To Overview the Mirror Zoo
EMIL requires more than 20 optical elements, like mirrors, which have to be accurately positioned. So it is important to know about the exact angles of the mirrors in the beamline and to adjust them. To realize a very high precision, a computer-based system controls the modulation. Therefore, it is necessary to translate the analogue data of the angles into a digital code. So-called encoders are doing this job at the interface between analogue and digital world. They can electronically monitor the position of the mirrors and process such data.
The technical realization of encoders happened in many ways. There are multiple technical implementations, such as mechanical, optical, magnetic and capacitive encoders. The way of the binary code production differs from type to type. Imagine for example an optical encoder. This one is working with a light source, an optical disk and a light source receiver. As a shaft, which is connected to the measured device (e.g. mirrors), rotates, the optical disk passes and breaks the light beam coming from the light source. The light receiver and associated electronic circuitry convert the on/off light states into an electrical signal, the binary code.
A schematic drawing can be seen here.
Such encoders can either be absolute or incremental. The signal of an absolute encoder gives a distinct position without requiring knowledge of any previous position. In contrast, the signal of an incremental encoder is cyclical and requires counting of cycles to maintain absolute position. Both provide the same accuracy. However the absolute encoder even provides position regardless of power interruptions. So it remembers the last adjusted position even if the system is temporarily out of energy.
What EMIL needs
EMIL uses both kinds of encoders. But it's the first time EMIL implements absolute encoders associated with monochromators and mirrors. These absolute encoders use a so-called single-track-system, developed by the company Renishaw. In this case, the encoder is scanning a tape measure which reminds of a barcode. You can think of it as a digital camera which takes photos of the barcode. Then, the encoder analyses the photos and is able to compute its own position with a precision up to one nanometer (one billionth of a meter). That process is done in just 40 microseconds. Some quite impressive numbers!
For a better visualization, watch this video.
As we know, the beamline requires a very clean environment. So the whole encoder system is designed to work under ultra-high vacuum (UHV) conditions. That means, the materials of the encoder and tape measure won’t contaminate the UHV. For example, there are no vapours seeping out from the materials. A clean way to control the mirror zoo.