Why does a polygon scan head need Start-Of-Scan (SOS) detection? Where is the encoder?
Most laser system designers are familiar with galvanometer scanner technology. A galvo scanner uses an absolute encoder to provide mirror position feedback. Polygon scanners work completely differently. Most polygon scanners do not have an encoder. Those that do, typically operate at low speeds and have in incremental encoder to give feedback to a BDC motor controller so the motor will rotate smoothly. A shaft mounted encoder cannot give highly accurate mirror position on a polygon scanner. One reason is that shaft runout causes encoder error. Another reason is that there is facet to facet error on the polygon mirror. Further, typical encoders will not work at the speed of the fastest polygon scanners.
The good news is that a SOS detection system solves all these problems! Galvo scanners need to start, stop and reverse on command. Polygon scanners are designed to run at a constant speed. So, if you know where the beam is at the start of a scan, a clock will tell you where it is throughout the scan.
Start of scan detection works this way. Just before the beam enters the work area, it crosses a detector known as the Start-Of-Scan (SOS) detector. The SOS pulse is used to start a clock timing that scan. When the next facet on the polygon comes around another SOS pulse is triggered. There is facet to facet angle error but it does not matter. The SOS pulse tells your system where the beam is. So, every working scan will line up very accurately with the other scans.
PRECISION mini-SOS LASER DIODE MODULE™
It is common practice in low power polygon scanner systems to use the working beam to trigger a Start-Of-Scan (SOS) detector. Polygon Scan Heads, used for material processing, employ high power and ultra-fast lasers. SOS detectors require a low power CW beam so SOS triggering becomes problematic with a high power or pulsing beam. The best solution is a separate low power laser out of plane from the working beam.
Precision Laser Scanning offers a custom CW laser diode module designed specifically for Start-Of-Scan (SOS) trigger in polygon scan heads. The Precision mini-SOS Laser Diode Module has very narrow divergence so there is no need for manual focusing or external focusing optics. The beam is visible red and eye safe which makes for easy scan head alignment. The new Precision mini-SOS Laser Diode Module is only 5 mm in diameter and 12 mm long. The compact size allows easy integration into a compact Polygon Scan Head or an AV LIDAR. The SOS laser is typically used out of plane from the working beam to avoid interference. By using a red SOS laser when the working beam is IR, Green or UV makes it easier to filter out interference from the bright working beam.
PRECISION SOS DETECTOR™
This is the first commercially available Start-Of-Scan detector designed for the challenging environment inside a Polygon Scan Head. It can reject the scattered light and reflections of a powerful working beam. The optical detector is a bi-cell design which assures that stability is immune to variation over a wide temperature range.
An SOS detector is required to achieve accurate line to line registration with any polygon scanner. It is used to synchronize a CW or pulsed laser to the scanner. The output pulse from the Precision SOS Detector has a fall time of <7 ns to give excellent accuracy.
While the Precision SOS Detector has features critical to use in a high power Polygon Scan Head, it works with any polygon scanner in a high or low power application.
The Precision SOS Detector is used out of plane from the high powered beam. It is designed to be triggered by a low power CW laser such as the Precision mini-SOS Laser Diode Module above.