What influences accuracy?

A number of factors have an effect on the accuracy. Temperature changes will have the greatest effect in most cases. This is accordingly more severe the longer the system is, and naturally also depends on the temperature difference.

The second most influential factors are regularly pitch and yaw. No table really moves completely straight, which means that there is always a certain curve movement. The further the object for positioning is from the guide track or scale, the stronger the effect of this curve passage will be (outer curve longer than inner curve).

In third position, are faults in the measuring system, followed by mechanical clearance and controller Hysteresis.

How can I achieve accuracy?

The influence of temperatures has to be minimized. Prerequisite for this is sufficient air-conditioning. If this cannot be achieved, then it is possible to use temperature insensitive measuring systems. The measuring systems work either interferometric or have scales made from temperature insensitive materials such as glass, Cerodur or as the best option Invar.

Furthermore, the object to be positioned has to be fitted at as small a distance as possible to the guide track and the solid measures. This condition is known as the Abbe comparator principle. For adherence to this principle, the influence of pitch and yaw can be eliminated. Since a positioning in the measuring and track level is often not practical, a track system with minimal procedure errors is necessary.

If a powerful controller is available, then repeatable faults can also be compensated. Often this is the more affordable and up-to-date method. The known accuracy deviations after measuring are saved in the controller and the movement is then corrected position-dependent.

Why is repeatability more cost-efficient than accuracy?

Accurate tables must cover across the route a fraction of the distance “meter” as defined in ISO. This means that a part, whose length is known as precisely as possible, must be fitted on the linear table. Such parts are expensive. An example would be the installation of highly accurate and thus expensive scales or the use of precision spindles. A highly accurate table also always places high demands on the tracking system due to the influence of pitch and yaw.

If a table is, however, only designed for repeatability, then you can use significantly cheaper measuring systems and the demands into the track system are reduced. Repeatable systems can also be precisely designed for the application in the overall system by using teaching or compensation.