Tech Foucs: Machining Accessories Top three ways to increase positioning system’s 3D accuracy

| Author / Editor: RJ Hardt / Steffen Donath

Positioning systems are vital for automated manufacturing processes. Aerotech offers three methods to increase the 3D accuracy of positioning systems, enabling manufacturers to meet tolerance goals.

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An XY positioning system using interferometer feedback at the working plane to minimise abbe error influences at the work point.
An XY positioning system using interferometer feedback at the working plane to minimise abbe error influences at the work point.
(Source: Aerotech)

Positioning systems are used to automate almost every high-precision manufacturing process. Many of these processes occur in 3D. Examples include positioning a part’s surface in 3D to be 100 % inspected and laser-cutting a feature along a complex 3D path. For these types of processes, it’s exponentially harder to achieve manufacturing tolerances in terms of microns and nanometres due to the additional errors that positioning systems exhibit within a 3D space.

However, when speaking in terms of spatial positioning accuracy or 3D accuracy, the positioning specialist tells of some fundamental considerations that can be put into effect to increase a machine designer’s chances of successfully meeting tolerance goals.

1. Add more stiffness

Stiffness is to a positioning system what sleep is to a new parent. There are ways to compensate for the lack of it, but the job becomes much harder without it. Adding stiffness to a mechanical design will increase motion repeatability, which in turn makes achieving high 3D accuracies much easier. As the term implies, non-repeatable motion is motion that cannot be predicted and compensated by software calibrations and correction tables (see Point 3).

Furthermore, there are many directions in which stiffness needs to be improved. There are error motions in six Degrees Of Freedom (D.O.F.) for each motion axis you add to a positioning platform. Adding more stiffness can reduce the five sources of spatial positioning error.

2. Add feedback at the work point

Another way is to incorporate feedback mechanisms that reduce the amount of abbe error. Reducing abbe error is as easy as reducing the distance between the positioning mechanic’s feedback device and the working point in space that you are using for the process. For example, let’s say you have an XY positioning system carrying a part, and let’ also say that you care what happens to that part at a point in space that is 100 mm above the XY positioning system. Adding a second feedback source that is 100 mm above the XY mechanics will provide valuable information to the motion axes and allow them to compensate for abbe errors that may be present.

3. Calibration and correction tables

When all else fails, you can try to correct 3D positioning errors by using calibration techniques and correction tables. In practice, a high-resolution measurement device would be used to measure the actual position of a motion axis as it is commanded through travel. The difference between the actual measurement and the commanded measurement is calculated and added to a correction file. Now, the next time the axis is commanded to move the same distance it will correct itself based on the actual measurement that was taken by the external measurement device. Calibration is an offline process and only works on repeatable errors.