Embodiments of the present invention generally provide a load cup used in the transfer of substrates in a chemical mechanical polishing system. The load cup includes an improved substrate edge sensing mechanism to ensure a substrate is present and correctly positioned in the load cup for transfer to a polishing head. In one embodiment, a lever actuated edge sensing mechanism is provided. In one embodiment, the edge of a substrate contacts a lever, which contacts a sensor to detect that the substrate is present and correctly positioned for exchange with a polishing head. Embodiments of the present invention provide reliable detection, while reducing contact with the feature side of the substrate during substrate transfer.