Research On The On-line Dimensional Measurement System For Large Forgings

Manufacturing technology of large forgings is one of the key technologies ofmanufacturing major equipment. And the large forgings are widely used in the field ofelectric power, metallurgy, petrochemical, shipbuilding, heavy machinery and aerospace.With the rapid development of science and technology, the demand for bulky forgings isgrowing, and the quality is becoming more and more important. But the presentmanufacturing technology of forgings, especially the technology of bulky forgings’dimensional measurement, still have the problems of low accuracy and inefficiency. Thedimensions are measured by laborers operating the mechanical measuring tools under thecondition of high temperature with large measurement error. So, in order to reducing therejection rate, they have to increase the cold machining allowance. And it increases theconsumption of resource. Hence, the dimensional measurement technology of largeforgings urgently needs to be improved.This paper researches on the dimension measurement method of large forging. And itaims to develop an on-line measurement with high precision, wide application, simpleoperation and high speed. The main subjects of the work are completed as the followings:(1) This paper proposes a dimension measurement system of large forgings based onlinear structured light and linear CCD camera scanning jointly. The basic idea of the systemlikes this: we see the intersection as the feature point which is the cross point of thehorizontal vision of linear CCD camera and the vertical structured light stripe. And largenumber of points on the surface of measuring object will be obtained through structuredlight stripe and CCD camera scanning driven by high-accuracy Pan-Tilt (PTZ) respectively.After getting the information of pixels, locations and angles, the3D coordinate data cloudcan be calculated based on the analytic geometry model of structured light vision.(2) A new system of image sampling and data processing is designed based on FPGA.In the system, the scanning of PTZ and the image capture shall be synchronized bydetecting the pulses sent by PTZ. Based on the feature of parallel processing of FPGA, theimages that have been captured can be processed, and other images can be captured in themeantime.(3) In this paper, the acquisition and processing of point-cloud data is completed. Thecamera’s parameters can be obtained through the method of the vector and the three collinear points. Then the pixels and angles are converted into3D coordinates based on theprinciple of spatial analytic geometry. After we get the data cloud, the ellipse centers areobtained by ellipse fitting, and using the centers makes line fitting to get forging’s axis. Atlast, the diameters of shaft can be obtained. In order to verify the reliability of fittingarithmetic, we design an algorithm to generate data cloud.(4) The software of measuring system is designed. It contains the setting of camera’sparameters, image display, the control of PTZ and the processing of data, and so on. Thecommunication between computer and controller of PTZ is completed via Ethernet. Andthe control of motion is programmed based on application programming interface (API).(5) The experiments were carried out in the laboratory. The precision of the threecoordinates of every measuring point were less than8mm. In the experiment of measuringshaft with the diameter of245mm, the fitting precision of shaft is lower than10mm.