STRONGER, COOLER, FASTER EXPLAINED
Stronger, Cooler, Faster – Explained
In modern optical scanning applications, there is often an interest in scanning as fast as possible or as wide as possible, and sometimes both. Although these goals are more achievable than ever before, the heat generated during intense scanning remains an obstacle to be overcome.
Similar to computer processors, where there is always a demand for faster performance without sacrificing reliability, there is a significant need for faster and more reliable galvo scanners. Applications such as Optical Coherence Tomography (OCT), Microscopy and LIDAR depend on faster, reliable laser scanners to continue to innovate. Unfortunately, while computer processor performance has continued to increase over time, optical scanning development and performance began to stagnate.
Given the need for fresh innovation, ScannerMAX laser optical scanning systems have been designed with modern applications in mind. While manufacturers of conventional galvanometers have used similar design and construction strategies since 1976, ScannerMAX engineers devised an entirely new way to manufacture a galvo, with stronger internal components and lower operating temperatures, which equates to faster scanning. Faster scanning provides for optical scanning systems to be used in applications previously considered infeasible. ScannerMAX scanners have been used as replacements for resonant scanners, polygon scanners and MEMS devices, typically at a lower cost.
ScannerMAX scanners are stronger than conventional galvos due to the construction of the rotating assembly. In a galvanometer, the rotating assembly includes the magnet, shafts, bearings, mirror mounts and the mirror. Conventional scanner manufacturers drill a hole through the output shaft and install a stopping pin, which dramatically reduces the stiffness of the rotor assembly and adds inertia to the rotating assembly.
ScannerMAX scanners use a larger diameter output shaft, which significantly increases the stiffness of the rotor assembly. In addition, we do not install a stopping pin in the output shaft, further contributing to stiffness.
Figure 1: Conventional scanner rotor assembly (left) vs ScannerMAX rotor assembly (right)
Conventional scanners often require the mirror to be mounted in a mirror mount fixture. These mirror mounts clamp to the output shaft, which adds to system inertia. Because of the stopping pin, the mirror- and mount- assembly is projected further away from the bearing, further reducing stiffness and subjecting the rotor to higher reaction torques. In addition, the mirror mount typically only bonds to the bottom of the mirror, leaving a significant portion of the mirror unsupported.
ScannerMAX scanners typically do not need a separate mirror mount assembly to attach the mirror. Rather, the mirror is mounted directly in to a slot in the output shaft. The additional inertia from the mirror mount is eliminated, and more of the mirror is supported, as seen in the figure below.
Figure 2: Conventional scanner rotor assembly (left) vs ScannerMAX rotor assembly (right)
By combining larger diameter shafts, extended output shafts to support the back side of the mirror, and the elimination of the stopping pin and mirror mount, resonances typically are not a concern with ScannerMAX scanners. When resonances occur during scanning action, they appear as vibrations or “wiggly lines” in a projected image. More violent resonances can significantly reduce scanner lifespan or destroy the mirror.
In addition, system inertia is significantly reduced in ScannerMAX scanners by removing unnecessary and redundant features. Lower system inertia is directly correlated to the amount of electrical current required to drive the galvo. The more electrical current required, the more generated by the galvo.
The largest barrier to faster scanning is related to heat generated by the scanner. All motors generate heat in order to produce torque, but the way the torque is generated and the heat is managed in ScannerMAX scanners is unique.
Conventional scanners have a construction similar to Figure 3, below. A copper coil is placed between an outer steel shell and the concentric rotating magnet. Heat must be removed from the coil as efficiently as possible, to prevent overheating of the coil and magnet and associated damage to the scanner. However, with conventional construction techniques, the heat in the magnet and the coil must overcome large air gaps to be able to escape to the steel outer shell and away from the system.
Figure 3: Cross section of a conventional scanner
In a ScannerMAX scanner, the copper wire is placed in slots in the stator, which allows for better heat transfer from the copper coil to the steel body, and away from the scanner. This also allows for the magnet to be closer to the steel body, which greatly increases the flux density. Combined, these techniques allow for the use of thicker copper wire, which lowers total coil resistance. Heat in the scanner is directly proportional to coil resistance – this means that ScannerMAX scanners run much cooler for a given amount of torque production.
As demonstrated above, by designing a stiffer rotor, ScannerMAX scanners are able to both reduce system inertia and also push out the system resonances. Lower inertia means less torque (and therefore less heat) is required to move the mirror. By pushing out resonances, the frequencies at which distorted images occurs is raised.
By redesigning the stator, ScannerMAX scanners have a lower coil resistance, and more efficient paths for heat generated to be removed from the scanner. Heat is typically the limiting factor of scan speeds, and overheating is the most common way which conventional scanners are destroyed.
By combining stiffer rotating assemblies with cooler stator operation, ScannerMAX scanners are able to scan faster than their conventional counterparts. These advantages can be seen in the figure below, which compares the scanning speed of a ScannerMAX galvo with the performance of a leading conventional galvo. For modern applications such as OCT, Confocal Microscopy, LIDAR, and many others, these advantages cannot be understated, because scan frequency, stability and reliability are paramount.
Figure 4: ScannerMAX performance vs Conventional galvo performance
For more detailed analysis on the design of ScannerMAX scanners and their advantages to their conventional counterparts, please Click Here to obtain a copy of our book LASER SCANNERS: Technologies and Applications by President and CTO Mr. William R. Benner Jr.