Patent ID: 8692978

Claim:
A method of determining a defect in a grid plate of an encoder-type position measurement system, the method comprising: providing an encoder position measurement system to measure a position of a movable object with respect to another object, the encoder position measurement system comprising a grid plate and an encoder head, wherein the grid plate is mounted on a first of the movable object and the other object, and the encoder head is mounted on a second of the movable object and the other object, the encoder head comprising a radiation source configured to emit a radiation beam towards a measurement location on the grid plate and two or more detectors each configured to measure a quantity of said radiation beam reflected by the grid plate; measuring a quantity of said reflected radiation beam received on each of the two or more detectors; using a combined radiation beam intensity of the reflected radiation beam on the two or more detectors to determine a reflectivity signal representative of a reflectivity of the grid plate at the measurement location, and determining a presence of a defect at the measurement location based on the reflectivity signal of the grid plate.