Robust Low-cost Planar Positioning Stage

Abstract

In 2018, there were more than 200 million reported cases of malaria worldwide, most of which were in Africa. Adequate diagnostics are required to properly treat the disease. According to the WHO, microscopic examination of a blood smear is the Gold Standard of malaria diagnosis. Currently, it is a labor-intensive process requiring trained personnel, expensive equipment and a lab-environment, which makes it unsuitable for use in field environments. Most of these problems can now be tackled with a smartphone microscope, which automatically identifies malaria parasites in a bloods smear. This solution is low-cost, suitable for use in field environments and excludes the need of a trained microscopist. However, it is still a labor-intensive process: 300 unique fields of view of a blood smear must be examined and doing this by hand takes a lot of time. To solve this problem, an (semi-)automated planar positioning stage is needed, which moves the smear in 2-dof, such that 300 unique images of the surface are acquired. The stage must be low-cost, robust and suitable for field environments. At the moment, there is still a lack of such a stage. Therefore, this research aims to fill that technology gap. The stage is designed to perform a motion pattern in 300 steps, whilst ensuring that the sample remains in focus and that there is no overlap between images. The design encompasses a coarse motion stage stacked atop a fine motion stage (combined, it is an $Rθ stage). The fine motion stage consists of a compliant rotary stage actuated by a stepper motor, generating cyclic motion of 40 steps (220 μm step size). After each cycle, the hand-actuated coarse motion stage displaces the sample one step (500 μm step size). The operation is completed after 8 cycles. A demonstrator is built to investigate whether the stage meets the requirements. The 3σ step precision of both stages ensure no overlap between images (11 μm and 30 μm) and the 3σ focus error during operation is small (δz ≤ 2 μm$). Consequently, we have successfully designed a low-cost and robust stage capable of meeting the requirements for this application. For future work, the stepper motor can be replaced 1-on-1 by a mechanical variant, eliminating the need of a power source and electronic components, whilst significantly reducing costs.