Vacuum Technology Book, Volume II

3.7.2 Accuracy, repeatable precision and resolution

Manipulators and mechanical feedthroughs have many different tasks. They range from continuous linear and rotational movements with specified speeds up to precise positioning tasks. Transport samples for analysis, move series products between process chambers or position measuring heads at their measuring point – such tasks often require only position accuracies of 0.1 to 1 mm. Other positioning processes require the most accurate repeatable precision of only a few micrometers or for tasks such as scans, the finest resolutions of 1 µm.

In order to be able to correctly identify requirements for the selection of a manipulator or a mechanical feedthrough, it is important to understand the following definitions, as well as, the equipment's properties and sources of errors.

The accuracy (precision) describes the deviation between the desired position and the actual position reached. Starting from one position, manually read from a scale or driven by a motor, ending at a position determined by a defined number of scale movements or motor steps.

Therefore the accuracy is determined by a variety of factors. Without claiming to be complete, here are some aspects that need to be considered frequently: How exact is the reading of the scale? How exact can the wheel be moved? How great is the division of the motor steps? Does the movement follow the motor step exactly? Furthermore, the properties of the guide and drive components are critical for accuracy: How big is the pitch error and the play of a spindle drive? How great is the deviation from the ideal leader of a linear guidance? Furthermore, attachments or air pressure of external loads cause deformations in the drive and guiding unit. If manipulators do not ensure adequate stability, the movable flange tilts during evacuation towards the connection flange.

The repeatable precision describes the deviation, with which a certain position can be achieved again. It is better or equals the precision, since not all errors are recorded. For example, unlike the play of components, the spindle-pitch error or leader deviation have no impact.

The resolution describes the smallest possible increment of movement. Pitch errors or guidance accuracy have also no impact. The readability of the scale or the smallest increment of the motor are crucial here. The requirement for this is, that the guide and drive unit follow the specifications. If, for example, due to the elasticity of the components and the difference between static and dynamic friction, it comes to a release among the sliding friction partners (stick-slip effect), such effects can determine the resolution. Recirculating ball systems are free from such effects and for demanding positioning tasks they are preferred over a slide unit.

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