A CMP system accurately measures eccentric forces applied to carriers for wafer or polishing pad conditioning pucks. An initial coaxial relationship between wafer axis of rotation and a carrier axis is maintained during application of the eccentric force, such that a sensor may measure the eccentric forces. Such initial coaxial relationship is maintained by a linear bearing assembly mounted between the carrier and the sensor. The linear bearing assembly is provided as an array of separate linear bearing assemblies, and may be assembled with a retainer ring in conjunction with a motor for moving the ring relative to the wafer mounted on the carrier so that an exposed surface of the wafer and a surface of the retainer ring to be engaged by the polishing pad are coplanar during the polishing operation.