Patent ID: 8614783

Claim:
An encoder-type measurement system configured to measure a position dependent signal of a movable object, said measurement system comprising: a light source and a sensor, said light source and said sensor being mounted on one of said movable object or a substantially stationary frame; a reference object comprising a grating or grid mounted on the other of said movable object or said substantially stationary frame, said light source being configured to emit a light beam towards the reference object, said sensor being configured to detect light of said light source reflected by said reference object, wherein a position of the movable object is determined based on one or more signals provided by said sensor using said detected light; and an error detector configured to receive the one or more signals and capable of detecting errors in or on said grating or grid of said sensor target object during a continuous production process, said errors being detected using the one or more signals received from said sensor, said light source and said sensor to be used to detect said errors, said error detector comprising a monitor configured to monitor one or more variables, and a comparator configured to provide one or more comparison signals on the basis of a comparison of the one or more monitored variables with one or more reference values of said variables, the one or more comparison signals being representative of a presence of an error on or in said grating or grid.