupled Device

As important members of fasteners, threads’ qualities will affect the reliability ofthe whole mechanical system, which depend on whether the thread geometricparameters meet the requirements or not. Therefore, it is very important to measure theparameters accurately. However, the domestic and overseas devices all have the sameproblem, namely the shading of the teeth，which adds a principle error to the imagemethod when measuring the parameters of the thread. Aiming at the characteristics ofthe threads, this paper adopt a machine vision measurement based on the universal toolmicroscope and CCD (Charge-coupled Device) to measure the threads’ geometricalparameters.Firstly, definitions of the thread parameters are showed and the set-up scheme ofthe mechanical system is carried out. System design idea is based on the platform of theuniversal tool, with the eyepiece replaced by a CCD camera and suitable illuminationmethod, optical parameters and the camera lens selected. Grating displacementtransducers are installed to measure the moving distances of the X, Y workbench torealize large size measurement. ActiveDAQ Counter Control (DAQCounter) is used toimplement data transmission from the grating displacement transducer to the computer.Then, after image obtained by the CCD, outline of the thread teeth is extractedafter binary and filtering. In order to realize thread parameters measurement, straightline fitting should be done firstly based on the original definitions of thread teeth typeparameters. Improved Hough transform is used to realize straight line fitting. Functionsfor corner detection of Opencv are adopted to extract the corner point from the threadoutline. Lastly, aiming at the definition of the parameters, final results are acquired afterprocessing the image pixels with math methods. The focus of the paper is researchingthe algorithms of straight-line fitting and the measurements of thread parameters. Also,this paper gives a correction method, aiming at resolving the thread teeth shadingproblem. This paper analyzes the causes and influence factors of this shading problemand gives focus on how to solve this shading problem.Finally, measuring thread parameters software based on machine vision parametersis design and interface processing is carried out, too. The whole system is divided intoseveral parts, such as information input, image acquisition, image processing,parameters measurement, grating data operation and the final data processing module.In order to expand application scope of the program, the paper also gives some normal parameters measurements, such as the distance between the straight lines, linestraightness, the distance between the two circles, diameter of a circle, roundness of acircle, et al. After debugging and running the measuring software, this paper also makessome final data error analysis.