A positioning control device and method in which the relative position error in tracking is reduced by using the periodicity of position shift. The light source receives a voltage signal causing the light source to be illuminated. Each wafer in the processing sequence is assigned a priority level depending on its processing stage, and this priority level is used to sequence the movement of wafers between chambers. The tube has a sidewall defining an inner diameter surface and an outer diameter surface, with at least one aperture through the sidewall adjoining the inner diameter surface and the outer diameter surface.