Resolution

Definition: The smallest achievable positioning increment that can be achieved by the motion control system.

Contributing factors: Drive screw pitch, gear drive ratio, motor/encoder combination perfromance

The threshold of minimum motion a positioning system can realize or sense defines its resolution.

Elasticity and play in the train of mechanical drive components-factors affecting repeatability  -may limit the positioning output in responding to commanded motion inputs.

Systems that must demonstrate very fine resolution normally require direct positioning from an encoder or other sensor directly coupled to the output motion element; for example, the translating carriage of a linear table.

A high-performance servomotor system coupled with a fine-pitch linear encoding scale can provide resolution approximating 0.1 mm.

Accuracy:

Definition: The maximum error between the expected (target) position and the position actually achieved by the motion control system.

Contributing factors: motor/encoder combination accuracy, ball and leadscrew error, repeatbility.

Accuracy is required when the motion system will be used as a gauge or measurement instrument.

It is qualitatively defined as the difference between the target position and the position the motion system actually achieves.

Quantitatively, it is the distance between the target point and the mean of the position dispersion plus the width of the dispersion that yields the largest absolute sum.

Positioning accuracy is constrained by system repeatability. To have an accurate positioning system, it is first necessary to have a repeatable positioner.

Accuracy, like repeatability, may be defined for either unidirectional or bidirectional motion. A linear motion system's positioning accuracy is a function of several factors, including the accuracy of drive screw thread leads,

repeatability, angular errors in the bearing ways, and the accuracy of the position measurement encoder (if one is used in the system).

High-performance, non-contact-type linear encoders, mated with electromechanical positioners of comparable precision, can provide accuracy to better than one part per million.

Understanding the sources of positioning precision is essential in specifying mechanical motion elements correctly.