Research On Digitized Scanning Control System For Electron Beam Selective Melting

Electron Beam Selective Melting (EBSM) deals with metal rapid manufacturing.Manufactured parts by EBSM have been applied in medical, defense, aeronautics, etc.To broaden the application field, the largest manufacturing size, accuracy and qualityneed to be raised. Currently electron beam has a relatively large deflection error, andfocus size of electron beam will enlarge with increased deflection distance, whichreduces electron beam power density and lowers manufacturing precision. A digitizedcontrol system is developed to solve the said problems.The hardware for deflection and focus control of required quality is discussed,and a four-channel, high speed arbitrary waveform generator is selected. Digitizedautomatic manufacturing software for deflection error correction and dynamic focuscontrol is developed; meanwhile, new data storage form is developed, breaking thelimits of simple line arrangement as scanning path, with more choices for customers.To improve deflection accuracy, nonlinear magnetic deflection is analyzed.Harris Corner Detect algorithm is used to get the coordinates of corners in the samplegird, and further a look up table for correcting deflection error is generated.Experiments prove that such digitized error correction method could greatly improvethe deflection precision, and reduce pincushion distortion.To get stable power density and focus size, electron beam magnetic focus isanalyzed. Based upon theoretical computation and experimental results, thecorresponding focus current for each target point is calculated, and the focuscompensation table for each point is built up. Experiments show that digitizeddynamic focus control could largely improve quality of electron beam focus.A raster scanning method based upon dynamic focus is explored. In rasterscanning method, the scanning frequency in the X, Y direction keeps constant, whichcould eliminate the deflection error caused by frequency change as well as theadverse effect of scanning over line segment of different length. Specific data storagefor raster scanning is developed, and the feasibility of applying raster in electronbeam selective melting in the future is approved.