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UBC Theses and Dissertations

Modelling of electron beam deflection system for beam position control in metal additive manufacturing Parks, Scott


Additive manufacturing (AM) is a layer-based process for producing parts. Metal AM is an attractive technology for the aerospace and biomedical industries due to its ability to produce complex geometries from difficult to cut materials. Electron beam melting (EBM) is a form of metal AM, which uses an electron beam to melt metal powders into fully dense parts. The position and velocity of the electron beam are important parameters in determining the success of production in EBM. In order to provide robust control of the beam position, a model for real-time prediction of the electron beam position has been developed. The electron beam’s position is controlled by an electron beam deflection system, which uses electromagnetic poles to deflect the beam to a desired position on the build plate. This thesis presents an electron beam deflection system model, which can predict the beam position during EBM operation. The current behavior within the deflection coils is modelled using an equivalent circuit to determine the effective current within the coils. The prediction of the magnetic flux density distribution generated by the coils based on the effective current in the coils is described. The interaction between the generated magnetic flux density and the electron beam gun structure is modelled as a first order system, to predict the lag induced by eddy currents on the beam’s position. With the magnetic flux density distribution, the position of the electron beam was predicted using a discrete-time domain simulation. Crosstalk between the axes of the system was modelled with an empirical model. The proposed model was validated through FEM simulations and experimentation on a single-axis prototype as well as an EBM machine. Recommendations for hardware alterations within the EBM machine are made, which would reduce error in the beam’s position. Additionally, a pole-zero cancellation controller is designed to compensate for errors caused by eddy currents. A feed forward controller is designed, which predicts the crosstalk between the system’s axes and compensates for the error in real-time.

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