Due to large temperature gradients and high thermal conductivities, additive manufacturing processes for metals have challenging dynamics. A high process reliability and repeatability hinges upon the base layer control of the melt pool and the resulting weld bead geometry. The material feed rate has proven to be an appropriate manipulated variable for weld bead height control, but it can only be varied slowly and is subject to deadtimes. We propose to use the distance of the process head to the build surface (the working distance) as an alternative manipulated variable. We derive a simple nonlinear dynamic model that captures the effect of the working distance on the weld bead height. We show scanning velocity fluctuations, which can be treated as known disturbances, can be compensated with working distance feedforward control. We apply the proposed controller to a real DED process and demonstrate working distance control is an alternative to material feed rate control.
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