Additional Error Causing Conditions
Abbe errors result from a position measuring device (typically a linear encoder) being offset from the point of work (end-effector). In these situations, motion errors caused by bearing system irregularities are neither sensed nor accounted for. As shown below, the magnitude of Abbe errors depend upon where the feedback sensing is with respect to the end-effector and the bearing system.
Cosine errors occur when a positing feedback scale is not in perfect parallel alignment with the motion of the moving carriage. Since the misalignment projects such a small error in the direction of motion, it is of miniscule consequence. These errors are typically linear in nature, so they are usually corrected by a one-time resolution adjustment.
Lost Motion (hysteresis)
Any system that reacts and recovers differently when varying directional forces are applied will exhibit lost motion. Mechanical structures often exhibit these stress memories such that when forces are removed they don't always fully recover to their original state. It is therefore vitally important to utilize sound structural concepts and mechanical elements when designing high-precision motion systems. Ballscrew backlash is an example of a typical source of lost motion. Sensing the motion at the point of work also enables local corrections to be made. Direct-drive linear servo motors, combined with stiff mechanical structures and tight servo control systems minimize the causes and effects of hysteresis. Sensing the motion at the point of work also enables local corrections to be made.
Thermal Expansion Errors
Thermal expansion coefficients of motion elements must be considered when selection high precision platforms. Besides the internal stresses generated by temperature transients on dissimilar elements, relative deflections between position feedback sensors and the elements being sensed will adversely affect performance and precision.