Linear encoders are usually classified according to their scanning technology - optical or magnetic. Magnetic encoders use metal scales (commonly referred to as "magnetic tapes"), while optical encoders use scales made of metal (mainly steel strips) or glass. Despite significant improvements in resolution and accuracy in magnetic encoders in recent years, optical encoders are still a better choice for single micron and sub micron measurement resolutions. Among the two substrates available for optical encoders (metal and glass), glass substrates are mainly used for linear encoders that are typically classified according to their scanning technology - optical or magnetic. Magnetic encoders use metal scales (commonly referred to as "magnetic tapes"), while optical encoders use scales made of metal or glass. Despite significant improvements in resolution and accuracy in magnetic encoders in recent years, optical encoders are still a better choice for single micron and sub micron measurement resolutions. Among the two substrates available for optical encoders (metal and glass), glass substrates are mainly used for applications that require resolution as low as nanoscale.

 

It can be said that the most common application of glass grating linear encoder is ultra precision machining, especially for grinding machines and lathes for optical components and parts in the aerospace industry. It is also necessary to inspect parts processed to single digit or submicron accuracy, and glass scales enable measuring equipment such as coordinate measuring machines (CMMs) to perform this task.

 

In addition to providing higher resolution than the magnetic version, the glass grating ruler linear encoder also has a much lower thermal expansion coefficient, which means that the grating ruler is almost unaffected by temperature changes, which can change the spacing of the grating ruler and reduce measurement accuracy. Glass scale encoders can also be used in vacuum environments, including high vacuum and ultra-high vacuum.

 

But glass gratings are still a niche product - in most cases, they are only used when absolutely necessary. This is partly due to cost, but also because the glass grating linear encoder faces different challenges in application and integration compared to the magnetic version. Compared to most magnetic encoders, the grating spacing is finer, and any errors in installation will have a more significant impact on the accuracy of the optical ruler.

 

Glass rulers are harder than tape rulers, but the flatness of the installation surface is more important for glass substrates than metal tapes. Installing the reading head requires alignment on five axes to achieve the correct air gap between the reading head and the scale - this is a labor-intensive process.

 

Once the grating ruler is installed, it is extremely important to protect it from pollution. In most cases, magnetic scales are only sensitive to magnetic contamination, while optical encoders are sensitive to any debris that interferes with light transmission. This may be a difficult problem to overcome, as the linear scale is installed on the travel axis where work is being performed, typically placing the encoder directly at the source of chips, liquids, and other debris. The most common method to protect optical encoders (especially glass grating rulers) from contamination is to enclose the grating ruler in a casing and optionally air blow the casing to better prevent dust and liquid from entering. However, the casing will increase cost, wear points, and hysteresis, which in itself will affect measurement accuracy. Therefore, installing the tape in an aluminum extrusion with a lip shaped seal is the most effective method to prevent contamination.

 

Heidelberg's DIADUR glass matrix 3D chrome wire grid typically has a grid spacing of 8 µ m. PRECIZIKA float glass and BK7 optical glass, with an accuracy level of ± 1.

 

The scanning principle of glass grating adopts interference scanning in photoelectric scanning. The glass scale is marked with very thin lines, called gratings or scales, with grating periods (spacing) as small as a few micrometers. Interference scanning is typically used for gate pitches of 5 micrometers or smaller - using a three-dimensional structure on the scale and relying on light diffraction and interference. When light passes through the reticle, it is diffracted into three sub waves. When these sub waves touch the scale, the 3D grating on the scale will diffract these waves. Then the waves meet again at the reticle, where they further diffract and interfere with each other. When the markings and scales move relative to each other, alternating light intensities are generated. This change in light intensity is monitored by photovoltaic cells and converted into electrical signals.

 

Overall, due to cost reasons and limitations in installation and usage conditions, there are still relatively few projects in China that use glass grating rulers of this type.

 

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