A method and apparatus is provided for determining substrate drift from its nominal or intended position. The apparatus includes at least two fixed reference points. The reference points can be fixed with respect to the processing tool, or with respect to the end effector. As a robotic arm moves the end effector and substrate along a path, a camera captures images of the edge of the substrate and the reference points. Two or more cameras can also be provided. A computer can then calculate positional drift of the substrate, relative to its expected or centered position on the end effector, based upon these readings, and this drift can be corrected in subsequent robotic arm movement.